Induction heating quenching refers to a process in which a workpiece is placed in an induction coil with sufficient power output. Under the action of a high-frequency alternating magnetic field, a strong induction current is formed on the surface of the workpiece, which causes the workpiece surface to rapidly heat up and then the surface is quenched. . Because of its high heating efficiency, short heating time, the surface of the workpiece is not easy to be oxidized and decarburized, and the heating speed is fast, so the austenite grains are refined, and the workpiece has excellent mechanical properties after quenching, and only the surface heating is performed. The quenching deformation is small, the equipment is easy to realize automatic production, and the labor productivity is high, so it has been widely used. The following is the application of induction heat treatment process methods on the fixed sleeves of labor products in our factory and the related troubleshooting procedures.

1. Part technical requirements

The structure of the fixing sleeve (also known as the inner spline shaft sleeve) is shown in Figure 1. The length of the inner spline is not less than 60mm , the small diameter of the outer contour of the fixing sleeve is 60mm , and the diameter of the large end is 84mm . There are three M18 × 1.5 -7H female threaded hole. The material is 40Cr (GB / T3077 — 2015) , and the whole quenching and tempering treatment is performed first. The hardness requirement is 28 to 32HRC . According to the national standard GB / T 13320 — 2007, the metallographic rating is 1 to 4 and it is evaluated as passing. Tensile strength ≥ of 980 MPa , a yield strength ≥ 785 MPa , elongation ≥ . 9% . The technical requirements are induction hardening of the surface of the internal spline, surface hardness of 50 to 56HRC , effective hardening depth of induction hardening ≥ 3mm , the depth of internal hardening of the internal spline requires measurement from the root of the tooth, and the hardness of the internal spline requires the internal spline Take measurements in the middle. Parts must be 100%Magnetic particle inspection is not allowed to have cracks, folds, white spots, and other harmful defects that degrade performance. The complete process route is: blank inspection → storage → rough turning → quenching and tempering → finishing turning → milling flat → drilling → tapping, chamfering → cut the internal spline → cut the small diameter → the internal spline induction heat treatment → magnetic particle inspection → Phosphating treatment → Final inspection → Cleaning, rust prevention, packaging → storage.

2. Part induction heat treatment process

The induction heat treatment process is based on the existing conditions, using profiled inductors, ZTTP series IGBT 250kW / ( 4 ~ 30kHz and 20 ~ 50kHz ) domestic intermediate frequency power supply and ZTVC100-2 CNC induction hardening machine for the inner of the fixed sleeve. Spline holes are induction hardened at intermediate frequencies.

Process parameters of heat treatment induction hardening: heating power 25 ～27kW ; frequency 20 ～23kHz . The inner spline starts to heat from the lower end and stays heated for 5 seconds . The sensor then scans the inner spline from bottom to top at a speed of F60 (about 1 mm per second ), and continuously sprays and cools the inner spline while heating. , Until the upper end of the inner spline, and stop heating, continue to cool for 20 seconds and then stop cooling, and then rely on the internal surface heat of the part to generate a temperature of 200 ~ 230 ℃ for self-tempering.

3. Quality problem description

During the intermediate frequency induction hardening of the fixed sleeve, three M18 × 1.5-7H internally cracked hardened cracks appeared in the internal thread, and the inspection showed quality problems such as magnetic marks. According to the metallographic analysis of the sample at the crack of the fixed sleeve, it was found that Overheating characteristics. The solid photo of the sleeve is shown in Figure 2 .

4. Analysis and troubleshooting

The depth and surface hardness of the induction hardening layer vary with frequency, power loss per unit area, the shape and size of the inductor, the material and shape and size of the part being processed, the matrix structure before the treatment, the heating method, and the heating time, and The main influencing factors such as the type of cooling agent and cooling method change. In order to solve these problems, the structure design, process parameter selection, heating positioning and cooling method of the inductor were investigated one by one in order to optimize the process.

( 1 ) Chemical composition

The material requirement of the fixing sleeve is 40Cr (GB / T 3077 — 2015) . After analysis by direct reading spectrometer, the chemical composition is shown in the attached table, which meets the standard requirements.

( 2 ) Selection of electrical parameters

The selection of the heating power frequency of the fixed sleeve mainly depends on the geometry of the fixed sleeve and the depth of the heating layer. The actual depth of heating is determined by the heating time, power density and frequency. The depth of current penetration into the surface of the workpiece is mainly related to the frequency of the current. Fixing sleeve internal splines hole ZTTP series the IGBT 250kW / ( . 4 ~ 30kHz and 20 is ~ 50kHz domestic and frequency power supply) ZTVC100-2 intermediate-frequency induction hardening on the CNC induction hardening machine. Heating power: 25 ～27kW , frequency 20 ～23kHz .

( 3 ) Analysis of special structure at the orifice of internally threaded hole

The shape of the intersecting line formed by the three M18 × 1.5-7H internally threaded holes on the internal spline surface and the internal spline surface is relatively sharp, and it is difficult for the machining process to have a proper way to chamfer the intersection. This results in a sharp corner effect during the induction heating process, which causes the temperature to rise sharply, which is higher than the temperature of other parts, and the heating rate is faster than other parts, so it is easy to cause overheating and cracking. In order to ensure that the workpiece can be heated uniformly, the sharp corner effect at the sharp edges of the workpiece should be avoided as much as possible.

( 4 ) Heating positioning

Because the heating position of the fixed sleeve induction hardening heating must be accurately controlled, the positioning fixture of the fixed sleeve uses the positioning tool shown in Figure 3 . The bottom end face is used for clamping and positioning. The small end of the outer contour of the fixed sleeve is assembled into a cavity of D 1 = 60mm (positive tolerance) at a diameter of 60mm to achieve positioning. The quenching cooling medium is sprayed to the inner spline and the diameter is evenly distributed from 6 12mm drain holes. Manually adjust the initial heating position to form the initial coordinate input program. The subsequent heating speed and terminal position are entered into the program after observation and adjustment. After that, the process can always be used to ensure stable production.

( 5 ) Medium cooling

When the quenching cooling medium concentration, temperature, cooling time, spray angle and spray pressure are not properly controlled, it can also cause quality defects such as quench cracking of the fixed sleeve. Test by the process, I plant is at a concentration of 5% - 8% of the PAG water-based quenching liquid, temperature of 20 is ~ 45 deg.] C, injection pressure of 0.1MPa , liquid ejecting angle to the vertical splined inner surface of the shower, The cooling process is proper.

5. Countermeasure implementation

To prevent the problem of sharp corner overheating, the relative height and relative clearance between the induction coil and the workpiece are limited under the existing working conditions. In order to avoid the three M18 × 1.5-7H internally threaded holes and internal splines The temperature at the intersection formed by the surface is too high due to the sharp angle effect. Considering that the thermal conductivity of carbon steel and medium carbon alloy steel is 47 to 58 λ / W · m ﹣ 1 · K ﹣ 1 , the thermal conductivity of pure copper The rate is as high as 384 λ / W · m ﹣ 1 · K ﹣ 1 , which can quickly conduct heat from the area of the screw hole. Our factory uses pure copper rods to match the internal threaded hole structure with three copper bolts as shown in Figure 4 to block the threaded hole, which can greatly reduce the current density at sharp corners or play a certain magnetic field. Shielding effect.

The copper bolts block the threaded holes, as shown in Figure 5 .

Figure 5 Copper bolts blocking the threaded holes 

Because the structure of the sensor mainly includes the outer diameter of the effective ring of the sensor (determines the gap between the fixed sleeve), height, and spray angle. According to the shape of the fixing sleeve, our factory commissioned a professional sensor manufacturer to make a special profile sensor. During induction hardening, the fixed sleeve adopts the form of vertical clamping, that is, the axis of the inner spline hole is perpendicular to the ground. The axial and radial clearances of the effective outer ring of the inductor and the spline hole of the fixed sleeve are reasonably controlled to ensure the effective circle. The gap between the outer diameter and the spline hole in the fixed sleeve is 1mm . Make the fixed sleeve heating temperature as uniform as possible to avoid quality problems such as the difference in effective hardened layer depth. The induction hardening state of the fixed sleeve is shown in FIG. 6 .

6. Process verification

( 1 ) Inspection of surface hardness and hardened layer depth

According to customer requirements, the quenching depth of the internal spline must be measured from the root of the tooth, and the hardness of the internal spline is required to be measured in the middle of the internal spline. After cross-sectioning the induction hardened fixing sleeve along the center of the inner spline hole, wash and rinse with gasoline, and perform wire cutting on the cross section, and grind away the wire cutting influence layer, and then use a concentration of 3% ～Corrosion of 5% nitric acid, the effective hardened layer depth (hardness method) and the surface hardness index are 54.5HRC and 3.2mm , respectively . The effective hardened layer depth of the fixed sleeve after induction hardening meets the technical requirements of the pattern. And the layer depth is uniform.

( 2 ) Tempering and magnetic particle inspection

Induction hardening uses controlled spray cooling time, so that the residual heat in the hardened zone is transferred to the hardened layer, so that it reaches a certain temperature for tempering, that is, self-tempering based on the residual heat of the workpiece, and the self-tempering temperature is 200 ~ 230 ℃. No overheating or cracking was found at the tapped hole openings. No defect-like magnetic marks were found on the magnetic particle inspection by the CJW-2000E magnetic particle inspection equipment.

7. Conclusion

( 1 ) Through the application of positioning tooling, to ensure the precise control of the heating position, and to ensure the stable depth of the effective hardened layer while ensuring the stable production of the batch.

( 2 ) Reasonably control the axial and radial clearances between the effective outer ring of the inductor and the spline hole in the fixing sleeve, so that the heating temperature of the fixing sleeve tends to be uniform.

( 3 ) By using pure copper rods with better thermal conductivity to make copper bolts to block the threaded holes, the current density at sharp corners is greatly reduced, magnetic field shielding is achieved, and the internally threaded holes of the induction sleeve of the fixed sleeve are effectively solved. Quenching crack quality problem at the mouth.

http://www.songdaokeji.com/plus/list.php?tid=47