Stabilization of retained austenite in Cr4Mo4V bearing steel by pulsed magnetic field

[Objective] The primary objeetive of this research is to meticulously examine how pulse magnetic field assisted deep cryogenic (MDC) treatment affects the transformation and stabilization of retained austenite in Cr4Mo4V bearing steel. This study aims to elueidate the underlying mechanisms by which the pulse magnetic field influences the microstructural changes in bearing steel, particularly focusing on the stabilization of retained austenite, which plays a crucial role in determining the mechanical properties and Overall Performance of the steel.[Methods] To achieve a comprehensive understanding of how retained austenite transformed under various treatment conditions, this study utilized several material characterization techniques, including X-ray diffraction (XRD), vibrating sample magnetometry (VSM), and electron backscatter diffraction (EBSD). The use of EBSD analysis allows for a detailed comparison of variations in the dislocation density among samples processed under different conditions. For comparative analysis, the experimental set-up was divided into two distinet treatment processes: the conventional deep cryogenic (DO treatment and the MDC treatment. Following these treatments, the samples were subjected to high-temperature tempering to evaluate the thermal stability of the retained austenite.[Results] The XRD analysis revealed a reduction in the volume fraction of retained austenite from (23.8±0.6)% to (21.5%±0.9)% following the DC process. A relatively smaller reduction to (22.5%±0.5)% was observed with the MDC process. These results, supported by VSM and EBSD analyses, highlight the capacity of the pulse magnetic field to partially inhibit the transformation of retained austenite. Further examination of the high-temperature stability of austenite in samples treated with DC and MDC revealed that MDC samples demonstrated improved retention, maintaining 7. 1% of retained austenite after high-temperature tempering, compared to 4. 9% in DC-treated samples. This indicates that the retained austenite in Cr4Mo4V bearing steel exhibits improved high-temperature stability following treatment with the MDC process. Furthermore, the dislocation density analysis revealed that the DC process led to a 9. 8% increase in the dislocation density, whereas the MDC process moderated this increase to only 6. 5%. This difference suggests the magnetic field's role in inhibiting dislocation diffusion, which in turn reduces martensite nucleation sites, thereby stabilizing retained austenite. The dislocation density change of the samples treated with DC and MDC after a high-temperature tempering validates this point. The dislocation density in DC-treated samples was approximately 1.23X1015 m-2, while it decreased to 1.13X1015 m-2 in MDC-treated samples. The dislocation density change reflects the extent of phase transformation.[Conclusions] This study provides a thorough analysis that clearly demonstrates the significant impact of applying a pulse magnetic field during deep cryogenic treatment on the microstructural evolution of Cr4Mo4V bearing steel. The magnetic field not only moderates the increase in the dislocation density but also enhances the mobility of dislocations. This contributes to the stabilization of retained austenite, which is crucial for improving the mechanical properties and Performance of bearing steel. The findings of this research lay a solid foundation for optimizing heat treatment processes using the magnetic field assisted deep cryogenic treatment. © 2024 Tsinghua University. All rights reserved.