AH36 vs DH36 – Composition, Heat Treatment, Properties, and Applications

Introduction

AH36 and DH36 are high‑strength, low‑alloy (HSLA) shipbuilding steels specified for hull structural members and offshore components. Engineers, procurement managers, and manufacturing planners often face the selection dilemma of balancing strength, toughness at service temperature, weldability, and cost when choosing between these grades. Typical decision contexts include whether a structure will operate in cold environments, whether thick sections or complex welds are required, and how much post‑fabrication testing and qualification is acceptable.

The principal practical distinction between AH36 and DH36 lies in their impact toughness qualification: DH36 is qualified to demonstrate higher impact toughness at lower temperatures than AH36. Because both grades share similar chemical strategies and strength levels, comparison typically centres on toughness in service, weld‑fabrication practices to preserve that toughness, and the additional testing or processing that DH36 may require.

1. Standards and Designations

• Common standards and designations:
• ASTM/ASME: ASTM A131 / A131M — "Shipbuilding Steel" (includes AH36, DH36, EH36).
• EN: EN 10025 family does not directly use AH/DH nomenclature; equivalents are sought via mechanical and impact requirements.
• JIS/GB: National standards may provide similar HSLA ship grades; local designations should be cross‑referenced.

• Classification societies (ABS, DNV, LR, etc.) publish acceptance criteria referencing ASTM A131 grades.

• Metallurgical class:

• AH36 and DH36 are HSLA carbon steels used for structural applications (not stainless, not tool steel).
• They are specified primarily for strength and notch toughness rather than corrosion resistance.

2. Chemical Composition and Alloying Strategy

Both AH36 and DH36 are produced using low carbon content with controlled additions of alloying and microalloying elements to achieve required strength and toughness. Exact composition limits are set by the governing standard and mill certificates; the table below provides representative elements of interest and their typical presence in these grades. Always verify the mill certificate for specific batches.

Table: Typical alloying elements and their role

Element	Typical presence / control (representative)	Primary metallurgical role
C (Carbon)	Low (controlled to limit hardenability)	Strength (solid solution), affects weldability and HAZ hardening
Mn (Manganese)	Moderate (major alloying element)	Strength, hardenability, deoxidation
Si (Silicon)	Low to moderate (deoxidizer)	Deoxidation, contributes slightly to strength
P (Phosphorus)	Strictly limited (trace)	Impairs toughness if high
S (Sulfur)	Strictly limited (trace)	Reduces ductility and toughness
Cr, Ni, Mo	Usually low to trace (not bulk alloyed)	Hardenability and strength when present; generally limited
V, Nb, Ti (microalloying)	Often present in small amounts	Precipitation strengthening, grain‑size control, toughness
B (Boron)	May be present in ppm	Improves hardenability in minute amounts
N (Nitrogen)	Controlled	Interacts with microalloying elements (Ti, Nb)

Explanation: The alloying strategy in AH36 and DH36 centers on keeping carbon low to preserve weldability and HAZ toughness, while using controlled manganese and trace microalloying (Nb, V, Ti) to obtain yield strength through grain refinement and precipitation strengthening. For DH36, mill practice and thermo‑mechanical control may be tightened to meet more stringent low‑temperature impact requirements.

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• As‑rolled and normalized HSLA ship steels exhibit a ferrite–pearlite or bainitic matrix with fine acicular ferrite depending on cooling rate and microalloying.

• Microalloyed steels (Nb, V, Ti) produce fine carbides/nitrides and restrict grain growth, improving strength and toughness.

• Effect of processing routes:

• Normalizing: Produces a refined ferrite–pearlite/bainitic structure and improves toughness compared with slow cooling; often used for moderate thicknesses.
• Quench & temper: Not typical for AH36/DH36 — these grades are normally delivered in the as‑rolled (thermo‑mechanically controlled) or normalized condition rather than quenched and tempered.

• Thermo‑mechanical control processing (TMCP): Widely used to produce fine‑grained microstructure and high toughness at lower temperatures. TMCP is particularly useful for achieving DH36’s lower‑temperature impact performance without high carbon/hardenability.

• Practical implication:

• DH36 often requires more stringent control of rolling and cooling to ensure the microstructure provides high absorbed energy at lower temperatures. That may affect mill selection and lead times.

4. Mechanical Properties

Both grades are specified to provide elevated yield and tensile strength for ship construction. The numbers below are typical ranges used for engineering comparison; verify contract and mill test reports for guaranteed minima.

Table: Representative mechanical property ranges (representative — check standards and mill certificates)

Property	AH36 (typical)	DH36 (typical)
Minimum Yield Strength	~355 MPa	~355 MPa
Tensile Strength (range)	~490–620 MPa	~490–620 MPa
Elongation (A%)	~18–24%	~18–24%
Impact Toughness (Charpy V‑notch)	Qualified at a higher test temperature than DH36	Qualified at a lower test temperature (greater low‑temperature toughness)
Hardness	Moderate (suitable for welding)	Similar to AH36 when processed similarly

Explanation: Strength levels are very similar for AH36 and DH36; the differentiation comes from impact toughness requirements at specified temperatures (DH36 retains toughness at lower temperatures). Elongation and hardness are comparable and largely a function of thickness and processing route.

5. Weldability

Weldability must be considered in two aspects: propensity to form hard, brittle HAZ microstructures and the ability to meet post‑weld toughness requirements.

• Carbon and hardenability:
• Low carbon and limited alloying reduce the risk of HAZ hardening; microalloying elements are used sparingly to avoid excessive hardenability.
• For weldability assessment engineers often use carbon equivalent indices. One common formula: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• A more detailed predictive formula is: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

• Interpretation:

• Both AH36 and DH36 typically have low $CE_{IIW}$ and $P_{cm}$ values relative to higher‑alloy steels, indicating generally good weldability.
• DH36’s tighter toughness requirements at lower temperatures mean that weld procedure qualification, preheat, interpass control, and post‑weld heat treatment (if specified) must be verified to preserve low‑temperature toughness — especially in thick sections or when using high heat input.
• Practical advice: For DH36, minimize HAZ hardening by controlling interpass temperature, selecting appropriate filler metal with matched toughness, and using procedures that avoid excessive cooling rates and martensitic microstructures.

6. Corrosion and Surface Protection

• These grades are non‑stainless carbon steels and do not provide intrinsic corrosion resistance beyond general carbon steel.
• Typical protection strategies:
• Surface coatings: primers, epoxy paints, and marine coatings.
• Galvanizing: applicable for some components (check thickness and fabrication sequence).

• Cathodic protection: frequently used for submerged or offshore structures.

• PREN (Pitting Resistance Equivalent Number) is not applicable to non‑stainless carbon steels; the PREN formula is relevant only for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$

• Clarification: Because AH36 and DH36 contain low Cr and negligible Mo or N, corrosion resistance must be provided by coatings, coatings maintenance, or sacrificial protection.

7. Fabrication, Machinability, and Formability

• Cutting and burning: Both grades cut and thermally cut (oxyfuel, plasma) similarly to other low‑carbon steels; edge heat‑affected zones should be removed or dressed if required for fatigue or coating adherence.
• Machinability: Moderate; typical HSLA steels machine satisfactorily with standard tooling but can be tougher than plain low‑carbon steels due to fine precipitates. Adjust speeds and tooling accordingly.
• Formability and bending: Good ductility allows bending and forming within standard limits. For tight radii or deep drawing, check bend radii and perform tests — DH36’s toughness focus does not improve formability but ensures performance at low temp.
• Welding fabrication: For DH36 thicker plates, more rigorous welding controls and notch toughness verification in weld and HAZ are recommended.

8. Typical Applications

Table: Typical uses by grade

AH36	DH36
General ship hull plating and structural members for temperate to milder cold service	Ship hulls, offshore topside and subsea structures intended for colder climates or where low‑temperature toughness is critical
Deck and framing where standard impact performance is sufficient	Arctic and high‑latitude vessels, LNG support structures requiring verified low‑temperature impact toughness
Barges, inland waterway craft, and auxiliary structures	Offshore jackets, risers, and equipment exposed to subzero ambient conditions
General marine fabrication where cost and availability are primary concerns	Critical welded details requiring verified low‑temperature HAZ performance

Selection rationale: Choose AH36 where standard ship‑grade toughness is adequate and cost/availability are primary constraints. Choose DH36 where operations or regulatory requirements demand demonstrated toughness at lower service temperatures or where brittle fracture prevention at low temperature is critical.

9. Cost and Availability

• Cost: Base material cost for AH36 and DH36 is comparable because they are produced from similar steelmaking routes. DH36 may incur slightly higher cost when tighter TMCP, additional microalloy control, or extra testing is required to meet low‑temperature impact criteria.
• Availability: Both grades are widely available from plate mills and stockists in standard shipbuilding thicknesses. Very thick plate or specific production conditions for DH36 may require longer lead times or selection of mills with appropriate TMCP capability.
• Product forms: Plates and coils are common; availability by thickness and dimension varies by mill and market.

10. Summary and Recommendation

Table: Quick comparison

Attribute	AH36	DH36
Weldability	Good (standard ship practice)	Good, but requires stricter procedure control for low‑temperature toughness
Strength–Toughness balance	High strength with standard impact qualification	High strength with enhanced low‑temperature impact qualification
Cost	Moderate (generally slightly lower)	Moderate to slightly higher (processing/testing premium possible)

Conclusion and selection guidance: - Choose AH36 if: - The structure operates in temperate environments where the standard impact qualification is sufficient. - You prioritize marginally lower material cost and broad availability. - Typical fabrication processes and standard welding procedures suffice without additional low‑temperature qualification.

• Choose DH36 if:
• The application will experience low ambient or service temperatures, or classification/regulatory requirements call for demonstrated toughness at lower temperatures.
• You require extra assurance against brittle fracture in the HAZ and base metal under cold conditions.
• You accept potentially tighter mill selection, welding procedure control, and possible marginal premium for the material or testing.

Final note: AH36 and DH36 are closely related HSLA shipbuilding steels; the decisive factor is the verified impact toughness at service or test temperatures. Always confirm the applicable standard edition, review mill test reports and impact‑test certificates, and qualify welding procedures for the exact thicknesses and joint details specified in the project.