Optimizing the austenite stability in a ferritic lightweight steel through thermomechanical processing

The present work deals with the effect of friction stir processing on the stability of the austenite during quenching and also its mechanical stability in the course of subsequent room temperature straining in a duplex ferritic lightweight steel. Interestingly, it was found that the austenite fraction increased owing to the dynamic phase transformation of ferrite to the austenite and enhancement in austenite stability. In this regard, the influence of dislocation density variations, partitioning in solute elements (Mn and C) and grain refinement on austenite stability was discussed in details. The optimum austenite stability during quenching was achieved in the case of processed microstructure holding finer grain size, and higher dislocation density. These provided fast track diffusion for austenite stabilizer elements, which decreased the thermally activated martensite starting temperature and finally led to stabilization of austenite. These factors have directly influenced the mechanical stability of the austenite during subsequent deformation and increased the strain range for the occurrence of strain induced martensitic transformation which led to a significant strength/elongation balance. The capability of transformation induced plasticity (TRIP) effect was deteriorated in the case of microstructures with lower austenite stability. Surprisingly, despite the appreciable decrease in grain size the yield strengths of the processed specimens were not influenced significantly. The deviation was justified considering the fact that the critical stress of TRIP effect was lower than of predicted through Hall-Perch relationship.