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.

The effects of cryogenic treatment and low temperature tempering on the microstructure, mechanical properties and distortion of the 20Cr2Ni4A and 17Cr2Ni2MoVNb carburized gear steels were investigated. The results showed that the case hardness of the experimental steels was increased after the cryogenic treatment, due to the decrease of the retained austenite content and the precipitation of the tiny carbides. The wear resistance of the two steels after cryogenic treatment was improved, although the wear mechanisms were different for 17Cr2Ni2MoVNb and 20Cr2Ni4A steels. The distortion of the Navy C-ring specimens underwent shrinkage before expansion during the cryogenic process, and the distortion of 17Cr2Ni2MoVNb steel was smaller than that of 20Cr2Ni4A steel.

This paper focuses on the effect of gear steel on distortion due to phase transformation in carburizing and quenching. The carburizing and quenching process of C-rings under suspension was studied by using the finite element method based on the thermo-mechanical theory, considering phase transformation. The phase transformation kinetics parameters, depending on different carbon contents, were measured by Gleeble-3500. The distortion behavior of the carburized C-ring during the cooling stage was analyzed, as well as the carbon concentration distribution and martensite volume fractions. The accuracy of the simulation was also verified by comparing the experimental data with the simulated result of the distortion and microstructure. A reliable basis is provided for predicting the distortion mechanism of gear steels in carburizing and quenching.

In this work, the solid solution product of [Nb][C] in the Nb-microalloyed steels with various carbon contents in the range of 0.20~1.80 wt.% was investigated by means of the extraction phase analysis method. The results showed that the Nb content in austenite tended to first decrease and then increase with the increase of carbon content in the steels. A unified solid solution product of [Nb][C] in austenite at different temperatures was obtained according to the results of the experimental steels. The Nb content in austenite of the experimental steels with high carbon contents was lower than that calculated by Ohtani’s equation. The existence of NbC precipitates in the case and the core of the specimens carburized at 930 °C and 980 °C were verified by transmission electron microscopy (TEM) observations. The pinning effect of NbC precipitates on austenite grain growth was calculated according to the size and amount of NbC precipitates in the carburized case and the core of the carburized specimens. The calculated results of prior austenite grain sizes were in good agreement with the experimental results, which indicated that the unified solid solution product of [Nb][C] in Nb-microalloyed steels with various carbon contents was applicable for the low-pressure carburizing process.

The hydrogen embrittlement susceptibility of a newly developed 1700 MPa-grade ultra-high-strength steel with a primary austenite grain size of 4 μm was studied and the mechanical properties and microstructure were characterized. The results show that the hydrogen content in the steel increases with the extension of charging time: the value reached 0.35 wppm with a charging time of 96 h. On the contrary, the fracture mode of the experimental steel remained ductile after hydrogen charging, and the elongation and the section shrinkage showed little difference, indicating an excellent resistance to hydrogen embrittlement, which could be ascribed to the refined microstructure and good cleanliness of the experimental steel.