Abstract
To study the effect of the surface properties on the bending fatigue performance of heavy-duty gear steel, the authors of this paper used the ultrasonic surface rolling process (USRP) to strengthen 20Cr2Ni4A carburized gear steel. USRP is a novel technique in which the ultrasonic technology is incorporated into the concept of conventional deep rolling. In this study, we illustrated how the surface properties and cross-section mechanical property influence the three-point bending fatigue life of the samples before and after USRP treatment. At the same time, the predicted failure probability-stress-number of cycles (P-S-N) curve was drawn, and the fatigue fracture was analysed. The results show that the fatigue limit increased from 651.36 MPa to 918.88 MPa after USRP treatment. The fatigue source is mainly from the sample interior or surface scratches, and the fatigue performance is positively correlated with the results of the material surface roughness, surface residual stress and surface hardness. At the same time, combined with the change in the phase structure, dislocation structure, residual stress and hardness of the cross section of the material, it is found that the USRP process turns the steel into a gradient material with five layers. Finally, the coupling mechanism between the ultrasonic surface strengthening deformation layer and the carburized layer of 20Cr2Ni4A carburized gear steel is presented, and the grain structure distribution diagram of the section of the 20Cr2Ni4A model after surface strengthening treatment was simulated. The mechanism that influenced the fatigue performance after USRP treatment is explained from the perspectives of the surface and cross section of the samples.
Highlights
The gear transmission system is a key component in the fields of aerospace, energy, transportation, and large machinery transmission, and its reliability affects the safety of the entire transmission system and even of the entire mechanical system [1]
The probability-stress-number of cycles (P-stress and number of cycles (S-N)) curve can comprehensively express the relationship between fatigue stress and fatigue life at various reliability levels and determine the degree of life dispersion of the three materials under different stresses
In the calculation of the P-S-N curve, it is generally considered that when life is constant, the material fatigue limit obeys a normal distribution and log-normal distribution
Summary
The gear transmission system is a key component in the fields of aerospace, energy, transportation, and large machinery transmission, and its reliability affects the safety of the entire transmission system and even of the entire mechanical system [1]. There are still three outstanding problems of heavy-duty gear manufacturing. They are short life, heavy structural load and poor reliability [2]. The core problem of manufacturing high-end heavy-duty gears is how to improve their fatigue performance while ensuring reliability and light weight [3]. This requires a heavy-duty gear with a high-strength surface and a high-toughness core. The key to this requirement is whether the surface structure of the gear teeth can effectively improve the gear bearing capacity and bending fatigue life. For the strengthening of the gear tooth surface, lots of surface treatments, such as plasma spraying [4], acid etching [5], sandblasting [6], high energy shot peening [7], surface mechanical attrition treatment [8], ultrasonic shot peening [9,10], ultrasonic surface rolling (USR) [11], surface ultrasonic impact [9], etc., were applied for surface modification or strengthening to better suit some specific situations
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