TRAIL GAS-SHIELDING SAVES THE DAY . . . AGAIN
Trail gas shielding devices have been shown time-and-again to have a dramatic effect on weld surface mottling. Here’s why.
A few years back, a customer sought to improve the appearance of welds obtained in production of their line of flange yokes. They were specifically looking to eliminate the splotchy, gray look that was thought to indicate problems with the weld.
Drawing on our company’s wealth of “tribal knowledge,” the application team of which I was a member was successful in helping our customer improve the appearance of its welds. And years later, in my current role as a Nuclear Certification Engineer, I’ve had the opportunity to re-apply the knowledge I gained in that endeavor to an entirely new situation.
The flange yokes in question were to be welded in a rotating positioner in the 1G position while a fixture was used to hold a welding torch fixed at the appropriate contact-tip-to-work distance and push angle. The surface of the 1040 grade steel was, by most shop fabrication standards, considered “clean” and free from oil and contaminants. The weld joint consisted of a single-sided V-groove weld with a 45 deg included angle and the distance around the circumference of the flange yoke was nearly 28 in.
To join the sections of 1040 steel, 0.045 in (1.2 mm) SuperArc® LA-75 was selected as a continuous solid wire electrode with an AWS classification of ER80S-Ni1 that is intended for use in the gas metal arc welding (GMAW) process. In this application it was used in conjunction with 90 percent argon/10 percent CO2 shielding gas and the standard soft pulse waveform on a Power Wave® 455M.
The idea behind using a pulsing power source was that we had provided ourselves with the potential for reducing the welding heat input that we initially thought was the main culprit behind the not-so-aesthetically-pleasing weld surface the customer had been experiencing.
After some trial-and-error, we discovered that a wire feed speed of 350 ipm – about 9 lb per hour – was our upper limit as to how much weld metal could be carried on the top of the flange yoke. At a surface travel speed of about 30 ipm, the weld was given enough time to wet-out to the edges of the joint and provide the optimum convex bead shape. As a frame of reference this works out to be about the equivalent weld nugget of just under a 3/16 in fillet weld – which can be considered to be a relatively high arc speed.
So, after tweaking the output mode, wire feed speed, torch angle, and the gas flow rate (ultimately we settled at 40 cu ft per hour), we were unfortunately not quite yet where we needed to be regarding surface appearance in order to show a true process improvement.
The problem inherent with a high arc speed is that the weld puddle is still molten or “near-solidified” as it leaves the protection of the gas shielding of the welding torch. Conventional wisdom would advise the welding operator to simply slow down the travel speed so that the shielding gas doesn’t “out-run” the solidifying puddle. However, the obvious drawback is that a reduction in travel speed results in a reduction in productivity.
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.