Cryogenically martensitic transformation and its effects on tempering behaviors of bearing steel

Herein, the cryogenically martensitic transformation and its effects on the tempering behaviors of bearing steel was investigated. An in-situ magnetic method was used for obtaining the thermodynamics of the martensitic transformation at sub-zero Celsius temperature from in respects of athermal and isothermal conditions. Microstructure evolution was also characterized systematically to investigate the mechanisms of the martensitic transformation. The results showed that the athermally martensitic transformation of samples experienced three different stages, where the martensite formed slowly at 300–230 K, rapidly at 230–120 K, and slowly again at 120–77 K during the continuous cooling process. With the increase of cooling rate, both the start and finish temperatures of the transformation rise, while the maximum transformed rate decreases. The content of retained austenite transformed isothermally is much smaller as compared to the athermal condition. The amount of isothermally formed martensite does not show linear relationship with the temperature, where an optimal isothermal temperature for GCr15 steel is 193 K in this work. XRD measurement result showed that the content of retained austenite decreased with the decrease of treating temperature in practical DCT, where the content was reduced up to 4.2% after DCT at the lowest temperature combined with tempering. DCT enhanced the precipitation of θ-carbides during tempering and altered the precipitation behavior by advancing the precipitated temperature of θ-carbides from 340 °C to 315 °C. The tempered hardness of samples was improved by DCT, which can be attributed to the enhanced carbides precipitation during tempering and lower content of retained austenite after DCT. The improvement was also affected by tempering and austenitizing temperatures. It was concluded that the performance improvement induced by DCT was most evident if DCT was conducted between higher temperature quenching and lower temperature tempering. DCT also affected the decomposition of retained austenite during tempering by decreasing the activation energy of decomposition by up to 38.13 KJ/mol, which indicated a lower thermal stability of retained austenite after DCT. Microstructure observation indicated that the substructure of the new formed martensite was dominated by ultra-fine twins, which derived from the accommodated strain induced by martensitic transformation during DCT. The transformation strain induced by DCT also causes more dislocations in the microstructure, which provides more nucleation sites for the precipitation of θ-carbides during tempering.