Influences of Mo and W on the Precipitation of Secondary Phases and the Associated Localized Corrosion and Embrittlement in 29 percent Cr Ferritic Stainless Steels

摘要

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 phase formed first in grain boundaries in an early stage of aging. and it was gradually substituted by sigma phase with further aging. The precipitation rate of sigma phase appears to be determined by both the diffusion rates of W and Mo for the formation of the sigma phase as well as by the affinity of chi phase, as a competitor, for the elements. Due to the high affinity of chi phase for W with & slow diffusion rate, the nucleation of sigma 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 sigma phase, which induced significant depletion of Cr and W (or Mo) around the phase, was prominent in the W-containing alloys. The W-containing alloys exhibited effective delay of sigma phase formation.