TRAIL GAS-SHIELDING SAVES THE DAY . . . AGAIN
At its most destructive, inadequate weld shielding can produce visible pores on the surface of the weld. However, viewed in a different way, visible porosity might sometimes be considered a blessing because visible porosity alerts the welder to damage caused to the weld by nitrogen and oxygen from the atmosphere.
But often times, porosity is not visible from the surface. Rather, these discontinuities lie beneath the surface and are not readily detectable without other non-destructive examination (NDE) techniques, such as radiography. Fortunately (or unfortunately, depending upon your perspective), we were provided with this less common evidence of excess level of nitrogen that, in some circles, has been called weld surface “mottling” – a grayish, splotchy and overall inconsistent surface appearance.
Had surface mottling not been present, our next best alternative to detecting excess levels of nitrogen would have been to conduct a chemical analysis of the weld. Ordinarily the go-to NDE technique would have likely been X-ray, but because of the geometry of the flange yokes, radiographic inspection would not have been feasible here.
Now that we had diagnosed the source of the problem, what was the remedy? Having pored over the abundance of our Application Engineering literature and procedures crafted by my predecessors, I came across a comprehensive report written by a gentleman by the name of Jeff Nadzam, a former member of our Application Engineering department that had detailed a very similar phenomenon.
In that effort, he was charged with the task of developing a Tandem MIGTM procedure for an offshore fabricator that would reduce the amount of nitrogen pick-up. And hopefully, not only would the level of surface mottling decrease, but the weld impact properties would be also be greatly improved.
The mottling effect that had been observed in this application was being caused by the intense arc energy of the high-deposition rate Tandem MIGTM process, causing the puddle to stay molten for a considerable length of time after the gas shielding of the welding torch had progressed further along the joint. So, what was Jeff’s solution? In addition to tweaking the welding parameters, he put into service a “trail-gas shielding device”.
Trail gas shielding devices such as the ones shown in Figure 1 have been shown time-and-again to have a dramatic effect on the observed surface mottling. These devices mitigate the absorption of nitrogen from the atmosphere as the weld cools and solidifies. They are designed in such a way that the gas flow is spread evenly along the entire surface area of the device, resulting in low level of gas turbulence. The key to their implementation is placing them as close as possible to the welding torch, but not so close as to aspirate air into the molten weld.
Now the only decision left to make was which shielding gas to use. For our flange yokes, we chose welding-grade argon, although this was not mandatory. We could have just as easily picked the same shielding gas that was being used to protect the arc as the one to be dispersed through the trail gas shielding device. In fact, if the same gas is to be used for both, a “splitter” can be applied so that the same regulator and gas bottle can be used.
At the end of the day the welds obtained during procedure development on our customer’s flange yokes were a vast improvement over the welds obtained with their current process. The mottled surface seen previously was replaced by a surface nearly free of discoloration. That being said, however, word of caution bears mentioning. If a welding procedure has previously been qualified, a change in the volume of shielding gas flow may require re-qualification or a change to the welding procedure specification. This can be of particular importance in an audit.
Now let’s fast-forward to 2012.
About the Author: Regis Geisler III is a registered Professional Engineer, Specials Certification Engineer and AWS Certified Welding Inspector who has worked the last eleven years at The Lincoln Electric Company, 22800 Saint Clair Avenue, Cleveland, OH 44117-8542, 216-481-8100, Fax: 216-486-1751, www.lincolnelectric.com. For questions or comments on this column, contact Regis at firstname.lastname@example.org.