Comparative study on sigma phase precipitation of three types of stainless steels: Austenitic,superferritic and duplex

Abstract

The present work studies, in a comparative manner, the sigma phase precipitation of three stainless steels: austenitic type 316L (17Cr–12Ni–2·5Mo, wt-%), superferritic type DIN W. Nr. 1·4575 (Nb–28Cr–4Ni–2Mo, wt-%) and duplex type DIN W. Nr. 1·4462 or UNS S31803 (22Cr–5·5Ni–3Mo–0·14N, wt-%). In austenitic stainless steel, the formation of sigma phase occurred both at austenite grain boundaries and inside delta ferrite islands. In superferritic stainless steel, sigma phase occurred both at grain boundaries and in the grain interior. In the ferrites, both in the duplex and in the austenitic steel, sigma phase formation occurred by eutectoid reaction ferrite→sigma + austenite. The tendency towards precipitation of the sigma phase in the three types of steel investigated can be placed in the following sequence: duplex>superferritic>austenitic.