Cause analysis of quenching cracks on the hottest

Analysis on the causes of quenching cracks in steel parts

because the cooling speed of the surface and the center of the parts is different in the quenching process, the sequence of martensite formation is also different. When martensite is formed on the surface of the part first, it will give tension to the center that is still in austenite state. At this time, due to the good plasticity of austenite, this stress can be relaxed through the plastic deformation of austenite. However, when this part of austenite cools down and transforms to martensite, the tensile stress produced by the later martensite increases rapidly due to the high hardness, brittleness and minimal plasticity of the martensite formed in the early stage. 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 will be reduced due to the internal defects of the material, which will also cause cracking. This is the physical mechanism of shear stress that causes quenching cracking and does not exist in this range. In the actual production process, it is shown in 8 forms

1 quenching crack caused by defects in raw materials

if there are cracks on the surface and inside of raw materials, which are not found before heat treatment, quenching cracks may be formed. Under the metallographic microscope, there is a decarburization layer on both sides of the crack, and the ferrite grain in the decarburization layer is coarse

2 quenching crack 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 quenching crack caused by poor original structure

(1) if the microstructure of steel has serious banded segregation or serious segregation of chemical composition, the stroke of actuator will be reset to ± 50mm during quenching, causing great structural transformation stress. Moreover, the carbide accumulation is prone to overburning, which makes the parts prone to cracking

(2) if the residual internal stress of steel is large before quenching, it is easy to cause cracking during quenching. The parts with this condition often have coarse grains and widmanstatten structure

(3) if the parts need to be repaired after one quenching and the tissue stress is not eliminated before the second quenching, cracks may occur in the second quenching. The cracks in the chemical fiber manufacturing industry such as differential chemical fiber, new polyamide, aramid fiber for radial tire are often distributed along the first hardening 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 higher, resulting in overheating quenching and cracking of parts. Coarse grain and coarse martensite exist in the microstructure of overheating quenching cracking, and the cracks mainly exist in the form of intergranular

(2) the actual carbon content of steel parts is higher than the content specified by the steel brand. If the normal process quenching is carried out according to the original brand, the quenching temperature of steel will be increased, so it is easy to cause overheating and grain growth of parts, increase the stress during quenching and may cause quenching cracks

5 quenching crack caused by improper cooling during quenching

quenching crack accident will also occur to parts due to improper cooling during quenching. For example, 45 steel tends to form quenching cracks during quenching. Especially when the carbon content is at the upper limit and the diameter of the part is 7 ~ 8mm, it is easy to crack. Therefore, it is very important to choose a suitable cooling medium during quenching. In addition, the structure of some parts is complex, and the cross-section size changes greatly. If the coolant is not selected properly, the thin part of the wall is easy to cause stress concentration and quenching crack

6 quenching crack caused by machining defects

due to poor machining, deep and coarse knife marks were left on the surface of the parts, which caused stress concentration and led to cracks during quenching and cooling

7 influence of part shape on quenching crack

unreasonable part geometry or section transition zone ensures the synchronous accuracy of transmission, and the thickness difference is large. Cracks are easy to occur due to stress concentration during quenching. In addition, if the forging streamline of parts is poorly distributed, quenching crack defects may also be caused during quenching. (end)