Effect of Mo substitution by W on high temperature embrittlement characteristics in duplex stainless steels

Abstract

The influence of molybdenum substitution by tungsten on the impact toughness of the duplex stainless steel base metals aged at 850 degrees C was investigated. The impact toughness was related to the precipitation kinetics of intermetallic phases. Only sigma phase was observed in 3Mo-containing alloy, whereas chi phase and eta (Laves) phase besides sigma phase were precipitated in 3W-containing alloy. The chi and sigma phases were formed intergranularly, and the eta phase was formed intragranularly within a gamma grain. In 3W-containing alloy, the chi and eta phases were precipitated first, followed by the formation of the sigma phase. In addition, unlike 3Mo-containing alloy, the amount of the intermetallic phases (chi, eta and sigma) increased exceedingly slowly, resulting in the retardation of an embrittlement. The chi phase had a lesser effect on impact toughness compared with the sigma phase, due to its fine size and uniform distribution. Aging temperature of 850 degrees C in 3W-containing alloy was located at the upper part of the C-curve denoted by 10% of sigma/gamma intensity ratio in the time-temperature-transformation (TTT) diagram, indicating that sigma phase formation in 3W-containing alloy was delayed by the decrease in driving force for the formation of sigma phase. (C) 1998 Elsevier Science S.A. All rights reserved.