History & glossary
A brief history of metal
The history of metallurgy does not begin with the Iron Age. Long before then, people had already learned to work certain metals, particularly copper. Initially used in its native form, copper was gradually extracted from ores and processed through an increasing mastery of fire and metallurgical techniques.
The Bronze Age
The Bronze Age marks a major step in this development. Bronze is an alloy made mainly of copper and tin. Harder and generally better suited to making tools and weapons than pure copper, it enabled the development of new manufacturing techniques.
In Europe and the Near East, the Bronze Age extends approximately from the third millennium to the beginning of the first millennium BCE. These dates should nevertheless be treated with caution: different regions of the world did not experience the same technological developments at the same time.
Some societies had already mastered copper or bronze metallurgy while others still relied mainly on stone tools.
Metallurgy also had significant economic and social consequences. Deposits of copper and, especially, tin were not necessarily close to the places where objects were made. Using these resources therefore required organised networks linking mining, transport, trade and processing.
Metal thus transformed more than tools: it also helped develop exchanges between communities and the specialisation of crafts.
The Iron Age
The Iron Age began gradually as different civilisations learned to produce and work iron effectively.
This development presented a considerable technical challenge. Pure iron has a melting point of approximately 1,538 °C, compared with around 1,085 °C for copper. In fact, early ironworkers generally did not melt the iron completely: they used processes that reduced the ore and then worked the resulting metal by forging.
The Iron Age therefore did not begin simultaneously everywhere in the world. Its emergence depended, among other things, on available resources, exchanges between communities and mastery of furnaces and metallurgical processes.
Over time, the ability to control iron and its carbon content would lead to the development of a material that now plays a central role in construction: steel.
From iron to steel
Unlike iron, steel is not a pure metal but a family of iron–carbon alloys.
The carbon content of steels generally ranges from approximately 0.02% to 2% by mass. Although this proportion may seem very small, it strongly influences the material’s properties.
In general, increasing the carbon content makes steel harder and stronger, but also tends to reduce its ductility and can make welding more difficult.
Other elements can be added to obtain particular properties: chromium, nickel, manganese, molybdenum, silicon and vanadium, among many others.
This explains the wide variety of steels available today. Some are designed for easy forming and welding, while others are intended to withstand substantial mechanical loads, wear, high temperatures or corrosion.
Steel for a metal structure is therefore not selected solely for its strength. A range of physical and mechanical properties suited to the intended use must be considered.
A brief history of welding
Joining metals is almost as old as metallurgy itself. Long before electricity, blacksmiths discovered that heating two pieces sufficiently and hammering them together could create a lasting bond: this is forge welding, performed without completely melting the pieces.
Modern welding developed particularly from the nineteenth century onwards, with advances in electricity and the electric arc. An arc concentrates a large amount of energy over a small area, quickly reaching the temperatures needed to melt steel. In technical language, welding refers to the joining operation and a weld to the resulting joint.
The twentieth century saw the development of the main processes still used today: manual metal arc welding, TIG, and then MIG/MAG. Mechanisation and robotics subsequently expanded their applications.
What happens during welding?
In the fusion welding processes commonly used for steel, the surfaces of the pieces are heated locally until they melt. They form a weld pool, to which filler metal may be added. Other processes, such as forge welding or friction welding, join materials in the solid state.
As the heat source moves forward, the metal cools and solidifies behind it to form the weld bead.
The effect is highly localised: a small region rapidly heats from ambient temperature to melting, while the neighbouring metal is heated without melting. The unmelted part whose structure and properties are changed by this thermal cycle is called the heat-affected zone (HAZ).
The electric arc and plasma
In MIG/MAG, TIG and manual metal arc welding, heat comes mainly from an electric arc established between an electrode and the workpiece.
The electric field and temperature ionise the gas between them: electrons separate from atoms. This mixture of charged and neutral particles allows current to flow and is called plasma.
The current sustains temperatures of several thousand degrees within the arc, above those needed to melt steel, which depend on its composition.
Plasma is also found in stars, including the Sun: it is the most widespread state of visible matter in the Universe.
Why does a weld need protection?
Melting steel presents a challenge: at high temperatures, the metal becomes particularly reactive with its surroundings.
Air contains approximately 21% oxygen and a large amount of nitrogen. Their interaction with the weld pool can cause oxidation, porosity, inclusions or other changes that may impair the weld’s properties.
The metal must therefore be heated while the weld pool is protected from the atmosphere. This is one role of shielding gases in MIG/MAG and TIG welding. In manual metal arc welding, the electrode coating produces protective gases and slag that covers the deposited metal.
The choice of gas distinguishes MIG, which uses an inert gas, from MAG, which uses an active gas. Both use a wire electrode that melts into the pool. TIG uses a non-consumable tungsten electrode; any filler metal is added separately. These processes are described in the glossary.
Some essential properties of steel
Mechanical strength
A material’s ability to withstand loads without breaking. A metal component may be subjected to tension, compression, bending, shear or torsion.
Elasticity
The material’s ability to return to its original shape after a load is removed. As long as it remains within the elastic range, the deformation is reversible.
Yield strength
The stress at which the material begins to undergo permanent deformation. This is a particularly important property in the design of metal structures.
Did you know? A car that crumples can offer its passengers better protection.
A strong car was long associated with bodywork that barely deformed. Yet a car showing little deformation after a crash does not mean its occupants were well protected: a very abrupt stop can subject them to severe deceleration.
Modern cars have crumple zones at the front and rear. During a collision, some components in these zones are loaded beyond their yield strength and deform permanently. This plastic deformation absorbs part of the crash energy and extends the time over which the vehicle slows down, helping to reduce the forces experienced by occupants alongside seat belts and airbags.
The principle is to combine zones that can deform with a very strong passenger compartment. The aim is therefore not simply to use a low yield strength everywhere: component geometry, thickness, steel strength and ductility are selected to suit the role of each zone.
Ductility
A material’s ability to deform plastically before breaking. A ductile steel can undergo considerable deformation before fracture, unlike a brittle material, which can break with relatively little deformation.
Hardness
A material’s ability to resist local deformation, indentation or wear. Hardness should not be confused with mechanical strength or toughness.
Toughness
A material’s ability to absorb energy and deform before breaking. It combines strength and the capacity for deformation and plays an important role when a component may be subjected to impacts.
Weldability
A material’s suitability for joining by welding while maintaining satisfactory properties in the component and the welded zone. Not all steels have the same weldability.
Corrosion resistance
A material’s ability to resist chemical or electrochemical reactions with its environment. Carbon steel can, for example, be protected by painting, powder coating or galvanising, while the corrosion resistance of certain stainless steels is notably due to their chromium content.
A short glossary of metallurgy
Alloy
A material made up of several chemical elements, at least one of which is a metal. Steel is primarily an alloy of iron and carbon, while bronze is primarily an alloy of copper and tin.
Iron
The chemical element with the symbol Fe. It is the main constituent of steels.
Steel
A family of alloys whose main constituent is iron, containing a controlled amount of carbon and potentially other alloying elements.
Cast iron
An iron–carbon alloy with a higher carbon content than steels. Its properties and manufacturing methods differ from those of the steels commonly used in metal construction.
Metallurgy
The techniques and knowledge involved in extracting, producing, processing and studying metals and their alloys.
Forging
A process that deforms a metal, generally when hot, by applying mechanical forces.
Rolling
A process in which metal passes between rolls to change its shape or thickness. Many sections, sheets and products used in metal construction are produced by rolling.
Galvanising
A process that protects steel against corrosion by applying a layer of zinc.
Powder coating
A finishing process that generally involves applying a powder paint to a metal component before curing it with heat. It provides a durable finish while helping to protect the component.
Welding and weld
Welding is the operation that permanently joins pieces using heat, pressure or both, with or without filler metal. A weld is the resulting joint.
Forge welding
Joining heated metal pieces by applying pressure, particularly through hammering, without completely melting them. It is one of the oldest welding processes.
Electric arc
A sustained electrical discharge through an ionised gas between two conductors. In arc welding, it provides the heat needed to melt the metal.
Plasma
A state of matter consisting of a partly or fully ionised gas containing ions and free electrons. It is the most widespread state of visible matter in the Universe, notably in the Sun and other stars. Read more about plasma.
Electrode
An element that conducts current to the arc. It melts in MIG/MAG and MMA welding, but remains non-consumable during normal TIG operation.
Filler metal
Metal added to the joint during welding, for example as wire or rod. Its composition is selected to suit the workpieces and the required properties.
Weld pool
The volume of molten metal formed during welding, originating from the workpieces and, where used, the filler metal.
Weld bead
The trail of solidified metal left by a welding pass. Several passes may be needed to complete a joint.
Heat-affected zone (HAZ)
The part of the base metal that has not melted but whose structure and properties have been altered by the heat of welding.
Shielding gas
Gas that protects the weld pool from the surrounding air. It may be inert, such as argon, or active, such as a mixture of argon and carbon dioxide (CO₂).
Slag
A non-metallic layer formed from flux or electrode coating that protects deposited metal in certain processes. It is removed after cooling and between passes where necessary.
MMA — Manual Metal Arc
Manual arc welding using a consumable coated electrode. The coating provides gas shielding and forms slag. Also known as SMAW or stick welding.
MIG — Metal Inert Gas
Arc welding using a continuously fed wire electrode and an inert gas, usually argon, helium or a mixture of both. Commonly used for aluminium and other non-ferrous metals.
MAG — Metal Active Gas
The same continuous wire electrode principle as MIG, but with an active gas, such as an argon–CO₂ mixture. Widely used for steels. MIG and MAG belong to the GMAW family.
TIG — Tungsten Inert Gas
Arc welding under inert gas using a non-consumable tungsten electrode. Any filler metal is added separately. Valued for precise weld control; also known as GTAW.
Explore the processes further: TWI — arc welding, MIG/MAG and the differences between MIG and TIG.