Common problems with heat treatment
Superheat microstructure superheat 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 Cr15 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. Severe overheating may even cause hardening cracks. If the temperature of underheat quenching is low or the temperature of underheat quenching is poor, the microstructure will produce more than the standard tostenite tissue, called underheat tissue, which will reduce the hardness, wear resistance and affect the service life of roller parts. When the quenching crack is high or the cooling is too fast, the microstructural stress of the hot stress and metal mass volume change is greater than the breaking strength of steel.
The original defects of the working surface (such as micro-cracks or scratches on the surface) or internal defects of the steel (such as slag inclusion, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration when quenching; 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. The quenching crack is deep and slender, the fracture is flat and straight, and the fracture section has no oxidation color.
It is usually vertical flat crack or ring crack on bearing sleeve. The shape on the bearing steel ball is S, T or ring. There is no decarbonization on both sides of the crack, which is different from forging crack and material crack.
NACHI bearing parts during heat treatment, heat treatment deformation are 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. In the heat treatment process of surface decarbonization bearing parts, if heated in oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the surface of 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. The method of measuring surface microhardness distribution curve can be used as arbitration criterion.
The phenomenon that the surface hardness of roller bearing parts is not enough due to insufficient heating, poor cooling and improper quenching operation is called quenching soft spot. Like surface decarbonization, it can cause severe reduction in surface wear resistance and fatigue strength.