We investigated the effects of the austenitizing temperature on the microstructure, hardness, and tensile properties of case-carburized steel after vacuum carburization at 930 °C and then re-austenitization at 820–900 °C followed by oil quenching and tempering. The results show that fractures occurred early with the increase in the austenitizing temperature, although all the carburized specimens showed a similar case hardness of 800 HV_0.2 and case depth of 1.2 mm. The highest fracture stress of 1919 MPa was obtained for the experimental steel when the austenitizing temperature was 840 °C due to its fine microstructure and relatively high percentage of retained austenite transformed into martensite during the tensile tests. We also found that the stress–strain behavior of case-carburized specimens could be described by the area-weighted curves of the carburized case and the core in combination. The strain hardening exponent was about 0.4 and did not vary with the increase in the austenitizing temperature. We concluded that the optimum austenitizing temperature was around 840 °C for the experimental steel.

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.

In order to control microstructure and performances of steel products, the investigation has been taken to the ways to refine grains in steels and the mechanisms, the availability in industrial production of ultrafine grain steel products, and the applications of ultrafine grain steel products. It is shown that strength of steels can be increased without detriment to ductility when ferrite grains are refined to less than 4 μm in low carbon steels. Based on the study of deformation induced transformation, a new rolling technology has been developed to refine ferrite grains in plain low carbon steel rebar and strip. Plain low carbon steel rebars of 400 MPa grade, with ferrite grains in the range of 4–8 μm, have been developed as the candidate to replace microalloyed steel rebars. High strength plain low carbon steel strips in the yield strength range from 345 to 420 MPa have been used for components conventionally made of microalloyed steels. Ultrafine grained microalloyed steel strip has also been developed. A new on-line cementite spheroidization softening technology has been studied for the wire rod production of medium carbon bolt steel. It is obvious that deformation could also induce pearlite transformation in eutectoid steel.

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.

As research of the high cycle fatigue of carburized gear steel could not meet the status quo of longer and longer service lives, research of very high cycle fatigue (VHCF) performance has become the focus of current research. The VHCF properties of case-hardening steel 18CrNiMo7-6 after being carburized with gradient hardness distribution were investigated by means of ultrasonic fatigue tests. The results showed that the carburized specimens with a case hardness of 705 HV and core hardness of 530 HV showed VHCF phenomenon, and the fatigue lives continuously increased to even 10⁹ cycles as the stress amplitude decreased to about 500 MPa. Observations of the fracture surfaces of the fatigue specimens showed that the fatigue crack initiation sites were located in the transition area with the hardness at about 580 HV. It was found that the transition area had low VHCF properties, since the core did not show VHCF phenomenon, and the case had a higher hardness. A fine microstructure was observed in the granular bright facet (GBF) area, and the stress intensity factor $\Delta$K_GBF was measured to be 3.04 MPam^−1/2. The 10⁹ cycles fatigue life was predicted based on the inclusion size, and the 10¹⁰ cycles fatigue life was 490 MPa based on the prediction model.