High frequency induction quenching heating

There are two ways of high-frequency quenching heating:

The first is simultaneous quenching, which involves heating the surface of the workpiece to be quenched at the same time, followed by rapid cooling.

The second is sequential continuous heating quenching, that is, induction heating a small part of the surface of the workpiece, at the same time the workpiece moved from the top down, so that the surface successive heating and cooling.


For the production of multi-variety and small-batch parts, different materials may need to use different quenching media, so the quenching method of simultaneous heating is mostly adopted. If the parts with large surface area are subject to the limitation of equipment power, continuous heating is considered.


1. High-frequency surface quenching of inner hole of martensitic stainless steel workpiece


(1) processing difficulties


The high frequency surface quenching of the inner hole of martensitic stainless steel workpiece adopts the method of simultaneous heating.


In the process of high-frequency induction heating, when the temperature exceeds the magnetic loss point of the material (the temperature of the magnetic loss point of the steel material is generally in 700 ~ 800℃), the electromagnetic induction capacity of the material decreases, the heating speed decreases several times, further heating is difficult. And stainless steel heat treatment temperature is high, are above 1000℃, heating to the material quenching temperature is more difficult. On the other hand, due to its high heat treatment temperature, close to the melting point of the material, although the heating speed above the magnetic loss point is reduced, but the heating speed is still faster than the conventional heat treatment, and it is difficult to control, there is a risk of overheating of parts surface melting.


The ring effect is one of the three main effects of induction heating and the reason of the difficulty of inner hole heating. That is, when the induction ring is used to heat the workpiece, the current through the induction ring is concentrated on the inner surface of the induction ring. When heating the outer surface of the workpiece, the inner surface of the induction ring is opposite to the outer surface of the workpiece, which is conducive to the heating of the workpiece. When heating the surface of the inner hole of the workpiece, the direction is just opposite, which will significantly reduce the electrical efficiency of the inductor and not conducive to the heating of the workpiece. Moreover, during the induction quenching of inner hole, the heating surface is inside the workpiece, and the operator is not easy to observe directly from the outside, which increases the operation difficulty to some extent.


The spherical bearing of a certain product (see figure 1) requires sf28mm spherical quenching. The material is 20Cr13 martensitic stainless steel, and the quenching hardness requires 35 ~ 45HRC. In addition to the above heating difficulties, the heating surface of the workpiece is a sphere, rather than a straight through hole, which will inevitably cause the gap between the inductor and the heating surface of the workpiece to increase, further reducing the electrical efficiency. In order to overcome the adverse effect of the annular effect on the workpiece heating, the inductor is set up to change the distribution of the magnetic field, forcing the current flow close to the surface distribution of the workpiece to be heated, so as to improve the heating effect. However, the workpiece has a small inner hole, so the clearance distance between the sensor and the workpiece and the sensor's own size are removed. The inner diameter of the sensor is less than 13mm, so it is impossible to install magnetic permeability. The induction quenching of the workpiece can only be done by optimizing the process parameters and improving the heating process to maximize the capability of the equipment.


(2) quenching process


The quenching process includes heating time, quenching temperature and quenching medium.


Many people believe that high frequency induction quenching belongs to instantaneous heating, can be in a few seconds to reach the quenching temperature, this understanding reflects the general situation, is not comprehensive. In some cases, the heating speed will be slower, and in some special cases, by reducing the voltage output means, slow down the heating speed of parts, can meet some special workpiece or special technical requirements. For the workpiece, due to the existence of many adverse factors, rapid heating is not practical, considering the need of visual temperature change and prevent overheating or even the occurrence of surface melting phenomenon, in order to ensure the quenching quality, must be based on a slower heating speed. If the heating speed is too slow, the advantage of surface quenching will be lost, and the hardened layer will be too large due to heat conduction. The practice has proved that the heating time of the workpiece should be controlled between 2.5 and 3min.


The quenching temperature of the workpiece should be determined according to the steel type, the original structure and the heating rate in the phase change zone. The faster the heating speed is, the higher the required quenching temperature is. The high-frequency quenching heating speed is much higher than the conventional heat treatment. Therefore, the high-frequency quenching temperature is generally higher than the conventional heat treatment quenching temperature. Because of various reasons, spherical bearing heating is difficult, the quenching temperature should not be too high, the higher the quenching temperature, the more difficult to achieve, which is one of the reasons for choosing a slower heating speed. Although the slower heating speed is chosen, it is still fast heating. Considering the slower heating speed, the time of austenitization is longer than that of fast heating. After the comprehensive analysis of a variety of factors, the quenching temperature should be similar to the conventional heat treatment or slightly higher.


Good hardenability of martensite stainless steel, workpiece size is not very large, air cooling can be completely quenched. The effective thickness of spherical bearing is less than 10mm, and it is surface quenching. At the same time, considering the special case that the quenching temperature is relatively low, in order to ensure the quenching effect of the workpiece and meet the hardness requirements, the air cooling quenching inevitably has some uncertain factors, so it is inevitable to choose the quenching medium with faster cooling speed to make up for the possible defects of the quenching temperature. Oil cooling speed is obviously better than air cooling, in all kinds of quenching medium belongs to a slow, the workpiece is heated to the quenching temperature immediately after the oil quenching can achieve the quenching effect. The slower cooling speed will not produce cracks and other defects, stable and effective to meet the technical requirements.


(3) practical effect


After quenching the spherical bearing according to the above scheme, the spherical hardness is above 45HRC, and after tempering at 480℃, the hardness is still stable above 40HRC, and the hardness distribution of each workpiece and parts of the workpiece is uniform and stable, indicating that the workpiece fully meets the quenching requirements. The successful quenching of the workpiece provides a useful reference for the surface quenching of the stainless steel workpiece and the inner hole which is difficult to heat.


2. High-frequency surface quenching of deep quenching layer of larger size parts


(1) processing difficulties


The high frequency quenching of the workpiece also adopts the method of simultaneous heating. The processing difficulty is mainly limited by the power and current frequency of the equipment.


High frequency quenching is short time and fast heating. It needs to be heated to very high temperature in a very short time. Sufficient heating power is needed as the basis. The larger the surface of the workpiece to be heated, the greater the power required. When the heated surface is large enough, it will be difficult to achieve simultaneous heating due to the power limitation of the equipment.


When the workpiece is heated by induction, the penetration depth of current is determined by the current frequency. The current frequency of high-frequency quenching equipment is generally fixed. For example, the current frequency of high-frequency equipment is 200 ~ 300kHz, and the corresponding heat penetration depth is 0.9 ~ 1.1mm, which limits the further deepening of the hardened layer depth.


The traction pin of a certain product (the quenching position is shown in figure 2) is the key part of the product. The material is 40Cr alloy structural steel, which requires high-frequency quenching on the outer circle surface of f 89mm. The quenching hardness requires 50 ~ 60HRC, and the depth of the hardened layer is 2.5 ~ 4.5mm. The workpiece needs a large size of quenching surface, in addition to the need for a large power heating, the heating impact is greater for the quenching part of the groove part of the workpiece, the production of inductor is also a big difficulty. Such as production of sensors by conventional method, namely the sensor inner diameter slightly larger than quenching surface diameter, the sensors must be on-site production, very trouble, and workpiece quenching have to damage the sensor to each workpiece surface of the high frequency quenching must make corresponding a sensor, there is also the production of each sensor error; If the inner diameter of the inductor is greater than the diameter of the adjacent section, that is, it is greater than 111mm, then the distance between the inductor and the quenching part increases by 11mm, and the induction heating efficiency will be significantly reduced. In terms of hardened layer, the depth range of 2.5-4.5mm is 2.5-4.5 times of the normal depth of thermal penetration. In order to improve the depth of hardened layer, generally the principle of heat conduction can be appropriately used, that is, the thickness of the heated layer can be increased by taking advantage of the characteristic of heat conduction from the surface to the center. However, the method of solely relying on heat conduction requires a large temperature difference from the surface to the inside, and when the required hardened layer depth reaches the quenching temperature, the surface temperature is too high, resulting in surface tissue overheating, overburning and other defects.


(2) quenching process


In order to complete the quenching of the workpiece, sensors are specially made, the process control is strengthened and the intermittent heating method is adopted.


Many characteristics, combined with traction pin production way, change the traditional sensors will make semicircle for sensors and overcome the traditional sensor for high frequency quenching of the above difficulties, the workpiece can be as small as possible distance between sensors and heating surface, and can easily make the workpiece quenching with sensors from. In the specific operation, the workpiece is made to rotate concentric relative to the inductor, so as to achieve the special effect of heating the semicircle instantly, and heating all the hardened surface as a whole.


Previously, steel materials heated to a certain temperature, will lose magnetic, heating speed down several times. In the actual heating process, when there is a thin layer on the surface that exceeds the magnetic loss point, the strength of the eddy current at the internal junction adjacent to the thin layer will suddenly rise and become the part with the fastest heating speed. The phenomenon of the decrease of the heating speed of the high temperature surface layer, the increase of the temperature at the junction and the passage of the inner part will occur. This phenomenon is beneficial to increase the depth of the hardened layer, but the heating speed of the surface high temperature area is much faster than that of the part within the boundary, and the surface overheating and overburning tendency is still very serious. At this time, it is necessary to find out the optimal configuration of voltage, heating speed and other parameters, strictly control the heating process, and increase the depth of the hardened layer as much as possible on the premise of ensuring the quality.


Traction pin requires a large depth of hardened layer, and the simple parameter control is still insufficient to fully meet the technical requirements, and some other techniques are needed. Intermittent heating, that is, when the quenching temperature is not reached, temporarily stop heating, so that the workpiece surface heat more inward conduction, and then start heating again. This is equivalent to increase the heat conduction time, reduce the temperature gradient from the surface to the interior, repeated several times, the surface temperature is not too high to produce overheating, overburning. From the surface to the inside of 2.5 ~ 4.5mm more uniform to achieve the purpose of quenching temperature.


(3) practical effect


After taking measures such as improving sensor design, optimizing process parameters, intermittent heating, etc., the hardness of traction pin surface after high-frequency quenching can be stabilized to about 55HRC, and the depth of hardened layer is more than 3mm. The method of high-frequency quenching can meet the requirements of hardened layer depth that is suitable for intermediate frequency quenching. And because of the improvement of inductor, the workpiece can be continuously quenched one by one, which effectively improves the working efficiency.


3. Matters needing attention


In order to ensure the processing quality, the following matters should be paid attention to:


(1) equipment maintenance is extremely important. The distance between the high frequency inductor and the workpiece should be as small as possible to reduce its power loss and maximize the need for simultaneous heating.


(2) the most common form of inductor is the spiral formed by bending red copper pipe. When designing and manufacturing such inductors, the larger diameter copper tube should be used as far as possible and the number of turns should be reduced to reduce the inductive reactance and ensure the heating efficiency.


4. Conclusion

High frequency induction quenching is a complex process, which also belongs to the special heat treatment category in the heat treatment, and the realization of simultaneous heating is more difficult. In specific operations, must consider equipment power, working frequency, sensors and heat treatment parameters, organization transformation, quenching medium and material factors such as the cooling way, to achieve the best fit of these factors, maximize equipment potential, as far as possible to satisfy many varieties, small batch workpiece quenching needs at the same time.



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