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

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