High-frequency Welding Principle of Longitudinal Welded Pipe

The emergence and maturity of high-frequency welding technology, directly promoted the development of Longitudinal Welded Pipe industry, it is the key processes of Longitudinal Welded Pipe(ERW steel pipe) production. High-frequency welding originated in the fifties of last century, it is a new welding process that uses the skin effect and adjacent effects produced by high-frequency current to make the steel plate and other metal materials docked.Good or bad quality of high-frequency welding directly has an impact on the overall strength, quality grades and production speed of welded pipe products. As a manufacturer of welded pipe, you must understand the basic principles of high-frequency welding, understand the structure and working principle of high-frequency welding equipment, and understand the high-frequency welding quality control points.

Relatively the 50Hz AC current frequency speaking, high frequency is generally refers to 50KHz ~ 400KHz high frequency current. When high-frequency current flow through the metal conductor, it will produce two strange effects: skin effect and proximity effect. And high-frequency welding just uses these two effects to weld the steel pipe. So, what are the those two effects?

Skin effect

Skin effect refers to a certain frequency of AC current flow through the same conductor, the current density is not evenly distributed in all sections of the conductor. It will mainly focus on the surface of the conductor, that is, the density of the current in the conductor surface is big, the density of the conductor inside is small. So we vividly called it "skin effect". Skin effect is usually measured by the penetration depth of the current, the smaller the penetration depth, the more obvious the skin effect. And the penetration depth is proportional to the square root of the resistivity of the conductor, and is inversely proportional to the square root of the frequency and permeability. In layman's terms, the higher the frequency, the more concentrated current will be on the surface of the steel plate; the lower the frequency, the more dispersed current will be on the surface. It must be noted that, although the steel is a conductor, its permeability will drop with the temperature rise, that is, when the steel plate temperature increases, the permeability will decline, skin effect will be reduced.

Proximity effect

The proximity effect refers to the fact that when the high frequency current flows counter currently in two adjacent conductors, the current will flow collectively toward the edges of the two conductors. Even if there is a shorter side in two conductors, the current does not flow along the shorter route. We call this effect "proximity effect". Proximity effect is essentially affected by inductive reactance, inductance plays a leading role in the high frequency current. The proximity effect increases as the frequency increases and the spacing of adjacent conductors approaches. If adding a core around the conductor, the high frequency current will be more concentrated on the surface of the workpiece.

These two effects are the basis for achieving high frequency welding of metals. High-frequency welding just used the skin effect so that the energy of high-frequency current concentrated in the workpiece surface, and used the proximity effect to control the location of high-frequency current flow and range. The speed of the current is very fast. It will heating and melting adjacent to the edge of the steel plate in a very short period of time, and achieve docking through the extrusion.

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