DH36

Higher-strength weldable carbon-manganese hull structural steel · IACS Unified Requirement W11, Revision 9 (current through December 31, 2026) · Applicable rules of the selected classification society · IACS Unified Requirement W2 for mechanical testing procedures · IACS Unified Requirement W13 for plate and wide-flat thickness tolerances · IACS Unified Requirement W14 when through-thickness properties are specified

DH36 is a class-certified higher-strength hull steel combining a minimum yield strength of 355 MPa with mandatory Charpy V-notch toughness at −20 °C. It is a killed, fine-grain-treated, weldable steel whose compliance depends not only on chemistry and tensile properties, but also on product form, thickness, delivery condition, impact-test orientation, approved manufacturing route and classification-society certification. The designation alone is therefore insufficient for purchasing or substitution.

355 MPa minimum yield strengthStrength level · IACS W11 higher-strength level 36; tensile strength is 490–630 MPa.
Charpy V-notch tested at −20 °CToughness class · The letter D identifies the impact-test-temperature class; required energy depends on thickness and specimen orientation.
Killed and fine-grain treatedMetallurgical practice · Al, Nb, V or another approved grain-refining practice is required.
Plate and wide flat through 100 mm; sections and bars through 50 mmPrincipal scope · Greater thickness requires case-specific consideration under the applicable class rules.
Ceq ≤0.38% through 50 mm; ≤0.40% over 50–100 mmTMCP carbon equivalent · IACS maxima for A36/D36/E36/F36 produced by thermo-mechanical rolling.
IACS W11 Rev.9 remains current until January 1, 2027Rule status · Revision 10, issued in September 2025, comes into force on January 1, 2027; the contract and certification dates determine the applicable rule edition.

Overview

Designation system: IACS unified hull structural steel designation implemented through classification-society rules
Product forms: Hot-rolled plate, Wide flat, Structural sections, including bulb flats, Bars for hull construction
Condition: As rolled, only within permitted thickness and fine-grain-practice limits, Normalized (N), Controlled or normalizing rolled (CR/NR), Thermo-mechanically rolled or TMCP (TM), potentially with approved accelerated cooling
Density: 7850 kg/m³ (Typical engineering value for carbon-manganese steel; not a grade acceptance requirement.)

What the designation means

DH36 is not simply a generic 355 MPa structural steel. In the IACS system, H identifies higher-strength hull steel, 36 identifies the 355 MPa minimum-yield strength level, and D identifies the toughness class requiring Charpy testing at −20 °C. IACS tables commonly list the grade as D36 and permit the H to appear before or after the basic grade mark, producing HD36 or DH36. DH36 is the most widely recognized commercial form.

IACS Unified Requirements establish minimum harmonized requirements that member classification societies incorporate into their own rules. Material for a classed vessel must comply with the society and rule edition applicable to that vessel; an unendorsed commercial certificate stating only “DH36” is not automatically equivalent to class-certified material.

Scope, product form and manufacturing route

Normal scope of IACS W11 for DH36

Product form	Normal thickness scope	Important qualification
Plate and wide flat	Up to 100 mm	Mechanical and impact requirements vary with thickness; plate delivery condition is controlled by thickness and grain-refining practice.
Sections and bars	Up to 50 mm	Availability of DH36 sections is more limited than plate and must be confirmed with an approved manufacturer.
Products above the stated scope	Case-specific	Variations or additional requirements require consideration and acceptance by the classification society.

The steel must be made at works approved by the classification society for the relevant grade, product form, thickness range and manufacturing process. The grade is killed and fine-grain treated. Basic oxygen and electric-furnace steelmaking are established routes; another route requires society approval.

A DH36 certificate must identify the actual condition of supply. Normalized, controlled-rolled and TMCP material can all satisfy the grade, but they are not metallurgically identical. TMCP properties arise from a controlled rolling and cooling schedule and cannot necessarily be restored by later normalizing or other heat treatment.

Chemical composition and weldability controls

IACS W11 ladle-analysis limits applicable to D36

Element or control	Requirement, mass %	Qualification
C	≤0.18	Maximum carbon.
Mn	0.90–1.60	Minimum may be reduced to 0.70% for thickness up to 12.5 mm.
Si	≤0.50	Maximum.
P	≤0.035	Maximum.
S	≤0.035	Maximum.
Al, acid soluble	≥0.015	Total Al ≥0.020% may be used instead.
Nb	0.02–0.05	One possible grain-refining addition.
V	0.05–0.10	One possible grain-refining addition.
Ti	≤0.02	May form part of the approved fine-grain practice.
Nb + V + Ti	≤0.12	Combined maximum.
Cu	≤0.35	Residual/alloy limit.
Cr	≤0.20	Residual/alloy limit.
Ni	≤0.40	Residual/alloy limit.
Mo	≤0.08	Residual/alloy limit.

Al, Nb, V or another suitable grain-refining element may be used singly or in an approved combination. When elements are combined, the individual minimum for a fine-graining element does not necessarily apply. TMCP chemistry may be varied or further restricted by the classification society.

IIW carbon equivalent

Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

All terms are mass percent. The formula gives a general weldability indication for carbon-manganese steels; it does not by itself establish preheat, consumable or welding-procedure requirements.

Maximum Ceq for TMCP 36-strength steels

Nominal thickness	Maximum Ceq
t ≤ 50 mm	0.38%
50 < t ≤ 100 mm	0.40%

The purchaser and manufacturer may agree on more stringent limits. At the society's discretion, Pcm may be used to assess cold-cracking susceptibility, particularly for low-carbon TMCP steel.

Cold-cracking susceptibility parameter

Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B

All terms are mass percent. Any acceptance limit must come from the applicable class rules, approved manufacturing specification, welding procedure or purchase requirements.

Specified mechanical properties

Room-temperature tensile requirements

Property	IACS W11 requirement	Test qualification
Yield strength, ReH or Rp0.2	≥355 MPa	Either upper yield strength or 0.2% proof strength may be used as prescribed by W11.
Tensile strength, Rm	490–630 MPa	Specified range for the 36 strength level.
Elongation	≥21%	Proportional specimen with gauge length 5.65√S₀.
Elongation on 25 × 200 mm full-thickness flat specimen	13–21%	Thickness-dependent alternative values apply through 50 mm; these must not be confused with the 21% proportional-specimen requirement.

Charpy V-notch requirements for DH36 at −20 °C

Product thickness t	Longitudinal average, min.	Transverse average, min.
t ≤ 50 mm	34 J	24 J
50 < t ≤ 70 mm	41 J	27 J
70 < t ≤ 100 mm	50 J	34 J

Values are minimum averages for a set of three specimens. Longitudinal specimens are generally used; transverse values must be guaranteed when transverse testing is required by the purchaser or classification society. For products over 40 mm, the impact specimen is taken at the quarter-thickness position.

The D toughness class is defined by testing at −20 °C; it is not, by itself, a universal minimum service-temperature rating. Structural detail, stress state, thickness, strain rate, fatigue, welding, residual stress and the vessel's material-selection rules remain relevant.

Delivery condition is part of the grade specification

Permitted conditions for D36 under IACS W11 Rev.9

Fine-grain practice	Thickness	Permitted condition
Nb and/or V	t ≤ 12.5 mm	Any permitted condition.
Nb and/or V	12.5 < t ≤ 100 mm	Normalized, controlled rolled or TMCP.
Al alone or with Ti	t ≤ 20 mm	Any permitted condition.
Al alone or with Ti	20 < t ≤ 25 mm	Any condition; as-rolled supply requires special society approval.
Al alone or with Ti	25 < t ≤ 100 mm	Normalized, controlled rolled or TMCP.

“Any” does not remove the requirement to meet all mechanical, impact and approval conditions. Quenched-and-tempered delivery is not the normal Table 5 route for D36.

For procurement and fabrication, DH36-N, DH36-CR and DH36-TM should be treated as distinct supply conditions even though each may meet the same grade-level acceptance values. The condition influences achievable chemistry, carbon equivalent, forming behavior, response to heating and welding-process limits.

Welding, forming and fabrication implications

DH36 is intended to be weldable, but the grade designation does not constitute a welding procedure. Production welding requires a procedure qualified under the applicable classification-society rules, with approved consumables providing the required weld-metal strength and notch toughness. Preheat and interpass limits must be based on the actual ladle chemistry or Pcm/Ceq, plate thickness, heat input, joint restraint, hydrogen level and ambient conditions.

Particular care is required with TMCP plate. Hot forming, stress relieving or other heating can alter the microstructure that produced the certified properties. High heat input can also reduce heat-affected-zone toughness. These operations should be checked against the mill's approved manufacturing specification, the welding-procedure qualification and the classification society's acceptance.

For welding above 50 kJ/cm, ordinary DH36 certification alone should not be taken as proof of suitability. Confirm the mill's class approval for the intended heat-input range, plate thickness and manufacturing route, as well as the shipyard's separate welding-procedure qualification. IACS W11 Revision 10 introduces an explicitly optional mill approval scheme with heat-input notation, effective January 1, 2027.

Higher base-metal yield strength does not automatically improve the fatigue strength of a welded detail. Weld geometry, defects, residual stress, stress range and detail category commonly control hull fatigue performance.

Inspection, testing and certification

Acceptance testing is organized by cast, product form, thickness and delivery condition. Plate and wide-flat tensile specimens are normally transverse to the rolling direction. Charpy specimens are taken near the rolled surface, or at quarter thickness when product thickness exceeds 40 mm. Test frequency and batch definition vary with product form, condition and rule provisions; verification therefore requires the governing certificate rather than a generic grade table.

Items to verify on the class material certificate

Certificate item	Why it matters
Classification-society endorsement and applicable rule edition	Confirms that the material was accepted under the project classification regime.
Grade and complete suffixes	Identifies DH36 and any TM, corrosion-resistant, Z-quality or other approved notation.
Manufacturer and approved works	Approval is specific to the works, product, thickness and process route.
Cast and piece identification	Maintains traceability from the finished product to the ladle analysis and tests.
Dimensions and product form	Determines the applicable scope, test requirements and tolerances.
Condition of supply	Separates normalized, controlled-rolled and TMCP material.
Ladle analysis and Ceq/Pcm where specified	Needed for chemistry compliance and welding assessment.
Tensile and Charpy results	Check test temperature, specimen orientation, thickness represented and actual values.
Purchaser order and hull number where applicable	Links the certificate to the intended classed construction.

Ultrasonic examination is not automatically implied by the designation. When internal-soundness requirements are needed, the inspection standard, acceptance class, coverage and reporting must be stated in the order. Likewise, through-thickness properties are separate: plates or wide flats at least 15 mm thick may be ordered to IACS W14 Z-quality requirements where lamellar-tearing resistance is needed.

Specifying and purchasing DH36

Minimum practical ordering information

Specify	Typical decision required
Governing authority	Named classification society, project rule edition and whether dual certification is required.
Designation	DH36, including required condition and any additional suffixes.
Product	Plate, wide flat, section or bar; dimensions and quantities.
Delivery condition	N, CR/NR or TM/TMCP as required; do not leave this ambiguous when fabrication depends on it.
Impact testing	Required orientation and any project requirements beyond standard D-class testing.
Weldability controls	Maximum Ceq or Pcm, if the project requires tighter limits than the base rule.
High-heat-input use	Required maximum heat input, thickness and mill approval or notation.
Internal soundness	UT standard, acceptance class, scanning pattern and reporting.
Through-thickness properties	Z-quality level where highly restrained through-thickness loading is present.
Surface and dimensional requirements	Applicable class rules, tolerances and any project-specific flatness or surface class.
Certification and marking	Class certificate type, traceability, marking and documentation requirements.
Additional project requirements	Cold-forming qualification, CTOD, brittle-crack arrest, corrosion-resistant designation or other supplementary testing when actually required.

Standard DH36 is not inherently corrosion-resistant steel. Coating, cathodic protection and corrosion allowance remain design matters. IACS corrosion-resistant cargo-oil-tank variants require separate approval and RCU, RCB or RCW identification; ordinary DH36 must not be represented as such.

Use and substitution boundaries

DH36 is widely used where hull scantling design benefits from the 355 MPa strength level while D-class notch toughness is sufficient under the applicable material-selection rules. The designer must still establish whether a colder toughness class such as EH36 or FH36 is required for the member, thickness, structural category and design temperature.

Substitution cannot be decided from nominal yield strength alone. A proposed replacement must be checked for governing specification, toughness temperature and orientation, tensile range, chemistry and weldability, product form, thickness, delivery condition, manufacturing approval, testing frequency, certification and any special properties. A commercial “S355” steel, for example, is not automatically acceptable as DH36 because it has a similar minimum yield strength.

Sources

IACS Unified Requirement W11 Rev.9 — Normal and Higher Strength Hull Structural SteelsInternational Association of Classification Societies
IACS UR W11 Version Status and Revision InformationInternational Association of Classification Societies
IACS Unified Requirement W11 Rev.10 — Normal and Higher Strength Hull Structural SteelsInternational Association of Classification Societies / ClassNK
IACS Unified Requirements — Purpose and ImplementationInternational Association of Classification Societies
Rules for the Manufacture, Testing and Certification of MaterialsLloyd's Register
ASTM A131/A131M — Standard Specification for Structural Steel for ShipsASTM International

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D36	IACS table designation for the same grade family	The H identifying higher strength may be placed before or after the basic grade mark; commercial and class markings should be checked.
HD36	Alternative permitted arrangement of the unified designation	Same nominal IACS strength and toughness class as DH36, subject to the governing society's marking practice.
AH36	Related 355 MPa hull grade	Same strength level but Charpy testing is at 0 °C rather than −20 °C; it is not an identical toughness grade.
EH36	Related 355 MPa hull grade	Same strength level with Charpy testing at −40 °C.
FH36	Related 355 MPa hull grade	Same strength level with Charpy testing at −60 °C.
ASTM A131/A131M Grade DH36	Parallel shipbuilding specification carrying the DH36 designation	Do not assume automatic interchangeability with class-certified IACS material; verify the specified edition, numerical requirements, product scope, testing and class endorsement.
S355 structural steels	Approximate strength-level comparison only	EN structural grades are not substitutes for DH36 without a complete technical and classification review.