Influences of Mo substitution by W on the precipitation kinetics of secondary phases and the associated localized corrosion and embrittlement in 29% Cr ferritic stainless steels

Influences of molybdenum (Mo) substitution by tungsten (W) on the formation kinetics of secondary phases and the associated localized corrosion and embrittlement of Fe–29Cr–4Mo, Fe–29Cr–4W, and Fe–29Cr–8W ferritic stainless steels were investigated. Fine chi (χ) phase formed first in grain boundaries in an early stage of aging, and it was gradually substituted by sigma (σ) phase with further aging. The precipitation rate of σ phase appears to be determined by both the diffusion rates of W and Mo for the formation of the σ phase as well as by the affinity of χ phase, as a competitor, for the elements. Due to high affinity of χ phase for W with slow diffusion rate, the nucleation of σ phase was significantly delayed in Fe–29Cr–4W and Fe–29Cr–8W alloys compared with that in Fe–29Cr–4Mo alloy. In addition, the deterioration of ductility and localized corrosion resistance by the precipitation of secondary phases was significantly retarded in Fe–29Cr–4W alloy compared with that in Fe–29Cr–4Mo alloy, due to the delayed precipitation of secondary phases in Fe–29Cr–4W alloy. In particular, retardation of degradation in localized corrosion resistance by the formation of σ phase, which induced significant depletion of Cr and W (or Mo) around the phase, was prominent in the W-containing alloys. These alloys exhibited effective delay of σ phase formation.