Effects of tempering mode on the phase transformation and mechanical properties of 30CrMnSiNi2A steel within the tempered martensite embrittlement regime

The effects of conventional furnace tempering (FT) and induction tempering (IT) on the microstructure, carbide precipitation behavior and mechanical properties of 30CrMnSiNi2A high strength steel were systematically studied in this work. The equivalent tempering degree was defined either by the Hollomon-Jaffe tempering parameter (TP) or the Semiatin modified TP. The results show that both tempering methods retained the original martensitic lath morphology and produced similar tempering microstructure under equivalent tempering parameters. Electron backscattering diffraction analysis revealed that IT resulted in a higher dislocation density compared to FT due to the inhibiting effect of short IT time on dislocation recovery and subsequent dislocation annihilation. As TP increases, the shape of carbides change from fine needle-like to spherical, regardless of the tempering method applied. However, IT requires higher TP to transform ε-carbide into θ-carbide. Under the same TP, the two tempering methods produced similar hardness, but the tempering method affects the room temperature tensile properties and impact toughness. IT slightly reduced the tensile properties, but by suppressing the formation of networked or coarse carbides, it prevented embrittlement of tempered martensite and increased impact toughness by up to 52 %.