No 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.

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.

The microstructure, interfacial characteristics, and corrosion resistance of Fe-W-Mn-Al-B alloys in molten zinc at 520 °C have been investigated using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and electron probe micro-analysis (EPMA). The experimental results indicate that the Fe-B alloy with 11 wt.% W, 7 wt.% Mn, and 4 wt.% Al addition displays a lamellar eutectic microstructure and excellent corrosion resistance to molten zinc. The toughness of M2B-type borides in the hyper-eutectic Fe-4.2B-11W-7Mn-4Al alloy can be more than doubled, reaching 10.5 MPa·m1/2, by adding Mn and Al. The corrosion layer of the Fe-3.5B-11W-7Mn-4Al alloy immersed in molten zinc at 520 °C comprises Fe3AlZnx, δ-FeZn10, ζ-FeZn13, and η-Zn. The lamellar borides provide the mechanical protection for α-(Fe, Mn, Al), and the thermal stability of borides improves as the fracture toughness of the borides increases, which jointly contribute to the improvement of the corrosion resistance to the molten zinc.

After T5 (forging + aging) and different T6 (forging + solution + aging) heat treatments, the AZ80A Mg alloys exhibited microstructures with different fractions of continuous precipitate (CP) regions and discontinuous precipitate (DP) regions. The effects of the fractions of DP regions and CP regions on the tensile properties and fracture behaviors were investigated using microstructural characterizations and analysis. The results showed that increasing the fraction of DP regions enhanced the yield strength and tensile strength at room temperature. However, at the same high temperature, increasing the fractions of DP regions improved the elongation but deteriorated the tensile strength significantly. The different resultant tensile properties at different temperatures were caused by the different precipitation-strengthening effects in the CP and DP regions. The strengthening contribution of the DP regions was more effective at room temperature but became inferior to the effect brought about by the CP regions at high temperatures. Micro-cracks were usually initiated and propagated in the CP regions at room temperature. At high temperatures, however, micro-voids formed more easily in the DP regions, and the fracture path preferred to locate there.