The weld looked decent at a glance, but once the grinder touched it, tiny holes started showing up and the bead chipped in spots. Frustrating, especially after putting time into getting the settings right.
That’s when I started digging into different types of weld defects and realized most problems don’t happen by accident — they leave clear signs if you know what to look for.
In real workshop conditions, defects like porosity, undercut, lack of fusion, and cracks can sneak in fast. Sometimes it’s bad technique, sometimes it’s dirty metal, and other times it’s something as simple as wrong settings. Ignore them, and you’re risking weak welds that can fail under load or get rejected during inspection.
Understanding these defects isn’t just about fixing mistakes — it’s about preventing them before they happen. That means stronger joints, cleaner finishes, and less time wasted reworking bad welds.
I’ll break down the most common defects, what causes them, and how to fix them on the spot. Here’s what actually makes the difference when your weld doesn’t turn out the way it should.
Image by choongngaiengineering
Cracks: The Rejectable Defect That Propagates Under Load
Cracks rank as the most severe discontinuity because they act as sharp stress risers capable of catastrophic propagation in both weld metal and heat-affected zone (HAZ). AWS D1.1 and ISO 5817 classify any crack—regardless of size—as unacceptable, demanding excavation and full repair before service.
Solidification (Hot) Cracks During Weld Pool Cooling
Hot cracks form in the mushy zone as the weld solidifies, typically between 1,200–1,500°C, when low-melting-point impurities like sulfur or phosphorus segregate at grain boundaries and shrinkage stresses pull the metal apart.
They appear longitudinal along the centerline in high-restraint joints or in austenitic stainless steels with high nickel content. Travel speeds above 12 ipm on 1/4-inch plate exacerbate them by narrowing the weld pool and increasing centerline solidification rate.
Prevention requires filler metals with matching chemistry (low sulfur <0.015%) and joint designs that reduce restraint, such as wider root gaps of 1/16–3/32 inch in butt joints. Back-step welding sequences further distribute stresses without altering base parameters.
Hydrogen (Cold) Cracks in the HAZ of Hardenable Steels
Cold cracks develop hours or days after welding when diffusible hydrogen—picked up from moisture in electrodes, flux, or base metal rust—combines with a susceptible martensitic microstructure and tensile residual stresses. Common in steels with carbon equivalent (CE) above 0.40, they initiate in the HAZ underbead or at the toe.
For a 1/2-inch thick 4130 chrome-moly plate, preheat to 200–300°F and maintain interpass temperatures at 250°F minimum to slow cooling below 10°F per second through the 800–500°F range.
Low-hydrogen electrodes (H4 or H8 designation) baked at 500°F for two hours drop hydrogen below 4 ml/100g, eliminating the risk when paired with stringer beads instead of wide weaves.
Crater Cracks at Weld Termination Points
Crater cracks initiate from rapid solidification at the weld end where the pool shrinks without filler addition, creating a concave depression prone to centerline fractures. They appear star-shaped or transverse in GTAW and GMAW.
Filling the crater with a 1–2 second dwell and ramp-down of current by 20–30% prevents them entirely. In multi-pass work, always terminate on previously deposited weld metal rather than base plate to avoid isolated stress concentrations.
Porosity: Gas Voids That Reduce Tensile Strength and Toughness
Porosity consists of spherical or elongated gas pockets trapped as the weld solidifies, cutting effective cross-section and creating leak paths in pressure applications. Surface pinholes signal contamination; subsurface clusters show up only on radiographs or UT.
Minor scattered porosity may meet AWS D1.1 limits (individual pores ≤3/32 inch, sum of diameters in any 12-inch weld length ≤3/4 inch for static loads), but clustered or wormhole types fail outright.
Surface Versus Subsurface Porosity Across Welding Processes
Surface porosity appears as pinholes or tunnels on the bead face, while subsurface remains hidden until sectioning or NDT. In GMAW on carbon steel, surface pores cluster when shielding gas flow drops below 15 CFH or when base metal holds oil residue.
Subsurface wormhole porosity in aluminum GTAW stems from hydrogen absorbed from hydrated oxide layers, migrating as the pool cools. Identification relies on visual for surface types and dye penetrant for near-surface; radiographic testing quantifies internal volume fraction. Both degrade impact toughness by 20–30% at levels above 2% porosity by area.
Shielding Gas, Contamination, and Parameter Controls That Prevent Gas Entrapment
Porosity traces to nitrogen, oxygen, or hydrogen sources: damp electrodes in SMAW, leaky hoses in GMAW, or inadequate back-purge in stainless root passes. Set argon/CO2 mixtures at 20–25 CFH with a 5/8-inch nozzle-to-work distance to achieve laminar flow; exceed 35 CFH and turbulence pulls in air.
Clean base metal to bright metal with a dedicated grinder—never reuse abrasives contaminated with paint. For MIG on dirty plate, increase voltage by 1–2 volts and shorten stick-out to 3/4 inch to stabilize the arc and drive gas out before solidification. In TIG, maintain 15–20 CFH pure argon with a gas lens for root protection on pipe welds.
Lack of Fusion and Incomplete Penetration: Bond Failures That Slash Joint Efficiency
These fusion-related discontinuities prevent full metallic continuity, reducing throat thickness and creating planes for crack initiation under shear or tension. Lack of fusion leaves unbonded interfaces between weld metal and base or prior passes; incomplete penetration fails to reach the joint root.
Sidewall and Interpass Fusion Failures in Multi-Pass Welds
Sidewall lack of fusion occurs when heat input stays too low to melt the bevel face, common at travel speeds over 10 ipm or electrode angles steeper than 10 degrees from perpendicular. In FCAW on 3/4-inch plate, a 25° drag angle with 220–260 amps ensures sidewall wash; narrower angles direct the arc inward, leaving 1/16-inch unfused gaps detectable by UT as linear reflectors.
Interpass fusion fails when slag from prior beads remains or when interpass temperature drops below 150°F, causing the new pool to solidify before bonding. Grind or chip every pass clean and maintain consistent heat input of 25–35 kJ/inch for carbon steels.
Root Penetration Shortfalls in Groove and Fillet Configurations
Incomplete root penetration leaves an unfused gap at the joint root, directly subtracting from effective throat in fillets or full-penetration requirements in butts. Causes include tight root gaps under 1/16 inch, low amperage (below 140 amps on 1/4-inch plate), or misdirected arc in open-root pipe welds. For a single-V groove with 60° included angle, set root pass at 160–180 amps DCEN with 1/8-inch E7018 to achieve 1/16-inch penetration beyond the root face.
Backing bars or ceramic tapes allow higher heat without burn-through while ensuring full fusion. Measure root bead width visually—minimum 1/4 inch wide confirms adequate penetration.
Surface Profile Defects: Undercut and Overlap That Create Fatigue Initiation Sites
Undercut and overlap alter weld geometry at the toe, concentrating stress and reducing fatigue life by up to 50% in cyclically loaded members per AWS D1.1 cyclic criteria.
Undercut Formation from Excessive Arc Energy and Poor Toe Wash
Undercut appears as a groove melted into the base metal parallel to the weld toe, typically 1/32–1/16 inch deep. High voltage (above 28V in MIG) or long arc lengths pull the pool away from the toe before filler fills the depression. Steep electrode angles (>45°) and travel speeds over 15 ipm worsen it on vertical-up fillets.
Correct by dropping voltage 2–3V, slowing travel to 8–10 ipm, and pausing 1/2 second at each toe during weave. AWS D1.1 limits undercut to 1/32 inch for cyclic loads and 1/16 inch for static; exceed this and deposit a small repair pass at 120 amps with a 1/16-inch electrode to blend smoothly.
Overlap Creating Stress Risers in Lap and Fillet Joints
Overlap, or cold lap, forms when weld metal flows onto unmelted base without fusing, creating a ledge at the toe. Low travel speed combined with excessive wire feed or incorrect polarity produces it in GMAW short-circuit mode. The unfused interface acts as a crack starter under bending loads.
Prevent by increasing travel speed 20% and directing the arc force into the joint rather than the toe. In practice, a 10–15° push angle on horizontal fillets keeps the pool from rolling over. Grind overlap flush only after confirming no underlying lack of fusion with magnetic particle testing.
Inclusions: Non-Metallic Particles Disrupting Metallurgical Continuity
Inclusions interrupt the uniform metallic structure, lowering ductility and creating fracture paths under impact or fatigue.
Slag Inclusions in SMAW and FCAW Multi-Pass Applications
Slag inclusions trap non-metallic flux remnants between passes or at the root when interpass cleaning skips or weave width exceeds 4x electrode diameter. Restricted groove access in narrow V-preps (under 50°) prevents slag float-out.
Remove every pass with chipping hammers and wire brushing, then inspect for dark lines before the next deposit. For 1/8-inch E7018 electrodes, limit weave to 3/8 inch maximum and maintain 20–25V to keep the slag fluid long enough to escape. UT easily detects elongated slag as linear indications exceeding code length limits.
Tungsten and Oxide Inclusions Specific to GTAW Setups
Tungsten inclusions occur when the electrode contacts the pool or filler rod, transferring refractory particles detectable on radiographs as high-density spots. Limit to 2% thoriated or 1.5% lanthanated electrodes sharpened to 30° points and maintain 1/8–3/16 inch arc length at 120–180 amps.
Oxide inclusions in aluminum arise from incomplete removal of the tenacious oxide layer before welding; stainless requires full back-purge at 10–15 CFH to prevent root-side oxidation. Both reduce toughness and must be ground out completely before re-welding.
Distortion Defects: Shrinkage-Induced Misalignment in Fabricated Structures
Distortion manifests as angular change, longitudinal bowing, or buckling when uneven heating and cooling create residual stresses exceeding yield strength. It forces fit-up problems in assemblies and secondary straightening operations.
Control angular distortion in T-joints by balancing fillet welds on both sides with equal leg sizes and using back-step sequences that deposit 6–8 inch segments in opposite directions. For 1/4-inch plate, limit heat input to 20 kJ/inch maximum via pulsed GMAW at 180 amps peak.
Longitudinal shrinkage in 10-foot butt seams reaches 1/8–1/4 inch without restraint; preset joints with 1/16-inch gap taper or clamp with strongbacks spaced every 12 inches. Post-weld flame straightening on low-carbon steel requires heating to 1,100°F in narrow bands while monitoring with dial indicators to avoid introducing new stresses.
On-the-Job Inspection Strategies for Different Types of Weld Defects
Visual testing catches surface undercut, overlap, porosity, and cracks within AWS D1.1 limits before NDT. Use 10x magnification and white light at 500 foot-candles minimum for toe details. Liquid penetrant reveals surface-breaking cracks and porosity after 10-minute dwell.
Magnetic particle testing excels on ferromagnetic materials for subsurface cracks and inclusions up to 1/4 inch deep when using wet fluorescent particles under black light.
Ultrasonic and radiographic methods quantify internal lack of fusion, slag, and porosity volume against code tables—apply them on critical joints after visual clearance.
Record travel speed, voltage, and amperage per pass to correlate parameters with any rejectable indications for immediate correction on subsequent work.
Wrapping Up
Matching preheat to carbon equivalent, calibrating voltage within 1V of WPS values, and enforcing strict interpass cleaning let fabricators hit zero-defect rates on repeat production.
The pro-level edge comes from pulsed-current or waveform-controlled inverters that synchronize droplet transfer to eliminate short-circuit spatter and gas entrapment simultaneously—delivering porosity below 0.5% and fusion quality that constant-voltage machines cannot achieve even on marginal material.
