Microstructure and hardness of sub-zero treated and no tempered P/M Vanadis 6 ledeburitic tool steel

• The γ to α′ transformation is more completed due to sub-zero treatment.
• The α′ formed on SZT is finer and contains more defects than that formed on CHT.
• Sub-zero treated Vanadis 6 contains higher portion of small globular carbides.
• The small globular carbides are of the nature of orthorombic cementite.
• As-quenched hardness of the Vanadis 6 steel is increased moderately due to SZT.

Microstructure, phase constitution and hardness of Cr–V ledeburitic steel Vanadis 6 subjected to sub-zero treatment with a soaking time of 4 h and at a temperature of −196 °C have been examined with reference to the same material after conventional heat treatment. Microstructures have been characterized using light microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Hardness has been evaluated by Vickers method. As-annealed material consists of the ferritic matrix and of two carbide phases, namely M7C3 and MC. After the heat treatment, the matrix is martensitic with certain amount of retained austenite. Sub-zero treatment results in more complete martensitic transformation. The martensite contains areas with extremely enhanced carbon content. Sub-zero treatment refines the size of martensitic domains and increases the dislocation density inside the domains. Microstructure of sub-zero treated steel contains enhanced amount of small globular carbides, as compared to conventionally heat treated material. These particles have a size of around 100 nm in most cases. Small globular carbides are of the cementitic nature for both the no sub-zero treated steels and the sub-zero treated material. Bulk hardness manifests a moderate increase due to the sub-zero treatment. The increase in hardness is more significant when higher austenitizing temperature is used for the treatment, which is associated with more pronounced reduction of the retained austenite content.