Carbon steels and tool steels as well as other steels and alloys with a high carbon content are primarily characterized by high strength properties. The yield strength, tensile strength and apparent hardness increase with an increasing carbon content, and correspondingly the elongation decreases.
It is hardly possible to produce a structural part from a carbon steel powder having a carbon content of above about 0.1% by pressing and sintering, as this powder is very hard and the resulting green density of the green body obtained by pressing will be very low. In order to get a good compressibility the powder could be soft annealed, but this is a very costly operation that has to be performed in a protective atmosphere. The carbon is therefor usually added as a graphite powder before the pressing. For the manufacturing of parts of a high carbon steel from a metal powder said powder is mixed with up to 1% by weight of carbon in the form of fine grain graphite. The graphite powder which is used for the addition of carbon to a steel or an alloy is made by grinding of either natural or synthetic graphite. Natural graphite has for long been dominating owing to a higher reactivity during the sintering process, but today there are also synthetic ones having said properties.
The “green density”, that is the density of the green body reached after the pressing operation, is an important property. A high green density will give better mechanical properties, higher final density, and better tolerances after sintering. In order to obtain a high green density the ductility of the powder must be high, as the pressure, which can be applied during the compacting, should normally not be higher than 800 MPa due to the tool life. However, due to the fact that graphite is added, the final density after sintering will be low. This is due to the fact that graphite has a low density and takes up volume during pressing. When it diffuses into the part as carbon the density increase is restricted. It is also well known that when mixing graphite and plain carbon steel it is difficult to obtain a perfect mixing, which leads to inhomogeneities with areas higher and lower in carbon, which gives uneven results and different properties of the sintered body. This is especially true for irregular powders like water atomized powder.
Today structural parts of carbon and tool steels are therefore, when good mechanical properties are required, mainly produced by forging, casting or hot isostatic pressing followed by machining.
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