Industrial scale extrusion performance of cryogenically processed DIN 100 Cr6 and DIN 21NiCrMo2 steels

Abstract

The effects of diferent heat and cryogenic (sub-zero) treatment parameters such as temperature and holding time on the microstructure (amount of retained austenite) and hardness of extrusion molds produced from the 21NiCrMo2 and 100Cr6 steels were investigated. The 21NiCrMo2 grade extrusion die was carburized for 22.5 h in an endogas (25% CO, 35% N-2 40% H2) atmosphere at 920 degrees C. At the end of the carburization process, the temperature was kept at 850 degrees C, which is the austenitization temperature, for 2 h, followed by cooling in oil at 80 degrees C and remaining in oil for 45 minutes. The carburizing process was not performed for the extrusion molds made of 100Cr6 steel grade. Only the austenitizing heat treatment at 850 degrees C (holding for 2 h) was carried out in this steel. The steel molds which were produced with 21NiCrMo2 and 100Cr6 steels were cryogenically treated at -120 degrees C for 2 h and subsequently tempered at 150 degrees C for 1.5 h. As a result of the cryogenic treatment, the hardness of 21NiCrMo2 steel increased to 840 Hv and the wear resistance of the extrusion die surface was improved. The amount of residual austenite decreased from 20% to 6% after the cryogenic treatment. Due to the effect of the cryogenic process, the surface hardness of the 100Cr6 steel sample increased to similar to 870 Hv, which implies an increase of 4.5%, due to the transformation of residual austenite to martensite. The mass loss, during the wear tests, of the hardened extrusion dies was reduced from 0.1420 mg to 0.0221 mg. The notch impact strength value measured in this condition was 20 J. The 100Cr6 steel after the cryogenic treatment was used to extrude 12 tons of Al alloy in an industrial press. This amount of material is 30% lower than for hot work tool steel. On the other hand, the 100Cr6 steel is more economical and heat treatment is more practical. The extrusion performance of 21NiCrMo2 steel was 50% lower than the hot work tool steel.