EH36 vs FH36 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
EH36 and FH36 are high-strength shipbuilding steels commonly specified for hull and structural members where a balance of strength, toughness, and weldability is required. Engineers, procurement managers, and manufacturing planners frequently weigh trade-offs such as low-temperature toughness vs. cost, weldability vs. allowable thickness, and fabrication ease vs. in-service performance when choosing between the two.
The principal practical distinction encountered in industry is how each grade performs in very thick plate sections and under demanding low-temperature service conditions. Because both are high-strength low-alloy (HSLA) steels designed for marine structures, they are often compared side-by-side when specifying heavy plates for primary hull members, offshore structures, and other critical fabrications. Exact composition and mechanical guarantees vary by specification and mill, so selection should be based on the relevant code and mill test certificates.
1. Standards and Designations
- Common specification contexts where EH36 and FH36 appear:
- Classification societies and shipbuilding rules (e.g., ABS, DNV/GL, Lloyd’s Register).
- National and international steel/product standards and equivalents (examples include ASTM/ASME shipbuilding categories, EN, JIS, and various GB standards). Exact cross-references depend on the authority and the product form (plate, coil).
- Material type: both EH36 and FH36 are High-Strength Low-Alloy (HSLA) carbon steels tailored for marine structural use (not stainless or tool steels). They are not stainless steels and require surface protection for corrosion resistance.
2. Chemical Composition and Alloying Strategy
The following table gives typical compositional ranges frequently seen in shipbuilding HSLA grades of the “36” series. These are illustrative ranges; mill certificates and the controlling specification should be consulted for project-critical values.
| Element | Typical range, EH36 (wt%) | Typical range, FH36 (wt%) |
|---|---|---|
| C | 0.08 – 0.18 | 0.08 – 0.20 |
| Mn | 0.70 – 1.60 | 0.70 – 1.60 |
| Si | 0.10 – 0.50 | 0.10 – 0.50 |
| P | ≤ 0.035 (max) | ≤ 0.035 (max) |
| S | ≤ 0.035 (max) | ≤ 0.035 (max) |
| Cr | 0.00 – 0.30 | 0.00 – 0.30 |
| Ni | 0.00 – 0.50 | 0.00 – 0.50 |
| Mo | 0.00 – 0.10 | 0.00 – 0.10 |
| V | trace – 0.08 | trace – 0.08 |
| Nb (Nb/Ta) | trace – 0.06 | trace – 0.06 |
| Ti | trace – 0.02 | trace – 0.02 |
| B | trace (ppm) | trace (ppm) |
| N | ≤ 0.012 (typical) | ≤ 0.012 (typical) |
Notes: - Both grades rely on microalloying (Nb, V, Ti) and controlled chemistry to achieve a fine-grained ferrite–pearlite/microalloyed ferritic microstructure that delivers high yield strength with acceptable toughness. - Slight chemistry variations (e.g., tighter control of sulfur, use of microalloy additions, or small Ni/Cr additions) are used to tailor hardenability, toughness at low temperatures, and through-thickness performance for very thick plates. - FH36 formulations are often tuned for guaranteed properties in thicker sections; EH36 may be produced with chemistry and processing optimized for enhanced low-temperature toughness in service.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both grades are produced to yield a primarily fine-grained ferritic microstructure with distributed pearlite and microalloy precipitates. The microalloy elements (Nb, V, Ti) refine grain size and provide precipitation strengthening. - In as-rolled and thermomechanically rolled (TMCP) plates, a bainitic/ferritic mixture can appear depending on cooling rates and alloy content.
Heat treatment and process routes: - Normalizing: can be used to refine grain size and improve uniformity in heavy sections but is not always performed for large ship plates due to cost. - Quenching & tempering: not typical for standard EH36/FH36 ship plates—these are primarily thermomechanically processed and controlled-rolled steels, not quenched-and-tempered alloy steels. - Thermo-mechanical control processing (TMCP): common for both grades to achieve high strength and good toughness without an additional heat treat. TMCP gives favorable through-thickness properties in properly controlled plates. - Very thick plate routes: for ultra-thick sections, controlled cooling and tailored chemistry (slightly lower carbon, microalloying strategy) are critical to avoid coarse-grained bands and to maintain through-thickness toughness; FH36 designations and deliveries are commonly optimized for these conditions.
4. Mechanical Properties
Mechanical properties depend strongly on plate thickness, rolling route, and the controlling specification. The table below summarizes typical target ranges and qualitative trends rather than single guaranteed numbers—project specifications and mill test reports must be the source of contractual values.
| Property | Typical EH36 (typical range / behavior) | Typical FH36 (typical range / behavior) |
|---|---|---|
| Yield strength (min) | ≈ 300–380 MPa (designed as a high-strength HSLA; actual min depends on spec and thickness) | ≈ 300–380 MPa (similar nominal levels; FH36 may be guaranteed at similar yields in heavier sections) |
| Tensile strength | ≈ 460–620 MPa (depends on thickness and processing) | ≈ 460–620 MPa |
| Elongation (A%) | ≈ 18–26% (reduced with increasing thickness) | ≈ 16–24% (thicker plates may show slightly lower elongation) |
| Impact toughness (Charpy V-notch) | Specified for lower temperatures for EH grades (improved low-temp toughness); typical acceptance energies depend on temperature and thickness | FH36 often specified and qualified for thicker plates where through-thickness impact may be lower at the same temperature—specification controls test temperature/energy |
| Hardness | Typically moderate (HB in typical HSLA ranges); not used as primary acceptance criteria | Similar, tailored to avoid excessive hardness that would impair weldability |
Interpretation: - Strength levels are broadly comparable between the two grades when produced to typical shipbuilding specifications. The meaningful differences are usually in guaranteed toughness at specified temperatures and in how properties are maintained through very thick sections. - EH36 is often associated with enhanced low-temperature toughness performance; FH36 is frequently selected where consistent properties (including toughness and strength) must be guaranteed in very thick plates.
5. Weldability
Weldability is primarily controlled by carbon content, combined alloying (hardenability), impurity levels (P, S), and microalloy additions.
Useful empirical indices: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Pcm index: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both EH36 and FH36 target relatively low carbon and controlled alloying to keep $CE_{IIW}$ and $P_{cm}$ in ranges that support conventional welding with preheat controls where required. - Microalloying (Nb, V, Ti) slightly increases hardenability and can raise the risk of HAZ (heat-affected zone) cracking if welding procedures are not controlled, especially in thicker sections. This is why preheat, interpass temperature control, and post-weld heat treatment (PWHT) strategies may be specified for very thick FH36 plates. - For heavy/ultra-thick plates (often a reason to choose FH36), weld procedure specifications typically require more rigorous preheat/interpass control and weld consumables to manage HAZ toughness and residual stresses.
6. Corrosion and Surface Protection
- Both EH36 and FH36 are plain carbon/HSLA steels (non-stainless). Corrosion protection is provided by coatings, cathodic protection, or metallic coatings as appropriate.
- Typical protection strategies:
- Hot-dip galvanizing (for some structural components, though not common for submerged hull plating).
- Organic coatings (epoxy/urethane systems) and primer/topcoat systems used for ship hulls and offshore structures.
- Cathodic protection (sacrificial anodes or impressed current) for immersed structures.
- PREN (Pitting Resistance Equivalent Number) is relevant only for stainless/ferritic stainless steels: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ This does not apply to EH36 or FH36 because they are not stainless alloys.
7. Fabrication, Machinability, and Formability
- Machinability: EH36 and FH36 are not designed for high machinability; machinability is typical of HSLA steels—adequate with correct tooling and cutting parameters. Slightly higher strength or hard spots in thicker plates can reduce tool life.
- Formability: Controlled rolling/TMCP improves formability for hot and cold forming when compared to quenched-and-tempered steels. Bending and forming limits decrease with increasing thickness; FH36 supplied for very thick plates may require higher forces and special bend radii.
- Cutting and welding: Plasma and oxy-fuel cutting are common. Welding requires appropriate consumables matching the plate strength and toughness requirements; preheat and interpass temp control are more critical on thick FH36 plates.
8. Typical Applications
| EH36 — Typical uses | FH36 — Typical uses |
|---|---|
| Hull plates and structural members where enhanced low-temperature toughness is required (polar or cold-water ships, deckhouses) | Very heavy hull plating, bulkheads, and primary structural members where large plate thicknesses are required and through-thickness assurances are critical |
| Plates where impact resistance at lower temperatures is a priority | Offshore platform components and heavy fabrication elements requiring consistent properties through ultra-thick sections |
| Areas requiring good weldability with toughness guarantees | Situations where the fabrication route includes large single plates and where specification-driven production for thickness is needed |
Selection rationale: - Choose the grade that matches the project’s required minimum toughness temperature, guaranteed thickness limits, and weld procedure capabilities. For cold-service hulls, EH36-like specifications are attractive; for very heavy plates with tight through-thickness requirements, FH36-like deliveries are often preferred.
9. Cost and Availability
- Relative cost: Both grades are in the same family; pricing differences are typically small and driven by plate thickness, processing (TMCP, normalizing), and certification/testing requirements. Plates specially rolled/controlled for ultra-thick sections (often FH36 deliveries) can carry a premium because of longer production runs, stricter testing, and greater scrap handling.
- Availability: Common shipbuilding grades (AH36/DH36/EH36 variants) are widely available from major mills worldwide. FH36-type plates intended for very heavy thicknesses may have more limited availability and longer lead times—particularly for large single-piece panels or projects requiring specific mechanical property guarantees through thickness.
10. Summary and Recommendation
| Attribute | EH36 | FH36 |
|---|---|---|
| Weldability | Good (low C, controlled alloys); optimized for standard fabrication | Good, but thicker sections may require stricter preheat/PWHT controls |
| Strength–Toughness balance | High toughness focus, particularly at lower temperatures | High-strength with emphasis on property retention in very thick plates |
| Cost | Typical HSLA pricing; moderate depending on processing | Potential premium for ultra-thick, specially processed plates |
Recommendations: - Choose EH36 if: you need a high-strength shipbuilding steel that is optimized for low-temperature toughness and standard-to-heavy plate thicknesses where impact resistance at colder service conditions is a priority and where routine welding procedures are acceptable. - Choose FH36 if: your application requires very heavy or ultra-thick plates with guaranteed through-thickness strength and mechanical properties, and you are prepared to follow more stringent welding and fabrication controls (and to accept possible longer lead times and incremental cost).
Final notes: - Always specify the controlling standard, required thickness range, test temperatures for impact toughness, and welding procedure qualifications in purchase documents. Mill certificates, including chemical analysis and mechanical test reports taken from the actual plate thickness, are essential for verifying that the delivered material meets the chosen grade’s performance intent.