55SiCr弹簧钢中的残留奥氏体与碳化物

Retained austenite and carbide in 55SiCr spring steel

通过扫描电镜(SEM)、电子背散射衍射(EBSD)、透射电镜(TEM)、X射线衍射仪(XRD)、热膨胀仪、洛氏硬度计等手段研究了弹簧钢55SiCr的组织和相变点以及残留奥氏体和碳化物在热处理过程中的组织演变。结果表明:55SiCr弹簧钢淬火后残留奥氏体以块状分布在基体上;随回火温度的升高,残留奥氏体减少并呈粒状和薄膜状分布;C在残留奥氏体中富集,使其稳定性增强;Si抑制了碳化物的析出,提高了残留奥氏体的稳定性。低温回火时,Si延缓了渗碳体析出;高温回火时,C原子扩散速率提高,促进渗碳体析出,引起体积的收缩。慢速加热回火时,C有足够的时间扩散,从而促进渗碳体的形成,使渗碳体的形成温度提前;快速加热回火时,C来不及扩散,抑制了渗碳体的析出。回火加热速率一样时,试验钢的硬度随回火温度的提高而下降。当回火温度为400℃时,硬度值最大为51HRC;当回火温度为650℃时,硬度值最小为37HRC。当加热速率为0. 1℃/s时,硬度值最小为33HRC;当加热速率为200℃/s时,硬度值最大为40HRC。

The microstructure and phase transition point of the spring steel 55SiCr,the microstructure evolution of retained austenite and carbide during heat treatment were studied by means of scanning electron microscopy( SEM),electron backscatter diffraction( EBSD),transmission electron microscopy( TEM),X-ray diffractometry( XRD),thermal expansion,and Rockwell hardness tester. The results show that the retained austenite of 55SiCr spring steel is distributed in blocks on the substrate after quenching. As the tempering temperature increases,the retained austenite is reduced and distributed in a granular and film form. Carbon is enriched in retained austenite,which enhances its stability. Since Si suppresses the precipitation of carbides,the stability of retained austenite is improved. Si delays the precipitation of cementite when tempered at low temperature. When the steel is tempered at high temperature,the diffusion of carbon atoms is increased,and the precipitation of cementite is promoted to cause volume shrinkage. Carbon has enough time to diffuse,which promotes the formation of cementite and advances the formation temperature of cementite when tempered at low heating rate. Carbon does not have enough time to diffuse and inhibits the precipitation of cementite when tempered at high heating rate. When the tempering heating rate is the same,the hardness of the tested steel decreases as the tempering temperature increases. The hardness value is 51HRC when the tempering temperature is 400 ℃ and the hardness value is 37 HRC when the tempering temperature is 650 ℃ . The hardness value is 33HRC when the heating rate is 0.1 ℃/s and the hardness value is 40HRC when the heating rate is 200 ℃/s.