Effect of R Phase Formation on the Mechanical Properties of 25Cr-7Ni-2Mo-4W Super Duplex Stainless Steel

Abstract

In this study, we investigated the precipitation behavior of the R-phase precipitated at the initial stage of aging and its effects on the mechanical properties of 25%Cr-7%Ni-2%Mo-4%W super duplex stainless steel. The R-phase was mainly precipitated at the interface of ferrite/austenite phases and inside of the ferrite phase during the initial stage of aging. It was transformed into the <TEX>${\sigma}$</TEX>-phase with an increase of the aging time. The ferrite phase was decomposed into a new austenite(<TEX>${\gamma}_2$</TEX>)phase and the <TEX>${\sigma}$</TEX>-phase by an aging treatment. The R phase was an intermetallic compound showing higher molybdenum and tungsten concentrations than the matrix and also showed higher molybdenum and tungsten concentrations than the <TEX>${\sigma}$</TEX> phase. In the initial stage of aging, precipitation of the R-phase did not change the hardness, the strength and the elongation. The hardness and the strength increased upon a longer aging time, but the elongation rapidly decreased. These results show that the R-phase did not significantly affect the hardness and the strength, though it did influence the elongation.