Technical Glossary

A fuel gas comprising two carbon atoms and two hydrogen atoms, C[2]H[2]. If acetylene is compressed on its own it can lead to a highly exothermic reaction. For this reason acetylene is supplied dissolved in acetone. Acetylene cylinders are made up of a porous mass capable of storing the acetone-acetylene solution.

A highly flammable organic compound in which acetylene is dissolved to enable it to be safely stored and transported.

Hardness in metal alloys occurs as a result of 'resistance to dislocation motion' in the alloy. If the alloy is held, or 'aged', at a suitable temperature, the hardness may increase. Age hardening may occur gradually at room temperature(natural ageing) and more rapidly at elevated temperatures(artificial ageing). It will be greater with increasing ageing treatment ( i.e. an increase in temperature and/or time), until a point is reached when softening, or 'over-ageing occurs.

A heat-treatment designed to fully soften a metal or alloy.

A method of artificially thickening the oxide film on an aluminium alloy by an electrolytic process. The component to be treated is made the anode in a tank of electrolyte, and on the passage of electric current, oxygen is produced which reacts with the metal surface to form a tightly adherent oxide coating. Dyes may be added to the electrolyte to produce colour anodised surfaces.

A competitive process to Flame Gouging. It uses a hollow carbon electrode through which compressed air is passed. The electrical arc generated between the electrode and workpiece melts the metal which is then blown away by the compressed air. Arc air gouging has poor environmental performance.

See Heat Input.

Arc stability is the term used to describe the regularity at which droplets detach from the consumable wire or rod during welding. Processes with good arc stability have droplet detachment rates that are regular, and of consistent size, these create a minimal amount of disturbance to the weld pool. Similarly processes that have poor arc stability are characterised by irregular droplet rates, with droplets of different sizes. One consequence of this is an increase in the amount of spatter produced.

Movement of the arc column caused by the arc attempting to find the path of least resistance to earth. Exhibits itself as the arc moving from one edge of the welded joint to the other.

A colourless and odourless monatomic gas, density relative to air 1.38. Its most useful property is its total inertness to all substances at all temperatures and pressures. It is the most widely used shielding gas in arc welding.

An age-hardening process which takes place at elevated temperature.

Unconsciousness or death produced by failure of the blood to become properly oxygenated. This failure may be due to inability of air to reach the lungs, as in suffocation, or because of chest injuries.

Atmospheric Pressure is the pressure exerted by the air in the atmosphere on the earth's surface.
Standard atmospheric pressure is quoted in a variety of units: 1.01325 kPa, 760 mm Hg (Mercury), 1.01325 bar or 1 atmosphere.

A solid solution of iron and carbon, with the carbon diffused uniformly throughout the structure. Austenite has a face-centred cubic (F.C.C.) structure.

Welding with equipment that performs the welding operation without adjustment of the controls by a welding operator. The equipment may or may not load and unload the workpieces. See also machine welding.

Backfire occurs when the flame burns back into the torch. It is characterised by a loud bang and is often a sign that there is a fault in the equipment or gas supply e.g. incorrect pressure settings; gas cylinder running low; large pressure drops in the hoses; dirt or carbon blocking the nozzle and restricting the gas flow or the equipment becoming overheated.

The removal of weld metal and base metal from the other side of a partially welded joint to facilitate complete fusion and complete joint penetration upon subsequent welding from that side.

A material or device placed against the back side of the joint, or at both sides of a weld in electroslag and electrogas welding, to support and retain molten metal. The material may be partially fused or remain unfused during welding and may be metal or non-metal.

An SI unit of pressure widely used in the gas industry to describe the filling pressure of cylinders and the rating of gas equipment such as regulators. A bar is defined as #100,000 Newtons / m{2} and 1.01325 bar equals one atmosphere.

Usually applied to steel MIG wire which does not have a copper-coating. The wire does have rust-inhibiting treatments applied to the wire surface. An alternative description for Non-coppered MIG wire.

The metal to be welded, brazed, soldered, or cut.

The angle formed between the prepared edge of a member and a plane perpendicular to the surface of the member.

A term sometimes used to describe a tangle of wire in the wire feed unit of a MIG welding machine.

The process whereby a liquid becomes a gas as the result of the input of heat.

The temperature at which a liquid starts to boil to form a vapour. This temperature is dependent on the pressure the liquid is under.

A non-fusion joining process which uses capillary action to draw a filler material into the joint to form a bond. The properties of the joint are not the same as the parent metal. Brazing uses a filler material which melts at above 450 degrees Celsius.

The British Standards Institute. Standards published by this organisation are denoted BS and are generally followed by a number and date e.g. @BS341. Part 1 1991 - Valve fitting for compressed cylinders. Over a hundred standards are relevant to cutting and welding and relate to design, inspection, testing and maintenance of cylinders and equipment ; safety clothing; consumables and welding practice. British Standards are increasingly being replaced by European standards in which BSI inputs the UK view. These standards are denoted BS EN.

A packaging method whereby large quantities of continuous MIG wire can be supplied to enable robots or automatic welding machines to continue welding for long periods without interruption. The packs may contain, typically, 200, 250 or 350 kg of wire, compared with the 15kg or so on a reel.

Butane is a gaseous component of natural gas, much like gasoline is a component of crude oil. While petroleum products like gasoline are refined, natural gas products like butane are extracted. Butane can also be produced from crude oil, but in much smaller quantities. Butane is often added to regular gasoline to boost performance without creating a highly volatile product. Other common uses for butane include refrigeration, heating and fuel for cigarette lighters.

The type of joint formed when two plates are joined edge to edge in the same plane. A Butt joint can only be made effectively by welding.

Heat required to raise the temperature of 1 gram of water by 1oC (4.18 joules).

The quantity of heat produced by the complete combustion of a given mass of fuel. Calorific value is measured as Joules or Calories per unit of volume.

The process by which liquid is drawn into the joint by molecular attraction. Capillary action can operate against the force of gravity. Brazing and soldering rely upon capillary action to draw the filler material into the joint.

Hardenability is usually expressed by means of a 'Carbon Equivalent', (CE), an empirical term representing the effects of alloying elements in a steel on its hardenability, which allows welding engineers to estimate requirements for pre-heat, interpass, cooling rate, etc. to achieve a successful weld in that steel. As the CE increases, so too does the degree of difficulty in welding.
The IIW equation for determination of CE is as follows:
CE = %C + %Mn/6 + %Cr/5 + %Mo/5 + %V/5 + %Ni/15 + %Cu/15

A flame containing an excess of fuel gas.

This is the diameter a wire naturally takes when a length is throw on the floor. It is used as a rough guide as to how the wire is likely to perform in service.

Tungsten electrodes used for TIG welding containing a small quantity of cerium to aid arc initiation.

A hard grey metallic element used in steel alloys and electroplating ti increase hardness and corrosion resistance. Symbol Cr; atomic number 24; atomic weight 51.996

The sum of ferrite forming elements, and used in conjunction with Nickel Equivalent, plotted in constitution diagrams (Schaeffler, DeLong, and WRC) for the estimation of phases in stainless steel weld metal, and calculated from various equations, such as:
Creq = %Cr + %Mo + 1.5x%Si + 0.5x%Nb

The process of burning a fuel with oxygen in which the products are energy, in the form of light and heat, and by-products such as water and carbon dioxide.

A collective term for gaseous chemical (in the gaseous or the liquefied state) when compressed in excess of 500mbar in a container or system.

The process of forming a liquid from its vapour under a change in temperature or pressure.

An enclosed or small space often without adequate ventilation. Area of potential hazard from asphyxiation if it becomes filled with an inert or unreactive gas. Also an area of potential explosion if filled with a flammable or explosive gas mixture. Follow this link to further information on the subject.

A weld that extends continuously from one end of a joint to the other. Where the joint is essentially circular, it extends completely around the joint.

Usually applied to steel MIG wire, but also applicable to TIG rod, which has a protective copper coating applied to the wire surface to prevent or slow down rusting of the wire. It is also maintained, in some circles, that electrical pick-up is improved by the coppering.

A joint between two members located approximately at right angles to each other.

A product which will attack many substances it comes into contact with, such as metals, polymers or tissue.

The union of two or more atoms by the sharing of one or more pairs of electrons.

A chemical bond in which two or more atoms are held together by the interaction of one or more pairs of electrons.

The temperature, below which pitting corrosion does not occur during testing (usually of Duplex and Super Duplex stainless steel) in ferric chloride, according to ASTM G48.

The temperature and pressure at which the gas and liquid phases of a substance are identical.

he absolute pressure at which both liquid and gas phases co-exist at the critical temperature.

The temperature above which a gas cannot be liquefied by pressure alone.

The volume occupied by 1.0 gram-mole of the substrate at its critical temperature and pressure.

A flashback arrestor which does not give any indication that a flashback has occurred.

Constitution Diagram used for the estimation of ferrite and other metallurgical phases present in stainless steel weld metal derived from Chromium and Nickel contents and their equivalents.

A weld containing one or more defects.

The sudden combustion of a gas, generally accompanied by a flame and a cracking sound. The speed of the flame is less than the speed of sound.

The weight of a substance per unit volume.

Where the combustion wave or flame travels into the un-reacted gas as a steep pressure wave faster than the speed of sound.

Dip, also known as Short Arc Transfer. In this mode of transfer the welding wire is feed down to the parent plate were it dips into the weld pool, causing a short circuit to occur. The welding current rises rapidly and the short circuit ruptures to detach some of the wire and re-establish the arc.

The composition of weld metal is a result of a mixture of the consumable and melted parent metal taken into the weld pool. The metal deposited from the consumable is said to have been 'diluted' by the parent metal, and 'dilution' is expressed as 'the percentage of parent metal included in the weld bead'. It should be noted that previously deposited weld metal can also 'dilute' a subsequent weld bead.

An interruption of the typical structure of the weldment such as a lack homogeneity in the mechanical or metallurgical or physical characteristics of the material or weldment. A discontinuity is not necessarily a defect.

A steel filler metal which relies on two elements in its make up, usually manganese and silicon, for deoxidation of the weld pool.

The percentage of time the welder is welding or welding equipment is being used, measured again a standard working day.

Welding process in which welds between metals are made using the heat from a concentrated beam of high velocity electrons. EB welding normally operates in a vacuum, but a variation of the process allows it to be carried out at atmospheric pressure.

A term used in tensile testing as an indication of the ductility of the material under test. It is evaluated by comparing the distance between two marks ( the gauge length, l ) on a test specimen before testing begins, with the distance (L) between the same two marks after fracture of the specimen.
It is quoted as a percentage, and calculated from the equation:
Percentage Elongation = 100 x ( L - l ) / l , where l = original gauge length,
and L = length after fracture.

A torch design in which the pressure of the oxygen and fuel gas are equal on the inlet side of the torch. This design is less suitable for equipment fitted with long hoses (greater than 50m) or if the fuel gas pressure is low.

A state of balance. A condition in which opposing activities are balanced so that no overall change in state occurs.

A fuel gas and a member of the paraffin family. A hydrocarbon molecule comprising two carbon atoms and six hydrogen atoms, C2H6.

See Exothermic Reaction

A chemical reaction in which energy is released as heat and light. The opposite of an endothermic reaction in which energy is absorbed.

An iron-carbon solid solution, stable up to 910oC, and at 723oC contains a maximum of 0.02wt% carbon. Its structure is body-centred cubic (bcc).

The material added to a welded, brazed and soldered joint. The mechanical and chemical properties of the filler affect the strength and corrosion resistance of the joint. Filler can be supplied as wire, rod, rings or shapes.

A joint formed when one plate is joined to another at right angles to form a 'T'. This type of joint is most widely used in arc welding.

The Flame Intensity is the amount of energy released per unit time per unit area of primary flame front. It is used to evaluate the effectiveness of the fuel.

A measure of how fast the flame will travel through an unburned gas mixture. Flame speed has a major impact on the burner design and controls, as well as the heat transfer rate.

The temperature at the hottest part of the flame.

Gas that forms a flammable mixture with air at a concentration of 13% or less, determined at normal atmospheric pressure.

The range over which a gas will form a flammable mixture with air.

Flashback is the action of the flame and it's pressure wave travelling back through the torch and into the gas delivery system.

An essential safety device which prevents flashbacks travelling into the regulator or cylinder. Flashback arrestors are specific to each gas and must not be interchanged.

A weld produced in the flat or 1G position

A substance applied to the surface of metals to be soldered to inhibit oxidation and aid the flow of filler. Also used to describe substances used in the smelting of metals to assist in the removal of impurities as slag.

An arc welding process that uses an arc between a continuous filler metal electrode and the weld pool. The process is used with shielding gas from a flux contained within the tubular electrode, with or without the additional shielding from an externally supplied gas, and without the application of pressure.

A composite tubular filler metal electrode consisting of a metal sheath and a core of various powdered materials, producing an extensive slag cover on the face of a weld bead. External shielding may be required.

The process whereby a liquid becomes a solid under a change in temperature or pressure.

A welding process that uses a rotating tool moving along a joint to produce a sufficiently high temperature to forge two components.

The airborne particles of consumable or parent plate produced by the welding or cutting processes.

A measurement of how much particulate welding fume is produced, by a process or a consumable, in a unit of time, expressed as g/min or g/hr.

The melting together of filler metal and base metal (substrate), or of base metal only, which results in coalescence.

The state of matter in which molecules move freely, so allowing it to expand completely to fill any space that it occupies.

A gas, or a mixture of gases, produced as a result of a welding or cutting operation. Gaseous fume tends to be process dependent, and does not normally come from the consumables or parent plate.

Pressure relative to atmospheric pressure.

A mode of transfer falling between dip and spray. In globular transfer the wire melts and the droplets fall into the weld pool under the force of gravity. This mode is characterised by the high spatter and particulate fume levels produced and by it's poor arc stability.

Gas Metal Arc Welding, a common US term for the welding process known in Europe and UK as MIG (Metal Inert Gas) welding. See MIG for more information.

Gas Tungsten Arc Welding, a common US term, also known as TIG (Tungsten Inert Gas welding). See TIG for more information.

A surface variation in which surfacing material is deposited to reduce wear.

A term used to describe the propensity of an alloy to harden when subjected to rapid cooling or quenching from elevated temperatures, for example, in the heat affected zone of a weld. Normally applied to steels, hardenability increases with increase in carbon and alloy content, by causing a lowering of the transformation temperature. In welding the effects are usually off-set by application of pre-heat to susceptible materials.

Hardness may be defined as 'resistance to indentation'. It is usually measured by pressing an indenter into a prepared, flat surface of the material under examination, using a known static load, and measuring the size of the impression produced. The hardness number is then obtained by reference to conversion tables for the specific test method being used. These numbers are based on a stress value obtained by dividing the load by the area of the impression.
The most common hardness test method is the Vickers, which uses a diamond pyramid indenter. Other tests include Rockwell (diamond cone indenter), and Brinell (hard steel ball indenter). The Shore Scleroscope is a dynamic test using a falling hammer to make the indent.

That portion of the base metal which has not been melted, but whose mechanical properties or microstructure have been altered by the heat of welding, brazing, soldering, or cutting.

Quantity of heat required to raise the temperature of a unit mass of material by 1°C.

Heat input is generally defined by the formula:
A x V
TS
Where A = Current (amps)
V = Voltage (volts)
TS = Travel Speed (mm/s)
The value is normally quoted in kJ/mm.
However, some of the energy generated by the arc fails to reach the weld pool because of radiation losses. To take these loses into account the heat input formula is multiplied by a
thermal efficiency factor related to the type of welding process used.
The thermal efficiency factors are:
Submerged arc 1.0
MIG, MMA and FCAW 0.8
TIG and plasma 0.6

Alloys which can be strengthened or hardened by application of heat.

Is the amount of spring a welding wire exhibits. It is measured by throwing a length of wire onto a flat floor and then measuring how high the wire end is from the ground. Ideally the wire should have no helix at all. If the wire exhibits a helix then it may spiral as it leaves the torch contact tip.

A weld made horizontally with the parent plate lying in the vertical plane. Designated as the 2G welding position.

Non-return valves are fitted to the end of the gas hoses. They prevent the backfeeding of one gas into another. For example if the oxygen working pressure is much higher than that for the fuel gas and a hose check valve is not fitted, oxygen could backfeed into the fuel gas hose creating an explosive mixture.

Also known as solidification cracking, it is a discontinuity produced by tearing of the metal while it is at an elevated temperature.

Compounds that are made up of mainly hydrogen and carbon that when combusted produce energy.

A weld crack caused by the presence of hydrogen in the weld.

The action of firing an explosive mixture of gases or vapours by means of a flame, electrical spark, heating or sudden pressure change.

A substance which does not form compounds with any other element. The noble gases, which occupy Group 8 of the Periodic Table are totally inert.

The impact strength of a material is a measure of its ability to withstand shock loads.
In the laboratory, it is evaluated by dynamic test methods, such as, the Charpy V Notch, and, less commonly nowadays, the Izod test. These tests measure the energy absorbed in breaking a pre-notched specimen, held at a known test temperature This absorbed energy is quoted in 'Joules' or 'Foot-pounds', and reference is always made to the temperature at which the test was carried out.

Electromagnetic energy with wave lengths from 770 to 12,000 nanometers.

A torch design which is ideal for use with low fuel gas pressures. The gases are mixed by allowing the oxygen flow to suck the fuel gas through the injector nozzle. This type of torch is ideal for equipment fitted with long hoses (greater than 50m).

In multipass welding, the temperature of the weld metal before the next pass is started.

A chemical bond in which one element loses an electron to another element. Elements in Group 1 of the Periodic Table, for example Lithium are extremely reactive because they readily give up their single electron in the outermost energy level to another element with an incomplete energy level thereby forming a stable compound.

The process by which gas molecules are converted into positively and negatively charged particles called ions. This process occurs when an arc is struck between an electrode and the workpiece.

International Standards Organisation. This organisation develops standards for worldwide use. Standards are denoted as ISO followed by a number and date.

The junction of members or the edge of members that are to be joined or have been joined.

The depth a weld extends from its face into a joint, exclusive of reinforcement.

The material and detailed methods and practices employed in the welding of a particular joint.

A term used to describe the squareness of a cut. It is defined as the angle generated from the difference in widths between the top and bottom of the cut.

A joint between two overlapping members in parallel planes.

A thermal cutting process which uses the heat obtained from a focused laser beam, often in conjunction with gas jets, to cut and remove metal.

A welding process which uses the heat from a concentrated coherent beam of light to effect melting and fusion.

A method of reeling MIG wire so that the wire is wound onto the spool in layers, one on top of the other, to ensure it feeds off smoothly during welding.

Liquation cracking is caused by the melting of low melting-point grain boundary constituents which lead to small micro-cracks. These micro-cracks may not prove to be a serious problem, providing that they do not provide sites for more serious cracks to occur.

A substance which is gaseous at ambient temperature and atmospheric pressure but has been transformed into liquid by changing its temperature and/or pressure. If the Critical Temperature for the substance is above ambient temperature it can be liquefied by either lowering the temperature or increasing the pressure, or both. If it's Critical Temperature is below ambient it cannot be liquefied by applying pressure alone, it must also be cooled.

The phase of a substance in which the constituent molecules, atoms or ions are relatively free to move but are restricted by forces to the extent that the liquid maintains a fixed volume.

The minimum temperature at which a metal or alloy is completely liquid.

This is the general term used to describe the easily liquefiable gases contained in crude oil and natural gas - propane, butane, ethane, propylene and butylene.. LPG sold for fuel use is generally a mixture of two or more of these products.

Welding with equipment which performs the welding operation under the constant observation and control of a welding operator. The equipment may or may not load and unload the workpieces. See automatic welding.

A header with outlets or branches to which a number of cylinders of gas may be connected.

A welding operation performed and controlled completely by hand. See automatic welding, machine welding and semi-automatic welding.

A trade name for a fuel gas which is a stabilised mixture of Methylacetylene and Propadiene. It comprises three carbon atoms and four hydrogen atoms, C3H4.

A hard brittle microstructure which forms if steel is heated and then cooled very rapidly.

The process whereby a solid becomes a liquid under a change in temperature or pressure

An opaque, lustrous, elemental chemical substance that is good conductor of heat and electricity, usually malleable, ductile, and more dense than other elemental substances.

A composite tubular metal electrode consisting of a metal sheath and a core of various powdered materials, producing no more than slag islands on the face of a weld bead. External shielding may be required.

A fuel gas and a member of the paraffin family. It is the simplest hydrocarbon molecule comprising one carbon atom and four hydrogen atoms, CH4.

A small crack in weld metal or HAZ, only visible under a microscope.

MIG welding stands for Metal Inert Gas welding. The MIG process uses the heat generated by an electric arc to fuse the joint area. The arc is formed between the tip of a consumable, continuously fed filler wire and the workpiece.

A term used to describe the general range of carbon and carbon manganese steels used for fabrication. These steels typically contain - 0.05 - 2% carbon, 0.3 - 2% manganese and up to 0.6% silicon.

MMA stands for Manual Metal Arc, an electric arc welding process in which the arc is struck between a covered metal electrode and the workpiece. Shielding of the weld pool is provided by the decomposition of the electrode covering and the electrode is consumed to provide filler metal.

Weight of one molecule of a substance in units of atomic weight. Molecular weight may be computed by summing the atomic weights of the component atoms of a molecule.

An age-hardening process which takes place at room temperature.

A mixture of hydrocarbon gases which occur naturally and are recovered for commercial use. LPG is extracted from natural gas because of its greater commercial value. Natural gas used for industrial applications is predominantly methane.

A technique in which components are machine cut to minimise wastage and distortion.

Also called a normal or working flame, it occurs when there is neither excess oxygen or fuel present.

The sum of austenite forming elements, and used in conjunction with Chromium Equivalent, plotted in constitution diagrams (Schaeffler, DeLong, and WRC) for the estimation of phases in stainless steels weld metal, and calculated from various equations, such as:

Nieq = %Ni + 30x%C + 0.5x%Mn

Alloys which cannot be strengthened or hardened by application of heat.

A group of hydrocarbons with general formula C[n]H[2n] .They are also known as Alkenes and include ethylene, C[2]H[4] and propylene, C[3]H[6].

The position in which welding is performed from the underside of the joint

The protrusion of weld metal beyond the weld toe or root weld.

The source of oxygen in a chemical reaction e.g. air or oxygen gas.

DEFN = "Containing free oxygen atoms which combine with metal to form metal oxides in the reaction called oxidation."

Caused by an excess of oxygen in the flame.

Oxygen is a colourless, odourless gas. It is represented by the symbol O. It's most important property for use in industrial processes is that it supports combustion. Oxygen forms 21% by volume of the atmosphere. Oxygen has a relative density of 1.105.

A group of cutting processes used to sever or remove metals by means of the chemical reaction between oxygen and the base metal at elevated temperatures. In the case of oxidisation-resistant metals, the reaction is facilitated by the use of a chemical flux or metal powder.

A group of hydrocarbons with the general formula CnH2n+2. They are also known as Alkanes and include methane CH4, propane C3H8 and octane C8H18.

Particulate fume is the discrete solid particles produced during welding and cutting. It consists of the complex oxides and silicates of metals in the parent plate and filler wire which are generally under 0,5 microns in diameter. This falls into the "respirable " size range meaning that the particles can be breathed in and reach the lungs.

Penetration area is the measurement of the area of the weld bead below the level of the plate surface.

Penetration depth is the measurement of the distance between the level of the plate surface and the bottom of the weld.

Penetration profile describes the shape of the weld bead below the level of the plate surface.

A table of the chemical elements arranged in order of atomic number in rows, so that elements with similar atomic structure (and hence similar chemical properties) appear in vertical columns.

A Phase Diagram shows the effect of changes in temperature and pressure has on the physical state of a product. A phase diagram may also show the effect of temperature and composition on an alloy of two or more metals.

A number indicating the resistance of a stainless steel to pitting corrosion, related to chemical composition and derived from the equation:

PRE = %Cr + 3.3x%Mo + 16x%N

MIG wire can be supplied as a continuous coil wound onto cylindrical reels or spools of various designs and diameters. Traditionally, MIG reels have been made of plastic. Reel sizes vary from the standard 300mm diameter reels to small, 50mm diameter, for spool-on-gun equipment. The amount of wire will vary with reel diameter and type of alloy, a standard reel holding about 15kg of steel MIG wire, or 6kg of aluminium MIG wire.

A large compound which is formed from repeated units of smaller molecules.

Also called Polyethane. A large hydrocarbon molecule formed by the polymerisation of ethylene (ethane).

A process in which the double or triple bonds in hydrocarbon molecules break and the molecules combine together to form very large molecules.

Cavities formed in the weld metal by gas entrapped during solidification.

A weld made in any position other than flat or horizontal,i.e. overhead.

Any heat treatment after welding.

An apparatus for supplying current and voltage suitable for welding, thermal cutting, or thermal spraying.

Parts per billion (1 x 10-9)

Parts per million (1 x 10-6). A convenient means of expressing very low concentrations of a substrate in a mixture, or a low level contaminant in a pure product.

An alloy consisting of an intrinsically soft metal matrix, stiffened by finely dispersed particles of harder substances, to give a combination of strength and ductility.

The application of heat to the base metal immediately before welding, brazing, soldering, thermal spraying, or cutting.

A specified temperature that the base metal must attain in the welding, brazing, soldering, thermal spraying, or cutting area immediately before these operations are performed.

The filling pressure of gas cylinders is far greater than the working pressure required at the torch. Pressure Regulators reduce the gas pressure to the working pressure and maintain it as the cylinder empties.

A fuel gas. A member of the di-olefin family. A hydrocarbon molecule comprising three carbon atoms and four hydrogen atoms, C3H4. Each carbon atom forms a double covalent bond with another carbon atom in which two pairs of electrons are shared.

A fuel gas. A member of the paraffin family. A hydrocarbon molecule comprising three carbon atoms and eight hydrogen atoms, C3H8.

A fuel gas. A member of the olefin family. A hydrocarbon molecule comprising three carbon atoms and six hydrogen atoms, C3H6. Two of the carbon atoms form a double covalent bond in which two pairs of electrons are shared.

Pounds per square inch absolute.

Pounds per square inch gauge

A procedure in which a gas or liquid is forced into a component to remove contaminants from the system.

A type of MIG welding torch containing a separate set of drive rolls within the head of the torch to assist with pulling the filler wire through the torch liner. This type of torch is most widely used for feeding soft wires such as aluminium which may buckle within the liner causing snags or bird-nesting at the wire feed unit.

A radical is a molecule or atom which has an unpaired outer electron. It is very reactive because it needs to gain an electron. Radicals may exist independently for short periods during the course of a chemical reaction, such as in the combustion of a fuel gas.

A method of reeling MIG wire in which the wire is wound onto the spool without attempting to ensure it forms in regular, controlled layers.

The act in which a substance brings about the chemical change of reduction, the process in which an electron is added to an atom or ion such as removal of oxygen from a molecule, in another substance, itself being oxidised.

Also called a carburising flame. Caused when there is an excess of fuel gas to oxygen in the oxy-fuel gas mixture.

A term used in tensile testing, comparing the original diameter of a test specimen with its diameter after fracture, and used as an indication of the ductility of the material under test.
It is quoted as a percentage, and is calculated from the equation:

%Reduction of Area = 100 x (A - a) / A, where A = original cross-sectional area,
and a = cross-sectional area at fracture.

The excess weld metal which stands proud of the surface of the plate. Excessive reinforcement induces stresses where the weld meets the parent plate and can lead to cracks forming.

Reinforcement Height is the measurement of the distance between the level of the plate surface and the top of the weld bead, or the distance between a line stretched between the attachment points of the weld on the parent plate and the top of the weld bead.

Reinforcement area is the measurement of the area of the weld bead above the level of the plate surface.

A flashback arrestor designed to provide a visual indication that a flashback has occurred, prompting the operator to check the system for faults. As the pressure wave hits the non-return valve it pushes up a button or lever, stopping the pressure wave from entering the regulator.

Small quantities of undefined impurities.

Welding processes which produce welds between two faying surfaces by means of the heat produced by resistance to electric current, combined with pressure.

That fraction of welding fume that can be breathed in, and is capable of reaching the lungs. This includes all gases, and particulate welding fume which is less than about 7 microns in diameter.

A method of restricting undue or excessive movement of components during welding, often by means of clamping, jigging, or tack-welding. Too much restraint of some alloy systems can result in cracking.

A term used to describe the amount of surface irregularity on the cut surface. Gouges and styrations on the cut face are normally caused by the use of incorrect parameters.

A variation of the spot gouging technique which is used to remove surface defects from the surface of steel billets before rolling.

Constitution Diagram used for the estimation of ferrite and other metallurgical phases present in stainless steel weld metal derived from Chromium and Nickel contents and their equivalents.

A joint which comprises two pieces of metal which fit together to form a gas tight seal. As distinct from metal components that are screwed together using a thread.

Arc welding with equipment that controls only the filler metal feed. The advantage of the welding is manually controlled.

Shielding gases are the gases and mixtures used to protect the weld pool from contamination from the atmosphere. They play a significant role by affecting the physical and metallurgical characteristics of the weld, the arc characteristics and the speed and ease of welding.

Sigma is an iron-chromium (roughly 50-50) phase, which can form from ferrite in austenitic stainless steel weld metal at elevated temperatures ( approximately between 450 and 850°C). It, therefore, forms during high temperature service and its presence in the stainless steel is only apparent at room temperature, during plant shutdown, when the sigmatised material becomes extremely brittle and will crack readily with low toughness.

In arc and gas welding, any joint welded from one side only.

A vitreous or glass like material containing oxides of materials such as manganese, silicon and aluminium. The slag produced solidifies on the surface of the weld protecting it from atmospheric contamination as it cools and is removed later.

SMAW stands for Shielded Metal Arc Welding (another name for MMA welding), an electric arc welding process in which the arc is struck between a covered metal electrode and the workpiece. Shielding of the weld pool is provided by the decomposition of the electrode covering and the electrode is consumed to provide filler metal.

The practice of momentarily opening a cylinder valve to remove moisture and dirt. There must be no possible source of ignition in the vicinity and the operator should wear eye protection and stand clear of the gas stream. Snifting must never be carried out on a hydrogen cylinder because spontaneous ignition can occur.

A non-fusion joining process which uses capillary action to draw a filler material into the joint to form a bond. The properties of the joint are not the same as the parent metal. Soldering uses a filler material which melts below 450 degrees Celsius, and often as low as 300 degrees Celsius.

The phase of a substance in which the constituent molecules, atoms or ions have no translational motion but vibrate about fixed positions

Solid solutions are solid phases containing more than one component and are the basis of the ability of metals to form alloys. There are two general classes of solid solution, 'substitutional' and 'interstitial'. Substitutional solid solutions are those in which the solute atoms are substituted for solvent atoms in the crystal structure. Interstitial solid solutions are those in which the solute atoms occupy interstices in the crystal structure of the solvent.

Also known as hot cracking it is a discontinuity produced by tearing of the metal while it is at an elevated temperature.

The maximum temperature at which a metal or an alloy is completely solid.

The small particles of weld metal ejected from the weld pool which can adhere to the surface of the metal.

A compound containing a mixture of one or more metal oxides.

A type of MIG welding torch where the welding wire is carried on a small spool located on the torch itself. This type of torch is used to feed soft wires such as aluminium that could suffer from wire feed problems if fed through the length of the torch liner.

The mode of transfer in which small droplets of consumable wire are thrown across the arc by means of the forces generated by the arc. Also known as 'free flight transfer'.

A corrosion resisting steel which contains a minimum of 13% Chromium. The corrosion resisting characteristics are caused by the formation of a thin layer of chrome oxide which adheres very strongly to the surface of the material. A range of stainless steels with different compositions and metallurgical structures are produced to deliver different mechanical properties.

Plain Cr-Ni stainless steel may be susceptible to weld decay due to formation of chromium carbides in the HAZ during welding. Weld decay can be avoided in several ways, one being to add 'stabilising' elements, such as niobium or titanium to the steel. These elements form carbides preferentially to chromium over a wide temperature range, freeing the chromium to provide a corrosion-resistant passive film. Stainless steels containing niobium or titanium are said to be 'stabilised'.

A relative term used to describe the arc stability characteristics of a process or product. Lower the Stability Index the more stable the process. In MIG welding, products with high Stability Indexes normally produce high levels of spatter.

 technique used to increase the productivity of a machine cutting operation. Several plates are laid on top of each other and cut at the same time. For the process to work efficiently there must be no gap between the plates. Plates are often clamped together to prevent this from happening.

A technique used to increase the productivity of a machine cutting operation. Several plates are laid on top of each other and cut at the same time. For the process to work efficiently there must be no gap between the plates. Plates are often clamped together to prevent this from happening.

The distance between the end of the contact tip or electrode and the workpiece.

The amount of a substance which is consumed when a chemical reaction proceeds to completion.

A post-weld heat-treatment designed to relieve residual stresses and restore mechanical properties, particularly toughness and ductility, in a welded structure. It normally involves holding at a specified elevated temperature for a set time, followed by slow cooling.

The process whereby a solid becomes a gas under a change in temperature or pressure without going through a liquid phase.

A joint formed when one plate is joined to another at right angles to form a 'T'. This type of joint is most widely used in arc welding.

A term which generally describes the mechanical properties of an alloy. Many steels may be hardened by quenching from high temperature and then softened by re-heating at a lower temperature to give the required properties, or 'temper'. Other alloy systems, like aluminium and copper, for example, start in the soft or annealed condition. With cold work, the strength of the alloy may be increased, accompanied by a reduction in ductility. Different degrees of cold work produce different properties, or 'tempers', e.g. annealed (soft), half-hard, fully hardened.

The tensile strength of a material is the value obtained by dividing the breaking load by the cross-sectional area.

When heated, most metals and alloys expand. The coefficient of thermal expansion is a second-order tensor, which describes the degree to which a particular metal or alloy expands when it is subjected to a uniform incremental temperature rise.

A tungsten electrode normally used for TIG welding containing a small percentage of thorium to assist arc initiation.

TIG stands for Tungsten Inert Gas welding, also known in the US as GTAW. TIG is an electric arc welding process which uses heat generated by an arc struck between a non-consumable tungsten electrode and the work piece.

A term used to describe the defect on the top edge of the cut. It is usually caused by using too intense a pre-heat flame.

The item of equipment in which the oxygen and the fuel gas are mixed and delivered to the nozzle.

A term denoting a condition intermediate between brittleness and softness. It is indicated in tensile tests by measurement of the ultimate tensile stress and elongation.

Gas which is injurious to health.

A steel filler metal which has additions of three elements, aluminium, zirconium and titanium, in addition to manganese and silicon, to effect deoxidation of the weld pool. May also be described as 'multiple deoxidised'.

The temperature and pressure at which the solid, liquid and gas phases of a substance co-exist.

A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal.

A substance in the gas phase that can be liquefied by increasing the pressure without altering the temperature.

The process whereby a liquid becomes a vapour under a change in temperature or pressure.

The pressure of a vapour in contact with it's liquid or solid form.

The position of welding in which the axis of the weld is approximately vertical.

A localised coalescence of metals or non-metals produced by heating the materials to the welding temperature, with or without the application of pressure and with or without the use of filler metal.

A weld deposit resulting from a pass.

Weld Bead Shape describes the physical profile of the weld metal.

Weld decay (intercrystalline corrosion) in austenitic stainless steel is an "in-service" corrosion problem caused by the formation of chromium-carbides at HAZ grain boundaries during welding. These effectively remove the chromium locally and prevent it from forming a corrosion resistant surface barrier, leading to corrosion on the chromium-depleted sites. Weld decay is a historic problem and arose during service in certain environments (most notably breweries). It has now been virtually eliminated by use of low carbon or stabilised stainless steels, modern low heat input processes and low carbon or stabilised welding consumables.

The exposed surface of a weld on the side of from which welding was done.

Description of way the component to be joined is configured. Examples would be a butt or fillet joint. Follow this link for a more detailed explanation.

A single progression of welding or surfacing along a joint or substrate. The result of a pass is a weld bead, layer, or spray deposit.

The shape to which the edges of the weld joint are cut or machined prior to welding. Follow this link for a more detailed explanation.

Weld metal in excess of the quantity required to fill a joint.

The junction of the weld face and the base metal.

A term describing the relative ease or difficulty with which a metal or alloy can be welded. A metal or alloy is considered to have good weldability if it can be welded without excessive difficulty, or the need for special, costly, welding procedures, and the resultant welded joints have suitable mechanical properties for the intended application.

An assembly whose component parts are joined by welding.

MIG wire can be supplied as a continuous coil wound onto cylindrical reels or spools of various designs and diameters. The amount of wire will vary with reel diameter and type of alloy. One form of reel/spool is manufactured entirely from wire. These are commonly described as 'wire baskets' and will hold about 15kg of steel MIG wire or 6kg of aluminium MIG wire. Baskets intended for use with stainless steel and non-ferrous metal MIG consumables have the wire covered by a plastic coating to prevent contamination of the welding wire. Some baskets require a 'reel adaptor' to be fitted to enable them to be attached to wire feed units (which are designed to take the traditional plastic reels), while others have such an adaptor built into them.

Some alloy systems will harden when subjected to cold work ( mechanical work carried out at temperatures too low for recrystallization to occur). This plastic deformation causes dislocations to become entangled and the material hardens and becomes more resistant to further working.

Welding Research Council constitution diagram for the estimation of weld metal ferrite from Chromium Equivalent and Nickel Equivalent. Used primarily with Duplex and Super Duplex stainless steels.

When a ductile metal or alloy is stressed beyond a certain point (the yield stress or elastic limit), it deforms plastically. The yield strength may be determined, during a tensile test, by dividing the load at the yield point by the original cross-sectional area of the test specimen.

Tungsten electrode used for TIG welding containing small quantities of zirconium to aid arc initiation. Primary use is for AC welding of aluminium.