Key Points of High Frequency Welding Process

Welding of Non-alloy Steel and Low-alloy High-strength Steel Pipes

Non-alloy steel tubes with a carbon equivalent (Ceq) of less than 0.2% have good high-frequency welding performance and do not require heat treatment after welding. Non-alloy steel tubes with a Ceq of 0.65%~0.7% have poor weldability, and the weld seam is prone to brittle fracture, so welding is prohibited. The Ceq value of low-alloy high-strength steel tubes is usually between 0.2%~0.65%.

After welding, the overheat zone has coarse grains and hardening structures are formed, leading to welding stress, so post-weld heat treatment is required. There are mainly two methods of post-weld heat treatment. For steel pipes with a larger diameter above Φ200mm, online normalizing heat treatment is usually adopted.

This involves heating the heat-affected zone of the weld seam to about 927℃ with a medium-frequency induction heating device after removing the external burr of the steel pipe, then air cooling to below 538℃ before water cooling and sizing. This method is efficient and does not cause significant oxidation of the pipe material, thus it is widely used.

For smaller diameter steel tubes, conventional overall treatment can be used, which involves heating the tube blank to over 900℃ using medium-frequency induction or flame heating methods, then air cooling or cooling in a controlled atmosphere cooling chamber. When welding tube blanks containing elements that easily form refractory oxides, such as chromium, welding defects are easily produced.

To reduce oxide inclusions in the weld seam, a neutral gas stream (N2) can be sprayed at the high-frequency welding device and inside the tube blank for gas protection.

Welding of Stainless Steel Pipe Seams

Stainless steel has poor thermal conductivity and high electrical resistivity. Compared with other steel materials of the same diameter and wall thickness produced by high-frequency welding, it consumes less thermal power and has a higher welding speed under the same input electric power.

However, due to the high-temperature strength of stainless steel, a larger squeezing pressure must be applied, usually 40~50MPa higher than when welding low-carbon steel pipes. The main issue with high-frequency welding of stainless steel pipe seams is that the corrosion resistance of the heat-affected zone is reduced due to carbide precipitation.

This problem can be avoided and the precipitation of carbides in the heat-affected zone can be limited by pre-solution treatment before welding, increasing the welding speed, and rapidly cooling the pipe material through a cooler after welding to obtain a joint with good corrosion resistance.

Welding of Aluminum and Aluminum Alloy Pipe Seams

Aluminum and aluminum alloys have a low melting point and are easily oxidized. During welding, the joining surface is quickly heated to the melting temperature, and intense oxidation occurs, forming a high melting point oxide film (Al2O3).

In order to extrude the oxide film from the weld point, it is necessary to increase the welding speed and squeezing speed, shorten the dwell time at the liquid temperature, reduce the temperature drop caused by heat dissipation, thereby promoting the extrusion of the oxide. When high-frequency welding aluminum alloy tube seams, the welding speed is relatively high, about twice that of steel pipe welding.

At the same time, aluminum alloy is a non-magnetic material, and the high-frequency current penetration depth is large. A higher power source frequency should be selected, and the voltage and power of the high-frequency power source should have a high degree of stability and a small fluctuation coefficient.

Welding of Copper and Copper Alloy Pipe Seams

Copper and copper alloys are non-magnetic materials with good electrical and thermal conductivity. Therefore, a higher frequency and welding speed must be used during welding to concentrate the electrical energy more strongly on the surface of the joint, reducing heat energy loss.

When high-frequency welding brass tube seams, the surface of the joint is heated to the melting temperature, and zinc is easily oxidized and vaporized. Therefore, rapid heating and squeezing are also required to thoroughly extrude the melted metal and oxide.

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