Reference library
One page per term: a definition that stands on its own, the symbol, unit and formula where the term has them, where you meet it in practice, and the calculator or standard it feeds. Terms are cross-linked, so a definition that depends on another one says so.
Acceptance criteria for a weld are the combined rule set that decides pass or fail: which quality level of imperfection applies, which acceptance level a specific inspection method invokes, and what extent of inspection is required before the two can be checked at all.
Alloying elements are the components of a steel's chemistry beyond iron and carbon — manganese, chromium, silicon, sulfur and others — each of which changes weldability through its own mechanism rather than through the single combined hardenability effect a carbon equivalent formula reports.
Unit: kJ/mm (also expressed as kJ/cm or kJ/in)
Arc energy is the electrical energy delivered by the welding arc per unit length of weld, calculated directly from arc voltage, current and travel speed, before any process thermal efficiency factor is applied.
An arc strike is a localized area of melting and rapid re-solidification on the parent material surface, caused by an accidental or momentary arc struck outside the intended weld joint.
Unit: % (weight percent, dimensionless)
Carbon equivalent is a single index calculated from a steel's chemical composition that estimates its hardenability and susceptibility to hydrogen cracking under a given welding cooling rate.
A welding certification body is a third-party organisation, itself accredited to assess conformity, that audits a company's quality system, a procedure or a person against a named scheme — such as ISO 3834, ISO 9606-1 or ISO 9712 — and issues the certificate; it does not write the technical standard it certifies against.
Cold cracking is a delayed fracture that forms after a weld has cooled, when diffusible hydrogen, a susceptible microstructure and tensile stress act together. In steel welding it is commonly called hydrogen cracking or delayed cracking.
In this glossary, a cold-lap thermal signature means a repeatable spatial or time-based anomaly in thermal weld data that flags conditions associated with poor fusion at a weld toe. It is a process-monitoring indication, not proof that a cold-lap imperfection exists.
Unit: s (seconds)
Cooling time t8/5 is the time a weld and its heat-affected zone take to cool from 800 °C to 500 °C, the interval in which austenite decomposes and the resulting microstructure is fixed.
Defect sizing is the measurement of an indication's dimensions once non-destructive testing has detected it — principally its length along the weld and its height or through-wall extent — and different NDT methods measure these two dimensions with very different accuracy.
Unit: kg/h (also expressed per shift, or kg per 1000 mm of weld)
Deposition rate is the mass of filler metal actually placed in the joint per unit time, the productivity measure that a procedure's heat input ceiling and travel speed limits constrain from the other side.
Unit: dimensionless (a ratio between material strength and allowable design stress)
The design factor is the margin a pressure-equipment code builds into the allowable design stress used to size a component, expressed as the ratio between the material's strength basis and the stress the design is permitted to use; for a welded joint that stress is further reduced by a joint efficiency the code sets from how much of the joint was examined.
Destructive testing of a weld is the group of tests — bend, tensile and macro examination chief among them — that cut, bend, pull or etch a sample to measure a property that visual and non-destructive inspection cannot reach.
Unit: %
Dilution is the proportion of deposited weld metal that originates from melted base metal rather than from the filler, expressed as a fraction of the total deposit.
A dissimilar metal weld (DMW) joins two base metals of different chemical composition or metallurgical family — most commonly a ferritic or low-alloy steel to an austenitic stainless or nickel-base alloy — so the fusion zone sits between two materials with different thermal, mechanical and microstructural behaviour rather than one.
Unit: % (percentage of a defined time period)
Duty cycle is the share of a defined time period — commonly ten minutes for a welding power source, or a full shift for a production cell — during which the arc is actually running rather than idle.
Unit: Dimensionless, between zero and one
Emissivity is the ratio between the thermal radiation emitted by a real surface and that emitted by an ideal blackbody at the same temperature, expressed as a dimensionless value between zero and one.
Execution class (EXC1 to EXC4) is the EN 1090-2 category that sets how tightly a steel structure's welding, non-destructive testing and coordination have to be controlled, assigned from the structure's consequence class and service category rather than chosen freely by the fabricator.
Welding fabrication is the sequence of controlled activities that turns material and components into a finished welded item, including preparation, forming or machining, fit-up, welding, inspection and release as applicable to the job.
A focal law is the calibrated set of per-element time delays applied to a phased array probe so the pulses from every element combine into one ultrasonic beam steered to a chosen angle and focused at a chosen depth in the part.
Fully automatic welding is an arc welding mode in which mechanized or robotic equipment carries out the entire welding cycle, including guiding the torch along the joint, without a person controlling any welding variable by hand once the arc has struck.
Unit: kJ/mm (EN and ISO practice); J/in (ASME and AWS practice)
Heat input is the energy delivered to a joint per unit length of weld, calculated from arc voltage, welding current and travel speed and multiplied by the thermal efficiency factor of the process.
The heat sink effect is the rate at which the surrounding parent material and any fixture in metal-to-metal contact with a joint conduct heat away from it, and it is the factor — alongside heat input — that fixes how fast a weld actually cools.
The heat-affected zone is the region of parent material adjacent to a weld whose microstructure and mechanical properties have been altered by welding heat, without the material itself having melted.
Hot cracking, also called solidification cracking, is an intergranular crack that opens in weld metal still finishing solidification, when a thin liquid film of low-melting segregates cannot carry the shrinkage strain the surrounding metal imposes as it contracts.
Hydrogen cracking is a delayed fracture in or near a weld caused by diffusible hydrogen combining with a susceptible microstructure and tensile residual stress, typically appearing hours or days after welding is complete.
An infrared camera is an imaging instrument that converts the infrared radiation emitted by a scene into a two-dimensional map of apparent temperature, displayed as a thermal image.
Unit: °C (°F in ASME and AWS practice)
Interpass temperature is the temperature of the weld metal and adjacent parent material in a multi-run weld immediately before the next run is started.
Lack of fusion is the absence of a fusion bond between weld metal and either the parent material or an adjacent weld pass, where the surfaces met without reaching fusion temperature.
Unit: Micrometres (µm), a wavelength
Long-wave infrared (LWIR) is the infrared band, roughly 8 to 14 micrometres — an atmospheric transmission window carrying usable signal from near-ambient temperature up to several hundred degrees Celsius on a low-cost uncooled microbolometer, with less scattering from welding fume than shorter bands — and it is the band most industrial thermal cameras use.
Martensite is a diffusionless transformation product that forms when austenite in steel cools fast enough to transform without the carbon redistribution associated with slower transformation products.
Unit: mm (in, in ASME and AWS practice) — measured on a transverse cross-section or macro-etch, not from the visible surface
Penetration is the depth to which weld metal and the fusion boundary extend into the parent material, measured from the original joint surface inward.
Phased array ultrasonic testing is a volumetric NDT method that fires a linear array of ultrasonic elements with programmable time delays to steer and focus the beam electronically, producing a sectorial image of a weld cross-section from a single probe position.
Porosity is a cavity or group of cavities in weld metal formed when gas trapped in the molten pool is unable to escape before the metal solidifies around it.
Unit: °C (°F in ASME and AWS practice)
Preheat temperature is the minimum temperature to which the parent material is raised, and held, immediately before welding starts, chosen to slow the cooling rate and give diffusible hydrogen time to leave the joint.
A procedure qualification record (PQR) is the documented evidence, including the actual test results, that a welding procedure performed under recorded conditions produces a weld meeting the required mechanical and soundness properties.
A pyrometer is a non-contact instrument that infers the temperature of a single point on a surface from the infrared radiation it emits, reporting one numeric value rather than an image.
A quality level is a graded band of acceptable weld imperfections, defined by a standard such as ISO 5817, that sets how much of a given imperfection type a weld may contain for a stated application.
Unit: Dimensionless optical density
Radiographic density, in industrial film radiography, is the logarithmic measure of how dark a processed radiograph is when viewed by transmitted light. It describes film blackening, not material density, flaw size or image sensitivity.
Residual stress from welding is the stress that remains locked into a joint once it has fully cooled to ambient temperature, produced by the non-uniform heating and cooling of the weld thermal cycle rather than by any load applied in service.
Resistance preheating is a welding preheat method in which electric current heats resistive pads or elements placed against the workpiece, and the elements transfer heat into the joint area before welding.
A responsible welding coordinator (RWC) — the designation EN 1090-2 names — is the person a manufacturer formally appoints to hold the welding-quality coordination tasks ISO 14731 assigns for a defined scope of work, at a technical-knowledge level matched to that scope's complexity and consequence.
A robotic welding cell is an automated welding station in which a robot arm or manipulator executes a programmed weld path and parameter set, typically instrumented with a fixed group of sensors that record the process for traceability and quality control.
Root cause analysis for weld defects is the structured process of tracing a rejected weld from its inspection classification back to the specific process variable that produced it, so the corrective action removes the mechanism rather than only the symptom.
A PAUT scanning plan is the documented set of probe, wedge, focal-law, scan-path and coverage decisions fixed before scanning starts, so a phased array ultrasonic examination can be repeated by another operator and its coverage of the weld volume demonstrated afterward.
Semi-automatic welding is an arc welding mode in which the equipment feeds the filler wire and regulates arc voltage and wire feed speed automatically, while a person continuously holds and guides the torch along the joint, controlling travel speed, travel angle and contact-tip-to-work distance by hand.
Unit: Micrometres (µm), a wavelength
Short-wave infrared (SWIR) is the infrared band immediately beyond visible light, roughly 0.9 to 1.7 micrometres by the common industrial convention (extended-range InGaAs detectors reach about 2.5 micrometres), used with narrow-band filtering to image very hot or self-luminous sources without the sensor saturating.
Spatter is small droplets of molten metal expelled from the arc or the weld pool during welding that solidify on the surrounding parent material rather than becoming part of the weld.
Unit: 1/K, commonly expressed as µm/(m·K)
The coefficient of thermal expansion is the fractional change in a material's length per degree of temperature change, stated for a defined material condition and temperature interval.
Unit: °C/mm (or K/m)
A thermal gradient is the rate of change of temperature with distance across or along a workpiece, and it is the quantity that drives heat flow, thermal stress and distortion during and after welding.
Thermal imaging is the capture and display of infrared radiation from a scene as a two-dimensional image whose pixels represent apparent temperature or radiation intensity. In welding, it shows where heat is distributed across the pool, bead and nearby material.
Thermography is the use of infrared imaging to record and display the surface temperature distribution of an object as a two-dimensional thermal image, rather than a single-point temperature reading.
Unit: mm (in, in ASME and AWS practice) — measured on the weld cross-section, not the visible leg
Throat thickness is the shortest distance from the root of a fillet weld to its face, the internal dimension used in structural calculations rather than the leg length shown on a drawing.
Welding traceability is the ability to identify a weld and follow its history through linked records for the joint, materials, consumables, procedure, personnel, equipment and inspection required by the job.
Unit: mm/min (in/min in ASME and AWS practice)
Travel speed is the rate at which the arc advances along the joint, the measurement that sets both weld bead geometry and, through the heat input formula, how much energy is delivered per unit length of weld.
Undercut is a groove or channel melted into the parent metal at the edge of a weld, most often at the toe, that is left unfilled by weld metal.
A weld map is a dimensioned or marked-up drawing on which every weld joint in a fabrication carries a unique identifier that links the physical joint to its applicable WPS, welder and inspection records.
Weld map traceability is the unbroken chain from a weld's unique identifier on the map to the WPS it was made to, the welder who made it and the NDT report examining it, such that any one link produces the other three.
Weld overlay, also called cladding, is a welding process that deposits one or more layers of a different — usually corrosion- or wear-resistant — alloy onto a base metal surface to protect it, rather than to join two separate parts.
The weld pool is the localized volume of molten metal created by the welding heat source, into which filler metal is added and from which the weld bead forms as it solidifies.
Unit: °C (a local value, not a single pool temperature)
Weld pool temperature is the temperature of the molten metal in the weld pool at a given point, which is not uniform: it spans from superheated liquid nearest the arc or beam to the solidification front at the trailing edge, where the metal sits between its liquidus and solidus.
The weld toe is the boundary line where the surface of a weld bead meets the surface of the parent material, on either side of the weld.
Welder continuity is the ongoing, dated evidence that a welder has kept using a qualified welding process within the activity window a governing standard such as ISO 9606-1 sets, separate from the original qualification test.
Welder qualification is the documented demonstration that a specific welder or welding operator can produce a sound weld within a defined range of processes, materials, thicknesses and positions.
A welding consumable is any material a welding process uses up in making the joint — filler metal as rod, wire or covered electrode, flux, shielding or backing gas, or backing material — as distinct from the base metal being joined.
Welding procedure qualification is the tested activity that establishes a welding procedure can produce a weld meeting the required mechanical and soundness properties, running from a preliminary specification through a qualification test to the qualified WPS a production welder follows.
A welding procedure specification (WPS) is the documented set of welding variables — process, materials, parameters and technique — that a welder or welding operator must follow to reproduce a qualified weld.
A welding supervisor is the person assigned to direct and check day-to-day welding work for a crew, shift or area within authority defined by the manufacturer. The title alone does not make that person the responsible welding coordinator.
Unit: m/min (in/min in ASME and AWS practice)
Wire feed speed is the rate at which continuous filler wire is delivered into the arc, the process variable that, together with wire diameter and material density, sets the deposition rate in wire-fed welding processes.
Progetto cofinanziato nell'ambito del PR Piemonte FSE+ 2021-2027,
Priorità I, Obiettivo Specifico a), Azione 4 – "Sostegno alla nascita delle start up"
