NK A40/AH40 Shipbuilding Steel Plate

Technical Reviewer: Jack | SHH STEEL Material Solutions Department · Updated: 2026-10-28

NK A40/AH40 is ClassNK’s 0 °C high-tensile hull plate at the 40 strength level. On the NK certificate it appears as KA40. Level 40 is the top of the UR W11 high-tensile range, chosen for thick deck and hatch-coaming plating where 355 N/mm² is not enough. It gives ReH ≥ 390 N/mm², Rm 510–650 N/mm² and A ≥ 20 %. The title uses the dual trade name (A40/AH40) because Japanese and Chinese buyers search for both forms, but the plate itself is stamped KA40.

Equivalent grades: ASTM A131 Grade AH40 · ABS AH40 · DNV NV A40 · LR AH40 · BV AH40 · CCS A40 / AH40 · KR RA40 · RINA AH40 · GB 712 Grade AH40.

In the ClassNK series the letter sets the test temperature (KA 0 °C, KD −20 °C, KE −40 °C, KF −60 °C) and the number sets the yield (32 → 315, 36 → 355, 40 → 390 N/mm²). KA40 is the A letter at level 40. Typical NK uses: main hull-girder plating of Capesize and Panamax bulk carriers, VLCCs and car carriers where 0 °C is enough.

SHH STEEL sources KA40 from Chinese mills that hold NK approval for high-tensile hull steel, with the NK survey at the mill. Stock is held in the common 36-level sizes and other sizes are rolled to order.

Commonly traded KA40 runs 8–60 mm thick in 2000–3000 mm widths. Heavier plate depends on the mill’s NK approval range for the delivery condition.

Product (check) analysis column: ClassNK Part K specify heat (ladle) limits; no numeric product-analysis tolerance is tabulated in the rule text reviewed, so product (check) limits are agreed on the order or taken from the approved mill specification and certificate.

The ladle values follow IACS UR W11 for high-tensile hull steel, which ClassNK adopts in Part K (not checked against a Part K edition — UNCERTAIN). KA, KD and KE share one chemistry column, so a single heat can often be certified to several of these letters if the impact results support it. For TMCP plate, ask for Ceq (and Pcm if your welding procedure uses it) on the certificate. Product analysis tolerances are agreed case by case.

Flat 200 mm test pieces: minimum elongation by thickness

KA40, KD40, KE40 and KF40 share these values. Only the Charpy temperature and, for KF, the chemistry differ.

Required energy rises with thickness. A 70 mm KA40 plate must reach 55 J longitudinal at 0 °C, against 39 J for plate up to 50 mm. The delivery condition (AR, CR, N or TM) affects batch size and test frequency. Most Chinese mills now supply TM plate for this grade, which also helps Ceq.

Minus tolerance follows IACS UR W13 as ClassNK applies it. Plus tolerance, flatness and dimensions follow the agreed standard (JIS G3193, EN 10029 or GB/T 709). Weight-optimised high-tensile designs leave less margin, so check thickness at several points on receipt.

Stock quantities change every week, so confirm before you book. Plates are sold as KA40 with the NK stamp and endorsed certificate. Sizes not in stock are rolled in about 4–6 weeks including survey.

Packing: bundled by size with steel strapping, painted with grade and heat number, shipped in containers (≤11.8 m) or break bulk.

All share the UR W11 AH40 requirements. Each society accepts only its own stamp, so dual certification must be booked before the plates are tested.

Read across for toughness (KA40 → KD40 → KE40 → KF40) and down for strength (KA32 → KA36 → KA40). ClassNK’s material-grade table fixes the letter from member category and thickness. The designer fixes the number from the scantlings. Substituting a higher letter of the same number is normally accepted; a lower letter is not.

Above 390 N/mm², ClassNK’s extra-high-strength grades start at KA420 (IACS UR W16). See NK Steel Plates for the full series.

Uses: main hull-girder plating of Capesize and Panamax bulk carriers, VLCCs and car carriers where 0 °C is enough. Welding: use high-tensile consumables approved by ClassNK for 0 °C toughness (grade 2Y or higher, typical — confirm in the WPS). Preheat and heat input come from the NK-approved procedure and the certificate Ceq. Large-heat-input electrogas or one-side SAW on thick KA40 needs a procedure qualified for that heat input.

Forming: TMCP plate gets part of its strength from controlled rolling. Keep line heating within the mill’s temperature limits, typically below about 650 °C for water cooling (UNCERTAIN, mill-specific).

The certificate lists ladle chemistry including microalloys, Ceq for TM plate, tensile results, Charpy results at 0 °C, delivery condition and plate numbers, with the NK endorsement. We can add EN 10204 3.2 third-party witness, UT to EN 10160 or ASTM A578, Z-direction testing, and dual class with ABS, DNV, LR or BV.

On receipt: check the stamp reads KA40, Charpy results are at 0 °C for the right thickness band, and the NK endorsement is present.

• Writing only “AH40” or “A40” without stating ClassNK, so the plate arrives with another society’s stamp
• Omitting the letter, so toughness is undefined
• Accepting mild-steel KA–KE certificates against a high-tensile order
• No Ceq on the certificate for TM plate
• Substituting KD40 without approval
• Charpy band for the delivered thickness not checked

State: NK A40/AH40 (KA40), delivery condition, sizes and quantities, UT/Z, dual-class, certificate type and port. Send to SHH STEEL.

Sample line: “Hull plate to ClassNK Rules Part K, grade KA40 (NK A40/AH40), TM, 25 × 2500 × 12000 mm, 20 pcs, NK-endorsed certificate with Ceq, EN 10204 3.2, CFR Busan/Imabari.”

NK A40/AH40 (KA40) is ClassNK high-tensile hull steel under Part K of the Rules for the Survey and Construction of Steel Ships: yield 390 N/mm², tensile 510–650 N/mm², Charpy 0 °C. An NK-classed hull accepts it only with the NK stamp and an endorsed certificate.

SHH STEEL supplies KA40 from NK-approved Chinese mills, with cutting, dual class, UT and EN 10204 3.2 on request. Send grade, sizes, quantities and port for a quotation.