Thermal decomposition characteristics of retained austenite and its influence on impact toughness of B-containing 9Cr1Mo1Co(FB2) steel during the two-step tempering

The decomposition kinetics of retained austensite (RA) and the effect of its decomposition on the impact toughness of FB2 steel during the two-step tempering were studied by employing differential scanning calorimetry (DSC), magnetic saturation measurement (MSM) and transmission electron microscopy (TEM). Furnace-cooling from 1100 °C produces 9.7vol.% filmy RA due to the contributions to Ms from the C enrichment (MS(Che) = 102 °C) and the high internal stress (ΔMS(stress) = -93.3 °C). With the increase of the first step tempering temperature from 570 °C to 630 °C for 1–10 h, RA involves into the different decomposition characteristics, forming θ phase, M3C particles, filmy M3C and M7C3, and also leading to the volume fraction of the carbon-depleted RA transformed into martensite fγRA→M decrease from 87.6% to 0.0% during the subsequent cooling process. During the second step tempering at 670 °C, these θ and M3C particles evolve into spherical M23C6, while film-like M3C and M7C3 evolve into filmy M23(C,B)6. Deteriorated by the filmy M23(C,B)6, the impact toughness increase from 72 J/cm2 to 118 J/cm2 with the increase of fγRA→M from 0% to 87.6%. While for the samples with fγRA→M=0, enhancing the second step tempering temperature to 690 °C for 10 h can further promote the spheroidization of filmy M23(C,B)6 and reduce hardness, resulting in the enhancement of impact toughness from 72 J/cm2 to 112 J/cm2. In summary, a two-step tempering process (570 °C for 1–5 h plus 670–690 °C for 10 h) for RA-containing FB2 steel is suggested to obtain excellent impact toughness and high hardness.