What is a quenching soft point?
Quenching soft point refers to the phenomenon that the hardness of the workpiece is low locally after quenching. Due to the low hardness of the soft point, it will directly affect the grinding quality, and contact fatigue damage will occur during use, resulting in early wear failure of the bearing and affecting the service life of the workpiece. The occurrence of soft point should be avoided in production.
There are two main common quenching soft points, one is decarburization: such as heating in the air furnace or heating furnace atmosphere is not good is easy to cause decarburization;Two non-martensite structure: 1) the heating temperature is insufficient, the insulation is not sufficient, so that the matrix is not fully austenitized, quenching can not all be converted into the stenitic structure, will cause local soft points;2) the local cooling rate is too low, the air bubbles or dirt attached to the surface of the workpiece, or the amount of methanol in the drip atmosphere is large and directly drops on the surface of the workpiece, or the place where the workpiece is in close contact with the workpiece, etc., can not reach the critical cooling rate, and can not be completely transformed into martensite structure.
What is the reason of bearing roller quenching soft point?
1. Macro morphology observation and soft point proportion analysis
After quenching and cold pickling of normal rollers, the surface of rollers should be uniform and light gray, as shown in Figure 1.However, after cold pickling, the rollers with soft quenching point appear black strips on the surface of the rollers, and the defects are all located on the rolling surface. The morphology is shown in Figure 2.
2. Reasons for bearing roller quenching soft points
Through the normal roller roller and soft point of macro morphology observation and chemical composition analysis, hardness testing and metallographic examination analysis shows that the soft point of roller surface layer microstructure of martensite, carbides and residual austenite in vitro, there are needle troostite and small patches, and significantly lower than the normal roller hardness, and uneven distribution of hardness.The main reason for the abnormal microstructure and hardness of the rolling surface may be that the cooling rate of the rolling surface is slow during quenching, and the subcooled austenite on the rolling surface does not completely undergo martensite transformation during quenching, leading to the partial transformation into acicular or massive trositic structure, so the soft point is formed after quenching.
By increasing the heating temperature, increasing the holding time, adjusting the parameters that affect cooling, increasing the stirring speed and the moving speed, the soft point of the trobucite of the roller will still appear, and the heating equipment uses propane protective gas, so there is no problem of drip.Analysis thinks, quenching process of bearing roller rolling surface cooling, the cause of the slow roller quenching could be the two rollers of the rolling surface contact each other, when the rolling surface contact area of quenching salt with poor liquidity, not in time will be away the heat generated by the rapid quenching process, resulting in slow cooling speed rolling surface contact area, a quenching soft point.
The reason that causes the soft point of the roller quenching is that in the quenching process, due to the mutual contact between the rolling surfaces of the rollers, resulting in the formation of exceeding the standard of the trobucite.In the actual roller heat treatment process, in order to avoid the appearance of soft spots on rollers, rollers should be evenly placed on the plate, and rolling surfaces are strictly prohibited to contact each other.