用钢箔渗碳法测定β系数的问题和建议

Problem and Suggestion in Relation to Determining β Coefficient by Steel Foil Carbonizing Method

用数值计算法模拟了钢箔的渗碳过程。根据模拟试验结果,按照钢箔渗碳试验的常用公式计算出工件表面传质系数β。结果表明,钢箔厚度、渗碳时间和钢箔的碳扩散系数对按常用公式计算的β系数值有明显的影响,该计算值总是小于模拟试验前的β系数设定值(即真值),而且随着钢箔中碳扩散系数的减小而减小。常用公式仅适用于钢箔的碳扩散系数为无限大的条件,否则所计算的β系数值是不正确的,并显示出受其他试验因素影响的虚假信息。常用公式的不当引用是造成长期来β系数的计算值不准确、可比性差的主要原因。根据钢箔渗碳方程的解析解,提出了新的计算β系数的方法,采用新方法可以求得接近真值的β系数,而且还能求得试验材料的碳扩散系数。

Carburizing process of steel foil was simulated by using numerical method.According to the simulation test result,mass transfer coefficient at workpiece surface,β,was calculated with a commonly used formula for steel foil carbonizing test.The results indicate that foil thickness,carburizing time and carbon diffusion coefficient in the foil will exert an appreciable influence on the value ofβcoefficient calculated by the commonly used formula,and that the calculated βvalue is always less than the set βvalue （i.e.,truth value）before the simulation test and reduces with the decrease in carbon diffusion coefficient in the foil.The commonly used formula is suitable only when carbon diffusion coefficient in the foil is unlimited,otherwise the calculated βvalue is not correct,and will give out false information that other test factors exert an influence on the calculated βvalue.Over a long period of time,the calculated βcoefficient values are inaccurate and difficult to be compared each other,which arise mainly from unsuitable quote of the commonly used formula.Based on analytical solution of carburizing equation of the steel foil,a new method of calculating the βcoefficient,by which the βcoefficient near truth value and the carbon diffusion coefficient in the tested material may be obtained,has been given.