High frequency welding
is a new welding process that uses the skin effect and adjacent effect generated by high-frequency current to connect steel plates with other metal materials. The emergence and maturity of high-frequency welding technology is a key process in the production of straight seam welded pipe (ERW). The quality of high frequency welding
directly affects the overall strength, quality grade and production speed of welded pipe products.
The so-called high frequency is relative to the frequency of the alternating current of 50 Hz, and generally refers to a high frequency current of 50 kHz to 400 kHz. When high-frequency currents pass through metal conductors, they produce two peculiar effects: skin effect and proximity effect. High frequency welding
uses these two effects to weld steel pipes.
The skin effect means that when an alternating current of a certain frequency passes through the same conductor, the density of the current is not uniformly distributed over all sections of the conductor. It concentrates the surface of the main conductor, that is, the current density of the conductor is large on the surface of the conductor, and the density inside the conductor is small, so we call it "the skin effect". The skin effect is usually measured by the penetration depth of the current. The penetration depth value is smaller, and the skin effect is more significant. 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 frequency is higher, and the current is more concentrated on the surface of the steel plate; the the frequency is lower, and the surface current is more dispersed. It must be noted that although steel is a conductor, its magnetic permeability will decrease with increasing temperature, that is, when the temperature of the steel plate rises, the magnetic permeability will decrease and the skin effect will decrease.
Proximity effect means that when a high-frequency current flows in opposite directions in two adjacent conductors, current will flow to the close edges of the two conductors. Even if the two conductors have a shorter side, the current does not follow. For shorter routes, we call this effect a "proximity effect."
The proximity effect is essentially due to the effect of inductive reactance. Inductive reactance plays a dominant role in high frequency currents. The proximity effect increases as the frequency increases and the spacing of adjacent conductors approaches. If a core is added around the adjacent conductor, the high frequency current will concentrate more on the surface of the workpiece.
These two effects are the basis for achieving high frequency welding
of metals. High frequency welding
uses the skin effect to concentrate the energy of the high-frequency current on the surface of the workpiece; the proximity effect is used to control the position and range of the high-frequency current flow path. The speed of the current is very fast, it can heat, melt and join the adjacent steel plate edges in a short time.