Prediction of ^|^sigma; Phase Precipitation in Type 316FR Stainless Steel Weld Metal

Abstract

Through the isothermal ageing treatments at 873, 923, 973, and 1023K, precipitation of sigma (σ) phase in type 316FR stainless steel weld metal was examined based on the kinetics approach. Microstructural examination was performed by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The dominantly precipitated phases were sigma (σ) and chi (χ), nucleated at δ-ferrite/austenite (γ) interface or interior of the δ-ferrite grains, consuming the δ-ferrite during isothermal holding at each of ageing temperature. The total amount of intermetallic phases precipitated during isothermal ageing sigmoidally increased as a function of the ageing time. Precipitation of these intermetallic phases could be approximately followed by Johnson-Mehl equation. Based on the determined kinetic equation, precipitation behavior of intermetallic phases at service-exposure temperature could be successfully predicted.