Effects of heat treatment on the phase ratio and corrosion resistance of duplex stainless steel

Abstract

The volume fraction of ferrite and austenite phase and corrosion resistance of 26.2Cr-6.99Ni-2.37Mo-2.88W-0.35N duplex stainless steel have been studied by the use of optical microscopy, Feritscope, EBSD, SEM and Anodic polarization test after solution treatment at every 20°C at 1050°C∼1200°C for 30 min. As the temperature of solution heat treatment increased, the content of ferrite that could be transformed to intermetallics such as σ and χ phase was higher, and besides, grain size increased and the number of grains decreased due to the growth of the phase and thus grain boundary and phase boundary that can be served as precipitation site of intermetallics were reduced, therefore the precipitation of intermetallics was suppressed. When PRE (Pitting Resistance Equivalent) values for the ferrite and austenite phases after solution heat treatment were calculated by weight percents of alloy elements using SEM-EDS, PRE value gap between the two phases was the smallest when heat treated at 1090°C. Because the increase over 1090°C of heat treatment temperature caused larger difference in PRE values between two phases, corrosion resistance between the two phases was out of balance and thus corrosion resistance was reduced when solution heat treated at the temperature over 1090°C. © 2009 The Japan Institute of Metals.