Welding of erw pipe

Welding of erw pipe

High frequency welding originated in the 1950s. It is a new type of welding process that uses the skin effect and adjacent effect generated by high-frequency current to connect steel plates and other metal materials. The emergence and maturity of high-frequency welding technology has directly promoted the huge development of the longitudinal welded pipe industry, and the longitudinal welded pipe is a key process in the production of longitudinal welded pipes. The quality of high-frequency welding directly affects the overall strength, quality level and production speed of welded pipe products.

Basic Principles of High Frequency Welding

The so-called high frequency is relative to the AC current frequency of 50Hz, and generally refers to the high frequency current of 50KHz~400KHz. When a high-frequency current passes through a metal conductor, two peculiar effects will occur: the skin effect and the proximity effect. High-frequency welding is to use these two effects to weld steel pipes. So, what are these two roles? The skin effect means that when an alternating current of a certain frequency passes through the same conductor, the current density is not evenly distributed on the cross-section of the conductor, but mainly concentrated on the surface of the conductor, that is, the density of the current on the surface of the conductor is high, and the density inside the conductor is small. So we vividly call it the "skin effect". The skin effect is usually measured by the penetration depth of the current. The smaller the penetration depth value, the more pronounced the skin effect. This penetration depth is proportional to the square root of the conductor resistivity and inversely proportional to the square root of frequency and permeability. In layman's terms, the higher the frequency, the more concentrated the current is on the surface of the steel plate; the lower the frequency, the more dispersed the surface current. It must be noted that although steel is a conductor, its magnetic permeability will decrease with the increase of temperature, that is to say, when the temperature of steel plate increases, the magnetic permeability will decrease and the skin effect will decrease.

The proximity effect means that when high-frequency current flows in opposite directions in two adjacent conductors, the current will concentrate on the edges of the two adjacent conductors. Even if two conductors have another shorter side, the current will not flow along the shorter path, we call this effect: "proximity effect". The proximity effect is essentially due to the effect of inductive reactance, which plays a dominant role in high-frequency currents. Proximity effects increase with increasing frequency and with increasing spacing between adjacent conductors. If a magnetic core is added around adjacent conductors, the high frequency current will be more concentrated on the surface of the workpiece. These two functions are the basis for realizing metal high-frequency welding. High-frequency welding uses the skin effect to concentrate the energy of high-frequency current on the surface of the workpiece; and uses the proximity effect to control the position and range of the high-frequency current flow path. The speed of the current is very fast, and the edges of adjacent steel plates can be heated and melted in a very short time, and the butt joint can be realized by extrusion.

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