AH36

Overview

Designation system

IACS/classification-society hull structural steel designation

Product forms

Hot-rolled plate, Wide flat, Rolled section, Bar

Condition

As rolled, only within rule-defined thickness and grain-refining limits, Normalized (N), Controlled or normalizing rolled (CR/NR), Thermo-mechanically rolled or TMCP (TM), with or without approved accelerated cooling

Density

7850 kg/m³ (Representative design value for carbon-manganese steel; density is not a grade acceptance requirement.)

What the designation means

AH36 belongs to the IACS family of higher-strength hull structural steels. “A” identifies the toughness category, for which Charpy V-notch testing is specified at 0°C. “H” denotes higher strength, and “36” identifies the nominal strength level corresponding to 355 MPa minimum yield strength in IACS rules. IACS permits the H to appear before or after the grade letter, so AH36, A36 and HA36 may identify the same class grade when the marine context is explicit.

An unqualified purchase order reading only “A36” is hazardous: ASTM A36/A36M is a different general structural steel. Use “AH36 hull structural steel” and state the governing class rules or ASTM A131/A131M as applicable.

IACS Unified Requirements are minimum requirements implemented through the rules of individual classification societies. The society classing the vessel remains the controlling authority and may impose additional or more stringent requirements. As of October 1, 2026, UR W11 Rev.9 remains the current IACS edition; Rev.10 is scheduled to enter into force on January 1, 2027. Contract date, certification application date and the society's implementation rules therefore need to be checked for each project.

Required mechanical properties and toughness

For plate thinner than 10 mm, impact testing may be waived by the classification society or performed using permitted sub-size specimens. Impact tests are generally not required below 6 mm nominal thickness. For full-thickness 25 mm-wide tensile specimens with 200 mm gauge length, separate thickness-dependent elongation minima apply, ranging from 13% for very thin product to 21% for thickness above 40 through 50 mm.

The 0°C Charpy test temperature is a grade qualification condition, not by itself a minimum allowable service or design temperature. Material selection for cold service depends on structural category, plate thickness, design temperature, stress concentration, welding details and the applicable hull rules.

Chemistry, fine-grain practice and weldability controls

Delivery condition is part of the grade specification

Normalized and normalizing-rolled products obtain their grain refinement through heat treatment or controlled finishing in the normalizing range. TMCP obtains strength and toughness through a tightly controlled rolling and cooling schedule; those properties cannot necessarily be recreated by later normalizing. Consequently, two AH36 plates meeting the same room-temperature acceptance values can respond differently to heating, forming and welding if their delivery conditions differ.

Quenched-and-tempered supply is not a standard UR W11 Rev.9 delivery route for AH36 within the normal thickness range. Do not infer its acceptability from requirements for other hull grades.

Welding and fabrication implications

AH36 is intended for welded hull construction, but grade approval does not replace welding-procedure qualification. Consumables must be approved for the applicable higher-strength hull-steel grouping and toughness category, and the WPS must address process, consumable, heat input, combined thickness, restraint, hydrogen control, preheat/interpass temperature and the actual Ceq or Pcm reported on the certificate. IACS UR W17 governs consumable approval, while UR W28 and the class society's rules govern welding-procedure qualification.

Uncontrolled heat input can coarsen the heat-affected zone and reduce toughness. This is particularly important for thick joints and TMCP plate. Under UR W11 Rev.9, steel intended for welding above 50 kJ/cm is subject to the dedicated high-heat-input approval scheme. The approval and any heat-input notation must be verified rather than inferred from the basic AH36 designation.

Before hot forming, line heating, flame straightening, stress relief or other significant reheating, confirm the permitted thermal cycle with the steelmaker, shipyard procedure and classification society. TMCP and accelerated-cooled plate is especially sensitive because excessive reheating may remove the processing benefits responsible for its certified properties. Cold forming also requires a qualified procedure appropriate to thickness, bend severity, edge condition and rolling direction.

Using higher-strength steel does not automatically improve the fatigue strength of a welded detail. Weld geometry, defects, residual stress and stress range commonly control fatigue performance; IACS specifically cautions that an AH36 welded joint may have no greater effective fatigue strength than the corresponding joint in normal-strength steel.

Surface quality, internal soundness and supplementary properties

For plate and wide flat, UR W11 connects surface acceptance to EN 10163 or an accepted equivalent, while thickness tolerances are governed by IACS UR W13 unless otherwise agreed. Cracks, laminations, shells, sharp seams and other injurious defects require approved repair or rejection. Grinding and weld repair are controlled operations; repair welding requires classification-society agreement and an approved procedure.

Standard AH36 does not inherently include ultrasonic inspection or guaranteed through-thickness ductility. Ultrasonic examination must be ordered to an agreed standard and acceptance class. Where resistance to lamellar tearing is needed, plate at least 15 mm thick may be ordered with Z-quality under IACS UR W14, commonly using Z25 or Z35 requirements. These supplementary properties should be selected from the joint geometry and through-thickness stress, not added indiscriminately.

AH36 is not intrinsically corrosion-resistant. Marine durability normally depends on coating systems, cathodic protection where applicable, corrosion additions and inspection. RCU, RCB and RCW are separate approved corrosion-resistant hull-steel designations for defined cargo-oil-tank locations and must not be assumed from an AH36 certificate lacking the relevant suffix.

Specifying and accepting AH36

Material identification must remain traceable to the original cast and piece. The certificate is expected to identify the steelworks, grade, dimensions, ladle analysis, test results and delivery condition, and to carry the applicable classification-society identification. Acceptance should confirm that the physical marking, certificate and purchase order agree; a chemistry-and-strength match without valid class traceability is not normally an adequate replacement for certified hull steel.

Substitution and dimensional limits

A proposed substitute must be assessed against more than nominal yield strength. Relevant factors include governing product standard, toughness category and test temperature, thickness-dependent impact energy, chemistry and weldability, delivery condition, product form, manufacturing approval, test frequency, through-thickness requirements, dimensions, certification and class acceptance. Commercial comparison tables are screening aids, not substitution authority.

UR W11's baseline scope ends at 100 mm for plate and wide flat and 50 mm for sections and bars. IACS Recommendation 197, published in 2026, gives harmonized guidance for plate and wide flat above 100 through 200 mm, including modified testing and toughness provisions. It is a recommendation rather than an automatic extension of every AH36 approval, so thick material remains subject to the classification society's acceptance and the steelworks' approved manufacturing range.

Related designations

A related designation or cross-standard counterpart is not a declaration of equivalence; confirm the standard, product form and condition before substituting.