一、绪言在生产实践中,渗碳硼钢(20MnVB)原材料常出现粗晶和混晶现象,致使齿轮渗碳淬火后,心部组织粗大(附图1)。我们知道当钢加热到临界温度之上时,开始形成奥氏体晶粒。随着温度升高,时间延长,由于奥氏体晶粒的形成,长大和并吞,在形成之后存在一定的混晶,直到最后为粗晶,使不同材料有其不同的粗化温度(图1)。通常人们把930℃作 I. INTRODUCTION In the production practice, coarse carburizing and mixed crystal often appear in the carburized boron steel (20MnVB) raw material, causing the heart tissue to be coarse after the carburizing and quenching of gears (Figure 1). We know that as the steel heats above the critical temperature, austenitic grains begin to form. As the temperature increases and the time increases, there is a certain amount of mixed crystals after the formation due to the formation, growth and misappropriation of austenite grains until the final coarse grain makes the different materials have different roughening temperatures (Fig. 1 ). People usually put 930 ℃