The influence of HIP design and product size on sigma phase formation in a P/M duplex stainless steel

Duplex stainless steels are a class of relatively low cost alloys that offer high strength and good corrosion resistance. Hot isostatic pressing (HIP) of duplex stainless steel powders is one method of producing a pore-free near net shape possessing a fine, equiaxed dispersion of phases with mechanical properties superior to those of equivalent wrought alloys. Unfortunately, slow cooling of a duplex stainless steel from typical processing temperatures causes the formation of the detrimental sigma phase. During the production of a HT powder metallurgy (P/M) part, the combination of cooling rate, component cross section, and sigma phase transformation kinetics determine the maximum size part which can be processed without requiring subsequent heat treating. This paper addresses the interrelationship between these factors for typical commercial applications of the duplex stainless steel alloy Duplok 27.