The microstructure overheating after quenching can be observed on the rough mouth of fag imported bearing parts. However, the microstructure must be Heating Problem 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 overall overheating caused by too high Heating Problem quenching heating temperature or too long heating and holding time;
It may also be due to the serious banded carbide in the original structure and the formation of local martensite acicular coarseness in the low-carbon Heating Problem region between the two bands, resulting in local overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases.
Due to overheated quenching structure and coarse crystal of steel, the toughness and impact resistance of parts will be Heating Problem reduced, and the service life of fag imported bearings will also be reduced. Severe overheating may even cause quenching cracks
Low quenching temperature or poor cooling will produce troostite structure exceeding the standard in the microstructure, which is called underheated structure. It will reduce the hardness and Heating Problem sharply reduce the wear resistance, and affect the service life of fag imported bearings
The crack formed by internal stress during quenching and Heating Problem cooling of fag imported bearing parts is called quenching crack.
The reasons for this kind of crack are as follows: because the Heating Problem 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;
(such as surface micro cracks or scratches) or internal defects of steel (such as slag inclusion, Heating Problem serious non-metallic inclusions, white spots, shrinkage cavity residue, etc.) form stress concentration during quenching; Severe surface decarburization and carbide segregation;
The tempering of parts after Heating Problem quenching is insufficient or not tempered in time; Excessive cold punching stress, forging folding, deep turning tool marks, sharp edges and corners of oil groove caused by the previous process.
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 fag imported bearing ring; The shape of fag imported 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.
When fag imported bearing parts are heat treated, 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. Knowing and mastering its change law can put the deformation of fag imported bearing parts (such as the ellipse of ferrule, 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 this deformation can be reduced and avoided by improved operation.
If fag imported bearing parts are heated in oxidizing medium during heat treatment, oxidation will occur on the surface, 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.
The phenomenon of insufficient surface hardness of fag imported 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.