Quantifying deep cryogenic treatment extent and its effect on steel properties

Authors: Funk, Paul; KANAAN, AHMED - New Mexico State University; SHANK, CHRISTINE - New Mexico State University; COOKE, PETER - New Mexico State University; SEVOSTIANOV, IGOR - New Mexico State University; Thomas, Joseph; PATE, MICHAEL - Down River Cryogenics, Llc

Submitted to: International Journal of Engineering Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2021
Publication Date: 7/1/2021
Citation: Funk, P.A., Kanaan, A., Shank, C., Cooke, P., Sevostianov, I., Thomas, J.W., Pate, M.O. 2021. Quantifying deep cryogenic treatment extent and its effect on steel properties. International Journal of Engineering Science. 167. Article 103521. https://doi.org/10.1016/j.ijengsci.2021.103521.
DOI: https://doi.org/10.1016/j.ijengsci.2021.103521

Interpretive Summary: Deep cryogenic treatment (DCT) is a thermal process where tools or parts are slowly lowered to the temperature of liquid nitrogen and held there for 1 to 3 days before slowly being brought back to room temperature. It has been used to extend the life of cutting tools for nearly a century, but, as researchers don’t agree on which changes brought about by DCT result in better performance, there is no measurable property to quantify the extent of DCT. Easily measured physical properties including yield strength, Young’s modulus, and hardness are not affected by DCT, and scanning electron micrographs only provide qualitative results. This paper reports a breakthrough. The electrical resistance of spring steel specimens with and without DCT were measured and found to correlate to changes in steel’s crystal structure (austenite becomes martensite during DCT). Resistivity is a quantitative measure of the extent of DCT.

Technical Abstract: Deep cryogenic treatment (DCT) has been known since the 1930s to improve hardness, fatigue resistance, and wear resistance of steel. While the effect of DCT on wear properties has been well documented, there is no consensus regarding the causal mechanisms, nor a widely accepted quantitative description of them. DCT transforms retained austenite into martensite and triggers the precipitation of fine carbides, among other things. We observed that DCT had a negligible effect on Young’s modulus and the yield limit of high carbon spring steel. The observed microstructural changes (presence of specific dendritic inhomogeneities typical for inclusions of austenitic phase in non-treated specimens and homogeneous microstructure of treated ones) can serve for qualitative purposes only. However, we found that DCT led to a decrease in steel electrical resistivity which can be explained by noticeable difference between the resistivities of the martensitic and austenitic phases. We propose a micromechanical model for electrical resistivity which allows monitoring of the content of retained austenite and postulate that it can be used for other materials as well. We also observed increased resistivity after mechanical loading of the specimens, correlating with increased dislocation density caused by loading. This quantity can be used to assess the average dislocation density.