Heat treatment deformation

During the heat treatment of bearing parts, there are thermal stress and structural stress. Stress can be superimposed or partially canceled, which is complex and changeable, because it can change with heating temperature, heating rate, cooling method, cooling rate, shape and size of parts, so heat treatment deformation is inevitable. Knowing and mastering its changing law can make the deformation of bearing parts (such as the ellipse of the ferrule, the size increase, etc.) placed in a controllable range, which is beneficial to the production. Of course, the mechanical impact during the heat treatment will also deform the part, but this deformation can be reduced and avoided with improved operations.

Overheating

The overheating of the microstructure after quenching can be observed from the rough mouth of the bearing parts. To accurately judge the degree of overheating, the microstructure must be observed. If coarse acicular martensite appears in the quenched structure of GCr15 steel, it is a quenched superheated structure. The reason for the formation may be the overall overheating caused by the quenching heating temperature is too high or the heating and holding time is too long; it may also be due to serious banded carbides in the original structure, forming local martensitic needle-like thick in the low-carbon area between the two bands, localized overheating. The retained austenite in the superheated structure increases and the dimensional stability decreases. Due to the overheating of the quenched structure and the coarse crystals of the steel, the toughness of the parts will be reduced, the impact resistance will be reduced, and the life of the bearing will also be reduced.

Underheated

If the quenching temperature is too low or the cooling is poor, it will produce a trosteite structure in the microstructure that exceeds the standard, which is called underheating. Thermal structure, it reduces the hardness and wear resistance sharply, which affects the bearing life.

Soft spots

The phenomenon of insufficient local hardness on the surface of bearing parts due to insufficient heating, poor cooling, and improper quenching operations is called quenching soft spots. . Surface decarburization can also cause a serious decrease in surface wear resistance and fatigue strength.

Surface decarburization

During the heat treatment process of bearing parts, if they are heated in an oxidizing medium, the surface will oxidize and cause the carbon on the surface of the parts to be decarbonized. The mass fraction decreases, resulting in surface decarburization. The depth of the decarburized layer on the surface exceeds that of the final processed left-hand maid and is scrapped. Determination of the depth of the surface decarburization layer can be used in metallographic examination metallographic method and microhardness method.

Quenching cracks

The cracks formed by the internal stress of the bearing parts during the quenching and cooling process are called quenching cracks. The reasons for this crack are: due to the high quenching heating temperature or the rapid cooling, the thermal stress and the structural stress of the metal mass and volume change are greater than the fracture strength of the steel; the original defects of the working surface (such as surface fine cracks or scratches) marks) or internal defects of the steel (such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration during quenching; severe surface decarburization and carbide segregation; parts are insufficiently tempered after quenching Or not tempered in time; the cold stamping stress caused by the previous process is too large, forging folding, deep turning tool marks, sharp edges and corners of oil grooves, etc.

In a word, the cause of quenching cracks 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.