What the O Temper Designation Covers
In the international temper classification, O designates the annealed condition: a wrought product that has been heated to complete recrystallisation and then cooled so that the work hardening left by rolling or stretching is removed. The designation is defined alongside the H (strain hardened) and T (thermally treated) families in ASTM B601 and in GB/T 16475, and it is the only family applied without any deliberate cold reduction after the final anneal. Plate supplied as 1060-O, 3003-O, 5052-O or 5083-O is therefore certified at the lowest strength and the highest ductility the alloy can deliver, together with the anneal cycle used to reach that condition.
Annealing Practice and Microstructure
A full anneal for non-heat-treatable alloys sits in the 340-400 C window: hot enough for deformed grains to recrystallise, cool enough to avoid the grain growth that roughens surfaces during deep drawing. Soaking time is short, typically one to three hours at temperature for plate up to 6 mm, and is followed by controlled cooling. Magnesium-bearing grades such as 5052 and 5083 are cooled faster than commercially pure grades, because slow passage through the 150-250 C range can precipitate beta phase at grain boundaries and reduce both corrosion resistance and formability. The finished structure is equiaxed with a low dislocation density, which is what makes O temper plate behave predictably in drawing, spinning and bending.
Mechanical Property Limits of Common O Temper Alloys
The table below summarises ranges normally certified for sheet and plate under the ASTM B209 series. Values are typical for fully annealed material and are quoted as alloy ranges rather than as single guaranteed figures.
| Alloy and temper | Tensile strength (MPa) | Minimum elongation (%) | Full anneal range (C) | Typical application |
|---|---|---|---|---|
| 1060-O | 60-95 | 25 | 340-400 | Deep drawn parts, heat shields, transformer strip |
| 3003-O | 95-130 | 20 | 340-400 | Formed components, packaging, radiator tanks |
| 5052-O | 175-225 | 16 | 340-360 | Fuel tanks, hydraulic panels, marine trim |
| 5083-O | 275-350 | 16 | 340-360 | Hulls, cryogenic vessels, welded structures |
Composition explains the spread. 1060 is unalloyed with 99.60% minimum aluminium; 3003 carries 1.0-1.5% manganese; 5052 contains 2.2-2.8% magnesium with 0.15-0.35% chromium; 5083 raises magnesium to 4.0-4.9% with 0.40-1.0% manganese. Because the O temper removes work hardening instead of changing the alloy, the strength ranking of these grades never changes - only the gap between them widens.
Where O Temper Plate Is Specified
O temper is chosen when forming severity rather than strength governs the design. Typical cases are prototypes and inner panels in automotive tooling trials, spun components such as reflectors and vessel ends, deep drawn housings, and 5083-O plate for shipbuilding and cryogenic service where notch toughness and weldability matter more than yield strength; ASTM B928 is the reference specification for the higher-magnesium sheet and plate grades in marine work. For heat-treatable alloys, annealed material is usually an intermediate step only, delivered as O before solution treatment and ageing.
Purchasing and Tolerancing Notes
Annealing adds roughly 10-15% to conversion cost, which is normally recovered through lower scrap in downstream forming. Buyers should state alloy, temper, thickness and tolerance, width, edge condition and the applicable standard; dimensional tolerances for sheet and plate are listed in the relevant part of the ASTM B209 series or in the EN 485-3 and EN 485-4 parts for European supply, and flatness is inspected with the material at rest. Soft material marks easily, so edge protection, interleaving and vacuum lifting belong in the purchase agreement. Because the anneal does not strengthen the alloy, moving from H or T temper to O is a redesign decision rather than a substitution: the part must be re-qualified against the lower proof stress.
Frequently Asked Questions
Q: Does O temper mean the plate is at its lowest possible strength?
A: Yes. Full anneal removes work hardening and any precipitation strengthening, so the mill certificate shows the minimum condition of that alloy. Intermediate soft conditions are designated H1x or H2x tempers instead, which retain part of the strain hardening.
Q: Is O temper the same as H temper?
A: No. O is fully annealed with no target strain hardening; H tempers are strain hardened and optionally stabilised, and are certified to higher strength ranges for the same alloy.
Q: Which alloy should be specified for a marine structure in the soft temper?
A: 5083-O to ASTM B928 for hulls and welded structures, or 5052-O for lighter panels and trim. Both resist chloride attack better than 3003-O and retain ductility at low temperature.
Q: How is O temper evidenced on a mill certificate?
A: By the temper designation, the anneal cycle, the chemical analysis against the alloy specification and mechanical tests taken from the delivered lot, usually one tension test per lot and size.
Q: Can O temper material be welded?
A: Yes. Non-heat-treatable alloys such as 1060, 3003, 5052 and 5083 do not age harden, so the heat affected zone softens less than in a heat-treatable alloy and the joint retains most of the parent strength when the correct filler is used.
Q: Why does 1060-O show higher elongation than 5052-O?
A: Unalloyed aluminium recrystallises to a coarse, soft structure with few obstacles to dislocation movement, while magnesium in solid solution in 5052 still raises strength and restricts elongation.