Troubleshooting Defects: Welding right
- October 29, 2015
Detecting weld defects (and how to avoid them in the first place)
Welds hold the world together. From the pipelines of Prince George to the chemical plants of Quebec, welding is used to erect bridges and shopping centres, fabricate cars and consumer goods, and join the steam tubing that makes turbines turn. Unless those welds have a long and predictable lifespan, however, all of our taken-for-granted infrastructure will one day come crashing down. This is where the following welding and inspection equipment suppliers come in: by providing safe, strong welds, and making sure they’ll stay that way.
Avoid costly downtime
“First and foremost, fabricators need to produce quality welds,” says Nino Mascalco, director of application engineering at ESAB North America, Florence, SC. “When identified, weld defects must be repaired, an expensive exercise that costs about three times more than making the product—once to weld it, then remove the defective weld by grinding or machining, and finally weld it again correctly. Absorbing the cost of these defects is often the difference between a company making or losing money, and is something fewer companies can survive for long in today’s increasingly competitive market.”
Adding insult to injury is the fact that, while the rework is underway, production effectively stops, disrupting schedules and further damaging profitability. Mascalco makes the following recommendation to avoid this:
• The correct weld procedures must be developed and followed in accordance with job conditions and applicable code requirements for that project.
• Skilled welders are needed—either find them and hire them (easier said than done in today’s environment) or train them. And once they become skilled, budget for additional training as new/higher deposition welding processes and materials become available.
• Purchase advanced welding equipment that will keep the operator compliant within job specifications via preset welding parameters limits. This method is effective in situations where strict procedural compliance is required.
• Use the right tools for the job. High quality welding equipment and filler materials give the welder a better chance of making a quality weld, the ultimate goal.
Lastly, you must have quality control procedures and the proper equipment to verify weld quality.
ESAB technical consultant Greg Stauffer agrees. “Poor weld quality is expensive in many ways. If you don’t weld it right the first time, labor and material costs skyrocket. And repeated problems can lead to lost business and damage to the company’s reputation.” Fortunately, there’s technology available to avoid this, he says. Improvements in electronics have made it easier to control welding parameters, and more capable power supplies make it possible to produce more consistent, higher quality welds. There’s also software available now that allows welding engineers and supervisors to track and monitor weld conditions.
Use wire-fed welding processes
Keith Packard, director of oil and gas consumable solutions at Miller Electric Mfg. Co., Appleton, WI, says one way to reduce welding defects is by switching to wire-fed welding processes. “Stick welding, and specifically stick welding on pipes, takes a certain amount of skill. The people who’ve been doing it a long time know how to make things work in less than ideal conditions. An inexperienced welder, on the other hand, may not know how to make those adjustments, which can lead to lack of fusion, slag entrapment, excessive reinforcement on the ID, porosity, and other types of defects.”
That’s not to say one type of welding joins metal better than the other, Packard explains, only that wire feed is an easier process to master. This is an important consideration in today’s market, where the lack of skilled stick welders is a real problem. With wire feed, the starts and stops associated with stick welding are reduced. There is little to no spatter, a greater tolerance for misalignment, and gaps are easier to bridge. And improved electronics in today’s wire feed equipment makes process control simpler, compensating somewhat for less experienced welders.
One industry that faces special challenges with weld quality is oil and gas. Not only are the consequences of a weld failure extreme, but the increased use of tough-to-weld duplex and chrome moly steels, as well as higher quality and on-time delivery expectations, is simply taking all the fun out of pipeline welding. “There are a lot of variables,” Packard says. “Proper heating and back purging, high performing consumables, operator technique and skill set—all of these contribute to weld quality.”
Good joint preparation is one way to alleviate some of this pain. Packard recommends ditching those old flame cutters and manual pipe beveling methods in favour of machined, or “cold cut” edges. A requirement in the nuclear industry and other critical applications, cold cutting provides far greater accuracy and repeatability to welds, in turn reducing the number of defects and subsequent repairs.
Use ultrasonics for inspection
Even the best welds need to be inspected. Those responsible for quality on a large pipeline installation might turn to the Olympus Pipe Wizard, an automated girth weld inspection system designed for both onshore and offshore work. For the rest of the welding world, however, simpler and far less expensive solutions are available.
Meindert Anderson, corporate director for marketing communications at inspection and measurement manufacturer Olympus Scientific Solutions Americas, Waltham, MA, says portable ultrasonic flaw detectors are a popular approach to weld inspection. “These are suitable for aerospace and automotive applications, infrastructure such as bridges and construction work, and even for the typical fab shop that’s welding pieces of plate together.”
Ultrasonic inspection falls under two basic categories—conventional ultrasound and phased array. Both work by transmitting a sound beam into a solid material, which reflects the beam back to a transducer probe. In the case of phased array, multiple transducers are used, producing a 3D image very similar to the ultrasound pictures many of us have seen in the doctor’s office. “Phased array produces a better image, but either system provides immediate results on weld quality, and allows you to store or transmit those inspection images.” The cost for one of these portable devices ranges between $7,000 to $20,000 and up, Anderson says, and is a far simpler and faster method than the other volumetric inspection alternative: radiography, or X-ray inspection.
Subcontracting out inspection
“There is a growing trend in the industry to replace radiographic inspection,” says Patrick Tremblay, sales manager at NDT inspection equipment provider Zetec Inc., Quebec, QC. “This is primarily because of the radiation hazard associated with X-rays. There’s no chance of inspecting a weld when people are working in the area, so it’s usually done at night, after everyone goes home. This means inspection results won’t be available until the next morning, which can be an expensive surprise if the operator was laying down defective welds the entire day.”
Tremblay agrees that ultrasound, and especially phased array ultrasound, is an excellent method of weld inspection. Yet ultrasonic product manager Federico Zottig says that, even though the cost of this equipment has come down such that it is an affordable solution for most shops, many still prefer to subcontract their weld inspection to an outside company. “There’s more to it than buying a device. You need to have trained operators and maintain the appropriate certifications. If you have the work to justify all that, it makes sense, but it’s often more cost-effective to bring in a specialist and let them print out the results.”
Sometimes, ultrasonic isn’t enough. Zottig says the more critical the workpiece, the more inspection that must be done. Welding used in power plants or aerospace components, for example, often calls for some type of surface inspection—eddy current, magnetic particle, or penetrant inspection—together with ultrasonic and even visual checks to look for large cracks and surface abnormalities.
Still, it’s hard to beat ultrasonic for the majority of welding inspection jobs, as there are virtually no depth limitations (eddy current devices only “see” a few millimeters deep), is very fast, and can be performed mostly unattended—happy news for those technicians who must don a radiation suit before performing steam pipe inspection in a nuclear facility.
“Weld failures in nuclear, steam, and offshore oil and gas environments can be catastrophic, so it’s critical to have reliable and efficient weld inspection methods,” says Zottig.
Inspection for robotic welding
Many welds today are performed by robots, which to the layman might seem failure proof. Not so, says Tom Graham, key accounts group manager at welding supply and automation provider Abicor Binzel Corp., Mississauga, ON. “With automated welding in general, you have a highly repeatable robot that goes to the same point and does the same thing every time. The problem is, the parts you put in front of that robot might not be located consistently. There might be part size variation, positional changes in the weld joint, height differences, and so on. You can have the best welding robot in the world and end up with weak joints, poor penetration—all kinds of bad things can happen,”
The solution, Graham says, is automated seam tracking. “There are a few methods available, but we use optical, which works under the theory of laser triangulation. There’s a mechanical sensor sitting out near the end of the torch. A set of laser diodes emit three separate beams that reflect off the workpiece and back onto a sensor, which in turn are converted into the positional data needed to drive the robot. It tells the robot to move a half millimeter this way, or adjust the weld angle—whatever is needed to achieve an accurate weld.”
Systems like these can also be used in semi-automated applications, where an operator is driving a joystick for remote welding. Graham says there are also systems available to make teaching a robot easier and more accurate. The company’s ABIDOT system uses a series of laser diodes to determine the proper head to workpiece distance. This prevents wire bending during the programming process, and ultimately leads to better weld quality.
“Weld quality is always important, but with automotive and other high-volume automated applications, the scrap can pile up pretty fast if you don’t have the right systems in place to control quality,” Graham says. “Much of it comes down to seam tracking and finding the joint. Regardless of the approach taken, there’s always a balance between the amount of automation investment vs. product quality and production efficiency. I’m of the opinion that, the more you do right on the front end, the more money you make on the back end.” SMT