One Alloy Family, Two Stabilised Tempers
5083 is a non-heat-treatable aluminium-magnesium alloy with a nominal magnesium content of 4.0-4.9%. Its strengthening phases cannot be dissolved and re-precipitated, so 5083 plate is strengthened by cold work and then thermally stabilised. 5083-H32 and 5083-H321 therefore share identical chemistry and are separated only by the amount of cold reduction applied before stabilisation and by the stabilisation cycle itself. That difference controls yield strength, formability and corrosion performance in service.
Typical composition of both tempers, in mass percent:
Magnesium (Mg): 4.0-4.9%
Manganese (Mn): 0.40-1.0%
Chromium (Cr): 0.05-0.25%
Iron (Fe): 0.40% maximum
Silicon (Si): 0.40% maximum
Copper (Cu): 0.10% maximum
Zinc (Zn): 0.25% maximum
Titanium (Ti): 0.15% maximum
How Each Temper Is Produced
Both tempers are strain hardened and stabilised, but the cold reduction differs. 5083-H32 is cold rolled to a quarter-hard condition, which gives the higher guaranteed yield strength. 5083-H321 is held at a lower cold reduction with a controlled stabilisation soak, so the plate keeps a balanced combination of strength and resistance to intergranular and exfoliation corrosion under sustained load. The marine temper 5083-H116 defined in ASTM B928 follows the same philosophy, which is why H321 and H116 plate behaves similarly in seawater structures.
Stabilisation is not solution heat treatment. It is a controlled low-temperature soak applied after the final cold pass that relaxes rolling stress and fixes the temper. If it is under-run, properties drift with time and corrosion behaviour is no longer predictable.
Mechanical Properties Side by Side
| Property | 5083-H32 | 5083-H321 |
|---|---|---|
| Tensile strength, Rm | 305-380 MPa | 305-385 MPa |
| 0.2% proof strength, Rp0.2 | 250 MPa minimum (EN 485-2) | 215 MPa minimum (ASTM B209) |
| Elongation, A50 mm | 12% minimum | 12% minimum |
| Cold work level | Quarter hard | Lower, corrosion optimised |
| Typical use | General fabrication, transport structures | Hulls, decks, superstructures |
A widely repeated claim is that 5083-H321 carries a higher yield strength than 5083-H32. The published minimum proof strengths point the other way: H32 holds the higher guaranteed yield value, while H321 is chosen for sea service because of its corrosion behaviour rather than a higher strength floor.
Corrosion Behaviour in Marine Service
Magnesium-bearing 5083 is sensitive to sensitisation, the precipitation of beta phase at grain boundaries after prolonged exposure to elevated temperature, for example during welding. Sensitised plate can suffer intergranular and exfoliation corrosion, so marine tempers are qualified by laboratory tests: ASTM G67 (NAMLT) measures mass loss from intergranular attack, and ASTM G66 (ASSET) assesses exfoliation resistance. For marine grades the accepted limit is a NAMLT mass loss no greater than 15 mg/cm2.
H321 and H116 plate keeps a fine, recrystallisation-controlled grain structure and a stabilised matrix, so it tolerates long immersion, splash-zone cycling and repeated weld thermal cycles better than a merely strain-hardened temper of the same strength level.
Forming, Welding and Selection
H32 bends and rolls predictably across a wide thickness range; H321 is formed with generous bend radii and checked for property drift in critical parts.
Both tempers weld readily by MIG and TIG with ER5183 or ER5356 filler, using controlled heat input to limit sensitisation in the heat-affected zone.
Machining produces a gummy chip, so sharp tooling, firm feed rates and generous coolant keep surface finish consistent.
Specify H321 or H116 for hull plating, decks, bulkheads, tanks and other permanently immersed structures.
Specify H32 for general plate work where higher guaranteed yield strength, tighter price and easy forming matter more than sustained seawater exposure.
Keep the mill certificate, heat number and temper designation traceable to the delivered plate, and request impact data for low-temperature service.
Frequently Asked Questions
Q: Is 5083-H32 stronger than 5083-H321?
On minimum guaranteed values, yes. H32 normally carries the higher minimum 0.2% proof strength, while H321 is selected for corrosion performance in permanent marine service.
Q: Can 5083-H321 be heat treated to increase its strength?
No. 5083 is not heat treatable. Strength comes from cold work, and thermal cycles can only anneal or stabilise the product, which lowers rather than raises strength.
Q: Which temper should be used for a boat hull?
H321 or H116. Their stabilised structure and tested corrosion resistance are specified for hull plating, decks and superstructures in continuous seawater contact.
Q: Does stabilisation remove all residual stress?
No. It reduces and stabilises the rolling stress pattern. Where dimensional stability is critical, a stress-relieved condition should be specified and its acceptance method agreed in advance.
Q: Are 5083-H321 and 5083-H116 interchangeable?
They share the same alloy and a similar stabilisation route, but H116 is defined with mandatory corrosion testing for marine service, so substitution should be checked against the design specification.
Q: What thickness range is normally available?
5083 plate is commonly supplied from about 3 mm upward to heavy plate for marine and tank work, in widths and lengths sized to the fabrication drawing.
