To investigate the relationship between inclusions and bending fatigue behaviors in 20Cr2Ni4 steel under different stress concentrations. This paper designs a new experimental method to prefabricate different size stress concentrations near the inclusions, and then conducts a new type of bending fatigue test to study the inclusions and their surrounding stress distributions in 20Cr2Ni4 steel. A microhardness tester was combined with laser etching equipment to realize the prefabrication of different stress concentrations at arbitrary positions around any inclusion on the gear steel surface. This method provides an experimental basis for the quantitative analysis of the relationship between stress distribution and fatigue life around the inclusions of heavy-duty gear steels. We also predict the bending fatigue lives of heavy-duty gear steels with different types of inclusions, stress states, and spatial distributions. Then, based on the prefabricated notch parameters and the state of inclusions in the steel, a mathematical model of quantitative analysis is proposed, which can accurately predict the fatigue limit of heavy-duty gear steel. The research results can be applied to the actual use of heavy-duty gears and to the accurate life estimation based on the state of gear stress, thereby providing a quantitative reference model for subsequent gear steel production and gear part processing.

To study the effect of the surface properties on the bending fatigue performance of heavy-duty gear steel, the authors of this paper used the ultrasonic surface rolling process (USRP) to strengthen 20Cr2Ni4A carburized gear steel. USRP is a novel technique in which the ultrasonic technology is incorporated into the concept of conventional deep rolling. In this study, we illustrated how the surface properties and cross-section mechanical property influence the three-point bending fatigue life of the samples before and after USRP treatment. At the same time, the predicted failure probability-stress-number of cycles (P-S-N) curve was drawn, and the fatigue fracture was analysed. The results show that the fatigue limit increased from 651.36 MPa to 918.88 MPa after USRP treatment. The fatigue source is mainly from the sample interior or surface scratches, and the fatigue performance is positively correlated with the results of the material surface roughness, surface residual stress and surface hardness. At the same time, combined with the change in the phase structure, dislocation structure, residual stress and hardness of the cross section of the material, it is found that the USRP process turns the steel into a gradient material with five layers. Finally, the coupling mechanism between the ultrasonic surface strengthening deformation layer and the carburized layer of 20Cr2Ni4A carburized gear steel is presented, and the grain structure distribution diagram of the section of the 20Cr2Ni4A model after surface strengthening treatment was simulated. The mechanism that influenced the fatigue performance after USRP treatment is explained from the perspectives of the surface and cross section of the samples.

This study designs a pulsed magnetic field assisted supersonic plasma spraying (PM-SPS) device. The instrument is divided into magnetic field generation and spraying modules, and they are connected by a dual control system and professional fixture. The PM-SPS system is simple to operate and has no contact with the spraying process. In addition, it can achieve high field intensity and stable pulse frequency in a short time. It has a strengthening effect on the whole coating forming process. The porosity, roughness, hardness, and tribological properties of the coatings prepared using the PM-SPS system were improved. The reported experiments and results can be generalized to other coating applications, which require high quality surfaces.

In order to study the effect of magnetic field on the spraying process, a thermal spraying technique of ferromagnetic metal powder assisted by pulsed magnetic field is presented. The Ni60 coating with different Fe content was prepared by pulsed magnetic field assisted supersonic plasma spraying. This process can improve the microstructure, mechanical properties and tribological properties of the coating. In this paper, a diagram depicting the mechanism for coating formation by pulsed magnetic field assisted spraying was prepared according to the test results for the coatings. We found that the pulsed magnetic field formed at the edge of the poles, and, compared with the Ni base material, the Fe phase material was easily attracted by the magnetic poles, which accelerated its downward movement. During this downward movement, a mixing effect developed within the coating that decreased the coating porosity, made the Cr strengthening phase more uniform, and, as the matrix provided many nucleation sites, further promoted the production of amorphous nanocrystalline phases. Finally, the service performance of the coatings was continuously improved. This process provides a new approach to the design and development of advanced coatings, and it shows great promise for applications in next-generation ferromagnetic metal-based coatings.

In order to study the effect of magnetic field on the spraying process, a thermal spraying technique of ferromagnetic metal powder assisted by pulsed magnetic field is presented. The Ni60 coating with different Fe content was prepared by pulsed magnetic field assisted supersonic plasma spraying. This process can improve the microstructure, mechanical properties and tribological properties of the coating. In this paper, a diagram depicting the mechanism for coating formation by pulsed magnetic field assisted spraying was prepared according to the test results for the coatings. We found that the pulsed magnetic field formed at the edge of the poles, and, compared with the Ni base material, the Fe phase material was easily attracted by the magnetic poles, which accelerated its downward movement. During this downward movement, a mixing effect developed within the coating that decreased the coating porosity, made the Cr strengthening phase more uniform, and, as the matrix provided many nucleation sites, further promoted the production of amorphous nanocrystalline phases. Finally, the service performance of the coatings was continuously improved. This process provides a new approach to the design and development of advanced coatings, and it shows great promise for applications in next-generation ferromagnetic metal-based coatings.

In this study, utilising magnetron sputtering technology and by precisely controlling the structural parameters of amorphous/nanocrystalline multilayer films, we successfully fabricated a composite film system with varying modulation ratios and systematically elucidated the coupling mechanism between the film’s microstructure and tribological properties. The residual stress state of the film and the distribution characteristics of the amorphous and nanocrystalline phases have a decisive influence on its friction and wear behaviour. The amorphous phase, owing to its excellent toughness and stress buffering capacity, is capable of effectively suppressing the propagation of microcracks under alternating loads; the nanocrystalline phase, acting as a hard reinforcing phase, provides the film with superior load-bearing support through grain boundary strengthening and dispersion strengthening mechanisms; whilst the high-density, well-defined heterogeneous interfaces facilitate the effective dissipation of strain energy during friction via interfacial shear slippage and stress redistribution, thereby delaying the film failure process. Tribological performance tests indicate that the wear rate of the optimised film was significantly reduced by 28.7% and 22.1% compared to films (1#) and (3#), respectively, demonstrating excellent friction stability. By establishing a design strategy of ’stress regulation–structural optimisation–performance synergy’, this study provides a new theoretical basis and technical approach for the development of high-performance wear-resistant films.