5083-H116 vs 5083-H321: Marine Aluminum Selection Guide

Few specification decisions in shipbuilding generate as many emails as the temper line on a 5083 plate order. The alloy is the same, the standard is the same and the certificate looks nearly identical - yet H116 and H321 are processed differently, and a substitution made because the properties are the same can invalidate a class approval. This article separates what actually differs between 5083-H116 vs 5083-H321 from what only appears to.

Both tempers are strain-hardened marine conditions of the same Al-Mg4.5 alloy, covered by ASTM B928 and accepted by DNV, ABS, LR, BV, CCS and the other classification societies. The minimum mechanical properties are identical: 305 MPa ultimate, 215 MPa yield, 10% elongation on 3-50 mm plate. What differs is the processing route - and one route adds a low-temperature stabilization treatment that changes how the material behaves in the shipyard and across a 25-year service life. Conglin Aluminum-Tech has produced marine aluminum profiles since 1992 under DNV certification; here is how the temper decision looks from the production floor.

I. Marine Aluminum Temper Selection: What H116 and H321 Actually Mean

1. Same Alloy, Different Processing History

The chemistry is identical in both conditions: magnesium 4.0-4.9%, manganese 0.40-1.0%, chromium 0.05-0.25%, copper limited to 0.10%. 5083 cannot be strengthened by heat treatment, so every MPa of strength comes from magnesium in solid solution plus cold work. That is why the temper designation - not the alloy - carries the engineering information.

H116 is strain-hardened with controlled cold work and defined exfoliation-corrosion requirements, delivered without a separate stabilization soak. H321 is strain-hardened and then given a low-temperature stabilization treatment that moves the microstructure toward a stable point on the strain-hardening curve. Neither temper is stronger; they are two processing routes arriving at the same mechanical floor.

2. ASTM B928: The Standard Governing Both Tempers

ASTM B928 marine aluminum plate is the governing specification for high-magnesium 5083, 5086 and 5456 plate, cross-referenced in practice to EN 485, ASTM B209 and ISO 12215-5 for small-craft scantlings. B928 requires both H116 and H321 to pass exfoliation-corrosion testing per ASTM G66 (ASSET) plus an intergranular corrosion test with maximum attack depth held to grade 3 or better.

That requirement is not decorative. Magnesium above roughly 3.5% makes 5xxx alloys vulnerable to beta-phase precipitation on grain boundaries when the metal sits warm, which is exactly the failure mode the marine tempers exist to control. A 5083 product supplied in H112 or ordinary H111 does not carry the same guaranteed exfoliation resistance, which is why classification societies do not accept it for hull shell plating.

II. 5083 H116 Mechanical Properties and Service Behaviour

1. The Numbers: One Floor for Both Tempers

Under B928, 5083 H116 mechanical properties and 5083 H321 mechanical properties share the same minimums: 305 MPa ultimate, 215 MPa yield, 10% elongation across the 3-50 mm plate range. H321 is typically delivered with a slightly tighter ceiling, around 305-385 MPa, and marginally higher minimum elongation, reflecting the controlled strain-hardening-plus-stabilization route.

The wrong engineering reflex is to read identical minimums as interchangeable tempers. Minimums describe the worst plate either route may deliver, not the distribution around it. Stability during fabrication - how much the plate moves when it is cut, formed and welded - is where the routes genuinely diverge.

2. Where H116 Is the Right Call

H116 dominates thinner hull plate in North American practice and is the default temper on many US-build specifications. For hull bottoms, tank plating and any surface in long-term seawater immersion, its guaranteed 5083 aluminum exfoliation corrosion resistance per G66 testing is exactly what the designer wants, at slightly lower processing cost than the stabilized route.

It is also the more forgiving temper on schedule: one fewer thermal cycle between mill and certificate. Programmes that move fast and buy plate against ASTM-based drawings usually specify H116 for shell and decks, and stock it as the house standard.

III. 5083 H321: Stabilization and Heavy-Plate Fabrication

1. What Thermal Stabilization Changes

H321's low-temperature stabilization does three things the certificate cannot show: it reduces residual stress from the heavy cold-work pass, it makes the through-thickness microstructure more uniform, and it moves the material toward a state that does not age-soften in warm service. The consequence in the shipyard is plate that moves less when it is cut and welded.

That is why European and classification-society-driven specifications lean toward H321 for heavier structural plate. On thick sections and large welded assemblies - deckhouses, ramp doors, tank boundaries - the difference between a stabilized and a non-stabilized plate shows up as distortion control, not as a tensile number.

2. Forming, Residual Stress and Weld Distortion

For forming, the stabilized route gives H321 a modest edge in bendability and ductility at equal strength, which matters when panels are pressed or rolled to tight radii. H116 forms perfectly well, but fabricators running severe cold work on thicker plate report more springback variation and more straightening after welding.

Welding itself does not separate the tempers, which answers most questions about H116 vs H321 weldability: both are welded with 5356 or 5183 filler under the same qualified procedures, and both lose the same share of strength in the heat-affected zone, settling near the O-temper level across a 15-25 mm band. Post-weld design values, not the delivered temper, govern the joint - which is why societies qualify procedures per alloy, not per temper.

IV. 5083 Aluminum for Ship Hulls: Buying Without Regret

1. The 65 C Sensitization Envelope

The one limit both tempers share is thermal. Sustained exposure above roughly 65 C starts precipitating beta phase on grain boundaries - the mechanism behind 5083 sensitization 65 degrees warnings in every marine design guide - and the exfoliation resistance that B928 certifies erodes from there. Engine-room boundaries, exhaust plenums and deck plating over heated tanks need thermal isolation or a conversation with the surveyor.

H321's stabilization was designed partly for this: a microstructure already pushed toward equilibrium precipitates beta more slowly, so for service profiles that live near the top of the temperature envelope it is the conservative call. For ordinary hull service in the 0-40 C band, the two tempers behave the same.

2. Substitution: The One Mistake That Costs an Approval

Because the certificate minimums match, substitution requests arrive constantly - H116 offered against an H321 drawing because stock is available, or the reverse. On an approved drawing the answer is no. The temper was named by a naval architect or a class society for a reason, and swapping it re-opens the material approval, the weld procedure qualification and sometimes the scantling calculation.

When the specification is genuinely open - repairs, non-classed structures, small craft outside ISO 12215 scope - marine aluminum temper selection reduces to three practical questions: who wrote the drawing, which standard edition governs, and what the fabrication shop is set up to handle. Answer those three and the temper picks itself.

3. DNV Certified Marine Aluminum: Documents to Demand

Whatever the temper, the paperwork survives the survey longer than the plate. Demand mill test certificates per heat with G66 exfoliation results, class society endorsement where the project requires it, and traceability that connects billet to finished part. Certificates cannot be issued retroactively, which is why they are worth more than any discount.

Conglin Aluminum-Tech produces DNV certified marine aluminum with 30 years of 5xxx alloy experience, from casting through extrusion, surface treatment and MIG/TIG/FSW fabrication, backed by 15 lines from 1,000 T to 10,000 T. Marine profiles in 5083, 5086 and 6082 ship with full batch documentation as standard output; industrial and marine extrusion products are produced under one roof rather than brokered between subcontractors.

FAQ

Q1: Which is stronger, 5083-H116 or 5083-H321?

A: Neither. Under ASTM B928 both tempers carry the same minimums - 305 MPa ultimate, 215 MPa yield, 10% elongation on plate up to 50 mm. The difference is processing route and stability, not strength tier.

Q2: Can H116 be substituted for H321 on an approved drawing?

A: Not without re-approval. The temper on the drawing was chosen by the designer or the class society; swapping it re-opens the material approval and often the weld procedure qualification too. For non-classed work, choose by governing standard, forming severity and service temperature.

Q3: Why is 5083 restricted to about 65 C in service?

A: Above that temperature, beta phase starts precipitating on grain boundaries, eroding the exfoliation and stress-corrosion resistance the marine tempers guarantee. H321's stabilization slows the process; no temper survives sustained high heat unchanged.

CTA

If you are specifying 5083 plate or marine extruded profiles and need the temper decision documented properly - G66 exfoliation results, class endorsement and batch traceability - send the drawing package to the Conglin Aluminum-Tech team. Email: Eric@aluminum-pm.com. Thirty years of DNV certified marine aluminum production, in-house casting through fabrication, and 5083, 5086 and 6082 output sit under one roof in Dalian.

Conclusion

5083-H116 vs 5083-H321 is a processing-route decision wearing a mechanical-property disguise. The B928 minimums are identical, the G66 exfoliation guarantee applies to both, and neither temper is stronger than the other. What differs is stability: H321 arrives thermally stabilized and moves less in fabrication, while H116 arrives one thermal cycle earlier at slightly lower cost.

So let the drawing govern, keep substitution off the table, and treat the certificate package - exfoliation results, class endorsement, batch traceability - as part of the material itself. With 30 years of DNV certified marine alloy production behind it, Conglin Aluminum-Tech supplies 5083 in whatever temper the project specifies, with documentation that survives the survey.