Microstructure and mechanical properties of Si-rich H13 steel processed via austempering and tempering

Nanobainite shows high tempering stability. However, studies of the multi-tempering behavior of nanobainite hot-working die steel are rare. In the present work, the microstructure and mechanical properties of Si–H13 nanobainite steel, subjected to multi-tempering processes, have been investigated via microstructural characterization and mechanical tests. The results show that the filmy retained austenite decomposed into ferrite and granular V2C during the first tempering process. The size of V2C increased slightly after the second tempering. VC precipitated from bainitic ferrite of the third tempered sample. Because of hindering effects that decomposition products of filmy retained austenite have on the combination of bainitic ferrite laths, the increment of thickness of bainitic ferrite lath is only 29% (26 nm) after multi-tempering processes. The yield strength of multi-tempered samples is obviously higher (by more than 50%) than that of austempered samples. With an increase in number of tempering, the dominant strengthening contribution changes from grain boundary strengthening and dislocation strengthening to precipitation strengthening. Moreover, the precipitation of carbides results in secondary hardening during the tempering process. Although the thickness of bainitic ferrite lath became coarse, the hardness of the multi-tempered sample was approximately the same as that of austempered sample. Hence, the Si–H13 nanobainite steel exhibits good tempering stability.