Microstructure – Fracture toughness relationship in a sub-zero treated 0.9C-7.8Cr sub-ledeburitic tool steel

The Sleipner steel (0.9C-7.8Cr sub-ledeburitic tool steel) is a widely utilized tool steel currently being adopted to produce tools used in fine blanking , shearing, forming, coining, deep drawing, and others. In these branches, tailoring the final mechanical properties, such as hardness and toughness, to specific application is highly appreciated. The Sleipner steel was subjected to sub-zero treatments (at –140 °C for 17 h and 36 h) in the current work. The resulting microstructures, hardness variations, and changes in fracture toughness were analyzed and discussed. It was observed that sub-zero treatments reduced the retained austenite amounts by 14–15 % and slightly refined the martensite. However, the impact of this treatment on carbide count was marginal. The hardness of the sub-zero treated steel increased when tempered at temperatures up to 400 °C, but it decreased after tempering at 520 °C compared to cryogenically treated specimens. Sub-zero treatment reduced the fracture toughness in the steel tempered up to a temperature of 400 °C, but an increment in this property was found after 520 °C tempering. Nevertheless, the obtained results indicate that it is impossible to simultaneously enhance both the hardness and fracture toughness of this particular steel grade. Therefore, it is necessary to carefully choose the principal goal of the treatment (either hardness or toughness) even before subjecting the tools to the heat/sub-zero treatment.