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FAG bearing grinding the cause of the metamorphic layer
Source: China Bearing Network Time: 2014-04-24
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1. Grinding heat in the grinding process; grinding wheel and workpiece touch area; cost a lot of energy; many grinding heat occurs; part of the instantaneous high temperature that constitutes the grinding zone. Use the linear motion heat source heat transfer theory formula to derive, account or The infrared temperature method and the thermocouple method are used to measure the instantaneous temperature under the test conditions; the instantaneous temperature in the grinding zone can be as high as 1000-1500 Â° C in the range of 0.1 to 0. 001 ms. Such instantaneous high temperature is sufficient for the operation appearance to be certain High-temperature oxidation of the outer surface layer; amorphous arrangement, high temperature tempering, secondary quenching; and even burn cracking and other changes.
.1) The effect of the outer surface of the oxide layer on the appearance of oxygen in the steel and the oxygen in the air; the thin layer of iron oxide which is raised to the pole of 20 to 30 nm. It is worth noting that the thickness of the oxide layer and the total thickness of the enthalpy layer of the external grinding The test results are in a corresponding relationship. This indicates that the thickness of the oxide layer is directly related to the grinding process; it is an important mark of the grinding quality.
.2) Amorphous arrangement of the instantaneous high temperature of the grinding zone causes the workpiece to reach the molten state; the molten metal molecular flow is uniformly applied to the work surface; and is cooled by the base metal at a very fast rate; Very thin layer of amorphous arrangement. It has high hardness and resistance; but it only needs about 10nm; it is very simple to be removed in fine grinding.
.3) The instantaneous high temperature in the grinding zone of the high temperature tempering layer can be heated to a temperature higher than the tempering heating of the workpiece at a certain depth (10 to 100 nm). In the case where the austenitizing temperature is not reached; The improvement of the heating temperature; the appearance of layer-by-layer will occur with the re-tempering or high-temperature tempering corresponding to the heating temperature; the hardness will also decrease. The higher the heating temperature, the more fierce the hardness is.
.4) Two-layer quenching layer When the instantaneous high temperature of the grinding zone heats the outer surface of the workpiece to above the austenitizing temperature (Ac1); then the austenitizing of the layer is arranged in the subsequent cooling process; Quenching into martensite arrangement. However, any workpiece with secondary quenching burn; under the secondary quenching layer must be a very high temperature tempering layer.
.5) Grinding cracks Secondary quenching burns will change the stress of the outer surface of the workpiece. The secondary quenching zone is under pressure; the maximum tensile stress exists in the high temperature tempering zone below; here is the most likely crack center The local crack is most simply conveyed along the original austenite grain boundary. Severe burns will cause cracks (greater cracks) in the entire grinding surface to form the workpiece.
2. The moirÃ© layer formed by the grinding force is in the grinding process; the outer surface of the workpiece will be affected by the cutting force, the tightening force and the friction force of the grinding wheel. Especially the effect of the latter two; the outer surface of the workpiece is very directional. Plastic deformation layer and work hardening layer. These metamorphic layers must affect the change of residual stress in the outer layer.
.1) The cold plastic deformation layer is in the grinding process; each moment the abrasive grain is equivalent to a cutting edge. However, in many cases, the rake angle of the cutting edge is negative; the abrasive grain is in addition to the cutting effect; The appearance of the kneading effect (ploughing effect); leaving a significant plastic deformation layer on the surface of the workpiece. The degree of deformation of this deformation layer will increase with the degree of grinding wheel blunt and the increase of grinding feed.
.2) Thermoplastic deformation (or high temperature deformation) The instantaneous temperature at which the layer grinding heat is formed on the outside of the work; the elastic limit of the outer surface of the workpiece at a certain depth is drastically reduced; and the degree of elasticity is not reached. At this moment, the outer surface of the work is under grinding force. Especially under the effect of tightening force and friction; the resulting free expansion; bound by the base metal; the outer surface is tightened (more plowed); the plastic deformation is formed on the outer surface. The high temperature plastic deformation does not change during the grinding process. Lower; as the workpiece surface temperature increases.
.3) The work hardening layer can sometimes be found by microhardness method and metallographic method; the hardness of the outer layer is increased due to processing deformation.
In addition to the grinding process; the outer decarburization layer formed by casting and heat treatment heating; if it is not completely gone in the future processing; the residual on the surface of the workpiece will also constitute the surface softening and transformation; promote the early failure of the bearing.
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