1. Quenching cracks caused by defects in raw materials
If there are cracks on the surface and inside of the raw material, which are not found before the heat treatment, quenching cracks may form. Observed under a metallographic microscope, there are decarburized layers on both sides of the crack, and the grains of ferrite in the decarburized layer are coarse.
2. Quenching cracks caused by inclusions
If the inclusions in the parts are serious, it is easy to cause stress concentration, and cracks may occur during quenching.
3. Quenching cracks due to poor original structure
(1) If the microstructure of the steel has severe band-like segregation or severe chemical composition segregation, it will cause great structural transformation stress during quenching. Furthermore, the over-burning phenomenon is easy to occur in the carbide aggregates, which makes the parts prone to cracking.
(2) If the residual internal stress of the steel is large before quenching, it is easy to cause cracking during quenching. Parts in this situation tend to have coarse grains and Widmanners structures.
(3) If the parts need to be repaired after one quenching, and the structural stress is not eliminated before the second quenching, cracks may occur in the second quenching, and the cracks are often distributed along the first quenching layer.
4. Two kinds of quenching cracks caused by improper quenching temperature
(1) The indicated temperature of the instrument is lower than the actual temperature of the furnace, which makes the actual quenching temperature on the high side, resulting in overheated quenching and cracking of the parts. Coarse grains and coarse martensite exist in all microstructures cracked by superheat quenching, and the cracks produced mainly exist in the form of intergranular.
(2) The actual carbon content of the steel is higher than the content specified by the steel grade. If the original grade is quenched by the normal process, the quenching temperature of the steel is increased, so it is easy to cause the parts to overheat and grain grow, which makes the quenching process. The increased stress may cause quench cracking.
5. Quenching cracks caused by improper cooling during quenching
Due to improper cooling during quenching, quenching cracking accidents will also occur in the parts. For example, No. 45 steel tends to form quench cracks during quenching. Especially when the carbon content is at the upper limit and the diameter of the part is 7-8mm, cracking is easy to occur. Therefore, it is extremely important to choose a suitable cooling medium during quenching. In addition, the structure of some parts is more complex, and the cross-sectional size changes greatly. If the coolant is not selected properly, the thin wall parts are likely to cause stress concentration and lead to quenching cracks.
6. Quenching cracks caused by machining defects
Due to poor machining, deep and coarse tool marks are left on the surface of the parts, and during quenching and cooling, stress concentration is caused there and cracks are caused.
7. The influence of part shape on quenching cracks
If the geometry of the parts is unreasonable or the thickness of the transition zone of the section varies greatly, cracks are likely to occur due to stress concentration during quenching. In addition, if the forging streamlines of the parts are poorly distributed, quenching cracking defects may also be caused during quenching.
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