A new hybrid surface strengthening method for 42CrMo steel based on ultrasonic shot peening and low-temperature nitriding

42CrMo steel, a high-strength chromium–molybdenum alloy steel, is widely used in the manufacture of high-end key parts that must withstand high loads and stresses over extended service times. Under extreme service conditions, such as high speed, heavy loading, and frequent impact, the failure of 42CrMo steel components typically initiates at the surface and then extends inward. Single shot peening treatment can refine grains and introduce high compressive residual stress, but the improvement of surface hardness is limited. Conversely, nitriding treatment alone markedly increases surface hardness, but it is usually accompanied by surface cracking and grain coarsening. In this study, seven types of surface treatment processes were applied to 42CrMo steel, single shot peening (S-SP), single high-temperature nitriding (S-HT), single low-temperature nitriding (S-LT), shot peening followed by high-temperature nitriding (S-SP + HT), high-temperature nitriding followed by shot peening (S-HT + SP), shot peening followed by low-temperature nitriding (S-SP + LT), and low-temperature nitriding followed by shot peening (S-LT + SP). Untreated samples were designated as S-UT. The surface properties after the different treatments, including morphology, microstructural features, mechanical properties, and tribological performance, were systematically compared and analyzed. The results show that the greatest nitriding-layer depth (121 μm, an increase of 31.0 % and 9.1 % compared with S-LT and S-HT, respectively) is obtained under the method combining shot peening pretreatment and low-temperature nitriding (S-SP + HT), as accompanied by the highest compressive residual stress (−1024.5 MPa, an increase of 22.6 % and 72.4 % compared with S-SP and S-LT, respectively) and the lowest wear rate (1.7 mm3/N·m, a decrease of 91.8 %, 45.2 %, and 55.3 % compared with S-UT, S-LT, and S-HT, respectively). This is because the dislocations and other crystallographic defects introduced by the shot peening pretreatment facilitate the penetration and diffusion of nitrogen ions into the steel and promote the formation of nitrides. Moreover, compared with S-SP + HT, S-SP + LT effectively inhibits the recovery of dislocation defects and grain-size coarsening at elevated temperatures. This work thus provides a theoretical basis and practical process guidance for the surface strengthening of 42CrMo steel under extreme service conditions.