High strength, corrosion resistant, seamless tubular components have many commercial applications. For example, durable tubular components having high strength and resistance to failure under stress, corrosive, and erosive environments are used in oil country tubular goods (OCTG) and other types of tubular components used in the production of oil, gas or other fluids from a well. These durable components are needed due to the severe downhole conditions in the wells and/or the hostile environments surrounding the wells. However, as wells become deeper, the downhole conditions in the well may limit the choice of tubular components capable of withstanding these environments. Typically, deeper wells contain higher temperatures and pressures and may have corrosive atmospheres, e.g., hydrogen sulfide, carbon dioxide, chlorides, associated hydrocarbons, and/or acidic environments. Weight considerations may also be a concern since more tubular components must be used and joined together in the deeper wells.
As such, the material selection criteria for these tubular components has become increasingly important since they may fail in a relatively short time due to such factors as stress corrosion cracking, corrosive pitting, erosive wear and general wall loss, e.g., by lowering the component's burst and collapse pressures. Currently, high strength, corrosion resistant alloys have been used, rather than the traditional carbon steels, for the downhole tubular components in these kinds of severe conditions. These tubular components are typically made out of stainless alloys, duplex (austenitic-ferritic) stainless alloys, and nickel-based alloys, e.g., alloys such as alloy 28, 625, 718, 825, 925, G-3, 050, C-276, 22Cr, 25Cr, Nickel 200, Monel 400 and Inconel 600. A component's resistance to failure may be influenced by a number of factors that include the component material's chemistry, the nature and amount of alloying elements, the component's dimensions, such as thicker wall thickness to withstand higher burst and collapse pressures, and the material's microstructure which is influenced by the manufacturing process of the component, e.g., mechanical processing and the nature of any heat treatments of the component.
Tubular components may be formed by a number of different manufacturing processes. One type of manufacturing process is casting, by which a liquid material is usually poured into a mold and then allowed to solidify. The mold contains a hollow cavity having the desired shape of the component. The solidified part is also known as a casting, which is then removed from the mold usually once it has sufficiently cooled. Metals and alloys may be formed by this process. However, the as-cast component typically includes large grain sizes and may contain casting defects, such as porosity and nonmetallic inclusions.
A slight variation to this manufacturing process is centrifugal casting. In centrifugal casting, a mold is rotated about its axis at various speeds (e.g., 300 to 3000 rpm) as molten material is poured into the mold. The speed of the rotation and material pouring rate vary with the material used, as well as the size and shape of the component being cast. When the molten material is poured into the rotating mold, the molten material is thrown towards the mold wall, which is typically held at a much lower temperature than the molten material, where it begins to solidify upon cooling. Near the mold wall, heterogeneous nucleation occurs relatively rapidly, and a fine, equiaxed grain structure is usually obtained in the outer diameter of the component adjacent to the mold, in an exterior zone. This rapid cooling effect of the mold induces directional solidification across the component's wall. A columnar zone begins to form with a dendritic growth direction in each columnar grain parallel to the heat flow direction. The growth of these crystals stops when they meet the grains growing from the inner diameter of the component in an interior zone. As the component's inner diameter is in contact with air, the solidification rate in the interior zone is much lower than in the exterior zone, resulting in coarser grains in the inner diameter are than in the middle area or the outer diameter of the component. Consequently, centrifugal casting usually results in a finer grain structure than regular casting with a fine-grained outer diameter, but with an inner diameter usually having more impurities and inclusions.
The resulting centrifugally cast component, however, presents many challenges for subsequent metal forming processes due to its different grain sizes in the various zones, along with its radially-oriented columnar grain structure. Due to these difficulties, cast and centrifugally cast components are frequently subjected to subsequent warm or hot forming manufacturing processes, that are conducted above the recrystallization temperature of the material, or are subjected to numerous annealing steps in between the metal forming processes. However, warm and hot forming processes affect the mechanical properties and the dimensional accuracies of a component, making it difficult to meet requirements with tight tolerances. In addition, centrifugal cast components have not been acceptable for applications with high internal pressure or where corrosive and/or erosive products are present, such as in environments where OCTG components are used. The centrifugal casting process tends to produce a microstructure with undesirable porosity causing crack initiation sites. Centrifugally cast components also may exhibit segregation of the solute alloying elements to Laves and carbide phases, depending upon the mold speed used during casting. Microstructural results show that the predominant crack or fracture path in centrifugal castings is frequently associated with the carbide or Laves phases in the interdendritic regions. It is well known that alloy inhomogeneities are responsible for the reduction of the tensile and creep-rupture performance of materials at room temperature and elevated temperatures. One of the main problems of centrifugal cast components is the non-uniform microstructure through the cross section of the wall thickness. The banded structure downgrades the physical and mechanical properties of the material and results in stratification.
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