IIW IWI-C key terms

The thermal energy introduced per unit length of weld, usually expressed in kJ/mm and calculated from arc voltage, current and travel speed. It strongly influences cooling rate, microstructure and toughness.

The time taken for a weld to cool from 800 to 500 degrees Celsius. It governs transformation products in the heat-affected zone and is used to predict hardness and cracking risk.

A documented set of qualified welding parameters and conditions that a welder must follow to produce a joint meeting specified requirements. It is supported by a procedure qualification record.

The record of actual variables and test results produced when a welding procedure is tested to demonstrate its suitability. It provides the evidence behind a WPS.

Testing of an individual welder's ability to produce a sound joint to a given standard, using specified process, position and material. It certifies the person, not the procedure.

A welding parameter that, if changed beyond permitted limits, invalidates a qualified procedure or welder qualification and requires requalification. Examples include process, filler classification, thickness range and position.

The parent metal adjacent to the fusion boundary that has been thermally altered by welding but not melted. Its microstructure and properties often control joint performance and cracking susceptibility.

Delayed cracking in weld metal or HAZ caused by the combined presence of diffusible hydrogen, tensile stress and a susceptible hardened microstructure, typically occurring hours after welding. Prevention relies on low-hydrogen consumables, preheat and controlled cooling.

Hydrogen absorbed by the weld that remains mobile in the lattice at room temperature and can migrate to stress concentrations, promoting cold cracking. Its content is measured in millilitres per 100 grams of deposited metal.

The minimum temperature of the parent metal before welding and the maximum temperature allowed between passes. These control cooling rate, hydrogen escape and residual stress.

Cracking that occurs at elevated temperature during solidification or shortly after, often along grain boundaries. It is associated with high restraint, unfavourable composition and segregation, and is common in some austenitic and nickel alloys.

A form of hot cracking that develops in the weld metal during freezing, when low-melting films or segregated elements cannot accommodate shrinkage strain. It is influenced by weld shape, dilution and travel speed.

Step-like cracking beneath a weld in rolled plate, caused by through-thickness tensile strain acting on flattened, elongated inclusions. It is managed by joint design, low-sulphur steel and improved through-thickness ductility.

Internal stress locked into a component after welding because of non-uniform heating, cooling and restraint, without any external load. It can drive distortion, cracking and reduced fatigue life.

The resistance of a structure or fixture to free movement of the joint during welding. High restraint raises residual stress and the risk of cracking.

The planning and techniques used to limit unwanted shape change from welding, such as balanced sequences, backstep welding, presetting and jigs. It treats shrinkage and residual stress together.

The planned order in which passes, joints and assemblies are welded. A sound sequence distributes heat and shrinkage to control distortion and residual stress.

The proportion of parent metal that melts into the weld metal, affecting composition, microstructure and properties. Excessive dilution can cause cracking or loss of corrosion resistance.

Gas supplied around the arc or weld pool to exclude atmospheric oxygen and nitrogen and stabilise the arc. Its composition affects penetration, bead shape, spatter and alloying element loss.

An arc process using a non-consumable tungsten electrode and an inert gas shield, with filler added separately if required. It gives precise control and is widely used for root passes and thin sections.

A gas-shielded consumable electrode process using a reactive gas mixture, typically argon with carbon dioxide or oxygen. The active component improves arc stability and penetration in ferrous metals.

An arc process in which the arc and molten pool are covered by a granular flux layer, giving high deposition rates and deep penetration. It is normally mechanised and used for long straight joints.

A granular or coated material that protects the molten weld from the atmosphere, stabilises the arc and can add alloying elements. Its composition and handling affect weld quality and hydrogen content.

A consumable electrode process in which a flux-coated rod provides both filler and shielding through decomposition of the coating. It is versatile and widely used for site and repair work.

The capacity of a material to be welded under given fabrication conditions into a joint that performs satisfactorily in service. It depends on composition, thickness, restraint and process choice.

A single value combining the effects of carbon and other alloying elements to estimate hardenability and cracking risk in steel. It is used to set preheat and select consumables.

A stainless steel family with a face-centred cubic structure, typically containing chromium and nickel. It offers good toughness and corrosion resistance but is sensitive to sensitisation and hot cracking.

The precipitation of chromium carbides at grain boundaries in austenitic stainless steel during heating in a critical range, leaving chromium-depleted zones that are prone to intergranular corrosion.

Corrosion that progresses along grain boundaries, often because of chromium depletion or impurity segregation. It can severely weaken a welded joint even when the surface looks sound.

A volumetric non-destructive method that transmits high-frequency sound into a component and analyses echoes to detect and size internal discontinuities such as lack of fusion, cracks and porosity.

A volumetric non-destructive method that passes ionising radiation through a component onto a detector, producing an image in which density differences reveal internal discontinuities. It is strong for volumetric defects but weaker for planar cracks aligned with the beam.

A surface and near-surface method in which magnetisation and applied particles reveal leakage fields at discontinuities in ferromagnetic material. It cannot be applied to non-ferromagnetic metals.

A surface method that applies a liquid penetrant, removes the excess, then draws penetrant out of surface-breaking discontinuities with a developer. It works on non-porous materials regardless of magnetism.

A planar discontinuity where weld metal fails to bond to the parent metal or to a previous pass. It acts as a sharp crack-like feature and is particularly damaging under fatigue or impact loading.

Cavities formed by gas trapped in the solidifying weld metal, appearing as rounded internal or surface pores. Excessive porosity reduces cross-section and can breach leak-tightness requirements.

The defined limits on defect type, size and frequency that determine whether a weld conforms to a code or specification. They link inspection results to fitness-for-purpose decisions.

Inspection methods that assess material or joints without damaging them, used to verify quality against acceptance criteria. Method selection depends on defect type, geometry and material.

An engineering evaluation of whether a component with known defects can safely perform its intended function for a defined period. It uses fracture mechanics and inspection data rather than fixed defect limits alone.

Local exhaust ventilation that captures welding fume near the arc before it enters the welder's breathing zone. Correct hood position and airflow are essential to control exposure to metal oxides and gases.

An engineered system that captures airborne contaminants at or near their source and removes them from the workplace. It is the primary control for welding fume and should be checked for effectiveness.

Worn equipment such as helmets, filters, gloves and flame-resistant clothing that protects the individual from residual hazards. It is the last line of defence after elimination and engineering controls.

Work inside an enclosed or poorly ventilated area with restricted access, requiring risk assessment, atmosphere testing, ventilation, a rescue plan and an attendant. Welding in such spaces creates serious fume and oxygen-deficiency risks.

The practices governing storage, handling and use of oxygen and fuel gases, including flashback arrestors, segregation of cylinders and prevention of oil contamination. Oxygen enrichment makes clothing and materials burn violently.

A person responsible for coordinating welding production and quality activities under a quality management system, including procedure and personnel qualification. The role is defined in several fabrication standards.

A project document listing the inspections and tests to be performed, who performs them, and the hold and witness points. It provides the audit trail linking specification to delivered quality.