1. Methods to prevent quenching cracks
(1) Reasonably design the shape of parts. Try to use equal wall thickness design to avoid sudden changes in cross-sectional shape: ①Don’t make parts with large wall thicknesses into one piece, but use a mosaic structure so that each piece has the same strength as possible; ②The thick parts should be designed as hollow as possible Structure; ③Open process holes when necessary; ④Avoid sharp corners at the corners of the contour, and add a larger four-corner radius R as much as possible, and the fillet radius of heat-treated tool steel parts should not be less than 2mm.
(2) Strictly control the heat treatment process. ①According to the size, shape, characteristics and material of the part, the correct heat treatment specifications should be formulated, and the heating speed, temperature, and time should be selected reasonably; ②Perfect temperature control measures should be taken to grasp the real temperature of the part in real time, and the temperature control device should be checked regularly , Strictly prevent overheating caused by thermocouple aging or inaccurate placement position.
2. Methods to prevent tempering cracks
Tempering crack refers to: the structure of the quenched steel Martens part is in an expanded state, and the first shrinkage occurs at about 100 ℃ during tempering. At this time, if the quenched part is rapidly heated, the surface of the part will shrink, while the interior is in an expanded state, resulting in Cracked. The measure to prevent this kind of cracking is to not perform rapid heating before the parts are heated to 300°C.
Tempering and quenching also produce tempering cracks. This is because the retained austenite in the quenched structure becomes the same state as quenched when it is quenched. The method to prevent such cracking is to perform air cooling from the tempering temperature.
3. Methods to prevent spontaneous cracks
Because the surface structure of the quenched parts is retained austenite, when placed at room temperature, the transformation of austenite into martensite will cause phase transformation and expansion, which will cause the parts to crack. The measures to prevent this kind of cracking are: tempering the parts immediately after quenching. *The time interval between quenching and tempering should not exceed 3h. If tempering is not possible on the same day, the parts should be kept in a furnace or water at 100°C for heat preservation until the normal tempering treatment is performed the next day.
4. Measures to prevent grinding cracks
Grinding cracks are mainly caused by grinding heat. The reason is the same as that of tempering cracks. It is the cracking caused by the shrinkage of the surface quenched structure by the grinding heat. The direction of cracking is generally at right angles to the grinding direction. The preventive measures are: ①Before grinding the quenched parts, pre-process the parts with low temperature tempering at 150℃ or high temperature tempering at around 300℃ (when the grinding volume is large); ②Strictly perform grinding wheel dressing and maintain The grinding tool is sharp, and the grinding wheel is strictly prevented from clogging to reduce the grinding heat; ①Select the grinding wheel particle size reasonably, and the processing practice shows that choosing a coarser particle size grinding wheel can reduce the occurrence of cracks; ④Select the appropriate grinding amount, use a higher grinding wheel speed, and increase Workpiece speed, proved by practice: it can avoid grinding burns and reduce the probability of workpiece cracking.
5. Methods to prevent decarburization and cracking
The reason for the decarburization is that the heating temperature is too high or the heating in the air does not take proper protection. To prevent decarburization, vacuum furnace can be used to heat the workpiece or the workpiece can be heated in a protective atmosphere to strictly control the temperature to prevent overburning.
6. Measures to prevent low temperature cracks
Since the residual stress of the quenched parts will increase during cold treatment, resulting in low-temperature cracking, therefore, a low-temperature tempering process should be added before cold treatment to reduce the occurrence of cracks.
7. Measures to prevent cracks in EDM
The instantaneous high temperature and rapid cooling of electric discharge machining can easily form micro-cracks on the surface of the quenched part. The method to prevent it is to use a smaller electrical gauge for processing (the processing speed will be affected), or to perform necessary polishing on the deteriorated layer after processing deal with.