PROCESS DESCRIPTION OF HIGH FREQUENCY WELDING
The high frequency current is used as the heat source to heat the contact surface and the area to be welded, and at the same time, the upset force is applied on the outside of the fin tube, so as to realize the solid phase welding of the contact surface.
Preparation before welding
(1) There shall be no obvious pits on the surface of the pipe material. Sand blasting shall be carried out to remove oil stains, rust and other contaminants.
(2) The preparation of the upset wheel shall ensure that the clamping of the steel strip is moderately tight and even in the circumferential direction, and the internals can rotate flexibly.
(3) The welding between electrode body and contact shall be firm and reliable, with smooth cooling and no leakage.
It mainly includes the tensile strength and welding rate of welded joints. The indicators of these parameters must meet the national standards, such as Q/SH.MM.25-2000.
Appearance inspection: conduct appearance inspection after welding, and the inspection results shall meet the standard requirements.
Welding rate inspection: strip the finned tube for 2 to 3 turns 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 examination: from the joint surface, there are incomplete normalized structure and original normalized structure. Check whether there are macro defects such as inclusions, incomplete fusion and cracks.
Connotation of high-frequency welding
High frequency welding is a kind of welding technology, including induction welding and contact resistance welding.
There are three welding systems:
- Solid plastic pressure welding - heat the junction area at the edge of the tube blank to a plastic state of 1300 ℃ - 1350 ℃, and then extrude the oxide film from the weld under the pressure of the extrusion roller. At the same time, weld the two edges together through high temperature. The extrusion strength is required to be greater than 40-50MPa.
- Semi melting welding - heat the junction area at the edge of the tube blank to the semi melting state, extrude the liquid film with oxide from the weld through the extrusion of the extrusion roller, and complete the welding at the same time. The extrusion strength is generally 20-30MPa.
- Melting welding - heat the junction area at the edge of the tube blank to the melting state, and the welding temperature is above 1400 ℃. At this time, the molten metal is partially gasified, and then through the extrusion of the extrusion roller, periodic spark splashing occurs in the entire liquid lap zone, and the extrusion strength is close to that of the semi melting welding.