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Principle of High Frequency DC Resistance Welding (ERW)

Date:2020-08-06    keywords:high frequency DC resistance welding, principle
High-frequency welding originated in the 1950s. It is a new type of welding process that uses the skin effect and proximity effect generated by high-frequency current to connect steel plates and other metal materials. The emergence and maturity of high frequency welding technology directly promoted the huge development of the longitudinally welded pipe industry, which is a key process in the production of longitudinally welded pipe (ERW pipe). 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 high frequency mentioned is relative to the frequency of the alternating current of 50Hz, generally refers to the high frequency current of 50KHz~400KHz. When high-frequency current passes through a metal conductor, two peculiar effects are produced: skin effect and proximity effect. High-frequency welding uses these two effects to weld steel pipes. So, what are these two effects?

Skin effect: When a certain frequency of alternating current passes through the same conductor, the current density is unevenly distributed on all cross-sections of the conductor. It is mainly concentrated on the surface of the conductor, that is, the current density on the surface of the conductor, and the density inside the conductor is very small, so we vividly call it the "skin effect". The effect of the skin is generally measured by the penetration depth of the current: the lower the penetration depth, the more significant the effect of the skin. The penetration depth is proportional to the square root of the conductor resistivity and inversely proportional to the square root of frequency and permeability. Simply put, the higher the frequency, the more current is concentrated on the surface of the steel plate; the lower the frequency, the more scattered the surface current is. It should 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 increases, the magnetic permeability will decrease and the effect of the skin will also decrease.

Proximity effect: When high-frequency current flows in opposite directions between two adjacent conductors, the current will flow intensively to the edges of the two conductors. Even if the two conductors have another shorter side, the current does not follow. Shorter route flow, we call this effect: "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 distance between adjacent conductors becomes closer. If a magnetic core is added around the adjacent conductor, the high-frequency current will be more concentrated on the surface of the workpiece.


Tips: ASTM A53 Gr.B is the grade in ERW high frequency welded pipe, divided into A and B grades. ASTM is only a set of specifications. ASTM steel pipe A53 A corresponds to the Chinese standard GB8163 raw material is 10# steel, and A53 B corresponds to the Chinese standard GB8163 raw material 20#.
For example: ASTM (standard) A53 (grade) gr is grade (abbreviation of grade) b (grade B)


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