高温渗碳齿轮钢铝氮含量对奥氏体尺寸的影响

轨道交通用高端齿轮钢往往要求长时间高温渗碳处理以提高其表面硬度与耐磨性，利用合适的铝、氮含量实现AlN粒子对奥氏体晶界的有效钉扎对保证齿轮的晶粒度、力学性能与尺寸精度至关重要。在通常的渗碳温度下，AlN已经发生了部分固溶，为了保证高温渗碳后奥氏体晶粒细小，齿轮钢中的酸溶铝质量分数一般需要保持在0.02%～0.055%以保证析出足量细小的AlN第二相粒子来钉扎晶界，且氮质量分数要求为0.01%～0.016%。这一元素含量范围较广，因此有必要研究钢在高温渗碳时所需要的恰当铝氮积与铝氮比，也就是钢中w(Al)与w(N)的乘积和比值的取值范围，还需要研究AlN粒子对于奥氏体的钉扎作用。针对不同含铝含氮轨道交通用齿轮钢进行了伪渗碳试验与AlN第二相粒子Ostwald熟化和Gladman钉扎模型计算研究，揭示了奥氏体晶粒不均匀性因子Z与加热温度T的定量关系式。研究了含铝含氮齿轮钢高温保温过程奥氏体晶粒半径R_A的变化规律，以及不同铝氮积和铝氮比对奥氏体晶粒生长的影响。结果表明，加热温度T在1 173～1 273 K范围内，此类微合金高强钢的奥氏体晶粒长大不均匀性因子服从线性规律...

Effect of Al and N contents on austenite grain size for high-temperature carburized gear steels

As high-end gear steels for rail transit need a long-time and high-temperature carburizing heat-treatment to improve their surface hardness and wear resistance,the key to ensure a fine microstructure and geometrical accuracy for the steels is to select appropriate Al and N contents to pin austenite grain boundary with their precipitated AlN particles. AlN particles have undergone partial solution at the usual carburizing temperature. In order to ensure a fine austenite grain size after high temperature carburizing,the content of acid-soluble aluminum in gear steel generally needs to be kept between 0.02% and 0.055%,and the content of N is required to be 0.01%-0.016% to ensure the precipitation of sufficient fine AlN second phase particles to pin grain boundaries. As the ranges of Al and N content are wide,the study of appropriate Al and N contents,the ratio of Al and N mass percents and the pinning effect of AlN particles on austenite during high temperature carburizing of steel is necessary. Pseudo carburizing experiments of the steels with various Al and N contents were conducted,and the ripening model of the second phase particle and its Gladman pinning model were studied. As a result,a quantitative expression for the non-uniformity factor Z and temperature T was revealed. Furthermore,the growth trends of austenite grain radius R_A during high temperature carburizing process were observed,and the effect of different products and ratios of Al and N on austenite grain-coarsening behavior were analyzed accordingly. The results show that for carburizing temperature range of 1 173-1 273 K,the Z factor of austenite grains obeys a linear law of Z=3.742 97-0.001 76T. With a given holding time t,lnR_A has a quadratic polynomial relationship with 1/T,but lnR_A is linearly related to lnt while under a constant heating temperature T,and the time exponent of austenite grain growth is 0.33. When the product of w(Al) and w(N) exceeds 4.77×10-4,namely the austenite-grain coarsening temperature exceeds 1 263 K,the austenite grains can outnumber grade 7 even after 6 h holding at T=T_C-10 K provided the ratios of w(Al) and w(N) of the steels are between 1.5-3.8. With a given product of w(Al) and w(N),the size of austenite grains after the same heating history shows a linear positive correlation with its ratio of w(Al) and w(N),and the size difference of the austenite grains is within grade 1 for steels with Al-N ratios ranged from 1.5 to 3.8.