Effect of high temperature aging on ferrite transformation kinetics and tensile properties of type 316L stainless steel weld metal

Abstract

Tensile samples of AISI type 316L weld metal, deposited by the gas tungsten arc welding process, were heat treated at 873,973, and 1073 K, before being subjected to tensile loading at an initial strain rate of 5·5 × 10−5 S−1 at a temperature of 427 K. The transformation kinetics of δ ferrite was studied in detail. The activation energy for the transformation of δ ferrite was found to be 272 kJ mol−1. Theferrite transformation kinetics datafor the above three temperatures were thenfitted into a master plot using the Dorn parameter, and its validity in this temperature range was verified by superimposing the ferrite transformation kinetics data at 948 and 1023 K on to the master plot. From examinations by optical microscopy, the weld metal was found to undergo competing transformation reactions during high temperature aging, namely, dissolution of δ ferrite, precipitation of carbidesj carbonitrides and σ phase, changes in σ phase morphology, and spheroidisation of σ phase. The effect of the ferrite transformation kinetics and σ phase precipitation kinetics on the tensile properties of the weld metal were also studied in detail. The yield stresses of all the aged weld metals were lower than that of the as deposited weld metal. The σ phase had no direct influence on changes in the yield stress. The ultimate tensile strength was affected only by the quantity and morphology of σ phase. All the competing processes (except changes in σ phase morphology) significantly influenced ductility. The increase in work hardening exponent n was dependent only on the amount of σ phase and not on its morphology.MST/1983