Common quality defects of parts after heat treatment are: overheating, underheating, quenching cracks, insufficient hardness, heat treatment deformation, surface decarburization, soft spots, etc. during quenching.

overheat

The overheating of the microstructure after quenching can be observed from the rough parts of the bearing parts, but the microstructure must be observed to determine the degree of overheating. If coarse needle martensite appears in the quenched structure of gcr15 steel, it is a quenched and superheated structure. The formation may be due to the overall overheating caused by too high quenching heating temperature or too long heating and holding time; it may also be due to the severe band carbide of the original structure, and the formation of local martensite needle-like coarseness in the low-carbon region between the two bands. Caused by local overheating.

Underheat

If the quenching temperature is too low or if the cooling is not good, the microstructure will produce a bainite structure over the standard, which is called underheated structure. It will reduce the hardness, sharply reduce the wear resistance, and affect the bearing life.

Quench crack

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 kind of cracks are: due to the high heating temperature or the rapid cooling during quenching, 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 on the working surface or internal defects of the steel are quenched Stress concentration occurs at the time; severe surface decarburization and carbide segregation; insufficient or untempered tempering after quenching; excessive cold stamping stress caused by the previous process, forging folding, deep turning tool marks, sharp edges of oil grooves Wait. In short, 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.

Heat treatment deformation

During the heat treatment of bearing parts, there are thermal stresses and tissue stresses. Such internal stresses can be superimposed or partially offset each other, which is complicated and changeable, because it can change with the heating temperature, heating rate, cooling method, cooling rate, and part shape And size changes, so heat treatment deformation is inevitable. Knowing and mastering its changing law can make the deformation of the bearing parts in a controllable range, which is beneficial to the production. Of course, mechanical collision during the heat treatment process will also cause deformation of the part, but this deformation can be reduced and avoided with improved operations.