Effects of Plastic Deformation and Thermal History on σ Phase Precipitation in Duplex Phase Stainless Steels

Abstract

Effects of hot or cold deformation and thermal history on theσ phase0 precipitation in (A) a 25Cr-7Ni-3Mo and (B) a 23Cr-6Ni steels with ferrite/austenite (α/γ) duplex structure have been studied. Results obtained are as follows:(1) The incubation time for the σ phase precipitation in Steel B was a couple of orders of magnitude longer than that in Steel A. This should be attributed to the difference in chemical composition between two steels, especially in a phase, where the majority of σ phase is known to precipitate.(2) σ phase precipitation is enhanced by hot work or thermal strain as well as cold work. The enhancing effect of cold work is predominant, and the precipitation of σ particles is observed in γ grains as well as in α grains.(3) The precipitation of σ phase is retarded by the prior heating history before aging; (a) heating at the α single phase region or (b) remelting. Especially the effect of the treatment (b) is remarkable. These retardation effects can be ascribed to homogenization of alloying elements by these heat treatments. The difference between the cases of (a) and (b) is thought to result from the degrees of their homogeneities.