Study On Bending Fretting Fatigue Characteristics Of 18CrNiMo7-6 Alloy Steel

Fretting fatigue is a type of fatigue strength and fatigue lifetime issues, which is associated with the fretting wear. It can cause the cracks initiation and propagation in advance, which will lead the components to failure untimely. Fretting fatigue exists widely in all kinds of fastening components of machinery, transportation, electric power, aerospace, nuclear power and other industrial fields, which is the main factor of failure for many key components. Through series of bending fretting fatigue tests, the bending fretting fatigue characteristics of the material were investigated in detail by taking the 18CrNiMo7-6 alloy steel as the study object, to provide some theoretical guidance and reference for its applications in practical engineering.The tests of the thesis, whose contact mode was cylinder to cylinder orthogonal point contact, were carried out on the bending fretting fatigue experiment table. By series of tests, the S-N curve of 18CrNiMo7-6 alloy steel was built up. Then on this basis, by means of the microscopic analysis and detection equipment, such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron probe microanalysis (EPMA), the damage morphology, fracture morphology and crack section of the material contact area were deeply analyzed. The main conclusions are as follows:(1) The S-N curve of 18CrNiMo7-6 alloy steel presented a shape of "ε", and the fretting regime was transformed from partial slip regime (PSR) to mixed regime (MR) and slip regime (SR) with the increase of bending fatigue stress. In the MR, the cracks were easily to initiate and propagate, and the fatigue lifetime of material was the lowest correspondingly.(2) The overall damage morphology of the material contact area showed a shape of asymmetrical elliptical ring. The central part was in a relatively static adhesion state, and was almost no wear there. Distinct damaged zones could be seen in both ends of contact area. Bending fatigue stress and test cycles number were two important factors influencing the bending fretting fatigue characteristics of material. With the increase of bending fatigue stress, the tangential stress and fretting displacement amplitude between the material contact interface were enlarged, causing a widened and deepened damaged zones in both ends of contact area, and enlarging the power of crack initiation and propagation. With the increase of test cycles number, the damage degree of the material contact area was aggravated, which could promote the crack initiation and propagation.(3) It was found that, the bending fretting fatigue cracks of 18CrNiMo7-6 alloy steel all initiated in the damaged zone on the fixed end side of contact area. By the crack section analysis and fracture analysis, it could further confirm the crack initiation position located in the sub-surface of the contact area which was about 50～100μm deep from the surface.(4) The process of the crack initiation and propagation could be divided into three stages. The first stage was mainly controlled by the contact stress, where the crack nucleated in the sub-surface of the contact area, and began to propagate along the 50° angle. The second stage was controlled by the contact stress and bending fatigue stress together, where the crack propagation angle turned to perpendicular to the contact surface. Because the crack propagation depth exceeded the maximum effect depth of contact stress, the third stage was controlled by the bending fatigue stress completely. Therefore, the crack propagation direction was perpendicular to the contact surface, and the material would fracture to failure soon.