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°C. Upon aging, the major intermetallic phase of the duplex stainless steel containing Mo was σ phase. In duplex stainless steels containing W, χ phase precipitation was enhanced at the early stages of aging and σ phase precipitation retarded. When aged at 900°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 25Cr duplex stainless steel. Despite a thin layer of χ phase at the phase boundary having a more deleterious effect on impact toughness than σ phase, W substitution for Mo in the duplex stainless steel did not degrade the overall toughness. The growth rate of χ phase was retarded owing to supersaturation of W in the 7chi; phase. Since σ phase precipitation depended strongly on Mo 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.
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