What Welders Must Know: Macro Analysis of Weld Defects

The quality requirements for welding structure, welding products and welding joints are various, including the internal requirements of joint performance and organization.

At the same time, there should be no defects in the appearance, shape and dimensional accuracy, weld formation, surface and interior.

In order to find and solve problems as soon as possible, macro analysis is often used first, and if necessary, detailed microscopic analysis is carried out.

The most important content of macro analysis is defect analysis of welded joints.

The metallographic microscope low magnification structure analysis method is mainly used to determine the cause of the internal defects of the welded joint through the metallographic low magnification inspection, together with the high magnification microstructure analysis, find out the methods to avoid and eliminate, and improve the quality of the welded joint.

Through sampling, grinding, etching and low magnification photography, we can clearly and intuitively check the macro defects of the welded joints, and combine the corresponding welding standards to judge whether the welding process, welding workers and welding structure can meet the relevant use requirements.

According to the formation causes and defect forms, the macro defects of welds can be mainly divided into the following categories:

1. Stoma

During the crystallization process of the welding pool, some gases may remain in the welding mirror before they escape to form stoma.

Stomas are common defects in welded joints.

Stoma not only appear on the surface of the weld, but also often appear in the interior of the weld.

They are not easy to be detected by simple methods in welding production, which will cause serious harm.

Welding pores produced inside the weld are called internal pores, and pores opened outside are mostly called surface pores.


2. Slag inclusion

Slag inclusion is the slag or other non-metallic inclusions in the weld, which is a common defect in the weld.

In welding with flux filled wire, such as submerged arc welding, slag can be formed by dust due to poor deposition, or in CO2 welding without flux, slag can be formed by deoxidized products and remains in the multi-layer welding metal.

Slag inclusion

3. Incomplete penetration and fusion

Incomplete penetration refers to the part left by incomplete penetration of the root of the joint during welding.

Incomplete fusion is a common defect, which refers to the local residual gap between the fusion welding metal and the base metal, or between the fusion welding metal and the adjacent weld bead and the weld layer.

The part between the base metal and the base metal that is not completely fused during spot welding is called incomplete fusion.

Incomplete fusion
Incomplete penetration

4. Cracks

Welding cracks can be divided into hot cracks (crystallization cracks, high-temperature liquefaction cracks, multilateralization cracks), cold cracks (delayed cracks, hardening embrittlement cracks, low plastic cracks), reheat cracks, lamellar tears, etc. according to their morphology and causes.

Root crack

5. Undercut

Undercut is sometimes called undercut, which is the groove below the surface of the base metal at the toe of the weld because the deposited metal does not completely cover the melted part of the base metal during welding.

It is the gap left after the welding arc melts the edge of the weldment without being supplemented by the molten metal of the welding rod.

Too deep undercut will weaken the strength of the joint, and may also cause structural damage at the undercut.


6. Other defects

In addition to the above defects, the common defects of the weld are porosity, cold lap, burn through, weld beading, shrinkage cavity, pit, collapse, uneven weld leg size, excessive concavity / convexity, and unequal weld toe angle.

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