During machining, the surface temperature of the workpiece will rise due to cutting heat in the machining area. When the temperature exceeds the critical point of metallographic structure change, metallographic structure changes will occur. For general cutting, most of the cutting heat is taken away by the chips, and the impact is not serious.

For grinding, since the cutting heat per unit area is dozens of times greater than that of general cutting, the surface temperature of the workpiece can be as high as 1000℃, which will inevitably cause changes in the metallographic structure of the surface layer, reduce the surface hardness, and generate residual tensile stress and cracks, thereby greatly reducing the service life of the workpiece. This phenomenon is called grinding burn. When grinding burn occurs, the surface layer of the workpiece often appears yellow, brown, purple, blue and other burn colors. They are the colors of the oxide film caused by the instantaneous high temperature on the surface of the workpiece.

Factors affecting grinding burn

1. Grinding amount

Mainly includes grinding depth, longitudinal feed of workpiece and workpiece speed. When the grinding depth increases, the surface temperature of the workpiece and the temperature at different depths below the surface will increase accordingly, and the grinding burn will increase, so the grinding depth should not be too large; the increase in the longitudinal feed of the workpiece reduces the surface contact time between the grinding wheel and the workpiece, improves the heat dissipation conditions, and reduces the grinding burn; although increasing the workpiece speed increases the temperature of the grinding zone, the metallographic structure has no time to change due to the reduced action time of the heat source, which can generally reduce the grinding burn.

For the increase in surface roughness caused by increasing the feed rate and workpiece speed, it is generally compensated by increasing the grinding wheel speed and a wider grinding wheel.

2. Cooling method

Using cutting fluid to take away the heat during grinding can avoid burns, but the cooling method currently applicable is less effective because the cutting fluid fails to enter the grinding zone.

In order to enable the cutting fluid to enter the grinding zone better to play a cooling role, the main methods currently used are internal cooling, jetting, intermittent grinding, and old oil grinding wheels. The internal cooling method is to introduce the cutting fluid into the central cavity of the grinding wheel through the hollow spindle of the grinding wheel. Because the grinding wheel is porous, when the grinding wheel rotates at high speed, the strong centrifugal force throws the cutting fluid out along the gap of the grinding wheel to the surroundings, so that the grinding area is directly cooled.

3. Workpiece material

The higher the hardness of the workpiece material, the more heat is generated during grinding; but if the material is too soft, it is easy to clog the grinding wheel, which will cause the surface temperature of the processing to rise sharply.

The strength of the workpiece material can be divided into high temperature strength and room temperature strength. The higher the high temperature strength, the more power is consumed during grinding. For example, at room temperature, the strength of 45 steel is 65n/mm2 higher than that of 20crmo alloy steel, but at 600℃, the latter is 180n/mm2 higher than the former, so the grinding heat of 20crmo steel is greater than that of 45 steel.

The greater the toughness of the workpiece material, the greater the grinding force required and the more heat is generated. Materials with low thermal conductivity, such as bearing steel and high-speed steel, are more likely to produce changes in metallographic structure during grinding.

4. Selection of grinding wheel

Grinding wheel with too high hardness has too strong bonding force and poor self-sharpening property, which will increase the grinding force and easily cause grinding burns, so softer grinding wheels are often used. Improving the hardness, toughness and strength of the grinding wheel abrasive grains helps to maintain the sharpness and self-sharpening of the blade tip, thereby inhibiting grinding burns. Diamond abrasive is an ideal abrasive because of its high strength and hardness, and its friction coefficient is only 0.05 in the absence of cutting fluid.

The bonding agent of the grinding wheel should be a material with a certain elasticity, such as resin. In this way, when the grinding force increases for some reason, the abrasive grains can produce a certain elastic concession, reducing the cutting depth; at the same time, due to the poor heat resistance of the resin, the bonding performance decreases significantly at high temperature, and the abrasive grains are easy to fall off. All of these help to avoid grinding burns.

When using a coarse-grained grinding wheel for grinding, it can reduce heat generation and avoid clogging of the grinding wheel when grinding soft and plastic materials.