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:bearing 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 segregation is easy to cause metal fatigue fracture.Low Magnification Defect of Bearing Steel
Low Magnification Defect of Bearing Steel
(1) Remove the decorative cover and dust cover of the bearing head;
(2) Remove the tire nut and tire, and be careful not to damage the thread of the tire bolt. If it is a disc brake, remove the brake and then use it to remove the lock ring or lock pin.
(3) Remove the bearing with special tools;
(4) Scrape off the old grease in the bearing, journal and bearing cavity, clean the bearing and journal with cleaning agent and wipe dry with cloth, and then wipe the bearing cavity with cloth.
(5) Check the bearing and bearing race. If cracks, fatigue peeling and loose bearing rollers are found, replace the bearing. If pitting is found on the bearing race, the bearing should also be replaced.
(6) Check the fit between the inner diameter of the bearing and the journal. The fit clearance shall not be greater than 0.1omm. When measuring the journal, it shall be measured at the upper and lower parts of the vertical ground (where there is a large wear part). If the fit clearance exceeds the specified service limit, replace the bearing to restore the normal fit clearance. It is not allowed to reduce the clearance by burring and pitting on the journal.
(7) After all parts meet the requirements, apply grease to the inner bearing and put it into the bearing.
(8) When applying grease to the inner cavity of the bearing, pay attention to squeeze the grease into the bearing until the grease comes out from the other side of the bearing. Apply a thin layer of grease in the bearing cavity and the shaft head cover to prevent rust. Pay attention not to apply too much grease in the bearing cavity, otherwise it will affect heat dissipation and braking.
(9) Install the bearing and outer bearing back on the journal, screw the shaft head adjusting nut by hand, and then tighten the adjusting nut with the shaft head wrench according to the specified torque.
After tightening the nut, turn the bearing left and right for several turns to see the installation of the bearing; On the other hand, rotate to make the bearing and race fit correctly. At this time, the bearing tightness is appropriate, and the wheel rotates freely without axial clearance.Low Magnification Defect of Bearing Steel
(10) Install the locking plate, fixing nut, tire, dust cover, decorative cover and other parts in turn.
(11) After the bearing is adjusted, drive for a certain mileage (about 1okm) and stop for inspection to wipe the temperature of the bearing. If the heating is caused by too tight adjustment of the bearing, it should be readjusted and the bearing tightness should be properly relaxed.Low Magnification Defect of Bearing Steel
The necessity of bearing lubrication is also confirmed by the test and practice. The 42724 bearing is tested at the speed of 14.3t and equivalent to 140km / h. When 5g grease is added, the bearing can run for 7h, while when there is no grease, it can only run for 10min, and the service life difference is 42 times;
According to the statistics of the vehicle department, due to poor sealing and mixing with water and sand dust, the bearing lubrication is damaged, resulting in the replacement proportion of different types of bearings as high as 30-60%. The above situation fully shows that normal lubrication conditions are an important prerequisite for giving full play to the internal life of the bearing.
Taken together, rolling bearing grease will play the following essential roles:
1. Reduce friction. The lubricating grease forms an oil film between the rolling element and the inner and outer rings, and between the cage and the outer ring of the rolling element. This liquid lubricating film can reduce and prevent metal contact, which will greatly reduce the wear between the friction parts of the bearing and reduce the heat generated by their mutual friction.Low Magnification Defect of Bearing Steel
2. Abrasion resistance. Elastic deformation will occur between the rolling faces of the bearing under base load; When there is eccentric load in the bearing, there will be relative slip between the rolling working surfaces. The liquid lubrication film can greatly reduce the scratch caused by the above phenomena, so as to prolong the fatigue life of the bearing.Low Magnification Defect of Bearing Steel
3. Heat dissipation. Grease can dissipate the heat generated by rolling working face through rolling, flow and conduction.Low Magnification Defect of Bearing Steel
4. Shock absorption and noise reduction. The lubricating oil film can absorb part of the energy and partially convert the concentrated load into distributed load, which plays the role of shock absorption and noise reduction.Low Magnification Defect of Bearing Steel
5. Seal. The grease also prevents dust and moisture from invading the rolling working surface of the bearing. The bearing grease applied in some oil seals is completely for filtering.Low Magnification Defect of Bearing Steel
6. Rust and corrosion prevention. Rolling bearing grease is generally a good anti rust and anti-corrosion agent, which can prevent bearing corrosion.Low Magnification Defect of Bearing Steel