According to the formation mechanism of the grinding bearing metamorphic layer on the working surface of the imported bearing, the main factors affecting the grinding of the metamorphic layer are the effects of grinding heat and grinding force. Below we will analyze the reasons for bearing failure.
1. KOYO bearing grinding heat
In the grinding process of KOYO bearings, the grinding wheel and the workpiece contact area consume a large amount of energy, generating a large amount of grinding heat, resulting in local instantaneous high temperature in the grinding zone. Using the linear heat transfer heat transfer theoretical formula to derive, calculate or apply the infrared temperature and thermocouple method to measure the instantaneous temperature under experimental conditions, it can be found that the instantaneous temperature in the grinding zone can be as high as 1000-1500 °C in 0.1-0.001 ms. Such instantaneous high temperature is sufficient to cause high temperature oxidation, amorphous structure, high temperature tempering, secondary quenching, and even burn cracking of the surface layer at a certain depth on the working surface.
(1) Surface oxide layer
The surface of the steel under transient high temperature acts on the oxygen in the air and rises into a very thin (20-30 nm) thin layer of iron oxide. It is worth noting that the thickness of the oxide layer is corresponding to the test results of the total thickness of the surface-grinding metamorphic layer. This shows that the thickness of the oxide layer is directly related to the grinding process and is an important indicator of the quality of the grinding.
(2) amorphous layer
When the instantaneous high temperature of the grinding zone causes the surface of the workpiece to reach a molten state, the molten metal molecular flow is uniformly applied to the working surface, and is cooled by the base metal at a very fast speed to form a very thin layer of amorphous state. Organizational layer. It has high hardness and toughness, but it is only about 10 nm and is easily removed in precision grinding.
(3) high temperature tempering layer
The instantaneous high temperature in the grinding zone allows the surface to be heated to a temperature above the tempering heating of the workpiece at a certain depth (10 to 100 nm). In the case where the austenitizing temperature is not reached, as the temperature to be heated is increased, the surface layer will be re-tempered or high-temperature tempered in accordance with the heating temperature, and the hardness will also decrease. The higher the heating temperature, the more the hardness decreases.
(4) Two-layer quenching layer
When the instantaneous high temperature of the grinding zone heats the surface layer of the workpiece above the austenitizing temperature (Ac1), the austenitized structure of the layer is re-quenched into martensite structure during the subsequent cooling process. For workpieces with secondary quenching burns, the secondary quenching layer must be a high-temperature tempering layer with extremely low hardness.
(5) Grinding crack
The secondary quenching burn will cause the surface layer stress of the workpiece to change. The secondary quenching zone is under pressure, and the material in the high temperature tempering zone below it has the largest tensile stress, which is the place where the crack core is most likely to occur. Cracks are most likely to propagate along the original austenite grain boundaries. Severe burns can cause cracks in the entire grinding surface (more cracking) and scrap the workpiece.
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