纳米贝氏体/马氏体钢的热稳定性

将低温贝氏体相变前淬火得到由马氏体、贝氏体铁素体和残余奥氏体组成的纳米贝氏体钢,使用扫描电镜(SEM)、X射线衍射(XRD)和透射电镜(TEM)等手段观察在不同温度回火的纳米贝氏体钢的显微组织和硬度变化,研究了预相变马氏体对纳米贝氏体钢热稳定性的影响。结果表明:含有马氏体的纳米贝氏体钢在中低温(473~773 K)回火后其硬度比回火前的高,回火温度高于823 K其硬度迅速下降到266.2HV(923 K)。预形成的马氏体在473~573 K回火后向附近的残余奥氏体排碳,后者的碳含量提高到峰值1.52%,提高了残余奥氏体的热稳定性,延迟后者在高温时的分解,从而提高了纳米贝氏体钢的高温热稳定性;回火温度高于723 K则残余奥氏体分解成碳化物,贝氏体铁素体粗化、回复形成新的铁素体晶粒。

Thermal Stability of Nanoscale Bainite/Martensite Steel

The effect of the prior martensite on the thermal stability of the nano-bainitic steel was investigated. The nano-bainitic steel composed of prior martensite, nano-sized bainitic ferrite and retained austenite was obtained by quenching and followed by bainite transformation at low temperature. The microstructure and hardness variation were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and hardness tester etc. for the nano-bainite steels after tempering at different temperatures. Results show that after tempering at 473~773 K the hardness of the nano-bainite steel containing prior martensite is higher than that of the untampered ones. However, after tempering at temperatures above 823 K, its hardness decreased rapidly and which down to 266.2HV at 923 K. The carbon was discharged from prior martensite to the retained austenite when the steel was tempered at 473~573 K. The carbon content of the later increased to a peak value, i.e. 1.52%, which improved the thermal stability of retained austenite, and further delayed the decomposition of the later, thus improved the thermal stability of the nano-bainitic steel at high temperature. The retained austenite decomposed into carbides, and the bainitic ferrite coarsened and recovered, formed new ferrite grain when the tempering temperature exceeded 723 K.