Grade D
Weldable ordinary-strength carbon-manganese hull structural steel · IACS UR W11 Rev.9 — Normal and higher strength hull structural steels (current on October 1, 2026) · Rules of the vessel's nominated classification society implementing IACS UR W11 · ASTM A131/A131M Grade D, when that specification is explicitly invoked
Grade D is the −20 °C toughness grade in the IACS ordinary-strength hull-steel series. It retains the 235 MPa minimum yield strength and 400–520 MPa tensile range common to Grades A, B, D and E, but requires verified Charpy V-notch toughness at −20 °C. Its acceptability depends on product form, thickness, delivery condition, class approval and certification—not on the designation alone.
- 235 MPa minimum yield strengthStrength class · Grade D is ordinary-strength hull steel; it is not DH32, DH36 or another higher-strength grade.
- 400–520 MPaTensile strength · Specified range under IACS UR W11 for normal-strength grades.
- −20 °CImpact test temperature · This is the distinguishing toughness level of Grade D.
- 27 J longitudinal; 20 J transverseCharpy energy, t ≤ 50 mm · Minimum average for a set of three standard 10 × 10 mm Charpy V-notch specimens.
- Killed; fine-grain treated above 25 mmDeoxidation and grain practice · Alternative approved grain-refining practice may be used.
- Any permitted condition through 35 mm; N, CR or TM above 35 mmDelivery condition threshold · Applies within the IACS W11 thickness scope; special provisions exist for sections.
Overview
- Designation system
- IACS/classification-society hull structural steel grading system
- Product forms
- Hot-rolled plate, Wide flat, Structural sections, including bulb flats, Bars
- Condition
- As rolled where permitted, Normalized, Controlled rolled/normalizing rolled, Thermo-mechanically rolled (TM/TMCP)
- Density
- 7850 kg/m³ (Representative density used for carbon-steel mass calculations; not a Grade D acceptance requirement.)
What the designation means
The letter D identifies a toughness grade, not a strength level. Within the ordinary-strength series A, B, D and E, the specified tensile properties are essentially the same; the principal distinction is the Charpy V-notch test temperature. Grade D is tested at −20 °C, compared with 0 °C for Grade B and −40 °C for Grade E. Grade A has the least demanding toughness regime.
Do not confuse Grade D with DH32, DH36 or DH40. The latter are higher-strength hull steels with minimum yield strengths of 315, 355 and 390 MPa respectively; the letter D in those designations still denotes the −20 °C toughness level.
IACS Unified Requirements are minimum requirements incorporated into member-society rules. The nominated classification society may impose additional conditions, and the applicable rule edition is normally tied to the vessel contract date, material-certification application or project rules. As of October 1, 2026, UR W11 Rev.9 remains current; Rev.10 is scheduled to enter into force on January 1, 2027.
Scope and delivery condition
| Product form | Primary W11 thickness scope | Grade D delivery condition |
|---|---|---|
| Plate and wide flat | Up to 100 mm | t ≤ 35 mm: any permitted condition; 35 < t ≤ 100 mm: normalized, controlled rolled or TM/TMCP |
| Sections and bars | Up to 50 mm | Same general thickness rule; as-rolled Grade D sections above the normal threshold require special class approval and consistently satisfactory impact results |
| Products beyond the stated scope | Case-by-case | Variations or additional requirements require classification-society consideration |
“Any” does not mean that manufacturing condition is irrelevant. The material must still come from an approved works, meet all tests and be certified in its actual condition.
For plate and wide-flat thickness above 100 mm, Grade D should not be specified solely by extending the ordinary W11 table. Class approval and project-specific requirements are necessary. IACS Recommendation 197, published in 2026, provides harmonized guidance for normal- and higher-strength plates and wide flats over 100 mm through 200 mm, but it does not remove the need to apply the nominated society's rules and approval process.
Chemical composition and steelmaking controls
| Element or control | Requirement | Applicability or qualification |
|---|---|---|
| Carbon, C | 0.21% maximum | All Grade D products |
| Manganese, Mn | 0.60% minimum | All Grade D products |
| Silicon, Si | 0.35% maximum | All Grade D products |
| Phosphorus, P | 0.035% maximum | All Grade D products |
| Sulfur, S | 0.035% maximum | All Grade D products |
| C + Mn/6 | 0.40% maximum | Composition control applying to ordinary-strength grades |
| Acid-soluble aluminium | 0.015% minimum | Grade D over 25 mm when aluminium is used for fine-grain treatment |
| Total aluminium alternative | 0.020% minimum | May replace acid-soluble aluminium determination; other approved grain refiners may be used |
Values are standard requirements, not a typical mill analysis. TM/TMCP material may use composition variations allowed or required by the classification society.
| Thickness | Required practice |
|---|---|
| t ≤ 25 mm | Killed steel |
| t > 25 mm | Killed and fine-grain treated |
The grain-refining requirement is important to achieving reliable notch toughness in thicker material.
The W11 composition table is not a complete welding-consumable or procedure-selection specification. Residual elements and any intentionally added elements are to be reported or controlled as required by the society. For critical welding, the actual certified analysis, product thickness, hydrogen control, restraint, heat input and applicable welding-procedure qualification must be considered.
Mechanical properties and toughness
| Property | Requirement | Test qualification |
|---|---|---|
| Upper yield strength ReH or 0.2% proof strength Rp0.2 | 235 MPa minimum | Compliance may be based on either value as applicable |
| Tensile strength Rm | 400–520 MPa | Applicable across the normal-strength grades |
| Elongation A5 | 22% minimum | Proportional specimen with gauge length 5.65√S₀ |
| Elongation on 200 mm gauge-length full-thickness specimen | 14–22% minimum | Minimum increases with thickness from ≤5 mm to 40–50 mm |
The reported elongation requirement depends on specimen type and dimensions; values from different specimen geometries should not be compared without qualification.
| Product thickness t | Test temperature | Longitudinal minimum average | Transverse minimum average |
|---|---|---|---|
| t ≤ 50 mm | −20 °C | 27 J | 20 J |
| 50 < t ≤ 70 mm | −20 °C | 34 J | 24 J |
| 70 < t ≤ 100 mm | −20 °C | 41 J | 27 J |
Values apply to standard 10 × 10 mm specimens. Longitudinal specimens are normally used, but transverse properties must be guaranteed and may be required for special applications.
A Charpy result is evaluated as the average of three specimens. One individual result may fall below the specified average, but it must not be less than 70% of that average. Impact testing is generally not required for nominal plate thickness below 6 mm. For material below 10 mm, testing may be waived at the society's discretion or sub-size specimens may be used under IACS UR W2.
A −20 °C Charpy test does not by itself establish a permissible ship service temperature or fracture-safety limit. Hull material selection also depends on structural category, plate thickness, location, design temperature, stress level, welding details and the applicable hull rules.
Testing, traceability and certification
Grade D is a surveyed and certified marine product. The steel must be made at a works approved by the classification society for the grade, product form, manufacturing process, delivery condition and thickness range concerned. Batches presented for acceptance are to comprise the same product form, cast and delivery condition.
| Control | Practical significance |
|---|---|
| Tensile testing | Normally at least one test for each batch or 50 tonnes or fraction, with additional testing for thickness or diameter variation |
| Impact testing | A set consists of three Charpy V-notch specimens; normal batch frequency is commonly based on 50-tonne units, with increased frequency for some rolling conditions or special approvals |
| Sampling orientation | Plate and wide-flat tensile specimens are normally transverse to rolling; Charpy specimens may be longitudinal or transverse |
| Thick-product Charpy location | Above 40 mm, the impact specimen longitudinal axis is taken at approximately quarter thickness |
| Heat traceability | Finished pieces must remain traceable to their original cast |
| Marking | Includes the society brand, grade, steelworks identification and cast or piece identity |
| Certificate content | Includes product description, dimensions, grade, cast and piece identity, ladle analysis, delivery condition and test results |
Exact survey and certification arrangements must follow the nominated society's rules and the purchase order.
Ultrasonic examination for internal soundness is not automatically included merely because the material is Grade D. If required for critical details, it must be ordered to an accepted standard and acceptance class. Likewise, through-thickness ductility is not inherent: plates or wide flats of at least 15 mm may be ordered with Z-quality under IACS UR W14.
Fabrication implications
Grade D is intended to be weldable, but satisfactory fabrication depends on an approved welding procedure and compatible class-approved consumables. Grade D's lower-temperature toughness requirement affects weld-metal and heat-affected-zone qualification; it does not authorize unrestricted heat input or eliminate preheat and hydrogen-control considerations.
| Issue | Engineering consequence |
|---|---|
| High heat-input welding | Base material is not automatically approved for heat inputs above 50 kJ/cm; manufacturer approval and procedure qualification must cover the intended process |
| TM/TMCP material | Subsequent hot forming, normalizing, stress relief or uncontrolled heating can alter properties that cannot simply be restored by normalizing |
| Cold forming | Bend radius, strain, edge condition and any need for post-forming testing should follow the society-approved fabrication procedure |
| Flame cutting and welding | Cut quality, hydrogen control, consumable classification, preheat/interpass limits and inspection must be established by the applicable fabrication rules and WPS |
| Corrosion protection | Grade D is not intrinsically corrosion resistant; coating systems, corrosion additions and maintenance remain part of hull design |
| Lamellar-tearing risk | Where heavy through-thickness loading occurs, consider joint redesign, improved through-thickness properties and appropriate inspection rather than relying on Grade D alone |
Specifying and purchasing Grade D
| Order item | Why it matters |
|---|---|
| Nominated classification society and rule edition | Determines approval, survey, marking and certification requirements |
| Full designation | State ordinary-strength hull structural steel Grade D; avoid the ambiguous wording “D steel” |
| Product form and dimensions | Requirements and sampling differ among plate, wide flat, section and bar |
| Required delivery condition | Especially important above 35 mm and whenever fabrication includes significant heating |
| Intended vessel or hull number | Supports certification and material traceability |
| Impact-test orientation or additional toughness requirements | Transverse testing or project-specific toughness is not implied unless required |
| Internal-soundness inspection | Specify UT standard, zone and acceptance level where needed |
| Through-thickness properties | Specify the applicable Z-quality where relevant |
| Surface condition and dimensional tolerances | State any requirement exceeding the standard W11/W13 provisions |
| Welding and forming route | Identify high heat input, hot forming, stress relief or other processing needing material-producer and class confirmation |
A certificate showing only “Grade D” is insufficient if the approving society, product identity, heat number, delivery condition or required supplementary properties cannot be established.
Material-grade selection for a classed hull is normally made from the society's structural material tables according to member category, thickness and design conditions. Grade D should not be downgraded to Grade A or B merely because their strength values are the same.
Substitution and comparison limits
A general structural steel with similar yield strength and −20 °C Charpy energy is not automatically an acceptable substitute. Marine acceptance also depends on chemistry, deoxidation and grain practice, thickness-dependent toughness, delivery condition, approved manufacturing route, survey, traceability and classification-society certification. Even ASTM A131/A131M Grade D should be treated as compliant only when the invoked edition, dimensions, testing and class-acceptance requirements are all satisfied.
Sources
- IACS UR W11 Rev.9 — Normal and higher strength hull structural steelsInternational Association of Classification Societies
- IACS UR W11 Rev.9 — Clean PDFInternational Association of Classification Societies
- IACS UR W11 Rev.10 — Normal and higher strength hull structural steelsInternational Association of Classification Societies / ClassNK
- IACS Recommendation 197 — Hull structural steels over 100 mm and up to 200 mmInternational Association of Classification Societies
- ASTM A131/A131M — Standard Specification for Structural Steel for ShipsASTM International
- ABS Rules for Materials and Welding, Part 2American Bureau of Shipping
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