DNV AH36 vs EH36 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
DNV (and other marine classification societies) list AH36 and EH36 among the commonly specified high‑strength shipbuilding steels used for hulls, decks, and other primary structural members. Engineers, procurement managers, and manufacturing planners routinely weigh trade‑offs such as cost versus guaranteed low‑temperature toughness, weldability versus strength, and production route (TMCP versus conventional rolling) when selecting between these two grades.
The central practical distinction is that EH36 is qualified for significantly better low‑temperature impact performance than AH36; both grades deliver comparable static strength, but EH36 carries explicit toughness testing requirements for colder service conditions. Because their chemical compositions and yield/tensile envelopes are close, the decision often hinges on intended service temperature, welding and fabrication practices, and budget.
1. Standards and Designations
Major standards that define AH36 and EH36 (or their equivalents) include: - ASTM A131 / ASME: Shipbuilding steel grades AH36, DH36, EH36. - DNV (Det Norske Veritas) class notation uses equivalent designations and acceptance criteria consistent with marine structural requirements. - EN / JIS / GB: equivalent EN/ISO / JIS shipbuilding steels exist (e.g., S355 type HSLA steels) but direct one‑to‑one mapping requires attention to impact test temperatures and thickness limits. Classification: Both AH36 and EH36 are high‑strength low‑alloy (HSLA) structural carbon steels optimized for shipbuilding — not stainless, not tool steel.
2. Chemical Composition and Alloying Strategy
The following table lists typical composition ranges (%) commonly associated with AH36 and EH36 as manufactured to shipbuilding specifications. Exact limits vary by standard version and mill practice; consult the governing standard or mill certificate for guaranteed values.
| Element | AH36 (typical range, wt%) | EH36 (typical range, wt%) |
|---|---|---|
| C | 0.12–0.20 | 0.10–0.18 |
| Mn | 1.00–1.60 | 1.00–1.60 |
| Si | 0.10–0.50 | 0.10–0.50 |
| P | ≤ 0.035 | ≤ 0.035 |
| S | ≤ 0.035 | ≤ 0.035 |
| Cr | trace – 0.30 | trace – 0.30 |
| Ni | trace – 0.30 | trace – 0.30 |
| Mo | trace – 0.08 | trace – 0.08 |
| V | trace – 0.06 | trace – 0.06 |
| Nb (Cb) | trace – 0.05 | trace – 0.05 |
| Ti | trace – 0.02 | trace – 0.02 |
| B | trace | trace |
| N | trace | trace |
Notes: - AH36 and EH36 are typically produced by thermomechanical controlled processing (TMCP) or rolling with tight control of carbon and microalloying to achieve the strength/toughness balance. - EH36 may be processed with slightly lower carbon equivalents and tighter control of inclusion and grain size to meet low‑temperature impact requirements. - Alloying strategy: low carbon + controlled Mn and microalloying (Nb, V, Ti) promote fine ferrite–pearlite or bainitic microstructures, improving strength without greatly increasing hardenability that would hurt weldability.
How alloying affects properties: - Carbon: raises strength/hardenability but reduces weldability and low‑temperature toughness when increased. - Manganese: strengthens but increases hardenability; controlled levels assist toughness. - Microalloying (Nb, V, Ti): enables precipitation strengthening and grain refinement — improves yield strength and toughness without high carbon. - Low P and S and controlled inclusions are critical for Charpy impact performance, especially for EH36.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both grades are designed to exhibit fine ferritic or bainitic microstructures depending on plate thickness and TMCP schedules. The target is a fine distribution of acicular ferrite, bainite, and controlled pearlite rather than coarse pearlite or martensite. - TMCP: controlled rolling and accelerated cooling refine grain size and produce bainitic/fine ferrite microstructures that give high strength with good toughness. - Conventional normalized plate: coarser ferritic/pearlitic structures may be acceptable for AH36 in thicker sections, but meeting EH36 low‑temperature impact targets generally requires TMCP or stricter processing.
Heat treatment response: - Normalizing can improve toughness and homogenize properties but is seldom used on a production scale for heavy ship plate because of cost. - Quenching & tempering (Q&T) is not typical for AH36/EH36 plate — these are chiefly controlled‑rolled HSLA steels designed to meet properties in the as‑rolled or controlled‑cooled condition. - Thermomechanical processing (TMCP) is the preferred industrial route to achieve EH36 toughness at low temperature while maintaining strength and weldability.
4. Mechanical Properties
Key mechanical property ranges (typical/minimums as per shipbuilding specifications):
| Property | AH36 | EH36 |
|---|---|---|
| Yield strength (min) | ~355 MPa | ~355 MPa |
| Tensile strength (typical range) | 490–620 MPa | 490–620 MPa |
| Elongation (typical) | ≥ 18–22% (depending on thickness) | ≥ 18–22% (depending on thickness) |
| Impact toughness (specified) | Not required at extreme sub‑zero; may be tested at higher temperatures | Specified at lower temperatures (e.g., −40 °C) for full qualification |
| Brinell hardness (typical) | ≤ ~200–230 HB (depends on plate and process) | similar, controlled to avoid brittle behavior |
Interpretation: - Static strength (yield and tensile) is essentially comparable between AH36 and EH36 when produced to the same thickness and processing route. - The primary differentiator is impact toughness under low‑temperature conditions: EH36 is qualified to sustain significant Charpy V‑notch energy at substantially lower temperatures than AH36. That makes EH36 preferable for cold‑climate or high‑latitude service. - Ductility (elongation) is similar across both when thickness is comparable; toughness depends on microstructure and cleanliness more than bulk chemistry.
5. Weldability
Weldability considerations hinge on carbon level, carbon equivalent (hardenability), and microalloy content. Two commonly used indices are provided here as examples.
Display the IIW carbon equivalent formula: $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$
And the more comprehensive Pcm formula: $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$
Qualitative interpretation: - Both AH36 and EH36 are designed with relatively low carbon and controlled alloying so their $CE_{IIW}$ and $P_{cm}$ values are moderate, supporting good weldability with standard procedures. - EH36’s need for improved low‑temperature toughness does not necessarily increase bulk carbon but may require tighter compositional control and processing. Consequently, the weldability of EH36 can be similar to AH36, but preheat, interpass temperature, and weld procedure specifications are often more strictly enforced to preserve low‑temperature toughness in the heat‑affected zone (HAZ). - Practical welding guidance: low hydrogen consumables, controlled preheat for thick sections, and post‑weld heat treatment only where specified. Avoid excessive hardenability in HAZ by maintaining low carbon and limiting alloying additions.
6. Corrosion and Surface Protection
- Both AH36 and EH36 are carbon‑based HSLA steels (not stainless); corrosion resistance in marine environments depends on protective systems.
- Typical protection strategies: fusion‑bonded epoxy, multi‑coat marine paint systems, galvanizing (where appropriate), and sacrificial anodes for immersed applications.
- Because neither grade is stainless, PREN (pitting resistance equivalent number) is not applicable for their corrosion classification. For reference, PREN is defined as: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
- Surface preparation, seam sealing, and cathodic/anodic protection design are primary design levers. EH36’s enhanced toughness does not provide intrinsic corrosion resistance benefits over AH36.
7. Fabrication, Machinability, and Formability
- Machinability: both grades are typical structural steels; machinability is average and dependent on microstructure, hardness, and thickness. The addition of microalloying elements in EH36 can slightly reduce machinability relative to very low‑alloy steels, but differences are usually modest.
- Formability/bending: comparable for both grades when in equivalent temper and thickness; EH36’s tighter toughness and strength control may require slightly larger bend radii for heavy sections to avoid cracking in the finished part.
- Hole punching and cold forming for thick plates should follow supplier guidelines; cryogenic or very cold forming is not recommended without qualification.
8. Typical Applications
| AH36 — Typical Uses | EH36 — Typical Uses |
|---|---|
| Ship hull plating in temperate service | Ship hull/deck plating for polar/arctic service |
| Offshore platform topside structures in milder climates | Arctic offshore structures and ice‑capable hulls |
| Bulk carriers, general cargo vessel structural members | Vessels operating in sustained low temperatures, LNG carriers’ jacket structures where low‑temp toughness needed |
| Decks, framing, and general structural plate where low‑temp impact is not critical | Critical primary structures subject to brittle fracture risk in sub‑zero service |
Selection rationale: - Choose AH36 where structural strength is required but ambient/service temperatures remain moderate and impact requirements at very low temperatures are not mandated. - Choose EH36 when there is a firm requirement for assured impact toughness at substantially sub‑zero temperatures (e.g., high‑latitude operations, Arctic/regional regulations), even if cost and production controls are higher.
9. Cost and Availability
- Cost: EH36 typically commands a premium versus AH36 because of the stricter processing, testing, and potentially tighter chemistry/control needed to meet low‑temperature impact criteria. The premium varies by mill, order size, and market conditions.
- Availability: AH36 is widely available in standard plate sizes and thicknesses. EH36 is also common among ship plate producers but availability can be more constrained for very thick plates or unusual dimensions because of the need for controlled processing and additional impact testing.
- Product form: plate, welded sections, and cut‑to‑size plates are common; delivery times for EH36 may be longer if specific impact testing at low temperatures is required.
10. Summary and Recommendation
Summary table (qualitative)
| Attribute | AH36 | EH36 |
|---|---|---|
| Weldability | Very good (standard procedures) | Very good but stricter WPS control recommended |
| Strength–Toughness balance | High strength; adequate toughness at moderate temps | High strength; superior low‑temperature toughness by specification |
| Cost | Lower | Higher (premium for low‑temp qualification) |
Final recommendations: - Choose AH36 if you need a high‑strength, readily available shipbuilding plate for temperate environments where extreme low‑temperature impact resistance is not required and you want lower material cost and simpler procurement. - Choose EH36 if the structure will operate in cold or Arctic conditions, if regulations require demonstrated Charpy toughness at low temperatures, or if the design has brittle fracture sensitivity (thin sections, high restraint, high residual stress). The extra cost is justified by reduced fracture risk and regulatory compliance.
Concluding note: AH36 and EH36 deliver comparable static strength; the practical selection should be driven by required impact performance at the service temperature, weld procedure constraints, and lifecycle risk. Always verify the exact chemical and mechanical acceptance criteria with the project specification and the mill test certificate before final selection.