Welding

Two components are joined by melting them together, usually with a filler of similar composition, so the joint is continuous metal rather than a mechanical connection. It made the all-welded ship, the pressure vessel and the modern steel frame possible, because a welded joint can be as strong as the parent metal and a riveted one cannot.

The part that matters metallurgically is not the weld. It is the heat-affected zone: the band either side that did not melt but was heated far enough to change, and which now has a structure nobody designed. It can be harder and more brittle than the parent metal, softer than it, or sensitised to corrosion, depending on the alloy and the cooling rate — and it is where welded structures fail.

A welded structure is also continuous in a way a bolted one is not, which is exactly why a crack in one can run a long way. The Liberty ships that broke in half in cold water are the standing lesson: brittle steel, and a welded hull that gave the crack an uninterrupted path.

Ships, pipelines, pressure vessels, structural steelwork, vehicle bodies and almost every fabricated metal assembly. Arc processes dominate by tonnage; resistance spot welding assembles car bodies at a few thousand welds each; friction stir welding joins aluminium without melting it and is what made welded aluminium aerospace structure practical.

Weldability is a real and separate material property. It is why most structural steel is low-carbon, why 2000 and 7000 series aluminium alloys are largely riveted or bonded rather than welded, and why titanium is welded under inert gas or not at all.

Forge welding — hammering hot iron together — is ancient. Fusion welding is not: the carbon arc dates from the 1880s, coated electrodes from the 1900s, and the process only became structurally trusted between the wars.

The Second World War is the turning point, in the American Liberty ship programme, which welded rather than riveted in order to build fast. It worked, and it also produced the field's defining failure: hulls cracking in cold northern water, which is what taught the industry about the ductile-to-brittle transition and made fracture toughness a design property rather than a curiosity.

It removed weight and labour from every large steel structure at once. A riveted joint needs overlapping plate, holes that reduce the section and a gang to close each rivet; a welded one needs none of that.

It also created an inspection industry. Because the joint's integrity is invisible and consequential, radiography, ultrasonic testing and welder qualification are all requirements of codes rather than optional care, and the certification of welders is one of the few places where an individual's skill is formally a structural safety control.