Common Quenching Cracks In Bearing Parts And Prevention Measures

(1) Cracks caused by quenching overheating: The quenching heating temperature is too high and the holding time is too long, which causes the austenite grains to become coarse, which increases the brittleness of the martensite after quenching, reduces the strength and causes cracks. The crack characteristics are: fine cracks along the circumferential direction of the ferrule, often occurring at the junction of thickness and thickness.

(2) Cracks caused by excessive cooling rate. Parts are cooled in a medium with an excessive cooling rate or dropped into an oil tank with water at the bottom during quenching. Due to the excessive cooling rate, the tissue stress is significantly increased and cracks are formed. This kind of crack often occurs at the junction of thickness and thickness.

(3) Cracks caused by the original stress before quenching. If the cold working stress is not fully eliminated before the parts are quenched or the previous quenching stress is not removed before the parts are repaired, then these unrelieved stresses will be superimposed with the quenching to cause cracks.

(4) Cracks caused by stress concentration. Cracks are formed due to stress concentration during processes such as too deep ring typing, too deep turning marks, too deep (sharp) oil grooves, and steel ball filing fatigue.

(5) Cracks caused by material defects. Material defects in steel, such as looseness, white spots, pores, inclusions, and uneven distribution of carbides, will cause quenching stress concentration and produce quenching cracks.

(6) Cracks caused by surface decarburization. Surface decarburization not only reduces the surface strength of the part, but also makes the Ms point temperature of the surface layer and the core different. The different transformation times of martensite during cooling cause greater internal stress and produce discontinuous fines. Not deep network quenching cracks.

(7) Cracks caused by failure to temper in time after quenching. Under the long-term action of quenching stress, the fracture strength of quenched martensite decreases with time. Therefore, if the quenched parts are not tempered in time, cracks will occur.

(8) Cracks caused by impact: After the ferrule is quenched, the oil outlet temperature is high. If it is cleaned immediately or is impacted before tempering, wide and neat penetrating cracks will occur along the longitudinal direction due to excessive quenching stress and mechanical impact force.

In order to prevent the occurrence of quenching cracks, the following measures should be taken according to the causes:

(1) Strengthen raw material acceptance inspection and strictly control steel quality.

(2) Choose a reasonable quenching temperature and holding time to prevent the workpiece from overheating, especially for fine annealed structures and secondary quenched parts. Pay more attention to this.

(3) Choose the appropriate cooling medium and cooling method to prevent water from being mixed in the quenching oil (the moisture content in the quenching oil is less than 0.1%), and the temperature of the quenching cooling medium should be controlled (the temperature of the quenching oil is around 90°C); for products that are prone to cracking Complex parts with thick walls are quenched in stages.

(4) It should not be left alone after quenching or cold treatment, especially the parts that have been quenched twice should be tempered immediately after quenching, and the tempering should be sufficient.