1) Heat treatment deformation
When the bearing parts are heat treated, there are thermal stress and tissue stress.
This internal stress can be superimposed or partially offset, which is complex and variable.
Since it can vary with heating temperature, heating rate, cooling method, cooling rate, part shape and size, heat treatment deformation is inevitable.
Knowing and mastering the law of change can make the deformation of the bearing parts (such as the ellipse of the ferrule, the size of the ferrule, etc.) in a controllable range, which is conducive to the production.
Of course, mechanical collisions during heat treatment can also cause deformation of the part.
However, this deformation can be reduced and avoided with improved operation.
Overheating of the microstructure after quenching can be observed from the rough mouth of the bearing part.
However, the exact microstructure must be observed to determine the degree of overheating.
If coarse needle-shaped martensite appears in the quenched structure of GCr15 steel, it is quenched and superheated.
The formation may be caused by excessive heating of the quenching heating temperature or excessive heating and holding time;
It may also be due to the fact that the original structure of the banded carbide is severe, and the localized martensite needle-like coarseness is formed in the low carbon zone between the two zones, causing local overheating.
The retained austenite increases in the superheated structure, and the dimensional stability decreases.
Due to the overheating of the quenched structure, the coarse crystal of the steel causes the toughness of the part to decrease, the impact resistance is lowered, and the life of the bearing is also lowered.
Low quenching temperature or poor cooling will produce a tortite structure in the microstructure that exceeds the standard, called underheated structure.
It reduces hardness and sharply reduces wear resistance, which affects bearing life.
4) The soft point
The phenomenon that the surface hardness of the bearing parts is insufficient due to insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft point.
Surface decarburization can cause a serious drop in surface wear resistance and fatigue strength.
5) Surface decarburization
During the heat treatment process of the bearing parts, if it is heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the part, resulting in surface decarburization.
The depth of the surface decarburization layer exceeds the final processing allowance and the part is scrapped.
Determining the depth of the surface decarburization layer The metallographic method and the microhardness method can be used in the metallographic examination.
6) Quenching crack
The crack formed by the internal stress of the bearing part during the quenching and cooling process is called quenching crack.
The causes of this crack are:
Since the quenching heating temperature is too high or the cooling is too fast, the microstructure stress when the thermal stress and the metal mass volume change is greater than the fracture strength of the steel;
The original defects of the working surface (such as surface cracks or scratches) or internal defects of the steel (such as slag inclusions, severe non-metallic inclusions, white spots, shrinkage residuals, etc.) form stress concentration during quenching;
Severe surface decarburization and carbide segregation;
Insufficient tempering or quenching in time after quenching of parts;
The cold punching stress caused by the previous process is too large, forging and folding, deep turning tool marks, sharp edges and corners of oil grooves, etc.
In short, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main reason for the formation of quenching cracks.
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