Precipitation of σ Phase in a 25Cr-7Ni-3Mo Duplex Phase Stainless Steel

Abstract

Precipitation of 6 phase in an Fe-25Cr-7Ni-3Mo alloy with α/γ duplex structure has been studied systematically and the following results were obtained:(1) TTP diagram for the a phase precipitation was a C-type with a nose at 820°C.(2) σ phase nucleated on the α/γ interfaces and grew into α grains as a eutectoid reaction: α→σ+γ. Enrichments of Cr, Mo and W atoms into σ phase were remarkable.(3) Hardness increase by the precipitation of σ was not so large, while impact value markedly decreased even at a few percent precipitation. σ phase was also found to decrease hot ductility.(4) The precipitation range during cooling was 700-900°C and σ phase precipitation could be avoided if cooling rate was larger than about 20°C/ min after solution treatment.