The four basic tempers
F is as fabricated — no control over the thermal or strain-hardened condition. You will see it on some extrusions and it is generally not a specification, it is an admission that nothing was controlled. Avoid it where properties matter.
O is annealed: fully softened, maximum ductility, minimum strength. This is what you want for deep drawing and tight bending, and what you never want for a structural part.
H is strain hardened, and it only applies to non-heat-treatable alloys — the 1xxx, 3xxx and 5xxx families. Cold work raises strength and costs ductility, in that trade.
T is thermally treated, and it only applies to heat-treatable alloys — 2xxx, 6xxx, 7xxx and some 8xxx. Solution treatment, quenching and ageing develop the precipitate that carries the strength.
Reading the H codes
The first digit after H says what else was done. H1 is strain hardened only. H2 is strain hardened and then partially annealed, which recovers some ductility at some cost in strength. H3 is strain hardened and then stabilised — a low-temperature treatment that locks the properties in, which is what makes H3 tempers behave predictably when something downstream warms them up.
The second digit says how much. Hx2 is quarter hard, Hx4 is half hard, Hx6 is three-quarter hard, Hx8 is full hard, and Hx9 is extra hard. So H32 is stabilised quarter hard, H14 is strain-hardened half hard, and H18 is full hard — the temper used forfoil and for can bodies where stiffness matters and forming is shallow.
A third digit appears occasionally, as in H116 or H321. These were developed for marine 5xxx plate, and they carry requirements for corrosion performance in addition to the mechanical properties. H116 and H321 are not interchangeable with H32 on a hull, even though the strength numbers look similar.
Reading the T codes
T1 is cooled from a shaping process and naturally aged — the starting point for many extrusions. T4 is solution heat treated and naturally aged to a stable condition; it is the softest of the common T tempers and the one you form in. T6 is solution heat treated and artificially aged; it is the strong one.
T5 sits between them: cooled from shaping and then artificially aged, without a separate solution treatment. It is common on extrusions because the extrusion press exit temperature does the work of solution treatment if the alloy and the process are set up for it. T5 is weaker than T6 for the same alloy, and that is not a defect — it is what the process produces.
A fifth digit indicates stress relief. T651 is T6 plus stretching to relieve residual stress. If you are machining 6061 plate from solid and the part moves as you cut it, T651 is usually the answer; the stress was there before you started cutting.
What this means when you order
- Always give the temper. "6061" is not a specification; "6061-T6" is.
- If the part is formed, ask whether the temper should be the soft one and the hardening should happen afterwards.
- If the part is machined from thick plate, ask for the stress-relieved variant.
- If the part is marine, name H116 or H321 and say why. Do not accept a substitution on similar strength numbers.
- If you are unsure, tell us the process. Temper selection is a process question, not a catalogue question.
Comparison table
| Code | Meaning | Typical use |
|---|---|---|
| F | As fabricated, no property control | Some extrusions; avoid where properties matter |
| O | Annealed, softest, most ductile | Deep drawing, tight bending |
| H12 / H22 / H32 | Quarter hard (variants: as-worked / part-annealed / stabilised) | Formed panels, roofing, general sheet |
| H14 / H24 / H34 | Half hard | General fabrication, signage, tread plate |
| H16 / H26 / H36 | Three-quarter hard | Stiffer panels, limited forming |
| H18 / H38 | Full hard | Foil, can body, shallow-drawn parts |
| H116 / H321 | Marine variants with corrosion requirements | Hull and marine structure in 5xxx |
| T1 | Cooled from shaping, naturally aged | General extrusions |
| T4 | Solution treated, naturally aged | Form-then-age parts |
| T5 | Cooled from shaping, artificially aged | Extrusions hardened at press exit |
| T6 | Solution treated, artificially aged | Structural and machined parts |
| T651 | T6 plus stretch stress relief | Machining stock that must stay flat |
| T7 | Solution treated, overaged (stabilised) | Where dimensional stability outranks peak strength |
The temper code is where most ordering mistakes happen, and it is the cheapest thing to get right. Give us the process and the environment and we will name the temper on the quote rather than leaving it to be discovered at the bending brake.