The failure modes of KOYO bearings include surface contact fatigue, abrasive wear, adhesion wear and corrosion wear. They always occur on the working surface and surface layer of KOYO bearings. Obviously, the quality of the working surface layer is important to the reliability and service life of KOYO bearings. Critical.
KOYO bearing working surface quality research includes: surface topography analysis; surface material and surface metamorphic layer analysis; surface stress state analysis; surface wear state and corrosion state analysis, etc.
Due to the influence of cold and hot processing conditions and lubricating media, the microstructure, physical, chemical, and mechanical properties of the working surface of KOYO bearings are often very different from the core. The surface layer on which the microstructure, physical, chemical, and mechanical properties of the KOYO bearing surface has changed is called the surface metamorphic layer. If the surface deterioration layer is caused by the grinding process, it is called the grinding surface deterioration layer. KOYO bearing working surface metamorphic layer analysis is the main component of KOYO bearing surface quality analysis, and of course it is also one of the important components of KOYO bearing failure analysis.
According to the formation mechanism of the degraded layer on the working surface of the KOYO bearing, the main factors affecting the degraded layer are the effects of grinding heat and grinding force.
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1. Grinding heat
In the grinding process, a large amount of energy is consumed in the contact area between the grinding wheel and the workpiece, and a large amount of grinding heat is generated, resulting in a local instantaneous high temperature in the grinding area. Using linear motion heat source heat transfer theory formula to derive, calculate or use infrared method and thermocouple method to measure the instantaneous temperature under experimental conditions, it can be found that the instantaneous temperature of the grinding zone can be as high as 1000-1500℃ within 0.1-0.001ms. Such instantaneous high temperature is enough to cause high temperature oxidation, amorphous structure, high temperature tempering, secondary quenching, and even burn and cracking of the surface layer at a certain depth of the working surface.
(1) Surface oxide layer
The steel surface under the action of instantaneous high temperature reacts with oxygen in the air, and it rises into a very thin (20-30nm) thin layer of iron oxide. It is worth noting that there is a corresponding relationship between the thickness of the oxide layer and the total thickness of the surface ground modified layer. This shows that the thickness of the oxide layer is directly related to the grinding process and is an important indicator of grinding quality.
(2) Amorphous structure layer
When the instantaneous high temperature of the grinding zone makes the surface of the workpiece reach a molten state, the molten metal molecule flow is evenly coated on the working surface, and is cooled by the base metal at a very fast speed, forming a very thin layer of amorphous state Organizational level. It has high hardness and toughness, but it is only about 10nm, which can be easily removed during precision grinding.
(3) High temperature tempered layer
The instantaneous high temperature in the grinding zone can heat the surface to a temperature higher than the tempering heating temperature of the workpiece within a certain depth (10-100nm). In the case of not reaching the austenitizing temperature, as the heated temperature increases, the surface layer will undergo a re-tempering or high-temperature tempering structural transformation corresponding to the heating temperature, and the hardness will also decrease. The higher the heating temperature, the greater the decrease in hardness.
(4) Two-layer quenching layer
When the instantaneous high temperature of the grinding zone heats the surface layer of the workpiece to above the austenitizing temperature (Ac1), the austenitized structure of this layer is re-quenched into a martensitic structure in the subsequent cooling process. For all workpieces with secondary quenching burns, the secondary quenching layer must be a high temperature tempered layer with extremely low hardness.
(5) Grinding cracks
The secondary quenching burn will change the stress of the surface layer of the workpiece. The secondary quenching zone is in a compressed state, and the material in the high temperature tempering zone below it has the greatest tensile stress. This is the place where the crack core is most likely to occur. Cracks are most easily propagated along the original austenite grain boundaries. Severe burns will cause cracks (mostly cracks) on the entire grinding surface and cause the workpiece to be scrapped.
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2. Metamorphic layer formed by grinding force
In the grinding process, the surface layer of the workpiece will be affected by the cutting force, compression force and friction force of the grinding wheel. Especially the effect of the latter two makes the surface layer of the workpiece form a highly directional plastic deformation layer and a work hardening layer. These metamorphic layers inevitably affect the changes in the residual stress of the surface layer.
(1) Cold plastic deformation layer
In the grinding process, every tick is equivalent to a cutting edge. However, in many cases, the rake angle of the cutting edge is negative. In addition to the cutting action, the abrasive particles are subjected to the squeezing action (plough action) on the surface of the workpiece, leaving an obvious plastic deformation layer on the surface of the workpiece. The degree of deformation of this deformation layer will increase with the degree of blunt grinding of the grinding wheel and the increase of the grinding feed.
(2) Thermoplastic deformation (or high temperature deformation) layer
The instantaneous temperature formed by the grinding heat on the working surface makes the elastic limit of the surface layer of the workpiece at a certain depth drop sharply, even to the extent that the elasticity disappears. At this time, the working surface layer is freely stretched under the action of grinding force, especially compression force and friction force. Due to the limitation of the base metal, the surface is compressed (more ploughed), causing plastic deformation in the surface layer. The high-temperature plastic deformation increases with the increase of the surface temperature of the workpiece under the condition of the same grinding process.
(3) Work hardened layer
Sometimes microhardness and metallographic methods can be used to find that the hardness of the surface layer increased due to processing deformation.
In addition to grinding processing, the surface decarburization layer caused by casting and heat treatment heating, if not completely removed during subsequent processing, remains on the surface of the workpiece will also cause the surface to soften and deteriorate, leading to the early failure of KOYO bearings.
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