Centrifugal compressor impellers are most commonly made from alloy steel forgings of AISI 4140 or 4340. Materials such as AISI 410 stainless steel and precipitation hardened stainless steels (including Armco 17-4 pH or 15-5 pH) may be used in situations where corrosion resistance is required. Austenitic stainless steels, monel, and aluminum, although somewhat limited in their application, are used in some special cases. Figure200-74 identifies the chemical analysis of various impeller materials. Figure200-75 provides a listing of mechanical properties.