The effects of P and S on the toughness and heat-checking resistance of the 5%Cr hot work die steel.

This study was carried out to investigate the effects of phosphorus and sulfur contents on the Charpy impact toughness and heat-checking resistance of 5% Cr hot working die steel. Segregation behavior and morphology of fracture surface were observed by means of AES, SEM and optical microscopy.Results are summarized as follows:(1) Charpy impact toughness at 20°C and 400°C are greatly improved by lowering of phosphorus and sulfur contents to very low level.(2) The decrease in directionality of Charpy impact toughness accompanying the lowering of these impurities is due to the increase in Charpy impact toughness transverse to the forging direction.(3) The total number and mean length of heat checks decrease with decreasing content of these impurities.Such improvements of Charpy impact toughness and heat-checking properties are considered to result from the decrease in the segregation and MnS content respectively.