GB 712 Grade AH40 shipbuilding steel plate

Technical Reviewer: Jack | SHH STEEL Material Solutions Department · Updated: 2026-10-27

GB 712 Grade AH40 is the 0 °C high-tensile hull steel of GB 712 at the 40 level. In GB 712 the 40 level is the top of the UR W11 high-tensile range. It goes into upper decks, sheer strakes and hatch coamings of large container ships, where 355 N/mm² plate would become too thick to weld economically. Minimum yield is 390 N/mm², tensile 510–650 N/mm², elongation 20 %.

Equivalent grades: ASTM A131 Grade AH40 · ABS AH40 · DNV NV A40 · LR AH40 · BV AH40 · CCS A40 / AH40 · NK KA40 · KR RA40 · RINA AH40.

Current edition: GB/T 712-2022 (Structural steel for ships and ocean engineering) (replaced GB 712-2011; implementation date 2023-05-01). English page titles keep the form “GB 712 Grade …”. Chemistry and tensile figures on this page are verified against the GB 712-2011 OCR tables and Caishuku consensus; full 2022 tables were not obtained free — do not assume 2022-only UHSS grades beyond this pack.

GB 712 names high-tensile hull steel the IACS way: H for higher strength, the toughness letter in front (A 0 °C, D −20 °C, E −40 °C, F −60 °C) and the strength level behind (32 → 315, 36 → 355, 40 → 390 N/mm²). AH40 is the A letter at level 40. Typical uses: decks, side shell and bottom of bulk carriers, tankers and container ships built at Chinese yards, and hull girder members of river–sea vessels where 0 °C toughness is enough.

China builds more merchant tonnage than any other country, and large integrated mills such as Baosteel, Ansteel and Wisco (now part of China Baowu) roll AH40 on wide-plate mills with controlled rolling and accelerated cooling (general market context, not a claim about any one mill’s approvals). The GB 712 grade is the common base; class certificates are added on top for export or classed hulls.

Compared with the GB/T 1591 structural grade Q355, GB 712 Grade AH40 has a similar yield level but is a different product: hull-steel fine-grain practice, Charpy grading by letter, a carbon-equivalent discipline for TMCP plate and hull-steel marking. Do not let a buyer or a mill swap one for the other.

SHH STEEL supplies GB 712 Grade AH40 from Chinese mills, from stock in common sizes or rolled to order, with class certification on the same heat when required.

How GB 712 names it. AH40 is written exactly as on an IACS drawing; Chinese buyers sometimes shorten it to “A40” or “Level 40”, but the full form belongs on the order and the stencil.

Level 40 is almost always TMCP plate with accelerated cooling, and fewer mills produce it in heavy gauges than level 36.

Traded GB 712 Grade AH40 is mostly 6–60 mm thick and 2000–3500 mm wide. Chinese wide-plate mills roll AH40 routinely up to about 4000 mm wide, which helps yards cut the number of butt welds.

Product (check) analysis column = heat (ladle) limit adjusted by GB/T 222-2006 Table 1 (non-alloy and low-alloy steel), as invoked by GB 712 clause 6.1.3 (upper limit + upper deviation, lower limit − lower deviation; row chosen by the specified upper limit; values for cross-sections up to 6 500 mm², larger sections by agreement); arithmetic shown in brackets.

These are the IACS UR W11 / GB 712-2011 OCR limits for higher-strength hull steel (Caishuku-aligned). Current national edition is GB/T 712-2022; full 2022 chem tables not obtained free — confirm any edition deltas on the mill certificate. AH40, DH40 and EH40 share one chemistry column; Chinese mills often cast one heat for all three and sort plates by Charpy result.

For TMCP plate ask for Ceq on the certificate, and Pcm too if the welding procedure is qualified on it. At level 40, TMCP with accelerated cooling and Nb–V–Ti is the norm and Ceq is the number to watch. Product-product analysis tolerances follow GB/T 222 (referenced by GB 712-2011 §6.1.3); confirm any agreed offsets on the order and mill certificate.

For AH40, check that Nb, V and Ti are printed even when small, because the Nb+V+Ti cap and the Ceq both depend on them. Chinese certificates often give Ceq to two decimals using C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15.

AH40, DH40, EH40 and FH40 share these tensile values; only the Charpy temperature and, for FH, the chemistry differ. Against mild Grade A the yield gain is 155 N/mm², which the designer turns into thinner plate. Chinese certificates give results in MPa; the yield/tensile ratio is not limited by the hull grade itself but is often reported, and some owners ask for it on export orders.

Tensile specimens are taken transverse to the rolling direction for plate, following UR W11/W16 practice; the certificate should show the specimen orientation and type. For TMCP plate the yield/tensile ratio is often around 0.8–0.9; no limit applies unless the order adds one (indicative).

The required energy rises with thickness: 70–100 mm AH40 plate must average 55 J longitudinal at 0 °C against 39 J up to 50 mm. AH plate in moderate thickness may be AR or controlled-rolled; TMCP is usual above about 20–25 mm.

TMCP is the dominant route for high-tensile hull plate in Chinese mills; the certificate should state it explicitly, because test frequency and the Ceq cap depend on it.

GB 712 plate is normally rolled to GB/T 709 dimensions with a hull-steel minus tolerance (IACS UR W13 practice, often quoted as −0.3 mm — confirm on the order). Export orders may specify EN 10029 or ASTM A6/A20 instead. High-tensile designs leave little margin, so measure AH40 at several points on receipt and record the results with the plate number.

If the plate is ordered with a classification certificate, the class minus tolerance applies whatever dimension standard is named, so state both on the order. For thin plate under 10 mm, also state the flatness requirement; GB/T 709 flatness classes differ from EN 10029.

Quantities change weekly — confirm before booking. Stock plate is sold as GB 712 Grade AH40 with the original mill certificate. Other sizes go into the next mill campaign: about 4–6 weeks.

Packing and shipping: bundled by size with steel strapping, stencilled with standard, grade and heat number, and shipped from Shanghai, Tianjin, Qingdao or other Chinese ports in containers (≤ 11.8 m) or break bulk.

Processing before shipping. We cut AH40 to drawing by plasma or oxy-fuel, bevel edges, shot-blast and prime, and transfer grade, heat and plate number to every piece so that GB 712 traceability survives cutting. Cut parts ship with a cutting list cross-referencing each piece to its mother plate and certificate.

GB 712 and the CCS Rules. GB 712 was drafted to mirror the IACS unified requirements, which the China Classification Society (CCS) also applies in its Rules for Materials and Welding. In practice a Chinese mill rolls one heat to the shared chemistry and test plan; the plate becomes GB 712 Grade AH40 through the national standard and CCS AH40 through CCS survey. The two are parallel documents, not one replacing the other (CCS chapter numbering not quoted — UNCERTAIN).

Class certification on a GB 712 base. Export buyers often order “GB 712 AH40 with CCS, ABS or DNV certificate” (or another IACS society). The class surveyor must witness or audit the tests at the mill before shipment; a plain GB 712 certificate cannot be converted into a class certificate afterwards. Several marks on one plate are possible when every society is booked in advance and the mill holds the relevant approvals.

Read the series as a grid: across (AH40, DH40, EH40, FH40) the test temperature falls, down (AH32, AH36, AH40) the yield rises. The drawing fixes the letter from structural category and thickness; the scantling calculation fixes the number. A higher letter at the same number is normally accepted as a substitute; a lower letter is not.

AH40 is the default high-tensile plate for hull girders in temperate service. Above 390 N/mm², GB 712 continues into the extra-high-strength series starting at AH420. See GB 712 shipbuilding steel plates.

Level 40 saves roughly 9 % thickness over level 36 in a yield-governed member, but fatigue-sensitive details may not allow the full saving.

Typical uses: decks, side shell and bottom of bulk carriers, tankers and container ships built at Chinese yards, and hull girder members of river–sea vessels where 0 °C toughness is enough.

Welding: use high-tensile consumables of grade 2Y or higher (typical UR W17 mapping — confirm in the WPS), with preheat and heat input set by the qualified procedure and the Ceq on the certificate. Chinese yards weld thick AH40 by one-side SAW and electrogas; those high-heat-input processes need a procedure qualified on plate from the same route.

Forming: TMCP plate gets part of its strength from controlled rolling and cooling; keep line heating below the mill’s stated limit (often quoted around 650 °C — UNCERTAIN, mill-specific).

Why the A letter. AH40 is tested at 0 °C and chosen for temperate trades and members above the waterline. An AH plate that happens to test well at −20 °C is still only AH unless it was ordered and tested as DH. Where blocks mix letters, keep AH40 plates separated and colour-marked so that a lower letter is never fitted by mistake.

The mill certificate lists heat analysis including microalloys, Ceq for TMCP plate, tensile results, Charpy results at 0 °C, condition of supply and plate numbers. A bilingual Chinese/English certificate is standard for export.

On receipt: check the stencil shows GB 712 and AH40, the Charpy temperature and thickness band match, and the heat numbers tie to the certificate. UT to GB/T 2970 (or EN 10160 / ASTM A578 for export), Z-direction tests and third-party witness can be added if booked before rolling.

Mill quality system. Chinese plate mills supplying GB 712 normally run ISO 9001-certified quality systems and hold their own national product certification; when a class certificate is wanted, the mill must also hold that society’s works approval for the grade and thickness (details per mill — UNCERTAIN). Ask for that approval scope before ordering; it is the single most useful check a buyer can make.

• Writing “Q355” or “Q345” when the drawing says AH40
• Writing only “40 grade” so the toughness letter is undefined
• Accepting a mild A/B/D/E certificate against a high-tensile order
• No Ceq printed for TMCP plate
• Assuming a GB 712 certificate equals a class certificate
• Charpy band for the delivered thickness not checked

A clean order line — standard, grade, condition, certificate, class — prevents nearly all of them.

With AH40 the costly errors are mixed letters within one delivery, missing Ceq for TMCP plate, and plates whose delivered thickness falls into a higher Charpy band than the test covered.

If you have not bought GB 712 plate from China before, we can send sample certificates in advance, arrange a trial lot, and let your inspector visit the mill during rolling and testing. For buyers outside China we also check that the bilingual certificate wording matches the purchase order line by line before shipment.

State: GB 712 AH40, condition of supply, sizes and quantities, tolerance standard, UT / Z-direction, class certificate if needed, certificate type and port, and send to SHH STEEL.

Sample line: “Hull structural steel plate to GB 712, grade AH40, TMCP, 16 × 2500 × 12000 mm, 40 pcs, bilingual mill certificate with Ceq, EN 10204 3.1, CFR Jebel Ali.”

Order checklist:

• Standard: GB 712 (state edition if your contract requires one)
• Grade: AH40
• Condition of supply (AR / N / TMCP as allowed)
• Sizes and quantity per size; tolerance standard and flatness class
• Certificate: bilingual GB 712 mill certificate, EN 10204 3.1 or 3.2; class certificate if required
• Extras: UT, Z-direction, shot-blast/prime, cutting
• Delivery terms, destination port and documents (CO, Form E)

GB 712 Grade AH40 is the higher-strength hull structural steel of the Chinese national standard GB 712, on the IACS UR W11 basis: yield 390 N/mm², tensile 510–650 N/mm², Charpy 0 °C. Order it by standard and grade, and add a classification certificate on the same heat when the hull is classed.

SHH STEEL supplies GB 712 Grade AH40 from Chinese mills with bilingual certificates, cutting, UT and EN 10204 3.2 on request. Send grade, sizes, quantities and port for a quotation.