Quantitative analysis of phase fraction and mechanical behaviour of friction stir processed carbon steels

Abstract

Duplex phase microstructure was produced by friction stir processing (FSP) in two steel plates of low and medium carbon wt.%. FSP at the same rotational and traverse velocity resulted in two different combinations of micro constituent phases. For processed low carbon steel (PLCS), it is the combination of 65.9% Ferrite, 9.6% Bainite, and 23% Martensite and for processed medium carbon steel (PMCS), it is 21.7% Ferrite and 74.3% Martensite. Microstructure and X-ray diffraction analyses confirm the presence of Retained austenite in both cases. In addition, it reveals the effect of carbon wt.% of base material on martensitic transformation in the form of lath size, volume fraction, and strength of Martensite. Further, the tensile response (yield strength) is found to be in good agreement with rule of mixture for PLCS only. However, by fixing the volume fraction of Martensite (fM) to 0.60 and neglecting the adverse effect of excess fraction of Martensite, contribution of micro constituent phases to yield strength satisfies the rule of mixture for PMCS too. Finally, yield strength contribution of different constituent phases is 43.5% by Ferrite, 29.4% by Bainite, and 31.2% by Martensite including an negligible error of 0.43% for PLCS, while it is 9.8% by Ferrite and 96.8% by Martensite including an error of 6.6% for PMCS.