ABS AQ51 Shipbuilding Steel Plate Supplier

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

Equivalent grades: ABS AH50 / AQ51 ≈ ASTM A131 (no direct grade at this strength) ≈ DNV NV A500 ≈ LR AH50 ≈ BV A500 ≈ CCS AH500 ≈ NK KA500 ≈ KR RA51 ≈ RINA A500 ≈ GB 712 Grade AH500 (all class-approved, stamped by the issuing society).

Naming note: commercial and many yard enquiries use ABS AH50 for the ~500 N/mm² yield band. In ABS Rules Part 2, Section 2-1-8 Extra High Strength Steel, the official designation is AQ51 (category 51 = 500 N/mm² yield; letter AQ sets Charpy temperature). This page uses the title grade AH50 and states the ABS rule grade AQ51 wherever requirements are cited.

AQ51 provides minimum yield 500 N/mm², tensile 610–770 N/mm², elongation 16 % min., and Charpy at 0 °C with average energy 50 J longitudinal or 33 J transverse (2-1-8/5.11 Table 5A). It sits above the AH–FH 40 higher-strength family (2-1-3, max yield 390 N/mm²).

SHH STEEL sources Extra High Strength plate from mills with ABS approval for the grade, under Surveyor attendance. Related: ABS AH40 / ABS FH40 (higher-strength 40 band); live ABS Grade A; BV FH36 / BV FH40.

Category 51 (500 N/mm²) sits mid Extra High Strength. Vacuum degassing applies whenever thickness exceeds 50 mm, regardless of category.

Maximum plate thickness depends on delivery condition (Table 3): e.g. N/QT up to 250 mm, TM/NR/CR up to 150 mm — actual approval limit is as per standard and mill certificate.

Product (check) analysis column = heat (ladle) limit adjusted by ASTM A6/A6M-19 Table A “Permitted Variations in Product Analysis”, as required by ABS Rules Part 2, 2-1-2/5.3 (maximum + “over”, minimum − “under”; row chosen by the specified upper limit); arithmetic shown in brackets.

Source: ABS 2-1-8/4 Tables 4A and 4B. Exact ladle limits depend on delivery condition (N/NR/CR vs TM vs QT) and on AQ/DQ vs EQ/FQ. Values above are the rule envelope — the purchase order and mill approval sheet govern. Product analysis tolerances: as per standard and mill certificate (ABS references general sampling; numeric product limits are not tabulated like ASTM A6 offsets in 2-1-2). All intentionally added elements and Ceq (or CET/Pcm) are to be reported.

Alternative B specimen elongations: 2-1-8 Table 5B (thickness-dependent). For t > 100 mm, tensile reductions may be considered except MOU leg/rack/chord “R” material (Note 3). Test frequency: each heat-treatment batch of same cast/condition/thickness for plates (2-1-8/5.7).

Charpy temperature is fixed by the grade letter: AQ → 0 °C. Energy does not increase with thickness in Table 5A the way 2-1-3 Table 4 does — the listed 50 / 33 J applies, with mid-thickness rules for t > 100 mm (at least 2/3 energy or +10 °C test option).

Allowed delivery conditions and max thicknesses are in Table 3 (N, NR/CR, TM, QT). Vacuum degassing is mandatory when t > 50 mm and for category 70. State N / TM / QT on the PO; do not assume AR for Extra High Strength.

Tolerances per 2-1-1/15 unless agreed otherwise (hull plates: 0.3 mm minus). Internal soundness: ultrasonic examination required for thickness ≥ 15 mm (EN10160 / ASTM A578 or agreed). Surface per EN 10163 or equivalent agreed with ABS. Edge discontinuities > 25 mm need investigation (2-1-8/9.1).

Quantities are approximate and change daily; all stock plates carry AB/AQ51 marking and ABS-endorsed mill certificates. Sizes outside the table are produced in the next mill rolling cycle, typically 3–6 weeks including ABS survey (as per standard and mill certificate for exact lead time). Send your size list and we will reply with stock vs roll.

Packing. Plates are bundled by size with steel strapping, marked with heat number, grade and ABS stamp, and shipped in containers (up to about 11.8 m) or break bulk for longer plates.

Extra High Strength AQ51 is less commonly stocked than AH36; many sizes are mill-roll only. Confirm available approved thicknesses with us before fixing the cutting plan.

Comparisons, not substitutions. Dual classification for Extra High Strength needs mills approved by each society for that strength level.

Use Extra High Strength when design yield exceeds the 390 N/mm² of AH–FH 40. Category steps: 43→420, 47→460, 51→500, 56→550, 63→620, 70→690 N/mm². Letter AQ/DQ/EQ/FQ (commercial AH/DH/EH/FH) sets 0/−20/−40/−60 °C.

Do not substitute ABS FH40 for AH50 without design approval — yield and rule section differ (2-1-3 vs 2-1-8).

When ABS AH50 is the right choice: typical uses are higher-strength hull girder plating (deck, bottom, side shell) of bulk carriers, tankers and container ships in normal service where 0 °C toughness suffices. The Charpy temperature (0 °C) is fixed by the letter, and the strength by the number (ReH 500 N/mm²). Pick the strength from the scantling calculation and the letter from the material class and design temperature on the approved drawing. Swapping to a different letter or strength needs class approval of the drawing change, not just a new purchase order.

Typical uses: weight-critical hull and offshore members, MOU legs/racks/chords (order “R” designation when required), high-stress deck and bulkhead plating specified at 500 N/mm². See offshore platform steel plates.

Fabrication: welding and PWHT windows are narrower than for AH36; use WPS qualified for the strength level and follow mill Ceq/CET/Pcm and hydrogen-control guidance. Incorrect post-weld or flame-straightening heat can damage toughness (2-1-8/13). Parameters from weldability tests may be requested from the mill (2-1-8/15).

Project pattern for category 51: Typical projects: high-stress bulkhead plating, crane pedestal foundations on classed vessels, and selected MOU rack material. Compared with ABS AH36 / ABS FH40, this grade changes both rule section (2-1-8 vs 2-1-3) and yield.

Order checklist unique to AH50: write ABS AQ51 on the PO; Charpy 0 °C; yield 500 N/mm²; UT level for t≥15 mm; vacuum degassing if t>50 mm; delivery condition N/TM/QT; optional R suffix for MOU leg/rack/chord.

Processing notes for ABS AH50: AQ51 owes its strength to TM or QT processing. Hot forming or stress relieving above the tempering temperature can lower the properties, so any heat treatment after delivery must be agreed with the mill and ABS. Cold forming and bending radii should follow the mill’s data sheet.

Surveyor witnesses mechanical tests and verifies process/NDE. Certificates include cast/batch/plate ID, grade, chemistry + Ceq/CET/Pcm, delivery condition with heat-treatment temperatures, mechanical results, surface and UT reports (2-1-8/23). Optional EN 10204 3.2 and multi-class.

Charpy acceptance for ABS AH50 by thickness (from 2-1-8; actual results as per standard and mill certificate):

On receipt, check that the test temperature on the certificate is 0 °C and that the energy matches the band for the delivered thickness, not the thinnest band. Also check that the plate number on the stamp matches the certificate line, and that the ABS Surveyor endorsement is present. For thickness ≥ 15 mm, 2-1-8 requires ultrasonic examination; check the UT report and the agreed acceptance level. If the PO asked for 3.2, the inspector’s report must name the witnessed tests and plate numbers.

• Using title AH50 on the PO without ABS grade AQ51 / 2-1-8 reference — mill may mis-route to 2-1-3
• Assuming A131 / GB 712 automatically cover Extra High Strength
• Omitting delivery condition (N/TM/QT) and UT class
• Skipping vacuum-degassing / t>50 mm requirements
• No “R” suffix when ordering MOU leg/rack/chord plate
• Substituting FH40 for EHS without design change
• How does ABS AH50 differ from ABS FH40?: FH40 is higher-strength 2-1-3 steel at 390 N/mm² yield and −60 °C. AH50 is Extra High Strength 2-1-8 (official AQ51) at 500 N/mm² and 0 °C. Chemistry, UT rules and vacuum-degassing requirements differ — do not substitute without design approval.

State both ABS AQ51 (rule) and commercial AH50 if used in the yard BOM; delivery condition; thickness × width × length × qty; UT level; Z/R if needed; multi-class; certificate type; port. Email SHH STEEL.

Sample PO line: “Steel plate to ABS Rules Part 2 2-1-8, Grade AQ51 (commercial AH50), delivery condition [AR/CR/N/TM/QT], 20 × 2500 × 12000 mm, 30 pcs, marked AB/AQ51, certificate endorsed by ABS Surveyor, EN 10204 3.2 [if required], UT [if required], CIF [port].” Each bracketed item changes price, lead time and which mills can supply, so fix them at enquiry stage.

ABS AH50 maps to rule grade AQ51: yield 500 N/mm², tensile 610–770 N/mm², Charpy 0 °C (50 J L / 33 J T). Fix on the order: official grade, delivery condition, UT, vacuum-degassing/t>50 needs, R-suffix if MOU, multi-class and 3.2.

SHH STEEL quotes only ABS-approved EHS mills, arranges Surveyor attendance, and can add third-party 3.2 and multi-class stamps.

T/T or L/C with inspection before balance; dedicated photo follow-up; workshop cutting/bevelling/drilling/machining.

Request a quotation: send SHH STEEL ABS AQ51 (title AH50), sizes, qty, delivery condition, port and inspection needs.