Effects of alloying elements on σ phase precipitation in δ–γ duplex phase stainless steels

The effects of alloying elements on σ phase precipitation in δ–γ duplex phase stainless steels have been studied with particular emphasis on the partitioning of alloying elements. Sigma phase precipitation occurred mainly through the eutectoid decomposition of δ into σ and γ phases. The concentrations of Cr, Mo, W, and Si in each phase decreased in the order σ, δ, then γ phase, while those of Ni and Mn decreased in the reverse order. The increase of Cr and Mo contents increased both the rate and the maximum amount of σ phase precipitation, while the increase of Ni content reduced the maximum amount of σ phase precipitation, though the rate of the precipitation was also accelerated. The effect of Ni can be explained in terms of the reduction in the δ phase fraction before aging, resulting in the enrichment of σ forming elements in the ferrite.