This paper analyzes the causes of the welding defects in the production process of the high-frequency straight seam welded steel pipe and the reasons of the crack in the flattening test of the welded pipe. Combining the production process parameters of the high-frequency straight seam welded steel pipe and the welding quality The control method of its production process parameters is put forward to improve the welding quality of welded pipe.
High-frequency longitudinal welded pipe is the use of high-frequency current skin effect and the proximity effect, forming a good tube to be welded quickly heated to a certain temperature, the role of the squeeze roll to complete the welding. It can be seen that the welding quality of welded pipe by the input heat, welding pressure, open angle, the edge of the shape of the tube, the electrode and the impedance of the placement, water cooling conditions and other factors. These factors are affected by the production environment and other factors have fluctuations, making the welding quality of high-frequency welded pipe is not easy to control.
The market needs high-quality welded pipe, and welding quality is the decisive factor affecting the quality of welded pipe. From the data we can see, the production of high-frequency longitudinal seam weld cracks can be divided into nine categories, most of which can be observed by hydrostatic test or the naked eye.
Welded pipe plant production of raw materials used mainly for Q235A, Q235B and other carbon structural steel hot-rolled strip, 60 or 114 forming sizing unit by the molding, welding for high-frequency welded steel pipe. In the flattening test, there are mainly two cracks on the weld, which are fusion line and 1 ~ 3mm away from the fusion line, the former only accounts for 10% of the total number of unqualified samples, while the latter accounted for 90 / RTI & gt; In the case of cracks, the low magnification structure was observed after acid etching.
Most of the cracks appearing on the weld fusion line are cracks. This indicates that the metal of the two edges of the tube is not sufficiently bonded to each other, and there may be air, inclusions, voids or the like on the bonding surface, that is, the metal atoms on the fusion line are separated by a distance exceeding the distance between the normal metal crystals, Thus, when the tension on both sides of the weld line, the first cracks in the weld line, resulting in weld cracking.
The reasons for this phenomenon are:(1) the lack of heat input, resulting in welding speed is too low, the weld metal to be welded edge metal heating temperature is too low, the side of the metal does not melt, while the width of the heat affected zone but increased, resulting in oxidation of the edge to be welded Metal, not all of the weld, the metal to be welded edge of the binding force reduction. (2) in the welding process of erw steel pipe, the metal to be welded side of the heating temperature is too high, the opening angle is too small, the formation of the metal lintel and squeeze roller centerline too far away, that is, after the molten metal is no longer beam after the current through the start cooling, oxide reflux, squeeze pressure in the squeeze roller, the oxide can not be all discharged from the weld, the formation of reflux inclusions, the weld zone morphology.