ITI/NCVT Welder (Fab & Fit) (AITT Theory) key terms

The percentage of a 10-minute period a welding power source can run at a stated current without overheating. A 60% duty cycle at 200 A allows 6 minutes of welding and 4 minutes of cooling.

The voltage present at the welding machine output terminals before the arc is struck, with no current flowing. It is higher than arc voltage and is limited for operator safety.

Deflection of the welding arc away from the intended joint by magnetic forces, causing spatter, poor penetration and uneven beads. It is worst with DC welding near the earth clamp or at the ends of long joints.

The letter and number designation printed on an SMAW electrode, such as E6013. It states minimum tensile strength, usable welding position and coating type, letting the welder select a suitable electrode for the job.

A covered electrode whose coating is rich in cellulose, producing a forceful shielding gas of hydrogen, carbon monoxide and water vapour. It gives deep penetration and is used for vertical-down root passes on pipelines.

A covered electrode containing a high proportion of titanium dioxide in the flux. It strikes and re-strikes easily, gives a smooth bead and low spatter, and suits general fabrication and sheet metal work.

An oxy-acetylene flame with approximately equal volumes of oxygen and acetylene, giving a clearly defined inner cone. It is the standard flame for welding steel and for most brazing work.

A flame produced with excess acetylene, showing a bright inner cone with a feathery secondary envelope. It adds carbon to the weld and is used for hardfacing and welding some non-ferrous metals.

A flame produced with excess oxygen, having a short, pointed inner cone and a hissing sound. It is used for cutting and for brazing some metals, but it oxidises steel and damages it.

A momentary pop as the flame goes out or snaps back to the tip, after which it may relight. It is usually caused by a dirty tip, loose tip nut or incorrect gas pressures.

A flame that travels back inside the torch, hose or regulator, producing a hissing or whistling sound. The operator must immediately close the oxygen valve, then the acetylene valve, and allow the equipment to cool.

The gap left by the cutting oxygen stream as it removes molten metal. Correct cutting speed and tip size keep the kerf narrow and the cut faces square and clean.

A joining process in which a filler metal with a melting point above 450 °C but below that of the parent metal is drawn into the joint by capillary action, without melting the workpieces.

A gas or gas mixture supplied around the arc to exclude air and protect the molten weld pool from oxidation and nitrogen absorption. Argon, helium, carbon dioxide and their mixtures are common.

A MIG/MAG metal transfer mode at low current in which the wire electrode short-circuits into the pool and detaches many times per second. It suits thin sheet and out-of-position work with low heat input.

A MIG/MAG transfer mode at high current in which fine droplets are projected axially from the wire into the pool. It gives deep penetration and high deposition but is used mainly in the flat position.

A non-consumable electrode made largely of tungsten, often with thorium, lanthanum or cerium oxide additions to improve arc starting. It carries the arc in TIG welding and is shielded by inert gas.

The connection of the electrode and workpiece to the positive and negative terminals of a DC source. Electrode negative concentrates heat on the workpiece, while electrode positive gives cleaning action in some processes.

The region of parent metal beside the weld that has been heated but not melted. Its microstructure and properties change, and it is often where hardness, cracking or reduced toughness occurs.

Heating the parent metal before welding to slow cooling, reduce hardness and help hydrogen escape. It is essential for thick sections, high-carbon steels and many alloy steels.

Delayed cracking in the heat affected zone caused by hydrogen absorbed during welding combined with restraint and hard microstructure. Low-hydrogen electrodes, preheat and dry consumables reduce the risk.

The ease with which a metal can be welded to give a sound joint under normal shop conditions. It depends on composition, thickness and the welding process used.

A calculated value that expresses the combined hardening effect of carbon and other alloying elements in steel. Higher values mean greater risk of hard, crack-sensitive heat affected zones and the need for preheat.

Internal stress locked into a fabricated part by uneven heating and cooling or by restraint during welding. It can cause distortion, cracking or reduced load capacity and may be relieved by heat treatment.

Unwanted change of shape or size in a welded assembly caused by shrinkage of the weld and adjacent metal. It is controlled by balanced sequences, tacking, pre-setting and correct fit-up.

A technique in which short beads are deposited opposite to the general direction of progress, so each new bead ends where the previous one began. It limits distortion and heat build-up in thin sections.

A drawing notation consisting of a reference line, an arrow and icons that specify the joint type, weld size, length and position. Supplementary letters and tails give finish and process details.

The side of the joint to which the arrow of a weld symbol points. A symbol placed below the reference line applies to the arrow side; one above applies to the other side.

The optional fork at the far end of a weld symbol's reference line. It carries a process or specification reference, such as SMAW, so the drawing states how the weld is to be made.

The accurate alignment and spacing of parts before welding, including root gap, root face and edge preparation. Good fit-up gives correct penetration and reduces distortion and defects.

The small clearance left between the two members at the root of a joint before welding. It allows the root pass to penetrate fully and is checked with feeler gauges or a gap gauge.

The un-beveled portion of the edge left at the bottom of a prepared weld groove. It supports the root pass and prevents burn-through while allowing complete fusion.

A defect in which the weld metal does not reach the root of the joint, leaving unfused parent metal. It results from too small a root gap, too large a root face or low current.

A defect where weld metal fails to bond to the parent metal or to a previous pass. Causes include low heat input, poor cleaning, incorrect electrode angle or travel speed.

Gas bubbles trapped in solidified weld metal, appearing as rounded voids on a radiograph or fracture surface. Damp electrodes, dirty surfaces and poor gas shielding are common causes.

A groove melted into the parent metal along the edge of a weld bead and left unfilled. Excessive current, long arc or wrong electrode angle usually cause it, and it reduces section thickness.

Non-metallic slag trapped between weld passes or between the weld and parent metal. It is prevented by thorough chipping and brushing between passes and by correct bead shape.

A surface inspection method in which a coloured or fluorescent liquid is applied, cleaned off and drawn from surface-breaking flaws by a developer. It detects cracks and porosity in non-porous materials.

A surface and near-surface inspection method that magnetises a ferromagnetic part and applies iron particles, which gather at flux leakage around defects. It works only on magnetic materials such as steel.

A volumetric inspection method in which X-rays or gamma rays pass through the weld and expose a film or detector. Internal defects show as density changes, giving a permanent record of the weld.

A volumetric inspection method that sends high-frequency sound waves into the weld and analyses reflected echoes. It detects internal planar defects such as cracks and lack of fusion and needs a trained operator.