Wang Bin, He Yanping, Liu Ye, Tian Yong, You Jinglin, Wang Zhaodong, Wang Guodong
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China.
State Key Laboratory of Advanced Special Steel, Shanghai Key Laboratory of Advanced Ferrometallurgy, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China.
Materials (Basel). 2020 May 20;13(10):2352. doi: 10.3390/ma13102352.
In this study, vacuum low-pressure carburizing heat treatments were carried out on 18Cr2Ni4WA case-carburized alloy steel. The evolution and phase transformation mechanism of the microstructure of the carburized layer during low-temperature tempering and its effect on the surface hardness were studied. The results showed that the carburized layer of the 18Cr2Ni4WA steel was composed of a large quantity of martensite and retained austenite. The type of martensite matrix changed from acicular martensite to lath martensite from the surface to the core. The hardness of the carburized layer gradually decreased as the carbon content decreased. A thermodynamic model was used to show that the low-carbon retained austenite was easier to transform into martensite at lower temperatures, since the high-carbon retained austenite was more thermally stable than the low-carbon retained austenite. The mechanical stability-not the thermal stability-of the retained austenite in the carburized layer dominated after carburizing and quenching, and cryogenic treatment had a limited effect on promoting the martensite formation. During low-temperature tempering, the solid-solution carbon content of the martensite decreased, the compressive stress on the retained austenite was reduced and the mechanical stability of the retained austenite decreased. Therefore, during cooling after low-temperature tempering, the low-carbon retained austenite transformed into martensite, whereas the high-carbon retained austenite still remained in the microstructure. The changes in the martensite matrix hardness had a far greater effect than the transformation of the retained austenite to martensite on the case hardness of the carburized layer.
在本研究中，对18Cr2Ni4WA渗碳合金钢进行了真空低压渗碳热处理。研究了渗碳层在低温回火过程中微观组织的演变及相变机制及其对表面硬度的影响。结果表明，18Cr2Ni4WA钢的渗碳层由大量马氏体和残余奥氏体组成。马氏体基体的类型从表面到心部由针状马氏体转变为板条马氏体。随着碳含量的降低，渗碳层的硬度逐渐降低。采用热力学模型表明，低碳残余奥氏体在较低温度下更容易转变为马氏体，因为高碳残余奥氏体比低碳残余奥氏体热稳定性更高。渗碳和淬火后，渗碳层中残余奥氏体以力学稳定性而非热稳定性为主导，深冷处理对促进马氏体形成的作用有限。在低温回火过程中，马氏体的固溶碳含量降低，残余奥氏体上的压应力减小，残余奥氏体的力学稳定性降低。因此，在低温回火后的冷却过程中，低碳残余奥氏体转变为马氏体，而高碳残余奥氏体仍保留在微观组织中。马氏体基体硬度的变化对渗碳层的表面硬度影响远大于残余奥氏体向马氏体的转变。
