Welding Imperfection and Cavities: Causes and Prevention23 November 2018
Inferior welds don’t just look bad, they deliver poor structural performance, too. The shoddy work won’t pass muster as a mechanically sound metal joint. There are weld imperfections in the seam, which have been visually or instrumentally detected. Cavities are undermining the joint, so it cannot deliver material integrity. If we’re to stop such process defects from sabotaging a welding team’s work, let’s dig into the causative factors.
The Metallurgical Detectives
Checking out both the equipment and its consumable components, the welding inspector gets his bearings. There’s a series of cavities on a length of load-bearing steel. The metal is inordinately strong and durable, but the cavities are weakening it structurally. First off, is the welding gear in optimal condition? Wet flux rods and damaged electrodes cause problems, which appear as weld cavities. Replace the equipment electrodes. If the issue persists, adjust the shield gas supply. Stubbornly, the cavities hang around, so the inspector advises a secondary HAZ cleaner, one that’ll remove trace hydrocarbons, dirt, and oils. Finally, the welding cavities disappear, the steel beams anchor, and the strength-crippling weld discontinuities are eliminated.
Crack and Micro Fracture Sleuths
Trapped process gasses are under control. The porosity problems are managed, and the operation is in full flow once more. Only, there are cracks troubling a second area on the same project. Off the weld inspector goes, dispatched promptly to the site of the cracking incidents. Pulling out a notebook and camera, this is the moment an initial report is recorded. Crack types are consulted. Among them, one of the following fracture types is propagating:
- Transverse cracks
- Longitudinal fractures
- Radiating discontinuities
- Cratering splits
- Forking cracks
What’s causing the weld-compromising fractures? Impurities are a problem. Phosphorous or carbon, perhaps sulphur, the contaminants are causing solidification stress. As a weld cools, hot cracking effects result in inter-granular formations, which interfere with the solidification process. Other crack producing agents include precipitation induced fractures, temperature stress, hydrogen levels, and nearby surface contaminants.
It’s the weld inspector’s job to visually examine the imperfections. Tools come out, so does a camera and a surface-penetrating instrument. If there are cavities or porosity problems that are impacting joint integrity, the inspector’s report will mandate action. Then, with the equipment or heat zone problem corrected, the work continues. As for a weld seam that appears purely amateurish, it can still be accepted if its underlying structure is mechanically sound. Granted, a run through with a surface grinder will likely be recommended. Finally, for cracks, the fracture cause must be found, identified and addressed.
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