① Shrinkage cavity: during the condensation process of molten steel after pouring, a hole is formed in the center of the ingot due to volume shrinkage, which is called shrinkage cavity. In order to reduce the harm of shrinkage steel, a reasonable process should be adopted in the process of liquid steel pouring and crystallization to move the hole formed by volume shrinkage to the head of the ingot. After the ingot is opened, the shrinkage part should be cut off.
However, due to the improper pouring and cooling process, such as unreasonable setting, insufficient heat preservation at the head of the ingot, and less removal at the head of the ingot after the billet is opened, the shrinkage hole remains in the steel. During low magnification inspection, Will show up.Low Magnification Defect of Bearing Steel
② White spot: the center of the transverse section of the steel sample after pickling or its adjacent area presents short, discontinuous, hair like open joints generally distributed in a radiation state, or silver white spots with smooth surface and approximate circular or oval shape appear on the longitudinal fracture of the steel, which are called white spots.
The reasons for the formation of white spots are: first, the existence of hydrogen in the steel; second, the steel is not slowly cooled at 600 ~ 300 ℃ after forging, and the hydrogen is not fully diffused, resulting in structural stress and cracking. The longitudinal and transverse mechanical properties of steel or parts with white spots have decreased significantly, so the steel or parts with white spots have no use value.Low Magnification Defect of Bearing Steel
③ Overburning: when the ingot or billet is forged and heated, the temperature is too high, and the surface layer is invaded by oxygen along the grain boundary to produce oxide. Some low melting point compounds at the grain boundary and between the dendrite axes melt, resulting in cracks or holes after condensation. This phenomenon is called overburning.
After the steel is over burned, it will cause cracking during re forging. Even if it does not crack, the strength and impact toughness under the sun will be greatly reduced, so it can not be used.Low Magnification Defect of Bearing Steel
④ Bubbles: the ability of steel to dissolve gas in liquid state is greater than that in solid state. During the condensation process of liquid steel, the gas escapes from the liquid steel.
If it is too late to discharge, it will form pores. In addition, if the ingot mold is not baked well, there will be water or gas on the surface of the steel mold, and the coating on the inner surface of the ingot mold is poor, forming a large amount of gas. If these water or gas are too late to discharge the liquid steel, subcutaneous bubbles will be formed. The existence of bubbles greatly reduces the strength of steel.Low Magnification Defect of Bearing Steel
⑤ Segregation: in the process of liquid steel condensation, the phenomenon of uneven chemical composition formed due to different crystallization and diffusion rates of various chemical components such as carbon, chromium, tungsten and phosphorus in steel is called segregation.
The existence of segregation will make it difficult for deformation and processing in the future. Sulfur segregation is easy to produce hot embrittlement and phosphorus segregation is easy to produce cold embrittlement. The existence of metal segregation is easy to cause fatigue fracture.Low Magnification Defect of Bearing Steel
⑥ Porosity: the small pores caused by volume shrinkage during the condensation of liquid steel are called porosity. The dispersed fine pores are called general porosity. The small pores distributed in the center of the steel are called central porosity. Porosity reduces the density of steel, significantly reduces the mechanical properties and reduces the service life of bearings.Low Magnification Defect of Bearing Steel
Due to the relative movement of the rolling element and the raceway and the invasion of pollutants and dust, the surfaces of the rolling element and the raceway are worn. When the wear is large, the bearing, noise and vibration increase, which reduces the operation accuracy of the bearing, thus directly affecting the accuracy of some main engines. Therefore, the daily maintenance of bearings for some silent machines is mainly to prevent the invasion of pollutants and dust.Low Magnification Defect of Bearing Steel
Low Magnification Defect of Bearing Steel
1. Effect on bearing life
The cleanliness of the bearing has a great impact on the service life of the needle roller bearing. A special test has been carried out on the bearing, and the result is that the difference is several times or even dozens of times. The higher the cleanliness of the bearing, the longer the service life. The tests of others show that the lubricating oil with different cleanliness has a great impact on the service life of the ball bearing. Therefore, improving the cleanliness of the lubricating oil can prolong the service life of the bearing. In addition, if the dirt particles in the lubricating oil are controlled below 10um, the service life of the bearing will increase several times.Low Magnification Defect of Bearing Steel
2. Influence of noise on Vibration
The test results of the bearing test center show that the cleanliness seriously affects the vibration level of the bearing, especially in the high frequency band. The bearing with high cleanliness has low vibration speed, especially in the high frequency band. The impact of dust in needle roller bearing grease on noise has been tested. It is proved that the more dust, the greater the noise.Low Magnification Defect of Bearing Steel
3. The impact on lubrication performance the decline of bearing cleanliness will not only affect the formation of lubricating oil film, but also cause the deterioration of grease and accelerate its aging, thus affecting the decline of lubrication performance of grease.Low Magnification Defect of Bearing Steel