UNS S31803 stainless steel phase transformations during 475°C and 650°C aging and its influence on the pitting potencial in a 0.6M NaCl solution

Abstract

This work evaluates the phase transformations during aging at 475°C or 650°C of UNS S31803 duplex stainless steel and its influence on the pitting potential in a 0.6M NaCl solution. The material was solution treated at 1175°C and water quenched, and then aged at 475°C or 650°C for times up to 360h. Samples were a nalyzed by optical microscopy through quantitative metallography, and ferrite quantification was conducted with a ferritscope. Vickers microhardness measurements were also performed. Cyclic polarization tests were then conducted in a 0.6M NaCl solution, at a scan rate of 1 mV/s. It is found that aging at 475°C leads to a continuous increasing of material microhardness, probably related to the presence of alpha prime phase, and reduction in the pitting potentials were observed for samples aged for more than 12 h. Samples aged at 650°C show the presence of sigma and chi phases, which increased with aging time, and pitting potential is reduced for aging times greater than 4h. The pitting potential reduction in the aged samples is probably associated with the presence of ferrite and/or austenite impoverished in Cr and Mo, as a result of alpha prime formation at 475°C or sigma and chi formation at 650°C.