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

Yongqiang Zhang a, Jianfeng Gu a,*, Lizhan Han a, Gang Shen a,b, Chuanwei Li a

a Institute of Materials Modification and Modelling Shanghai Jiao Tong University, Shanghai, 200240, China
b School of Mechanical Engineering, Suzhou University of Science and Technology, Suzhou, Jiangsu, 215009, China

ABSTRACT

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 (M_S(- Che) = 102 °C) and the high internal stress (△M_S(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 Ɵ phase, M₃C particles, filmy M₃C and M₇C₃, and also leading to the volume fraction of the carbon-depleted RA transformed into martensite f^RA→M  decrease from 87.6% to 0.0% during the subsequent cooling process. During the second step tempering at 670 °C, these Ɵ 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 f^RA→M from 0% to 87.6%. While for the samples with f^RA→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.