Shear band and α′-martensite induced by stress in laser melting carbide-free bainite layer

By means of laser melting and following isothermal transformation (LMFIT), the shear band and α′-martensite in mid-carbon alloy steel layer induced by stress were analyzed. The residual stress, microstructures and nanohardness were analyzed by X-ray stress analyser, X-ray diffraction (XRD), transmission electron microscopy (TEM) and nanoindentation. The results indicate that the carbide-free baninte layers with high hardness are formed on the surface of mid-carbon and high-carbon alloy steels. By LMFIT, the hardness of mid-carbon alloy steel layer can be increased by the carbon supersaturation in bainite ferrite (BF). Meanwhile, shear band and α′-martensite can be induced by thermal stress in mid-carbon alloy steel layer to further improve the hardness. The gradient change of residual stress in the bainite layer leads to that of the nanohardness in bainite layer.