Blog: GammaFoundries
Stainless Steel Castings Composition and Microstructure The term stainless steel (SS) is used to encompass a wide array of materials with varied physical, chemical and mechanical properties. For the design engineer, it is important to identify the different alloys available, their properties and the equivalent casting grades that correspond to the well-known wrought formulations. In more detail, stainless steel may be categorized according to its microstructure: Ferritic: The main alloying element for these grades is chromium in the range of 10.5-29%. As in all stainless steels, chromium creates a tough passive layer in the surface that protects the metal from further corrosion. The microstructure of these alloys corresponds to the ferrite (body centered cubic) configuration. Due to their low nickel content these grades are usually cheaper than the nickel-alloyed alternatives below. Some alloys of this type are AISI 431 and 446 for wrought components and casting equivalents are ASTM A743 CB-30 and CC-50 respectively. Austenitic: By far the most common stainless-steel variant, the main alloying elements are chromium and nickel. The high nickel content on these alloys stabilizes the austenite (face centered cubic) configuration at all temperatures. Since the microstructure is invariant with temperature, these alloys cannot be strengthened by heat treatment. Consequently, these alloys have a relatively low yield strength but high corrosion resistance and excellent ductility. These alloys are identified by the AISI 200 to 300 series, commonly used designations for wrought components are 304 and 316 and their casting equivalents are ASTM A743 CF8 and CF8M. Castings of these alloys are commonly used for impellers, volute casings and other food and fluidhandling corrosion-resistance service where high yield strength is not a requirement.