Fracture toughness of ledeburitic Vanadis 6 steel after sub-zero treatment for 17 h and double tempering

The influence of sub-zero treatment on the fracture toughness of Cr-V ledeburitic steel Vanadis 6 was examined, in comparison with the same material processed without the sub-zero period.The microstructure of the material consists of the matrix and several types of carbides -eutectic carbides (ECs), secondary carbides (SCs) and small globular carbides (SGCs).Small amounts of retained austenite were also present in the microstructure, but only in the untempered or low-temperature tempered steel.Sub-zero treatment increases the amount of small globular carbides.On the other hand, tempering results in a decrease in the population density of these particles.The fracture toughness of conventionally heat-treated steel firstly increases with the tempering, but then it decreases rapidly when the steel is tempered at the temperature of secondary hardening.In the case of the sub-zero treated material, the fracture toughness is correspondingly lower when the material is tempered at low temperatures, but it becomes slightly higher in the temperature range normally used for secondary hardening.Generally, one can say that the fracture toughness follows well the values of the hardness of the material, except in the narrow temperature range in the case of the sub-zero treated steel, where a "window" for a simultaneous enhancement of hardness and toughness exists.Keywords: