Six key heat treatment technology problems in bearing manufacture

If the improper heat treatment, bearing heat treatment is the core of the bearing manufacturer, one of the key technology of heat treatment process and the physical properties of raw materials determines the bearing the final hardness, uniformity of hardness, abrasion resistance and contact fatigue life.What problems will arise?

Heat treatment deformation
Bearing parts in heat treatment, there is the thermal stress and organization stress, the internal stress can overlay or partially offset each other, is complex and changeable, because it can as the heating temperature, heating rate, cooling, cooling speed, parts of the change of the shape and size changes, so the heat treatment deformation is inevitable. Understanding and mastering the change rule of bearing parts can make the deformation of bearing parts (such as the ellipse of the ring, the size increase, etc.) in a controllable range, which is conducive to the production. Of course, mechanical collision during heat treatment will also cause deformation of parts, but such deformation can be reduced and avoided by improved operation.



Overheating
The microstructure overheating after quenching can be observed from the roughness of bearing parts.But the microstructure must be observed to determine the degree of overheating.If the coarse needle martensite appears in the quenching tissue of GCr15 steel, it is the quenching superheat tissue.The reason may be the overheat caused by too high quenching temperature or too long heating time.It may also be due to the severe carbonation of the original tissue ribbon, and the formation of local martensite needle thickness in the low carbon zone between the two bands, resulting in local overheating.The residual austenite in the superheated tissue increased and the dimensional stability decreased.Because the quenching structure is overheated and the steel crystal is thick, the toughness of the parts will be decreased, the impact resistance will be decreased, and the service life of the bearings will also be reduced.

Owe hot
Quenching temperature is low or poor cooling will generate more than standard stipulated in the microstructure of austenite organization, called owe hot organization, it decreased the hardness, wear resistance reduced sharply, affect the bearing life.

Soft point
Due to insufficient heating, poor cooling and improper quenching operation, the phenomenon of insufficient local hardness on the surface of bearing parts is called quenching soft spot.Surface decarbonization can also result in severe reduction in surface wear resistance and fatigue strength.

Surface decarburization
In the heat treatment process of bearing parts, if it is heated in oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of the parts, resulting in surface decarbonization.The depth of the decarbonizing layer on the surface exceeds the amount left in the final machining and the parts will be scrapped.The depth of surface decarbonization layer can be determined by metallography and microhardness.

Quenching crack
The cracks formed by internal stress during the quenching process of bearing parts are called quenching cracks.The reasons for this crack are as follows: because the temperature of quenching is too high or the temperature of quenching is too high or the temperature of cooling is too fast, the tissue stress of the change of thermal stress and metal mass volume is greater than the breaking strength of steel;The original defects on the surface of the work (such as surface microcrack or scratches) or steel internal defects such as slag, severe non-metallic inclusions, white spots, shrinkage residual, etc.) during quenching form stress concentration;Severe surface decarbonization and carbide segregation;Insufficient tempering or delayed tempering of parts after quenching;Cold impact stress caused by the previous process is too large, forging folding, deep turning tool marks, sharp corners of oil groove, etc.

In a word, 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 crack.