Property Improvement of Subzero/Cryogenic Heat Treated Camshafts made of 8620H, 16MnCr5 and 100Cr6 Steels

Abstract

Several types of camshafts that made of 8620H, 16MnCr5 and 100Cr6 steel were subzero/cryogenic heat treated and the effect of these heat treatment parameters such holding duration and temperature on the microstructure, retained austenite volume ratio and hardness. 8620H and 16MnCr5 grade camshafts were carburized at 925 • C in endogas atmosphere (20% CO, 40% N2, 40% H2) for 1440 min. At the end of carburing process the diffusion temperature was decreased to 840 • C, which is the austenizing temperature, and held for 90 min. Next the samples were hardened in oil at 60 • C (held for 30 min). Samples were subzero treated at −100 • C for 210 minutes and were tempered at 185 • C for 120 minutes. The microstructure was revealed that the subzero/cryogenic heat treatment increased the hardness up to 62 HRc and increased the wear resistance of camshafts surface. The decrease in the retained austenite ratio was observed from 25% to 5-10% after cryogenic heat treatment. All of steels the hardness values were increased with the transformation of retained austenite into martensite. This increase is the result of the transformation of martensite from retained austenite and the carbide precipitation mechanism. In this study, the wear strength values of the hardened camshaft have been brought from the level of 2.2851 mg to 0.5239 mg and SEM analyzes were performed.

Cryogenic Treatment of Martensitic Steels: Microstructural Fundamentals and Implications for Mechanical Properties and Wear and Corrosion Performance

“…Various research groups have also reported extensive RA transformation during cryogenic treatment for carburised steels (see the overview of investigated steels in Table 1). Furthermore, these results are mostly consistent with the abovementioned general tendency; the use of −196 • C (or a lower temperature, combined with sequence A) is much more effective than the use of CT temperatures in the range from −70 to −120 • C [33,180,[225][226][227][228].…”

“…For steel subjected to CT at −196 °C for 24 h, this reduction was slightly higher (60%) [246]. In other work [228], Karaca et al reported that an application of −100 °C cryogenic treatment for 3.5 h improved wear performance by only 10%, but the treatment at −120 °C gave a 40% improvement in wear rate [234]. A CT duration of 24-36 h (at −185 or −196 °C) was recommended as optimal to achieve the best wear performance of bearing steels [24,151,244].…”

“…However, the treatments at higher temperatures can also bring undisputable benefits with respect to the hardness values obtained. Treatment at −100 °C for 210 min (following sequence A), for instance, resulted in a 60 HV hardness increase for the given steel grade [228]. Cryogenic treatment at −120 °C for 2 h increased the hardness by 60 to 70 HV [234].…”

“…The extent of the improvement depends on both the CT temperature and the duration. The maximum wear resistance for En353, 20CrNi2MoV, 16MnCr5, 20MnCr5, and 17Cr2Ni2MoVNb steels was obtained by treatments at −196 °C (or close to this temperature) [18,33,46,180,229,239], even though the use of higher temperatures (e.g., between −80 and −103 °C) can also significantly improve this property [181,225,228,239]. Two diagrams in Figure 29 clearly delineate an enhanced wear performance of cryogenically treated 20CrNi2MoV carburised steel as compared with the same steel without applying cryogenic treatment.…”