Influence of laser surface hardened layer on mechanical properties of re-engineered low carbon steel sheet

In the present work, a C-Mn low carbon automotive steel sheet has been used for laser surface hardening treatment. The steel sheet has been successfully surface hardened using high power diode laser up to certain depth (250-300 mu m) to make layered composite structure through its thickness. The microstructure of the treated layered steel sheet have been evaluated by different characterization techniques such as optical microscopy, scanning electron microscopy, X-ray diffraction. The hardness at the treated surface improved significantly (133-145% increase as compared to base hardness) due to formation of harder phases, which are identified as a mixture of martensite, bainite and prior-ferrite using EBSD technique. The tensile property of the layered steel sheet was also evaluated and found to yield significant improvements in both Yield Strength (YS) (25-40%) and Ultimate Tensile Strength (UTS) (20%) due to the sandwich effect of a composite layer constituting hardened layer and base accomplished by a strengthening mechanism associated with rule of mixtures concept. Correspondingly, the Young's modulus of the layered steel sheet determined from the slope of tensile stress-strain diagram was also found to be consistent with ultrasonic test results. Additionally, the crystallographic texturing effects of the hardened layer and untreated base measured using standard XRD technique re-confirmed their effect on Young's Modulus and r-bar values obtained.