Effect Of Cryogenic Treatment On Low Alloy Steels

Abstract

Cryogenic Treatment, as an additional step to hardening and tempering heat-treatments, has proven to be beneficial in improving mechanical properties of various steel grades. To transform retained austenite into martensite in structural steels, cryogenic treatment is conventionally performed immediately after quench-hardening and prior to the tempering treatment. Contrary to this well- established conventional practice, some industrial heat-treaters often carryout the cryogenic treatment as the last step after the tempering treatment without comprehensively knowing the consequence of the same on the properties and performance of steel components. While published literature on the cryogenic treatment performed after the tempering of steels is sparse, a direct comparison of the effects of conventional and unconventional cryogenic treatments on common structural steels is almost nonexistent. This work presents a comparative analysis of deep cryogenic treatment performed on five commercial steel grades, EN8, EN18, EN47, EN24 and EN45, and highlights the effect of cryogenic exposure carried out before and after the tempering step separately. Through detailed experimentation, testing and characterization involving, several heat treatment cycles, tensile test, impact test, hardness test, wear test, fractography, metallography, and microstructural analysis, this work suggests that the cryogenic treatment carried out as an intermediate step between quenching and tempering yields much better mechanical properties than that carried out after tempering. The results of the experimental work in this study shows that, the unconventional cryogenic treatment, that is carried out after the tempering step, has almost no influence on hardness and strength of EN8, EN18 and EN47 steels and very little influence on EN24 and EN45 steels. However, the conventional cryogenic treatment that is carried out prior to the tempering step shows significant improvements in the hardness (1-3 HRC), yield strength (4-17.4%) and tensile strength (3.9-16.8%) in the above steel grades in general. In particular, this work shows that the conventional cryogenic treatment is very effective in improving the overall mechanical properties of EN45 and EN24 grades including hardness (2-3 HRC), tensile strength (13.2- 16.8%), resilience (up to 24%), and wear resistance (36-42%). Thus, this thesis conclusively demonstrates and cautions commercial heat-treaters that the conventional cryogenic treatment that is applied as an intermediate step between hardening and tempering is much more effective than the unconventional industrial practice.