Heat treatment
Change the properties without changing the composition — the reason two pieces of identical steel can differ by a factor of five in hardness.
Metal is heated and cooled on a controlled schedule to change its internal structure. Nothing is added and nothing is removed: the composition before and after is the same, and the properties can differ by a factor of five.
This is the single most important fact about engineering alloys and the one most often missed. A steel's carbon content sets what it *can* become; the heat treatment sets what it *is*. Quench a medium-carbon steel and it is glass-hard and brittle; temper it and it is a spring; anneal it and it is soft enough to machine — one alloy, three materials.
The mechanisms differ by family. Steel relies on a diffusionless transformation to martensite on rapid cooling; aluminium, nickel and titanium alloys rely on precipitation hardening, where a solution is quenched and then held warm so a fine second phase forms and obstructs dislocation movement. Both are ways of making it harder for the metal to deform internally.
Uses
Hardening and tempering of tools, springs, bearings, gears and cutting edges. Annealing to soften metal between forming operations, and stress relief after welding and machining. Solution treatment and ageing of aluminium, nickel and titanium alloys — the T6 in an aluminium temper designation is exactly this. Case hardening, which gives a gear a hard surface and a tough core by changing only the composition of the outermost skin.
Also normalising to refine the grain of a casting or forging, and the controlled cooling that decides whether a cast iron ends up grey, white or ductile.
History
Empirical for at least three thousand years and understood for barely one. Quench hardening of steel was known across the Iron Age world and passed down as procedure — quench in oil, in urine, in the body of a slave in one particularly persistent classical instruction — with no theory whatsoever behind it.
The explanation arrived with metallography from the 1860s onward and the iron-carbon phase diagram at the turn of the twentieth century. Precipitation hardening in aluminium was discovered accidentally by Alfred Wilm in 1906, who left a quenched alloy over a weekend and found it had hardened by itself; that observation produced Duralumin and, eventually, the all-metal aircraft.
Economic significance
It buys properties with time and energy rather than with alloying elements, which is usually far cheaper — a plain carbon steel correctly heat-treated outperforms an expensively alloyed one that is not.
It is also where a great deal of manufacturing goes wrong: distortion, quench cracking and decarburisation are heat treatment problems, they appear at the end of a part's manufacturing route when all the value has been added, and they scrap finished components rather than raw stock.
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
- 6000 series aluminium alloy — alloy · solution treated, quenched and aged to T6, which roughly doubles the strength of the annealed material
- 7000 series aluminium alloy — alloy · solution treated and then aged either to peak strength or deliberately past it, trading perhaps a tenth of the strength for resistance to stress corrosion cracking
- Ti-6Al-4V — alloy · annealed, or solution treated and aged where the extra strength is worth the reduced ductility
- Tool steel — alloy · the entire point of the family — supplied soft, machined, then hardened and tempered, because a steel hard enough to cut with cannot itself be cut
- High-speed steel — alloy · hardened from around 1200 °C and tempered two or three times, each temper transforming the austenite the previous one left behind
- Steel — alloy · and the same steel becomes a spring, a cutting edge or a machinable bar depending only on the schedule
- Nitinol — alloy · shape setting is a heat treatment — constrained in the desired geometry at around 500 °C, and that is the shape it remembers
- Invar — alloy · stress relieved and thermally cycled, because residual machining stress relaxes over time and moves the part, which defeats the entire purpose
- Electrical steel — alloy · a box anneal of more than a day above 1,100 °C, during which a few correctly oriented grains grow enormously and consume the rest
is used in
- Automotive manufacture — industry · gears, shafts and springs, and case hardening to give a gear a hard face and a tough core
produces
- Carbon fibre composite — material · laid up as fibre and resin and cured under heat and pressure, usually in an autoclave — and the cure schedule is as much of the specification as the fibre is
- Glass fibre composite — material · the same, and mostly without the autoclave, which is why it costs a fraction as much
- Plywood — material · veneers peeled from a log, laid with the grain of each at right angles to the last, and pressed with adhesive under heat
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing