A defect in welding is any flaw in or around a weld joint that compromises its strength, appearance, or structural integrity. These flaws range from tiny gas pockets trapped inside the weld metal to visible cracks running along the surface. Some defects are cosmetic nuisances; others can cause a welded structure to fail under load. Understanding the most common types, what causes them, and how they’re caught is essential for anyone working with or inspecting welded joints.
Porosity: Trapped Gas in the Weld
Porosity happens when gas bubbles get trapped in the molten weld metal as it solidifies, leaving behind small voids. Think of it like bubbles frozen inside an ice cube. These voids weaken the weld by reducing the amount of solid metal carrying the load.
Porosity takes several forms. Surface porosity shows up as visible open gaps on the weld’s face. Subsurface porosity sits beneath the surface and is difficult to spot with the naked eye. Wormholing produces elongated tunnel-like voids that look like a worm burrowed through the metal. Cratering porosity appears as uneven, crater-shaped depressions, usually caused by not enough filler material to fill the cavity at the end of a weld pass.
The usual culprits are poor joint preparation, incorrect shielding gas flow, and excessive welding speed. Shielding gas exists to keep oxygen and other atmospheric gases away from the molten weld pool. If the gas type is wrong for the application, or the flow rate is too high or too low, excess oxygen enters the weld and gets trapped. Surface contaminants like oil, rust, moisture, and mill scale on the base metal also release gas when heated. Cleaning the metal thoroughly before welding and using anti-corrosive treatments on rusty surfaces addresses most of these causes before they become problems.
Cracking: The Most Serious Defect
Cracks are generally considered the most dangerous welding defect because they can propagate under stress, leading to sudden structural failure. They fall into two broad categories based on when they form.
Hot cracks develop while the weld is still cooling from high temperatures, typically forming in the weld metal itself. They occur when the solidifying metal doesn’t have enough strength to resist the shrinkage stresses pulling it apart.
Cold cracks, also called delayed cracks or hydrogen cracks, are a different problem entirely. They occur at or near room temperature, sometimes hours or even days after welding is finished. Three factors combine to cause them: hydrogen introduced by the welding process, a hard and brittle microstructure in the heat-affected zone next to the weld, and tensile stress acting on the joint. The base metal’s composition plays a major role here. High-carbon and alloy steels are more prone to forming that brittle microstructure, especially when cooling happens too quickly. Hydrogen migrates through the steel into these hardened, stressed areas and initiates cracking.
Prevention focuses on controlling all three contributing factors. Preheating the steel slows the cooling rate, producing a less brittle structure. Using low-hydrogen electrodes that have been properly stored in a dry oven limits the hydrogen source. Allowing the weld to cool slowly, especially in cold or windy conditions, further reduces risk. Covering a fresh weld with an insulating blanket is a common practice for critical joints.
Incomplete Fusion and Penetration
These two defects sound similar but describe different problems. Incomplete fusion means the weld metal failed to bond to one side of the joint, leaving an unfused gap between the weld and the base metal. Incomplete penetration means neither side of the root (the deepest part of the joint) was reached by the weld at all, leaving the bottom of the joint entirely unjoined.
Both defects dramatically reduce joint strength because the weld simply isn’t connected where it needs to be. The causes overlap: too little heat input, an excessively thick root face on the joint, too narrow a gap between the parts being joined, or a bevel angle that’s too small for the electrode to reach the root. In MIG welding, too low a current for the material thickness produces shallow penetration. Paradoxically, too high a current can also cause problems: the welder moves faster to compensate, and the weld pool bridges over the root without actually fusing into it.
Joint design matters as much as technique. A root gap that’s too small or a root face that’s too thick physically prevents the arc from reaching the bottom of the joint, regardless of the welder’s skill.
Undercut: A Groove Along the Weld Edge
Undercut is a groove or channel melted into the base metal along the edges of the weld that doesn’t get filled back in with weld metal. It creates a stress concentration point, essentially a notch where fatigue cracks can start under repeated loading.
The causes come down to welding parameters and technique. Excessive voltage, excessive amperage, travel speed that’s too fast, and incorrect electrode angle all contribute. In flux-cored arc welding, dragging the gun on a flat fillet weld tends to produce undercut. Pushing the gun often eliminates it. A general guideline is to use a 10 to 15 degree drag angle for MIG and stick welding, and a push angle for TIG. Travel speed matters too: moving too fast doesn’t give the molten metal time to fill the edges of the weld pool before it solidifies.
Slag Inclusions: Trapped Flux Particles
Welding processes that use flux (like stick welding and flux-cored arc welding) produce a layer of slag on top of the weld. This slag is supposed to be chipped or brushed off between passes. When it isn’t removed completely, tiny particles of flux get trapped inside the next layer of weld metal. These inclusions act like voids, weakening the joint and preventing full penetration.
Four primary causes drive slag inclusions: incorrect bead placement, wrong travel angle or speed, improper heat input, and failing to clean between weld passes. Traveling too slowly is a particularly common mistake. It causes the arc to trail behind the weld puddle, and the slag floats forward and gets buried under new weld metal instead of rising to the surface where it belongs. Using the correct heat input and following the wire manufacturer’s guidelines for diameter and settings helps keep the slag where it’s supposed to be: on top.
How Welding Defects Are Detected
Not all defects are visible. Inspection methods range from simple visual checks to techniques that can see inside a completed weld without cutting it apart. These are collectively called non-destructive testing (NDT).
- Visual testing is the most basic method. An inspector examines the weld with the naked eye or magnifying aids, looking for surface cracks, porosity, undercut, and other visible flaws. It catches a surprising number of problems but misses anything below the surface.
- Liquid penetrant testing reveals surface-breaking defects on non-porous materials. A colored or fluorescent dye is applied to the surface, drawn into any cracks or voids by capillary action, then made visible with a developer.
- Magnetic particle testing locates surface and near-surface flaws in magnetic metals like steel. A magnetic field is applied, and iron particles sprinkled on the surface cluster around any discontinuities, making them visible.
- Ultrasonic testing sends high-frequency sound waves into the weld. When the waves hit an internal flaw, they bounce back differently than they would from solid metal, revealing the location and size of subsurface defects.
- Radiographic testing uses X-rays or gamma rays to create an image of the weld’s internal structure, similar to a medical X-ray. It’s one of the most reliable methods for finding internal porosity, inclusions, and cracks hidden inside the weld.
The method chosen depends on the material, the type of defect suspected, and the criticality of the joint. Structural steel on a bridge gets a different inspection protocol than a handrail.
Preventing Defects Before They Happen
Most welding defects trace back to one of a few root causes: contaminated surfaces, wrong settings, or poor technique. A quick pre-weld routine catches the majority of these issues. Before striking an arc, experienced welders verify that the metal is clean and properly prepped (wire-brushed, ground, or solvent-wiped), the shielding gas type and flow rate are correct (roughly 20 to 25 cubic feet per hour for MIG welding, with 100% argon for aluminum), voltage and amperage are within specification for the material thickness, consumables like electrodes and wire are in good condition and stored dry, and the torch nozzle is clean.
Filler metal should match the base material in composition and be appropriate for the thickness being welded. Joint design plays a role too. Bevel grooves on thicker stock help spread stress and give the arc access to the root. For crack-prone materials, preheating and controlled slow cooling are standard practice rather than optional extras.
Taking 30 seconds to run through these checks before every weld is a habit that, by some industry estimates, prevents up to 70% of common defects. The cheapest fix for any weld defect is never making it in the first place.