Abstract
The formation mechanism of vermicular ferrite and lacy ferrite observed in austenitic stainless steel weld metal solidified in the FA mode was investigated. The formation of vermicular ferrite or lacy ferrite is not decided by chemical compositions but it is decided by both the crystallographic orientation relationship between ferrite and austenite established at the stage of ferrite nucleation and the relationship between the welding heat source direction and the preferential growth directions of ferrite and austenite respectively. On the basis of the results, the effect of ferrite morphology on a cryogenic impact toughness and a pitting corrosion resistance of weld metal was investigated. In the case of almost the same amount of ferrite, as the proportion of lacy ferrite increases, the cryogenic impact toughness and the pitting corrosion resistance increase. Consequently, even though the identical chemical composition of stainless steel, it is possible to improve a cryogenic toughness and pitting corrosion resistance by controlling the ferrite morphology.
