Inspection analysis and preventive measures for sensor burnout of medium frequency induction heating machine

The rear axle half shaft of a car is an important part that drives the wheel movement. The half shaft mainly bears bending and torsional compound moments, and also bears a certain impact load. The half shaft is usually made of medium carbon alloy steel and is surface hardened by medium frequency induction heating and quenching after being integrally quenched and tempered in a medium frequency induction heating furnace. The depth of the hardened layer is 4-6mm. The hardness is 50-55HRC. During production, it was found that the burnt areas of the sensor were located at around 225°C of the induction coil, and expanded to both sides to form regular failure characteristics. When the axle shaft is heated in the inductor, the heat generated by the inductor has three main ways of dissipating heat in addition to heating the workpiece.

(1) Radiate heat to all sides. The induction coil is equipped with magnets, insulating paint and glass filaments. This form dissipates less heat and dissipates heat evenly, which has little impact on sensor burnout.

(2) The cooling water takes away heat, and this heat cools down the sensor, ensuring safe operation of induction heating. The test found that when the water inlet is between 0-180°, the water temperature is low, indicating that the heat exchange of the cooling water is good, the water flow is smooth, and the water temperature does not rise much; after exceeding 180°, due to the long water flow path, the water temperature increases greatly, and the water temperature The rise is due to the deterioration of heat exchange, which can easily cause the temperature of the next section of the sensor to rise, which is the main reason for the burnout of the induction coil.

(3) The heat of the induction coil is conducted to the link plate. The current density at the cheek plate is small and the water cross-sectional area is large. The low water temperature in this area causes the heat on the induction coil to be conducted to the link plate. At 0°-90° and 270°-360°, the induction coil is close to the link plate, and the heat conduction is good; while at 90°-270°, the induction coil is far away from the link plate, and the heat conduction effect is poor, causing the temperature of the induction coil in this area to rise. The height is relatively large, and it is easy for the induction coil to burn out in this section. 

After comprehensive analysis, we further conducted production tests on the relationship between workpiece induction heating water supply volume, induction heating power, sensor impedance and other process parameters, and based on this, we proposed the following technical measures to prevent the half-axis induction heating sensor from burning out:

(1) Requirements for safe operation of the sensor. The ratio of cooling water volume (KG/MIN) to equipment output power (KW) ≥ 0.2KG/(KW.MIN), or cooling water volume (KG/MIN) to sensor impedance (Ω) The ratio is ≥3.2KG/(Ω.min). If the above conditions are met, the sensor will operate safely and normally, but when it is lower than the above ratio, the life of the sensor will decrease.

(2) Inductor impedance <5Ω. When the water supply volume per unit surface area (cm2) of the induction coil is 5KG/(KW.CM2), the sensor is prone to burnout and should be avoided. When using impedance>Ω. When the water supply volume per unit surface area (cm2) of the induction coil and unit time (min) is 7KG/(KW.CM2.MIN), measure the cooling water outlet temperature to be 60°C. The sensor works safely and normally, so there will be no burnout, failure or damage, and the sensor has a long service life.