Process analysis of heat treatment of starter brush spring using high-frequency induction heating machine

The starter brush spring is a part used to swing back and forth. It is required to have a certain hardness and good elasticity during the working process. The main form of failure during use is fracture. The brush spring is wound from T8A cold-rolled steel strip with a width of 6mm and a thickness of 0.5mm. In order to meet the working requirements of the starter brush spring, we often use high-frequency induction heating machines for heat treatment, and the effect is very good. Today, the editor will tell you about the high-frequency heat treatment process of the starter brush spring.

(1) Mechanical processing process of brush spring. The processing procedures of this type of small spring are: cold steel plate blanking - selection of molds - winding and forming - quenching - tempering - checking hardness and elasticity - shaping treatment - shot peening - rust-proof packaging. It adopts a heat treatment process after rolling. This is because the T8A brush spring itself is a slender strip. If it is heat treated first, it will be very difficult to roll and shape it, and it will be difficult to ensure its technical requirements.

(2) Heat treatment of brush spring

①Technical requirements. T8A thin cold strip has a hardness of 71.5-74.5HRA after heat treatment, no fracture, and no decarburization layer on the surface.

②Heat treatment process. According to the traditional heating temperature range of T8A steel, a high-frequency induction heating machine is used for heating at 790°C, oil cooling and quenching, and tempering at 330-350°C. After tempering, the hardness is qualified, but the elasticity is insufficient, and the brush spring breaks, etc. fatal flaw, so the correct heat treatment process should be explored.

From this point of view, according to the technical requirements of the T8A material brush spring, heat treatment should start from the direction of lowering the quenching temperature and increasing the tempering temperature. The choice of quenching cooling medium has an important impact on controlling the hardness and elasticity. The hot-out process that has been tested repeatedly is: heating with a high-frequency induction heating machine, the process parameters are (745-755) ℃ × (5-6) min, graded cooling in a nitrate solution of 160-180 ℃, and each brush spring is the technical indicators meet the requirements.

(3) Analysis and implementation key points of heat treatment process of brush spring

① T8A material with eutectoid composition is prone to overheating during quenching and heating, and its crystal grains are easy to grow even at 780-790°C, so its strength, toughness, plasticity, etc. are poor. Appropriately lowering the quenching temperature will reduce the tendency of oxidative decarburization, improved surface quality.

②Although the hardenability of thin steel sheets (O.5mm) made of T8A is worse than that of alloy steel, it can obtain higher quenching hardness using oil or nitrate salt, so using a lower quenching temperature reduces quenching overheating. The tendency is that the austenite grains are fine, and cryptocrystalline martensite is obtained after quenching, so the toughness of the brush spring is improved. In addition, the carbon content of the austenite dissolved in T8A is reduced, and the carbon content in the quenched martensite is reduced. The degree of supersaturation decreases, partially reducing the brittleness of martensite.

③ Using a lower quenching temperature reduces the temperature difference between the quenching heating temperature and the cooling medium, which is beneficial to reducing thermal stress and structural stress, and reducing brittleness.

④ Tests show that by using low quenching temperature, the martensite structure is obtained. By adjusting the subsequent tempering temperature, the hardness and elasticity of the brush spring can be changed. Therefore, it is possible to reasonably adjust the quenching temperature and tempering temperature. Better heat treatment process parameters are found, that is, the hardness decreases as the tempering temperature increases, but its elasticity increases and the fracture phenomenon is gradually eliminated. The quenching temperature is about 750°C and the tempering temperature is in the range of 380-400°C, which not only ensures that the hardness meets the requirements and its elasticity is within the specified range, but more importantly, the problem of brush spring breakage is completely solved.

⑤ From this point of view, low quenching temperature can appropriately improve the toughness of the brush spring. Increasing the tempering temperature can completely eliminate the structural stress after quenching. On the basis of ensuring the elasticity requirements, the reduction in hardness greatly improves the martensite. body toughness. This is quite useful for parts that require elasticity.