冷却速度对5Ni钢组织和相变温度的影响

应用Formastor-FⅡ相变膨胀仪测定并研究了5Ni钢的静态CCT曲线，测定了相变临界点Ac1与Ac3，为制定热处理工艺提供依据.通过万能硬度计及OM，SEM和EPMA分析了冷却速度对5Ni钢硬度、显微组织及微观成分偏析的影响.结果表明:随着冷却速度的增大，试样显微组织由多边形铁素体和珠光体变成贝氏体和马氏体.从CCT曲线上可以看出，淬火热处理时为了避免铁素体和贝氏体相变，临界冷却速度要大于20℃/s；在不同冷速下，Ni元素都存在偏析，这有助于回火时产生逆转奥氏体，进而改善5Ni钢的韧性.

Effects of Cooling Rate on Microstructure and Phase Transformation Temperature of 5Ni Steel

Using the Formastor FⅡphase transformation thermal expansion instrument， the static CCT diagram and critical points Ac1 and Ac3 of 5Ni steels were investigated in order to provide the basis for the process plan of the heat treatment. Microstructures， Vickers microhardness and composition segregation were investigated by means of durometer， optical microscopy(OM)， scanning electron microscope(SEM) and electron probe micro analyzer(EPMA). It was found that with the increase of the cooling rate，microstructures of the specimens transformed from polygonal ferrite and pearlite to bainite and martensite. The CCT diagram revealed that the critical cooling rate need exceed 20 ℃/s to avoid ferrite and bainite transformation in quenching treatment. The composition segregations of Ni were found under different cooling rates that were beneficial to the formation of retained austenite during the tempering，which improved the toughness of 5Ni steels.