Welding Glossary — A to Z Welding Terms & Definitions | Welders Supply — Welding Guides, Reviews & Technical Reference

A

Amperage

The measure of electrical current flowing through a welding circuit, measured in amps (A). Higher amperage produces more heat and deeper penetration.

Arc Blow

Deflection of the welding arc from its intended path caused by magnetic fields in the workpiece. Common with DC welding on magnetized steel.

Arc Length

The distance between the tip of the electrode and the surface of the weld pool. Too long causes porosity and spatter; too short causes sticking and poor bead shape.

Auto-Darkening Lens

A welding helmet lens that's light enough to see through until the arc strikes, then darkens to a protective shade within milliseconds. Adjustable shade range (typically 9-13) and sensitivity settings. Lets you position the torch accurately before striking the arc.

B

Back Purge

Filling the backside of a weld joint with inert gas (usually argon) to prevent oxidation during welding. Required for stainless steel, titanium, and chromoly pipe and tubing.

Base Metal

The metal being welded or cut. Also called the parent metal or workpiece.

Bead

A single pass or deposit of weld metal on a joint. A finished weld may consist of one or multiple beads (multi-pass welding).

Burnback

When MIG welding wire melts back into the contact tip, fusing to it. Caused by too-slow wire feed speed relative to voltage, or stopping the trigger before releasing the wire.

Brazing

A joining process that uses a filler metal with a melting point above 840°F (450°C) but below the melting point of the base metals. The filler flows into the joint by capillary action. Doesn't melt the base metal, so it's great for joining dissimilar metals.

Butt Joint

A joint where two pieces of metal are placed edge to edge in the same plane. The most common joint type in plate welding. May include a groove (bevel) preparation on thicker material to allow full penetration.

Burn-Through

When the arc melts completely through the base metal, leaving a hole. Common on thin material or open root joints with too much heat. You'll see it as a blown-out hole on the backside. Fix it by reducing amperage, increasing travel speed, or using a backing bar.

See also: Troubleshooting

Bevel

An angled edge preparation cut or ground into base metal before welding a groove joint. Common angles are 30 or 37.5 degrees per side (60 or 75 degree included angle). Required on material too thick for a single-pass square butt weld. Provides access for the arc to reach the root.

C

Cap Pass

The final weld pass on a multi-pass joint, forming the visible surface. Must meet appearance and profile requirements per code.

Contact Tip

The copper alloy tip at the end of a MIG gun through which the welding wire feeds and makes electrical contact. A consumable component that wears over time.

Crater

The depression at the end of a weld bead where the arc was terminated. Craters are weak points prone to cracking if not filled properly.

CTWD

Contact Tip to Work Distance. The distance from the end of the MIG gun contact tip to the workpiece. Typical CTWD is 3/8" to 3/4" depending on process and parameters.

Corner Joint

A joint formed when two pieces meet at a right angle, edge to edge. Can be welded from the outside (open corner) or inside (closed corner) depending on access and strength requirements.

Cold Lap

A defect where weld metal rolls over the base metal surface without fusing to it. Looks like a good weld on the surface but there's no bond underneath. Caused by too little heat, too fast travel speed, or wrong torch angle.

See also: Welding techniques weld defects

Collet

A split brass or copper sleeve inside a TIG torch that grips the tungsten electrode. Tightened by the torch cap. Sized to match the tungsten diameter. A worn collet won't hold the tungsten tight, causing it to slip during welding.

Carbon Equivalent

A calculated value that estimates a steel's tendency to harden and crack during welding based on its chemical composition. Higher CE means more preheat is needed. Steels with CE above 0.45 need careful attention to avoid hydrogen cracking.

D

DCEN

Direct Current Electrode Negative (straight polarity). Current flows from the electrode to the workpiece. Used for TIG welding on steel and stainless. Concentrates heat in the workpiece.

DCEP

Direct Current Electrode Positive (reverse polarity). Current flows from the workpiece to the electrode. Used for MIG welding and most stick electrodes. Provides deeper penetration.

Dilution

The mixing of base metal into the weld deposit. Expressed as a percentage of the total weld metal that comes from the base metal rather than filler. Affects weld chemistry and properties.

Duty Cycle

The percentage of a 10-minute period that a welding machine can operate at its rated output without overheating. A 60% duty cycle at 200A means 6 minutes of welding, 4 minutes of cooling.

Drive Rolls

The rollers inside a wire feeder that grip and push the welding wire through the liner to the gun. Come in V-groove (solid wire), U-groove (soft wire like aluminum), and knurled (flux-cored wire). Wrong type or tension causes wire feeding problems.

Ductility

A metal's ability to deform permanently without breaking. Measured by percent elongation in a tensile test. Mild steel is ductile, cast iron isn't. Excessive heat or fast cooling can reduce ductility in the HAZ, making the joint brittle.

E

Edge Joint: A joint where two pieces of metal are set parallel with their edges aligned and welded along the shared edge. Mostly used for sheet metal, flanges, and non-structural applications where loads are light.
Related terms: butt joint, sheet metal
Excessive Reinforcement: Weld metal built up above the surface of the base metal beyond what's allowed by code. A tall, humped bead that creates stress concentration at the toes. Most codes limit reinforcement to 1/8" or less. Grind it down to pass inspection.
Related terms: cap pass, weld profile, weld defects
Electrode Holder: The clamp that holds the stick electrode during SMAW. Called a "stinger" in the shop. Grips the bare end of the electrode and conducts welding current to it. Rated by amperage. Spring-loaded jaws need periodic cleaning and replacement.
Related terms: stick welding, SMAW, electrode, amperage

F

FCAW

Flux-Cored Arc Welding. A wire-feed process using tubular wire filled with flux. Available as self-shielded (no gas) or gas-shielded (dual-shield). Common in structural and field welding.

Filler Metal

Metal added to a weld joint during welding. Includes MIG wire, TIG rod, stick electrodes, and brazing alloys. Selected to match or exceed base metal properties.

See also: Welding consumables

Fill Pass

Weld passes deposited after the root pass to fill the joint to near-flush. May require multiple fill passes on thick material.

Flux

A material used to prevent oxidation, stabilize the arc, and form slag during welding. Present in stick electrode coatings, flux-cored wire fill, and submerged arc welding granules.

Flux-Cored Welding

Common name for FCAW. Runs like MIG but uses a tubular wire filled with flux instead of solid wire. Self-shielded versions need no gas bottle, making it popular for outdoor and field work. Gas-shielded (dual-shield) versions produce higher quality welds.

Fillet Weld

A roughly triangular weld deposited in the internal corner of a tee, lap, or corner joint. Sized by the leg length (e.g., a 1/4" fillet weld has 1/4" legs). The most common weld type in structural steel fabrication.

Fish Eye

A small, shiny, circular spot visible on a fractured weld surface, usually surrounding a gas pore or inclusion. Named for its appearance. Often a sign of hydrogen-related issues in the weld metal.

Foot Pedal

An amperage control for TIG welding that lets you vary heat with your foot while welding. Press harder for more amps, ease off for less. Gives you real-time heat control for thin material, corners, and tacking without stopping.

Flash Burn

A painful eye injury caused by UV radiation from the welding arc hitting unprotected eyes. Feels like sand in your eyes, usually hitting you hours after exposure. Also called arc eye or welder's flash. Even a brief exposure can cause it. Always wear a proper welding helmet.

Fire Watch

A person assigned to watch for fires during and for at least 30 minutes after hot work (welding, cutting, grinding) in areas with combustible materials. Required by OSHA and most fire codes. The fire watch carries an extinguisher and knows the alarm location.

G

Gas Lens

A TIG torch component that replaces the standard collet body and uses a layered mesh screen to produce laminar gas flow. Improves shielding coverage and allows longer tungsten extension for better visibility.

GMAW

Gas Metal Arc Welding. The technical designation for MIG welding. Uses a continuously fed solid wire electrode and external shielding gas.

See also: Welding processes mig welding

GTAW

Gas Tungsten Arc Welding. The technical designation for TIG welding. Uses a non-consumable tungsten electrode, external shielding gas, and optional filler rod added by hand.

See also: Welding processes tig welding

Groove Weld

A weld made in a groove between two pieces of metal. The groove shape (V, J, U, bevel) is cut or machined into the base metal before welding. Used on butt joints when full penetration is required on thicker material.

Grain Structure

The crystalline arrangement of atoms in a metal, visible under a microscope. Welding heat changes grain size in the HAZ. Fine grains are generally tougher, coarse grains are weaker. Controlled cooling and heat treatment restore grain structure.

H

HAZ

Heat-Affected Zone. The area of base metal adjacent to the weld that was heated enough to change its metallurgical properties but did not melt. Can be harder, softer, or more brittle than the original base metal depending on material and cooling rate.

Heat Input

The amount of energy per unit length transferred to the workpiece during welding. Calculated as (voltage × amperage × 60) ÷ (travel speed in inches per minute). Measured in joules per inch (J/in) or kilojoules per millimeter (kJ/mm).

See also: Tools heat input calculator

Hot Pass

The second weld pass applied immediately after the root pass at higher heat input to burn out any slag trapped in the root and ensure fusion. Common in pipe welding with E6010 root / E7018 fill sequences.

Hot Cracking

Cracks that form in the weld metal while it's still solidifying. Usually runs down the centerline of the bead. Caused by high sulfur or phosphorus content in the base metal, too-fast travel speed creating a teardrop-shaped weld pool, or high restraint.

Hydrogen Cracking

Cracks that form in the HAZ or weld metal hours or days after welding, also called cold cracking or delayed cracking. Caused by hydrogen trapped in the weld, high hardness, and residual stress. Prevented with preheat, low-hydrogen electrodes, and proper storage of consumables.

Hardness

A metal's resistance to indentation, measured on scales like Rockwell or Brinell. In welding, hardness testing checks the HAZ for excessive hardening that could lead to cracking. High carbon and alloy steels are prone to high HAZ hardness.

Hot Work Permit

A written authorization required before welding, cutting, or grinding in areas not designated as permanent welding stations. Documents hazards, precautions, and fire watch requirements. Valid for a single shift or specific time period. Required by OSHA 29 CFR 1910.252.

I

Interpass Temperature: The temperature of the weld area between successive weld passes. Maximum interpass temperatures are specified by welding codes to control metallurgical properties and prevent defects.
Related terms: preheat, multi-pass welding, HAZ

L

Lack of Fusion

A weld defect where the weld metal does not fully bond to the base metal or previous weld pass. Caused by insufficient heat, incorrect angle, or contamination. Also called incomplete fusion.

See also: Welding techniques weld defects

Lap Joint

A joint where two overlapping pieces of metal are welded along the edge of the overlap. Common in sheet metal work. Welded with fillet welds on one or both sides.

Lamellar Tearing

A step-like crack that forms in the base metal parallel to the rolled surface, typically under tee joints in thick plate. Caused by through-thickness stress pulling apart weak planes of sulfide inclusions in the steel. Prevented by using Z-grade (through-thickness tested) steel or redesigning the joint.

Liner

The flexible conduit inside a MIG gun cable that guides the welding wire from the drive rolls to the contact tip. Steel liners for steel wire, Teflon liners for aluminum. A kinked, dirty, or wrong-size liner causes most wire feed issues.

Low-Hydrogen Electrodes

Stick electrodes (like E7018) with coatings that contain very little moisture, minimizing hydrogen in the weld. Must be stored in a rod oven at 250-300°F after opening. Used on medium and high-carbon steels where hydrogen cracking is a risk.

M

MIG Welding

Metal Inert Gas welding (technically GMAW). A semi-automatic process using a continuously fed solid wire electrode and external shielding gas. The most widely used welding process for mild steel fabrication.

See also: Welding processes mig welding

Multi-Pass Welding

Completing a weld joint using more than one pass. Required when material thickness exceeds what a single pass can fill. Passes include root, hot pass, fill passes, and cap pass.

Martensite

A hard, brittle phase that forms when steel cools too quickly from high temperature. In welding, it forms in the HAZ of high-carbon or alloy steels when cooling is too fast. Preheat and controlled cooling prevent it. If it forms, PWHT can temper it.

N

Nozzle: The tip at the end of a MIG gun or the ceramic/metal cup on a TIG torch that directs shielding gas over the weld pool. MIG nozzles collect spatter and need regular cleaning. TIG cups come in numbered sizes (4, 5, 6, etc.) measured in 1/16" increments.
Related terms: shielding gas, MIG welding, TIG welding, cup size

O

OFW: Oxyfuel Welding. Uses a torch burning a mix of oxygen and acetylene (or other fuel gas) to melt and join metal. Mostly replaced by arc processes for welding, but oxy-acetylene is still widely used for cutting, brazing, and heating.
Related terms: brazing, acetylene, cutting

P

Penetration

The depth that the weld metal extends into the base metal from the surface. Full penetration means the weld extends completely through the joint thickness. Critical for structural integrity.

Polarity

The direction of electrical current flow in a DC welding circuit. DCEP (electrode positive) drives deeper penetration. DCEN (electrode negative) concentrates heat in the workpiece. AC alternates between both.

Porosity

Gas pockets trapped in solidified weld metal. Appears as round or elongated voids. Caused by contamination, insufficient shielding gas, or moisture. Weakens the weld and is a rejectable defect in code work.

Post-Weld Heat Treatment

Controlled heating and cooling of a welded assembly after welding is complete. Used to relieve residual stress, temper the HAZ, and improve mechanical properties. Required by code for certain materials and thicknesses. Abbreviated PWHT.

Preheat

Heating the base metal to a specified temperature before welding begins. Slows cooling rate to reduce risk of hydrogen cracking and excessive HAZ hardness. Required for high-carbon and alloy steels.

PAW

Plasma Arc Welding. Similar to TIG but uses a constricted arc focused through a copper nozzle, creating a higher energy density plasma jet. Produces deeper penetration with narrower welds. Common in aerospace and automated pipe welding.

Plasma Cutting

A cutting process that uses a constricted arc and high-velocity ionized gas to melt and blow away metal. Cuts any electrically conductive material. Faster than oxy-fuel on steel under 1" and handles stainless and aluminum that oxy-fuel can't cut.

See also: Welding equipment plasma cutters

Plug Weld

A weld made through a circular hole in one piece to fuse it to the piece underneath. Used as an alternative to spot welding when you don't have a resistance welder. Common in auto body repair.

PAPR

Powered Air Purifying Respirator. A battery-powered respirator that blows filtered air into the welding helmet, protecting against welding fumes. More comfortable than standard respirators for long welding sessions. Required in many shops for stainless, galvanized, and chromium-bearing materials.

Push vs Drag

The two basic torch directions in MIG and stick welding. Dragging (pulling) the torch points it back toward the finished weld and gives deeper penetration. Pushing points it toward the unwelded joint and gives a wider, flatter bead with less penetration.

R

Root Pass: The first weld pass deposited at the bottom (root) of a joint. Must achieve full penetration through the root opening. The most critical and difficult pass in pipe and groove welding.
Related terms: hot pass, fill pass, cap pass, pipe welding
Resistance Welding: A group of processes that join metals by applying pressure and passing current through the workpieces. The resistance to current flow generates heat at the interface. No filler metal, no shielding gas, no flux needed.
Related terms: spot welding, seam welding, projection welding
Root Concavity: An inward-curving depression on the root side of a weld where the weld metal has sucked back from the backside surface. Caused by excessive back purge pressure or too much heat pulling the root inward. Called "suck-back" in the shop.
Related terms: root pass, back purge, penetration
Regulator: A device attached to a gas cylinder that reduces the high cylinder pressure (2,000+ PSI) down to a usable working pressure. Shows cylinder contents on one gauge and delivery pressure on the other. Flow-type regulators for MIG/TIG read in CFH (cubic feet per hour).
Related terms: shielding gas, gas flow rate, argon, CO2

S

Shielding Gas

Inert or active gas used to protect the weld pool and arc from atmospheric contamination. Common types: 100% argon (TIG, MIG aluminum), 75/25 Ar/CO2 (MIG steel), 100% CO2 (MIG economy), Ar/He mixes (aluminum).

Slag

A glassy byproduct that forms on top of a weld bead from flux decomposition. Must be completely removed between passes and before inspection. Slag inclusions are rejectable weld defects.

SMAW

Shielded Metal Arc Welding. The technical designation for stick welding. Uses a consumable flux-coated electrode. The oldest and most portable arc welding process.

See also: Welding processes stick welding

Spatter

Small droplets of molten metal expelled from the weld pool during welding. Lands on surrounding base metal and must be cleaned off. Excessive spatter indicates incorrect parameters.

Stickout

The length of MIG wire extending from the contact tip to the arc. Also called electrode extension. Typical stickout is 3/8" to 1/2" for short-circuit MIG. Longer stickout increases resistance heating and reduces penetration.

SAW

Submerged Arc Welding. An automatic process where the arc burns under a blanket of granular flux, completely hiding the arc. High deposition rates make it the go-to for long straight seams on thick plate, like pressure vessels and structural beams.

See also: Welding processes

Soldering

Joining metals with a filler that melts below 840°F (450°C). The base metal never melts. Used for electrical connections, plumbing, and electronics. Not a structural joining method for load-bearing applications.

Spot Welding

A resistance welding process where two copper electrodes squeeze overlapping sheet metal and pass current through to form a weld nugget at the contact point. Fast and cheap for production sheet metal work. Every car body is held together with thousands of spot welds.

Stick Welding

Common name for SMAW. Uses a flux-coated consumable electrode ("stick") held in a stinger. The most portable and forgiving arc process. Works outdoors in wind and on dirty or rusty metal where MIG and TIG can't.

See also: Welding processes stick welding

Slot Weld

Similar to a plug weld but made through an elongated slot rather than a round hole. Used to join overlapping members when edge access isn't available or when more weld area is needed than a plug weld provides.

Seal Weld

A weld made primarily to prevent leakage rather than to carry structural loads. Common on tanks, pressure vessels, and piping where you need a gas-tight or liquid-tight joint.

Spool Gun

A MIG gun with a small wire spool and drive motor built right into the handle. Feeds wire only a few inches instead of 10+ feet through a regular gun. Essential for welding aluminum with MIG because soft aluminum wire birdnests in a standard liner.

Stringer Bead

A weld bead made with a straight drag or push motion along the joint, with little or no side-to-side movement. Produces a narrow, consistent bead with good penetration. The standard technique for root passes and pipe welding.

T

TIG Welding

Tungsten Inert Gas welding (technically GTAW). Uses a non-consumable tungsten electrode, argon shielding gas, and optional filler rod added by hand. Produces the highest quality welds with the most control.

See also: Welding processes tig welding

Travel Speed

The rate at which the welding torch or electrode moves along the joint, measured in inches per minute (IPM). Affects heat input, bead width, penetration, and weld appearance.

Tungsten

The non-consumable electrode material used in TIG welding. Has the highest melting point of any metal (6,192°F / 3,422°C). Available in several alloy types identified by color-coded tip bands: 2% thoriated (red), 2% lanthanated (blue), 2% ceriated (gray), pure (green).

Tee Joint

A joint formed when one piece of metal is placed perpendicular to another, forming a T shape. Welded with fillet welds on one or both sides. One of the most common joints in structural fabrication.

Torch Body

The main housing of a TIG or MIG torch that the operator grips. On a TIG torch, it holds the collet, collet body, gas lens, and cup. Rated by amperage capacity. Air-cooled for lighter duty, water-cooled for higher amps.

Tensile Strength

The maximum stress a material can withstand while being pulled apart before it breaks, measured in PSI or MPa. E70XX electrodes produce weld metal with 70,000 PSI minimum tensile strength. Filler metal tensile strength should match or exceed the base metal.

Travel Angle

The angle of the electrode or torch measured along the joint in the direction of travel. A slight drag angle (5-15 degrees from perpendicular) is standard for most processes. Also called lead or trail angle depending on the direction.

Tack Weld

A small, temporary weld used to hold parts in position for final welding. Should be small enough to be consumed by the final weld but strong enough to resist distortion forces during welding. Place them at intervals along the joint. Bad tacks cause bad welds.

U

Undercut

A groove melted into the base metal at the toe of a weld that is not filled by weld metal. A stress concentration that weakens the joint. Caused by excessive amperage, travel speed, or incorrect electrode angle.

See also: Welding techniques weld defects

V

Voltage: The electrical potential difference in a welding circuit, measured in volts (V). In MIG welding, voltage primarily controls arc length and bead width. In stick welding, voltage is a function of arc length maintained by the operator.
Related terms: amperage, heat input, arc length

W

Weave Bead

A weld bead made with a side-to-side oscillating motion of the torch or electrode. Used to produce wider beads and fill wider joints. Contrast with a stringer bead (straight line, no weave).

Wire Feed Speed

The rate at which MIG or flux-cored wire is fed through the gun, measured in inches per minute (IPM). Directly controls amperage in constant-voltage MIG welding. Higher wire speed = higher amperage.

WPS

Welding Procedure Specification. A formal document that defines the essential variables for performing a specific weld: process, base metal, filler metal, preheat, interpass temperature, travel speed, amperage, voltage, position, and other parameters. Required for code welding.

Wagon Tracks

Slag inclusions running along both toes of a weld bead, parallel to the direction of welding. Named because they look like wheel ruts. Common in multi-pass welding when slag isn't fully removed between passes or the bead profile traps slag at the toes.

Wormhole

An elongated gas pore or porosity tunnel that extends through the weld metal, often following the direction of solidification. Looks like a worm burrowed through the weld on a radiograph. Caused by gas entrapment during solidification.

Wire Feeder

The mechanism that pulls wire off the spool and pushes it through the gun to the arc. Contains the drive rolls, tension adjustment, and feed speed control. Can be built into the welder or a separate bench or boom-mounted unit.

Weld Metal

The portion of a completed weld that melted and resolidified during welding. It's a mix of melted filler metal and melted base metal. Its chemistry and properties differ from both the filler and base metal due to dilution.

Welding Curtain

A fire-resistant screen placed around a welding area to protect nearby workers from arc flash and UV radiation. Required in shops where other people work near welding operations. Available in opaque or tinted transparent versions.

Whip Technique

A stick welding technique where you push the electrode forward into the puddle then whip it slightly ahead and pause, creating a series of overlapping "dimes." Used mainly with E6010 and E6011 cellulosic rods for root passes and open-root pipe welding.

Work Angle

The angle of the electrode or torch measured across the joint (perpendicular to the direction of travel). For a tee joint fillet weld, the standard work angle is 45 degrees. Incorrect work angle causes uneven leg sizes or undercut.

Wire Brush

A hand tool or angle grinder attachment used to clean the weld area before and between passes. Removes slag, oxidation, and light contamination. Stainless steel wire brushes are required for stainless steel and aluminum to prevent carbon steel contamination.

Y

Yield Strength: The stress level at which a metal starts to permanently deform (won't spring back). Always lower than tensile strength. Structural design is based on yield strength with a safety factor applied. Mild steel typically yields around 36,000-50,000 PSI.
Related terms: tensile strength, ductility, base metal