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Medium frequency induction hardening process optimization for camshaft

  This process test is under the existing production conditions, on the camshaft whose base circle radius is R80mm, using the profiling sensor, on the KGPS-250kW medium frequency power supply and KQCJC-1500 CNC hardening machine tool produced by our company. The cam profiles of the fuel injection camshaft section and the intake and exhaust camshaft sections are induction hardened. Through the optimization of the sensor and process control, the problems of the depth of the hardened layer (the depth of the top layer of the lift is too deep, and the depth of the base circle layer is too shallow), the soft zone in the transition zone, and the overheating and cracking of the top of the lift are solved.

1. Parts technical requirements

(1) Chemical composition The material of the camshaft parts is 50CrMo4 (German grade 1.7228), and its chemical composition is shown in Table 1.

Table 1: Chemical composition of 50CrMo4 steel (mass fraction ) %
Element C Si Mn Cr Mo P S Fe
Measured value 0.52 0.12 0.65 1.1 0.20 0.01 0.012 Surplus
DIN 10083 Standard 0.46~0.54 ≤0.40 0.50~0.80 0.90~1.20 0.15~0.30 ≤0.035 ≤0.035 Surplus

(2) Technical requirements for camshaft quenching The depth of the hardened layer on the cam profile is 5.5-8.5mm, and the surface hardness is 60-64HRC.

2. Problem description and solutions

  The depth of the hardened layer and surface hardness of induction hardening varies frequency unit area, shape, and size of the inductor, material, shape, and size of processed parts, matrix structure before treatment, heating method, heating time, coolant The main factors such as the type and cooling method vary.

  Usually, when the camshaft cam surface is induction hardened, the depth of the hardened layer at the cam lobe tip is too deep, and the depth of the hardened layer at the base circle of the cam is shallow; Quality problems such as uniformity and quenching cracks. To this end, we take the following measures to solve the above-mentioned possible problems.

  • Inductor structure optimization

  The structure of the sensor mainly includes the inner diameter of the effective circle of the sensor (determining the gap with the workpiece), height, and spray angle.

  According to the shape of the part, our company entrusts a professional sensor manufacturer to make a special profiling sensor, especially to increase the height of the two sides of the base circle of the sensor by about 4mm; to ensure that the gap between the effective circle and the part is 5-6mm (see Figure 1). Through the improved design of the inner gap and height of the inductor, the heating uniformity of the inductor is improved, and the depth of the hardened layer is effectively guaranteed.

Figure 1 Inductor structure

Camshaft inductor structure
  • Electrical parameter selection

  The selection of the frequency of the camshaft heating power supply mainly depends on the geometric shape of the cam and the depth of the heating layer. In induction heating, once the surface is heated, the heat is quickly conducted from the surface to the interior of the workpiece. The actual depth of heating is determined by heating time, watt density, and frequency. The depth to which the current penetrates into the surface of the workpiece is mainly related to the frequency of the current.

  Combined with the actual conditions of the test equipment, considering the depth of the hardened layer is deep, the induction heating is carried out by conduction. According to the technical requirements of the parts, the relationship between the depth of the heating layer and the frequency of the equipment used, the frequency is selected from 4.5 to 5.5kHz. According to the structural shape of the workpiece, the transformation ratio of the transformer at the output end of the power supply is 14:1. After process optimization, the intermediate frequency voltage is 550-600V, the DC voltage is 475-500V, the DC current is 220-240A, and the power is 90-100kW. The voltage and current used in the original process are relatively high, resulting in local overheating and even cracking of some parts.

  • Quenching process control

(1) Heating positioning Since the heating and quenching of the camshaft must accurately control the heating position, the requirements for the center holes at both ends of the workpiece during machining are relatively high. If the shape and depth of the top hole are not standardized, it will directly affect the positioning position of induction heating and affect the quenching quality.

(2) When the gap between the inductor and the workpiece is quenched, the cam is placed vertically. We optimize the height of both sides of the base circle of the inductor by 4mm and optimize the axial and radial gaps between the effective ring of the inductor and the part. Reasonable control. The gap between the sensor and the cam tip is adjusted from the original 7.5mm to 10mm, the gap between the sensor and the cam lifting portion is adjusted from the original 6mm to 4.5mm, and the gap between the sensor cam base circle is adjusted from the original 7.5mm to 5mm so that The heating temperature of the cam tends to be uniform, and the excessively large difference in the depth of the hardened layer and the quality problems of the soft belt in the transition zone is avoided.

(3) Medium cooling If the concentration, temperature, cooling time, spray angle, and spray pressure of the quenching cooling medium are not properly controlled, it is easy to cause quality defects such as quenching cracking of parts, soft belts, and peeling of edges and corners. Through tests, we use AQ251 water-soluble quenching medium with a concentration of 10% to 12%, an operating temperature of 20 to 30°C, and a spray pressure of 1.2MPa, which effectively eliminates the above defects.

3. Process verification

  • Depth inspection of the hardened layer

  Carry out wire cutting on the camshaft after induction hardening along the cross-section of the cam profile, and carry out axial cutting according to the cam lobe, lift, base circle, drop and other parts, grind away the wire cutting affected layer, and then pass through 3 % to 5% nitric acid alcohol corrosion, the profile of the induction hardened layer is shown in Figure 2, and the depth of the hardened layer (hardness method) and surface hardness testing are shown in Table 2. The depth of the hardened layer of the camshaft after quenching meets the process requirements, the layer depth is uniform and no soft belt is found.

Figure 2 Section of camshaft after induction hardening

Section of camshaft after induction hardening
Table 2: Hardened layer depth and surface hardness
Measuring item Technical requirements The tips of the camshaft Lift Base circle Drop
Case depth/mm 5.5~8.5 7.0~7.9 6.8 6.6 6.5~6.7
Surface hardness 60~64 60.5~64 60.5~63.5 61.5~63 61~63.5
  • Low-temperature tempering and magnetic particle inspection

  After induction hardening, carry out the first low-temperature tempering within 1 hour and then carry out the second tempering after cooling to room temperature. The two low-temperature tempering processes is 140℃×4.5h, and air cooling; the low-temperature tempering is to reduce the quenching stress and prevents cracking. No overheating and cracking of the edges and corners were found in the quenched camshaft, and no defect magnetic marks were found after magnetic particle inspection.

  The results show that through the improvement of the quenching process, the quality problems in the medium-frequency induction hardening of the camshaft are basically solved.

4. Conclusion

(1) Through the optimization of the sensor structure and precise control of the heating position, the quenching temperature and the depth of the hardened layer at the base circle can be increased, while the quenching temperature and the depth of the hardened layer at the tip of the cam can be reduced, effectively ensuring the hardened layer depth.

(2) By reasonably controlling the axial and radial gaps between the effective circle of the inductor and the parts, the heating temperature of the cam tends to be uniform, and the base temperature caused by the excessive temperature difference between the top of the lift and the base circle is avoided The depth difference between the circle and the cam lift part is too large and the quality of the soft belt in the transition zone.

(3) By effectively controlling the concentration, temperature, cooling time, and spray pressure of the quenching cooling medium, quality defects such as quenching cracking, soft bands, and peeling of edges and corners of parts are reduced.

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