Modern Steel Products (2014) - Major Applications: lecture 24
Cooman, Bruno C. de
Professor de Cooman describes some of the major applications of steel in the context of the production technologies and metallurgy of the alloys. This is a part of a course of lectures given at the Graduate Institute of Ferrous Technology, POSTECH, Republic of Korea.
 So, what's, and so some of these products represent the highest strength steels that you can possibly purchase today for applications. And so, for instance, all the tires have a, use these products either as the beads here, yes, that make sure your tire is airtight, yes, and then here the part of the construction of the tire, you have these belts of tire cord. Okay, very important is in all cases strength, yes, but what equally important is the cleanliness of the steel, cleanliness of the steel, yes, cleanliness, very important is the product has a small section, yes, and in particular applications such as springs, you have suspension springs, for instance, and also the valve springs, yes, you need very high fatigue resistance, yes, and that also requires a very high level of steel cleanliness in addition to strength. Okay, these were typical wire products, and then the bar products are shown here, yes, with the main applications are shafts, very big applications are these crankshafts for automotive, yes, heavy springs, carburized gears is also very big application area for bar, yes, and of course in many cases, for instance, like if you have this shafts, machine ability is an additional requirement, fatigue is important in addition to strength, yes, okay, so fatigue fracture is a very big issue in bar products, yes, for instance, this is a broken crank shaft and another shaft here that broke through torsional fatigue, yes, so big issue, and as you know, very often, you may notice from your courses in fatigue, and if you're interested in fatigue in general, there is a course that Professor Zhang Xuli teaches on fatigue, internal cleanliness of the steel is essential, so because that's usually where the fatigue cracks are initiated, yes, so have a look at these products, these are general, probably covers most of the products for wire, what are the most important ones are the so-called cold-headed quality products, the tire core products, the wire steel products, the bearing steels, and the free cutting steels, yes, that's probably what represents the largest tonnage, yes, and when we talk about cold-headed quality, we also talk about the products are called fasteners, and what are fasteners, well, they're bolts, rivets, screws, you have, of course, when you need to make wire for nails, fences, the material is softer, yes, and so you have soft wire rod that you use in this case, and what you see in terms of the composition, I do want to point this out to you, that the compositions are very much composition where we get the strength from the carbon, yes, get the strength from the carbon, and from what we do to the microstructure, when you have high carbon, you know that if you have the equilibrium situation, you'll have, for this kind of carbon levels, you have ferrite and pearlite, yes, so you can do something to increase the strength by refining the microstructure, smaller grain sizes and refining the interlaminar spacing in the pearlite, you can also turn this steel into martensite, yes, martensite, you can tell with what grades you typically would do this, yes, where you see the chrome content, so for instance, a chrome content on the higher end, for instance, you see here chrome contents that are close to 1% in the case of spring steel, bearing steels, yes, particular for bearing steels, that level of chromium points to the fact that we are making martensite, we're not using a prolitic microstructure, and the chromium that's added in this particular case is to make the steel hardenable, so you can easily get martensite, correct. Now it's also very important to realize that the product that's created in the steel plant doesn't really have the properties and the microstructure of the, for instance, the bearing steel or the cold-heading quality fastener, so there are usually many steps between the production of wire rod, yes, and for instance, a bolt or a bearing or steel cord that you will be able to use in a wheel, in a tire, yes. So let's have a look, for instance, we won't go into all of them, we'll see a few things here, and what we basically see here is that there can be multiple steps in the processor, yes, the steel company will bring this wire to a company that will do additional thermal treatments such as spheroidizing, before it actually gets to the company that will make bolts or bearings, yes, and give the steel its final application. For instance, Posco may be making, yes, the steel wire here, but the bolts are not made by, for instance, made by a company, a very famous company from Sweden, SKF, very huge, or Timken, whatever, these companies are specialized, they may not make steel themselves, yes, and they basically buy these steels from steel makers, yes. And what's really important here, yes, is that these wire steels belong to the strongest types of steels you can get. Steel cord in the car tires easily reach 2,000, 3,000 mega Pascal, bridge wires, yes, are easily 2 giga Pascal, yes, standard, yes. A few words again about the microstructure here. So when you make perlite, yes, you can make a fully, because we'll see, we'll make fully perlitic microstructures, yes, at compositions that are not exactly the perlitic composition, well. So what do I want to say here? If you look at the, again, steel is very simple most of the time, yes, you only need to look at the iron-rich corner of the face diagram. So obviously when you have exactly, for instance in this case, 0.8% of carbon, you expect to see, well, it's not strange that you could form a ferrite, cementite lamellas, yes. But it turns out that if I have a composition here or I have a composition there, I can also get a fully perlitic microstructure. And that sounds kind of strange. How can you get fully perlitic if you're not exactly perlitic microstructure? To understand this, you have to understand why, what makes perlite grow. So let's see what makes perlite grow and ask ourselves, why does perlite grow? I have here ferrite and I have here cementite. This ferrite has very, very little carbon, carbon very, very small. And here the carbon is about pretty high, close to 7 weight percent. And we know that in order to grow, this is gamma, yes, in order for this interface to go from here to here, will be cementite to grow, yes, I basically need to get carbon to go from here to here. Makes sense, right? I have to get rid of it. But then how would that work? Because there's no carbon here and there's lots of carbon there. Why would carbon go where there's lots of carbon already? Obviously, the reason is because the carbon goes from this region of austenite to this region of austenite, yes, from this area to this area. Now what is the carbon content here and the carbon content here? That is the question, yes. Not obviously the carbon content, I just told you the carbon content, what the carbon content is the ferrite and what the carbon content is the cementite, yes. But that's not what makes the carbon move from here to here. What makes the carbon move from here to here is the carbon content at the ferrite austenite boundary and here the carbon content at the cementite austenite boundary. So say I would like to know what these concentrations are, yes, what are these concentrations? If I look at this phase diagram, if I look at this phase diagram, no problems. Say I'm doing the pearlite transformation at this temperature, this is the temperature at which I do the transformation from austenite to ferrite plus pearlite, yes. What is the carbon content at the interface, alpha gamma interface and the cementite, yes. And so I look at this and I say, well, how am I supposed to know? I can't get the information, right. The reason is of course, is this phase diagram, yes, tells you what is here at equilibrium, yes, but we don't have equilibrium. When you do the transformation at this temperature, you still have austenite during the transformation, right. And in order to know what the concentrations are, these two concentrations, I need to extend these lines, yes. So I need to know where these lines would be if there was no pearlite transformation, basically, if you want to see that. So I basically need to extend, and I can do this computationally, it's not a problem, I need to extend the A3 line and I need to extend the ACM line, yes. And if you do this, yes, what is this line here? This line here, compositions here correspond to austenite cementite concentrations, yes. And compositions on this line correspond to, excuse me, austenite, cementite, also, corresponds to ferrite austenite, yes. And of course, that continues here, yes. So this is the carbon content at the boundary between austenite and ferrite during the transformation, yes, and this is the carbon content at the cementite austenite boundary during the transformation, yes. And now you can see that, yes, it makes sense, the carbon content at this interface, yes, is higher than at this interface, yes. And so the carbon will move from here to there, and it will be controlled by the diffusion coefficient of carbon in austenite, that's the controlling parameter. It also means, yes, that I can make perlite when this condition is achieved, when the concentration here, yes, when C theta gamma is smaller than C alpha gamma, yes. And so that means I can have, I can form perlite here inside this triangle, and I do not need to have exactly the perlite composition. I can have slightly less, slightly more. And you can see it's not necessary to have 0.8% of carbon, you can make perlite if you have 0.6% of carbon, and it will be fully perlitic, yes, or I can go to 0.9% or higher, and it will still be fully perlitic, yes, because perlite can grow, all right. And D, but you can also see, of course, that things will change depending on the temperature at which I work, okay. So that's important, yes. D, so I have control by the carbon content, yes, and the process of creating, so the start of this process here is gamma, gamma, grain boundaries, you start forming little ferrite content, nuclei, so it's a nucleation and growth process, yes. So if you look at the kinetics of the transformation, I see that very close to AE1, of course I don't get much, okay. At low temperature also don't get much in terms of transformation, that's because my diffusivity, carbon diffusivity becomes lower, but there is a maximum rate of transformation, yes. So that's important, and we'll see this back in the, when we look at the processing of these steels, yes, between 600, 550 and 600, we get the very fastest growth rate, yes. There it takes one second for the perlite transformation to start, so very fast, and that's where we also get very fine perlite, okay. What is fine perlite, that's where you have a small interlaminar spacing, yes. Small interlaminar spacing means that you have lots of interfaces, yes, lots of interfaces, so anytime you have an interface that means you have more energy, interfacial energy. So small interlaminar spacings are unstable, yes. So if you heat this up, it can easily coarsen, yes. In fact, from a purely thermodynamic point of view, even a coarse perlite is unstable, again because of all this interfacial energy, and if you keep perlite long enough at between 6 and 700 degrees C, it will spheroidize, the perlite will turn, excuse me, the cementite will turn into little balls, yes, and that will minimize the interfacial energy. However, from an application point of view, this is just what we want, yes, because that's where we have the best properties, yes, okay, and that's the reason why. So as we decrease the temperature at which we do the transformation, what we see is that, so again in the region of 600 to 550, you see here that the growth rate of the perlite is very, very high, yes, and the corresponding interlaminar spacing is also very small, so you can get interlaminar spacings which are between 0.1 and 0.2 microns, so that's 100 to 200 nanometers, okay. So if you would do, for instance, the transformation very close to the A1 temperature, you would have a interlaminar spacing which is 10 times larger, okay, so it's control of the temperature at which you do the transformation is very important, okay, and people, we have, I don't want to go into this kinetics here, so anyway, let's have a look now at what these implicate, what the implications are, so, and look, for instance, at a steel, a wire steel that contains for instance 0.6 percent of carbon, yes, okay, how would we process this steel, okay, but first of all we will look at our phase diagram, we will have to extend the AECM line, the A3 line, yes, okay, and we will have to look at the TTT diagram, okay, so what it would be the TTT diagram here, so you can see that as soon as I go below this temperature, I will form, I will start to form pro-utectoid ferrite, okay, okay, if I am in this range of temperatures here, yes, I basically continue the same thing, yes, I only start to make perlite below this temperature, right, so when I am in this triangular region, yes, so if I want to make perlite, yes, I have to make sure that I cool very quickly to this temperature range, yes, not just being below the AE1 line will be enough, I have to go below this line here, below the AECM line, then I will start making perlite, okay, again, we are talking about, so if you would choose a, excuse me, a transformation temperature around 600, you would be fine, okay, because that is where you would have the maximum rate of transformation, okay, good, so we know that we want to have fine microstructure, of course, when I have less carbon, yes, this means that, I am exaggerating here, sorry, that is the reverse, if I have less carbon than the eutectoid composition, I will have more ferrite in my perlite, if I have a carbon content that is higher than the eutectoid, I will have more cementite in my perlite, okay, that is of course, it does not, the amount of cementite will change, yes, so it comes as no surprise that if you plot the strength of these steels, yes, as a function of carbon content, yes, you will see an increase because you have more cementite, more of this very hard phase in the structure, however, and you can see this here, even if you have a eutectoid or a slightly off-eutectoid composition, you can still increase the strength considerably by strain, yes, so what happens when you strain the perlite, you can see here, if I strain the material, yes, I can have at 0.8 carbon, I can have the same strength as at 1.4 carbon, yes, and the reason is that if you strain, oops, if you strain the perlite, you can reduce mechanically the interlaminar spacing, okay, so if I take this material here, yes, I have a certain interlaminar spacing, yes, if I wire draw this material, yes, now I have material where the spacing is much smaller, yes, yes, much smaller, much smaller than this 0.1 or 0.2 microns, I can now go to 0.05 microns, yes, so I can first reduce it, yes, than what I get after the transformation, okay, so and that is being used very often, so let's now look at some products and first let's look at cold-heading qualities, so cold-heading qualities are typically made, used to make these bolts, yes, and the carbon content is typically 0.4 percent, the structure itself, yes, of these, when you start with the wire, yes, the wire has a perlite ferrite microstructure, yes, but the bolt here doesn't have a ferrite, it has a martensitic microstructure, okay, we'll go through some processing to make this, so first of all let's have a look at these steels, they're typically what we call medium carbon steels, and these are steels that are used for engineering applications, to make things with basically, yes, so we call them engineering steels, and they're alloyed with chrome and molly, yes, because we want to get a martensitic microstructure, and what are the typical ranges, 0.3 to, close to 0.4, about 1 percent of chromium, and about 0.2 percent of lead, yes, and these are typical examples here, this is equivalent, and you remember here, SIJ, S for steel, C for chrome, M for molly, and 40 stands for 0.4 percent of carbon, here the European normalization, 0.42 carbon, chrome and molly, and then the four means we have chrome, yes, and you know that the four here is not 4 percent, it means four divided by four, this is about 1 percent, yeah, and here the SAE, ISSA where we recognize the 0.4 percent of carbon, okay, right, and so for these steels, this is the composition range, okay, it's similar to what you have here, but this is a wider range because it covers all the products, yes, okay, right, so let's make, let's go through the steps of making a bolt, yes, so you start with wire rod from, yes, and it's for instance, one of these steels is typically used to make wire rod, SCM 435 for instance, or the equivalent European and American grade, so the first thing you do, yes, is this alpha plus perlite microstructure is going through a softened needle, yeah, so this softened needle is to soften this material, this wire, yeah, the material has typically, when it leaves the plant, about 900 to 1100 megapascal in strength, yes, so we're going to soften it, yes, so it can be drawn, it can be drawn, yes, and then sphero diced, so we draw it, so it goes to all our section, yes, and then we sphero diced it to make it really soft, yes, and that usually happens in a company that's specializing in heat treatments, yeah, they just heat treat materials, yes, get materials, heat treat them, give them back to their customers, yes, so this, so when you do the softened kneeling you just go to below A1, yes, sphero dicing you may go a little bit higher than A1, yes, and then this product is now very soft, 500 to 600 megapascal, we're going to use it, we can cold form it, yes, and that is a number of steps, you start with the wire, yes, you cut, upset it, extruding, you finally make, thread this bolt, and that usually happens in another customer, that's the actual maker of bolts, yes, he will quench and temper it, so we get, for this particular grade we'll get about 1000 megapascal in strength, and he will apply the quench and tempering treatment, so he gets a tempered martensite microstructure, yes, okay, so this is the sphero dicing, this material is turned into material that looks like this, it's very small particles, yes, because this material is chromium and molybdenum added, usually it takes a long time to sphero dize wire steel, so okay, so we will be using a process, yes, you cannot use a continuous annealing process, because it's just too slow, so you use what, something that looks very much like a batch annealing furnace that we've seen for strip recrystallization, so in this furnace you put stacks of these wires, yes, as they are drawn or as they are produced, yes, and you cover them with insulation cover, and then you put in, put over that your furnace, yes, okay, and then you heat it up, the wire heats up, the softer annealing, so usually that's done with the same company, this is the first softer annealing that's right below AE1, the sphero dicing annealing, yes, is much, first of all much longer, 30 hours, yes, and you go slightly above AC1, why would that be, okay, because this is another picture where you perhaps see it better, where the wire material is, yes, the reason is why do we go slightly above AC1 when we do the sphero dicing, that's because the cementite particles we want to destabilize them, yes, so we partially dissolve this cementite in the austenite, but we leave some cementite in the microstructure, so we get cementite seeds, yes, so that when this slowly cools down, yes, instead of making perlite, we now get these very nice rounded sphero dites shaped cementite, yes, so there are many theories and models about how it really happens, and you can see some of these models, but anyway, and it's very interesting, physical metallurgy, but however, whatever the process is, it's a slow process, okay, so once you have this annealed sphero diced wire, you basically cut it in little pieces, yes, and then you go through the process of making a threaded bolt, yes, once you have this threaded bolt, you austenitize it, you can do some extra forming, yes, sometimes necessary, and then you can quench and temper the material, yes, and you end up with a tempered martensite which can be very strong indeed, yes, now high quality, bolts are very important, yes, in engineering applications, bolts are no joke, and so, and you can always recognize a high quality bolt, yes, because it's got the name of the company who made the bolt, yes, and it's also got numbers, and the numbers help you not making the error of using the wrong bolt, right, so for instance here you have 5.6 and here 8.8, the 8.8 is used very much in engineering application, the 8.8 tells you that the yield strength is 8 times 8 times 10, yes, that's 640 megapascal, and it also tells you the 10 cells, the UTS, so that's 8 times 100, so you do the same on the 5.6, 5 times 300, and 500 megapascals as UTS, okay, and so again as I said, the serious business in engineering construction, you don't mix bolts, yes, even if they look the same, yes, why, because there could be serious consequences if you use a weaker bolt, yes, and the building collapses, or the machine breaks, or it's a high pressure vessel, and the bolts are not the ones you should be using, there may be explosions and things like this, so very serious things, and so that's how you can tell these are good bolts, and usually in engineering application they are also specified what kind of bolt you should be using, okay, another interesting application for wire seals are spring seals, and so springs can come in many different forms, right, you have strips, for instance this light spring here, yes, very commonly used for household items, but also this suspension spring here from a car, and of course motors have also many springs in the valves, yes, so typically about 45 to point 70 in carbon contents, yes, one of the things you will see in these spring steels is the high silicon, high silicon is because these steels are not aluminum kilts, yes, you want them to be silicon kilts, so you don't have these non-metallic inclusions that will impact the fatigue resistance, yeah, the chrome content, yes, points to the fact that we need hardenability here, okay, and in certain cases you have copper, so let's have a look, what's also very important here for spring steels is the control of the nitrogen content, yes, because you don't want nitrogen aging in these steels, it turns out that the nitrogen reduces the dectility of these carbon wires, and we add boron to this, these steels, to make boron nitride, yes, and that reduces the effect of the nitrogen, you can see the effect of nitrogen on the properties of the wire by twisting the wires, you twist the wires and if there's nitrogen aging, the number of twists is very small, yes, if you add boron you can have enough twists before the wire breaks, typically you will need to be able to do a minimum of 40 twists, okay, springs have their own technologies when they're made, for instance suspension springs can be hot processed or cold processed, yes, coil type springs are usually always cold formed, the coil type, yes, hot formed can be applied for coil type but also for leaf type springs, and this is a lightweight leaf type spring but this also applies to leaf type springs that you use for trucks, okay, so let's have a look at how we make engine valves, yes, again if the steel company makes wire, for instance typically that would be 5 to 13 millimeter section wire, yes, it goes through two steps, yes, so POSCO makes wire, doesn't make springs, yes, and first place the wire goes to is a company that does heat treatment, that prepares the steel so it can be processed by a spring manufacturer, okay, so the first one customer will coat it, will make the wire drawing, yes, so will reduce the section of the wire, will do patenting, what is patenting, when you deform the pearlite, yes, it's you reduce the interlaminar spacing but the material becomes very hard, so if you want to continue processing it, reducing the interlaminar spacing even further, you have to anneal it, yes, and that's this patenting process here, so you do wire drawing until you have the required section, yes, and then oil quenching and tempering, okay, so that means in this case you make the final microstructure and properties, right, now what happens then with the spring manufacturer for valve springs again, yes, you do cold forming into, yes, there will be stress relief after the cold forming, shot peening, you know that in order to improve fatigue resistance, it's good to have compressive residual stresses at the surface of parts that undergo fatigue conditions, yes, shot peening then, stress relief again to remove excessive residual stresses and then pre-setting of the spring, so you can, and then you're done basically. What kind of strength levels, well for instance if you use an SE 9254 here, about 2000 mega Pascal in strength, the wires for suspension bridges and another cables, yes, it's a very specialized business, wires and in particular cables because of the ways you thread the wires basically, you can have open spiral strands like this, usually hot dip galvanized steels, steel wires, yes, and they're usually twisted or helically wounded around the core, but you can have more complex versions like where the wires are shaped, for instance here in this case you have an outer, the outer cables here are locked, the strands are locked, so you cannot access these inner wires, yes, you have z-shaped wires and they are self-locking, corrosion protection is very important, so most of these wires will be galvanized and so it can be very complex, so suspension bridge cables are not just little wires next to each other and depending on the application, the stiffness you want, the flexibility you want, you will use different ways of organizing the wires around a central wire and also if you look carefully here, you can see that there is also, you also use a mixture of wire diameters, right, for instance you can see here that in between the larger wires you filled in the interstices with smaller wires, yes, okay, right, so let me skip this one here, very important also in the wires, yes, is because we use this, the ability to reduce, to refine the microstructure when we, when we draw the wire, yes, the wire diameter, yes, is an essential contributor to strength, yes, so if you use, so for instance same grade here, see this black grade, about 1% of carbon, yes, goes from here to here, you can see that if I use say 1, 2, 3, 4, 5 millimeter wire, yes, will have a strength of 1,000, but I can go up to 5,000, yes, if I can make the wire very, very fine, yes, okay, okay, so, and that's of course what you will use because you don't have to do anything to the composition basically of the, of the steel, alright, so, the wire drawing again doesn't, also doesn't have to be circular, it can be complex shapes, so you can design very complex cable sections, yes, and, and so, one of the things you will be, being as this patenting, to get the optimum fine perlite microstructure, yes, the, where you basically want very fine microstructure, and again, you know, the optimal range of transformation will be in the 600 to 550 degrees C, okay, in, so for the, for wire steels, what we see is a evolution from about eutectoid compositions, which give us about close to 3,000 megapascal, to higher hyper eutectoid compositions, yes, so, 0.8, the eutectoid composition can give me close to 4 megapascal, and I could definitely in hyper eutectoid compositions with about close to 1%, I can get 4,000 megapascal in strength for cables, yes, so typically in applications as high carbon bridge suspension wire or for tire cord reinforcement of tires, we will use these high carbon levels for strength, so, and the strength, the way we work to, the strength is decrease interlaminar spacing, you can also of course increase the strength of the ferrite, but increasing the amount of the cementite is the easiest way to have a very high strength, so, to up the carbon content, you can also increase the work hardening of the wire, yes, again, an increased amount of cementite will improve this, you can do this with higher carbon content, but also higher chrome additions, yes, and then drawing strains is very important, use very small sections, yes, smaller wire diameters, yes, you can see here by the way the effect of chrome additions to the strength here, so if you just look at a wire that is basically pearlitic, yes, you see that you can increase the strength by adding some chromium, so very important here is the drawing, right, because the drawing allows you to go beyond what the thermodynamics will give you in terms of the deformation, in terms of refining the grain size, yes, so, and it's also very important to see that in fact you could do this with any material, right, so if you would use like an IF steel, yes, and you would strain it as a wire for, and refine the microstructure this way, you would, you know, you would be able to achieve a 2 gigapascal without any alloying, yes, by basically refining the microstructure, but if you have a two-phase microstructure like pearlite, yes, the effect is even stronger, yes, you can see here at the same reduction you have about five times the impact of the deformation to smaller wire diameters, yeah, nowadays 100 nanometers and lower are very normal in industry in terms of pearlite refinements, okay, the high carbon wires, again, you start with wire rod, well for instance, be typically say five millimeters, you do first wire drawing to three millimeters, first pattern thing, so it means you're going to soften the microstructure again, yes, so you reheat it basically and then you cool down, you make fine pearlite, yes, so you do a second wire drawing to from three millimeters to one to two millimeters, now the strength goes up all the time to 1700, 1800, second pattern thing, yes, of course the thickness, the diameter hasn't changed, your strength is reduced now, 12, 39, you can do, when the wire gets really fine, it becomes difficult to do the wire drawing, so you need to know how to do it and one of the ways you do it is by coating the wire, for instance with brass, yes, and so now you are at, so you started with one millimeters, after in the third drawing step you can go to a fraction of a millimeter, a tenth to half a millimeter and the strength levels reach more than three megapascal and then you do the cabling, yes, there may be many additional steps which are not included here such as pickling, phosphating, remember maybe that I talked about phosphating is important in technology because it allows you to reduce friction, yes, it's not indicated but it's one important part of the step for the production steps in drawing and then of course many of these wires, they are extremely high strength, they're very thin, right, and they're made of steel, carbon steel, we don't, you don't want them to rust away, okay, so it's also important to coat these wires, yes, so galvanizing is a very, very often used to coat the wire, yes, bearing steels are steels which contain probably the highest levels of carbon, yes, so there are not many steel grades that contain one percent, around one percent of carbon, so bearing steels are typically around that level, so they're all hyper eutectoid steels, yes, okay, again you start making bearing steels by obtaining a wire steel, one percent carbon wire steel in the steel plant and then it goes to a company that will do, it will prepare the wire for the producer of the ball bearings or another types of bearings, yes, okay, and the, the company that prepares the wire is again mostly a thermal treatment company that will do a low temperature annealing, so to soften the material, to draw the material so you get the right dimensions, yes, and then it will do a spheroidizing annealing, again to make it very soft, why do we want to make this, the wire very soft, because we need to make very accurate metal balls, yes, from this wire, so we need to forge, do some cold forging of the wire to make these metal balls, yes, and then we need to heat treat the soft balls to a martensitic microstructure and there may also be surface treatment, so okay, so typically these bearing steels they will be divided into through hardening steels or case hardening steels, the through hardening steels, yes, have a very high carbon content, yes, and also tend to have a very high chrome and moly content, so what does that mean, that means that we're going to, when we heat treat these balls, yes, into like an oil bath, the entire structure of the bearing will be martensitic, will be fully martensitic, if we have lower carbon contents, yes, slightly lower chrome and moly, then only the outer side is martensitic and the central part can be, you know, bainitic, prolitic, depending, yes, so we want, in this case, that's why would you want to have a structure that's only case hardenable, that's because you want to have, for instance, a tougher, more ductile center part of the bearing, so here this is a typical bearing steel here, 1% of carbon and 1.4% of chromium, yes, for the hardenability, free cutting steels are also typically wire bar products, yes, free cutting refers to the fact that these products will be going through machining, yes, like a CNC machine, yes, and carbon contents are typically 0.1 to 0.5, yes, what is special about these is the sulfur content, yes, certain free cutting steels will require 0.2 to 0.4% of sulfur, that's a huge amount of sulfur, remember, for flat roll products, we went out of our way to have preferably no sulfur, yes, in this case you add the sulfur in it and the reason is because when you're working on a workpiece with a tool in the lathe, for instance, a CNC machine, yes, if you look here in detail, the tool cuts away chips, basically, of the metal, chips away the metal and the formation of the easy formation of these chips is very important, yes, and if you have manganese sulfides particles in your microstructure, this really helps chip formation, so your tools can be used longer and the cut, the quality of the cut is better, yes, okay, now I do have to say that these requirements are relatively high sulfur contents are really lately not so useful because in many cases the tool, the quality of the tool material has improved a lot, so the tools can take the higher heat and the higher stresses during the cutting, yes, and also the use of CNC machines has improved the machining a lot, so we don't really have to rely so much on sulfur inclusions in our material to get good machinability, it's still important when you drill small holes, I have to say, so if you have engineering parts that require small holes, this presence of the sulfur helps, very careful, however, when you add sulfur, you get all the negative problems of sulfur in that part, so you cannot add sulfur, yes, if the application doesn't allow, for instance, for instance, if you have seamless tubes, yes, and you need to make thread on these seamless tubes, yes, so the thread, the part that you will use to make the connection, yes, it will be machined, yes, that must be resistant to stress corrosion cracking in applications, yes, so that's no sulfur in this case, yes, okay, so very important limitation, finally, and this is not a very high carbon applications, very important in terms of tonnage are rebar, now you have to know that you can make rebar from any steel, right, so you can buy very this is people typically think of rebar as a cheap product, yes, however, there are, you can make rebar from very expensive stainless steels, okay, and it depends on the application, for instance, in Switzerland, all the, or most of the civil engineering constructions, like bridges and roads, in the mountains, they use very expensive stainless steel rebars, because it's difficult to do, to inspect roads and bridges in the Alps, you know, lots of snow and stuff, so you want to have guaranteed that, you know, there's going to be no deterioration of rebar, etc., so in a certain European countries, in particular in the mountainous regions, you will use stainless steel types of rebar, yes, but most of us think of rebar as, you know, very commodity product, that is for which we use plain carbon steels at 0.2% of carbon, less than 0.4% for the carbon equivalent, so you can weld them, yield strength is pretty low, 250, 500 for high strength steels, the strength can be because of the carbon level, or mechanically, sometimes you can get work hardened rebar, that's just harder because it's twisted, yes, or more expensive are vanadium micro alloyed grades, where the hardening comes from precipitation hardening, okay, again here the microstructure of these rebar is kind of nice, yes, typically nowadays you will try to make your rebar both strong, yes, and tough, yes, and that's happened usually this way, and when you make your rebar, you finish the rolling at 900, and then you have a three-stage process, in the first stage you make martensite at the surface of your rebar by strong cooling, yes, this would be what happens to the surface, it's quickly cooled to below the MS temperature, so the surface is basically quenched, that's the first stage that you do, and then the second stage you temper the martensite as a result of the temperature gradient equalization, so you do very quick cool of the surface, yes, and then you at this point here you stop the cooling, so the martensite at the surface is reheated, yes, and tempered, the layer below the surface doesn't get to be transformed to martensite and goes straight into the bainite transformation, yes, and in the third stage, yes, you also do the center, in the third stage the center is cooled, yes, and here we get either upper bainite or even ferrite perlite microstructure, so a rebar, yes, you can see here section of a typical rebar, 16 millimeter rebar, it's actually quite an interesting microstructure, very hard outer surface that progressively gets softer and more tougher as you go to the center, so at the outside we have lath martensite, and as you go inside you get bainite, and as you see here in the middle you can even see the perlite in the microstructure, so that makes for nice properties, all right, and so with this small review of products and their physical metallurgy we'll just stop the session here for today, so thank you for your attention.
 So, what's, and so some of these products represent the highest strength steels that  you can possibly purchase today for applications. And so, for instance, all the tires have a,  use these products either as the beads here, yes, that make sure your tire is airtight,  yes, and then here the part of the construction of the tire, you have these belts of tire  cord. Okay, very important is in all cases strength, yes, but what equally important  is the cleanliness of the steel, cleanliness of the steel, yes, cleanliness, very important  is the product has a small section, yes, and in particular applications such as springs,  you have suspension springs, for instance, and also the valve springs, yes, you need  very high fatigue resistance, yes, and that also requires a very high level of steel cleanliness  in addition to strength. Okay, these were typical wire products, and then the bar products  are shown here, yes, with the main applications are shafts, very big applications are these  crankshafts for automotive, yes, heavy springs, carburized gears is also very big application  area for bar, yes, and of course in many cases, for instance, like if you have this shafts,  machine ability is an additional requirement, fatigue is important in addition to strength,  yes, okay, so fatigue fracture is a very big issue in bar products, yes, for instance,  this is a broken crank shaft and another shaft here that broke through torsional fatigue,  yes, so big issue, and as you know, very often, you may notice from your courses in fatigue,  and if you're interested in fatigue in general, there is a course that Professor Zhang Xuli  teaches on fatigue, internal cleanliness of the steel is essential, so because that's  usually where the fatigue cracks are initiated, yes, so have a look at these products, these  are general, probably covers most of the products for wire, what are the most important ones  are the so-called cold-headed quality products, the tire core products, the wire steel products,  the bearing steels, and the free cutting steels, yes, that's probably what represents the largest  tonnage, yes, and when we talk about cold-headed quality, we also talk about the products are  called fasteners, and what are fasteners, well, they're bolts, rivets, screws, you have,  of course, when you need to make wire for nails, fences, the material is softer, yes,  and so you have soft wire rod that you use in this case, and what you see in terms of  the composition, I do want to point this out to you, that the compositions are very much  composition where we get the strength from the carbon, yes, get the strength from the  carbon, and from what we do to the microstructure, when you have high carbon, you know that  if you have the equilibrium situation, you'll have, for this kind of carbon levels, you  have ferrite and pearlite, yes, so you can do something to increase the strength by refining  the microstructure, smaller grain sizes and refining the interlaminar spacing in the pearlite,  you can also turn this steel into martensite, yes, martensite, you can tell with what grades  you typically would do this, yes, where you see the chrome content, so for instance, a  chrome content on the higher end, for instance, you see here chrome contents that are close  to 1% in the case of spring steel, bearing steels, yes, particular for bearing steels,  that level of chromium points to the fact that we are making martensite, we're not using  a prolitic microstructure, and the chromium that's added in this particular case is to  make the steel hardenable, so you can easily get martensite, correct.  Now it's also very important to realize that the product that's created in the steel plant  doesn't really have the properties and the microstructure of the, for instance, the bearing  steel or the cold-heading quality fastener, so there are usually many steps between the  production of wire rod, yes, and for instance, a bolt or a bearing or steel cord that you  will be able to use in a wheel, in a tire, yes.  So let's have a look, for instance, we won't go into all of them, we'll see a few things  here, and what we basically see here is that there can be multiple steps in the processor,  yes, the steel company will bring this wire to a company that will do additional thermal  treatments such as spheroidizing, before it actually gets to the company that will make  bolts or bearings, yes, and give the steel its final application.  For instance, Posco may be making, yes, the steel wire here, but the bolts are not made  by, for instance, made by a company, a very famous company from Sweden, SKF, very huge,  or Timken, whatever, these companies are specialized, they may not make steel themselves, yes, and  they basically buy these steels from steel makers, yes.  And what's really important here, yes, is that these wire steels belong to the strongest  types of steels you can get.  Steel cord in the car tires easily reach 2,000, 3,000 mega Pascal, bridge wires, yes, are  easily 2 giga Pascal, yes, standard, yes.  A few words again about the microstructure here.  So when you make perlite, yes, you can make a fully, because we'll see, we'll make fully  perlitic microstructures, yes, at compositions that are not exactly the perlitic composition,  well.  So what do I want to say here?  If you look at the, again, steel is very simple most of the time, yes, you only need to look  at the iron-rich corner of the face diagram.  So obviously when you have exactly, for instance in this case, 0.8% of carbon, you expect to  see, well, it's not strange that you could form a ferrite, cementite lamellas, yes.  But it turns out that if I have a composition here or I have a composition there, I can  also get a fully perlitic microstructure.  And that sounds kind of strange.  How can you get fully perlitic if you're not exactly perlitic microstructure?  To understand this, you have to understand why, what makes perlite grow.  So let's see what makes perlite grow and ask ourselves, why does perlite grow?  I have here ferrite and I have here cementite.  This ferrite has very, very little carbon, carbon very, very small.  And here the carbon is about pretty high, close to 7 weight percent.  And we know that in order to grow, this is gamma, yes, in order for this interface to  go from here to here, will be cementite to grow, yes, I basically need to get carbon  to go from here to here.  Makes sense, right?  I have to get rid of it.  But then how would that work?  Because there's no carbon here and there's lots of carbon there.  Why would carbon go where there's lots of carbon already?  Obviously, the reason is because the carbon goes from this region of austenite to this  region of austenite, yes, from this area to this area.  Now what is the carbon content here and the carbon content here?  That is the question, yes.  Not obviously the carbon content, I just told you the carbon content, what the carbon content  is the ferrite and what the carbon content is the cementite, yes.  But that's not what makes the carbon move from here to here.  What makes the carbon move from here to here is the carbon content at the ferrite austenite  boundary and here the carbon content at the cementite austenite boundary.  So say I would like to know what these concentrations are, yes, what are these concentrations?  If I look at this phase diagram, if I look at this phase diagram, no problems.  Say I'm doing the pearlite transformation at this temperature, this is the temperature  at which I do the transformation from austenite to ferrite plus pearlite, yes.  What is the carbon content at the interface, alpha gamma interface and the cementite, yes.  And so I look at this and I say, well, how am I supposed to know?  I can't get the information, right.  The reason is of course, is this phase diagram, yes, tells you what is here at equilibrium,  yes, but we don't have equilibrium.  When you do the transformation at this temperature, you still have austenite during the transformation,  right.  And in order to know what the concentrations are, these two concentrations, I need to extend  these lines, yes.  So I need to know where these lines would be if there was no pearlite transformation,  basically, if you want to see that.  So I basically need to extend, and I can do this computationally, it's not a problem,  I need to extend the A3 line and I need to extend the ACM line, yes.  And if you do this, yes, what is this line here?  This line here, compositions here correspond to austenite cementite concentrations, yes.  And compositions on this line correspond to, excuse me, austenite, cementite, also, corresponds  to ferrite austenite, yes.  And of course, that continues here, yes.  So this is the carbon content at the boundary between austenite and ferrite during the transformation,  yes, and this is the carbon content at the cementite austenite boundary during the transformation,  yes.  And now you can see that, yes, it makes sense, the carbon content at this interface, yes,  is higher than at this interface, yes.  And so the carbon will move from here to there, and it will be controlled by the diffusion  coefficient of carbon in austenite, that's the controlling parameter.  It also means, yes, that I can make perlite when this condition is achieved, when the  concentration here, yes, when C theta gamma is smaller than C alpha gamma, yes.  And so that means I can have, I can form perlite here inside this triangle, and I do not need  to have exactly the perlite composition.  I can have slightly less, slightly more.  And you can see it's not necessary to have 0.8% of carbon, you can make perlite if you  have 0.6% of carbon, and it will be fully perlitic, yes, or I can go to 0.9% or higher,  and it will still be fully perlitic, yes, because perlite can grow, all right.  And D, but you can also see, of course, that things will change depending on the temperature  at which I work, okay.  So that's important, yes.  D, so I have control by the carbon content, yes, and the process of creating, so the start  of this process here is gamma, gamma, grain boundaries, you start forming little ferrite  content, nuclei, so it's a nucleation and growth process, yes.  So if you look at the kinetics of the transformation, I see that very close to AE1, of course I  don't get much, okay.  At low temperature also don't get much in terms of transformation, that's because my  diffusivity, carbon diffusivity becomes lower, but there is a maximum rate of transformation,  yes.  So that's important, and we'll see this back in the, when we look at the processing of  these steels, yes, between 600, 550 and 600, we get the very fastest growth rate, yes.  There it takes one second for the perlite transformation to start, so very fast, and  that's where we also get very fine perlite, okay.  What is fine perlite, that's where you have a small interlaminar spacing, yes.  Small interlaminar spacing means that you have lots of interfaces, yes, lots of interfaces,  so anytime you have an interface that means you have more energy, interfacial energy.  So small interlaminar spacings are unstable, yes.  So if you heat this up, it can easily coarsen, yes.  In fact, from a purely thermodynamic point of view, even a coarse perlite is unstable,  again because of all this interfacial energy, and if you keep perlite long enough at between  6 and 700 degrees C, it will spheroidize, the perlite will turn, excuse me, the cementite  will turn into little balls, yes, and that will minimize the interfacial energy.  However, from an application point of view, this is just what we want, yes, because that's  where we have the best properties, yes, okay, and that's the reason why.  So as we decrease the temperature at which we do the transformation, what we see is that,  so again in the region of 600 to 550, you see here that the growth rate of the perlite  is very, very high, yes, and the corresponding interlaminar spacing is also very small, so  you can get interlaminar spacings which are between 0.1 and 0.2 microns, so that's 100  to 200 nanometers, okay.  So if you would do, for instance, the transformation very close to the A1 temperature, you would  have a interlaminar spacing which is 10 times larger, okay, so it's control of the temperature  at which you do the transformation is very important, okay, and people, we have, I don't  want to go into this kinetics here, so anyway, let's have a look now at what these implicate,  what the implications are, so, and look, for instance, at a steel, a wire steel that contains  for instance 0.6 percent of carbon, yes, okay, how would we process this steel, okay, but  first of all we will look at our phase diagram, we will have to extend the AECM line, the  A3 line, yes, okay, and we will have to look at the TTT diagram, okay, so what it would  be the TTT diagram here, so you can see that as soon as I go below this temperature, I  will form, I will start to form pro-utectoid ferrite, okay, okay, if I am in this range  of temperatures here, yes, I basically continue the same thing, yes, I only start to make  perlite below this temperature, right, so when I am in this triangular region, yes,  so if I want to make perlite, yes, I have to make sure that I cool very quickly to this  temperature range, yes, not just being below the AE1 line will be enough, I have to go  below this line here, below the AECM line, then I will start making perlite, okay, again,  we are talking about, so if you would choose a, excuse me, a transformation temperature  around 600, you would be fine, okay, because that is where you would have the maximum rate  of transformation, okay, good, so we know that we want to have fine microstructure,  of course, when I have less carbon, yes, this means that, I am exaggerating here, sorry,  that is the reverse, if I have less carbon than the eutectoid composition, I will have  more ferrite in my perlite, if I have a carbon content that is higher than the eutectoid,  I will have more cementite in my perlite, okay, that is of course, it does not, the  amount of cementite will change, yes, so it comes as no surprise that if you plot the  strength of these steels, yes, as a function of carbon content, yes, you will see an increase  because you have more cementite, more of this very hard phase in the structure, however,  and you can see this here, even if you have a eutectoid or a slightly off-eutectoid composition,  you can still increase the strength considerably by strain, yes, so what happens when you strain  the perlite, you can see here, if I strain the material, yes, I can have at 0.8 carbon,  I can have the same strength as at 1.4 carbon, yes, and the reason is that if you strain,  oops, if you strain the perlite, you can reduce mechanically the interlaminar spacing, okay,  so if I take this material here, yes, I have a certain interlaminar spacing, yes, if I  wire draw this material, yes, now I have material where the spacing is much smaller, yes, yes,  much smaller, much smaller than this 0.1 or 0.2 microns, I can now go to 0.05 microns,  yes, so I can first reduce it, yes, than what I get after the transformation, okay, so and  that is being used very often, so let's now look at some products and first let's look  at cold-heading qualities, so cold-heading qualities are typically made, used to make  these bolts, yes, and the carbon content is typically 0.4 percent, the structure itself,  yes, of these, when you start with the wire, yes, the wire has a perlite ferrite microstructure,  yes, but the bolt here doesn't have a ferrite, it has a martensitic microstructure, okay,  we'll go through some processing to make this, so first of all let's have a look at these  steels, they're typically what we call medium carbon steels, and these are steels that are  used for engineering applications, to make things with basically, yes, so we call them  engineering steels, and they're alloyed with chrome and molly, yes, because we want to get  a martensitic microstructure, and what are the typical ranges, 0.3 to, close to 0.4,  about 1 percent of chromium, and about 0.2 percent of lead, yes, and these are typical  examples here, this is equivalent, and you remember here, SIJ, S for steel, C for chrome,  M for molly, and 40 stands for 0.4 percent of carbon, here the European normalization,  0.42 carbon, chrome and molly, and then the four means we have chrome, yes, and you know  that the four here is not 4 percent, it means four divided by four, this is about 1 percent,  yeah, and here the SAE, ISSA where we recognize the 0.4 percent of carbon, okay, right, and  so for these steels, this is the composition range, okay, it's similar to what you have  here, but this is a wider range because it covers all the products, yes, okay, right,  so let's make, let's go through the steps of making a bolt, yes, so you start with wire  rod from, yes, and it's for instance, one of these steels is typically used to make wire  rod, SCM 435 for instance, or the equivalent European and American grade, so the first  thing you do, yes, is this alpha plus perlite microstructure is going through a softened  needle, yeah, so this softened needle is to soften this material, this wire, yeah, the  material has typically, when it leaves the plant, about 900 to 1100 megapascal in strength,  yes, so we're going to soften it, yes, so it can be drawn, it can be drawn, yes, and  then sphero diced, so we draw it, so it goes to all our section, yes, and then we sphero  diced it to make it really soft, yes, and that usually happens in a company that's  specializing in heat treatments, yeah, they just heat treat materials, yes, get materials,  heat treat them, give them back to their customers, yes, so this, so when you do the softened  kneeling you just go to below A1, yes, sphero dicing you may go a little bit higher than  A1, yes, and then this product is now very soft, 500 to 600 megapascal, we're going to  use it, we can cold form it, yes, and that is a number of steps, you start with the wire,  yes, you cut, upset it, extruding, you finally make, thread this bolt, and that usually happens  in another customer, that's the actual maker of bolts, yes, he will quench and temper it,  so we get, for this particular grade we'll get about 1000 megapascal in strength, and  he will apply the quench and tempering treatment, so he gets a tempered martensite microstructure,  yes, okay, so this is the sphero dicing, this material is turned into material that looks  like this, it's very small particles, yes, because this material is chromium and molybdenum  added, usually it takes a long time to sphero dize wire steel, so okay, so we will be using  a process, yes, you cannot use a continuous annealing process, because it's just too slow,  so you use what, something that looks very much like a batch annealing furnace that we've  seen for strip recrystallization, so in this furnace you put stacks of these wires, yes,  as they are drawn or as they are produced, yes, and you cover them with insulation cover,  and then you put in, put over that your furnace, yes, okay, and then you heat it up, the wire  heats up, the softer annealing, so usually that's done with the same company, this is  the first softer annealing that's right below AE1, the sphero dicing annealing, yes, is  much, first of all much longer, 30 hours, yes, and you go slightly above AC1, why would  that be, okay, because this is another picture where you perhaps see it better, where the  wire material is, yes, the reason is why do we go slightly above AC1 when we do the sphero  dicing, that's because the cementite particles we want to destabilize them, yes, so we partially  dissolve this cementite in the austenite, but we leave some cementite in the microstructure,  so we get cementite seeds, yes, so that when this slowly cools down, yes, instead of making  perlite, we now get these very nice rounded sphero dites shaped cementite, yes, so there  are many theories and models about how it really happens, and you can see some of these  models, but anyway, and it's very interesting, physical metallurgy, but however, whatever  the process is, it's a slow process, okay, so once you have this annealed sphero diced  wire, you basically cut it in little pieces, yes, and then you go through the process of  making a threaded bolt, yes, once you have this threaded bolt, you austenitize it, you  can do some extra forming, yes, sometimes necessary, and then you can quench and temper  the material, yes, and you end up with a tempered martensite which can be very strong indeed,  yes, now high quality, bolts are very important, yes, in engineering applications, bolts are  no joke, and so, and you can always recognize a high quality bolt, yes, because it's got  the name of the company who made the bolt, yes, and it's also got numbers, and the numbers  help you not making the error of using the wrong bolt, right, so for instance here you  have 5.6 and here 8.8, the 8.8 is used very much in engineering application, the 8.8  tells you that the yield strength is 8 times 8 times 10, yes, that's 640 megapascal, and  it also tells you the 10 cells, the UTS, so that's 8 times 100, so you do the same on  the 5.6, 5 times 300, and 500 megapascals as UTS, okay, and so again as I said, the serious  business in engineering construction, you don't mix bolts, yes, even if they look the  same, yes, why, because there could be serious consequences if you use a weaker bolt, yes,  and the building collapses, or the machine breaks, or it's a high pressure vessel, and  the bolts are not the ones you should be using, there may be explosions and things like this,  so very serious things, and so that's how you can tell these are good bolts, and usually  in engineering application they are also specified what kind of bolt you should be using, okay,  another interesting application for wire seals are spring seals, and so springs can come  in many different forms, right, you have strips, for instance this light spring here, yes, very  commonly used for household items, but also this suspension spring here from a car, and  of course motors have also many springs in the valves, yes, so typically about 45 to  point 70 in carbon contents, yes, one of the things you will see in these spring steels  is the high silicon, high silicon is because these steels are not aluminum kilts, yes,  you want them to be silicon kilts, so you don't have these non-metallic inclusions that  will impact the fatigue resistance, yeah, the chrome content, yes, points to the fact  that we need hardenability here, okay, and in certain cases you have copper, so let's  have a look, what's also very important here for spring steels is the control of the nitrogen  content, yes, because you don't want nitrogen aging in these steels, it turns out that the  nitrogen reduces the dectility of these carbon wires, and we add boron to this, these steels,  to make boron nitride, yes, and that reduces the effect of the nitrogen, you can see the  effect of nitrogen on the properties of the wire by twisting the wires, you twist the  wires and if there's nitrogen aging, the number of twists is very small, yes, if you  add boron you can have enough twists before the wire breaks, typically you will need to  be able to do a minimum of 40 twists, okay, springs have their own technologies when they're  made, for instance suspension springs can be hot processed or cold processed, yes, coil  type springs are usually always cold formed, the coil type, yes, hot formed can be applied  for coil type but also for leaf type springs, and this is a lightweight leaf type spring  but this also applies to leaf type springs that you use for trucks, okay, so let's have  a look at how we make engine valves, yes, again if the steel company makes wire, for  instance typically that would be 5 to 13 millimeter section wire, yes, it goes through  two steps, yes, so POSCO makes wire, doesn't make springs, yes, and first place the wire  goes to is a company that does heat treatment, that prepares the steel so it can be processed  by a spring manufacturer, okay, so the first one customer will coat it, will make the wire  drawing, yes, so will reduce the section of the wire, will do patenting, what is patenting,  when you deform the pearlite, yes, it's you reduce the interlaminar spacing but the material  becomes very hard, so if you want to continue processing it, reducing the interlaminar  spacing even further, you have to anneal it, yes, and that's this patenting process here,  so you do wire drawing until you have the required section, yes, and then oil quenching  and tempering, okay, so that means in this case you make the final microstructure and  properties, right, now what happens then with the spring manufacturer for valve springs  again, yes, you do cold forming into, yes, there will be stress relief after the cold  forming, shot peening, you know that in order to improve fatigue resistance, it's good to  have compressive residual stresses at the surface of parts that undergo fatigue conditions,  yes, shot peening then, stress relief again to remove excessive residual stresses and  then pre-setting of the spring, so you can, and then you're done basically.  What kind of strength levels, well for instance if you use an SE 9254 here, about 2000 mega  Pascal in strength, the wires for suspension bridges and another cables, yes, it's a very  specialized business, wires and in particular cables because of the ways you thread the  wires basically, you can have open spiral strands like this, usually hot dip galvanized  steels, steel wires, yes, and they're usually twisted or helically wounded around the core,  but you can have more complex versions like where the wires are shaped, for instance here  in this case you have an outer, the outer cables here are locked, the strands are locked,  so you cannot access these inner wires, yes, you have z-shaped wires and they are self-locking,  corrosion protection is very important, so most of these wires will be galvanized and  so it can be very complex, so suspension bridge cables are not just little wires next to each  other and depending on the application, the stiffness you want, the flexibility you want,  you will use different ways of organizing the wires around a central wire and also if  you look carefully here, you can see that there is also, you also use a mixture of wire  diameters, right, for instance you can see here that in between the larger wires you  filled in the interstices with smaller wires, yes, okay, right, so let me skip this one  here, very important also in the wires, yes, is because we use this, the ability to reduce,  to refine the microstructure when we, when we draw the wire, yes, the wire diameter,  yes, is an essential contributor to strength, yes, so if you use, so for instance same grade  here, see this black grade, about 1% of carbon, yes, goes from here to here, you can see that  if I use say 1, 2, 3, 4, 5 millimeter wire, yes, will have a strength of 1,000, but I  can go up to 5,000, yes, if I can make the wire very, very fine, yes, okay, okay, so,  and that's of course what you will use because you don't have to do anything to the composition  basically of the, of the steel, alright, so, the wire drawing again doesn't, also doesn't  have to be circular, it can be complex shapes, so you can design very complex cable sections,  yes, and, and so, one of the things you will be, being as this patenting, to get the optimum  fine perlite microstructure, yes, the, where you basically want very fine microstructure,  and again, you know, the optimal range of transformation will be in the 600 to 550 degrees  C, okay, in, so for the, for wire steels, what we see is a evolution from about eutectoid  compositions, which give us about close to 3,000 megapascal, to higher hyper eutectoid  compositions, yes, so, 0.8, the eutectoid composition can give me close to 4 megapascal, and I could  definitely in hyper eutectoid compositions with about close to 1%, I can get 4,000 megapascal  in strength for cables, yes, so typically in applications as high carbon bridge suspension  wire or for tire cord reinforcement of tires, we will use these high carbon levels for strength,  so, and the strength, the way we work to, the strength is decrease interlaminar spacing,  you can also of course increase the strength of the ferrite, but increasing the amount  of the cementite is the easiest way to have a very high strength, so, to up the carbon content,  you can also increase the work hardening of the wire, yes, again, an increased amount  of cementite will improve this, you can do this with higher carbon content, but also  higher chrome additions, yes, and then drawing strains is very important, use very small sections,  yes, smaller wire diameters, yes, you can see here by the way the effect of chrome additions  to the strength here, so if you just look at a wire that is basically pearlitic, yes, you see that you  can increase the strength by adding some chromium, so very important here is the drawing, right,  because the drawing allows you to go beyond what the thermodynamics will give you in terms of  the deformation, in terms of refining the grain size, yes, so, and it's also very important to  see that in fact you could do this with any material, right, so if you would use like an  IF steel, yes, and you would strain it as a wire for, and refine the microstructure this way,  you would, you know, you would be able to achieve a 2 gigapascal without any alloying, yes, by  basically refining the microstructure, but if you have a two-phase microstructure like pearlite,  yes, the effect is even stronger, yes, you can see here at the same reduction you have about  five times the impact of the deformation to smaller wire diameters, yeah, nowadays 100 nanometers  and lower are very normal in industry in terms of pearlite refinements, okay, the high carbon wires,  again, you start with wire rod, well for instance, be typically say five millimeters,  you do first wire drawing to three millimeters, first pattern thing, so it means you're going  to soften the microstructure again, yes, so you reheat it basically and then you cool down,  you make fine pearlite, yes, so you do a second wire drawing to from three millimeters to one  to two millimeters, now the strength goes up all the time to 1700, 1800, second pattern thing,  yes, of course the thickness, the diameter hasn't changed, your strength is reduced now, 12,  39, you can do, when the wire gets really fine, it becomes difficult to do the wire drawing,  so you need to know how to do it and one of the ways you do it is by coating the wire,  for instance with brass, yes, and so now you are at, so you started with one millimeters,  after in the third drawing step you can go to a fraction of a millimeter, a tenth to half a  millimeter and the strength levels reach more than three megapascal and then you do the cabling,  yes, there may be many additional steps which are not included here such as pickling,  phosphating, remember maybe that I talked about phosphating is important in technology because  it allows you to reduce friction, yes, it's not indicated but it's one important part of the  step for the production steps in drawing and then of course many of these wires,  they are extremely high strength, they're very thin, right, and they're made of steel,  carbon steel, we don't, you don't want them to rust away, okay, so it's also important to coat these  wires, yes, so galvanizing is a very, very often used to coat the wire, yes,  bearing steels are steels which contain probably the highest levels of carbon, yes,  so there are not many steel grades that contain one percent, around one percent of carbon,  so bearing steels are typically around that level, so they're all hyper eutectoid steels,  yes, okay, again you start making bearing steels by obtaining a wire steel, one percent carbon  wire steel in the steel plant and then it goes to a company that will do, it will prepare the  wire for the producer of the ball bearings or another types of bearings, yes, okay, and the,  the company that prepares the wire is again mostly a thermal treatment company that will do  a low temperature annealing, so to soften the material, to draw the material so you get the  right dimensions, yes, and then it will do a spheroidizing annealing, again to make it very soft,  why do we want to make this, the wire very soft, because we need to make very accurate metal balls,  yes, from this wire, so we need to forge, do some cold forging of the wire to make these metal balls,  yes, and then we need to heat treat the soft balls to a martensitic microstructure and there may  also be surface treatment, so okay, so typically these bearing steels they will be divided into  through hardening steels or case hardening steels, the through hardening steels, yes, have a very  high carbon content, yes, and also tend to have a very high chrome and moly content, so what does  that mean, that means that we're going to, when we heat treat these balls, yes, into like an oil bath, the  entire structure of the bearing will be martensitic, will be fully martensitic, if we have lower carbon  contents, yes, slightly lower chrome and moly, then only the outer side is martensitic and the central  part can be, you know, bainitic, prolitic, depending, yes, so we want, in this case, that's why would  you want to have a structure that's only case hardenable, that's because you want to have, for  instance, a tougher, more ductile center part of the bearing, so here this is a typical bearing  steel here, 1% of carbon and 1.4% of chromium, yes, for the hardenability, free cutting steels are  also typically wire bar products, yes, free cutting refers to the fact that these products will be  going through machining, yes, like a CNC machine, yes, and carbon contents are typically 0.1 to 0.5,  yes, what is special about these is the sulfur content, yes, certain free cutting steels will  require 0.2 to 0.4% of sulfur, that's a huge amount of sulfur, remember, for flat roll products,  we went out of our way to have preferably no sulfur, yes, in this case you add the sulfur in it and  the reason is because when you're working on a workpiece with a tool in the lathe, for instance,  a CNC machine, yes, if you look here in detail, the tool cuts away chips, basically, of the metal,  chips away the metal and the formation of the easy formation of these chips is very important, yes,  and if you have manganese sulfides particles in your microstructure, this really helps chip  formation, so your tools can be used longer and the cut, the quality of the cut is better, yes,  okay, now I do have to say that these requirements are relatively high sulfur contents are really  lately not so useful because in many cases the tool, the quality of the tool material has improved  a lot, so the tools can take the higher heat and the higher stresses during the cutting, yes,  and also the use of CNC machines has improved the machining a lot, so we don't really have to rely  so much on sulfur inclusions in our material to get good machinability, it's still important when  you drill small holes, I have to say, so if you have engineering parts that require small holes,  this presence of the sulfur helps, very careful, however, when you add sulfur, you get all the  negative problems of sulfur in that part, so you cannot add sulfur, yes, if the application doesn't  allow, for instance, for instance, if you have seamless tubes, yes, and you need to make thread  on these seamless tubes, yes, so the thread, the part that you will use to make the connection,  yes, it will be machined, yes, that must be resistant to stress corrosion cracking in  applications, yes, so that's no sulfur in this case, yes, okay, so very important limitation,  finally, and this is not a very high carbon applications, very important in terms of tonnage  are rebar, now you have to know that you can make rebar from any steel, right, so you can buy very  this is people typically think of rebar as a cheap product, yes, however, there are, you can make  rebar from very expensive stainless steels, okay, and it depends on the application, for instance,  in Switzerland, all the, or most of the civil engineering constructions, like bridges and  roads, in the mountains, they use very expensive stainless steel rebars, because it's difficult to  do, to inspect roads and bridges in the Alps, you know, lots of snow and stuff, so you want to have  guaranteed that, you know, there's going to be no deterioration of rebar, etc., so in a certain  European countries, in particular in the mountainous regions, you will use stainless steel types of  rebar, yes, but most of us think of rebar as, you know, very commodity product, that is for which  we use plain carbon steels at 0.2% of carbon, less than 0.4% for the carbon equivalent, so you can  weld them, yield strength is pretty low, 250, 500 for high strength steels, the strength can be  because of the carbon level, or mechanically, sometimes you can get work hardened rebar,  that's just harder because it's twisted, yes, or more expensive are vanadium micro alloyed grades,  where the hardening comes from precipitation hardening, okay, again here the microstructure  of these rebar is kind of nice, yes, typically nowadays you will try to make your rebar both  strong, yes, and tough, yes, and that's happened usually this way, and when you make your rebar, you  finish the rolling at 900, and then you have a three-stage process, in the first stage you make  martensite at the surface of your rebar by strong cooling, yes, this would be what happens to the  surface, it's quickly cooled to below the MS temperature, so the surface is basically quenched,  that's the first stage that you do, and then the second stage you temper the martensite as a result  of the temperature gradient equalization, so you do very quick cool of the surface, yes, and then  you at this point here you stop the cooling, so the martensite at the surface is reheated, yes,  and tempered, the layer below the surface doesn't get to be transformed to martensite and goes  straight into the bainite transformation, yes, and in the third stage, yes, you also do the center,  in the third stage the center is cooled, yes, and here we get either upper bainite or even ferrite  perlite microstructure, so a rebar, yes, you can see here section of a typical rebar, 16 millimeter  rebar, it's actually quite an interesting microstructure, very hard outer surface that progressively  gets softer and more tougher as you go to the center, so at the outside we have lath martensite,  and as you go inside you get bainite, and as you see here in the middle you can even see the perlite  in the microstructure, so that makes for nice properties, all right, and so with this small  review of products and their physical metallurgy we'll just stop the session here for today,  so thank you for your attention.
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So, what's, and so some of these products represent the highest strength steels that you can possibly purchase today for applications. And so, for instance, all the tires have a, use these products either as the beads here, yes, that make sure your tire is airtight, yes, and then here the part of the construction of the tire, you have these belts of tire cord. Okay, very important is in all cases strength, yes, but what equally important is the cleanliness of the steel, cleanliness of the steel, yes, cleanliness, very important is the product has a small section, yes, and in particular applications such as springs, you have suspension springs, for instance, and also the valve springs, yes, you need very high fatigue resistance, yes, and that also requires a very high level of steel cleanliness in addition to strength. Okay, these were typical wire products, and then the bar products are shown here, yes, with the main applications are shafts, very big applications are these crankshafts for automotive, yes, heavy springs, carburized gears is also very big application area for bar, yes, and of course in many cases, for instance, like if you have this shafts, machine ability is an additional requirement, fatigue is important in addition to strength, yes, okay, so fatigue fracture is a very big issue in bar products, yes, for instance, this is a broken crank shaft and another shaft here that broke through torsional fatigue, yes, so big issue, and as you know, very often, you may notice from your courses in fatigue, and if you're interested in fatigue in general, there is a course that Professor Zhang Xuli teaches on fatigue, internal cleanliness of the steel is essential, so because that's usually where the fatigue cracks are initiated, yes, so have a look at these products, these are general, probably covers most of the products for wire, what are the most important ones are the so-called cold-headed quality products, the tire core products, the wire steel products, the bearing steels, and the free cutting steels, yes, that's probably what represents the largest tonnage, yes, and when we talk about cold-headed quality, we also talk about the products are called fasteners, and what are fasteners, well, they're bolts, rivets, screws, you have, of course, when you need to make wire for nails, fences, the material is softer, yes, and so you have soft wire rod that you use in this case, and what you see in terms of the composition, I do want to point this out to you, that the compositions are very much composition where we get the strength from the carbon, yes, get the strength from the carbon, and from what we do to the microstructure, when you have high carbon, you know that if you have the equilibrium situation, you'll have, for this kind of carbon levels, you have ferrite and pearlite, yes, so you can do something to increase the strength by refining the microstructure, smaller grain sizes and refining the interlaminar spacing in the pearlite, you can also turn this steel into martensite, yes, martensite, you can tell with what grades you typically would do this, yes, where you see the chrome content, so for instance, a chrome content on the higher end, for instance, you see here chrome contents that are close to 1% in the case of spring steel, bearing steels, yes, particular for bearing steels, that level of chromium points to the fact that we are making martensite, we're not using a prolitic microstructure, and the chromium that's added in this particular case is to make the steel hardenable, so you can easily get martensite, correct. Now it's also very important to realize that the product that's created in the steel plant doesn't really have the properties and the microstructure of the, for instance, the bearing steel or the cold-heading quality fastener, so there are usually many steps between the production of wire rod, yes, and for instance, a bolt or a bearing or steel cord that you will be able to use in a wheel, in a tire, yes. So let's have a look, for instance, we won't go into all of them, we'll see a few things here, and what we basically see here is that there can be multiple steps in the processor, yes, the steel company will bring this wire to a company that will do additional thermal treatments such as spheroidizing, before it actually gets to the company that will make bolts or bearings, yes, and give the steel its final application. For instance, Posco may be making, yes, the steel wire here, but the bolts are not made by, for instance, made by a company, a very famous company from Sweden, SKF, very huge, or Timken, whatever, these companies are specialized, they may not make steel themselves, yes, and they basically buy these steels from steel makers, yes. And what's really important here, yes, is that these wire steels belong to the strongest types of steels you can get. Steel cord in the car tires easily reach 2,000, 3,000 mega Pascal, bridge wires, yes, are easily 2 giga Pascal, yes, standard, yes. A few words again about the microstructure here. So when you make perlite, yes, you can make a fully, because we'll see, we'll make fully perlitic microstructures, yes, at compositions that are not exactly the perlitic composition, well. So what do I want to say here? If you look at the, again, steel is very simple most of the time, yes, you only need to look at the iron-rich corner of the face diagram. So obviously when you have exactly, for instance in this case, 0.8% of carbon, you expect to see, well, it's not strange that you could form a ferrite, cementite lamellas, yes. But it turns out that if I have a composition here or I have a composition there, I can also get a fully perlitic microstructure. And that sounds kind of strange. How can you get fully perlitic if you're not exactly perlitic microstructure? To understand this, you have to understand why, what makes perlite grow. So let's see what makes perlite grow and ask ourselves, why does perlite grow? I have here ferrite and I have here cementite. This ferrite has very, very little carbon, carbon very, very small. And here the carbon is about pretty high, close to 7 weight percent. And we know that in order to grow, this is gamma, yes, in order for this interface to go from here to here, will be cementite to grow, yes, I basically need to get carbon to go from here to here. Makes sense, right? I have to get rid of it. But then how would that work? Because there's no carbon here and there's lots of carbon there. Why would carbon go where there's lots of carbon already? Obviously, the reason is because the carbon goes from this region of austenite to this region of austenite, yes, from this area to this area. Now what is the carbon content here and the carbon content here? That is the question, yes. Not obviously the carbon content, I just told you the carbon content, what the carbon content is the ferrite and what the carbon content is the cementite, yes. But that's not what makes the carbon move from here to here. What makes the carbon move from here to here is the carbon content at the ferrite austenite boundary and here the carbon content at the cementite austenite boundary. So say I would like to know what these concentrations are, yes, what are these concentrations? If I look at this phase diagram, if I look at this phase diagram, no problems. Say I'm doing the pearlite transformation at this temperature, this is the temperature at which I do the transformation from austenite to ferrite plus pearlite, yes. What is the carbon content at the interface, alpha gamma interface and the cementite, yes. And so I look at this and I say, well, how am I supposed to know? I can't get the information, right. The reason is of course, is this phase diagram, yes, tells you what is here at equilibrium, yes, but we don't have equilibrium. When you do the transformation at this temperature, you still have austenite during the transformation, right. And in order to know what the concentrations are, these two concentrations, I need to extend these lines, yes. So I need to know where these lines would be if there was no pearlite transformation, basically, if you want to see that. So I basically need to extend, and I can do this computationally, it's not a problem, I need to extend the A3 line and I need to extend the ACM line, yes. And if you do this, yes, what is this line here? This line here, compositions here correspond to austenite cementite concentrations, yes. And compositions on this line correspond to, excuse me, austenite, cementite, also, corresponds to ferrite austenite, yes. And of course, that continues here, yes. So this is the carbon content at the boundary between austenite and ferrite during the transformation, yes, and this is the carbon content at the cementite austenite boundary during the transformation, yes. And now you can see that, yes, it makes sense, the carbon content at this interface, yes, is higher than at this interface, yes. And so the carbon will move from here to there, and it will be controlled by the diffusion coefficient of carbon in austenite, that's the controlling parameter. It also means, yes, that I can make perlite when this condition is achieved, when the concentration here, yes, when C theta gamma is smaller than C alpha gamma, yes. And so that means I can have, I can form perlite here inside this triangle, and I do not need to have exactly the perlite composition. I can have slightly less, slightly more. And you can see it's not necessary to have 0.8% of carbon, you can make perlite if you have 0.6% of carbon, and it will be fully perlitic, yes, or I can go to 0.9% or higher, and it will still be fully perlitic, yes, because perlite can grow, all right. And D, but you can also see, of course, that things will change depending on the temperature at which I work, okay. So that's important, yes. D, so I have control by the carbon content, yes, and the process of creating, so the start of this process here is gamma, gamma, grain boundaries, you start forming little ferrite content, nuclei, so it's a nucleation and growth process, yes. So if you look at the kinetics of the transformation, I see that very close to AE1, of course I don't get much, okay. At low temperature also don't get much in terms of transformation, that's because my diffusivity, carbon diffusivity becomes lower, but there is a maximum rate of transformation, yes. So that's important, and we'll see this back in the, when we look at the processing of these steels, yes, between 600, 550 and 600, we get the very fastest growth rate, yes. There it takes one second for the perlite transformation to start, so very fast, and that's where we also get very fine perlite, okay. What is fine perlite, that's where you have a small interlaminar spacing, yes. Small interlaminar spacing means that you have lots of interfaces, yes, lots of interfaces, so anytime you have an interface that means you have more energy, interfacial energy. So small interlaminar spacings are unstable, yes. So if you heat this up, it can easily coarsen, yes. In fact, from a purely thermodynamic point of view, even a coarse perlite is unstable, again because of all this interfacial energy, and if you keep perlite long enough at between 6 and 700 degrees C, it will spheroidize, the perlite will turn, excuse me, the cementite will turn into little balls, yes, and that will minimize the interfacial energy. However, from an application point of view, this is just what we want, yes, because that's where we have the best properties, yes, okay, and that's the reason why. So as we decrease the temperature at which we do the transformation, what we see is that, so again in the region of 600 to 550, you see here that the growth rate of the perlite is very, very high, yes, and the corresponding interlaminar spacing is also very small, so you can get interlaminar spacings which are between 0.1 and 0.2 microns, so that's 100 to 200 nanometers, okay. So if you would do, for instance, the transformation very close to the A1 temperature, you would have a interlaminar spacing which is 10 times larger, okay, so it's control of the temperature at which you do the transformation is very important, okay, and people, we have, I don't want to go into this kinetics here, so anyway, let's have a look now at what these implicate, what the implications are, so, and look, for instance, at a steel, a wire steel that contains for instance 0.6 percent of carbon, yes, okay, how would we process this steel, okay, but first of all we will look at our phase diagram, we will have to extend the AECM line, the A3 line, yes, okay, and we will have to look at the TTT diagram, okay, so what it would be the TTT diagram here, so you can see that as soon as I go below this temperature, I will form, I will start to form pro-utectoid ferrite, okay, okay, if I am in this range of temperatures here, yes, I basically continue the same thing, yes, I only start to make perlite below this temperature, right, so when I am in this triangular region, yes, so if I want to make perlite, yes, I have to make sure that I cool very quickly to this temperature range, yes, not just being below the AE1 line will be enough, I have to go below this line here, below the AECM line, then I will start making perlite, okay, again, we are talking about, so if you would choose a, excuse me, a transformation temperature around 600, you would be fine, okay, because that is where you would have the maximum rate of transformation, okay, good, so we know that we want to have fine microstructure, of course, when I have less carbon, yes, this means that, I am exaggerating here, sorry, that is the reverse, if I have less carbon than the eutectoid composition, I will have more ferrite in my perlite, if I have a carbon content that is higher than the eutectoid, I will have more cementite in my perlite, okay, that is of course, it does not, the amount of cementite will change, yes, so it comes as no surprise that if you plot the strength of these steels, yes, as a function of carbon content, yes, you will see an increase because you have more cementite, more of this very hard phase in the structure, however, and you can see this here, even if you have a eutectoid or a slightly off-eutectoid composition, you can still increase the strength considerably by strain, yes, so what happens when you strain the perlite, you can see here, if I strain the material, yes, I can have at 0.8 carbon, I can have the same strength as at 1.4 carbon, yes, and the reason is that if you strain, oops, if you strain the perlite, you can reduce mechanically the interlaminar spacing, okay, so if I take this material here, yes, I have a certain interlaminar spacing, yes, if I wire draw this material, yes, now I have material where the spacing is much smaller, yes, yes, much smaller, much smaller than this 0.1 or 0.2 microns, I can now go to 0.05 microns, yes, so I can first reduce it, yes, than what I get after the transformation, okay, so and that is being used very often, so let's now look at some products and first let's look at cold-heading qualities, so cold-heading qualities are typically made, used to make these bolts, yes, and the carbon content is typically 0.4 percent, the structure itself, yes, of these, when you start with the wire, yes, the wire has a perlite ferrite microstructure, yes, but the bolt here doesn't have a ferrite, it has a martensitic microstructure, okay, we'll go through some processing to make this, so first of all let's have a look at these steels, they're typically what we call medium carbon steels, and these are steels that are used for engineering applications, to make things with basically, yes, so we call them engineering steels, and they're alloyed with chrome and molly, yes, because we want to get a martensitic microstructure, and what are the typical ranges, 0.3 to, close to 0.4, about 1 percent of chromium, and about 0.2 percent of lead, yes, and these are typical examples here, this is equivalent, and you remember here, SIJ, S for steel, C for chrome, M for molly, and 40 stands for 0.4 percent of carbon, here the European normalization, 0.42 carbon, chrome and molly, and then the four means we have chrome, yes, and you know that the four here is not 4 percent, it means four divided by four, this is about 1 percent, yeah, and here the SAE, ISSA where we recognize the 0.4 percent of carbon, okay, right, and so for these steels, this is the composition range, okay, it's similar to what you have here, but this is a wider range because it covers all the products, yes, okay, right, so let's make, let's go through the steps of making a bolt, yes, so you start with wire rod from, yes, and it's for instance, one of these steels is typically used to make wire rod, SCM 435 for instance, or the equivalent European and American grade, so the first thing you do, yes, is this alpha plus perlite microstructure is going through a softened needle, yeah, so this softened needle is to soften this material, this wire, yeah, the material has typically, when it leaves the plant, about 900 to 1100 megapascal in strength, yes, so we're going to soften it, yes, so it can be drawn, it can be drawn, yes, and then sphero diced, so we draw it, so it goes to all our section, yes, and then we sphero diced it to make it really soft, yes, and that usually happens in a company that's specializing in heat treatments, yeah, they just heat treat materials, yes, get materials, heat treat them, give them back to their customers, yes, so this, so when you do the softened kneeling you just go to below A1, yes, sphero dicing you may go a little bit higher than A1, yes, and then this product is now very soft, 500 to 600 megapascal, we're going to use it, we can cold form it, yes, and that is a number of steps, you start with the wire, yes, you cut, upset it, extruding, you finally make, thread this bolt, and that usually happens in another customer, that's the actual maker of bolts, yes, he will quench and temper it, so we get, for this particular grade we'll get about 1000 megapascal in strength, and he will apply the quench and tempering treatment, so he gets a tempered martensite microstructure, yes, okay, so this is the sphero dicing, this material is turned into material that looks like this, it's very small particles, yes, because this material is chromium and molybdenum added, usually it takes a long time to sphero dize wire steel, so okay, so we will be using a process, yes, you cannot use a continuous annealing process, because it's just too slow, so you use what, something that looks very much like a batch annealing furnace that we've seen for strip recrystallization, so in this furnace you put stacks of these wires, yes, as they are drawn or as they are produced, yes, and you cover them with insulation cover, and then you put in, put over that your furnace, yes, okay, and then you heat it up, the wire heats up, the softer annealing, so usually that's done with the same company, this is the first softer annealing that's right below AE1, the sphero dicing annealing, yes, is much, first of all much longer, 30 hours, yes, and you go slightly above AC1, why would that be, okay, because this is another picture where you perhaps see it better, where the wire material is, yes, the reason is why do we go slightly above AC1 when we do the sphero dicing, that's because the cementite particles we want to destabilize them, yes, so we partially dissolve this cementite in the austenite, but we leave some cementite in the microstructure, so we get cementite seeds, yes, so that when this slowly cools down, yes, instead of making perlite, we now get these very nice rounded sphero dites shaped cementite, yes, so there are many theories and models about how it really happens, and you can see some of these models, but anyway, and it's very interesting, physical metallurgy, but however, whatever the process is, it's a slow process, okay, so once you have this annealed sphero diced wire, you basically cut it in little pieces, yes, and then you go through the process of making a threaded bolt, yes, once you have this threaded bolt, you austenitize it, you can do some extra forming, yes, sometimes necessary, and then you can quench and temper the material, yes, and you end up with a tempered martensite which can be very strong indeed, yes, now high quality, bolts are very important, yes, in engineering applications, bolts are no joke, and so, and you can always recognize a high quality bolt, yes, because it's got the name of the company who made the bolt, yes, and it's also got numbers, and the numbers help you not making the error of using the wrong bolt, right, so for instance here you have 5.6 and here 8.8, the 8.8 is used very much in engineering application, the 8.8 tells you that the yield strength is 8 times 8 times 10, yes, that's 640 megapascal, and it also tells you the 10 cells, the UTS, so that's 8 times 100, so you do the same on the 5.6, 5 times 300, and 500 megapascals as UTS, okay, and so again as I said, the serious business in engineering construction, you don't mix bolts, yes, even if they look the same, yes, why, because there could be serious consequences if you use a weaker bolt, yes, and the building collapses, or the machine breaks, or it's a high pressure vessel, and the bolts are not the ones you should be using, there may be explosions and things like this, so very serious things, and so that's how you can tell these are good bolts, and usually in engineering application they are also specified what kind of bolt you should be using, okay, another interesting application for wire seals are spring seals, and so springs can come in many different forms, right, you have strips, for instance this light spring here, yes, very commonly used for household items, but also this suspension spring here from a car, and of course motors have also many springs in the valves, yes, so typically about 45 to point 70 in carbon contents, yes, one of the things you will see in these spring steels is the high silicon, high silicon is because these steels are not aluminum kilts, yes, you want them to be silicon kilts, so you don't have these non-metallic inclusions that will impact the fatigue resistance, yeah, the chrome content, yes, points to the fact that we need hardenability here, okay, and in certain cases you have copper, so let's have a look, what's also very important here for spring steels is the control of the nitrogen content, yes, because you don't want nitrogen aging in these steels, it turns out that the nitrogen reduces the dectility of these carbon wires, and we add boron to this, these steels, to make boron nitride, yes, and that reduces the effect of the nitrogen, you can see the effect of nitrogen on the properties of the wire by twisting the wires, you twist the wires and if there's nitrogen aging, the number of twists is very small, yes, if you add boron you can have enough twists before the wire breaks, typically you will need to be able to do a minimum of 40 twists, okay, springs have their own technologies when they're made, for instance suspension springs can be hot processed or cold processed, yes, coil type springs are usually always cold formed, the coil type, yes, hot formed can be applied for coil type but also for leaf type springs, and this is a lightweight leaf type spring but this also applies to leaf type springs that you use for trucks, okay, so let's have a look at how we make engine valves, yes, again if the steel company makes wire, for instance typically that would be 5 to 13 millimeter section wire, yes, it goes through two steps, yes, so POSCO makes wire, doesn't make springs, yes, and first place the wire goes to is a company that does heat treatment, that prepares the steel so it can be processed by a spring manufacturer, okay, so the first one customer will coat it, will make the wire drawing, yes, so will reduce the section of the wire, will do patenting, what is patenting, when you deform the pearlite, yes, it's you reduce the interlaminar spacing but the material becomes very hard, so if you want to continue processing it, reducing the interlaminar spacing even further, you have to anneal it, yes, and that's this patenting process here, so you do wire drawing until you have the required section, yes, and then oil quenching and tempering, okay, so that means in this case you make the final microstructure and properties, right, now what happens then with the spring manufacturer for valve springs again, yes, you do cold forming into, yes, there will be stress relief after the cold forming, shot peening, you know that in order to improve fatigue resistance, it's good to have compressive residual stresses at the surface of parts that undergo fatigue conditions, yes, shot peening then, stress relief again to remove excessive residual stresses and then pre-setting of the spring, so you can, and then you're done basically. What kind of strength levels, well for instance if you use an SE 9254 here, about 2000 mega Pascal in strength, the wires for suspension bridges and another cables, yes, it's a very specialized business, wires and in particular cables because of the ways you thread the wires basically, you can have open spiral strands like this, usually hot dip galvanized steels, steel wires, yes, and they're usually twisted or helically wounded around the core, but you can have more complex versions like where the wires are shaped, for instance here in this case you have an outer, the outer cables here are locked, the strands are locked, so you cannot access these inner wires, yes, you have z-shaped wires and they are self-locking, corrosion protection is very important, so most of these wires will be galvanized and so it can be very complex, so suspension bridge cables are not just little wires next to each other and depending on the application, the stiffness you want, the flexibility you want, you will use different ways of organizing the wires around a central wire and also if you look carefully here, you can see that there is also, you also use a mixture of wire diameters, right, for instance you can see here that in between the larger wires you filled in the interstices with smaller wires, yes, okay, right, so let me skip this one here, very important also in the wires, yes, is because we use this, the ability to reduce, to refine the microstructure when we, when we draw the wire, yes, the wire diameter, yes, is an essential contributor to strength, yes, so if you use, so for instance same grade here, see this black grade, about 1% of carbon, yes, goes from here to here, you can see that if I use say 1, 2, 3, 4, 5 millimeter wire, yes, will have a strength of 1,000, but I can go up to 5,000, yes, if I can make the wire very, very fine, yes, okay, okay, so, and that's of course what you will use because you don't have to do anything to the composition basically of the, of the steel, alright, so, the wire drawing again doesn't, also doesn't have to be circular, it can be complex shapes, so you can design very complex cable sections, yes, and, and so, one of the things you will be, being as this patenting, to get the optimum fine perlite microstructure, yes, the, where you basically want very fine microstructure, and again, you know, the optimal range of transformation will be in the 600 to 550 degrees C, okay, in, so for the, for wire steels, what we see is a evolution from about eutectoid compositions, which give us about close to 3,000 megapascal, to higher hyper eutectoid compositions, yes, so, 0.8, the eutectoid composition can give me close to 4 megapascal, and I could definitely in hyper eutectoid compositions with about close to 1%, I can get 4,000 megapascal in strength for cables, yes, so typically in applications as high carbon bridge suspension wire or for tire cord reinforcement of tires, we will use these high carbon levels for strength, so, and the strength, the way we work to, the strength is decrease interlaminar spacing, you can also of course increase the strength of the ferrite, but increasing the amount of the cementite is the easiest way to have a very high strength, so, to up the carbon content, you can also increase the work hardening of the wire, yes, again, an increased amount of cementite will improve this, you can do this with higher carbon content, but also higher chrome additions, yes, and then drawing strains is very important, use very small sections, yes, smaller wire diameters, yes, you can see here by the way the effect of chrome additions to the strength here, so if you just look at a wire that is basically pearlitic, yes, you see that you can increase the strength by adding some chromium, so very important here is the drawing, right, because the drawing allows you to go beyond what the thermodynamics will give you in terms of the deformation, in terms of refining the grain size, yes, so, and it's also very important to see that in fact you could do this with any material, right, so if you would use like an IF steel, yes, and you would strain it as a wire for, and refine the microstructure this way, you would, you know, you would be able to achieve a 2 gigapascal without any alloying, yes, by basically refining the microstructure, but if you have a two-phase microstructure like pearlite, yes, the effect is even stronger, yes, you can see here at the same reduction you have about five times the impact of the deformation to smaller wire diameters, yeah, nowadays 100 nanometers and lower are very normal in industry in terms of pearlite refinements, okay, the high carbon wires, again, you start with wire rod, well for instance, be typically say five millimeters, you do first wire drawing to three millimeters, first pattern thing, so it means you're going to soften the microstructure again, yes, so you reheat it basically and then you cool down, you make fine pearlite, yes, so you do a second wire drawing to from three millimeters to one to two millimeters, now the strength goes up all the time to 1700, 1800, second pattern thing, yes, of course the thickness, the diameter hasn't changed, your strength is reduced now, 12, 39, you can do, when the wire gets really fine, it becomes difficult to do the wire drawing, so you need to know how to do it and one of the ways you do it is by coating the wire, for instance with brass, yes, and so now you are at, so you started with one millimeters, after in the third drawing step you can go to a fraction of a millimeter, a tenth to half a millimeter and the strength levels reach more than three megapascal and then you do the cabling, yes, there may be many additional steps which are not included here such as pickling, phosphating, remember maybe that I talked about phosphating is important in technology because it allows you to reduce friction, yes, it's not indicated but it's one important part of the step for the production steps in drawing and then of course many of these wires, they are extremely high strength, they're very thin, right, and they're made of steel, carbon steel, we don't, you don't want them to rust away, okay, so it's also important to coat these wires, yes, so galvanizing is a very, very often used to coat the wire, yes, bearing steels are steels which contain probably the highest levels of carbon, yes, so there are not many steel grades that contain one percent, around one percent of carbon, so bearing steels are typically around that level, so they're all hyper eutectoid steels, yes, okay, again you start making bearing steels by obtaining a wire steel, one percent carbon wire steel in the steel plant and then it goes to a company that will do, it will prepare the wire for the producer of the ball bearings or another types of bearings, yes, okay, and the, the company that prepares the wire is again mostly a thermal treatment company that will do a low temperature annealing, so to soften the material, to draw the material so you get the right dimensions, yes, and then it will do a spheroidizing annealing, again to make it very soft, why do we want to make this, the wire very soft, because we need to make very accurate metal balls, yes, from this wire, so we need to forge, do some cold forging of the wire to make these metal balls, yes, and then we need to heat treat the soft balls to a martensitic microstructure and there may also be surface treatment, so okay, so typically these bearing steels they will be divided into through hardening steels or case hardening steels, the through hardening steels, yes, have a very high carbon content, yes, and also tend to have a very high chrome and moly content, so what does that mean, that means that we're going to, when we heat treat these balls, yes, into like an oil bath, the entire structure of the bearing will be martensitic, will be fully martensitic, if we have lower carbon contents, yes, slightly lower chrome and moly, then only the outer side is martensitic and the central part can be, you know, bainitic, prolitic, depending, yes, so we want, in this case, that's why would you want to have a structure that's only case hardenable, that's because you want to have, for instance, a tougher, more ductile center part of the bearing, so here this is a typical bearing steel here, 1% of carbon and 1.4% of chromium, yes, for the hardenability, free cutting steels are also typically wire bar products, yes, free cutting refers to the fact that these products will be going through machining, yes, like a CNC machine, yes, and carbon contents are typically 0.1 to 0.5, yes, what is special about these is the sulfur content, yes, certain free cutting steels will require 0.2 to 0.4% of sulfur, that's a huge amount of sulfur, remember, for flat roll products, we went out of our way to have preferably no sulfur, yes, in this case you add the sulfur in it and the reason is because when you're working on a workpiece with a tool in the lathe, for instance, a CNC machine, yes, if you look here in detail, the tool cuts away chips, basically, of the metal, chips away the metal and the formation of the easy formation of these chips is very important, yes, and if you have manganese sulfides particles in your microstructure, this really helps chip formation, so your tools can be used longer and the cut, the quality of the cut is better, yes, okay, now I do have to say that these requirements are relatively high sulfur contents are really lately not so useful because in many cases the tool, the quality of the tool material has improved a lot, so the tools can take the higher heat and the higher stresses during the cutting, yes, and also the use of CNC machines has improved the machining a lot, so we don't really have to rely so much on sulfur inclusions in our material to get good machinability, it's still important when you drill small holes, I have to say, so if you have engineering parts that require small holes, this presence of the sulfur helps, very careful, however, when you add sulfur, you get all the negative problems of sulfur in that part, so you cannot add sulfur, yes, if the application doesn't allow, for instance, for instance, if you have seamless tubes, yes, and you need to make thread on these seamless tubes, yes, so the thread, the part that you will use to make the connection, yes, it will be machined, yes, that must be resistant to stress corrosion cracking in applications, yes, so that's no sulfur in this case, yes, okay, so very important limitation, finally, and this is not a very high carbon applications, very important in terms of tonnage are rebar, now you have to know that you can make rebar from any steel, right, so you can buy very this is people typically think of rebar as a cheap product, yes, however, there are, you can make rebar from very expensive stainless steels, okay, and it depends on the application, for instance, in Switzerland, all the, or most of the civil engineering constructions, like bridges and roads, in the mountains, they use very expensive stainless steel rebars, because it's difficult to do, to inspect roads and bridges in the Alps, you know, lots of snow and stuff, so you want to have guaranteed that, you know, there's going to be no deterioration of rebar, etc., so in a certain European countries, in particular in the mountainous regions, you will use stainless steel types of rebar, yes, but most of us think of rebar as, you know, very commodity product, that is for which we use plain carbon steels at 0.2% of carbon, less than 0.4% for the carbon equivalent, so you can weld them, yield strength is pretty low, 250, 500 for high strength steels, the strength can be because of the carbon level, or mechanically, sometimes you can get work hardened rebar, that's just harder because it's twisted, yes, or more expensive are vanadium micro alloyed grades, where the hardening comes from precipitation hardening, okay, again here the microstructure of these rebar is kind of nice, yes, typically nowadays you will try to make your rebar both strong, yes, and tough, yes, and that's happened usually this way, and when you make your rebar, you finish the rolling at 900, and then you have a three-stage process, in the first stage you make martensite at the surface of your rebar by strong cooling, yes, this would be what happens to the surface, it's quickly cooled to below the MS temperature, so the surface is basically quenched, that's the first stage that you do, and then the second stage you temper the martensite as a result of the temperature gradient equalization, so you do very quick cool of the surface, yes, and then you at this point here you stop the cooling, so the martensite at the surface is reheated, yes, and tempered, the layer below the surface doesn't get to be transformed to martensite and goes straight into the bainite transformation, yes, and in the third stage, yes, you also do the center, in the third stage the center is cooled, yes, and here we get either upper bainite or even ferrite perlite microstructure, so a rebar, yes, you can see here section of a typical rebar, 16 millimeter rebar, it's actually quite an interesting microstructure, very hard outer surface that progressively gets softer and more tougher as you go to the center, so at the outside we have lath martensite, and as you go inside you get bainite, and as you see here in the middle you can even see the perlite in the microstructure, so that makes for nice properties, all right, and so with this small review of products and their physical metallurgy we'll just stop the session here for today, so thank you for your attention.
So, what's, and so some of these products the part of the construction of the tire, you fatigue in general, there is a course that the carbon content at the ferrite austenite for the production steps in drawing and then of carbon wire steel in the steel plant and then it