Microstructural and mechanical properties of low-carbon ultra-fine bainitic steel produced by multi-step austempering process

The multi-step isothermal austempering heat treatment to achieve an ultra-fine bainitic microstructure and maximum volume fraction of bainite was conducted on a steel containing 0.26 wt% carbon. The microstructural and crystallographic characteristics, as well as the mechanical properties and fracture behavior were studied. The results showed that the subsequent austempering heat treatment at a lower temperature, immediately after partial bainite formation at a higher temperature, would replace the coarse austenite/martensite areas with much refined bainite consisting nanoscale plates of bainitic ferrite and filmy austenite which ultimately leads to the refinement of the bainitic microstructure. This microstructural modification, in addition to the increased yield strength, causes a significant increase in the impact fracture toughness of the multi-step austempered steels. The EBSD analysis also showed that the subsequent austempering heat treatment at a lower temperature results in a finer structure of Bain groups and increase in the fraction of high angle grain boundaries leading to higher resistance against crack propagation and subsequently higher impact energy absorption.