• Categories:Industry news
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• Time of issue:2021-11-05
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(Summary description)1、 Appearance defect appearance defect (surface defect) refers to the defect that can be found from the workpiece surface without the help of instruments. The common appearance defects include undercut, weld bead, depression and welding deformation. Sometimes there are surface pores and surface cracks, and the root of single-sided welding is not fully welded.

A. Undercut

It refers to the depression or groove formed in the base metal part along the weld toe. It is the gap left by the lack of sufficient supplement of deposited metal after the arc melts the base metal at the weld edge.

The main causes of undercut are that the arc heat is too high, that is, the current is too large, the strip conveying speed is too small, the angle between electrode and workpiece is incorrect, the swing is unreasonable, the arc is too long, and the welding sequence is unreasonable. The magnetic bias blow of arc in DC welding is also a cause of undercut. Some welding positions (vertical, horizontal and vertical) will aggravate undercut, which reduces the effective cross-sectional area of base metal and the bearing capacity of the structure. At the same time, it will also cause stress concentration and develop into crack source.

Prevention of undercut: correcting the operation posture, selecting reasonable specifications and adopting good strip transportation methods will help to eliminate undercut. When welding fillet weld, AC welding instead of DC welding can also effectively prevent undercut.

B. Weld bead

The liquid metal in the weld flows to the base metal that is not melted due to insufficient heating or overflows from the root of the weld. The metal tumor formed after cooling that is not fused with the base metal is the weld bead. Too strong welding specification, too fast melting of welding rod, poor quality of welding rod (such as eccentric core), unstable characteristics of welding power supply and improper operation posture are easy to cause weld beading, which is easier to form weld beading in horizontal, vertical and overhead positions.

Weld beading is often accompanied by incomplete fusion and slag inclusion defects, which is easy to lead to cracks. At the same time, the weld bead changes the actual size of the weld and will bring stress concentration. The weld bead inside the pipe reduces its inner diameter and may cause blockage of flow animals.

Measures to prevent weld beading: make the weld in the flat welding position, correctly select the specification, select the non eccentric core electrode and operate reasonably.