On the local mechanical properties of isothermally transformed bainite in low carbon steel

Abstract

For bainite microstructure composed of ferritic matrix and carbides or martensite-austenite (MA) constituents, one of constituent phases sometimes plays a crucial role in the specific stage of overall mechanical behaviors. In this study, the nanoindentation tests were employed to examine the local mechanical behavior of each constituent phase that formed after isothermal annealing. Transmission electron microscopy observation was conducted to reveal the intricate structure of each constituent phase. The average nanohardness of MA constituents decreased sharply with increasing isothermal temperature, while the ferritic laths had relatively stable nanohardness over the whole range of bainite transformation. However, the nanohardness of ferritic laths was intrinsically lower than the nanohardness of bainite reported in the literature. For the bainite formed at lower isothermal temperature condition, the MA constituents exhibited twice the hardness of ferritic matrix. Based on the load-displacement curve, it was further revealed that the MA constituents have higher the maximum shear stress and smaller plastic zone underneath the indenter tip than the ferritic matrix, leading to the difference in plastic deformation ability between them.