Two-step austempering of high performance steel with nanoscale microstructure

It is crucial to eliminate the thermally and mechanically less stable austenite blocks as much as possible if enhanced mechanical properties are demanded in high performance nanostructured bainitic steels. Step-austempering would be an efficient procedure in this regard. This article aims to investigate the strength-ductility- impact toughness combinations in nano-bainite after two-step austempering process comparing with those obtained after conventional isothermal bainite transformation. It has been shown that large austenite blocks further decomposed to bainite after step-austempering process which in turn decreased the average volume fraction of austenite, increased its mechanical stability and generally refined the final microstructure. Step-austempering increased the hardness value and enhanced the yield strength and ultimate tensile strength properties. Additionally, the higher mechanical stability of retained austenite in step-austempered samples increased the elongation level and improved the impact toughness at earlier stages of the second step of transformation. The important point is, it has been found that ductility and toughness properties were influenced by transformation time at the second stage of austempering which approved the idea that not only the mechanical stability and morphology of retained austenite but also its volume fraction must be taken into account when applying a multi-step austempering heat treatment in nanobain steels.