Quenching cracks are the most common defect in the heat treatment process. Often times the product is finished and sent for heat treatment. When you come back, you will see that all have been cracked. All the previous procedures were wasted,The delivery time is also not up to date, and the consequences for us are very serious, and it is hard to say when facing customer’s accusations. At this time, we often attribute the responsibility to the heat treatment manufacturer, and all think that the heat treatment manufacturer has not controlled the process well so that have the problem. In fact, the real reason for heat treatment cracks is that hidden dangers are buried in the design stage. If you want to truly prevent cracks from occurring, you have to consider it from the design stage. But many machining companies are not qualified to make demands from customers. Then we can only avoid it from the craftsmanship. First, let’s understand the cause of cracks:
First: Material reasons.
1. The carbon content of the material. The higher the carbon content, the easier it is to crack during heat treatment. This point needs to be circumvented by design.
2. Alloying elements also have an effect on cracks, which are mainly reflected in the influence of hardenability, MS point, grain size growth tendency, and decarburization. Generally speaking, hardenability increases and Quenching cracks increases, but it can be neutralized with a quenching with weak cooling capacity. When designing and selecting materials, overheating sensitivity should be considered. Steels that are more sensitive to overheating are prone to cracks, so they should be avoided as much as possible.
Two: Reasons for the structural design of parts
1. Parts with uneven cross-sectional dimensions and sharply changed cross-sectional dimensions will crack due to internal stress concentration during heat treatment. When the
design is unavoidable, only craftsmanship can be used to avoid cracks. Reserve a step before heat treatment. After heat treatment, it will be processed to the drawing requirements.
2. Sharp edge. It is also the cause of cracks. When it is unavoidable in design, R can only be increased excessively at the sharp corner stress concentration. then do re-machining.
3. Thin-walled holes. In particular, there are thin-walled holes on the square parts, and other position are thick, only the hole wall is thin, the heating rate is inconsistent, and the cooling rate is inconsistent, which is most likely to cause cracks. When making the process, you should pay attention to the thickness that can be increased as much as possible and then wire cutting or other methods to remove it. The holes that cannot be increased can be made in correct after rough machining.
Four: Heat treatment process arrangement. Many materials need to be pre-heat treated. It can not save process. For example, for 45# steel, after the rough machining is completed, leave the margin to do a normalizing. After returning to work and then do the desired hardness, the probability of cracks will decrease a lot. I have personally verified many times and the effect is very good. It’s just the extra cost of normalizing.
Considering the above reasons, We should be done cracking prevention measures according to the specific parts material, shape and other characteristics, then the probability of basic crack will be reduced accordingly.
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