There are many reasons why steel parts produce scrap during the production process or fail during use, but the appearance of quenching cracks is an important reason.
Since the cooling rate of the surface and the core of the part is different during the quenching process, the sequence of forming martensite is also different. When martensite is formed on the surface of the part first, the core part that is still in austenite state is given tension. At this time, due to the good plasticity of austenite, this stress can be relaxed by plastic deformation of austenite .
However, when this part of austenite cools down and transforms into martensite, because the martensite formed earlier has high hardness, high brittleness, and extremely small plasticity, the tensile stress generated by the later formed martensite will increase rapidly. Big. Once the strength limit of the material is exceeded, it will cause cracking. In addition, even if the tensile stress does not exceed the strength limit of the material, the strength is reduced due to the internal defects of the material, which can also cause cracks. This is the physical mechanism that causes quenching cracking. In the actual production process, it manifests itself in 8 forms.
Quench 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 heat treatment, quenching cracks may form. Observed under a metallurgical microscope, there are decarburized layers on both sides of the crack, and the ferrite grains in the decarburized layer are coarse.
2 Quenching 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 Quench cracks caused by poor original organization
(1) If the steel microstructure has severe band segregation or severe chemical composition segregation, it will cause a great structural transformation stress during quenching. In addition, over-burning is prone to occur where carbides accumulate, which makes 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 with this condition often have coarse grains and Widmanstatten structure.
(3) If the parts need to be repaired after a 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 hardened 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 too high, causing overheating and quenching, resulting in cracking of the parts. All the microstructures cracked by overheating and quenching have coarse grains and coarse martensite, and the cracks produced mainly exist in the form of intergranular.
(2) The actual carbon content of steel parts is higher than the content specified by the steel grade. If the normal process is quenched according to the original grade, the quenching temperature of the steel is increased, so it is easy to cause overheating of the parts and grain growth, which will cause the quenching The increase in stress may cause quenching cracks.
5 Quenching cracks caused by improper cooling during quenching
Due to improper cooling during quenching, quenching cracks will also occur in parts. For example, No. 45 steel has a tendency to form quenching cracks during quenching. Especially when the carbon content is at the upper limit and the part diameter is 7-8mm, it is easy to crack. Therefore, it is extremely important to select a suitable cooling medium during quenching. In addition, the structure of some parts is more complicated, and the cross-sectional dimension changes greatly. If the coolant is not selected properly, the thin wall parts are likely to cause stress concentration and cause quenching cracks.
6 Quenching cracks caused by machining defects
Due to poor mechanical processing, deep and thick knife marks are left on the surface of the parts. During quenching and cooling, stress concentration in this place causes cracks.
7 The influence of parts shape on quenching cracks
Part geometry is unreasonable or the thickness of the cross-section transition zone is very different, and it is easy to crack due to stress concentration during quenching. In addition, if the forging streamline of the part is poorly distributed, it may also cause quenching cracks during quenching.
8 Cracking caused by not timely tempering
If the tempering cannot be performed in time after quenching, so that the structural stress cannot be eliminated in time, it may cause cracks due to excessive quenching residual stress. Especially for large-sized workpieces, although the surface has been cooled to room temperature after quenching, the core has not yet been completely cooled, and the core austenite structure is still transforming to martensite, and the stress is increasing, that is, the quenching process It is still going on inside the part, so that after a period of time at room temperature, the part cracks.