> Common Weld Defects <

(Note: You can dowload a printable PDF version of this section.)

Although welding is a straightforward process, metal is a dynamic material, so you can expect many twists and turns along the way. Your work pieces may expand when heated. The grain structure can weaken and cause brittleness. And the metal's shape may deform, causing cracks, which can spread over time and potentially break the weld.
Needless to say, such grisly possibilities keep inspectors on their toes and engineers lying awake
deep into the night wondering if their designs correctly anticipated everything that might go wrong. Structures like the Golden Gate Bridge (which was riveted, not welded), demonstrate that steel is the go-to building material when it comes to strength.


The Good, the Bad and the Ugly - Welding beads along a practice plate illustrate what can go wrong when a welder uses inproper technique or the wrong machine settings.

Low-carbon (aka mild) steel is widely used for structural work, since it’s more likely to retain its ductility than other metals when overheated . Even if you quench it in water too quickly, it manages (most of the time) to survive the shock of the ordeal . But life is not so breezy for aluminum, cast iron, titanium, stainless or high-carbon steels.
In addition to brittleness, other common weld defects like distortion, cracking and melt-through can compromise the durability of metals (including mild steel).

Consequently, codes and standards developed by the American Welding Society specify exactly how a joint must look when the job is completed. Depending on the application, other groups may have a say in the project as well, such as the American Society of Mechanical Engineers (ASME), the American Petroleum Institute (API), and the American Society for Nondestructive Testing (ASNT). Simply put, when the codes are followed, events like the failure of gusset plates along the I-35 bridge in Minneapolis might not have occurred in August 2007, causing the structure to collapse. Even more deadly was the collapse of two skywalks at the Hyatt Regency Hotel in Kansas City in 1981. (See photos below.)

Code specs for welding address the finished shape, dimensions and extent of any anomalies in the finished weld. A single crack is considered a weld defect, automatically failing an inspection. However, other problems, known as discontinuities, may be minute enough to escape the heavy hand of justice. What’s acceptable to the inspector is spelled out in the code.

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The skywalk collapse in Kansas City was attributed to engineering design mistakes excaserbated by further changes made on site during construction. The welds and other fasteners failed to sustain the weight of people crowding onto the walk during a tea dance.

Code specs for welding address the finished shape, dimensions and extent of any anomalies in the finished weld. A single crack is considered a weld defect, automatically failing an inspection. However, other problems, known as discontinuities, may be minute enough to escape the heavy hand of justice. What's acceptable to the inspector is spelled out in the code.

Take a look at the photo below. It shows a relatively uniform weld with good tie-in, a slighly convex face and no spatter, craters or other visible discontinuities. Creating welds like this requires both practice and an understanding of how metals react to heat and oxidation.

Here's an example of a relatively uniform weld with good tie-in, a slighly convex face and no spatter, craters or other visible discontinuities.

To master code welding, you should also get on a first-name basis with the defects that cause welds to be rejected. Keep in mind that most problems you’ll encounter have a single identifiable cause and solution. Moreover, that cause may have nothing to do with your ability to lay down a welding bead. Instead, the culprit will prove to be one of the following:

• poor joint design and fit-up

• incorrect machine settings

• wrong shielding gas or flow rate• inadequate pre or post-heat treatment

• using the wrong (or defective) rod /wire

• a hot or cold ambient temperature, high humidity, or other atmospheric conditions

As you acquire proficiency with various welding processes and work materials, you’ll learn to troubleshoot any difficulties and make adjustments quickly. Until then, be sure to stop welding as soon as you notice something’s wrong. Consult with a fellow worker, supervisor or your instructor before you continue. Remember, the more heat you apply to metal, the more its molecular or chemical structure becomes compromised. Do-overs should therefore be avoided whenever possible.

As you acquire proficiency with various welding processes and work materials, you’ll learn to troubleshoot any difficulties and make adjustments quickly. Until then, be sure to stop welding as soon as you notice something’s wrong. Consult with a fellow worker, supervisor or your instructor before you continue. Remember, the more heat you apply to metal, the more its molecular or chemical structure becomes compromised. Do-overs should therefore be avoided whenever possible.

Cracks

No matter how small, every crack is considered a defect, and it takes just one to fail a weld inspection. That’s because a crack spreads over time. Unlike carpentry, you can’t just fill the opening with a little glue and sawdust. Cracks must be ground out with a file or grinder, then a new weld performed. Here are four common types to watch for:

hot cracking - This crack appears soon after welding, usually inside the weld, as a result of something called hot shortness. Poor fit-up or design may be responsible, but the presence of sulphur in the base or weld metal can likewise cause problems, as can different rates of cooling within the weld. Often, the crack forms along the axis (center) of the joint as the two sides pull apart.

cold crack - This doesn’t show up at first, but within a day or so of the weld. It’s induced by hydrogen absorbed into the weld via the weld puddle. Hydrogen may be present due to moisture seeping into an electrode prior to welding, which is why it’s important to keep low-hydrogen rods in an oven until they’re need. Another cause for a cold crack is base metal contamination, so be sure to clean any old, soiled or discolored metal before you start your weld.

microfissure - This is more of a future crack which developes during the life of the weld. It’s generated by a seismic disturbance, metal fatigue or stresses in the heat-affected zone, or HAZ.

crater crack - This crack develops at the end of the joint whenever a welder neglects to backfill the gap. It’s standard practice to weld a little past the end of the joint or go backwards and on top of the weld metal to make sure there’s no crater left behind.

To avoid the potential for cracks in your welds, your fit-up should always be as spot-on as possible, just as if you were a master cabinet-maker. Don’t assume any gap can easily be eliminated by adding extra weld metal. Again, metal is a much different animal than wood. It expands when heated, making it difficult to compensate here and there for sides that don’t fit together well. Even though the joint might look OK right after you weld it, more often than not your work plates will reassume their original orientation once the weld cools. So here’s the recipe to get around that:

• Spend the necessary time grinding, cleaning, filing and/or deburring the edges of the plates so they easily fit together .

• Preheat both sides of the joint (if necessary).

• Clamp and/or tack your plates together.

• Before welding, test your machine settings to see if you have the right amount of heat dialed up.

Porosity

Porosity is the technical term for gas bubbles. These develop inside or on the face of welds because metal in a molten state is highly vulnerable to impurities. For this reason, some form of shielding gas (or gas ingredients in rods) are used in most welding processes. Porosity can be caused by one of the following:

• The flow meter setting on the shielding gas tank is too high.

• You’re using the wrong gas mixture or rod/wire.

• The weld puddle is contaminated due to unclean metal, surface moisture or contact between dissimilar metals.

• Your welding travel speed is too fast, not allowing enough time for the shielding gas or rod flux ingredients to do their job.

• A draft, or breezy conditions outdoors are blowing the gas away from the weld puddle.

Oxygen and hydrogen are the two big enemies of welders. Oxidized surfaces - which appear as rust, corrosion or mill scale in ferrous metals - should be removed from all weldable areas just prior to welding (and not the day before). Remember, water is composed of two parts hydrogen, one part oxygen. Once the hydrogen gets inside metal, it creates a condition known as hydrogen embrittlement.

A clogged gas nozzle on a MIG welding gun can also prevent shielding gas from reaching the weld puddle, so be sure to clean the orifice often. In stick welding, electrodes should be stored away from any source of moisture. In particular, low hydrogen rods like E7018 must be placed in a rod oven set to 250 degrees F, once the box has been opened. (Don’t heat other types of rods, however, as this will cause their cellulose coatings to crumble.)

In MIG welding, always check the flowmeter setting before starting your first weld, and make sure the gas mixture (argon, CO2, etc.) is the right one for your application.

Lack of Penetration and Fusion

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This weld lacks both penetration into the base metal and fusion with either side. Thus, it won’t serve its purpose of providing a solid, lasting connection between the two plates.

Depending on the joint, you may have to weld all the way down to the bottom or your plates to achieve what’s known as Complete Joint Penetration (CJP). Most fillet welds, on the other hand, require only Partial Joint Penetration (PJP). Either way, you’ll have to set your welding machine so you have sufficient voltage and current to get the job done.

A weld bead that simply rides the surface of the base metal is easy to spot because the bead looks like a bullet train. Sufficient penetration is achieved by setting the right amperage or voltage on your welding machine. Too fast a travel speed, or holding your torch too high above the joint, will usually limit penetration. (For some metals, such as aluminum and stainless steel, you’ll have to move faster because they’re both sensitive to heat.)

Fusion refers to tying the sides of the weld to the base metal so that there’s no dividing line. This requires a little more time and attention for groove welds with an open root, since there’s a lot of space there to fill. Any gap will produce cracks. The cracks will eventually expand and result in leaks (in the case of pipe) or detachment and inability to support a load (in structural steel).

There’s no penetration or fusion here with the bottom plate. On the standing plate, you see uneven penetration and a gap at the bottom.

To fuse the sides of the joint, a welder must pause briefly on each side while depositing the weld metal. As a general rule, you move quickly across the center of the weld to avoid globbing new metal in that area. But the sides of joints are prone to undercutting, so pausing is needed to prevent these gaps from forming. Thus, a flat or slightly convex bead is deposited with good tie-in at the toes of the weld.

In this weld, a much better job is done penetrating and fusing the sides of the base metal. The weld’s face is slightly convex, which is generally preferred over a flat or concave shape.

In stick welding mild steel, E6010 or 6011 electrodes are the standard rods used, because they’re designed for deep penetration. The welder then switches to another rod type, often a low-hydrogen E7018 for the filler and cap passes.

Undercutting

As noted earlier, failure to get good tie-in with both toes of a weld can result in undercutting. That’s because metal melts faster at its edges than in the middle. On a T-joint, like the one shown above, the standing plate is most often welded on its edge, where it’s more susceptible to meltthrough. Since the bottom plate is welded at the middle, it makes sense to focus more heat on it as you move along the joint. Many entry-level welders forget this basic rule of thumb about how metal responds to heat.

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The toes of this weld are badly undercut. This iscaused by the welder not pausing long enough on each side of the weld as he zig-zags (or rolls a curly-Q) along the joint. Also, the weld penetrated clear through the standing plate, which suggest to much heat (or current) was applied. -

You can also undercut the toe on the bottom plate by not spending enough time welding on that side of the joint. So in addition to focusing more heat on it, be sure to manipulate your electrode laterally, pausing on each toe, so the weld puddle covers both those edges of the joint. This is an essential skill to master, as it will come up repeatedly in almost every type of welding assignment you undertake.

Correct side-to-side movement insures fusion with the base metal on each welding pass. When you reach the final pass (cap), make sure the weld puddle reaches and covers either toe.

As you practice side-to-side movement, you’ll become adept at performing this manipulation at a sufficient speed to avoid overheating, and at the same time not deposit too much material across the throat of the joint.

Overlap

Whereas too much heat causes undercut, too little creates overlap, where two separate layers of metal are left behind along the toe. Welding to code requires thorough fusion of the base metal.

This is the opposite extreme from undercutting. Here, the weld metal flows across the base metal at the toe without producing any fusion between the two. This may be caused by insufficient heat to melt the base metal, and/or improper manipulation of the electrode. Make sure your rod or gun work angle is correct; otherwise the heat won’t be directed at the toe.

Distortion

Because metal expands when heated, then shrinks after cooling, the two sides of a joint may shift position in the course of welding. That’s why tacking and/or the use of clamps is an integral part of many weld operations. Stainless steel is especially prone to movement.

TWI

The base metal will fold inward towards the face of the weld unless clamped into place. Sometimes you can pre-position your plates with a little tilt in the opposite direction to compensate for the inevitable distortion.

On groove or other multi-pass welds, the two base plates may likewise start to shrink and fold in the direction of the joint, regardless of tacks or clamps. Control of heat (i.e. correct machine settings, a brisk travel speed, etc.) will help prevent distortion. Changing the sequence of welds, or the location of the joint, or making fewer passes, can also help to reduce the risk. On occasion, you may decide to start your first weld with the plates slightly tipped away from the direction you expect them to fold. That will compensate for the inevitable bending inward. As a rule, the bigger the weld, and the smaller the plates, the greater the chance for shrinkage, twisting or warping.

When prevention doesn’t work, try a post-heat treatment to relax any thermally-induced distortion. Sometimes you can get the plates to bend back to the way they were. And sometimes you can’t.

Click here for more suggestions.

Spatter, Arc Strikes and Other Surface Disruptions

When welding to meet code requirements, maintaining an unblemished surface on your work plates or pipe sections is a big deal. Even a single burr left behind after grinding can interfere with a mechanical assembly or catch another worker’s clothes or skin and cause injury. (Burrs can also block the flow of weld metal during a welding operation.)

A dent in the metal left by a wayward arc strike could be the start of a transverse crack. So always strike your arcs inside the joint ahead of your weld, or on the edge of your work plates or other area that will be ground or removed later.

Spatter describes the bits of molten metal that are sent flying up out a weld when using an arc welding process. This material hardens into little balls that affix to the surface of your weld plates. In stick welding, spatter is caused by excessive voltage or too long an arc. In MIG welding, too high a setting on the wirefeed will generate the same result. In some cases, you can grind or sand off any blemishes on metal surfaces before calling over the inspector. For other jobs, you can only use hand files to dress up the metal after welding.

Discoloration around welds is common and not usually considered a defect. However, with more heat-sensitive metals like aluminum and stainless steel, excessive discoloration in the HAZ may indicate that metal has been overheated, altering its mechanical properties or chemical composition. In this case, grinding off the color won’t eliminate the problem. Consult with a supervisor or instructor on how to proceed.

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