Effect of Solidification Cooling Rates and Subsequent Homogenization Treatment on Mn–Cr–Mo Element Segregation in Oil Casing Steels

An experimental investigation has been conducted with respect to the element segregation and microstructure evolution of a Mn–Cr–Mo alloyed oil casing steel from different solidification cooling rates (0.5 °C/s, 3 °C/s and 10 °C/s) and subsequent defined homogenization treatment using the confocal scanning laser microscope (CSLM). A quantitative relation between the secondary dendrite arm spacing and cooling rates is achieved through regression analysis. The experimental results shown that the micro-segregation area of Mn, Cr and Mo elements decreased significantly, but its concentration increased in the finer inter-dendritic region with the increase of solidification cooling rate. The distribution of the experimental elements determined by the grid point measurements (obtained with EPMA-WDS) processed by the weighted interval rank sort method is consistent with the modified V–B segregation model considering the proper partition coefficient by the mass balance analysis method. Moreover, the residual element segregation index apparently decreased and the element micro-segregation was effectively eliminated during the subsequent homogenization treatment especially for the sample prepared by a higher cooling rate in the as-cast period.