Mechanism of Widmanstätten Austenite Formation in a δ/γ Duplex Phase Stainless Steel

Abstract

Widmanstätten austenite laths forming from δ-phase in a δ/γ duplex phase stainless steel have been investigated metallographically. The austenite laths form in the temperature range between 1100 and 750°C with a C-curve in a T-T-T diagram. The formation of Widmanstätten austenite laths accompanies a sharp surface relief similar in appearance to that of martensite. Widmanstätten laths formed at 1 000°C where relatively large alloy partition occurs do not change the morphology by the further isothermal holding. Those formed at lower temperatures, 900 and 800°C, where the partition is relatively small, however, decompose into the rows of small austenite fragments, and the alloying elements are partitioned quite largely between the γ fragments and the δ-matrix. The driving force for this interphase boundary migration producing the γ-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. Such 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 resulting elastic strain is relaxed by lattice invariant shear. © 1995, The Iron and Steel Institute of Japan. All rights reserved.