脉冲磁场对Cr4Mo4V轴承钢残余奥氏体的稳定化作用

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

为研究脉冲磁场辅助深冷处理对Cr4Mo4V轴承钢中残余奥氏体组织的影响,采用X射线衍射、振动样品磁强计、电子背散射衍射等进行实验,结果表明:纯深冷(DC)处理工艺使Cr4Mo4V轴承钢中残余奥氏体体积分数从(23.8±0.6)%下降到(21.5±0.9)%,而在深冷处理过程中加入辅助脉冲磁场(MDC)仅使残余奥氏体体积分数从(23.3±0.3)%下降到(22.5±0.5)%。振动样品磁强计和电子背散射衍射的实验结果也验证了辅助脉冲磁场可以抑制部分残余奥氏体相变。将处理后的轴承钢样品进行一次高温回火后发现,MDC处理使残余奥氏体具有更好的高温稳定性。对材料中几何必要位错密度的对比分析发现,DC处理使位错密度增加了9.8%,而MDC处理下位错密度仅增加了6.5%,这表明脉冲磁场的加入抑制了DC处理时位错密度的增加,进而有效减少了马氏体的形核部位,使得残余奥氏体的转变得到抑制。脉冲磁场通过提高位错移动性降低材料中的位错密度,是Cr4Mo4V轴承钢中残余奥氏体稳定化的主要原因。

[Objective] The primary objective 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 elucidate 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 distinct treatment processes:the conventional deep cryogenic(DC) 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 1.23×10^(15) m^(-2),while it decreased to 1.13×10^(15) 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.