Influence of Various Factors on the Precipitation Behavior of σ Phase in Duplex Phase Stainless Steels

Abstract

Influence of various factors on the precipitation behavior of σ phase in Fe-25Cr-7Ni-3Mo (A), Fe-23Cr-6Ni (B) alloys have been studied. Results obtained in this study are as follows;
(1) In the time for the precipitation of σ phase, a longer time more than two order of magnitude is necessary in the steel B than in the steel A. This should be attributed to the difference in chemical compositions between two steels, especially in α phase, where most of σ phase is known to precipitate.
(2) σ phase precipitation is found to be enhanced by hot work of thermal strain as well as cold work. The enhancement effect of cold work is predominant, and the precipitations of σ phase are observed in γ grains as well as in α grains.
(3) The precipitation of σ phase is found to be retarded by the prior heating history; (a) heating at the temperature of α single phase or (b) remelting before σ phase precipitation. Especially, the effect of pretreatment (b) is remarkble. These retardation effects can be ascribed to homogenization of each element by these heat treatment. The difference in case (a) and (b) can result from the degree of their homogenities.