FAILURES AT WELDS: ARE THEY ALWAYS CAUSED BY WELD FLAWS?

PHILIPPA MOORE, PhD, CEng, FWeldl, FIMMM, IWE

Failures at weld locations can be initiated by other factors besides welding defects.

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In engineering failures, we often see examples of pressure vessels bursting along their seams or bridge decks where cracks have started at welds. While it might be tempting to consider faulty welding to be the cause of the deterioration, but there could also be other reasons for these failures. The welding shouldn’t always be the first to blame, and it’s important that a thorough forensic investigation is completed to objectively establish the origin of component failures.

Causes of Failures at Weld Locations

Failures at weld locations can be initiated by other factors besides welding defects. The surface profile of the weld acts as a geometrical stress concentration at the weld. Even in the highest quality welds, excessive structural loading or over-pressurization could create a rupture at the weld location. Vibrations, too, can initiate fatigue cracking at welds due to this stress concentration effect.

Residual stresses are also intrinsic to welded joints. As the small region of hot weld metal cools within the surrounding ambient temperature material, thermal strains are formed and become trapped as internal stresses. If exposed to a sufficiently corrosive environment, this residual stress can make components susceptible to stress cracking in the region close to weld. These failures might happen years later in the service life of the component, even in welds fabricated by the most skilled welder.

Investigating Failures at Welds

Forensic investigations of engineering failures should first determine whether the fabrication has followed the established quality procedures for welding. ISO 3834 is an internationally recognised standard that sets the quality requirements for fusion welding of metallic materials in the welding workshop and at field installation sites and covers the welding of safety critical equipment in oil, gas, power generation and manufacturing industries.

As part of ISO 3834, fabricators are required to keep quality records that could be requested to support an investigation into a failed weld. Such documents include:

+ welding procedure specification (WPS) listing the essential parameters for making that weld.

+ welding procedure qualification records (WPQR) comprising the test and inspection certificates that demonstrate suitability of the WPS to produce a quality weld.

+ welder qualification certificates demonstrating ability to perform the welding with sufficient quality.

+ inspection and non-destructive testing records demonstrating the presence or absence of flaws associated with the welds following fabrication.

Following a review of these relevant documents, the investigator can then effectively evaluate the conditions experienced by the weld to understand the cause of the failure both in terms of design and the actual situation during operation. An analysis of maintenance and inspection records and examination of the component itself will help uncover any evidence of degradation mechanisms such as corrosion, embrittlement, and stress-corrosion or fatigue cracking. Microscopy, metallography and testing may also be used to support the investigation where necessary.

Avoiding Failures in Welded Structures

No weld is perfect after it’s created, but fabrication following welding best practice is intended to ensure that any weld flaws are not detrimental to the performance of that weld. To avoid welding failures and ensure a fully fused weld has few flaws, fabrication needs to follow established weld quality systems. And because it cannot be assumed that best practices have been followed, records need to be carefully kept and checked.

The potential in-service degradation that might occur must be also considered, and an inspection and maintenance plan should be implemented throughout the structure’s service life. If all potential damage failure modes have been considered and avoided, welded structures can operate safely for decades.

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