High frequency welding definition
High-frequency welding is a common solid phase resistance welding method in the machinery industry.The principle of this method is as follows: high frequency current will produce resistance heat through steel, the heat generated can heat the surface of the steel to be welded to melt or close to the plastic state, and then apply the top forging force to realize the welding of steel. High-frequency welding can be divided into high-frequency induction welding and high-frequency contact welding according to the way that high-frequency current generates heat in the steel.
In high-frequency induction welding, the induction current is generated in the workpiece through the coupling of the outer induction ring. In high-frequency contact welding, the high-frequency current is directly introduced into the workpiece through mechanical contact with the workpiece. High-frequency welding requires a special equipment system, and its advantage is that the productivity is very high, and the welding speed can theoretically reach 400m/min.
Historical background and current situation of high frequency welding
After the 1950s, with the development of welding production technology, high-frequency welding has become one of the main methods of welded pipe production. However, the initial development of high-frequency contact resistance welding was slow, and it was not developed rapidly until 1960. During this period, with the large number of welded pipe units in the world, high-frequency contact resistance welding has gradually been widely used. Subsequently, with the introduction of the ring sensor, high-frequency induction welding also began to be applied to the production of welded pipes, and its application was gradually promoted and accepted.
High-frequency welding has many advantages, such as fast heating speed, small welding heat affected zone, and the ability to weld a variety of metal materials. Therefore, in the 1970s, high-frequency welding gradually became a common method for small and medium-sized diameter welded pipe units, and high-frequency welding technology was also modified for low-frequency welded pipe units to improve the production capacity and product quality of welded pipes.
In modern industrial production, high-frequency induction welding has become a very important seamless welding technology due to its high efficiency, high precision, environmental protection and other characteristics, which is widely used in industrial and production fields, and gradually replace the traditional welding method. According to the report, the technology has been widely used in the manufacturing of food heating equipment, shoe-making machinery, auto parts, pipes and metal products, furniture, building materials and other fields.
High frequency welding technology has two technical routes, series and parallel. In summary, the representative brand of parallel is the United States Sematur, etc., and the representative brand of series is Norway EFD, etc.; In China, the brands represented in parallel are Red Star (Bohong), Zhuo Si Hengchang, etc., and the representative brands in series are Sanzheng. At present, the use of series routes shows a new trend of SiC devices, and there is also a fusion between series and parallel routes.
Welding method: induction welding and contact welding
The common high-frequency welding methods are induction welding and contact welding.
1. Induction welding
induction welding is a welding method that generates heat in the workpiece through electromagnetic induction. Its basic principle is to use the induced current caused by the high-frequency electromagnetic field acting on the conductor to generate heat in the conductor, so that the surface or internal part of the workpiece is heated and welding is achieved. The speed and accuracy of induction welding make it widely used in areas such as pipe edge welding, especially for mass production. The new induction welding technology also has the advantages of low power consumption and high efficiency, controllability and repeatability, which can reduce costs.
2. Contact welding
contact welding is a welding method that uses heat and pressure to weld two metal parts. The basic principle of contact welding is that the contact head is in contact with two workpieces, and then the power is heated, and enough pressure is applied to the two contact workpieces, so that the contact is everywhere in a thermoplastic state, forming a melt pool and achieving extrusion welding. Contact welding is widely used in the connection of metal materials, welding of solid parts and lead welding on windings, especially in the field of electronic devices and automotive manufacturing with high air sealing requirements.
3. Comparison of induction welding and contact welding
From the point of view of energy consumption, contact welding can directly transfer electrical energy, reduce the electromagnetic transfer energy loss during induction welding, so the contact welding energy consumption is lower, higher production efficiency, is conducive to energy saving, can allow higher current and lower output power than induction welding, low investment, low operating costs.
From the point of view of welding speed, contact welding can improve the welding speed and increase the output per unit time. Under the condition of the same high-frequency output power, the welding speed of induction welding is only 1/2 to 2/3 of that of contact welding.
The contact welding electrode leg can be closer to the extrusion roll than the induction welding electrode leg, so the heating time is shorter, the thermal energy response zone contact is small, and the ineffective shunt contact is small, so the efficiency contact is high and the power consumption contact is low.
The contact welding electrode leg is more convenient to lift and press down, so it is more suitable for non-continuous production (that is, no looping and shear butt welding machine).
Contact welding is above the weld without any obstacles, so that the operator can observe the heating state, or the weld tracking system can control the welding quality of the weld.
The disadvantage of contact welding is that the contact parts need to transfer energy in the sliding state, which will cause the conductive parts of the contact workpiece to wear quickly, low service life, and become the necessary consumable parts of contact welding; The induction welding coil almost does not need spare parts, is water-cooled and non-contact with the contact workpiece, and the working life is much higher than that.
The contact pressure between the contact parts and the surface of the tube billet should be both moderate and consistent. When the contact pressure is small, the conduction effect is not good; If the contact pressure is too large, it will increase the wear, and even cause the deformation of the thin-walled pipe and damage the pipe surface. Inconsistent pressure at the two poles, in addition to causing uneven heating, sometimes also cause lap welding and ripple welding, can not guarantee the welding quality of the weld. The contact welding method is less affected by the outer diameter of the steel pipe, and is more suitable for large-diameter and thick-wall welded pipes.
Contact welding has higher requirements on the surface quality of the tube billet than induction welding, and the surface is uneven, rust and dirt will prevent the normal flow of current.
When welding coated materials (aluminized or galvanized), the touching parts will adhere to the coating, which also needs to be cleaned frequently.
Four effects of high frequency welding
The principle of high frequency welding is based on the Joule-Lenz law of electromagnetism, and makes full use of the characteristics of current proximity effect, skin effect, ring effect, and sharp Angle effect. When alternating current is passed into the induction ring, the variable magnetic flux will cause the tube blank to be welded to be subjected to electromagnetic induction, resulting in induction potential and induction current. The direction of these currents always obstructs the change in magnetic flux that causes the induced current, converting the electrical energy into electrical energy in the circuit of the tube billet to be welded, and then into the form of joule heat energy.
Especially under high frequency current, most of the induced current and Joule heat energy in the tube billet will converge to the surface of the tube billet and the two edges of the adjacent tube billet, and welding will be realized under the action of external extrusion pressure. Therefore, high-frequency welding uses the principle of electromagnetism, and gives full play to its own characteristics in the welding process, so as to achieve the purpose of welding.
The outstanding advantage of high-frequency welding is energy saving, thanks to the four effects of induced current:
Skin effect
Skin effect is the phenomenon that the current inside the conductor is unevenly distributed when there is an alternating current or an alternating electromagnetic field in the conductor. As the distance from the conductor surface gradually increases, the current density inside the conductor decreases exponentially. And as the current frequency increases, the current in the conductor will gradually concentrate on the conductor surface. From the cross section perpendicular to the direction of the current, there is almost no current flowing in the center of the conductor, and there is only a current flowing in the outer edge of the conductor, as if concentrated on the “skin” of the conductor, so the skin effect is named. If a conductor passes through a high-frequency current, then, due to the role of distributed inductance, the external inductance blocks most of the applied voltage, and only the resistance near the surface flows through a larger current (85% to 90% of the total current), and because of the distributed electroinduced voltage drop, the surface pressure drop is the largest, and the pressure drop from the surface to the center is gradually reduced, so that, From the surface to the center of the current is also smaller and smaller, or even no current, the skin effect of high-frequency current is more and more obvious, and most of the current is gathered to the conductor “skin”.
Proximity effect
When an equal and opposite alternating current is applied to two adjacent conductors, current flows through the adjacent inner surface layer of the two conductors. When two conductors pass through an alternating current of equal size and in the same direction, current flows through the outer surface layer of the two conductors. Proximity effect makes the current distribution in the conductor further uneven, it is this uneven achievement of high frequency welding.
Ring effect
When the high-frequency current passes through the cylindrical, circular or spiral cylindrical tubular parts, the maximum current density distribution is concentrated in the inner side of the cylindrical shape (circular or spiral cylindrical tubular parts), this phenomenon is called the ring effect;
The magnitude of the ring effect is related to the current frequency and the radius of curvature of the ring. The higher the frequency f and the smaller the curvature radius r, the more significant the ring effect is.
Sharp Angle effect
When the parts have sharp corners, and the height of the parts is less than the height of the sensor, in the induction heating, the eddy current intensity at the sharp corners of the parts is the largest, the heating is the fastest, and the overheating is caused, this phenomenon is called the sharp corner effect. Therefore, in the high-frequency welding, the burrs and sharp corners of the welding parts must be removed.
Influencing factors of high frequency welding
Frequency
Frequency is a very important parameter in high-frequency welding, which needs to be reasonably adjusted according to factors such as thickness, material and welding speed to achieve the best welding effect.
1. Energy consumption
High-frequency induction welding pipe heating and other induction heating, the electrical efficiency of the coil improves with the increase of frequency, so it is hoped to use a higher frequency under the premise of meeting the process requirements; In addition, the frequency determines the depth of the heating layer, that is, the width of the welding heating. The higher the frequency, the shallower the heating layer, and the lower the energy consumption, so from the perspective of energy saving, it is hoped to choose a higher frequency.
2. Welding speed
The choice of welding speed and frequency also has a certain relationship. The welding speed is fast, that is, the heating time of the weld is short, and the heat transfer time is short. When heating the same width is required for the process, the frequency of welding speed is lower when welding speed is fast than when welding speed is slow.
3. Billet thickness
The frequency is also closely related to the thickness of the tube billet wall. In order to diathermy along the wall thickness and heat a sufficient amount of high temperature metal for pressure welding, the thick wall tube should be used at a lower frequency than the thin wall tube.
Rendezvous Angle
The meeting Angle refers to the contact Angle of the welded joint of two workpieces. It is very important to determine the appropriate meeting Angle for high-frequency welding.
If the rendezvous Angle is too small, the proximity effect will significantly increase the current density of the preheating section’s surface and side, which is conducive to improving the welding speed. However, when the rendezvous Angle is too small, the preheating section and the melting section will become longer, which makes the flash process of the melting section unstable, easy to form deep pits and pinholes, affecting the welding quality of the weld.
If the rendezvous Angle is too large, the length of the melting section will be reduced, which makes the flash process of the melting section more stable. But at the same time, the proximity effect will weaken the current density of the preheating tube billet surface & side, so that the time to reach the welding temperature is extended, thus affecting the efficiency of high-frequency welding, and the power consumption will increase. Especially in the forming of thin-wall steel pipe, the meeting Angle is too large to cause the edge of the tube to elongate, appear wavy crease, and then affect the quality of high-frequency welding.
Therefore, we need to choose the appropriate meeting Angle according to the specific situation to ensure the stability and excellent high-frequency welding effect.
Power
It is very important to control the input power when performing high frequency welding. Improper power setting may lead to insufficient heating on the side of the preheating section tube billet and fail to reach the welding temperature, resulting in non-welding defects such as false welding, unwelding and pinch welding, or excessive heating may lead to serious burning defects such as spatter, pinhole and slag inclusion, affecting the welding quality and stability of the weld.
To solve these problems, it is necessary to adjust the input power according to the tube wall thickness and forming speed. Different forming methods, unit equipment and material steel grades need to be personalized welding area process preparation, the experience summary is applied to the first line of production, only in this way can ensure the weld quality and continuous stability of high-frequency welding.
Welding speed
In the process of high-frequency welding, when the side of the tube billet is heated from room temperature to welding temperature, the side of the tube billet has no protection and is completely exposed in the air, which inevitably reacts violently with oxygen and nitrogen in the air, resulting in a significant increase in oxygen and nitride in the weld. At the same time, a large number of alloy elements such as manganese and carbon, which are beneficial to the weld, are burned and evaporated, which reduces the mechanical properties of the weld. Therefore, for the welding speed, the slower the welding speed, the worse the quality of the weld.
In addition, the slower the welding speed, the longer the heated tube billet side will be exposed to the air, which will cause non-metallic oxides to be produced at a deeper level, and in the subsequent extrusion crystallization process, they are difficult to be completely extruded from the weld, so an obvious fragile interface is formed. Thus, the continuity of weld tissue is destroyed and the strength of weld is reduced. The faster the welding speed, the shorter the oxidation time, the less non-metallic oxides generated and limited to the surface layer, it is easy to be extruded from the weld in the subsequent welding, and the strength of the weld is improved.
Pipe blank groove
The cutting and design of groove are usually involved in the process of high frequency welding of tube billet. Some manufacturers in the cutting groove, directly into the process of high-frequency welding, the use of “I” groove, this shape of the groove in the plate thickness of more than 8~10mm, easy to produce high internal burrs, and the sharp Angle effect will cause the center layer and the outer layer of insufficient heating problem, affecting the strength of high-frequency welding weld.
Therefore, in the production of thick wall pipes, it is best to use edge planing or milling treatment, so that the groove presents an “X” shape, so as to achieve uniform heating and ensure the quality of the weld. At the same time, the selection of the groove shape will also affect the size of the adjustment of the meeting Angle, and it also has a very important role in controlling the internal quality of the weld and the manufacturing quality of the welding structure.
In general, the design of the groove must consider the factors such as the fusion ratio of the base material, welding space, welding position and comprehensive economic benefits, and the groove form is crucial to the manufacturing quality of the welding structure and the internal quality control of the weld.
Impedance device
Impedance is a device used to enhance the skin effect and proximity effect of high frequency current, thereby improving the welding efficiency. Usually made of M-XO/N-XO iron oxidation system, they are burned at high temperature and high pressure to form a magnetic bar impedance, and put into a heat-resistant and insulated magnetic bar protection sleeve, and must be cooled by cooling water to cool the magnetic bar impedance to maintain its stable working temperature.
The setup of the impedance device needs to match the diameter of the pipe to ensure the corresponding magnetic flux. In addition, in order to ensure the permeability of the impedance, in addition to the material requirements, the ratio of the cross-sectional area of the impedance to the cross-sectional area of the pipe diameter also needs to be large enough. When producing high-grade pipes, internal burrs need to be removed, so the impedance device can only be placed inside the internal burr knife. In this way, the cross-sectional area of the impedance will be much smaller, and the concentrated sector arrangement of the magnetic bar is more effective and saves the amount of magnetic bar than the ring arrangement.
The distance between the impedance and the welding point also affects the welding efficiency. The clearance between the impedance device and the inner wall of the tube is generally 6~15 mm, and the upper limit value is taken when the diameter of the tube is large. The impedance device should be concentrically placed with the pipe, the distance between the head and the welding point should be 10~20 mm, and the diameter of the pipe should be larger.
Welding pressure
Welding pressure is one of the important parameters of high frequency welding, which has a direct impact on weld quality. In high-frequency induction welding, the welding pressure increases the amount of extrusion on the short axis of the steel pipe, which allows better control of the pipe diameter and weld position. Proper pressure can improve the heat transfer efficiency in the welding process, promote the formation of metal melt pool and deep penetration, and effectively eliminate porosity, inclusion, cracks and other defects. However, if the welding pressure is too high, it will lead to excessive plastic deformation, making the weld narrow, and even cracking and fragmentation, thus affecting the reliability and durability of the weld. Therefore, in high-frequency welding, controlling the welding pressure is one of the keys to ensure the quality of the weld.
In summary, high-frequency welding is a welding technology widely used in metal material connection. The high frequency current is generated by the high frequency power supply, and the induced current is formed in the workpiece and the principle of resistance heating is used for welding. High-frequency welding has the advantages of strong joints, fast welding speed and low energy consumption, so it has been widely used in various industries. NEW VICTOR TUBE MILLS focuses on the research of high frequency straight seam welding pipe machine for more than 20 years, and is committed to serving high frequency welding pipe technology in various fields. We will provide high quality high frequency welded pipe production line and complete after-sales service, if you have purchase intention, welcome to contact us.