How can we prevent fracture defects caused by heat treatment of 35CrMo steel bolts using a medium frequency induction heating machine?

35CrMo steel bolts are used to connect the air conditioning compressor cylinder and cylinder head. The bolt blank is processed by cold heading and then threaded. The heat treatment process is quenching + tempering, and then bluing treatment is performed. The technical requirements are: hardness 32-38HRC. We often use a medium frequency induction heating machine for this heat treatment, but during production we found that some bolts broke at the thread root, causing the workpiece to be scrapped. Today, the editor will tell you about breakage defects in workpieces. How can we prevent them?

Metallographic examination revealed obvious cracks at the root and tip of the thread. Due to the high degree of stress concentration produced during thread processing and assembly, cracks can easily expand rapidly under the action of external forces, causing the workpiece to break and fail. Observation of the metallographic structure found that although the structure of the broken bolt after quenching and tempering was tempered sorbite, it was significantly affected by the orientation of the original martensite, which reduced the plasticity and toughness of the bolt material, increased brittleness, and increased the hardness of the workpiece. The workpiece hardness inspection confirmed this. The bolt hardness value is 37-41HRC, which is about 5HRC higher than the bolt technical requirement hardness value. High hardness is caused by insufficient or insufficient tempering.

Improvement and prevention measures are as follows:

1) After cold heading of bolts and before thread processing, it is advisable to supplement the tempering to eliminate residual stress in the workpiece and stabilize the structure, and prevent micro-cracks in the workpiece due to insufficient plasticity and toughness, which may cause crack propagation and breakage of the workpiece.

2) When using a medium frequency induction heating machine for tempering heat treatment, we should strictly check the tempering process and performance to avoid insufficient or insufficient tempering of the workpiece, which will cause the workpiece to have high hardness and increase brittleness, thereby accelerating crack expansion and making the workpiece brittle. fracture. It is also necessary to promptly check the hardness and other properties and take rescue measures.

Fracture is one of the common defects of workpieces during heat treatment. Unqualified raw materials and improper operation techniques may cause breakage defects in the workpiece and reduce the service life of the workpiece. In serious cases, it will cause the workpiece to be scrapped and cause a waste of resources. , Therefore, when we use a medium frequency induction heating machine for heat treatment, we should strictly operate the process.