How to avoid cracking at the weld of high frequency welded pipe

In high frequency longitudinal welded pipe, cracks include long cracks, local periodic cracks and irregular cracks. Some steel pipes do not produce obvious cracks immediately after welding, but cracks appear after straightening,flattening and hydrostatic test. So what are the factors causing cracks?

1、Poor quality of raw materials.

In the production of welded pipe, there are often some problems, such as large edge burr, too wide width of raw material and so on. If the width of raw material used and the pass of extrusion roll are toofull, peach shape will be formed, and the external weld scar islarge, which is easy to crack after straightening. When welding, theburr is outward, which is easy to produce intermittent cracks

2、Edgecorner connection status.

Edge corner joint of tube blank is a common phenomenon in welded pipe production. If the pipe diameter is smaller, the corner joint will be more serious. Improper forming adjustment is the premise of corner joint. Large outer cornerof pass, improper pass design of extrusion roll and adjust mentelevation angle of extrusion roll are the main factors affecting angular joint.

The pass of single radius extrusion roll can not eliminate the corner joint problem caused bypoor forming. In the later stage of production, the pass wear of extrusion roll is vertical ellipse, which intensifies the sharp peach welding state and leads to serious corner joint. Corner joint willmelt most of the metal and flow out from the upper part. At thistime, there will be large metal splash, resulting in overheating ofthe joint structure. The external burr has the characteristics of high heat, irregularity, large quantity, difficult scraping and smallinternal burr.

The large fillet on theextrusion roll makes the tube blank insufficient in the extrusionroll, and the edge contact state changes from parallel to V-shape,resulting in incomplete penetration of the internal weld. The wear of the extrusion base bearing and the long-term stress wear of the pressing roller shaft make the two shafts form an elevation angle,resulting in in sufficient extrusion force, vertical ellipse and serious angular joint

3、The selection of process parameters is unreasonable.

The production processparameters of high frequency welded pipe include welding temperature,welding current, welding speed, extrusion force, size and placement position of inductor and impedance, etc. Extrusion force, high frequency power, welding speed and opening angle are important process parameters, which need to be matched reasonably, otherwise the welding quality will be affected.