The properties of the heat-affected zone of the weld seam of the steel roll forming machine are closely related to the quality of the welded pipe. The width, metal streamline angle and metallographic structure of the weld heat-affected zone under different process parameters are studied by examples, and the microscopic approach that is conducive to stability and improving the welding quality of the welded pipe is explored.

When the output power of the steel roll forming machine is fixed, under the action of the skin effect and the proximity effect of the thinner tube wall, the isotherm of the edge of the tube blank before the welding point is more evenly distributed on the cross section, so the metal melting area is more uniform. The scope of the heat-affected zone and the heat-affected zone is small, and a small extrusion force is enough to remove the slag and weld reasonably, forming a narrow and ideal heat-affected zone; while for a thicker tube wall, the isothermal temperature of the edge of the tube blank The lines are distributed in a wide range on the cross-section. When the neutral layer reaches the welding temperature, the edges and corners have been partially over-burned. See Figure 3 for the isotherm diagram of welding points with different wall thicknesses

Obviously, the closer to the welding point, the higher the temperature, and when the tube wall is thick, ① due to the skin effect and proximity effect unique to the high-frequency current at the edge of the tube blank, the edges and corners of the neutral layer have not yet reached the welding temperature. Overheating may occur, and in order to implement reasonable welding, the melting zone must be widened so that the neutral layer can be properly welded. As a result, the heat-affected zone must be correspondingly widened, and the edges and corners may be over-burned; They are pursuing the contact between the two sides of the tube blank as much as possible during welding, and have thought a lot about the design of the forming roll and the adjustment of the extrusion force. However, in practice, the thicker the tube wall, the greater the deformation stress and the springback The phenomenon is also larger, especially in the production of thick-walled and small-diameter welded pipes. In fact, when welding thick-walled pipes, the inner side of the pipe wall is often contacted first, which is why the metal streamline angle of the thick-walled pipe is prone to asymmetry twice inside and outside the wall thickness. As long as this asymmetry is not serious and the angle is not less than 40°, the quality of the weld can still be guaranteed. As long as the extrusion force can extrude the overheated Widmanderman structure (Note 1) and slag into the burr, it can be To achieve the purpose of ensuring quality, without deliberately pursuing the heat affected zone, the wall thickness must reach 1/4~1/3

2.2 Solutions

When the steel roll forming machine is adjusted in the thick-walled tube process, the position of the induction coil should be moved back appropriately to lengthen the heating time; the V-shaped opening angle should be appropriately reduced to increase the proximity effect, so that the high-frequency current in the load loop is highly Concentrated, the heating efficiency can be significantly improved; at the same time, the extrusion force can be appropriately increased to effectively prevent reflow inclusions to ensure the welding quality.

In order to solve the disadvantages of thick-walled pipes in welding, the following 4 measures can be taken:

(1) When there is only one pre-bending roller in the row forming unit, one pre-bending roller can be added when the equipment layout allows;

(2) When the steel strip is longitudinally cut, the steel strip shall be coiled in a reasonable arrangement direction, that is, the adjacent two strip steels shall be coiled in opposite directions, so that each strip steel can enter the forming machine with a normal trapezoidal section. Make it easier for the two edges to be parallel before welding;

(3) When designing the extrusion roller, consider making the tube blank close to a micro-oblate circle at the welding point, leaving a space that tends to be parallel to the sides of the tube blank;

(4) A planing and milling device is installed in front of the forming machine, so that the edges and corners on both sides of the steel strip are planed and milled before entering the forming machine, so as to avoid the welding quality due to overburning of the edges and corners during the welding process.

Epilogue

The shape of the welding heat-affected zone of the steel roll forming machine is closely related to the selection of the welding process parameters. For welded pipes with different wall thicknesses, the width of the welding heat-affected zone and the metal streamline are allowed to have a certain difference. When the value of t/D ratio (Note 2) increases, corresponding technical measures should be taken to seek reasonable heat-affected zone properties, which is conducive to stably improving the quality of welded pipes.