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Quality defects often seen in imported bearing parts after heat treatment
Source: China Bearing Network Time: 2014-09-20
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1. Overheating The microstructure of the imported bearing parts can be observed to be overheated after quenching; however, it is necessary to observe the microscopic arrangement to accurately distinguish the level of superheat. If coarse needle-shaped martensite appears in the quenching arrangement of gcr15 steel, then Arrangement for quenching and overheating. The cause of the quenching can be the detailed overheating caused by the quenching heating overheating or the heating and holding time is too long. It can also be because the original arrangement of the banded carbide is severe; the low carbon zone between the two zones constitutes some horses. The acicular body is greatly enlarged; the composition is somewhat overheated. The retained austenite is increased in the superheating arrangement; the dimensional invariance is reduced. The quenching arrangement is overheated; the crystal grain of the steel is greatly enlarged; the tolerance of the part is lowered; The impact resistance is reduced; the life of the bearing is also reduced. The overheating is severe and may even constitute a quenching crack.
2. If the under-heat quenching temperature is low or the cooling is poor, a tortite arrangement exceeding the standard rule occurs in the micro-arrangement; it is called an under-heating arrangement; it reduces the hardness; the wear resistance is drastically reduced; affecting the bearing life.
3. Quenching cracked imported bearing parts are called quenching cracks due to internal stress in the quenching cooling process. The causes of such cracks are: due to quenching heating, excessive temperature or too fast cooling; thermal stress and metal mass volume change Arrange the stress greatly greatly to the fracture strength of the steel, the original defects of the mission surface (such as surface cracks or scratches) or the internal defects of the steel (such as slag inclusion, severe non-metallic inclusions, white spots, shrinkage) The remaining holes, etc.) constitute stress gathering during quenching, severe surface decarburization and carbide segregation, tempering after part quenching or lack of real-time tempering, the cold pressing stress of the previous process is too large, casting Folding, deep turning tool marks, oil groove Sharp edges, etc.. The cause of quenching cracks can be one or more of the above factors; the existence of internal stress is the primary cause of quenching cracks. The quenching cracks are deep and long; The fracture is straight; the fractured section has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring, and the shape on the bearing steel ball is s-shaped, t-shaped or ring-shaped. Characterized in that arranged on both sides of crack no crack decarburization; clear differences casting crack and crack materials.
4. Thermal treatment of deformed imported bearing parts during heat treatment; there are thermal stresses and arrangement stresses; such internal stresses can be superimposed or partially offset; they are complex and variable; because they can follow heating temperature, heating rate, cooling method, cooling Speed, part shape and size change; so heat treatment deformation is inevitable. The viewpoint and master the transformation rules can make the deformation of the bearing parts (such as the ellipse of the ring, the scale is greatly increased, etc.) Controllable restrictions; facilitate the production. Of course, mechanical bumps in the heat treatment process will also deform the parts; but this deformation can be reduced and avoided with improved operation.
5. Surface decarburization imported bearing parts are in the process of heat treatment; if it is heated in oxidizing medium; oxidation occurs on the surface to reduce the mass fraction of carbon on the surface of the part; surface decarburization is formed. The depth of the surface decarburization layer exceeds the end processing The amount of waste will make the parts invalid. The determination of the depth of the surface decarburization layer can be used in the metallographic examination. The metallographic method and the microhardness method can be used. The surface layer microhardness distribution curve measurement method is preferred; the ruling criterion can be made.
6. Soft spots are lacking due to heating; poor cooling; poor quenching operation, etc. The phenomenon of insufficient hardness on the surface of the bearing parts is called quenching soft point. It can form the same surface wear resistance and embarrassing strength as the surface decarburization.
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