Investigation of the Microstructural Changes and Hardness Variations of Sub-Zero Treated Cr-V Ledeburitic Tool Steel Due to the Tempering Treatment

The microstructure and tempering response of Cr-V ledeburitic steel Vanadis 6 subjected to sub-zero treatment at ??196 °C for 4 h have been examined with reference to the same steel after conventional heat treatment. The obtained experimental results infer that sub-zero treatment significantly reduces the retained austenite amount, makes an overall refinement of microstructure, and induces a significant increase in the number and population density of small globular carbides with a size 100-500 nm. At low tempering temperatures, the transient M₃C-carbides precipitated, whereas their number was enhanced by sub-zero treatment. The presence of chromium-based M₇C₃ precipitates was evidenced after tempering at the temperature of normal secondary hardening; this phase was detected along with the M₃C. Tempering above 470 °C converts almost all the retained austenite in conventionally quenched specimens while the transformation of retained austenite is rather accelerated in sub-zero treated material. As a result of tempering, a decrease in the population density of small globular carbides was recorded; however, the number of these particles retained much higher in sub-zero treated steel. Elevated hardness of sub-zero treated steel can be referred to more completed martensitic transformation and enhanced number of small globular carbides; this state is retained up to a tempering temperature of around 500 °C in certain extent. Correspondingly, lower as-tempered hardness of sub-zero treated steel tempered above 500 °C is referred to much lower contribution of the transformation of retained austenite, and to an expectedly lower amount of precipitated alloy carbides.