Effects of deep cryogenic treatment on static mechanical properties of 18NiCrMo5 carburized steel

In consideration of the raising interest about the application of the deep cryogenic treatment (DCT) on materials, its effects on the static mechanical properties of a commercial gear carburized steel (18NiCrMo5) are investigated through hardness and tensile tests followed by optical fractographic observations. The present study focuses on the comparison between the results given by different sequences of DCT and tempering performed after the conventional case hardening. In addition, the soaking time parameter influence is analyzed for the case of the pre-tempering DCT. A special attention is paid to the statistical analysis of the experimental data in order to avoid misinterpretations. The results point out substantial hardness increases (from +0.6 HRC to +2.4 HRC) for all the cryotreated groups and a remarkable enhancement of the tensile strength (+11%) in one case, indicating that different sequences and DCT parameters must be considered depending on the application requirements. The experimental results are then discussed in consideration of the literature about the DCT on similar steels with the aim of correlating the obtained improvements to the microstructural mechanisms that are commonly ascribed to this treatment.