退火对齿轮钢枝晶偏析及元素扩散的影响

Effect of annealing on dendritic segregation and elemental diffusion of gear steel billet

为了研究退火工艺对20CrMnTi齿轮钢铸坯的枝晶形貌、合金元素(碳,硅,铬和锰)分布及其扩散的影响,利用光学显微镜(OM)对退火前后铸坯枝晶形貌进形了比较,利用电子探针(EPMA)对元素的分布进行了定量分析,并使用Thermo-Calc软件中的Dictra模块进行了模拟验证。研究结果表明,退火工艺使20CrMnTi齿轮钢铸坯的枝晶形貌显示得更加完整清晰,测量退火后铸坯中的二次枝晶间距为55μm。铸坯中碳元素存在上坡扩散,偏析系数变大,偏析系数变化量?kC为0.666;硅元素存在轻微的上坡扩散,偏析系数变化量?kSi为0.041 5;锰元素扩散更加均匀,偏析系数变化量为?kMn为-0.141;铬元素扩散得更加均匀,偏析系数变化量为?kCr为-0.106。模拟结果显示,铸坯在保温温度为680℃,保温时间为2 h的退火工艺条件下,碳原子发生了上坡扩散,产生偏析现象;硅、铬、锰原子无明显扩散变化。

In order to study the effects of annealing process on the dendritic morphology of 20CrMnTi gear steel slab, the distribution of alloying elements(C,Si,Cr and Mn)and its diffusion,the optical microscope(OM)was used to shape the dendrites before and after annealing. The appearance was compared and the distribution of the elements was quantitatively analyzed using an electron probe(EPMA),and the simulation was performed using the Dictra module in the Thermo-Calc software. The results showed that the dendritic morphology of the 20CrMnTi gear steel slab was more complete and clear,and the secondary dendrite spacing in the slab after annealing was measured to be 55 μm. The C element in the slab had an upward slope,the segregation coefficient became larger,the variation of the segregation coefficient was ΔkC=0.666. The Si element had a slight uphill diffusion,the variation of the segregation coefficient was ΔkSi= 0.041 5. The Mn element diffusion was more uniform,and the variation coefficient of the segregation coefficient was ΔkMn=-0.141. The Cr element also diffused more uniformly and the variation coefficient of the segregation coefficient was ΔkCr=-0.106. The simulation results showed that under the annealing condition of slab temperature of 680 ℃ and holding time of 2 h,C atoms were up-slope and diffuse,resulting in segregation. Si,Cr and Mn atoms had no obvious diffusion changes.