Reasons for cracks in bolts quenched using medium-frequency induction heating power and their preventive measures

High-strength bolts are made of 35CrMo steel with a diameter of 24mm. In order to meet their working needs, they need to be tempered. The heat treatment is often carried out using medium frequency induction heating power. The bolt needs to have high mechanical properties, and oil quenching is used for the quenching and tempering process. The mechanical properties of the workpiece are not easy to achieve, so water quenching and oil cooling are used. During production, it was discovered that 40% quenching cracks occurred in a certain batch of workpieces during the water quenching and oil cooling treatment. Inspection and analysis found that the sulfur content in the chemical composition of the bolt exceeded the standard; metallographic examination found that there was severe segregation in the structure, with a banded ferrite morphology, and cracks extended along the banded ferrite.

Analysis suggests that the band-like structure is formed during rolling of raw materials, which is closely related to the high sulfur content in the workpiece and the fact that sulfide serves as the phase transformation core of ferrite. However, the preparatory heat treatment of the workpiece is not carried out before quenching and tempering to improve the defective structure, or the workpiece is forged to break up the sulfide and ferrite band-like redistribution, so that the workpiece obtains a good preliminary structure before quenching, thus causing batch quenching cracks during the quenching of the workpiece invalid.

In order to prevent the above defects, the improvement measures are as follows:

(1) Strictly inspect the quality of raw materials, do not allow impurities (such as sulfide, phosphide, etc.) to exceed the standard, and do not allow blanks with severe banded structures to enter the normal processing flow.

(2) For raw material blanks with high impurities or band-shaped raw materials, it is recommended to increase the forging process. The forging ratio should be higher to break up impurity compounds and band-like tissue distribution, so as to remove hidden dangers before quenching the workpiece.

(3) The structural defects of the above-mentioned raw materials can be reduced by adding a normalizing treatment before quenching with medium-frequency induction heating power supply. If normalizing is used as preparatory heat treatment, it must be tested according to the degree of defects before it can be used for production. For serious band-like structural defects caused by impurity segregation, normalizing treatment has little effect, but forging treatment has obvious effect.

Today, the editor briefly tells you the reasons and preventive measures for cracks caused by quenching bolts using medium-frequency induction heating power. I hope it will be helpful to everyone's work. However, the editor hopes that everyone can apply the theoretical knowledge they have learned to their work. Of course, if you have better measures, you can share them with everyone.