Effect of Cryogenic Treatment on the Microstructure and Wear Resistance of 17Cr2Ni2MoVNb Carburizing Gear Steel

Abstract

Cryogenic treatment as a process that can effectively improve the performance of steel materials is widely used because of its simplicity and speed. This paper investigates the effects of different low temperature treatments on the microstructure and properties of 17Cr2Ni2MoVNb steel. The low temperature treatment range is divided into cryogenic treatment (CT-80), shallow cryogenic treatment (SCT-150) and deep cryogenic treatment (DCT-196), all with a duration of 1 h. The retained austenite content and the change in carbide volume fraction at 0.2 mm in the carburised layer are studied. The microhardness gradient of the carburised layer, as well as the friction coefficient and wear scar morphology at 0.2 mm, was investigated. The results show that the low temperature treatment is effective in reducing the retained austenite content and increasing the volume fraction of carbide. The lowest retained austenite content and highest carbide volume fraction were obtained for DCT-196 specimens at the same holding time. Due to the further transformation of martensite and the diffuse distribution of carbides, the microhardness and frictional wear properties of DCT-196 are optimal. Therefore, low temperature treatment can change the microstructure of the case layer of 17Cr2Ni2MoVNb steel and effectively improve the mechanical properties of materials.

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

“…In SNCM 415, a hardness increase of almost 200 HV was observed due to CT at −85 • C for 24 h [225]. For 17Cr2Ni2MoVNb, 21NiCrMo2, and 20Cr2Ni4A steels, there was a hardness increase of almost 40, 80, and 120 HV1, respectively, after CT at −80, −150, or −196 • C (each of them for 1 h) [229,230] or after CT at −120 • C for 2 h [234]. The hardness increase due to cryogenic treatments is a result of a 30 to 50% reduction in retained austenite as well as more extensive precipitation of fine carbides due to cryogenic treatment [229].…”

“…For 17Cr2Ni2MoVNb, 21NiCrMo2, and 20Cr2Ni4A steels, there was a hardness increase of almost 40, 80, and 120 HV1, respectively, after CT at −80, −150, or −196 • C (each of them for 1 h) [229,230] or after CT at −120 • C for 2 h [234]. The hardness increase due to cryogenic treatments is a result of a 30 to 50% reduction in retained austenite as well as more extensive precipitation of fine carbides due to cryogenic treatment [229]. For Ni-containing steels, the hardness increase showed clear cryo-temperature dependence for 20CrNi2MoV steel [180] and time dependence for 18NiCrMo5 steel [235].…”

“…Cryogenic treatment is also an effective way to reduce the retained austenite in nickel-containing carburising steels. Even though they contain more than 20 vol.% of RA after conventional quenching, the retained austenite can be reduced to one-half [47,225,227,229] or one-fourth [180] by applying sequence A. A typical example is the work by Yan et al [229], who reported retained austenite reduction from 18.15% (CHT) to 12.92, 10.73, and 9.45% for CT at −80, −150, and −196 • C. On the other hand, the effect of cryogenic treatment on retained austenite reduction is suppressed when steels are pre-tempered prior to CT (sequences E or F), which is due to the stabilisation of retained austenite [230,231].…”

“…The principal explanation for the CT-controlled hardness increase is that cryogenic treatment reduces the retained austenite amount. This reduction is greater for Ni-containing steels [226,229,230] than for steels without Ni [225]. Tempering prior to CT thermally stabilises retained austenite [126,324].…”

“…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.…”