When weld defects form in a weld they can weaken the joint. In some cases, this results in complete failure of the weldment.
In serious cases, there can be severe consequences to a failing weld.
So, you need to understand the various defects.
But in addition, you must know how to prevent them from happening.
With that in mind, let’s dive in.
What is a weld defect?
In short, a weld defect is any flaw or imperfection that compromises the intended use of a weldment. These are classified according to ISO 6520.
This also implies a flaw or imperfection may not compromise the weld, and a weld is said to have a discontinuity when this occurs. So, a weld can have a discontinuity and not be considered defective. These acceptable limits are specified in ISO 5817 and ISO 10042.
However, if enough discontinuities exist (i.e., they exceed a limit as defined in an applicable code or specification), then the discontinuities become classified as defects, and the weld is rejected.
A weld needs to be strong enough for the intended purpose at the most basic level, and many defects can weaken a joint. But in some cases, your weld must be aesthetically pleasing, too. So, most defects either weaken a weld or make it look ragged and unpresentable.
We all want to avoid having our welds fail or be rejected. So, you need to know the types of defects that can occur and how to avoid them.
13 Common Types of Weld Defects
Irregular welds include too wide or too narrow, those with an excessively convex or concave surface, and those with coarse, irregular ripples.
These characteristics may be caused by poor torch manipulation, a speed of travel that is too slow, current that is too high or low, improper arc voltage, improper stick out, or improper shielding gas.
But when a particular defect occurs, you want to know which parameter needs adjusting so you can fix it. Therefore, a list by defect type, along with how to correct the problem, is helpful.
A list of every weld defect ever encountered would be long and unwieldy. But in general, the most common weld defects are:
- Lack of fusion
- Poor penetration
- Burn through
- Excess reinforcement
- Mechanical damage
We may as well start with one of the most obvious and serious defects in a weld – cracks. These weaken a weld, and even worse, cracks tend to grow at a rapid rate making the problem worse.
So, it goes without saying you do not want any cracks in your welds. But it can be a challenge, and there are three main types of cracks:
- Longitudinal cracks run along, or are parallel, to the length of the weld.
- Transverse cracks run across the width of a bead.
- Crater cracks usually occur at the end of a weld when the arc is terminated. They are often star-shaped and form when a dent or “crater” is formed at the end of a weld.
Cracks can further be categorized as hot or cold cracks.
Welds can be heated to over 10,000°C, and hot cracks occur as the weld cools and transitions from the liquid to the solid phase. Hot cracks tend to occur when the wrong alloy filler material is used.
Cold cracks occur after the weld has cooled. They can occur hours or days after the joint is made. This defect usually occurs when welding steel and is often caused by deformities in the base metal.
To prevent cracks:
- Use the correct alloy filler material for the metal being welded.
- Avoid welding high sulfur and carbon steel.
- Preheat your joint.
- Ensure the joint is filled and avoid a convex-shaped bead.
- Use sound, defect-free base metal.
- Avoid low currents coupled with high travel speeds.
- Do not use hydrogen shielding gas with ferrous metals.
- Keep a good depth to width ratio for your joint.
- Avoid craters at weld termination by placing adequate filler material when ending a bead.
- Allow for expansion and contraction of a weld joint during the weld and cool down.
Impurities can become trapped inside a weld, and these are referred to as inclusions. Contaminants trapped inside a weld dramatically weaken the joint.
Slag often forms when flux is used, such as brazing and stick, flux-cored, and submerged arc welding. The slag must be allowed to float to the top of the puddle and not become trapped inside the bead. That means the molten pool should not be allowed to cool too fast.
But it can occur with MIG welding as well. Bits of rust and even tungsten can be counted as slag and can cause contamination in your welds. So, MIG and TIG welding is not immune to inclusions.
To prevent inclusions in your welds:
- Prep and clean the base metal well.
- Avoid low amperage settings (prevent the weld pool from cooling too fast).
- Keep a proper torch speed (the welding and slag pools should not mix).
- Maintain a proper torch angle.
- Clean slag from previous welds between passes.
3. Lack of fusion
It may seem obvious, but the filler material must be well bonded to the base metal on both sides and to welds underneath during multiple passes. If there are voids, gaps, or poor adhesion, the joint will be structurally impaired.
Ways to prevent a lack of fusion are:
- Clean your base metal well and remove all impurities.
- Use the correct size electrode.
- Select the right electrode alloy for the metal being welded.
- Don’t move the torch too fast.
- Prevent the arc from getting too short.
- Keep the amperage high enough for the job.
Weld porosity (also know as wormhole weld) is where gas bubbles accumulate and get trapped inside a weld. This is also said to be porous. A cross-section of a porous weld bead will resemble a sponge with all the air bubbles trapped inside.
As you weld, gases like steam, hydrogen, and carbon dioxide can be generated, and they normally bubble out of the molten bead. But if the gas bubbles are trapped, they can weaken your joint, and the work is ruined.
Avoid porous welds by:
- Properly clean and prepare the base metal.
- Make sure the joint is dry.
- If used, set your shielding gas flow correctly (too low or high can create issues).
- Keep the amperage from getting too high (i.e., too “hot”).
- Use the correct electrode alloy for the job.
- Ensure the electrode coating is not damaged if it has one.
- Move your torch slow enough to keep a molten puddle, allowing the gas to bubble out.
- Avoid a long arc.
- Use low hydrogen electrodes.
When the welding process results in spots or sections being less than the original base metal, the defect is referred to as an undercut. This will often appear as a “notch” at the edge of a weld, either on the top or bottom of the weld.
A loss in thickness reduces the strength of the weldment and makes the joint susceptible to fatigue. This defect is often the result of too high a current or moving the torch too fast.
Techniques to prevent undercutting include:
- Do not move the torch too quickly.
- Use the proper amperage and avoid too high a setting.
- Keep the torch at the correct angle (and angle the heat to thicker areas when possible).
- Use a correctly sized electrode.
- Keep a shorter arc.
- Ensure you have the right shielding gas flowing at the correct rate.
- Use proper welding techniques.
- Employ multiple passes.
6. Poor Penetration
When the bead does not fill a butt joint all the way to the bottom, the weld achieves poor penetration. It is also referred to sometimes as incomplete penetration. Whatever you call it, this form of defect also compromises the integrity of a joint.
For good penetration:
- Use a properly sized electrode for the weld (avoid an oversized electrode).
- Don’t move the puddle too fast.
- Prepare V grooves for butt joints with 60 to 70 degree sloped sides.
- Align the workpieces, so there are no large or irregular gaps to fill.
- Keep your amperage, or heat, at an optimum setting and avoid too low a current setting.
7. Burn Through
If too much heat is applied during the weld, you can actually blow a hole through the metal. This defect is referred to as burn through, but sometimes it is also called melt through. Of course, creating a hole defeats the purpose of a weld and destroys the joint.
This type of defect is usually encountered with thin stock, material less than 1/4 inch thick. But it can occur with thicker stock if your welder settings are too high, if the gap between pieces is large, and/or you are moving the torch too slow.
In general, to prevent this type of defect:
- Do not let the current get too high.
- Avoid excessive gaps between plates.
- Ensure your travel speed is not too slow.
- Stay away from large bevel angles.
- Ensure the nose is not too small.
- Use the correct wire size; too small accentuates the problem.
- Provide adequate metal hold-down and/or clamping.
When the weld bead sits below the surface of the base metal, the weld is said to be under-filled. The bead itself is thinner than the base metal, which weakens the joint. This condition often appears as a “rut” that runs the length of the bead and is sometimes called a convex joint.
Method to prevent under-filled welds include:
- Avoid moving too fast.
- Use the right current setting.
- Use the correct size electrode/filler wire.
9. Excess Reinforcement
In contrast to an underfilled joint, a defect results when there is too much filler material in the joint. This is known as excess reinforcement or a “high” crown. Project specifications and codes often regulate what is considered too high.
At times, excess reinforcement may even come out the bottom of the joint. This is sometimes referred to as excess penetration.
Other variations of the defect include narrow, steep-sided beads caused by an insufficient coating of flux on your feed wire or low voltage.
Also, when excess reinforcement is uneven and ragged, it may be called “mountain range” reinforcement, and this is caused by excess flux on the feed wire or fast/uneven travel speed.
To avoid excess reinforcement:
- Keep the torch moving at a proper speed. Too slow, and excess filler will be placed. Too fast, and the bead becomes erratic.
- Set your amperage correctly and avoid excess heat.
- Adjust your voltage so that it is not too low.
- Align the pieces so that the gap is not too large.
While usually not a threat to structural integrity, spatter can be considered a defect. The aesthetics of a weld are sometimes as important as the weld’s strength. But nothing makes welded pieces look sloppy, like spatter stuck all over the surrounding metal.
Spatter frequently occurs with MIG welders but can occur with other welding processes, too. While you can never eliminate all spatter, there a few things you can do to minimize it:
- Clean the base metal well.
- Use the correct amperage, avoid “hot” settings.
- Use the correct voltage, avoid low settings.
- Ensure the polarity is set correctly.
- Keep a short arc.
- Increase the electrode angle.
- Check the feed wire to ensure it is unimpeded.
Related: How to reduce weld spatter
When the filler material at the weld’s toe covers the base metal without bonding, an over-roll or overlap defect occurs.
If you want to avoid this condition:
- Avoid letting your travel speed get too slow.
- Keep the correct torch angle.
- Do not use oversized electrodes.
- Set the correct amperage, avoid a high setting.
When MIG welding, whiskers are short lengths of electrode wire sticking through the weld on the root side of the joint. They are caused by pushing the electrode wire past the leading edge of the weld pool.
These protruding wires look bad, but they can also cause problems. For starters, whiskers are considered inclusions and weaken the joint. In pipes, they can even inhibit the flow or even break off inside and cause equipment damage downline.
Whiskers can be prevented by:
- Reducing your wire-feed speed.
- Keep an optimum travel speed, avoid going too fast.
- Increase the wire stick-out distance.
- Weaving the torch.
13. Mechanical damage
Once the perfect bead is installed, you are not out of the woods. Damage can be caused by chipping hammers, grinders, and other tools. Not surprisingly, the term used for this type of defect is mechanical damage.
Common sense guides you to prevent mechanical damage with cautions like:
- When removing slag or cleaning a joint, do not get too aggressive
- Avoid heavy hammer blows
- Do not let other large pieces of metal impact or grind over your welds
Wrapping It Up
As you can see from our shortened list of common defects (yes, there are more we could have discussed), there are good reasons why welders need to learn certain fundamentals. Defects usually occur when one of these basics is not followed. The cornerstones included:
- Prepare the base metal to ensure it is clean and contaminant-free
- Position the pieces correctly for the type of weld to be performed with no large gaps
- Create V grooves at the proper angle when needed
- Set both the amperage and the voltage correctly
- Maintain a proper arc length
- Move the torch at an optimum speed
- Use a correctly sized electrode
- Ensure your feed wire is not impeded
- Find and keep the right torch angle
- Properly weave the electrode when necessary
- Avoid mechanical damage to the parent metal and finished bead
Knowing how to identify the various defects and correct them makes your joints stronger and more presentable. It also makes you a better welder.
All these potential defects may seem overwhelming and impossible to avoid at first. But keep welding. It takes patience and lots of practice to become a great welder.