Phase transformation during friction stir processing of dual-phase 600 steel

Abstract

The objective of this work is to investigate the phase transformation during friction stir processing of dual-phase 600 steel. Friction stir processing is a microstructure modification process through high strain rate deformation at high temperature. The material undergoes appreciable microstructural changes, which reflects in the refined mechanical properties. The type of metallurgical phases and their fractions have direct impact on the mechanical properties. A coupled 3D thermo-mechanical and phase transformation model is developed to predict the temperature history, plastic deformation and phase transformation during friction stir processing. The modified Johnson–Mehl–Avrami Kolmogorov and Koistinen–Marburger equations are used to model the diffusional and non-diffusional phase transformation, respectively. Friction stir processing is performed with 1.4-mm-thick dual-phase 600 steel and pinless tool. Friction stir tool rotation frequency is varied across friction stir-processed samples, at constant traverse speed. Electron back-scattered diffraction and optical microscopy are used to analyse the phases and microstructure. Model predicts the maximum bainite and martensite volume fractions at stir zone centre. Further, more bainite and martensite formation is estimated at higher tool rotation speeds. These predictions compare well against the experimental observations. Numerical results suggest that the ferrite in the stir region is the untransformed ferrite during heating. During cooling, very small austenite grain size restricts the amount of austenite transformation to ferrite. The phases present in the stir region upon FSP are significantly affected by the amount of carbon content and the initial phases present in the steel.