In cast stainless steels, the structure may be austenitic, ferritic, martensitic or ferro-austenitic (duplex). The structure of a particular grade is largely determined by the ingredients. Chromium, molybdenum, and silicon promote the formation of ferrite (magnetic), while carbon, nickel, nitrogen, and manganese favor the formation of austenite (nonmagnetic).
Chromium (ferritic and martensitic promoters), nickel and carbon (austenitic promoters) are particularly important in determining the microstructure. In general, straight chromium grades of high-alloy cast steels are either martensitic or ferritic, chromium-nickel grades are either duplex or austenitic, and nickel-chromium steels are fully austenitic.
Cast austenitic alloys typically have 5% to 20% ferrite distributed in discontinuous pools throughout the matrix, with the percentage depending on the nickel, chromium, and carbon content. The presence of ferrite in austenite can be beneficial or detrimental, depending on the application.
Ferrite can be beneficial in terms of weldability, as fully austenitic stainless steels are prone to weldability problems known as hot cracking or microcracking. Intergranular cracks occur in the weld and/or in the weld heat-affected zone and can be avoided if the composition of the filler metal is controlled to produce about 4% ferrite in austenitic welds. Duplex CF grade alloy castings are not affected by this problem.
