On 2018-08-08 14:48:54
heat treatment common problems
Overheating the microstructure after quenching can be observed from the rough port of bearing parts. However, to determine the degree of overheating, the microstructure must be observed. If there is coarse acicular martensite in the quenched structure of GCr15 steel, it is the quenched overheated structure. The reason may be the overall overheating caused by too high quenching temperature or too long heating and holding time, or it may be the local overheating caused by the formation of local martensite needle like coarseness in the low carbon area between the two belts due to the serious banded carbide of the original structure.
The amount of retained austenite increased and the size stability decreased. As the quenched structure is overheated and the crystal of the steel is coarse, the toughness of the parts will be reduced, the impact resistance will be reduced, and the life of the bearings will also be reduced. Overheating can even cause quenching cracks. If the quenching temperature is too low or the cooling is not good, there will be more than the standard troostite structure in the microstructure, which is called the under heat structure. It will reduce the hardness, the wear resistance and the bearing life of the roller parts.
When the quenching crack is high or the cooling is too fast, the structure stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; when the quenching, the original defects (such as surface micro cracks or scratches) on the working surface or internal defects (such as slag inclusion, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) of the steel form the stress concentration; when the quenching, the serious surface decarburization and carbide bias occur Analysis; the parts are not tempered enough or not tempered in time after quenching;
The cold punching stress caused by the previous process is too large, forging folding, deep turning tool marks, sharp edges and corners of oil groove, etc. In a word, the reason of quenching crack may be one or more of the above factors, and the existence of internal stress is the main reason of forming quenching crack. The quenching crack is deep and slender, the fracture surface is straight, and the fracture surface has no oxidation color. It is usually a longitudinal straight crack or annular crack on the bearing ring; the shape of the bearing ball is S-shaped, T-shaped or annular.
The microstructure of quenching crack is characterized by no decarburization on both sides of crack, which is obviously different from forging crack and material crack. During the heat treatment of the deformed Nachi bearing parts, there are thermal stress and structural stress. This kind of internal stress can be superposed or partially offset with each other, which is complex and changeable. Because it can change with the change of the heating temperature, heating speed, cooling mode, cooling speed, shape and size of the parts, the heat treatment deformation is inevitable.
Knowing and mastering its changing law can make the deformation of bearing parts (such as the ellipse of ferrule, 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. During the heat treatment of bearing parts with surface decarburization, if heated in oxidizing medium, the surface will be oxidized, which will reduce the mass fraction of carbon on the surface of the parts and cause the surface decarburization.
If the depth of decarburization layer exceeds the final machining allowance, the parts will be scrapped. The measurement of the depth of the decarburized layer on the surface can be done by metallography and microhardness. According to the measurement method of microhardness distribution curve of surface layer, it can be used as arbitration criterion. The phenomenon of insufficient hardness on the surface of roller bearing parts caused by insufficient heating, poor cooling and improper quenching operation is called quenching soft spot. Like surface decarburization, it can cause serious decrease of surface wear resistance and fatigue strength.