Induction heating quenching refers to the process in which the workpiece is placed in the induction coil with sufficient power output and the workpiece surface forms a strong induced current under the action of high frequency alternating magnetic field, which makes the workpiece surface rapidly heat up and then the surface is quenched. Since it has high heat efficiency, short heating time, the surface is not easy oxidation decarburization, heating speed, thus refine austenite grain, make the workpiece after quenching has excellent mechanical properties, and only surface heating, quenching deformation is small, easy to realize automatic production equipment, high labor productivity, and thus has been widely used. The following is the application of induction heat treatment technology in the fixed set of labor products in our factory and the troubleshooting and solving process of related quality problems.
1. Technical requirements of parts
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 no less than 60mm, the diameter of the small end 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 internal threaded holes on the fixing sleeve. The material is 40Cr(GB/ T3077-2015), and the hardness is required to be 28 ~ 32HRC. According to the national standard GB/T 13320-2007, the metallographic rating is 1 ~ 4, and it is evaluated as qualified. Tensile strength ≥980MPa, yield strength ≥785MPa, elongation ≥9%.The technical requirements are as follows: the internal spline surface shall be induced quenching, the surface hardness shall be 50 ~ 56HRC, the effective depth of the induction hardening layer shall be ≥3mm, the internal spline quenching depth shall be measured from the root of the tooth, and the hardness of the internal spline shall be measured in the middle of the internal spline. The parts must be inspected with 100% magnetic particle and no cracks, folds, white spots, or other harmful defects that may impair the performance of the parts. The complete process line is blank inspection → warehousing → coarse truck → fine truck → milling → drilling → tapping and chamfering → cutting inside spline → cutting path → internal spline induction heat treatment → magnetic particle inspection → phosphating treatment → final inspection → cleaning, rust prevention, packaging → warehousing.
FIG. 1 Structure of fixed seat
2. Parts induction heat treatment process
The induction heat treatment process is based on the existing conditions, using copy inductor, ZTTP series IGBT 250kW/ (4~30kHz and 20~50kHz) domestic intermediate frequency power supply and ZTVC100-2 type CNC induction quenching machine tool for the fixed sleeve of the internal spline hole to carry out intermediate frequency induction quenching.
Process parameters of heat treatment induction quenching: heating power 25 ~ 27kW; The frequency is 20 ~ 23kHz.Internal spline start from the bottom heating, heating for 5 s, sensors to the speed of the F60 (less than 1 mm per second) internal spline scanning from the bottom of the heating, heating and continuous spray liquid cooling for internal spline, and until the top internal spline, and stop the heating, to keep the 20 s to stop after cooling, cooling and then rely on internal return parts surface heat generated to self tempering temperature 200 ~ 230 ℃.
3. Description of quality problems
During the induction quenching process of the fixed sleeve, three M18× 1.5-7h internal thread holes were found to have quenching cracks at the orifice, and the flaw detection showed magnetic trace and other quality problems. Moreover, through the sample cutting metallography analysis at the cracking point of the fixed sleeve, overheating characteristics were found. The solid photo of the fixed sleeve is shown in Figure 2.
Figure 2. Solid photo of a fixed set
4. Analysis and screening
The depth of hardening layer and surface hardness of induction quenching change with the main influencing factors such as frequency, power loss per unit area, shape and size of the induction coil, material and shape and size of treated parts, matrix structure before treatment, heating mode, heating time, as well as the type and cooling method of coolant. In order to solve these problems, the structure design of the induction coil, process parameter selection, heating location, and cooling mode are checked one by one, so as to optimize the process.
(1) Chemical composition
The material requirements of the fixed sleeve are 40Cr(GB/T 3077-2015), and the chemical composition of the fixed sleeve has been analyzed by the direct reading spectrometer, as shown in the attached table, which meets the standard requirements.
(2) Selection of electrical parameters
The choice of heating power frequency of fixed sleeves mainly depends on the geometric shape and heating layer depth of the fixed sleeve. The actual depth of heating is determined by heating time, power density, and frequency. The depth of the current penetrating into the surface of the workpiece is mainly related to the current frequency. The internal spline holes of the fixed sleeve are quenched in the ZTTP series IGBT 250kW/ (4 ~ 30kHz and 20 ~ 50kHz) domestic medium frequency power supply and ZTVC100-2 type CNC induction quenching machine tool.Heating power: 25 ~ 27kW, frequency: 20 ~ 23kHz.
(3) Analysis of special structure at the orifice of internal thread holes
Three on the surface of the internal spline M18 x 1.5-7 h internal thread hole curve formed by the internal spline surface with sharp, shaped and mechanical processing is very difficult to have the right way to the curve of chamfering processing, lead to the Angle appeared in the process of induction heating effect, the temperature will rise sharply, higher than other parts of the temperature, heating up speed faster than in other areas, so it is easy to cause overheating craze.In order to ensure that the workpiece can be evenly heated, the sharp Angle effect should be avoided as far as possible in the workpiece sharp edge.
(4) Heating positioning
As induction quenching heating of the fixed sleeve must be controlled accurately, positioning clamping of the fixed sleeve adopts positioning tooling as shown in FIG. 3.The bottom end surface clamping is adopted for positioning, and the small end diameter of the outer contour of the fixed sleeve is assembled into the cavity body D1=60mm (positive tolerance) to realize positioning. The quenching cooling medium is sprayed into the center internal spline and drained from the 6 evenly distributed diversion holes with a diameter of 12mm. The initial heating position can be manually adjusted to form the initial coordinate input program, and the subsequent heating speed and terminal position can be input into the program after passing the adjustment through observation. After that, the production stability can always be guaranteed through the process program.
FIG. 3 Schematic diagram of fixed sleeve positioning tooling
(5) Medium cooling
When quenching cooling medium concentration, temperature, cooling time, injection Angle, and injection pressure are not controlled properly, it can also cause the quality defects such as quenching cracking of the fixed sleeve. Through the process test, our customer’s factory USES PAG water-based quenching liquid with a concentration of 5% ~ 8%, the service temperature is 20 ~ 45℃, the injection pressure is 0.1mpa, the spray liquid Angle is perpendicular to the surface of the internal spline spray, and the cooling process is appropriate.
5. Implementation of countermeasures
To prevent overheating of the Angle, because under the condition of existing conditions by adjusting the relative height between the induction coil and the workpiece and the relative gap is restricted, in order to avoid the three M18 x 1.5-7 h internal thread hole interfingering lines with internal spline surface formation because of high temperature caused by Angle effect, given the carbon steel and medium carbon alloy steel thermal conductivity in 47 ~ 58 lambda/w. k. m – 1-1, and the thermal conductivity of pure copper is as high as 384 lambda/W, m – 1, K – 1, can quickly the threaded hole area of heat conduction. Our customer’s factory adopts a pure copper bar and plugs the threaded hole according to the structure of the threaded hole with three copper bolts as shown in FIG. 4, which can greatly reduce the current density at the sharp corner or play the role of magnetic field shielding.
FIG. 4 Copper bolts
Copper bolts plug threaded holes as shown in FIG. 5.
FIG. 5 Copper bolts plug threaded holes
The structure of the inductor mainly includes the size of the effective ring outside diameter of the inductor (determining and fixing the gap between the Chambers), the height, and the spray Angle. According to the shape of the fixed set, our factory commissioned the professional manufacturer of the induction coil to make a special copying inductor. During induction hardening, the fixing sleeve adopts the form of vertical clamping, that is, the axis of the internal spline hole is perpendicular to the ground. The axial and radial clearance between the effective outer ring of the inductor and the spline hole in the fixing sleeve is reasonably controlled to ensure that the clearance between the outer diameter of the effective ring and the spline hole in the fixing sleeve is 1mm. Make the heating temperature of the fixed sleeve as uniform as possible to avoid quality problems such as the difference in the depth of the effective hardening layer. The induction quenching state of the fixed sleeve is shown in FIG. 6.
FIG. 6 Induction quenching state of the fixed sleeve
6. Process verification
(1) Surface hardness and depth test of the hardened layer
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 shall be measured in the middle of the internal spline. After induction hardening of a fixed set of along the internal spline hole at the center of the crosscutting, washed with gasoline, and the cutting sample preparation for cross-section, and the cutting effect is ground to layer, then use alcohol concentration 3% ~ 5% nitric acid corrosion, the effective depth of hardening layer method (hardness) and surface hardness index for 54.5 HRC surface hardness testing conditions, effective hardening layer depth of 3.2 mm, after induction hardening of a fixed set of effective hardened layer depth conforms to the requirements of the design technology, and deep layer.
(2) Tempering and magnetic particle inspection
Induction quenching USES the control of injection cooling time to transfer the residual heat from the hardened layer to the hardened layer, so as to achieve a certain temperature for tempering, that is, relying on the waste heat of the workpiece for self-tempering, self-tempering temperature 200 ~ 230℃. No overheating and cracking were found at the orifice of the threaded hole. After magnetic particle inspection with cJW-2000E magnetic particle inspection equipment, no defective magnetic trace was found.
(1) Through the application of positioning tooling, the heating position can be precisely controlled to ensure the effective quenching and hardening layer depth as specified while ensuring stable batch production.
(2) By reasonably controlling the axial and radial clearance between the effective outer ring of the inductor and the spline hole in the fixing sleeve, the heating temperature of the fixing sleeve tends to be uniform.
(3) After plugging the threaded hole with copper bolts made of the pure copper bar with better thermal conductivity, the current density at the sharp corner was greatly reduced, the magnetic field shielding was realized, and the quality problem of quenching crack at the orifice of the fixed sleeve’s induction quenching internally threaded hole was effectively solved.
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