MECHANISM OF WIDMANSTATTEN AUSTENITE FORMATION IN A DELTA GAMMA DUPLEX PHASE STAINLESS-STEEL/

Widmanstatten austenite laths forming from delta-phase in a delta/gamm a duplex phase stainless steel have been investigated metallographical ly. The austenite laths form in the temperature range between 1 100 an d 750 degrees C with a C-curve in a T-T-T diagram. The formation of Wi dmanstatten austenite laths accompanies a sharp surface relief similar in appearance to that of martensite. Widmanstatten laths formed at 1 000 degrees C where relatively large alloy partition occurs do not cha nge the morphology by the further isothermal holding. Those formed at lower temperatures, 900 and 800 degrees C, where the partition is rela tively small, however, decompose into the rows of small austenite frag ments, and the alloying elements are partitioned quite largely between the gamma fragments and the delta-matrix. The driving force for this interphase boundary migration producing the gamma-fragments is thought to arise from the partial supersaturation of alloying elements in the initially formed laths. The interphase boundary migration with alloy partition does not produce additional surface reliefs, suggesting that a pure diffusional transformation does not induce surface reliefs. Su ch an austenite lath formation can be explained consistently in terms of a shear-assisted diffusional transformation model where the lattice change occurs via a diffusional individual atomic jumps and the resul ting elastic strain is relaxed by lattice invariant shear.