Forging
Deform it solid, and the grain flows with the shape instead of being cut through — which is why the critical parts of a machine are forged and not cut.
Metal is shaped in the solid state by compressive force, hot or cold. The immediate effect is a shape; the metallurgical effect is the reason it is used.
Forging breaks up the coarse structure a casting solidified into, closes internal porosity, and — most importantly — leaves a grain flow that follows the contour of the part. A machined component has its grain cut through wherever the tool went; a forged one has it running around the fillet where the stress concentrates. In fatigue, which is how most loaded components actually fail, that difference is large and repeatable.
That is why connecting rods, crankshafts, turbine discs, aircraft landing gear and hand tools are forged, and why a forged part is generally the more expensive and the one specified where failure is not acceptable.
Uses
Crankshafts, connecting rods and gears; turbine discs, which are among the most highly stressed components made; landing gear; hand tools, spanners and hammers; fasteners, which are cold-forged at enormous volume; and the forged billets that machined aerospace components start from.
Open-die forging for one-offs and very large sections; closed-die for volume, where the tooling cost buys near-net shape and little machining afterwards.
History
The blacksmith's process, and the oldest metalworking there is — copper was hammered before it was ever melted, and cold-worked native copper predates smelting by a considerable margin.
The whole of pre-industrial ironwork is forging, because bloomery iron was never liquid: the bloom came out of the furnace as a spongy solid mixed with slag, and hammering was how the slag was expelled and the metal consolidated. Wrought iron's fibrous structure and its resistance to sudden failure both come from that.
The steam hammer of 1839 is what made forging industrial, and what made large marine and railway components possible.
Economic significance
Expensive tooling and cheap parts, in the same shape as moulding: a closed die costs a great deal and is worth it over volume. Where it wins outright is in parts whose fatigue life is the product's life, because there the alternative is not a cheaper part but a heavier one.
A very small number of facilities worldwide can forge the largest sections — turbine rotors, reactor pressure vessel components — which makes those a genuine supply constraint on power generation projects.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
takes as input
- Ti-6Al-4V — alloy · into landing gear, engine components and the billets that airframe fittings are machined from
- Steel — alloy · for crankshafts, connecting rods and anything whose fatigue life is the product's life
- Wootz steel — alloy · and the forging is what aligns the carbide bands into the visible watered pattern — the pattern is made by the smith and out of structure that was already there
is used in
- Aerospace manufacture — industry · turbine discs and landing gear, among the most highly stressed components made
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing