Influence of sub-zero treatment and tempering on the microstructure of Cr-V ledeburitic steel Vanadis 6 has been examined. The samples were austenitized at the temperature of 1050 °C in a vacuum furnace, held at the final temperature for 30 min. and nitrogen gas quenched (5 bar). In SZT, conventionally heat treated specimens were cooled down immediately after quenching from the room temperature to the temperature of liquid helium (-269 °C), kept there for 17 h and then re-heated to the room temperature. Double tempering was performed at the temperatures from the range 170 – 530 °C, whereas each tempering cycle was realized with a hold of 2 h. Typical heat treated microstructure of ledeburitic steels consists, besides of the martensitic matrix with certain amount of retained austenite, of several types of carbides – eutectic, secondary and small globular carbides. In sub-zero treated steel the amount of retained austenite is significantly reduced. The population density of small globular carbides increases as a result of sub-zero treating as - possibly due to high internal stresses in martensite and material efforts of their relaxation. Tempering of the material resulted in decrease in population density of small globular carbides with increasing the tempering temperature. The hardness of sub-zero treated material is higher than that of conventionally quenched one. Also, this tendency is preserved when the steel is low-temperature tempered. On the other hand, the hardness of conventionally quenched steel becomes higher than that of SZT one when tempered at the temperature of secondary hardening.Keywords: Ledeburitic steel, sub-zero treatment, martensite, retained austenite, carbides
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