According to the formation mechanism of the grinding metamorphic layer on the working surface of the FAG bearing, the main factors affecting the grinding metamorphic layer of the FAG bearing are the effects of grinding heat and grinding force.
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 local instantaneous high temperature in the grinding area. The instantaneous temperature in the grinding zone can be as high as 1000-1500°C within 0.1-0.001 ms by deriving, calculating or using the infrared method and thermocouple method to measure the instantaneous temperature under the experimental conditions by using the heat transfer theory formula of the linear motion heat source. Such an instantaneous high temperature is enough to cause high-temperature oxidation, amorphous structure, high-temperature tempering, secondary quenching, and even burn cracking on the surface layer of the working surface at a certain depth.
(1) Surface oxide layer
The steel surface under the instantaneous high temperature reacts with the oxygen in the air to form a very thin (20-30nm) iron oxide thin layer. It is worth noting that there is a corresponding relationship between the thickness of the oxide layer and the total thickness of the surface grinding and deterioration layer. This shows that the thickness of the oxide layer is directly related to the grinding process and is an important indicator of the grinding quality.
(2) Amorphous tissue layer
When the instantaneous high temperature in the grinding area makes the surface of the workpiece reach a molten state, the molten metal molecular flow is evenly coated on the working surface, and is cooled by the base metal at a very fast speed, forming an extremely thin layer of amorphous organizational layer. It has high hardness and toughness, but it is only about 10nm and can be easily removed in precision grinding.
(3) High temperature tempering layer
The instantaneous high temperature in the grinding area can make the surface be heated to a temperature higher than the tempering heating temperature of the workpiece within a certain depth (10-100nm). When the austenitizing temperature is not reached, as the heated temperature increases, the surface will undergo retempering or high temperature tempering corresponding to the heating temperature layer by layer, and the hardness will also decrease. The higher the heating temperature, the greater the decrease in hardness.
(4) Second quenching layer
When the instantaneous high temperature in the grinding zone heats the surface layer of the workpiece above the austenitization temperature (Ac1), the austenitized structure of this layer is re-quenched into martensite structure during the subsequent cooling process. For workpieces with secondary quenching burns, there must be a high-temperature tempering layer with extremely low hardness under the secondary quenching layer.
(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 state of compression, and the material in the high temperature tempering zone below it has the largest tensile stress, and this is the place where the crack core is most likely to occur. Cracks propagate most easily along the original austenite grain boundaries. Severe burns can cause cracks (mostly cracks) to appear on the entire grinding surface and cause the workpiece to be scrapped.
2. Metamorphic layer formed by grinding force
During 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 role of the latter two makes the surface layer of the workpiece form a highly directional plastic deformation layer and work hardening layer. These metamorphic layers will inevitably affect the change of residual stress in the surface layer.
(1) Cold plastic deformation layer
In the grinding process, each moment of abrasive grain is equivalent to a cutting edge. However, in many cases, the rake angle of the cutting edge is negative. In addition to the cutting effect, the abrasive grains make the surface of the workpiece suffer from extrusion (plowing action), leaving an obvious plastic deformation layer on the surface of the workpiece. The degree of deformation of this deformed layer will increase with the degree of blunting of the grinding wheel and the increase of grinding feed rate.
(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 under the action of grinding force, especially compressive force and friction force, which causes free extension and is restricted by the base metal, and the surface is compressed (more plowed), causing plastic deformation on the surface layer. The high-temperature plastic deformation increases with the increase of the surface temperature of the workpiece under the condition of constant grinding process.
(3) Work hardened layer
Sometimes it can be found by microhardness method and metallographic method that the hardness of the surface layer increases due to processing deformation.
In addition to grinding, the surface decarburization layer caused by casting and heat treatment heating, if not completely removed in subsequent processing, will remain on the surface of the workpiece and cause surface softening and deterioration, leading to early failure of the bearing.
