二次超声滚压处理对 45CrNiMoVA 超高强度钢 低周扭转疲劳性能的影响

Ultra-high strength steels are typically characterised by high strength, high toughness and high hardenability. Due to their excellent comprehensive mechanical properties, they are often used in high-strength structural components such as aircraft engine crankshafts, landing gears, and rocket shells. They are also used in the manufacture of key important components that withstand harsh loads in heavy machinery, including torsion shafts, and gearbox shafts. Ultra-high strength steels usually have low damage tolerance and high surface defect sensitivity, and their service conditions are harsh. So, many components belong to low-cycle fatigue parts, and their service reliability and life are worse than those of low strength materials. For example, 45CrNiMoVA ultra-high strength steel torsion shaft, which is subjected to cyclical torsional loads during service, the maximum load is located on the surface of the material. Therefore, fatigue cracks can easily initiate on the surface, resulting in fatigue failure. Ultrasonic Surface rolling treatment can improve the fatigue performance of mechanical components. For conventional high-plastic materials, the fatigue life is often increased by nearly 10 times, while for ultra-high strength steel, the fatigue life is limited. Under low-cycle fatigue loading, the beneficial effects of the ultrasonic surface rolling process on ultra-high strength steel degraded. In particular, the surface roughness was seriously degraded, and stripes with the genetic characteristics of turning traces appeared, which significantly reduced the fatigue life. This study first discussed the degradation of the ultrasonic surface rolling effect under low-cycle fatigue loading. The stress and strain distribution in the near-surface region of the ultrasonic surface rolling specimen was analyzed by the finite element method. It was found that there were significant stress and strain inhomogeneities associated with turning marks on the surface of the turning sample after ultrasonic surface rolling. Under low-cycle fatigue loading, the strain and stress inhomogeneity of the surface structure would induce the appearance of surface stripes related to turning marks. The increase of surface roughness is beneficial to the initiation of fatigue crack, which is the main factor affecting the improvement of fatigue life. Then, the effect of re-apply ultrasonic surface rolling treatment on the low-cycle torsional fatigue behavior and mechanism of 45CrNiMoVA ultra-high strength steel was investigated. After ultrasonic surface rolling treatment, the roughness of the as-machined specimen decreased from Ra 937 nm to Ra 94 nm. However, under the torsional fatigue loading, micro-cracks with turning marks characteristics appeared on its surface. Compared to the as-machined specimen, the low-cycle torsional fatigue life increased approximately 5 times after ultrasonic surface rolling treatment, exceeding 200 000 cycles. After re-applying ultrasonic surface rolling treatment, the specimen did not break after 900 000 cycles, which was 19.65 times higher than that of the as-machined specimen. Compared with the USRP specimen, the life was increased by about 3.33 times. After re-applying ultrasonic surface rolling treatment, the stress / strain uniformity of the sample surface was improved, and the surface roughness showed extremely high stability under low-cycle fatigue loading without obvious roughening, which is the core mechanism of the improvement of low-cycle torsional fatigue life by re-applying ultrasonic surface rolling treatment. In addition, after re-applying ultrasonic surface rolling treatment, the strength of the gradient structure and residual compressive stress level were restored, and the relaxation rate of residual compressive stress was significantly reduced due to the improved surface stress and strain uniformity. These mechanisms also contribute to the improvement of low-cycle fatigue life. The Re-apply ultrasonic surface rolling treatment can well inhibit the deterioration of the surface roughness of the specimens during fatigue loading, and provide the specimens a prominent fatigue life. This study has reference significance for the life improvement and extension of ultra-high strength steel low-cycle fatigue parts.