Chi-phase precipitation in a duplex stainless steel

The aim of this study is to investigate the precipitation of intermetallic phases, especially the chi-phase, in a 4SN (type UNS S31803) duplex stainless steel through aging heat-treatments carried out at 700 and 750 degrees C. Two intermetallic phases are detected: chi (chi) and sigma (sigma). The chi-phase precipitates at ferrite/ferrite grain boundaries prior to the sigma-phase precipitation, which occurs preferentially at ferrite/austenite interfaces and at ferrite/ferrite grain boundaries. The sigma-phase precipitation is a eutectoid type reaction of ferrite leading to sigma-phase phase and austenite. The chi-phase is consumed in the sigma-phase precipitation after becoming completely surrounded by both the sigma-phase and the newly formed austenite. (C) 2009 Elsevier Inc. All rights reserved.