The influence of molybdenum substitution by tungsten on the impact toughness of the duplex stainless steel base metals aged at 850℃ 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 phase and σ phase were formed intragranularly, and the η phase was formed intragranularly within a γ grain. In 3W-containing alloy, the χ phase and η phase 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 slow, 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℃ in 3W-containing alloy was located at the upper part of the C-curve denoted by 10 % of σ/γ intensity ratio in time-temperature-transformation (TTT) diagram, indicating that the σ phase formation in 3W-containing alloy was delayed by the decrease of driving force for the formation of σ phase.
The influence of molybdenum substitution by tungsten on the pitting resistance of solution-treated duplex stainless steel base metals was investigated by the anodic polarization test. In terms of the pitting resistance, molybdenum could be successfully substituted by tungsten. However, it is suggested that molybdenum should be partially replaced to maintain good pitting resistance. AES depth profiling offered an explanation for the change in pitting resistance depending on the bulk content of molybdenum and tungsten. Molybdenum and tungsten were mostly located in the outer part of passive film. Furthermore, the concentration ratios of molybdenum and tungsten in the passive film were almost proportional to their bulk contents. Therefore, it can be concluded from the AES analysis that the bulk content of molybdenum and tungsten in duplex stainless steels affected the composition of passive film, resulting in a change of pitting resistance. The influence of molybdenum substitution by tungsten on the resistance to stress corrosion cracking (SCC) was also investigated by the slow strain rate test. Although molybdenum was substituted by tungsten, the susceptibility to SCC in $MgCl_2$ solution hardly changed and thus kept constant. The cracking was mainly transgranular irrespective of the bulk content of molybdenum and tungsten. The crack followed a path primarily through ferrite and austenite grains, often being stopped by the austenite or diverted into the phase boundary around the austenite grains.
Welding process can significantly modify the microstructure and mechanical properties of duplex stainless steels. And it is necessary to control the phase transformation during welding for obtaining good weldability. When molybdenum was substituted by tungsten, the impact toughness of heat affected zone (HAZ) kept nearly constant irrespective of the change of cooling rate. Therefore, it can be concluded that tungsten-containing duplex stainless steels show good welding characteristic. In the HAZ of tungsten-containing duplex stainless steels, the phase transformation from austenite to ferrite during heating was delayed. Accordingly, because partially dissolved austenite bands retarded the coarsening of ferrite grains, the grain size of ferrite phase remained small. Furthermore, because of the absorption of nitrogen by the partially dissolved austenite bands, there were relatively small amounts of chromium nitrides within ferrite grains. The type of chromium nitride was hexagonal $Cr_2N$ and the intragranular $Cr_2N$ particle had the following relationship with ferrite matrix: $[011]_δ$ // $[0001]_{Cr_2N}$, $($\bar{2}$2$\bar{2}$)_δ$ // $(1$\bar{1}$00)_{Cr_2N}$.
