Effect of Subzero Treatment on Microstructure and Material Properties of EN24 Steel

Abstract

Cryogenic treatment of steels has been widely used for enhancing mechanical properties like hardness, toughness and stable metallurgical structure. Application such as gears, kicker rods, bolts are made of medium carbon alloy steels like EN-24 steel. In these applications, percentage of retained austenite has considerable effects on the life of the material. A comparative study on conventionally heat-treated (CHT) and shallow cryogenic treated (SCT) EN-24 steel was done to evaluate the effect of shallow cryogenic treatment (SCT) on hardness, toughness and the amount of retained austenite present in the structure of EN24 steel. The microscopic structure of cryogenic treated EN24 steel revealed the formation of carbides, both primary and secondary carbides. An estimated amount of 15% retained austenite after CHT tempered condition was less than 2% after SCT tempered condition. Tensile test fractography of subzero treated (SCT) specimen revealed ductile fracture. The maximum hardness observed in case of SCT tempered samples was 415BHN, 15% increase from CHT tempered samples. The maximum impact strength observed in case of SCT tempered samples was 240kJ/m2, 11% increase from CHT tempered samples. Further SCT tempered samples, tempered at 650°C resulted in ductility increase by 55% as compared to CHT tempered samples without sacrificing hardness. Index Terms: EN24 Steel, hardness, shallow cryogenic treatment, retained austenite, tensile test fractography.