Intragranular Chromium Nitride Precipitates in Duplex and Superduplex Stainless Steel

Abstract

Intragranular chromium nitrides is a phenomenon with detrimental effects on material properties in superduplex stainless steels which have not received much attention. Precipitation of nitrides occurs when the ferritic phase becomes supersaturated with nitrogen and there is insufficient time during cooling for diffusion of nitrogen into austenite. Heat treatment was carried out at between 1060◦C and 1160◦C to study the materials susceptibility to nitride precipitation with varying heat treatment temperature, nitrogen content and microstructure. Microhardness tests and Charpy v-notch tests were also carried out to investigate the nitrides effect on material properties. No standard method of quantification of nitrides exist. A method of quantifying the precipitation based on area fraction of nitrides was therefore introduced. The results show an increased amount of nitride precipitation with increased heat treatment temperature. A coarse microstructure with a large austenite spacing was found to promote higher fractions of nitride precipitation while nitrogen content was found to affect the amount of precipitation in less extent. The intragranular nitrides cause precipitation hardening in the ferrite and the precipitation was found to be at its most severe in the center of the ferritic regions, with precipitation free zones close to the phase boundaries. The microhardness of the phase was affected accordingly, with increasing hardness towards the center. Charpy v-notch test results show that nitride precipitation causes an embrittlement of the steel while intragranular secondary austenite improves the impact toughness of the material as it shortens the dif- fusion distance of nitrogen, decreasing the materials susceptibility to intragranular nitride precipitation.