Metal inert gas welding, using its shielding gas and wire-feed electrode, is believed to be one of the easiest welding techniques to learn. That’s an immediate advantage. Expect hobbyists and motorsport enthusiasts to own a MIG rig. Stored in their garages, the equipment is pulled out everytime a big weekend project comes around. Meanwhile, for professional welders, there are other pros to list, plus a few cons.
MIG Welding: Listing the Many Pros
Some trainers say they can teach a student how to use MIG welding equipment in under an hour. That may be true for a weekend warrior, but experts require years of training to get the most out of their tools. Still, that’s a notable benefit. The training is carried out, the welder becomes familiar with the equipment, certifications are issued, and another welder enters the field. Of note, there’s still a need to clean the weld site. Next on the list, then, because of the wire feed system, Metal Inert Gas work is fast. There’s no pausing to grab more consumable materials, more welding rods, no stopping to clean away slag. When the welder gets going, a streamlined workflow comes naturally. Here’s a bulleted list of some more productivity-enhancing advantages.
The Cons of MIG Welding
It’s hard to imagine any drawbacks, especially after examining the above list, but there are a few disadvantages, some of which can be inferred simply by reading deeper into the pros. For instance, that thin metal suitability feature means MIG tools aren’t very good at fusing thick metal workpieces. Also, there are problems to mull over when the weld pool forms. It’s a liquid pool, so it’s not going to form properly if a weld is executed on a vertically oriented join. On the equipment side of things, there’s the shield gas bottle to lug around, plus wire and nozzles to replace. Still, considering the number of pros, this welding equipment more than earns its good name.
Most drawbacks are easily resolved. If the shielding gas is being blown around by a fierce wind, then a shelter of some kind will blunt this effect. Welders can establish artificial shelters in minutes. They ask for assistance, and the extra muscle builds the shelter. Then, if the equipment isn’t producing clean seams and uninterrupted welds, do remember that MIG welding is a cleanliness sensitive field. Clean the surfaces in and around the join.
It’s all very well having a welding inspector’s report in hand. The papers are typed, and they’ve been delivered to multiple engineering departments. It’s the contents that are alarming, though. There are weld discontinuities noted in the report, plus a series of appended photographs. They frame the trouble spots. To beat the problem, to execute a cost-effective solution, those project head honchos need to know all about weld discontinuity science.
What Is Weld Discontinuity?
The structure of a weld is interrupted. More than irregularly applied, the weld is physically disconnected. And this type of fusion breakage doesn’t just apply to cracks or propagating fracture networks. No, material porosity is another major issue, as are depth penetration problems. Worse still, it may not be immediately obvious, this lack of depth penetration. Likewise, porosity problems can be hard to detect, while cracks and fractures lie hidden below a weld’s surface. In fact, to a casual observer, metal fusing work can look perfect, but, as every welding inspector knows, looks can be deceiving.
Weld Discontinuities: The Causative Factors
More than deceiving, a weld discontinuity, concealed or not, undermines the structural integrity of two or more fusion-anchored elements. For structural beams, larger than life and made of hardened steel, cracks develop. They propagate and undermine weld coherence. So, what causes weld discontinuities? The inspector has done the job, hence the report and those photographs, but there’s no way to address the weld irregularities until a cause is determined. Well, dirty or oily base materials cause porosity problems, so the area must be prepped before the equipment sparks to life. Moisture and gas shielding errors are next on the list. Welders work dry, they don’t just expect the heat to burn away excess moisture. As for shielding gas glitches and other equipment malfunctions, entire articles have been written about such weld-undermining influences.
Conceiving Systematic Weld-Correcting Solutions
Instead of referring back to past posts, let’s think about weld discontinuities for a minute. Pages of troubleshooting tips could easily fill the next few lines, but there’s another way to conquer the issue. Understand weld discontinuities. Understand hot and cold cracks, the temperatures that cause those fractures to develop. On understanding the underlying cause, a truly effective process-diagnosing campaign dissects the flaw. The longitudinal or transverse fracture patterns are analyzed. Probability studies trace the weld interruptions, and a cause is discovered. Generally, the environment or the equipment or the equipment’s operator are found responsible. Every so often though, weld discontinuities occur because of substandard filler rods or structural preloading difficulties.
More than a source of frustration, weld discontinuities endanger entire structures. A tiny, almost invisible flaw on a project might seem unimportant, but that’s an unacceptable way of approaching a critically important job. After all, welding inspectors understand the weakest link principle. No weak links can be allowed to exist, not if an engineering inspector is doing his job right.
Inferior welds don’t just look bad, they deliver poor structural performance, too. The shoddy work won’t pass muster as a mechanically sound metal joint. There are weld imperfections in the seam, which have been visually or instrumentally detected. Cavities are undermining the joint, so it cannot deliver material integrity. If we’re to stop such process defects from sabotaging a welding team’s work, let’s dig into the causative factors.
The Metallurgical Detectives
Checking out both the equipment and its consumable components, the welding inspector gets his bearings. There’s a series of cavities on a length of load-bearing steel. The metal is inordinately strong and durable, but the cavities are weakening it structurally. First off, is the welding gear in optimal condition? Wet flux rods and damaged electrodes cause problems, which appear as weld cavities. Replace the equipment electrodes. If the issue persists, adjust the shield gas supply. Stubbornly, the cavities hang around, so the inspector advises a secondary HAZ cleaner, one that’ll remove trace hydrocarbons, dirt, and oils. Finally, the welding cavities disappear, the steel beams anchor, and the strength-crippling weld discontinuities are eliminated.
Crack and Micro Fracture Sleuths
Trapped process gasses are under control. The porosity problems are managed, and the operation is in full flow once more. Only, there are cracks troubling a second area on the same project. Off the weld inspector goes, dispatched promptly to the site of the cracking incidents. Pulling out a notebook and camera, this is the moment an initial report is recorded. Crack types are consulted. Among them, one of the following fracture types is propagating:
What’s causing the weld-compromising fractures? Impurities are a problem. Phosphorous or carbon, perhaps sulphur, the contaminants are causing solidification stress. As a weld cools, hot cracking effects result in inter-granular formations, which interfere with the solidification process. Other crack producing agents include precipitation induced fractures, temperature stress, hydrogen levels, and nearby surface contaminants.
It’s the weld inspector’s job to visually examine the imperfections. Tools come out, so does a camera and a surface-penetrating instrument. If there are cavities or porosity problems that are impacting joint integrity, the inspector’s report will mandate action. Then, with the equipment or heat zone problem corrected, the work continues. As for a weld seam that appears purely amateurish, it can still be accepted if its underlying structure is mechanically sound. Granted, a run through with a surface grinder will likely be recommended. Finally, for cracks, the fracture cause must be found, identified and addressed.
A weld coating inspection can’t just cut out a section of the coating, then dispatch it to a lab for testing. That’s an approach that would defeat the whole purpose of the weld overlay, after all. No, a non-destructive test and inspection system should be recruited here, with its wet comb filters and wheels leading the way. Then, for a truly in-depth inspection, we call in electronic instrumentation.
What Are Weld Coatings?
A full-girth pipe seam is being used to illustrate this hypothetical case study. The weld, perfectly applied, mates the two pipe sections together, and that seal will persevere, no matter the fluid pressure pushing away from inside the pipeline. Outside, protecting the seam from corrosion and environmental attack, a special coating has been applied. Painted on by a roller brush or chemically adhered to the pipe-encircling weld, this is no ordinary paint-based coating. To properly protect the seam from highly corrosive environments, strong polymers and chemically modified fluids cure or shrink-fit to form impressively durable material barriers.
Non-Destructive Weld Coating Inspections
Tested to destruction, the protective coating would be left seriously weakened. Clearly, we need a non-destructive approach. The initially wet, polymer-rich emulsion is first checked with wet film thickness tools. The special combs or wheels gauge the depth of the curing overlay. Dry film thickness instruments come next. Employing a flexible wire and probe, the device scans the dry coating for concealed discontinuities and other flaws. From here, the instrument readout measures the DFT, the Dry Film Thickness, as a minimum and maximum value from one side of the pipe to the other. And, since many polymer-based protective coatings use three-layered films, then the device can indicate a defect in any or all of those layers.
Internationally Accredited Thickness Measuring
Employed as a weld protecting blanket, the coatings referenced here can be as thin as a few millimetres, but they can also be as thick as several centimetres. To ensure dry measurement accuracy, the measurement instruments talked about in the above paragraph possess advanced self-calibrating features. However, since we’re talking about internationally recognized engineering codes and standards, weld coating inspections can’t always rely on such features. For example, in an arctic wasteland, a pipe inspection can be impacted by temperatures that affect automatically calibrated instruments. There are zero test plates and calibration films that’ll confirm instrument accuracy.
And we thought the inspection phase was done after all the welds had been exhaustively inspected for application discontinuities. That’s not the case, not when the welds are exposed to tough environmental conditions. Coated with advanced, polymer-reinforced emulsions, there are wet and dry film thickness tests still to conduct.
When NDT (Non-Destructive Testing) procedures don’t measure up, then weld inspectors opt for destructive weld testing methods. As the label implies, small samples of the subject weld are exposed to destructive energies. That’s not a task that can be addressed with a carefree demeanour. Unlike a radiographic test, this procedure affects weld zones, so these techniques must be utilized with caution. We’ll begin by looking at acid testing methods.
Destructive Weld Testing: Acid Etch Examinations
If present, concealed weld discontinuities come to light when an acid reacts with a fused material. When a sample is removed from a weld, it’s polished and cut into cross-sections. The acid, often formulated as a mild hydrochloric or nitric acid solution, then breaks down any concealing materials and reveals the clean edges of each sectioned area. Porosity issues are often discovered by adopting this technique. Transition sites, between base metals and weld materials, are also made clearer by acid etching.
Guided Bend Weld Tests
Mounted on a jig and die assembly, the welded metal is forcefully bent. The bending action continues until the specimen reaches the desired angle, for this means the weld area has assumed a predetermined bend radius. To be clear, this approach looks closely at the ductility and strength of a selected welding process. By using face bend tests, transverse bends, and other force-directed flexing actions, the inspection can also uncover liner fusing defects. If the ductility test does fail, weld cracks will appear.
Tensile Strength Tests
Pushing the weld beyond its material limitations, this is often the only way to determine the tensile properties of a fusion joint. A large piece of equipment applies massive amounts of mechanical energy to the specimen. It rips apart transversely or breaks at a critically weakened juncture. Used in conjunction with an acid etching test, failure analysis science delves down into the flaw to discover the cause of the discontinuity. This test usually involves sending the part to a stationary hydraulic press, although there are portable variants, machines that come equipped with movable crushing jaws and hydraulic power.
As stated several times during this post, the welds tested here are mostly specimens. They’ve been created to test a welder’s skills, to see how a technique will serve a new project, or for some other intelligently mandated reason. There are nicks and indentations to make, cross-sections to cut, and acids to carefully apply here, so the NDT approach isn’t applicable. In place of that approach, these “specimens” will be broken and tortured in every imaginable way, all the better to establish the skills of a welder or the limitations placed upon a new engineering project.