High frequency induction heating and temperature m

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High frequency induction heating and temperature measurement of thin plate samples

Abstract: for high frequency induction heating of thin plate samples, the structure of axial eccentricity between the sample and the plate ring is designed. The thermocouple is spot welded on the sample to measure the temperature, and the sample is reliably grounded, so as to solve the heating speed and accuracy of temperature measurement of thin plate samples, and overcome the interference of strong magnetic field

key words: high frequency induction heating thin plate sample spot welding thermocouple induction plate coil


simulating and improving the temperature process curve of thin plate heat treatment, it is often necessary to conduct simulation tests on single thin plate samples through simulation devices. According to the requirements of Baosteel users, a set of high-frequency induction heating test device integrating computer, electricity, atmosphere and machinery, which can automatically control the temperature process curve of thin plate by computer, is developed. The general block diagram is shown in Figure 1. After the sample is installed and the oxygen is driven by nitrogen and filled with protective gas, the device can automatically complete the whole process control under the computer command, and display and record the parameters (print). You can also intervene manually and modify or form a variety of process flows freely

Figure 1 general block diagram of high frequency induction heating test device

this paper will focus on the high frequency induction heating and temperature measurement of thin plate samples. The size of the heated sheet specimen is 50 × two hundred × About 1.5mm. The thin plate sample shall be heated evenly, and there shall be a 100mm soaking zone in the middle section of the sample, and the temperature fluctuation shall be ≤ 7 ℃

the temperature process curve of thin plate is shown in Figure 2. The maximum temperature of a-b-c thin plate in the heating and insulation section is 1000 ℃, and the average heating rate from room temperature to the maximum temperature is 0~40 ℃/s. The average cooling rate of cooling section c-d-e is 5 ~ 50 ℃/s. According to the requirements of the temperature curve of the thin plate sample, the heating device is the most critical part to realize the rapid heating rate and cooling rate

1 high frequency induction heating of thin plate samples

high frequency induction heating of thin metal plates can achieve the purpose of rapid temperature rise and achieve the effect of diathermy heating

the sample can be heated by the method of transverse magnetic field induction heating. However, it is difficult to accurately calculate the expression of the equivalent resistance of the thin plate workpiece, because the current path on the thin plate is very complex, and the magnitude of the equivalent resistance is related to the air gap between the workpiece and the induction coil, so the approximate formula is often used in engineering to express the equivalent resistance of the thin plate workpiece

where req is the equivalent resistance of the heated sample, W is the width of the thin plate, and l is the length of the induction coil, ρ Represents the resistivity of the workpiece, and T represents the thickness of the thin plate. Its heating efficiency can be expressed as

, where RC is the coil resistance

because the metal workpiece forms a circuit by itself, the induced current if is generated in the cross section, and its value depends on the induced electromotive force E and the impedance of the eddy current circuit z

where: e - the effective value (V) of the induced electromotive force E

r - resistance in eddy current circuit (Ω)

x - inductive reactance in eddy current circuit (Ω)

because the Z value is very small, the eddy current if can reach a very high value, so the sample can be heated to a very high temperature in a very short time

through the above analysis, the heating power, oscillation frequency, structural form of induction coil, gap size between coil and heated sample and other parameters of high-frequency induction heating device are reasonably designed, which meets the requirements of rapid heating of the sample as a whole. According to different sample cooling curves and sample characteristics, the power of high-frequency induction coil can be changed to cool down. The sample can also be further moved outside the ring, and nitrogen can be sprayed through the cooling nozzle to achieve faster cooling

heat treatment high-frequency induction heating device, the distance between the induction coil and the heated workpiece is generally within a few millimeters. Practice has proved that too large gap can not meet the requirements of rapid heating. However, because the length of the driving rod holding the thin plate sample in the heating device is about 1.2m and the diameter is about 4cm, if the driving sample rod is to pass through the ring, the ring must at least have an inner space for the sample driving rod to pass through, which is obviously contradictory to the requirements of rapid heating of the thin plate sample and reducing the inner space of the ring. For this reason, a structure is designed in which the driving rod of the sample is axially eccentric with the ring on the flat plate (the design of this structure has obtained a national patent). The specific structure is shown in Figure 3

Fig. 3 structural diagram of the eccentric shaft between the sample transmission rod and the plate ring

the ring is designed as a plate open type, and the gap between the two plates can be reduced to 11-15.5 mm. In the design, considering the bending deformation of the thin plate sample under the action of high current and strong electric field, in order to avoid the short circuit between the sample and the heating plate coil, high temperature resistant glass-ceramic strips are pasted on both sides of the heating plate

the driving rod of the sample and the ring of the flat plate have an eccentric structure, which is the most critical part of the thin plate heating device. Indeed, several key problems have been solved

first, the rapid heating of the sample is guaranteed. Because this structure greatly reduces the distance between flat plates and rings

secondly, the rapid cooling of the sample is also guaranteed. This structure makes the distance between the cooling nozzle and the sample very small. This can enhance the effect of nozzle cooling

finally, when applied to other aspects, such as hot galvanizing simulation device, the air knife can also be close. The schematic diagram is shown in Figure 4. This structure is beneficial to the adjustment of parameters such as air knife angle and the reduction of injection pressure

Figure 4 cooling nozzle and air knife

2 temperature detection of thin plate heating

in high-frequency induction heating, infrared (optical fiber sensor) fixed-point temperature measurement method is usually used to measure the temperature of the heated body. However, it is difficult to measure the temperature of a moving body. If the sheet sample is heated as a whole and moved to the cooling nozzle, or moved to the zinc pot for galvanizing, it is necessary to continuously detect the temperature of the sample. In order to accurately measure the real temperature of the moving heating sheet, the only way is to spot weld the thermocouple with the heating sample, so as to solve the temperature measurement problem of continuous tracking of the heating sample

2.1 temperature measurement thermocouple

because the temperature measurement range of the sample is ≤ 1000 ℃, K-type thermocouple with large output thermoelectric potential and relatively low price is used. The thickness of thermocouple wire has a certain influence on the thermal inertia of temperature measurement, so the thinner is better. However, on the premise of meeting the temperature measurement response, considering that the couple wire needs to have a certain strength to meet the requirements of thermocouple spot welding and convenient sample installation, it is more appropriate to select the couple wire with a diameter of 0.5 mm

capacitive energy storage welding is usually used between thermocouple and sample surface. During spot welding, the contact surface between the couple wire and the sample must be pressed tightly to weld firmly. As shown in Figure 5, the method of parallel spot welding of even wire and sample can be adopted. After repeated heating tests, not only the domestic engineering material suppliers of thermocouples can ensure the strength of wires and the fastness of welding points in terms of cost and delivery time, but also the temperature measurement speed of spot welded thermocouples can meet the requirements, and the effect is very good

Figure 5 Schematic diagram of parallel spot welding of thermocouples

2.2 anti interference measures of thermocouples

since the thermocouples spot welded on the sample are directly affected by high-frequency induction strong magnetic field, as shown in Figure 3, the thermocouples spot welded on the sample will also generate high-frequency induction electromotive force, and the value of the induced electromotive force is

e = - D φ/Dt

the induced potential generated here is a large AC potential, and what we need to measure the thermocouple signal is that their target capital is 4500 euros (US $4940), a much smaller DC level signal. For a given thermocouple, when its reference end temperature t0 is constant, EAB (T0) is a constant, then the thermoelectric EMF EAB (T, t0) is only a function of the measured end temperature T, That is,

therefore, the thermocouples spot welded on thin plate samples at high frequency are embodied in four aspects: cooperation, reconciliation, support and publicity; It is introduced that the second stage industry plan preparation plan is "planned activities, time arrangement of reserve projects, industry status and difficulties faced, and the second stage hpmp concept". Under the action of induction heating, the difference on the thermocouple is within ± 1, and the qualified measured signal is composed of two parts, that is,

the high-frequency induction potential on the thermocouple that is far greater than the DC level basically drowns the required DC level signal, If the measuring end of the heated sample is not well grounded, a very large interference voltage will be induced at the thermocouple output end. The thermocouple output signal we detected is actually the induced potential of the two electrodes of the thermocouple under the action of a strong magnetic field, which is greater than 500VAC when detected with the AC gear of the multimeter, and strong sparks will be generated when the two output ends are short circuited. Therefore, it is necessary to shield this large induced potential (interference signal)

therefore, a series of anti-interference measures have been taken for thermocouples

① thermocouple measuring end is grounded. The specific method is: the heating sample and the sample transmission rod are well contacted and fixed, and then the sample transmission rod is reliably grounded. In this way, the thermocouple spot welded with the sample is also grounded at the same time to ensure that the measuring end of the thermocouple is at the same potential to the ground. This is the most effective of many anti-interference measures

② since the outgoing line of the thermocouple output end welded with the sample is led out from the hollow sample transmission rod, and the sample transmission rod is grounded, the thermocouple outgoing line is shielded. After taking such measures, the interference in thermocouple temperature measurement is greatly improved. In particular, the high-frequency induction heating device must have a good grounding, and the grounding resistance is less than 4 Ω

3 sample heating experiment and analysis

the experimental results show that the actual heating curve of the sample sheet obtained after taking the above measures is basically consistent with the required heating process curve, smooth and free of burrs, as shown in Figure 6

Fig. 6 measured and required heating process curve

it can be clearly seen from Fig. 6 that the sample was heated to 800 ℃ in only more than 20 seconds, meeting the requirements of rapidity; Throughout the heating, insulation and cooling process of the curve, the curve is smooth without any burrs, indicating that the interference has been completely suppressed

4 Conclusion

the thin plate heating temperature control system composed of industrial PC, programmable controller YS-170, high frequency induction heating equipment, thermocouples, etc. has been applied in Baosteel. The actual operation test shows that the spot welding thermocouple temperature measurement and temperature control system on the induction heated sample is effective for the automatic control of high-frequency induction heating equipment, and its rapidity and accuracy meet the requirements of process test. At the same time, it shows that:

① the high-frequency induction heating plate coil has an eccentric structure with the sample transmission rod, which ensures that the sample transmission rod does not pass through the heating plate coil, so that the distance between the two heating plates has nothing to do with the diameter of the sample transmission rod, which meets the conditions and requirements of rapid heating of the sample

② for moving heating objects, the method of spot welding thermocouples on the heating body can be used to measure temperature; The good grounding of the heating body is the most effective measure to resist high-frequency interference in the temperature measurement of spot welding thermocouple


1 Huang Shenzhi, Fei Jianmin, Yu Hongyi Temperature measurement and control of thin plate heating by high frequency induction heating device The first National Annual Academic Conference on heat treatment automation, 1996

2 Huang Shenzhi, Fei Jianjian, Yu Hongyi Intelligent instrument communication and computer real-time management

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