Mechanism of δ → δ + γ phase transformation and hardening behavior of duplex stainless steel via sub-rapid solidification process

In this contribution, a duplex stainless steel (DSS) strip is obtained under the sub-rapid solidification condition by a recently developed fast dip tester, which is designed for simulation of the strip casting process. The mechanism of δ → δ + γ phase transformation and hardening behavior of DSS via the sub-rapid solidification process are investigated. The results suggest that the DSS under a rapid cooling rate would go through a shear/semi-shear phase transformation, which is different from the traditional diffusional transformation. There is no noticeable element precipitation content difference between the ferrite and austenite phases, and more dislocations are formed inside the austenite grains in the sub-rapid solidified sample. Furthermore, the austenite fraction of the sub-rapid solidified sample (4.08–4.32%) is substantially decreased compared with the slow solidified sample (36.12–39.37%). The length of grain/phase boundary, especially the dispersed small austenite particles is increased significantly. Besides, the hardness is increased to 311 HV and the wear-resisting property is enhanced with the wear consumption decreasing from 0.0291 to 0.0122 g, due to the multiple hardening effects under sub-rapid solidification process, including higher ferritic friction, grain refinement hardening, and dislocation hardening.