DH36 vs EH36 – Composition, Heat Treatment, Properties, and Applications

Introduction

DH36 and EH36 are high‑strength, low‑alloy (HSLA) structural steels widely used in shipbuilding, offshore structures, and heavy fabrication. Engineers and procurement professionals commonly face a selection dilemma between these two grades when balancing impact toughness at low temperatures against cost, weldability, and availability. Typical decision contexts include hull plating for different service climates, small‑boat vs. polar applications, and fabrication workflows that constrain post‑weld heat treatment.

The principal practical distinction between DH36 and EH36 lies in their specified low‑temperature impact performance and the production controls that support that performance. Both belong to the same family of HSLA ship steels and share similar chemical strategies, but EH36 is specified and processed to demonstrate superior notch toughness at lower temperatures than DH36, which affects processing, inspection, and cost.

1. Standards and Designations

• Common international standards where these designations appear:
• ASTM/ASME: ASTM A131 (shipbuilding steel) — AH36 / DH36 / EH36 are common A131 designations.
• ABS / DNV / LR / BV / NK: Classification societies reference equivalent requirements for plate grades.
• EN: EN 10025 family covers structural steels but does not use the AH/DH/EH nomenclature directly; EN grades such as S355 have comparable strength levels.
• JIS / GB: JIS and Chinese GB standards have analogous marine structural grades; national designations differ.
• Material type: HSLA (high‑strength low‑alloy) structural carbon steel with microalloying elements for strength and toughness.

2. Chemical Composition and Alloying Strategy

The following table gives representative composition ranges typical for AH/DH/EH36 family ship steels. Exact limits are standard‑ and mill‑specific; refer to the purchase specification for guaranteed values.

Element	Typical range / comment (wt%)
C	0.08 – 0.18 (kept low to preserve weldability and toughness)
Mn	0.7 – 1.6 (primary strength Mn contribution)
Si	0.10 – 0.50 (deoxidation; small amounts aid strength)
P	≤ 0.035 (controlled to avoid embrittlement)
S	≤ 0.035 (controlled to improve toughness & weldability)
Cr	≤ 0.40 (if present, improves hardenability and strength marginally)
Ni	≤ 0.50 (occasionally added to enhance low‑temperature toughness)
Mo	trace – 0.15 (can improve hardenability and creep resistance)
V	trace – 0.10 (microalloy, refines grain size)
Nb (Cb)	trace – 0.06 (microalloy, stabilizes fine grain structure)
Ti	trace – 0.02–0.05 (deoxidizer, grain refiner if used)
B	trace (ppm, can increase hardenability at very low levels)
N	controlled low levels (affects precipitation and toughness)

Alloying strategy: - Low carbon and controlled impurity levels maintain weldability and toughness. - Mn is the principal strength contributor; microalloying elements (Nb, V, Ti) are used in small amounts to refine grain size and increase yield strength through precipitation strengthening without raising carbon. - Small additions of Ni and Cr (or Mo) may be used to secure low‑temperature toughness or to marginally improve hardenability. EH36 typically requires tighter control of chemistry and thermomechanical processing to meet lower temperature impact criteria.

3. Microstructure and Heat Treatment Response

Typical microstructure: - As‑rolled or thermomechanically processed DH36/EH36 plates exhibit a refined ferrite–pearlite or ferrite with dispersed fine bainite/tempered martensite depending on cooling and alloy content. - Microalloyed steels with Nb/V/Ti promote a fine polygonal ferrite matrix with fine precipitates, improving yield strength and toughness.

Processing effects: - Normalizing: Raises toughness by producing a uniform fine grain structure; sometimes specified for very thick plates to assure homogeneity. - Quench & temper: Not normally applied to conventional ship plates due to cost; these grades are designed for as‑rolled or thermomechanically controlled processing where controlled cooling replaces full quench/temper cycles. - Thermo‑mechanical controlled processing (TMCP): Common route for DH36 and EH36 to achieve high strength and low‑temperature toughness without excessive alloy content. TMCP imparts favorable transformation sequences that produce acicular ferrite/bainite and limit coarse pearlite.

EH36 response: - To meet lower impact temperature requirements, EH36 is often produced with more rigorous TMCP schedules, lower carbon equivalent and stricter cleanliness to avoid embrittling inclusions; thicker plates may receive supplemental toughness testing or furnace normalization.

4. Mechanical Properties

Representative mechanical property ranges (typical; verify per standard and thickness):

Property	Typical requirement / range
Yield strength (min)	≈ 355 MPa (commonly specified for AH/DH/EH36 family)
Tensile strength	≈ 490 – 620 MPa
Elongation (A%)	≥ 20% (depends on thickness and standard)
Charpy impact	Grade‑dependent: DH36 tested at lower temperature than AH36; EH36 has specified impact energy at an even lower temperature
Hardness	Typically < 250 HB (varies with processing)

Interpretation: - Strength: DH36 and EH36 exhibit comparable nominal yield and tensile strength; differences are not primarily in static strength but in impact toughness at specified temperatures. - Toughness & ductility: EH36 is specified to retain higher notch toughness at lower temperatures than DH36. Achieving that typically requires tighter process control and sometimes reduced carbon equivalent, hence potentially slightly different mechanical trade‑offs. - Hardness: Both are not hardened steels; hardness is moderate and controlled by rolling and TMCP.

5. Weldability

Weldability considerations hinge on carbon content, carbon equivalent (CE), and presence of microalloying elements that increase hardenability.

Common weldability indicators (useful for qualitative assessment): - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Lower $CE_{IIW}$ and $P_{cm}$ values suggest easier weldability with less need for preheat and post‑weld heat treatment (PWHT). - DH36 and EH36 have low to moderate CE due to limited carbon and controlled alloying; therefore, they are generally considered weldable with standard procedures for structural steels. - EH36 may require more conservative welding practices for thick sections or very low ambient service temperatures because its production aims to ensure better low‑temperature toughness; microalloying that refines grain size can increase hardenability locally, so preheat and controlled interpass temperatures are sometimes recommended.

Practical welding guidance: - Use appropriate filler metals matching strength and toughness requirements. - For thick plates or cold service, qualify procedures and consider PWHT or controlled cooling to prevent HAZ hardening or hydrogen cracking. - Non‑destructive testing and coupon testing are prudent when substituting one grade for another in critical applications.

6. Corrosion and Surface Protection

• Neither DH36 nor EH36 is stainless steel; both are conventional carbon/HSLA steels and require surface protection for long‑term corrosion resistance.
• Typical protection systems: hot‑dip galvanizing (for some components), barrier coatings (epoxy primers, polyurethane topcoats), cathodic protection for offshore structures, and sacrificial corrosion allowances in design.
• Metal loss rates, maintenance frequency, and coating system selection depend on environment (marine splash, atmospheric, immersed seawater).
• PREN formula (for stainless assessment) is not applicable to these non‑stainless steels, but for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Use of PREN is only meaningful when evaluating stainless alloys.

7. Fabrication, Machinability, and Formability

• Forming: With moderate yield strength and decent ductility, both grades can be roll formed, bent, and pressed; bend radii must account for thickness and the lower temperature toughness requirements of EH36.
• Machinability: HSLA steels are less machinable than plain‑carbon steels due to microalloying and higher strength. Tooling wear is modestly higher; normal machining parameters and carbide tooling are typical.
• Cutting: Thermal cutting (oxyfuel, plasma) is common for plates; minimize HAZ size and perform post‑cut grit/blast for corrosion protection.
• Finishing: Grinding and surface prep follow standard practices; EH36 may require extra attention to avoid localized cold work that could affect toughness at low temperature.

8. Typical Applications

DH36 – Typical uses	EH36 – Typical uses
Hull plating and deck plates for vessels operating in temperate to cold climates (service down to about −20°C)	Hull and structural plating for vessels or offshore units intended for polar/sub‑arctic service (service down to about −40°C)
Offshore topsides and secondary structural members where moderate low‑temperature toughness is adequate	Critical structural members, braces, and low‑temperature impact‑sensitive components in arctic offshore platforms
Cargo decks, bulkheads, and general structural plate where cost‑efficiency and standard weld practice are prioritized	Structures where stricter qualification and toughness testing are required; areas with stringent notch‑toughness certification

Selection rationale: - Choose based on design temperature, required impact energy at that temperature, thickness (thick plates are more challenging to produce with uniform low‑temperature toughness), and lifecycle maintenance plans.

9. Cost and Availability

• Relative cost: EH36 is typically modestly more expensive than DH36 due to more stringent processing, tighter chemistry control, and additional testing to guarantee low‑temperature toughness.
• Availability: Both grades are widely available from major mills in plate form; however, very thick EH36 plates or specific thickness/width combinations may be less commonly stocked and subject to longer lead times.
• Product forms: Plate is the dominant form. Availability of cut‑to‑length, pre‑fabricated sections, or certified mill test reports should be confirmed at procurement.

10. Summary and Recommendation

Summary table (qualitative):

Characteristic	DH36	EH36
Weldability	Good (standard HSLA practices)	Good, but may require stricter welding control for thick sections
Strength–Toughness balance	High strength with good toughness at moderate low temps	Similar static strength, higher certified low‑temperature toughness
Cost	Lower	Higher (due to processing and testing)

Recommendation: - Choose DH36 if: the structure will operate in temperate or moderately cold environments (design service temperature around −20°C or higher), if cost and standard fabrication practices are priority, and when the thicknesses involved are within ranges that do not challenge toughness limits. - Choose EH36 if: the structure will be exposed to very low temperatures (arctic or sub‑arctic service), if regulatory or classification requirements mandate higher notch toughness at lower temperatures, or when critical welded details require guaranteed toughness margins despite thicker sections.

Final note: DH36 and EH36 belong to the same family of HSLA ship steels and are often interchangeable for many strength requirements, but the choice is governed by the specified impact temperature, processing controls, and inspection demands. For any critical application, always review the governing standard and mill material certificates, and qualify welding procedures and examination routines to the project specification.