The Influence of Heat Treatment on the Microstructure and Machinability of a Prehardened Mold Steel

The machinability performance of a modified AISI P20 steel, heat treated to have the same hardness but three different microstructures, lower bainite, tempered martensite, and primary spheroidized carbides in a tempered martensite matrix, was studied. The microstructures were characterized using light optical and scanning electron microscopy and X-ray diffraction, and mechanical properties were compared by means of tensile and Charpy V-notch impact tests. The influence of microstructure and the resultant mechanical properties on machinability was studied in the context of single tooth end milling operation. The results showed that the material containing primary spheroidized carbides exhibited a superior machinability at the expense of a marginal loss of tensile strength and impact toughness, with comparable yield strength to that of the material containing tempered martensite. By contrast, the material with bainitic mi- crostructure showed the lowest yield strength and the poorest machinability performance while having the highest uniform elongation