Ultrasonic surface rolling (USR) was applied to GCr15 steel with different static loads and passes to improve the friction and wear properties, and then the fretting wear mechanism of GCr15 steel after USR treatment was systematically investigated. The results showed that the specimens treated by the USR had lower surface roughness and significantly increased compressive residual stress and microhardness. Furthermore, severe plastic deformation occurred in the surface layer of the specimen, which refined the grains and increased the density of high- and low-angle grain boundaries. Besides, the results of the fretting test showed that the USR treated specimens had lower wear volume, dissipated energy, and steady-state friction coefficient. The fretting wear resistance increased with the static load and the number of passes. The fretting wear mechanism changed from abrasive wear and severe adhesive wear to slight fatigue wear and abrasive wear owing to the use of the USR treatment. Surface smoothing and hardening are responsible for the improvement in the fretting wear properties of GCr15 steel for USR treatment.

More and more cars are installing urea selective catalytic reduction (SCR) systems to solve the problem of exhaust emissions, which often operates at high temperatures in the exhaust system and is prone to failure of the exhaust pipe due to high-temperature oxidation. And niobium containing ferritic stainless steel has been widely used in the manufacturing of automotive exhaust pipes. In order to extend the service life of ferrite stainless steel exhaust pipes, niobium plays a crucial role as an added alloying element. The solid solution and precipitation of niobium in ferritic stainless steel will give ferritic stainless steel more excellent high temperature resistance. The precipitation of Nb can change the organizational structure in steel and refine the grains. However, if the content is not properly controlled, large particles of (Nb, Ti) C will precipitate, which will reduce the high temperature oxidation resistance. In this paper, the high temperature oxidation behavior of two kinds of ferritic stainless steels with different Nb content at 700 °C, 800 °C and 900 °C was studied. The microstructure of the oxide film on the surface of the material, the thickness of the oxide layer on the cross section, the distribution of chemical composition, the existence form and distribution of Nb element were analyzed by SEM, EDS, XRD and TEM. The results show that the higher the niobium content, the better the high temperature oxidation resistance, and the higher the temperature, the more obvious the high temperature oxidation resistance of niobium. This is because the high Nb content steel is easy to precipitate NbN and other Nb-containing precipitates at the grain boundary, which is helpful to the high temperature resistance. In the case of less Nb content, its ability to generate (Nb, Ti) C precipitates and coarsen at high temperatures reduces its high-temperature resistance.

Exhaust is generated by engine flows through a turbocharger, front exhaust pipe, and selective catalytic reduction (SCR) post-treatment device. The structure of the front exhaust pipe affects the temperature, velocity, and turbulent kinetic energy of exhaust and the Sauter Mean Diameter (SMD) of urea water solution (UWS). A high temperature and turbulent kinetic energy in the exhaust will promote the decomposition of UWS, and further accelerate the evaporation and atomization effect of the UWS droplets. Therefore, in order for the exhaust to reach a high temperature and turbulent kinetic energy, a double-layered pipe structure with air insulation was designed. The flow field and the atomization of UWS in the double-layered pipe based on hydroforming processing was investigated through numerical simulation. The thermal insulation simulation was verified by the temperature measurement system and the temperature drops between the double-layered pipe and the volcanic rock-wrapped pipe were also compared. The results indicate that the temperature at the outlet of the designed double-layered pipe was 3.5% higher than that of a single-layered pipe with the same structure, and the velocity at the outlet of the exhaust of the double-layered pipe was 16.1% higher than that of a single-layered pipe. The maximum turbulent kinetic energy in the double-layered pipe was 71 times that of the single-layered pipe. The design is not only conducive to the mixing of UWS and exhaust, but can also improve the atomization performance of UWS.

GCr15 bearing steel is a commonly used material for wind power bearings, and its material microstructure characteristics are the key indexes affecting the service life of wind power bearings. In order to analyze the influence of material microstructure characteristics on bearing performance and service life, this paper explores in detail the impact of microstructural characteristics of martensite/bainite bearing steel on its tribological properties under different loading conditions. The results show that compared to martensitic bearing steel, the martensitic bainitic composite bearing steel displays a substructural morphology of entangled dislocations, grains exhibiting an evident optimal orientation, and a notable increase in the density of large-angle grain boundaries. In addition, the distribution of spherical carbides and residual austenite is more dispersed and homogeneous in this microstructure. In tribological performance tests, it was found that the wear rates of bearing steel with martensitic bainite composite microstructure were significantly reduced by 35.9, 37.9, and 55.5% at 40N, 70N, and 100N load conditions, respectively, compared to those of bearing steels with martensitic microstructure. Changes in the microstructure of the bearing steel affect the performance of its strength and toughness, which in turn makes the martensitic bainite composite microstructure of bearing steel show outstanding advantages in wear resistance performance, thus extending the service life of bearings. The research results can provide a reference basis for optimizing the microstructure design of GCr15 bearing steel, to provide data support for the research of high-performance and long-life bearings.

In the study, laser-MIG hybrid welding of high-strength shipbuilding steel was conducted. The microstructure and impact toughness were studied. The microstructure of the Filling and capping weld metal (FCWM) is widmanstatten and acicular ferrite. The microstructure of the backing WM (BWM) and the heat-affected zone (HAZ) are bainite. The impact energy of the joints is greater than 110 J at different charpy impact test positions. The reason for the different toughness is related to the proportion and distribution of the microstructure at the V-notch of the impact specimen. Through the observation of the section, it is also found that the section corresponding to the microstructure of the different HAZ will show significant differences in fracture morphology.