Quenching cracks are cracks that occur during quenching or after quenching at room temperature. The latter is also called an aging crack. There are many reasons for quenching crack. When analyzing quenching crack, it should be distinguished according to the characteristics of the crack.
There are several common quenching cracks:
1. Longitudinal crack
As shown in Fig. 1, also known as axial crack, is a typical crack caused by organizational stress (tangential stress). The crack cracks inward from the surface and is deep and long. Occurs in a quenched workpiece. Defects such as severe banded carbide segregation or longitudinal nonmetallic inclusions increase the susceptibility to longitudinal crack formation.
2. Transverse crack or arc crack
As shown in Fig. 2, cracks often occur at the sharp corners of the workpiece. Unquenched high carbon steel parts or carburized parts are easy to produce peak tensile stress in the transition zone, such cracks often initiate in a certain depth of the surface or inside the workpiece. When there are soft points on the hardened steel parts, it is easy to form fine arc-shaped cracks.
3. The inner hole longitudinal crack
When the hardenability of the steel is large enough, the internal stress on the surface of the inner hole is mainly the organizational stress, and the tangential tensile stress is large, so it is easy to form cracks along with the longitudinal distribution on the inner hole wall, which is radially seen from the end face, as shown in Fig. 3.
4. Section thickness gap caused by quenching crack
During cooling, the time difference of martensitic transformation in the part with great thickness difference is large, resulting in the formation of large structural stress, resulting in the generation of cracks, as shown in Figure 4:
5. Cracks caused by stress concentration
When there are sharp angles and gaps on the steel parts, it is easy to cause stress concentration and crack during quenching. Especially under the combined action of stress concentration and sharp change in section size, the danger of quenching is greater. As shown in Figure 5, the root of the flange with 3mm thickness is easy to crack.
6. Network crack
This crack has an arbitrary direction independent of the shape of the workpiece, as shown in Fig. 6. The depth of reticular crack is generally in the range of 0.01-0.15mm, which is a kind of surface crack. Reticular crack is easily formed on the surface of high carbon tool steel and alloy tool steel after decarburization and quenching.
7. Cracks caused by defects of raw materials
Slag inclusion in raw materials, mesh carbides, surface folding during plastic forming, and overheated structures during heating may all be referred to as crack sources. When quenching, it will be exposed or further expanded. To solve this kind of crack, we should start by controlling the quality of raw materials before quenching.