图a、b和c分别为DZ（连铸）钢边部、1/2半径和心部三个位置的OM组织形貌，由图可知，三个位置上的组织均是由回火马氏体和贝氏体组成，且随着取样位置从边部到心部的马氏体含量逐渐减少。由于车轴直径较大，淬火时截面存在温度梯度场，从表面到心部冷却速度逐渐降低，因此心部贝氏体最高，1/2半径处次之，边部最少。

Figure A, B and C respectively show the OM microstructure morphology at the edge, 1/2 radius and center of DZ (continuous casting) steel. It can be seen from the figure that the microstructure at the three positions is composed of tempered martensite and bainite, and the martensite content gradually decreases from the edge to the center with the sampling position. Due to the large diameter of the axle, there is a temperature gradient field in the section during quenching, and the cooling speed gradually decreases from the surface to the center, so the bainite is the highest in the center, followed by the 1/2 radius, and the least at the edge.