Abstract
The effect of the holding time of the deep cryogenic treatment on the strain hardening of HY-TUF, a low-alloy medium-carbon steel, and a comparison with strain hardening of conventional heat-treated steel were investigated. For this purpose, a deep cryogenic treatment was done for different holding times of 12, 24, 48, and 72 h. The metallographic results indicate completion of martensitic transformation; η-carbide precipitation and decrement of carbon concentration in martensitic matrix happened while carrying out a deep cryogenic process. Observations of transmission electron microscopy indicate higher density of dislocations within martensite near the cementite compared to that near the η-carbides. The tensile flow stress data for this steel was examined in terms of Hollomon equation. The results show that strain hardening of conventional heat-treated steel takes place in two stages. This evidence is related to the co-deformation of austenite and martensite. The strain hardening takes place in one stage for the deep cryogenic-treated steels, and their strain hardening exponents increase (from ~0.29 to 0.47) with an increasing holding time up to 48 h. A further increase in the holding time of the deep cryogenic treatment is found not to vary strain hardening exponent. The increase of strain hardening exponent and then observation of plateau in this parameter show an optimum value for the holding time of the deep cryogenic treatment (48 h).
Highlights
The effect of the holding time of the deep cryogenic treatment on the strain hardening of HY-TUF, a low-alloy medium-carbon steel, and a comparison with strain hardening of conventional heat-treated steel were investigated
The carbon clouds around the dislocations serve as nuclei for the η-carbide, which is consistent with the previous studies by Oila et al (2014) who observed precipitates of η-carbide in the vicinity of dislocations in the deep cryogenic-treated (DCT) martensite of a low-alloy medium-carbon steel
The main conclusions emerging from the relationship between the holding time of the deep cryogenic treatment and strain hardening parameters as well as microstructural changes investigated in this paper are the following: 1. Completion of martensitic transformation and η-carbides precipitation occur during cryogenic treatment
Summary
The effect of the holding time of the deep cryogenic treatment on the strain hardening of HY-TUF, a low-alloy medium-carbon steel, and a comparison with strain hardening of conventional heat-treated steel were investigated. Material hardening is generally registered by the true stress–strain relationship gotten through the conventional tension test. The four most commonly used empirical strain hardening laws to fit the tensile data in the area of uniform plastic deformation to empirical equations were suggested by Hollomon (1945), Voce (1948), and Swift (1952). Among these equations, the segment of true stress–strain curve from yielding to ultimate stress for multi-phase steels can truly be matched by the following experimental Hollomon equation: σ 1⁄4 K εn ð1Þ. It is mentioned that the UTS/YS and UTS-YS can be considered as criteria for strain hardening
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