Effects of W substitution on sigma and chi phase precipitation and toughness in duplex stainless steels

Abstract

The effects of W on the precipitation behaviour of 25 Cr-6Ni base super duplex stainless steels with various Mo and W contents have been examined following heat treatments in the temperature range 700-1050 degrees C. Upon aging, the major intermetallic phase of the duplex stainless steel containing Mo was sigma phase. In duplex stainless steels containing W, chi phase precipitation Mins enhanced at the early stages of aging and sigma phase precipitation retarded. When aged at 900 degrees C for. 13.5 min the impact energies of 25Cr-6Ni-3.6Mo, 25Cr-6Ni-3.6Mo-3W; 25Cr-6Ni-2Mo-3.6W, and 25Cr-6Ni-1.5Mo-4.3W were reduced to 7, 3.6, 19.4, and 28.1 J respectively. Simple addition of W increased brittleness on aging in 25 Cr duplex stainless steel. Despite a thin layer of chi phase at the phase boundary having a mole deleterious effect on impact toughness than sigma phase, W substitution for Mo in the duplex stainless steel did not degrade the overall toughness. The growth I ate of chi phase was retarded owing to supersaturation of W in the chi phase. Since sigma phase precipitation depended strongly on Mb content, the total amount of intermetallic could be reduced for a given aging condition by W substitution. As W substitution increased, the total intermetallic phase content decreased.