Due to the excellent deformation hardening behavior, high manganese steel(HMnS) is widely used under strong impact wear conditions. The surface hardness can be increased from approximately 200 HB after water toughening treatment to 500–800 HB after deformation. Meanwhile, the core maintains good toughness, suitable for applications in mining machinery, railway, metallurgy, building materials, and other fields. However, the hardening ability of HMnS requires improvement under low stress wear. In this paper, the ultrasonic shock strengthening method is proposed to improve the hardening ability of HMnS under low stress wear. First, the surface of HMnS was treated via ultrasonic shock strengthening technology. The maximum output amplitude of the ultrasonic shock strengthening device used in this work was 90 μm. The vibration frequency was 20 kHz, and the diameter of the vibrating head was 6 mm. The MPX-3X wear tester was employed to conduct the wear test on the surface of HMnS. The load was 50 N, the spindle speed was 300 r / min, the wear time was 60 min, the friction radius was 5 mm, and the diameter of the Al2O3 grinding ball was 6.35 mm. Vickers hardness measurement and microstructure analysis of the sub surface before and after wear were performed using a Vickers hardness tester and scanning electron microscopy. The load of Vickers hardness measurement was 0.3 kg, the holding time was 15 s, and the hardness was the average value of five measurements. Then, the wear tests on the surface of HMnS before and after ultrasonic shocking demonstrated the good wear resistance of HMnS; the depth of wear scar and the mass loss were 51.7% and 57.68% less than those for the original HMnS, respectively. The microstructure and surface wear marks were examined using a three-dimensional profiler, X-ray diffractometry, scanning electron microscopy, and energy dispersive spectrometry to study the wear resistance of the HMnS surface before and after ultrasonic shock strengthening. As a result, only microplastic deformation and small flake delamination wear were observed on the surface of the wear scar. Finally, the wear resistance mechanism of ultrasonically shocked HMnS (US-HMnS) was revealed through measurements of the microstructure and Vickers hardness of the worn sub surface. When the HMnS was subjected to ultrasonic shocking, with propagation of stress wave from the surface to the interior, the HMnS undergoes plastic deformation from the surface to the interior, and deformation twins were continuously formed. At the frequency up to 20 kHz, high-density twins and secondary twins were formed below the surface of the HMnS, so that the HMnS had twin hardening. In addition, deformation twins continue to form, dividing the grains into several small areas, which leads to the accumulation of dislocations at the grain boundary, near the twin boundary and in the grains, greatly improving the dislocation density of the HMnS microstructure, increasing the hardening degree of the HMnS, and realizing dislocation strengthening. Grain refinement on material surface is also a typical phenomenon of ultrasonic shocking, and grain refinement strengthening is the mechanism of ultrasonic shock strengthening. Therefore, grain refinement, the high-density twin, and dislocation accumulation in the deformation process of HMnS produce a high hardening effect. Thus, ultrasonic shock strengthening technology is helpful to improve the hardening ability of HMnS under low stress wear conditions, to improve its wear resistance. © 2023 Chinese Mechanical Engineering Society. All rights reserved.
