Study on microstructure of 42CrMo steel by ultrasonic surface rolling process.

To explore the microstructure formation mechanism of 42CrMo steel under the strengthening of ultrasonic surface rolling process (USRP), the combination of theoretical analysis and experiment was used to conduct in-depth research on USRP. Firstly, according to contact mechanics and Hertz contact theory, the calculation model of contact stress distribution and elastoplastic strain between the rolling ball and the part during USRP is obtained. Secondly, the USRP processing test was carried out by single-factor experimental design method, and the microstructure of 42CrMo steel after USRP was analyzed by LEXT OLS5100 3D laser surface topography instrument and VEGA3 tungsten filament scanning electron microscopy, which found that with an increase in static pressure, residual stress and plastic strain gradually increase, the hardness firstly increases and then decreases, while surface roughness exhibits an initial decrease followed by an increase. The results show that USRP produces violent plastic deformation inside the material under the superposition of high-frequency impact and static pressure, at the same time, it refines the grains, so as to improve the surface performance of the part and improve its fatigue resistance.

Study on surface residual stress of 42CrMo steel treated by ultrasonic rolling extrusion.

To obtain the distribution law of residual stress of 42CrMo steel processed by ultrasonic rolling extrusion process (UREP), according to the characteristics of the material treated by ultrasonic rolling extrusion process, 42CrMo steel quasi-static experiment and dynamic experiment are carried out, the improved Johnson-Cook model was proposed. Based on the 42CrMo constitutive equation, the ultrasonic rolling extrusion process finite element method and experiment of 42CrMo steel were carried out. The results show that the simulation value and experimental value of the residual stress distribution after ultrasonic rolling extrusion process of 42CrMo steel have good consistency, with the increase of static pressure and amplitude, the residual stress increases, and with the increase of rotational speed and feed speed, the residual stress decreases, a more accurate residual stress distribution law of ultrasonic rolling extrusion of 42CrMo steel is obtained.

Development of a hydraulically controlled piston-pressurized pulsed water jet device and its application potential for hard rock breaking.

To improve the efficiency of hard rock breaking by a pulsed water jet (PWJ), a hydraulically controlled piston-pressurized PWJ (HCPPPWJ) device has been developed, by which the large amplitude pressurization of the jet could be realized through the motion coupling of the piston and the valve core inside the device without requiring additional control or ultra-high-pressure components. Under the continuous injection of low-pressure hydraulic oil, the device has a stable pressurization effect and controllable pulse pressure and pulse frequency. The jet pressure varies periodically with the alternation of high and low pressures; in the rising stage of the pulse pressure, the jet morphology presents an umbrella-like thin-layer structure, which ensures an effective initial impact force of the jet in contact with the target. With the addition of high-frequency stress waves and water wedge pressure, local flaky exfoliation was observed when the granite surface was eroded, and the maximum radius and volume of the erosion pit were greater than those in the case of employing a continuous water jet. Compared with the interrupted PWJ, the HCPPPWJ efficiently utilizes the jet energy during the erosion process, and the specific energy is lower. The results prove that the HCPPPWJ device is an advanced tool in the field of hard rock breaking.