Induction Quenching Process For Small Diameter Internal Splines

Test objects and technical requirements

  The structure of the internal spline is shown in Figure 1, the modulus of the internal spline is m=3mm, the thermal retention size of the quenched internal spline is 69.38mm, and the material is 42CrMo.

Structure of inner spline shaft

FIG. 1 Structure of inner spline shaft

Technical requirements: spline hardness 40 ~ 45HRC, hardened layer depth > 1.2mm.

Induction coil design

(1) Induction copper tube selection

The heating efficiency of the square copper pipe is higher than that of the round copper pipe, so the square copper pipe is selected. Since the equipment frequency is around 10kHz, the corresponding thickness of the copper pipe is 1.5mm. Considering the small diameter of the internal spline, the size of the square copper pipe is 10mm×12mm×1.5mm.In order to improve the heating efficiency of the inductor, the circular direction of the square copper tube of the inductor for the internal spline was inset with a magnetic guide body. The thickness was selected as 0.2mm according to the selected frequency.

(2) Design of water spray holes

For general surface quenching, the spraying density is 0.01 ~ 0.015l /cm2•s, the diameter of the spraying liquid hole for scanning quenching is 2.5mm, the spacing of the holes is 3mm cross-distribution, and the Angle between the center line and the new axis of the spraying liquid hole is 45°.

(3) Design of outside diameter of inductor

When the inner hole is heated, the gap between the effective ring and the workpiece is generally 2.0mm ~ 2.5mm. Considering the small diameter of the inner spline, the gap is designed according to 3mm, and the effective ring diameter of the inner spline is selected to be 68mm. The structure of the internal spline sensor is shown in Figure 2.

Internal spline sensor structure

Figure 2 Internal spline sensor structure

Selection of process parameters

(1) Frequency selection

The primary principle for selecting the current frequency is permeable heating. That is, the penetration depth of the current d is greater than the depth of hardened layer Ds, and the penetration heating method is adopted. The thermal energy on the surface of the workpiece is mainly generated by eddy induction. Compared with the heat conduction heating method, this heating method is energy-saving, with high heating efficiency, and the surface superheat of the workpiece is small. When the depth of the hardening layer is 1.2mm, the frequency f=10kHz is generally selected.

(2) Selection of heating power

Theoretical calculation of the required heating power, P=P0 Dh, where P is the output power of the equipment (kW), P0 is the specific power of the equipment (kW/cm2), the coefficient is generally 0.6 ~ 2.0, h is the height of the inductor, D is the workpiece diameter (cm). According to experience, when the frequency is 10kHz, the coefficient is generally 1.6 ~ 2.0, and the power output value required by theoretical calculation is 44 ~ 55kW. During the test, the quenching test is carried out with process parameters in Table 1.During the test, the workpiece is first stopped heating for a period of time and then sprayed (3 ~ 5s), and then scanned and quenched upward at a certain speed.

PlanMovement speed (mm/min)Rotation speed (r/min)Heating power(%)Seting time(s)Quenching fluid concentration(%)



Table 1 Internal spline quenching process parameters

Experimental results and analysis

1. Problems exist in the design of the internal spline structure

Using the designed sensor internal spline to quenching, when adjusting the spray pressure sensors, a solution is found that heat quenching process parameters, on the premise of guarantee the spray pressure, quenching liquid can not discharge in time, influence the heating effect of spline, in order to solve this problem, at the bottom of the internal spline from 20 mm hole size increased to 30 mm after the structure is improved, ensures that the quenching liquid can be ruled out in time.

2. Test results

After the internal spline structure is improved, the above four schemes are used for heating and quenching, and the results are shown in Table 2.

PlanPower(kw)Hardness after quenching(HRC)Size after quenching (mm)Deformation (mm)
140Heating temperature low

Table 2 Internal spline quenching process parameters

3. Result and analysis

(1) Scheme 1 was adopted for the test. When the heating power was set at 50%, the actual output was 40kW. Due to the low power during induction heating, the heating temperature could not reach the temperature required by quenching.

(2) Plan II was adopted for the test. When the heating power was set at 55%, the actual output was 48kW. The visual induction heating temperature was about 800℃. The main reason for the insufficient quenching hardness is that the heating temperature of the internal spline is low. Although the heating power reaches the theoretical calculated power, the actual heating power required by the internal spline is higher than that of the internal hole because the heating surface area of the internal spline is larger.

(3) Scheme 3 was adopted for the test. When the heating power was set at 60%, the actual output was 60kW. The visual induction heating temperature was about 870℃. For the spline in this structure, because the wall is thin, the deformation is large during quenching. In order to ensure that the size meets the drawing requirements after quenching, it is recommended to increase the size of rod spacing to 0.2mm in subsequent processes.

(4) Plan 4 was adopted for the test. When the heating power was set at 70%, the actual output was 75kW. Due to the high heating power, the internal spline could not pass the cooling water within 5s when the heating was stopped, causing the sensor to burn out.


(1) In terms of internal spline structure design, in order to ensure that the cooling quenching fluid can be removed in a timely manner during heating, the size of the process hole of quenching fluid should be designed according to the amount of quenching fluid discharged by the heating spline inductor. Generally, it is recommended that the size of the process hole should exceed 30mm.

(2) In the theoretical calculation of the power needed for heating, the power needed for internal spline quenching is about 10% higher than the theoretical calculation. In the selection of technological parameters, a smaller power should be selected first for the test, and then gradually increase to the required power, so as to avoid the situation of burning inductor caused by the selection of too high heating power.

(3) The internal spline induced quenching deformation is relatively complex, which is related to the structure of the heating workpiece (the wall thickness of the quenching part), the sensor structure and the heating process parameters, etc. When each workpiece is quenched, the specific analysis should be made according to the specific workpiece, and a reasonable machining allowance can be determined only after multiple quenching tests.

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