The steering knuckle is quenched by medium frequency induction heating machine, the defect analysis and process improvement of quenching cracks

Steering knuckles are subject to huge friction during work, therefore, high hardness, high wear resistance and long service life are required. We usually use medium frequency induction heating machine to quench and heat treat the workpiece. However, due to the influence of many factors, the steering knuckle will produce quenching crack defects. The failure and damage of the steering knuckle not only causes failures in the operation of the car, but also is prone to serious safety accidents. Therefore, its quality becomes more and more important. Today, Zhengzhou Gou's technicians will analyze its quenching crack defects and how to improve its process.

Steering knuckle workpiece material is usually 40Cr steel or 40MnB steel. Technical requirements The surface hardness after quenching is 52-63HRC, the hardness of the unhardened area is 241-285HBW, the hardening degree of journal, fillet and end face is 3-6mm, and the structure after quenching and tempering is tempered sorbite grade 1-4. After quenching, the structure is acicular or fine acicular martensite, and the surface quality is excellent. We usually use medium frequency induction heating machine for quenching heat treatment, but found in production, some workpieces have circumferential cracks in the fillet area after medium frequency induction heating and quenching, the depth is about 1.2mm, accounting for about 98.3% of the number of cracks, and the other There are a small number of cracks appearing in the unconnected orifices, which are radial cracks, and the cracked parts cause about 1.28% of the waste.

The steering knuckle tempering treatment process is 860°C x 50min, cooling in 0.2% (mass fraction) polyvinyl alcohol aqueous solution, and then tempering at 600°C x 3h. The power of the mid-frequency induction heating machine is 100kW, and the frequency is 8000Hz. It can be seen from the location and depth of the knuckle cracks that the cracks on the surface of the workpiece are caused by the superposition of tissue stress and thermal stress in the workpiece after intermediate frequency induction heating and quenching, which makes the tensile stress concentrated, and the tensile stress exceeds the tensile strength of the material.

(1) The tensile stress generated in medium frequency induction heating and quenching is the result of the combined effect of tissue stress and thermal stress, and its peak value is near the inner and outer sides of the hardened layer or the transition layer area. The magnitude of the tensile stress is closely related to the temperature gradient during heating. The larger the temperature gradient, the narrower the transition layer and the higher the peak value of the tensile stress. Another influencing factor is the change in cross-sectional area of the workpiece. Stress concentration is formed during quenching at the change in cross-sectional area, which is prone to cracks and breaks the workpiece. The third influencing factor is the depth of the hardened layer. As the depth of the hardened layer increases, the peak value of the maximum tensile stress moves to the center of the workpiece, and the peak value decreases, and vice versa. The depth of the hardened layer is an important cause of quenching cracks. The fourth influencing factor is the hardening transition zone, which is often the peak of the tensile stress. Quenching defects and cracks often appear in the transition zone, and it is also an area where fatigue cracks frequently occur.

(2) Overheating at the non-through hole, and half of the inner side of the non-through hole is in the hardened area. Due to the structural characteristics, the temperature at the non-through hole is often high or overheated, which increases the quenching stress of the workpiece and reduces the strength of the overheated area, which is easy to form quenching cracks.

(3) Insufficient heating at the rounded corners causes the hardened layer at the rounded corners of the steering knuckle to be too shallow, and the peak value of the tensile stress is near the rounded corners, which is easy to cause cracks and cracks at the rounded corners after intermediate frequency induction heating and quenching.

After continuous analysis, we propose process improvement measures as follows:

(1) Improve the design of the inductor, so that the diameter of the small bottom of the conical surface in the effective circle is 66mm, and the gap between the inductor and the workpiece is 4-4.5mm, so as to ensure that the quenching temperature is suitable, and the non-through hole is not overheated, and the quenching layer is deep. 3-6mm.

(2) The chamfer of the non-through hole is increased, and after quenching and heating, it is air-cooled for 1.0-1.2s to reduce the temperature of the non-through hole and reduce the quenching stress.

(3) Optimize the quality of preparatory heat treatment. Uniform tempered sorbite should be obtained after quenching and tempering, which is conducive to reducing quenching stress and reducing the tendency of quenching cracking.

(4) Induction self-tempering is carried out immediately after medium-frequency induction hardening. After the workpiece stops spraying water to cool, the residual heat of the workpiece rises to 220-250°C, so that the hardened layer structure is self-tempering. The temperature is slightly higher but the time is short. The surface of the workpiece after treatment The hardness and wear resistance are good, the structure is stable and the peak value of the tensile stress is reduced, eliminating the hidden danger of workpiece cracking.

(5) The use of half-circle inductors can realize the overall induction hardening of the hardened area of the workpiece at the same time, instead of continuous induction hardening. The advantage is that the overall induction heating speed is slow, and the heating time is 4 times that of the latter, so the gradient of the workpiece is small, the width of the transition layer of the workpiece increases after quenching, the peak value of the tensile stress moves to the center of the workpiece, and the peak value of the tensile stress decreases, which is beneficial to the reduction of cracks in the workpiece cracking is very beneficial.

Various defects always occur in the workpiece during heat treatment, so it is very important to master the measures to prevent defects and improve the workpiece. Zhengzhou Gou's has been engaged in the production and sales of medium frequency induction heating machines for fifteen years, and has a good understanding of the heat treatment of workpieces. For more information about the heat treatment of workpieces, you are welcome to inquire.