Because the parts in the quenching process, the surface and the heart of the cooling speed is not the same, so the formation of martensite is not the same. When the surface of the part first forms the martensite, it gives the tensile force to the heart part which is still in the austenite state, at this time, because the austenite plasticity is good, this stress can be loosened by the plastic deformation of the austenite. However, when this part of the austenite cooled down to the martensite transformation, due to the early formation of martensite hardness, brittleness, and plastic very small, so the formation of the martensite part of the resulting tensile stress rapidly increased. 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, but due to the internal defects of the material caused by the reduction of strength, will also cause cracking.
This is caused by the physical mechanism of quenching and cracking, in the actual production process, and specifically in 7 forms of expression.
1, the raw material has been defective and caused by the quenching crack If there is a crack in the raw material surface and inside, it is possible to form a quenching crack before heat treatment is found.
Under the Metallographic microscope, the two sides of the crack have decarburization layer, and the ferrite grains in the decarburization layer are coarse.
2, inclusions caused by the quenching crack
If the parts of the inclusions are serious, easy to cause stress concentration, quenching will likely produce cracks.
3, due to the poor original tissue caused by the quenching (1) If the microstructure of steel has serious banding segregation or serious segregation of chemical composition, it will cause great change stress in the tissue when quenching.
Moreover, the carbide gathering place is prone to excessive burning phenomenon, so that the parts are prone to cracking.
(2) If the steel in the residual stress before quenching, prone to cracking in quenching, the occurrence of the situation of the parts, there are often coarse grains, such as the organization of the Wei-Wei phenomenon.
(3) If the part is repaired after one quenching, before the second quenching and without eliminating the tissue stress, it is possible to produce a crack in the second quenching, and the crack is often distributed along the first hardening layer.
4. Two kinds of quenching caused by improper quenching temperature (1) The indicator temperature of the instrument is lower than the actual temperature of the furnace, so that the actual quenching temperature is high, resulting in overheating quenching, resulting in cracking of the parts.
There are coarse grains and coarse martensite in the microstructure of overheating quenching and cracking, and the resulting cracks are mainly in the form of crystal.
(2) The actual carbon content of steel is higher than the specified steel grade, if the original brand for normal process quenching, is equal to improve the steel quenching temperature, it is easy to cause the parts overheating and grain growth, so that when the stress increased when quenching may cause quenching cracking.
5. Quenching caused by improper cooling Quenching due to improper cooling, the parts will also cause quenching accident. For example, 45th Steel in quenching has the tendency to form quenching cracks. Especially when the carbon content is in the upper limit and the part diameter is 7~8mm, it is prone to crack. So it is very important to choose the proper cooling medium when quenching.
In addition, some parts of the structure is more complex, section size changes are large, if the coolant selection is not appropriate, the thin wall of the stress concentration can lead to quenching cracking.
6, machining defects caused by the quenching
Due to poor mechanical processing, the surface of the parts left a deep and coarse knife marks, in quenching and cooling, causing the stress concentration caused by cracks.
7. The influence of part shape on quenching crack The geometry of the part is unreasonable or the thickness of the transition zone of the section is large, the cracks are easily caused by stress concentration in the quenching, and if the forging streamline distribution of the parts is bad, it may cause the quenching crack defect in quenching.