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A number of competing failure mechanisms are involved in bearing failure initiation. For well manufactured bearings operating under clean and well controlled running conditions, sub-surface initiated fatigue is the classical initiation form. Three mechanisms dominate the concept of subsurface induced initiation and growth: (i) The well documented slow structural breakdown of the steel matrix due to accumulation of fatigue damage in a process superficially similar to tempering, (ii) stress induced generation of butterflies by a process enabling the growth of butterfly micro-cracks and accompanying wings at non-metallic inclusions, and (iii) surface induced hydrogen intrusion causing hydrogen-enhanced fatigue damage accumulation in the matrix. The development of butterflies as a function of contact stress, over-rolling, and non-metallic inclusion characteristics is presented, and the influence of metallurgical cleanliness and processing history on this progression is discussed. The results of laboratory conducted tests are compared to results from field applications where premature spallings have occurred. The progression from butterfly micro-cracks to extending cracks with non-etching borders has been studied. Special interest has been paid to the interaction between the non-metallic inclusion composition and morphology and their propensity to generate butterfly wing formations, as this may affect the way that inclusion harmfulness should be judged in rolling bearing steel quality assurance efforts. Complex oxy-sulfides are the main butterfly initiators in today's bearing steels.
Recently, it has been generally accepted that flaking life is dependent on the size of non-metallic inclusion under proper condition in bearing use. Statistics of extreme values to predict the maximum non-metallic inclusion size and ultrasonic test to assess large non-metallic inclusions in a large volume are widely used as practical methods for the evaluation of bearing steel cleanliness. Murakami's formula is well known, which describes the relationship between non-metallic inclusion size and fatigue strength. However, the formula cannot be directly applied to rolling contact fatigue because of the difference in applied stress mode. While the equation was invented to predict the fatigue limit in principal stress mode, rolling contact fatigue is mainly caused by shear stress. It is expected that the condition of bearing use will be more critical due to the downsizing tendency in various industrial or automotive machine units. Thus the research on the flaking mechanism becomes more important from the viewpoint of global ecology because that is beneficial to the improvement in bearing life and methods for cleanliness evaluation. Due to the difficulty in experimental observation, however, the details of the flaking mechanism in rolling contact fatigue caused by non-metallic inclusion has not yet been clarified. Focusing on the relationship between non-metallic inclusion and initiation of the crack, the flaking mechanism is proposed in this paper.
The most common failure in bearings under well-controlled lubricating conditions is internal-originated flaking. In contrast, surface-originated flaking, such as pitting, occurs in bearings and gears under rolling contact fatigue (RCF) with sliding contact. The friction force between steel parts in sliding contact causes surface damage. As a result, the parts' flaking initiation is changed from the inside to the surface. Nowadays, due to the two following changes in global environmental issues, steel parts are expected to endure more serious conditions. (1) To suit the needs for reduction in size and weight, the allowable stress of steel parts tends to increase. (2) To improve power-transmission efficiency, viscosity of lubricant oil tends to decrease. In order to respond to these changes, clarification of the flaking mechanism is very important. However, under RCF with sliding contact, surface conditions such as texture and roughness change from moment to moment. These changes make it difficult to clarify the mechanism of surface-originated flaking. In the present work, the influence of subsurface microstructure of carburized steel on surface-originated flaking life was investigated under RCF with sliding contact. Chemical compositions of steel were varied to transform the subsurface microstructure after gas carburizing. Surface-originated flaking life was changed depending on the subsurface microstructure, which consists of intergranular oxide and incomplete quenching structure. Some RCF tests were interrupted and the wear of raceway and cross-sectional microstructure around the surface cracks was observed. The results indicated that the surface-originated flaking life can be improved by suppression of harmful surface cracks originating from intergranular oxides.
The metallurgical results produced on balls tested in the rolling-contact fatigue spin rig were studied by metallographic examination. Origin and progression of fatigue failures were observed. These evaluations were made on SAE 52100 and AISI M-1 balls fatigue tested at room temperature (80 F) and 200 to 250 F. Most failures originated subsurface in shear; inclusions, structure changes, and directionalism adversely affected ball fatigue life. Structures in the maximum-shear-stress region of the balls of both materials were stable at room temperature and unstable at 200 to 250 F. Failures were of the same type as those found in full-scale bearings.
This proceedings gather a selection of peer-reviewed papers presented at the 8th International Conference on Fracture Fatigue and Wear (FFW 2020), held as a virtual conference on 26–27 August 2020. The contributions, prepared by international scientists and engineers, cover the latest advances in and innovative applications of fracture mechanics, fatigue of materials, tribology, and wear of materials. In addition, they discuss industrial applications and cover theoretical and analytical methods, numerical simulations and experimental techniques. The book is intended for academics, including graduate students and researchers, as well as industrial practitioners working in the areas of fracture fatigue and wear.
Improvements in the rolling contact fatigue resistance of bearing steels need to be made, and various researches to achieve this have been implemented. It is particularly important that, among the non-metallic inclusions, the quantity of large-size oxide inclusions be reduced. Therefore, studies looking at minimizing oxide inclusions have been carried out. More recently, however, a new approach to improving the rolling contact fatigue resistance, different from previous methods, has been undertaken. It has been reported that the condition of the contact at the interfaces between the matrix and non-metallic inclusions is important because it affects the stress state around the inclusions, and that this condition needs to be considered to improve the fatigue resistance of bearing steel. In this paper, to develop bearing steel with a rolling fatigue life better than is currently the case, the relationship between the rolling contact fatigue resistance and the composition of oxide inclusions is investigated. By controlling the composition of the oxide inclusions, fatigue failure was avoided in a thrust-type rolling contact fatigue test, and the fatigue life (L10) achieved was greater than 200 million revolutions. The reason for this improvement is considered to be the absence of voids at the interface between the matrix and the non-metallic inclusions, which would otherwise have a negative effect on the fatigue life.