Shipbuilding steel grades are chosen by strength, impact toughness, thickness, hull location, design loads, service temperature and classification rules—not by the grade name alone. In the common 36 family, AH36, DH36 and EH36 all have a minimum yield strength of about 355 MPa, but their Charpy impact tests are performed at different temperatures: 0°C, −20°C and −40°C respectively. EH36 is therefore tougher in colder service; it is not simply a stronger version of AH36.
For a compliant hull, the owner, designer, shipyard and classification society must match the grade to the approved scantlings, welding procedure, material certificate and installation location. The following guide explains the code, mechanical properties, cold-temperature choices, testing and documentation that control that decision.
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What the letters and numbers mean
Large commercial vessels use steels covered by classification-society rules and material standards. The designation combines a strength family with a toughness class. Normal-strength grades are identified as A, B, D and E. Higher-strength families are AH, DH, EH and FH.
The number identifies the nominal strength level within a higher-strength family. For example, the 32, 36 and 40 families step upward in minimum yield strength. The letters distinguish impact-toughness requirements. In the 36 family, the first letter remains A, D or E while the H indicates a higher-strength grade; the letter changes the temperature at which impact performance is verified.
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- A/B/D/E: normal-strength shipbuilding steel families.
- AH/DH/EH/FH: higher-strength families with several yield-strength levels.
- 32, 36 and 40: increasing minimum-yield-strength bands in the ABS table for plates up to 100 mm thick.
The code is not a complete purchasing specification. Thickness limits, delivery condition, impact results, chemistry, weldability, through-thickness quality and class approval still have to be checked.
Strength levels used in commercial-ship construction
American Bureau of Shipping data published in 2024 gives the following minimum mechanical-property bands for plates up to 100 mm. The figures are minimum yield strength and tensile-strength ranges; they do not describe every requirement in a class-approved material certificate.
| Grade family | Minimum yield strength | Tensile strength |
|---|---|---|
| A, B, D and E | 235 MPa | 400–520 MPa |
| AH32, DH32, EH32 and FH32 | 315 MPa | 440–570 MPa |
| AH36, DH36, EH36 and FH36 | 355 MPa | 490–630 MPa |
| AH40, DH40, EH40 and FH40 | 390 MPa | 510–660 MPa |
A higher yield grade can permit thinner scantlings or provide more capacity at the same thickness, but the permitted design is controlled by the applicable class rules. It is not safe to reduce plate thickness merely because a stronger grade is available. Buckling, corrosion allowance, fatigue, welding details, local loads and fabrication tolerances can govern before yield strength does.
AH36, DH36 and EH36: the practical difference
AH36, DH36 and EH36 are among the most common commercial-shipbuilding steels. All three are approximately 355 MPa minimum-yield steels, with a tensile range of 490–630 MPa in the ABS table for plates up to 100 mm. Their principal distinction is the Charpy V-notch impact-test temperature.
| Grade | Minimum yield strength | Charpy impact-test temperature | What the distinction means |
|---|---|---|---|
| AH36 | About 355 MPa | 0°C | Specified for service where the AH toughness requirement is adequate. |
| DH36 | About 355 MPa | −20°C | Provides verified impact performance at a lower temperature than AH36. |
| EH36 | About 355 MPa | −40°C | Selected when the design service condition requires the lowest of these three test temperatures. |
The test temperature is not the same thing as the vessel’s exact operating temperature. The designer must apply the governing class rule, including location, thickness, stress state and service environment, to determine which toughness class is acceptable. A vessel that occasionally encounters cold air or water is not automatically an EH36 vessel; the required grade follows the approved design basis.
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Which is strongest?
None of AH36, DH36 and EH36 is stronger in minimum yield strength than the others. Their nominal strength level is the same. EH36 has a more demanding low-temperature impact requirement, not a higher yield requirement. If greater strength is needed, the comparison moves to a different number family, such as the 40 family, subject to class approval and fabrication constraints.
Which is best for cold water?
Among these three grades, EH36 is the appropriate candidate when the applicable rule requires impact verification at −40°C. DH36 is tested at −20°C and AH36 at 0°C. “Best” still depends on the vessel’s route, structural member, thickness and classification society. The correct choice is the lowest-cost, weldable grade that satisfies every approved requirement—not automatically the coldest-rated grade.
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IACS Unified Requirement S6 addresses the use of steel grades in various hull members on ships 90 m in length and above. A class-approved material schedule can assign different grades to different members of the same vessel. The decision considers:
- Member location, such as deck, bottom, side shell, strength deck or highly stressed connection.
- Plate or section thickness.
- Hull-girder and local design loads.
- Design service temperature and expected exposure.
- Stress concentration, fatigue and fracture-critical details.
- Applicable classification-society rules and any owner or flag-state requirements.
Consequently, there is no single grade that is universally used for every commercial-vessel hull. A normal-strength grade may be adequate in one region, while a higher-strength or lower-temperature class is required in another. The approved drawing and material list, rather than a generic industry preference, control procurement.
Material factor in hull-girder calculations
IACS structural rules apply a material factor k in hull-girder strength calculations. The factor changes with the minimum yield strength:
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| Minimum yield strength | Material factor k |
|---|---|
| 235 MPa | 1.00 |
| 315 MPa | 0.78 |
| 355 MPa | 0.72 |
| 390 MPa | 0.68 |
These values are used within the relevant IACS calculation method; they are not a universal percentage discount that can be applied to any structural check. A naval architect must use the current rule set for the vessel and calculation being performed.
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ASTM A131/A131M covers structural steel plates, shapes and bars intended primarily for ship construction. It defines ordinary-strength A, B, D and E grades at a 34 ksi (235 MPa) minimum yield level. Its higher-strength AH, DH, EH and FH grades are organized at 46, 51 or 57 ksi minimum-yield levels, corresponding approximately to the 32, 36 and 40 families.
Mechanical properties are evaluated with tension tests and Charpy V-notch impact tests. The impact requirement is essential because a plate can meet tensile strength while still lacking the fracture resistance needed at the design temperature. The exact specimen orientation, test temperature, thickness adjustment and acceptance criteria must be taken from the applicable edition and class rule.
“When the steel is to be welded, it is presupposed that a welding procedure suitable for the grade of steel and intended use or service will be utilized.” — ASTM International, A131/A131M-19
ASTM compliance alone does not replace classification approval when a vessel is being built under class. The purchase order should identify the governing standard, class society, grade, thickness range, delivery condition, tests and documentation required.
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Impact toughness and cold-service selection
Charpy testing measures absorbed impact energy at a specified temperature. The AH, DH and EH sequence establishes progressively colder test temperatures in the commonly used 36 family. This distinction matters because steels that behave acceptably at 0°C can have less fracture resistance as temperature falls.
Temperature selection must reflect the rule-defined design condition, not a marketing label such as “arctic grade.” Check whether the rule uses sea temperature, air temperature, minimum design temperature, a thickness correction or a member-specific requirement. Also verify whether the plate’s actual certificate reports the required impact values at the required temperature.
Through-thickness quality and Z grades
Some welded details load a plate through its thickness, creating a risk of lamellar tearing caused by non-metallic inclusions and poor through-thickness ductility. In those locations, the design may require a Z-quality suffix in addition to the base grade.
For the AH36/DH36/EH36 Z25 specification described by SSAB, through-thickness testing requires a 25% minimum average reduction of area. The product information is available at SSAB’s AH36/DH36/EH36 Z25 specification. Z25 is not a higher-yield grade; it addresses ductility through the plate thickness. A drawing or class rule may require Z25 or Z35 for a particular joint, so the suffix must appear explicitly in the order and certificate.
Welding, filler metal and procedure qualification
Higher-strength and low-temperature steels are not interchangeable in the welding shop. AWS guidance states that AH36, DH36 and EH36 are common commercial-shipbuilding steels and that filler metals must be certified and selected for the specific base material. Filler choice therefore belongs with the approved welding procedure, classification requirements and mill certificate.
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The welding plan should establish, at minimum:
- Qualified welding procedure specification and supporting procedure qualification record.
- Approved process, joint preparation, preheat and interpass-temperature limits.
- Compatible certified filler metal and required strength or toughness class.
- Hydrogen-control, storage and handling requirements for consumables.
- Inspection, nondestructive testing and repair limits.
- Traceability from weld area to plate heat number and material certificate.
Do not select a consumable solely because its catalogue name contains “36.” The weld metal and heat-affected zone must satisfy the approved procedure for the actual thickness, restraint, position and service temperature.
How to specify and verify a plate
Use this sequence when preparing a procurement package or checking a delivery:
- Identify the structural location. Mark the hull member and connection on the approved drawing, including any fatigue- or fracture-critical designation.
- Confirm the governing rules. Record the classification society, rule edition and any project-specific material requirements.
- Set the strength level. Use the required yield band—235, 315, 355 or 390 MPa—after the scantling and buckling calculations are complete.
- Set the toughness class. Select AH, DH, EH or the required family from the rule-defined impact temperature and thickness.
- Check through-thickness needs. Add the required Z-quality suffix where the joint is susceptible to lamellar tearing.
- Define testing and certification. Require tension results, Charpy results, chemistry, thickness, heat number, delivery condition and class endorsement as applicable.
- Align welding documentation. Confirm that the WPS, PQR, filler metal certificate and inspection plan cover the ordered grade.
- Inspect on receipt. Match plate markings and heat numbers to the certificate, verify dimensions and review all required test results before cutting.
Common purchasing and design mistakes
Treating AH36, DH36 and EH36 as strength grades only
This can place a plate in service below its required fracture-toughness level. Always compare the Charpy temperature as well as the yield strength.
Using a stronger grade without redesign
A higher yield number does not automatically justify thinner material. Buckling, fatigue, corrosion allowance, minimum thickness and welding details may still govern.
Ignoring thickness limits
The ABS values cited above apply to plates up to 100 mm. Thicker material can have different permitted properties or additional testing requirements under the applicable standard and class rule.
Omitting the Z-quality requirement
A base grade such as EH36 does not by itself guarantee through-thickness ductility. If the detail requires Z25 or Z35, that suffix and test requirement must be included in the order.
Assuming a generic structural plate is class steel
A plate that resembles the grade designation but lacks the required heat traceability, impact results, class certification or approved delivery condition may not be acceptable for a commercial vessel.
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What to keep on the project file
For each heat and plate, retain the approved material certificate, heat number, grade and suffix, nominal thickness, tensile and yield results, Charpy temperature and energy results, chemical analysis, delivery condition, class inspection endorsement and receiving inspection record. Keep the welding procedure and filler-metal certificates linked to the same traceability system. This documentation allows the yard, surveyor and owner to prove that the installed steel matches the design.
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