Welding Types and Defects: The Complete Guide to Processes, Flaws, and Prevention

A fabricator once told me the story of a 40-foot steel beam that passed visual inspection, shipped 300 miles, and came back cut open because a single cold crack grew in a fillet weld no one could see. The repair cost more than the entire job earned. Welding types and defects are not academic trivia — they decide whether a bridge holds, a pipeline stays sealed, and a welder keeps their reputation. This guide walks you through every major welding process and every defect that can quietly ruin its work.

Why Welding Types and Defects Belong in the Same Conversation

A fabricator once told me about a 40-foot steel beam that passed visual inspection, shipped 300 miles to a jobsite, and came back cut open because a single cold crack had been growing in a fillet weld no one could see. The repair bill cost more than the entire job had earned. That story is this guide in one sentence: welding types and defects are not separate topics filed in separate chapters. They are two halves of the same question — which process did you pick, and what did that choice leave behind inside the metal?

Every welder who has burned rod for a living knows this instinctively. Choose the wrong process for the joint and the joint tells on you, sometimes immediately, sometimes three days later when the crack finally opens. Understanding that link separates someone who can lay a bead from someone trusted with structural work.

What People Actually Want When They Search This Topic

Whoever types “welding types and defects” into a search bar is rarely hunting for a textbook chapter. They are usually in one of four situations: a student prepping for certification, a new welder trying to figure out why a weld failed, a shop supervisor building a procedure, or a buyer or inspector deciding what to reject. This guide serves all four, and it is structured so you can read straight through or jump to the section matching your problem.

It flows from a taxonomy of welding processes into the major arc processes and the specialty processes such as resistance, laser, friction stir, and underwater welding. Then we separate discontinuities from defects and show how codes draw that line. The second half is what most readers come for: cracking, porosity, and geometric defects explained cause by cause, followed by inspection methods and a prevention playbook you can take to the shop floor.

What a Welding Defect Actually Is

Here is the most important idea in this article, and one beginners get wrong for years: every weld in the world contains discontinuities. Porosity, slight undercut, excess reinforcement, a restart — none make a weld “bad” on their own. A discontinuity is simply an interruption in the normal structure or geometry of the weld, a neutral and measurable observation.

A discontinuity becomes a defect only when it exceeds the acceptance limits of the governing code. That is not a philosophical distinction; it is a legal and contractual one. An undercut measuring 0.5 mm on a non-critical bracket may be perfectly acceptable under AWS D1.1 in one context and rejectable in another. Same flaw, same number, two verdicts depending on the standard, material thickness, and service conditions. We unpack that fully in the section on codes and acceptance criteria, but keep it in your head from here on.

Why Process Selection Drives Defect Type

Each welding process brings its own physics to the joint, and each physics has a signature failure mode:

• Shielded metal arc welding drags flux-coated electrodes that release hydrogen if damp, which is why stick work is tied to cold cracking and why electrode storage matters as much as amperage.
• Gas metal arc welding fuses wire through external shielding gas, so its signature problems are porosity from gas disruption and lack of fusion from wrong voltage or travel speed.
• Submerged arc welding deposits enormous heat input, bringing distortion and lamellar tearing into the conversation.
• Friction stir welding never melts the metal, eliminating an entire family of fusion defects while introducing its own, like wormhole defects from inadequate forging pressure.

This is why choosing the wrong process is itself a defect cause. Running a MIG setup on a thick, poorly fitted structural joint, or using a stick electrode of the wrong classification for the base metal, does not just make the work harder. It builds the failure mechanism into the weld from the start.

Before we go further, a note on the stakes. Welding is not a niche skill. The U.S. Bureau of Labor Statistics reports a median annual wage of $53,750 for welders, cutters, solderers, and brazers as of May 2025, with employment projected to grow roughly 2 percent from 2025 to 2035. Steady demand plus modest growth means the shops that win work are the ones with the lowest rework rates — and rework is almost always a defect problem, not a production problem.

Safety sits underneath everything here. OSHA 1910.252 requires welding, cutting, and brazing operations to address fumes and gases, fire prevention, electrical shock, and radiation hazards as a baseline. You cannot inspect, repair, or prevent defects safely without the right protective equipment, which is why we always recommend reviewing proper welding PPE before starting any new process work. The standard applies whether you are running your first bead or your ten-thousandth.

With that foundation set, let us look at the families of welding processes and what each one is physically doing to the metal — because once you understand the physics, most defects stop being mysterious.

The Major Families of Welding Processes Explained

Ask ten people what “welding” means and you’ll get ten answers: a stick arc, a MIG gun, a torch, a laser. All of them are right, and none of them are complete. Welding is not one process but a tree of processes grouped into families, and the family you pick quietly decides which defects you will fight for the rest of the job.

Each family joins metal through a different physical mechanism, and every mechanism has its own failure mode. A process that melts metal can produce solidification cracks; a process that never melts the metal cannot. Learning the taxonomy is the fastest way to predict trouble before you strike an arc.

Fusion Welding: Joining by Melting the Base Metal

Fusion welding is the broadest family. Heat melts the base metal, usually with filler, the puddle solidifies, and two pieces become one. It splits into three sub-groups:

• Arc welding — an electric arc as the heat source: shielded metal arc (SMAW), gas metal arc (GMAW/MIG), flux-cored arc (FCAW), gas tungsten arc (GTAW/TIG), and submerged arc (SAW). This family covers the widest thickness range, from sheet thin enough to blow through with a hot rod to heavy structural plate over an inch thick. Its classic defects are porosity, lack of fusion, undercut, and cracking.
• Gas welding — an oxy-fuel flame plus related torch processes. The heat is less concentrated, so the heat-affected zone (HAZ) is wide and distortion is common. Because the operator judges heat by eye, burn-through on thin material and overheating on thick plate are routine problems. Protective equipment matters here as much as at the arc; the basics are covered in Welding PPE: The Complete Guide to Protective Gear Every Welder Needs.
• Energy-beam welding — laser beam and electron beam. Both produce deep, narrow welds with a small heat-affected zone and low total heat input, which is why aerospace and battery manufacturers love them. The trade-off is tight joint fit-up: a gap the arc would bridge becomes a void. Common defects are keyhole porosity, lack of fusion from misalignment, and cracking in restrained joints.

A fourth cousin worth knowing is Railroad Thermite Welding, where a superheated chemical reaction pours molten steel into a mould. It is fusion welding without any power source at all, and it brings its own defect family: inclusions and porosity from a poorly prepared rail gap.

Solid-State Welding: Joining Without Melting

Solid-state processes join materials below their melting point using pressure, deformation, or friction instead of a puddle. Friction stir welding, ultrasonic welding, forge (pressure) welding, and explosion cladding all belong here. Because the base metal never becomes a liquid, solidification cracking and gas porosity are largely eliminated — the two defect families that plague fusion work simply cannot form.

The defects shift to mechanical problems instead: insufficient friction heat leaving a “wormhole” channel, incomplete bonding at the stir zone boundary, hooking at the interfaces of stacked sheets, or an unwelded interface where pressure was too low. The physics are unforgiving in a different way, and you inspect them differently too.

Resistance Welding: Pressure Plus Electrical Resistance Heat

Resistance welding passes a high current through the joint while squeezing the parts together. Heat comes from electrical resistance at the interface (I²R heating), and the parts fuse under pressure — no filler, no shielding gas, no flux. Spot welding, seam welding, and projection welding are its members, and they dominate automotive body production, where a single car body can carry several thousand spot welds.

Because it is a metallurgical and electrical process at once, its defects are distinctive: expulsion (“button splash”) from excessive current, undersized or “cold” nuggets from too little time or pressure, porosity inside the nugget from contamination or degraded electrode faces, and current shunting when welds are placed too close together. For the underlying physics and setup, see How Does a Spot Welder Work? Physics, Process, and Practical Setup Explained. Copper changes the rules entirely, since its conductivity robs the interface of heat — a separate challenge addressed in Spot Welding Copper.

Brazing and Soldering: Filler With a Lower Melting Point

Brazing and soldering melt only the filler metal, never the base metal. The filler flows into the joint by capillary action, so joint strength depends less on heat and more on gap control and surface cleanliness. Oxide, oil, or a gap too wide to hold the filler produces a weak, starved joint rather than a cracked one. Typical defects are flux entrapment, voids, incomplete wetting, and erosion of the base metal from overheating. Since brazing often involves fluxes, fumes, and torch work, the ventilation requirements in OSHA’s 1910.252 welding, cutting, and brazing standard apply squarely to this family too.

Two practical takeaways. First, the family narrows the defect list, but it does not excuse you from the rest of this guide: the same joint can be made three ways and fail differently each time. Second, family choice drives safety obligations as much as quality ones — every fusion process burns, sparks, and throws ultraviolet light, which is why skin and eye protection is non-negotiable whether you are MIG welding in a shop or cutting outdoors. With the map in hand, let us walk the arc-welding branch in detail, since that is where most readers will spend their working lives.

Arc Welding Types: SMAW, GMAW, FCAW, GTAW, and SAW

Every arc process does the same basic job: melt base metal with an electric arc so two pieces become one. What changes is how filler metal is delivered, how the puddle is shielded from the atmosphere, and how much heat the operator controls. Those three decisions determine the defects you will fight. Here are the five workhorse processes and the specific flaws each one tends to produce.

SMAW (Stick Welding): The Portable Workhorse

Shielded metal arc welding burns a consumable, flux-coated electrode. As the flux heats, it releases gas that shields the puddle and leaves a slag blanket over the finished bead. That is why a stick rig needs nothing but a power source, a holder, a ground clamp, and a box of rods. It performs well outdoors in wind that would wreck a gas-shielded weld, and it works in every position, including overhead. Shipyards, structural erection, pipeline maintenance, and repair shops all lean on it. Beginners can also start here, as covered in Is Stick Welding Hard? An Honest Beginner’s Guide.

Defect drivers: electrode storage and slag removal. Low-hydrogen rods like E7018 absorb moisture from humid air, and that hydrogen ends up in the weld as porosity or delayed cold cracking. Slag chipped out of the previous pass but not brushed away becomes a slag inclusion. Excessive amperage or fast travel produces undercut; too little heat produces lack of fusion. If you are dialing in rod settings, the 3/32 E7018 Amperage guide is a practical starting point, and What Are Electrodes in Welding? explains how to read the classification digits.

GMAW (MIG Welding): Speed and Deposition

Gas metal arc welding feeds a bare wire continuously through the gun while shielding gas flows from a cylinder around the arc. Deposition rates are high, spatter is low, and there is no slag to chip, which makes it the fastest process for fabrication work: automotive panels, sheet-to-medium plate, brackets, and general production. Setup matters more than people expect, so How to Set Up a MIG Welder walks through the variables.

Defect drivers: wire feed speed and stick-out. Wire feed that is too slow relative to travel starves the joint and causes lack of fusion; too fast causes spatter, cold lap, and burn-through on thin metal. Contact-to-work distance that is too long weakens gas coverage and arcs erratically, producing porosity. Gas flow disturbed by a shop draft, a clogged nozzle, or the wrong gas mix does the same. Because MIG produces fumes and UV radiation, the controls described by OSHA 1910.252 on ventilation and shielding apply directly; our MIG Welding Safety Precautions guide translates them into shop practice.

FCAW (Flux-Cored): Wind-Tolerant Power

Flux-cored arc welding feeds a hollow wire filled with flux. Self-shielded wire makes its own protective gas, so it tolerates wind, drafts, and lightly contaminated plate far better than GMAW. Gas-shielded (dual-shield) wire adds external gas for cleaner, higher-quality beads. Both deliver high deposition and support vertical-up welding on thick sections, which makes FCAW a favorite for structural steel and heavy equipment.

Defect drivers: slag. Every FCAW pass leaves a heavy slag layer that must be completely removed between passes, or it gets trapped as a slag inclusion. Self-shielded wire also requires a longer stick-out than MIG; set it too short and you get porosity, too long and the arc becomes unstable. Field technique for cleaner beads is covered in Flux Core Wire Welding Tips.

GTAW (TIG): The Highest-Control Process

Gas tungsten arc welding uses a non-consumable tungsten electrode, inert argon shielding, and a separate filler rod held in the other hand. The result is the most controllable arc in welding: no spatter, precise heat input, and full visibility of the puddle. It is the standard choice for root passes on pipe, sanitary stainless, aluminum, titanium, and thin sheet. The skill floor is high and the travel speed is slow, which is why shops reserve it for work that must be right. See The Complete Guide to TIG Welding Sheet Metal for settings and common mistakes.

Defect drivers: shielding and contamination. Losing gas coverage from wind, a long torch extension, or a porous hose causes porosity and, on stainless and titanium, sugaring inside the root. Touching the filler rod to the hot tungsten embeds tungsten particles in the weld, which is rejectable under most codes. Poor fit-up and too little heat cause lack of fusion at the joint edges. Multi-pass pipe work is detailed in Pipe Welding Procedures.

SAW (Submerged Arc): Thick-Plate Production

Submerged arc welding hides the arc under a layer of granular flux. Because the arc is buried, there is no visible flash, little spatter, and very high deposition, which lets SAW deposit heavy welds quickly on thick plate. You will find it on pressure vessels, ship hulls, I-beams, and spiral pipe, usually on a mechanized carriage.

Defect drivers: position and parameters. SAW is practical only in flat or horizontal fillet positions, so it cannot fix a joint that must be welded overhead or vertically. Wrong travel speed, arc voltage, or flux depth produces lack of fusion, slag inclusions, undercut at the weld toes, and excessive reinforcement. Rust, oil, or moisture under the flux causes porosity, so plate preparation is part of the process. Heavy-section joints also demand a documented procedure under AWS D1.1.

Specialty Welding Types: Resistance, Laser, Friction, and Underwater Welding

Most welders train on arc processes — stick, MIG, TIG, flux core — and most defect literature follows the same path. Yet a huge share of the welds holding the modern world together were never made by an arc. The spot welds down your car’s flank, the hermetic seal in a medical device, the aluminum battery tray, the repair on a harbor pier at 60 feet: all specialty processes, each with its own physics, its own advantages, and its own very particular way of going wrong. This section covers the families readers search for most — resistance, laser beam, electron beam, friction stir, plasma arc, and underwater welding — and the defects unique to each.

Resistance Welding: Spot, Seam, and Projection

Resistance welding uses no flame, no arc, and no filler. Two copper alloy electrodes clamp the workpieces together and force a controlled surge of current through the stack. Resistance at the faying surface generates heat, the metal softens, and applied forge force forms a nugget that fuses on cooling. The physics is explained in our guide to how a spot welder works, but the practical takeaway is simple: nugget quality is a function of current, time, and electrode force, and all three must stay in balance.

• Spot welding joins overlapping sheets at discrete points and dominates automotive body construction, where one vehicle can carry several thousand spot welds.
• Seam welding uses rotating wheel electrodes to produce a continuous leak-tight seam — fuel tanks, mufflers, drum assemblies.
• Projection welding localizes current at engineered bumps, making it ideal for fasteners, nuts, and multi-point welds.

The characteristic defects differ sharply from arc welding: stuck electrodes, expulsion (metal splash from excessive current or force), undersized nuggets from cold settings, indentation or burn-through on thin or highly conductive material, and current shunting, where current leaks through an adjacent finished weld instead of passing through the joint being made. Electrode dressing matters more here than anything else — a mushroomed or contaminated tip changes contact resistance and quietly pushes nuggets out of specification.

Working with conductive metals changes the game completely — copper and brass demand far higher currents and tighter control, as our practical guide to spot welding copper explains.

Laser Beam and Electron Beam Welding

Laser beam welding focuses coherent light onto the joint, producing a deep, narrow fusion zone with a small heat-affected zone. It runs at high speed with little or no filler, which suits sheet metal, battery tabs, medical instruments, and precision assemblies. Our TIG sheet metal guide covers the manual alternative when volumes do not justify a laser cell.

Electron beam welding works on the same deep-penetration principle but uses accelerated electrons in a vacuum chamber. Removing gas makes the process extraordinarily clean and lets it weld highly reactive metals like titanium. The trade-off is chamber cost, cycle time, and X-ray shielding.

The distinctive defects are keyhole porosity and spatter. Deep-penetration welding vaporizes a narrow channel that collapses as the beam advances; if travel speed, power density, or shielding gas flow are wrong, it collapses imperfectly and traps fine, scattered pores. Excessive power or poor gas coverage blows molten metal out, leaving spatter and craters. Crater cracks at run-off are common, so a beam-off delay or run-off tab is standard practice.

Friction Stir Welding: The Solid-State Breakthrough

Friction stir welding, developed by The Welding Institute in the United Kingdom, remains one of the most elegant ideas in joining: a rotating tool with a hardened pin plunges into the joint line and traverses along it, softening the material and mechanically stirring the two sides together without ever reaching the melting point.

Because nothing melts, FSW avoids the whole family of solidification defects. Porosity, shrinkage, and hot cracking are dramatically reduced, and the joint typically shows higher strength and lower distortion than a fusion weld in the same alloy. That is why FSW became standard for aluminum railcar bodies, marine deck panels, aerospace fuel tankage, and electric vehicle battery trays — alloys notoriously unfriendly to fusion welding.

The defects FSW does produce are mechanical: tunnel defects or wormhole voids from insufficient heat or travel that is too fast, pin holes at the trailing edge, surface galling or onion-ring irregularities, flash from excessive forge force, and lazy S oxide lines on aluminum that were never properly stirred. Tool wear is a defect driver of its own — a worn pin changes shoulder geometry long before anyone notices. A related high-temperature process, aluminothermic joining, works in a completely different setting: our feature on railroad thermite welding covers that exothermic reaction and its own defect profile.

Plasma Arc Welding

Plasma arc welding constricts the arc through a nozzle bore to produce a stiff, high-energy-density column. Compared with GTAW, the arc is more focused, standoff distance is more forgiving, and deep penetration at moderate current is achievable in one pass — useful for precision components, aerospace hardware, and machine parts needing narrow, repeatable welds.

Typical defects include arc instability when the orifice is contaminated, double arcing (the arc jumping to the nozzle instead of the work), porosity from inadequate trailing shielding, and melt-through in keyhole mode if travel speed drops. Argon-hydrogen and argon-helium mixes are common, and gas purity is unforgiving: a contaminated bottle can ruin a production run.

Underwater Welding: Wet and Hyperbaric Dry

Underwater welding splits into two very different technologies. Wet welding leaves the arc directly exposed to water, using specially coated electrodes that burn under hydrostatic pressure and slow gas-bubble formation. It is fast and portable, the default for in-situ repairs on ships, piers, offshore platforms, and pipelines. Hyperbaric dry welding places a habitat around the weld zone and pumps it full of dry gas, producing far better metallurgy — but the cost reserves it for high-value or code-critical work.

Water is brutal on metallurgy. Rapid quenching drives hydrogen pickup from dissociated water vapor, raising hardness in the heat-affected zone and susceptibility to hydrogen-induced cold cracking — the same delayed cracking risk covered earlier, amplified. Residual hydrogen in wet welds can run several times higher than in an equivalent shop weld.

Preheat is largely impossible underwater, so electrode selection does the heavy lifting: wet-service rods use thick coatings that burn stably under pressure while slowing hydrogen generation. Even so, wet welds are generally treated as temporary or emergency repairs, accepted only with supplementary inspection such as magnetic particle or ultrasonic testing of every weld rather than shop-style sampling. That caution is well founded: high hardness, absorbed hydrogen, and residual stress are a textbook recipe for delayed cracking, and a crack forming hours after the diver surfaces will fool a visual check. Dry welding sidesteps most of it, which is why it is preferred for permanent offshore and pipeline repairs.

One thing never changes: the protection requirements stay in force. A diver, a laser operator, and a spot-weld line worker face different hazards, not fewer, and our welding PPE guide covers what each of them needs before work starts.

That is why specialty welding belongs in the same conversation as SMAW and MIG. A shop that knows which defect family its process produces can design inspection, set acceptance criteria, and prioritize training accordingly. The next section covers where that acceptance line actually sits in writing — and why the difference between a discontinuity and a defect decides whether your weld gets repaired, accepted, or rejected.

Weld Discontinuities vs. Weld Defects: How Codes Draw the Line

Two welders can stand in front of the same fillet weld and reach opposite conclusions. One calls it scrap, the other calls it fine, and both feel certain. The argument only ends when somebody pulls out a code book and starts measuring. That is the whole point of this section: welding quality is not a matter of taste, and the difference between a discontinuity and a defect is where the money, the safety, and the reputation all sit.

A Discontinuity Is a Fact; a Defect Is a Verdict

A weld discontinuity is any interruption in the normal physical structure of a weld or base metal: a pore, a bit of undercut, some overlap, an arc strike, a rough stop-start, spatter. Nearly every weld ever produced contains some. Discontinuities are observations. You see one, you measure it, you record it.

A weld defect is a discontinuity that has been judged against an acceptance standard and found unacceptable. It is a verdict, not a sighting. The same 0.5 mm pore that is perfectly legal in one joint is a rejection in another, because the applicable code, the weld’s service condition, and the quality level chosen by the contract decided it. As Michigan Tech’s welding discontinuities training material puts it, the line between the two is drawn by a standard, not by an opinion (MTU welding defects webinar).

What AWS D1.1 Actually Asks

The AWS D1.1 Structural Welding Code — Steel is the working rulebook for most structural steel in the United States, and it is refreshingly blunt. It provides explicit, tabulated acceptance criteria for porosity, undercut, overlap, incomplete fusion, and cracks. It does not ask whether a weld “looks good.” It asks whether a specific measurable threshold has been crossed.

In practice that means:

• Undercut is measured with a depth gauge or a rule against a tabulated maximum, roughly 1/32 in (0.8 mm) for statically loaded members, with essentially no tolerance allowed where cyclic loading is involved.
• Porosity is counted and converted into an equivalent total pore diameter inside a fixed 2-inch (50 mm) gauge length, and the allowance tightens as material thickness increases.
• Incomplete fusion and lack of penetration are treated far more harshly because they remove load-bearing section rather than merely interrupting it.
• Cracks are generally unacceptable in any load-carrying weld regardless of size. There is no “small enough” crack, no gauge length, no grace period.

That last line is why cracking gets its own deep-dive chapter later in this guide, and why a fabricator who fixes a crater crack “later” has already lost. For a practical look at how a single discontinuity type gets its own limit table, see our breakdown of welding defects undercut: causes, acceptance limits, and how to fix it.

ISO 5817, ISO 10042, and the Quality Levels

Internationally the language changes but the logic does not. ISO 5817 covers fusion-welded joints in metallic materials and defines three quality levels:

ISO 10042 plays the same role for aluminium, copper, nickel and their alloys, where porosity and oxide inclusions behave differently than in steel. The project specification, not the welder, decides which level applies on a given job.

Pressure work runs through the ASME Boiler and Pressure Vessel Code — Section IX for welding qualification, Section VIII for vessels, B31.3 for process piping — while pipeline work runs through API 1104. Even OSHA’s welding standard points back to recognized codes rather than to personal judgment (29 CFR 1910.252). If you are writing or reading a procedure for pipework, our guide to pipe welding procedures: WPS, positions, passes and inspection shows how these criteria flow into the paperwork.

Why This Distinction Saves Real Money

Subjective inspection is expensive on both sides. Reject a legal weld and you pay for grinding, re-welding, re-inspection, and schedule slip. Accept an illegal one and you pay later, with interest, sometimes catastrophically. Measurable limits remove the argument: nobody’s ego is on the line when a gauge says 1.1 mm and the table says 0.8 mm.

That consistency also depends on who is holding the gauge. Inspection levels and personnel qualification — AWS Certified Welding Inspector, ISO 9712 visual and NDT certification — are built into code requirements precisely so the same discontinuity gets the same verdict on Tuesday as on Friday, in this shop or the next one. The standard decides, the inspector verifies, and the shop either fixes it or documents why it stays. No flashlight, no opinion, no surprise invoice.

Cracking Defects: Hot Cracks, Cold Cracks, Crater Cracks, and Hydrogen Damage

Every other weld flaw can be argued about. A little undercut, accepted within code limits. Scattered porosity, often tolerable. A crack? Not negotiable. Cracking is the one discontinuity that construction, structural, and pressure-vessel codes reject outright, because its sharp tips concentrate stress and let failure run under load. That 40-foot beam from this guide’s opening failed because of one cold crack, not porosity.

Solidification (Hot) Cracking

Hot cracks, also called solidification or centerline cracks, form while the weld pool is still partly liquid, usually in the last stage of freezing. As dendrites grow inward from the weld walls, liquid films trap along the centerline. Held rigidly by the surrounding joint, the shrinking bead tears that film open before it can heal.

Typical culprits:

• Metallurgy: sulfur and phosphorus in steel, or aluminum alloys with a wide freezing range (6xxx is the classic example).
• Restraint: heavy, tightly clamped joints and poor fit-up that give the shrinking bead nowhere to move.
• Technique: travel too fast, a deep narrow bead, a wrong weave, or a sequence that piles stress into one corner.

The counterintuitive fix is often a smaller, wider bead: lower current, faster travel, and stringer passes instead of stacking a narrow deep pool. Because hot cracking shows on the surface, visual inspection catches it before anything ships.

Hydrogen-Induced (Cold) Cracking

Cold cracking, or delayed cracking, is the dangerous one: it often appears hours or days after the weld has cooled and passed visual inspection. It forms in the heat-affected zone or weld metal and needs three factors at the same time — the hydrogen triangle:

1. Diffusible hydrogen present in the joint.
2. A hard, susceptible microstructure, typically untempered martensite in the HAZ of a rapidly cooled steel.
3. Tensile stress from joint restraint, residual stress, or misalignment.

Remove one leg and the risk drops sharply: prevention is a three-part job.

Where hydrogen comes from: moisture in electrode coatings and flux, damp or rusty plate, oil, grease, paint, primer, cutting fluids, and humid shop air. OSHA 1910.252 covers ventilation, but hydrogen control itself happens at the bench.

Low-hydrogen consumables: basic electrodes such as E7018 are classified by diffusible hydrogen level — H4, H8, H16, meaning milliliters of hydrogen per 100 grams of deposited weld metal. They must be baked to spec and sealed against moisture pickup, which returns within hours of leaving the oven. Rod ovens and heated quivers are cheap insurance. What Are Electrodes in Welding? Types, Numbers, Polarity & How to Choose covers the classification system, and 3/32 E7018 Amperage: Settings, Polarity & How to Run It shows how to run them without settings that pull in extra hydrogen.

Post-weld heat treatment does two jobs: it drives out trapped hydrogen and relieves residual stress. On critical joints — pressure pipework, heavy plate, quenched and tempered steels — PWHT or a controlled bake-out is specified alongside preheat and interpass limits. Our Pipe Welding Procedures: WPS, Positions, Passes & Inspection shows those requirements inside a real WPS.

Crater Cracks

A crater crack forms when the arc is broken abruptly at the weld stop. Shrinkage tears the starved pool open and a star-shaped or crescent crack appears at the tie-in. Common at every MIG start-stop. The fix: backfill the crater. Pause and let the puddle rise before breaking the arc, use current decay or downslope if the machine has it, and overlap the crater on restart. Our The Complete Guide to TIG Welding Sheet Metal treats crater backfilling as a core habit.

Reheat Cracking and Lamellar Tearing

Two less common but costly forms deserve names:

• Reheat cracking occurs during post-weld heat treatment or high-temperature service, in coarse-grained HAZ regions of steels alloyed with vanadium, niobium, chromium, and molybdenum. Grain-boundary precipitates pin the grains while boundaries slide, tearing them apart. Fine-grained practice, lower restraint, and peening help.
• Lamellar tearing is a stepwise subsurface crack running parallel to the plate surface, caused by sulfide stringers pulling apart through the thickness of heavily restrained sections. Prevention is design and material: specify through-thickness (Z-direction) plate, keep fillet sizes reasonable, prefer a T-joint to a deep edge weld, and use a more ductile filler.

Once a crack is found, the sequence never changes: gouge or grind it out completely, confirm removal by magnetic particle inspection, then reweld to a qualified procedure with correct preheat. Gouging and grinding throw sparks, UV, and debris, so suit up first with this welding PPE guide. For the acceptance framework that decides whether a crack-like indication is rejectable, AWS D1.1 and this MTU welding discontinuities webinar are the next reads.

Porosity, Gas Pockets, and Contamination Defects

Cracks get the headlines, but porosity is the defect that actually shows up on your job: the most common reason welds fail visual and radiographic inspection, and almost always self-inflicted. Understand where the gas comes from and it becomes one of the easiest flaws to prevent.

What Porosity Actually Is

As the arc melts base metal and filler, the molten pool dissolves hydrogen, nitrogen, and carbon monoxide. Liquid steel holds far more gas than solid steel, so when the pool freezes the excess has nowhere to go and forms bubbles. Bubbles that escape before solidification leave nothing behind; bubbles that are trapped become porosity. Michigan Tech’s welding discontinuities training material lists inadequate shielding, moisture, and surface contaminants as the most commonly cited causes.

The Four Faces of Porosity

• Uniform (dispersed) porosity: small, evenly scattered round voids, usually a steady shielding problem such as wrong gas, low flow, or a leaky hose.
• Wormhole porosity: elongated, tunnel-like cavities that look like worm trails on a radiograph, often traced to oil or to travel speed that seals the pool over escaping gas.
• Surface-breaking porosity: pinholes on the bead face, and the most common inspection rejection because anything open to the surface can leak or seed a crack.
• Blowholes: large, isolated cavities visible to the naked eye, caused by heavy contamination or total shielding collapse. They cut load-bearing cross-section dramatically.

Hydrogen usually comes from moisture in coatings, electrode flux, or the atmosphere. Nitrogen comes from air reaching the arc, and in steel welding it is what produces the blue-tinted, brittle, porous weld metal of lost shielding.

Shielding Gas, Flow, and Cleanliness

Gas selection matters more than most beginners realize. Straight CO₂ gives deep penetration but a violent, spatter-heavy arc that can pull atmospheric gas into the pool; an argon/CO₂ mix runs calmer and cleaner. Flow rate is a Goldilocks setting: too little and the envelope collapses, too much and turbulence sucks air in. Most shop conditions call for roughly 20–25 CFH on a MIG setup, verified with a soap-bubble leak check from regulator to gun. Our MIG welder setup guide walks through the whole procedure.

Contamination is the other half of the equation. Oil, mill scale, rust, cutting-fluid residue, and paint all release gas when the arc hits them, so grind, wipe, and degrease before striking an arc. For stick work, electrode conditioning is critical: low-hydrogen rods like E7018 must stay in a rod oven or heated quiver, because the flux absorbs atmospheric moisture within hours of exposure, and a wet rod causes hydrogen porosity and, worse, hydrogen cracking. Correct amperage and polarity settings matter too, since a cold, underheated pool traps gas it should have released.

Travel speed ties it together. Too fast and the solidifying crust freezes over bubbles still rising; too slow and you overheat the pool, boil off alloying elements, and invite atmospheric pickup. Flux-cored work has quirks of its own, covered in our flux core wire welding tips on polarity and stickout.

Oxide Films: Aluminum’s Hidden Trap

Aluminum’s oxide layer melts at roughly 2,070°C (3,760°F) while the base metal melts near 660°C (1,220°F), so the arc melts the aluminum long before it touches the oxide. Unless that film is removed first, it floats into the pool as an inclusion or causes lack of fusion right at the weld toes. Clean with a dedicated stainless steel brush reserved only for aluminum, or a chemical cleaner, and never let the part sit between cleaning and welding. TIG welding sheet aluminum is where this mistake is punished fastest.

Arc Strikes: The Defect Nobody Meant to Make

An arc strike is any arc started outside the weld zone: a dead stick scratching the plate, a stuttered MIG start, a ground clamp that slipped. The CWB Group identifies arc strikes as a distinct inspection issue because each one creates a small, rapidly cooled heat-affected zone that is hard, brittle, and crack-prone, and it counts as a discontinuity whether or not it sits in the weld. CWB’s training material flags it alongside porosity and undercut as a top avoidable flaw, and OSHA 1910.252 treats accidental arcing on base metal as a hazard to be engineered out of the work area. Start your arc on a run-on tab, a tack, or the joint itself.

The Prevention Checklist

1. Leak-check gas lines and set flow to spec each shift.
2. Screen drafts, or move to a self-shielded process.
3. Degrease, grind, and brush every joint.
4. Store low-hydrogen rods in a heated quiver or oven.
5. Match travel speed to heat input and pool fluidity.
6. Ban arc strikes on base metal; use run-on tabs or tacks.
7. Inspect every bead under good light.

Porosity costs shops far more in rework and reputation than skipped prep time ever saved. OSHA’s welding hazard guidance and the AWS D1.1 structural welding code both treat gas protection and surface condition as non-negotiable quality gates, and now you know exactly why.

Geometric and Structural Defects: Undercut, Lack of Fusion, Incomplete Penetration, and Distortion

A weld can be made from perfectly clean, correctly specified metal and still fail. Geometric defects are shape problems: the weld metal is there, the chemistry is fine, but the profile, penetration, or position of the bead is wrong. Some of these you can spot with your eyes. Others hide in the root of a joint until a radiograph or ultrasonic scan finds them. Both kinds cost money, and distortion alone accounts for a surprising share of rework in fabrication shops.

Undercut and Overlap: Too Much Heat Here, Not Enough There

Undercut is a groove melted into the base metal alongside the weld toe that the filler never refills. It usually comes from amperage that is too high, travel speed that is too fast, an arc angle pointed at the edge, or a weave that swings wide without pausing at the toes. That thin notch is a stress raiser, and it is one of the most common fatigue-crack initiation sites in welded structures. Our detailed guide on welding defects undercut: causes, acceptance limits, and how to fix it covers repair technique and code limits in depth.

Overlap, often called cold lap, is the opposite failure: molten metal spills over the base surface without fusing into it. Slow travel, low voltage, wrong electrode angle, or a dirty surface leaves a rolled, unfused lip. It is cosmetic on the surface and structural underneath, because the unfused edge carries no load.

Incomplete Fusion and Lack of Penetration

Incomplete fusion (lack of fusion) is a break in bonding between weld metal and base metal or between adjacent passes. Lack of penetration means the weld never reached the root of the joint. Both are frequently internal and completely invisible during visual inspection, which makes them among the most dangerous defects in service. Common causes include insufficient heat, travel speed too quick for the joint, a weave that doesn’t dwell on the sidewalls, poor fit-up with a gap that is too tight, and the habit of laying a tack weld in the middle of the joint without burning it through on the root pass. This is why pipe welding procedures are so strict about root passes and tacking practice: in a pressure pipe, an unfused root is a leak path or a crack starter.

Melt-through is the mirror image on thin material, where the arc burns a hole clean through the joint. Excessive reinforcement is a bulging bead that wastes filler and creates a sharp stress concentration at the toe rather than a smooth transition. Inconsistent bead width and height, often called an inconsistent weld bead, usually traces back to wandering arc length, unsteady travel speed, or a machine set incorrectly; a properly tuned feed and a stable arc fix most of it, as covered in how to set up a MIG welder.

Weld Distortion and Residual Stress

Distortion comes from uneven heating and contraction. As the weld pool solidifies and cools, it shrinks; if the surrounding metal resists that shrinkage, the part moves. Longitudinal shrinkage, transverse shrinkage, angular warping, and buckling are the four usual forms, and every one of them leaves residual stress locked in even when the part looks straight. AWS D1.1 and the distortion guidance in the Michigan Tech welding discontinuities webinar both treat excessive distortion as a rejectable condition when it pushes the assembly out of tolerance.

Control starts before the arc is struck:

• Joint design: balanced weld placement on both sides of a neutral axis, and avoiding oversized fillets that add shrinkage for no strength gain.
• Fit-up and restraint: tight, uniform root gaps, proper bevel angles, and strongbacks or fixtures that hold the part without over-constraining it.
• Tack welding: sufficient, evenly spaced tacks sized for the material. See the MIG tack weld guide for spacing and settings.
• Weld sequencing: back-step, skip, and balanced alternating passes distribute heat instead of concentrating it in one area.
• Heat input discipline: the smallest amperage and largest travel speed that still give full fusion, especially on thin metal where burn-through is a constant risk.

Pre-setting camber, using chilling blocks or heat sinks, and welding from the center outward all let contraction work for you rather than against you. On thicker sections, a controlled post-weld straightening pass or thermal stress relief can recover tolerance without cutting the assembly apart.

How Welds Are Inspected: Visual, Radiographic, Ultrasonic, Magnetic Particle, and Dye Penetrant

Before any weld leaves the shop or the job site, it must prove it is fit for service. Inspection is what separates a weld that looks fine from one that will hold under pressure. No matter how skilled a welder is, subtle flaws can slip in, which is why a layered approach to inspection exists. For this reason, the goal is not to find every single discontinuity, but to find the ones that matter based off of the code that governs the job.

Visual Inspection: The First and Most Cost-Effective Gate

The first line of defense for any fabricator is visual inspection (VT). AWS D1.1 and nearly every other welding code require a visual check before moving onto more expensive non-destructive testing (NDT) methods. This is due to the fact that visual inspection catches the majority of weld defects, ranging from undercut, overlap, and porosity at the surface, all the way down to poor fit-up.

Proper VT requires adequate lighting, a clean weld, and an inspector with trained eyes. It also goes hand in hand with safety, as inspectors must utilize welding PPE when examining hot or confined work. While VT cannot see inside of a joint, it prevents wasted time and money by filtering out rejectable work early on.

Radiographic Testing (RT): Seeing Through The Weld

When the concern lies beneath the surface, radiographic testing (RT) is one of the most utilized methods. RT uses X-rays or gamma rays to pass through the weldment, revealing internal porosity, slag inclusions, and cracks as density differences on film or digital detectors. These results can be preserved, making RT extremely useful for traceability on critical work, such as pipe welding procedures and gas line welding.

However, RT does have its limits. It requires strict radiation safety in compliance with OSHA 1910.252, is less effective at detecting tight, planar cracks oriented parallel to the radiation beam, and can be costly for large structures. Despite this, for volumetric flaws, very few methods compare to its reliability.

Ultrasonic Testing (UT): Precision For Planar Flaws

Ultrasonic testing (UT) uses high-frequency sound waves to detect both surface and subsurface discontinuities. Unlike RT, UT excels at finding planar defects, such as lack of fusion and cracking, making it a staple for thick joints and code-driven inspections. With the rise of phased array ultrasonic testing (PAUT), inspectors can now scan complex geometries with far greater coverage and accuracy than conventional UT.

With this level of responsibility, personnel qualification is non-negotiable. Ultrasonic testing personnel are commonly qualified to ASNT SNT-TC-1A or an equivalent written practice, and AWS D1.1 requires UT operators to meet such qualification. These requirements, paired with the code’s acceptance criteria, ensure results remain objective, not opinion based.

Magnetic Particle & Dye Penetrant: Surface-Focused Detection

Not every flaw is deep inside of the weld. Many of the most dangerous cracks begin at the surface, which is exactly where Magnetic Particle Testing (MT) and Dye Penetrant Testing (PT) shine. MT induces a magnetic field in a ferromagnetic material, attracting iron particles to surface and near-surface breaking discontinuities. PT, on the other hand, utilizes a colored or fluorescent penetrant to seep into surface-breaking flaws before being drawn out for inspection. Both are affordable, portable, and perfect for weld repairs or spot checks.

Emerging Methods & Code-Driven Quality

The industry is continuously evolving to increase both safety and efficiency. Real-time radiography (RTR) allows for live imaging without the lengthy film development process, while PAUT drastically reduces scan times. These advancements, however, still fall under the same umbrella: inspection level and personnel qualification must tie directly to code requirements. Standards from AWS, ASME BPVC, API, ISO 5817, and ISO 10042 all dictate measurable limits, not subjective judgment.

Ultimately, inspection isn’t about rejecting work, it’s about preventing failures. Whether you’re running a high volume production job, or working on a precise MIG setup, utilizing the correct NDT method at the correct time is what protects both the welder and the end user. The data that these methods provide directly feeds into the prevention strategies that will be covered in the final section of this guide.

Prevention Playbook: WPS, PQR, Welder Qualification, and the Top Mistakes to Avoid

Everything so far has explained how welds fail. This section is about how they don’t. Prevention is repeatable, cheap, and slightly boring, and most shops only adopt it after a rejection forces their hand.

Start With a Procedure, Not a Guess

The strongest defect-prevention tool available is a welding procedure specification (WPS) backed by a procedure qualification record (PQR). A WPS fixes the critical variables before the arc is struck: base metal group, filler classification, polarity, current range, voltage, travel speed, preheat, interpass temperature, joint geometry, and shielding gas. A PQR is the test data proving the procedure produces sound welds, typically from destructive bend and tensile testing of a coupon.

Without that paperwork, every welder improvises. Amperage drifts high, preheat gets skipped on a cold morning, travel speed doubles when the job runs late, and the result shows up later as undercut, cracking, or lack of fusion. For pipe work, where rework access is miserable, the procedure is the whole game. Pipe Welding Procedures: The Complete Guide to WPS, Positions, Passes & Inspection shows how a qualified procedure maps onto real passes and positions.

Qualify the Welder, Not Just the Paper

A welder performance qualification test confirms that a specific person can produce sound welds within a qualified WPS. Tests cover the positions the welder will actually work in, such as 3G and 4G plate or 6G pipe, followed by visual and often radiographic inspection plus bend testing. Under AWS D1.1, qualifications are process-specific and position-specific, and they lapse if the welder has not used the process for six months. That gap rule surprises a lot of shops the hard way.

Qualification also means setting the machine correctly rather than by feel. See How to Set Up a MIG Welder for the parameter chain, plus the specific rod charts in 3/32 E7018 Amperage: Settings, Polarity & How to Run It and 5/32 E7018 Amperage: Settings, Polarity & Technique.

Keep Consumables Dry and Controlled

Storage quietly prevents an entire class of cracking and porosity defects. Low-hydrogen electrodes like E7018 absorb moisture from humid air, and that hydrogen enters the weld pool as hydrogen-induced cracking. Keep them in a heated rod oven, issue them in small batches, and never bake a cellulosic electrode by mistake. Wire, backing rings, and gas deserve the same discipline: clean spools, verified gas certificates, and fresh bottles instead of running them dry. What Are Electrodes in Welding? explains how to read the classification digits so you know what you are loading.

Repair Smart, or Repaint the Problem

A shop repair costs a fraction of a field failure, but a bad repair makes things worse. Burning out a crack without a qualified repair procedure, gouging too deep, or re-welding over a contaminated root adds new hydrogen and new stress, and the crack often returns larger. Repairs need the same controls as the original weld: documented procedure, preheat, qualified welder, and re-inspection. Per OSHA 1910.252, cutting and welding operations also require fire protection and ventilation controls during any rework.

The Mistakes Shops Repeat Every Week

• Skipping fit-up checks and tacking too small, then fighting gap variation mid-weld. MIG Tack Weld Guide gives practical spacing and sizing.
• Running settings by ear instead of by procedure, which is how undercut happens. See Welding Defects Undercut for limits and fixes.
• Wet, contaminated, or out-of-date electrodes used because the oven was empty.
• No preheat on thick, restrained, or high-carbon steel joints.
• Welding over paint, mill scale, or oil and blaming the shielding gas later.
• No wind screen outdoors, so porosity appears only when the weather changes.
• Unqualified welders assigned to code work, discovered at inspection.

A Checklist You Can Use Tomorrow

1. Verify the current WPS matches the joint, process, and position on the travelers.
2. Confirm welder qualification is valid for that process and position.
3. Inspect fit-up: gap, land, misalignment, cleanliness, and tack quality.
4. Check and record preheat and interpass temperature.
5. Pull consumables from proper storage and check exposure time.
6. Verify gas flow, polarity, and machine settings against the WPS.
7. Confirm PPE and ventilation are in place per Welding PPE guidance.
8. Perform visual inspection before anyone calls for NDT.
9. Document any repair with procedure, welder, and re-inspection result.

Prevent defects at the procedure stage and inspection becomes confirmation instead of discovery. That shift, from finding flaws to proving quality, separates a shop that ships once from one that ships twice.

Frequently Asked Questions

What is the difference between a weld discontinuity and a weld defect?

A discontinuity is any interruption in the normal structure of a weld — porosity, undercut, or a small inclusion. It becomes a defect only when its size, type, or location exceeds the acceptance criteria of the governing code such as AWS D1.1 or ISO 5817. Every real weld contains some discontinuities; only unacceptable ones are rejected.

Which welding type produces the fewest defects?

GTAW (TIG) welding gives the operator the finest control over heat and filler deposition and generally produces the cleanest welds. However, the lowest defect rate really comes from matching the process to the joint, material, position, and environment, and welding to a qualified procedure — not from the process alone.

What is the most common welding defect?

Porosity is widely reported as the most frequently encountered weld defect across shops and field work, followed by undercut, lack of fusion, and incomplete penetration. Cracking is less common but is the most dangerous because it can propagate under load and cause sudden failure.

Why does porosity keep appearing in my welds?

Gas porosity is almost always caused by inadequate shielding, moisture, or contamination. Check your shielding gas flow and cylinder, use wind screens outdoors, clean the joint of oil, paint, and rust, dry your consumables, and verify correct travel speed and stick-out.

Can a cracked weld be repaired?

Cracks in structural welds are generally unacceptable and must be completely removed by grinding or carbon-arc gouging before re-welding with an approved repair procedure. Simply welding over a crack traps the flaw and usually makes it worse by adding stress and hydrogen.

How does preheat prevent welding defects?

Preheat slows the cooling rate of the weld and heat-affected zone, which reduces hardness, lowers residual stress, and gives dissolved hydrogen time to diffuse out before it can cause delayed cracking. Required preheat depends on steel chemistry, thickness, and ambient temperature per the applicable code.

What NDT method should I use to find internal weld defects?

Radiographic testing and ultrasonic testing are the primary methods for internal flaws. RT gives a direct image of porosity and inclusions, while UT is faster on long welds and better at detecting lack of fusion. Surface-breaking flaws are best found with magnetic particle or dye penetrant testing.

Does welding produce hazards beyond defective welds?

Yes. OSHA identifies welding hazards including burns, eye damage (arc radiation), electrical shock, cuts, and breathing hazardous fumes and gases, and requires adequate ventilation and protective equipment under 1910.252. Fire protection and confined-space procedures are also mandatory on most jobsites.