中碳Cr-Mo-V二次硬化钢奥氏体化温度及冷却速度对微观组织的影响

Effect of austenitizing temperature and cooling rate on microstructure of medium-carbon Cr-Mo-V secondary hardening steel

为了探究中碳Cr-Mo-V二次硬化钢在不同奥氏体化温度及连续冷却条件下的组织及硬度变化规律,利用热膨胀仪、扫描电子显微镜(SEM)、维氏硬度计和电子背散射衍射(EBSD)等方法进行了研究,探讨了不同冷速试样的晶界密度和KAM值(kernal average misorientation)与硬度的关系,并绘制了试验钢的连续冷却转变曲线(CCT曲线)。结果表明,试验钢硬度随奥氏体化温度变化呈现为2个阶段,当奥氏体化温度为900~1000℃时,随温度升高,晶粒尺寸缓慢增加,硬度由636HV快速增加至786HV;当奥氏体化温度为1000~1080℃时,随温度升高,晶粒尺寸由13.3μm快速增加至53.4μm,硬度变化不大,最大为791HV。连续冷却相变结果表明,试验钢在0.1~5℃/s冷速范围内,相变产物为贝氏体/马氏体复相组织;冷速大于5℃/s时,相变产物全部为马氏体;随冷速增加,大角度晶界密度(>15°)从1.66μm^(-1)增加至1.91μm^(-1),KAM值从1.062增加至1.407,硬度从671HV增加至788HV。

In order to explore the changes of microstructure and hardness of medium carbon Cr-Mo-V secondary hardening steel under different austenitizing temperature and continuous cooling transformation,the thermal dilatometer,scanning electron microscope(SEM),Vickers hardness tester and electron backscattering diffraction(EBSD)were used to study.The relationship between grain boundary density,KAM value(kernal average misorientation)and hardness of samples with different cooling rates were analyzed,and the continuous cooling transformation curve(CCT curve)of the experimental steel was drawn.The results show that the hardness of the experimental steel shows two stages with the increase of austenitizing temperature.When the austenitizing temperature is 900-1000℃,the grain size increases slowly and the hardness increases rapidly from 636HV to 786HV with the increase of temperature.When the austenitizing temperature is 1000-1080℃,the grain size increases rapidly from 13.3μm to 53.4μm with the increase of temperature,and the hardness changes little,the maximum value is 791HV.The results of continuous cooling indicate that the phase transformation product is bainite/martensite complex structure under cooling rate range of o.1-5℃/s,and the phase transformation product is fully martensite when cooling rate increases upon 5 C/s.As the cooling rate increases,the grain boundary density(>15°)increases from 1.66μm^(-1) to 1.91μm^(-1),the KAM value increases from 1.062 to 1.407,and the hardness increases from 671HV to 788HV.