The common quality defects of bearing parts after heat treatment are: quenching microstructure Quality Defects overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots and so on.
The microstructure overheating after quenching can be observed from the rough mouth of bearing parts. Quality Defects However, the microstructure must be observed to determine the degree of overheating.
If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is quenched superheated structure. The cause may be the Quality Defects overall overheating caused by too high quenching heating temperature or too long heating and holding time;
It may also be due to the serious banded carbide in the original Quality Defects structure and the formation of local martensite acicular coarseness in the low-carbon region between the two bands, resulting in local overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases.
Due to the overheating of quenching structure and the coarseness of steel crystal, the toughness of parts will be reduced, the impact resistance will be reduced, and the service life of Quality Defects bearings will also be reduced. Serious overheating may even cause quenching cracks.
2. Under heating
Low quenching temperature or poor cooling will produce troostite Quality Defects structure exceeding the standard in the microstructure, which is called underheated structure. It will reduce the hardness, sharply reduce the wear resistance and affect the service life of the bearing.
3. Quenching crack
The crack formed by internal stress during quenching and cooling of bearing parts is called quenching crack. The reasons for this kind of crack are as follows: because the Quality Defects quenching heating temperature is too high or the cooling is too urgent,
the structural stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; The original Quality Defects defects on the working surface (such as surface micro cracks or scratches) or internal defects of steel
(such as slag inclusion, serious non-metallic inclusions, white spots, Quality Defects shrinkage cavity residue, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation; The tempering of parts after quenching is insufficient or not tempered in time;
Excessive cold punching stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and 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 cause of quenching crack.
The quenching crack is deep and slender, the fracture is straight, and the fracture surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; The shape on the bearing steel ball can be S-shaped, T-shaped or ring-shaped.
The microstructure characteristic of quenching crack is that there is no decarburization on both sides of the crack, which is obviously different from forging crack and material crack.
During the heat treatment of bearing parts, there are thermal stress and structural stress. This internal stress can be superimposed or partially offset each other, which is complex and changeable. Because it can change with the changes of heating temperature, heating speed,
cooling mode, cooling speed, part shape and size, heat treatment deformation is inevitable. Understanding and mastering its change law can put the deformation of bearing parts (such as the ellipse of ferrule, size increase, etc.) in a controllable range, which is conducive to production.
Of course, mechanical collision during heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improved operation.
5. Surface decarburization
During the heat treatment of bearing parts, if they are heated in oxidizing medium, the surface will be oxidized, reducing the mass fraction of carbon on the surface of parts and causing surface decarburization. If the depth of surface decarburization layer exceeds the allowance of post-processing, the parts will be scrapped.
The depth of surface decarburization layer can be measured by metallographic method and microhardness method. The measurement method of microhardness distribution curve of surface layer shall prevail and can be used as arbitration criterion.
6. Soft spot
The phenomenon of insufficient local hardness on the surface of bearing parts caused by insufficient heating, poor cooling and improper quenching operation is called quenching soft spot. Like surface decarburization, it can cause a serious decline in surface wear resistance and fatigue strength.