High-frequency welding originated in the 1950s. It is a new 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 straight seam welded pipe industry, which is the key process of ERW production. The quality of high-frequency welding directly affects the overall strength, quality level and production speed of welded pipe products.

The basic principle of high frequency welding The so-called high frequency is relative to the 50Hz AC current frequency, 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 uses these two effects to weld steel pipes. So, what are these two effects? The skin effect means that when an alternating current of a certain frequency passes through the same conductor, the current density is not uniformly distributed on all sections of the conductor, and it will mainly concentrate on the surface of the conductor, that is, the current density on the surface of the conductor is high, The density inside the conductor is small, so we call it “skin effect” vividly. The skin effect is usually measured by the penetration depth of the current. The smaller the penetration depth value, the more significant the skin effect is. This penetration depth is proportional to the square root of the resistivity of the conductor and inversely proportional to the square root of the 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 permeability will decrease as the temperature increases, that is to say, when the temperature of the steel plate increases, the 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 conductors that are close to each other. Even if the two conductors have another shorter side, the current will not flow along the Shorter routes flow, we call this effect: the “proximity effect”. The proximity effect is essentially due to the role of inductive reactance, which plays a dominant role in high-frequency currents. The proximity effect increases as the frequency increases and the spacing between adjacent conductors increases. If a magnetic core is added around the adjacent conductors, the high-frequency current will be more concentrated on the surface of the workpiece. These two effects 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 location and range of the high-frequency current flow path. The speed of the current is very fast, it can heat and melt the edges of the adjacent steel plates in a very short time, and realize the butt joint by extrusion.