The paper presents properties of HS6-5-2 high speed steel subjected to deep cryogenic treatment (DCT) and subsequent tempering at different temperatures. DCT process of HS6-5-2 steel leads to shifting of maximum hardness peak to the lower temperature and the reduction of the obtained maximum hardness by about 1 HRC. These changes in hardness may be due to the shifting of the stage of nucleation and growth of carbide phases to lower temperatures or the changes taking place in the matrix, connected with the additional transformation of the martensite proceeding during the isothermal martensitic transformation occurring at cryogenic temperatures and more extensively occurring precipitation processes, lowering the content of the carbon in the martensite, determining thereby its lower hardness.

Aluminum alloys are widely used today in plastic injection molds in the automotive and aerospace industries due to their high strength and weight ratio, good corrosion and fatigue resistance as well as high feed rates. The 5754 aluminum alloy has high corrosion resistance and a structure suitable for cold forming. In this study, an AA 5754-H111 tempered aluminum alloy with the dimensions of 80×80×30 mm was used, and some of the materials were cryogenically heat treated. For the milling operations, ϕ12 mm diameter 76 mm height uncoated as well as TiCN and TiAlN coated end mills were used. Different levels of cutting depth (1.25, 2.0, 2.5 mm), cutting speed (50, 80, 100 m/ min), feed rate (265, 425, 530 m/ min) and machining pattern (concentric, back and forth and inward helical) were used. The number of experiments was reduced from 486 to 54 using the Taguchi L54 orthogonal array. The values obtained at the end of the experiments were evaluated using the signal-to-noise ratio, ANOVA, three-dimensional graphs and the regression method. Based on the result of the verification experiments, the processing accuracy for surface roughness was improved from 3.20 μm to 0.90 μm, with performance increase of 71.88%.

In this work, thermo-mechanically treated 42CrMo steel was subjected to cryogenic treatment conducted by means of orthogonal design method, followed by low-temperature tempering to investigate the effect of different parameters of cryogenic treatment on wear resistance of 42CrMo steel and to optimize parameters of cryogenic treatment for improving wear resistance. The results of hardness test and wear test show that cryogenic treatment significantly improves wear resistance with marginal changes in coefficient of friction and hardness. Specifically, cryogenic temperature has the largest impact on wear resistance of 42CrMo steel, holding time has medium impact, and the parameter of treatment cycles has the least impact. The optimum parameters of cryogenic treatment are −196°C for 12 hours with one cycle for improving wear resistance. The results of scanning electron microscopy (SEM) and X-ray diffractometry (XRD) analysis indicate that marginal changes in hardness and coefficient of friction may be owing to little amount of transformation of retained austenite, and the significant influence of cryogenic treatment on improving wear resistance of 42CrMo steel can be mainly attributed to segregation of carbon atoms promoted by cryogenic treatment resulting in more precipitation of carbides in subsequent tempering.

The machining residual stress produced in the cutting process of aluminum alloy parts can easily lead to a scrap of the processed parts. In order to reduce the residual stress of aluminum alloy in the milling process, based on the Taguchi-Grey relational approach, the effects of different milling parameters on the residual stress and surface roughness of 2A12 aluminum alloy were studied. To reduce the residual stress and surface roughness of 2A12 aluminum alloy, optimized milling parameters were obtained. To further reduce the milling residual stress of 2A12 aluminum alloy, the samples processed by the optimized milling parameters were treated by cryogenic treatment and artificial aging. The residual stress of the sample was measured by the blind hole drilling method, and the evolution mechanism of the microstructure to reduce the machining residual stress was revealed. The results show that the combination of deep cooling treatment and oil bath aging can effectively reduce the residual stress on the machined surface of the aluminum alloy and facilitate a more uniform distribution of the residual stress inside the specimen. The effect of the coarse second phase on the residual stress in the microstructure is not significant, and the fine and diffusely distributed precipitation phase is beneficial to the reduction of the residual stress in the aluminum alloy.

Deep cryogenic treatment (DCT) is gaining popularity as a treatment used to modify structures obtained during heat or thermo-chemical treatment. The article presents the influence of DCT, carried out during heat treatment before and after gas nitriding processes, on the formation of gas nitrided layers on X153CrMoV12 steel. It was found that the use of DCT between quenching and tempering performed prior to gas nitriding processes, increases the hardness, thickness and wear resistance of the nitrided layers. At the same time, if we apply cryogenic treatment during post-heat treatment of nitrided layers, we also get very high wear resistance and increased thickness of nitrided layers, in comparison with conventional gas nitriding of X153CrMoV12 steel. In this case, DCT significantly increases also the hardness of the core by the transformation of retained austenite and the precipitation of fine carbides of alloying elements.