From Ingot to Deck Plate: the Process Behind the Plate You Order
Two 5083 plates with the same certificate can behave completely differently in a hull. The difference is made in six process steps — and it is readable, if you know what each step does. This page walks through how 5xxx marine plate and sheet are actually made at the mills we source from, and what our QC team checks before a container is loaded.
What a Certificate Shows — and What It Does Not
Exfoliation resistance in the H116 temper, flatness that survives welding, edges that do not start cracks in a formed bracket — none of these show up as a single number on an MTC. They are decided upstream: how the mill casts and homogenizes the ingot, how the rolling schedule is set, and whether the plate is stretched or simply stacked.
- For buyers: knowing the route tells you which questions move price and which move risk
- For QC departments: each step below lists what should be verifiable on the documents you receive
- For designers: temper and stress-relief choices at order stage decide how the plate behaves in your fabrication shop
Six Steps, and What We Verify in Each
Material is sourced from certified mills running the equipment described below. What makes the difference for buyers is not the hardware alone — it is knowing what each stage contributes, and what should be checked before the plate ships.
STEP 01 Melting & Casting
The route starts with clean metal. Large rolling ingots — typically in the 10–30 tonne range — are cast from melt that has been degassed and filtered, then scalped and sawn ready for rolling. Ingot mass matters: bigger ingots allow heavier, more consistent plate programmes, while melt practice — hydrogen is typically held below about 0.15 ml per 100 g of aluminium, and inclusions are removed by filtration — decides whether internal soundness problems exist at all.
Hydrogen and inclusions that survive melting become laminations and porosity in heavy plate — defects no amount of downstream rolling can remove, and exactly what ultrasonic inspection is specified to catch.
Heat chemistry on the MTC matches the order grade; for orders where internal soundness is critical, UT testing is arranged at production stage, not at arrival.
STEP 02 Homogenizing
As-cast ingots are soaked in homogenizing furnaces, typically around 480–540 °C for several hours up to more than a day. This diffuses alloying elements — magnesium and manganese in the 5xxx series — into a uniform distribution and relaxes cast stresses, conditioning the ingot so it can take heavy hot-rolling reduction without cracking.
A shortened or uneven soak shows up as mechanical properties that differ between plates from the same heat, and as edge cracking during rolling.
Mechanical values on the MTC must be consistent across the heat, not just pass on one test coupon. Outliers in elongation or yield are flagged back to the mill before dispatch.
STEP 03 Hot Rolling — Including Thick Plate
The conditioned ingot is rolled hot, typically between about 400 °C and 500 °C, through reversing stands — wide four-high mills for heavy plate, narrower trains for sheet and coil stock. Each pass breaks down the cast structure into a fine, directional wrought grain. This is also where hot-rolled tempers such as H112 take their character.
The hot-rolling schedule sets the grain structure your welder and surveyor inherit. Hull plating needs consistent through-thickness properties — a product of controlled reduction, not just final thickness.
Thickness and width against the order and ASTM B209 / EN 485-2 tolerances; standard compliance confirmed on the paperwork, with DNV-recognised grades for 5083, 5086 and 5754 where specified.
STEP 04 Cold Rolling — Sheet & Coil
Below the hot-rolled stage, material passes to tandem and single-stand cold mills, where final gauge, and the work hardening that most H-tempers are built on, are achieved. Gauge control here is what makes batch-to-batch sheet feel identical in an automated shop.
Cold reduction is also strength: the difference between an H14 and an H116 sheet is written into how much cold work and what stabilization follows. Superstructure and interior sheet work depends on that repeatability.
Gauge along the full coil or sheet, not a single mid-coil reading; temper designation on the MTC matches what was ordered — a mismatch here is the classic silent substitution error.
STEP 05 Heat Treatment & Temper Control — Where H116 and H321 Are Made
H-tempers in the 5xxx series are strain-hardened and then stabilized in furnace lines. Marine tempers also carry corrosion requirements: exfoliation resistance assessed per ASTM G66 and intergranular corrosion mass loss per ASTM G67, as called up by the ASTM B928 marine specification. It is the stabilization treatment that lets the temper hold those properties through forming and welding heat.
Temper sets exfoliation resistance, formability and how the plate behaves around welds. It is also the line most often mis-specified on a marine order.
Mechanical results on the MTC sit inside the specified temper range for the ordered designation — and the designation on the certificate is the one on the order, with plate markings checked against the paperwork.
STEP 06 Stretching, Sawing & Final Check
Heavy plate is stretcher-levelled: a controlled permanent elongation of roughly 1–3 % releases residual rolling stress and locks in flatness. Plate is then sawn to length, and every unit goes through final dimensional, surface and edge checks before packing.
Residual stress only shows itself once the plate is cut out: panels released from unstretched plate can move on the cutting table. Stretcher-levelled plate holds its geometry through cutting and welding.
Our QC team checks dimensions, flatness, surface and edge condition against the order; plate markings (grade, temper, heat, standard) are photographed and matched to the document pack.
Temper, Production Route, and Where Each One Fits
The same alloy can be delivered into very different mechanical states. This is the map we use when reviewing a customer drawing — and the version of it worth keeping next to your material spec.
| Temper | How it is produced | Where it typically fits | Confirm on the MTC |
|---|---|---|---|
| O | Fully annealed — recrystallized, softest condition | Deep forming work, tank and compartment parts that need maximum ductility before fabrication | Tensile and elongation within O ranges |
| H112 | Hot worked with limited control of work hardening | General structural plate where moderate strength is acceptable | Mechanical values stated for H112 |
| H116 | Strain-hardened, with stabilization and corrosion-control requirements targeting exfoliation resistance | Hull and deck plating in seawater contact — 5083, 5086 | Temper designation + mechanical range; exfoliation-related requirements where specified |
| H321 | Strain-hardened and thermally stabilized to a defined strength range | Hull structure where drawings specify H321 — commonly 5083 / 5086 | Strength within the H321 band for the alloy and product form |
Guidance only — the drawing, classification rules and the mill test certificate for your specific order are the deciding documents. Every batch carries a mill test certificate issued by the producing mill; always confirm final values against it. Note on 5754: it is normally supplied in O, H111 or H22–H24 for marine work; the H116 and H321 designations apply to the high-magnesium marine grades such as 5083 and 5086.
The Pre-Shipment Verification Checklist
This is the checklist our QC team works through before packing — the same list you can hold us to when reviewing arrival condition.
Thickness, width and length measured against the order and the agreed tolerance class — not against memory.
Checked against the applicable ASTM B209 / EN 485-2 requirements, or the tighter value written into your order.
Visual inspection for handling marks, edge damage from sawing, and condition consistent with the specified finish.
Grade, temper, heat number and standard legible on each plate or pack — and photographed before loading.
MTC heat numbers matched against physical markings; order lines, quantities and sizes reconciled.
Ultrasonic testing or third-party witness inspection arranged at order stage and documented before dispatch.
Process & Specification FAQ
Is H116 the same as H321?
They sit in similar strength bands but are produced by different routes. H321 is strain-hardened and then thermally stabilized; H116 is strain-hardened under the additional controls the marine specification calls for. When marine corrosion performance is specified, the material is supplied to ASTM B928 requirements. Which designation is correct depends on the alloy and on what your drawing and classification rules state.
Which grades carry DNV recognition?
Material is supplied to ASTM B209 / EN 485-2, with DNV-recognised marine grades for 5083, 5086 and 5754. The rest of the range is supplied against its ASTM and EN standards. If your project needs class paperwork, tell us at enquiry stage so the right route is confirmed before ordering.
Can plate be supplied with ultrasonic testing?
Yes, by arrangement at order stage. Ultrasonic inspection is typically specified for thicker structural plate where internal soundness matters. Give us the acceptance class your project requires and it is written into the order before production — see the service model for how order confirmation works.
How flat can marine plate be supplied?
Flatness follows ASTM B209 / EN 485-2 tolerances as standard. Tighter flatness is achievable for plate that has passed through stretcher leveling — agree the value at order stage and it becomes a verified check before shipment.
Grade, Temper and Checks — Confirmed Before Order
Send your drawings and application — we will confirm grade, temper and verification steps in writing before anything is produced.
