8 Weld Quality Hot Spots
- October 11, 2020
Learning to recognize and address these weld quality issues will improve inspection results and boost productivity
The production of high quality, highly repeatable welds is the priority of any fabricator or workshop. The importance of welding quality control cannot be underestimated—it is often a question of structural and public safety rather than mere aesthetics. In some instances, the failure of a poor quality weld can result in damage to property, injuries, and even fatalities. We take a look at eight common quality issues found by welding inspectors.
Porosity occurs when excess gas is trapped inside the weld, which creates cavity formation, reducing the overall strength of the join and therefore the quality of the weld. While not all forms of weld porosity can be detected by a welding inspector (many forms require non-destructive testing techniques), porosity that is visible on the surface of the weld presents as small round holes. There are several reasons that surface porosity forms:
Electrodes and filler metals may had been handled improperly during preparation or the welding process itself
Shielding gas may be contaminated or its flow may be restricted
Welders may be using an incorrect welding technique
Lack of Fusion
Lack of fusion occurs when the weld metal does not fully fuse with the base metal during welding. This is especially prevalent at the overlap and slope out of a circumferential weld. For example, when completing a circumferential weld overlap tie-in with a conventional GTAW process, the lack of penetration makes it challenging to create a good fuse; the metal at the beginning of the weld can act as a barrier against full penetration at the point of overlap.
As there is no metallurgical bond between the weld and base metal, incomplete fusion drastically reduces weld strength. It can also cause higher localized rates of corrosion.
Lack of fusion can be caused by many variables, including the use of incorrect welding parameters and an incorrect welding technique.
Distortion is a standard welding problem in GMAW and GTAW. Both of these processes offer low energy density and slow travel speeds while requiring high heat input to create the weld. This high heat input can distort the metal and compromise the overall quality of the weld.
Inclusions occur more commonly when welding thick materials that require several passes, particularly when using flux coated cored rods. Each weld pass generates slag that covers the weld pool. If this slag is not removed before additional passes are made, it can enter the weld, causing contamination and weakness.
Some of the more common causes of inclusions are: a failure to clean a weld pass before applying the next pass; using too much flux; welding at an incorrect angle; and using insufficient amperage.
A weld undercut is an erosion of the base metal, which causes a surface discontinuity at the toe of the weld. The erosion is caused by energy being focused on an area where filler metal has not been deposited. A weld undercut is sharp in shape, which leaves it prone to stress if not treated and can lead to premature structural failure.
Excess energy is often caused by the arc voltage being too high, or an overly long arc. Incorrect angle and usage of electrodes or using a travel speed that is too quick can also cause this issue.
Welding inspectors often encounter inconsistency in the use of welding processes, which impact negatively on the final quality of the weld. It’s certainly possible to achieve process consistency with PAW, GMAW and GTAW, although there are several variables at play.
With PAW, a smooth and consistent keyhole can be created at a low power level, but if the power has to be dialled up to weld thicker sections, the process can drift due to thermal distortion and lens contamination.
The need for a root pass and multiple fill passes, as well as tungsten tip erosion and sensitivity to parameter variation also makes GTAW susceptible to process inconsistency.
Created by the rapid—and often accidental—discharge of an electrical arc, an ark strike is a mark outside of the weld zone on the component or structure being welded. If an arc strike occurs, the base material will heat and then cool rapidly, which can form a hard, brittle microstructure prone to cracking. An ark strike usually leaves a dent on the base material, reducing its original thickness and, therefore, strength.
Arc strikes are usually caused by incorrect welding techniques or improper grounding.
Welding is an extremely technical trade. PAW, SAW, GTAW, and GMAW forms of welding, are all notoriously difficult processes to master, requiring expert tradesmen. Automation takes much of the responsibility off the shoulders of the welder.
While poor preparation is not an issue that welding inspectors will see specifically, it causes quality issues that welding inspectors regularly deal with. GTAW, GMAW and SAW all require extensive joint preparation, which adds to the cost and time of each welding project. As the demand for productivity, efficiency and cost-cutting measures increases, preparation can be neglected, and weld quality suffers.
The ability to weld in a single, full penetration pass virtually eliminates the potential for lack of fusion and inclusions, and dramatically reduces the potential for porosity and other defects typical of multi-pass welding processes. SMT
Bill Kelly is director of market development at K-TIG, a welding technology manufacturer.