Welding knowledge of spiral finned tubes

High-frequency welding is generally used for the welding of spiral finned tubes.
1. Principle of spiral finned tube welding
The spiral finned tube uses high-frequency current as a heat source to heat the contact surface and the area to be welded, and at the same time applies a forging force to the outside of the finned tube to achieve solid-phase welding of the contact surface.

2. Preparation before welding of spiral finned tubes
(1) There should be no obvious pits on the surface of the tube material, and sandblasting should be performed to remove oil, rust and other dirt.
(2) The preparation of the forging wheel should ensure that its clamping of the steel strip in the circumferential direction is moderately tight and uniform, and the internal parts can rotate flexibly.
(3) The welding between the electrode body and the contact should be firm and reliable, cooling should be smooth, and there should be no leakage.

3. Welding parameters of spiral finned tubes
The high-frequency welding finned tube mainly includes the tensile strength and welding rate of the welded joint. The indicators of these parameters must meet the national standards, such as Q/SH.MM.25-2000, etc.

4. Welding inspection of finned tubes
Appearance inspection: Perform appearance inspection after welding, and the inspection results should meet the standard requirements.
Welding rate inspection: Peel the finned tube 2 to 3 times, and inspect it every 120 degrees. The welding rate can be obtained by dividing the actual weld width by the nominal thickness of the steel strip.
Metallographic inspection: From the joint surface, it is the incomplete normalized structure and the original normalized structure. Check whether there are macro defects such as inclusions, unfusion and cracks.

5. The connotation of high-frequency welding of finned tubes
High-frequency welding of finned tubes is a welding technology, which includes induction welding and contact resistance welding.
There are three welding systems:
Solid phase plastic pressure welding-heat the junction area of ​​the edge of the tube blank to a plastic state of 1300℃-1350℃, and then squeeze the oxide film out of the weld under the pressure of the extrusion roller. At the same time, the two edges are welded together by high temperature, and the extrusion strength requirement is greater than 40-50MPa.
Semi-molten welding – heat the junction area of ​​the edge of the tube to a semi-molten state, and squeeze the liquid film containing oxides into the weld through the extrusion roller, and complete the welding at the same time. Its extrusion strength is generally 20-30MPa.
Melting welding – heat the junction area of ​​the edge of the tube to a molten state, and the welding temperature is above 1400℃. At this time, the molten metal is partially vaporized, and then squeezed by the extrusion roller, periodic spark splashing occurs in the entire liquid overlap area, and its extrusion strength is close to that of semi-molten welding.