EH40 vs FH40 – Composition, Heat Treatment, Properties, and Applications

Introduction

EH40 and FH40 are two high-strength structural steels encountered most often in maritime, offshore and heavy plate industries. Engineers, procurement managers, and manufacturing planners commonly face a selection dilemma between the two: which grade provides the required through-thickness toughness and weldability for very thick plates, and which offers the best strength-to-cost balance for standard plate thicknesses and welded structures. Typical decision contexts include hull and deck plating for ships and offshore platforms, heavy fabrications where crack-arresting toughness is critical, and large welded structures where thermal cycles and post-weld properties govern performance.

The principal practical distinction between the two is their optimization for plate thickness and through-thickness performance: one grade is typically specified for conventional heavy-plate applications where standard thermomechanical processing yields the required strength and toughness, while the other is tailored for extreme plate thicknesses and improved through-thickness properties using different alloying and processing strategies. This is why designers commonly compare EH40 and FH40 when specifying very thick plates or when demanding exceptionally uniform toughness through plate thickness.

1. Standards and Designations

Both EH40 and FH40 are best described as high-strength, low-alloy (HSLA) structural steels used in shipbuilding and offshore construction. They are not single international test methods like an ASTM number but appear as grade families adopted or referenced by classification societies and national standards. Typical standards and classifications to consider:

• National and regional standards: GB (China), JIS (Japan), EN (Europe), ISO.
• Classification societies: ABS, DNV-GL, Lloyd’s Register — these societies include hull and structural steel designations that correlate to EH/FH families.
• Generic material standards: ASTM/ASME provide mechanical-property requirements and test procedures that may be used in conjunction with a class designation.

Material type identification: - EH40: HSLA structural steel (low-alloy carbon steel with microalloying and thermomechanical control). - FH40: HSLA structural steel, typically optimized for very thick-plate applications with enhanced through-thickness toughness and specific alloying/processing modifications.

Note: Exact chemical and mechanical requirements vary among standards and mill certificates; always refer to the contract specification or classification society notation for procurement.

2. Chemical Composition and Alloying Strategy

The table below shows representative composition ranges typically encountered for EH40 and FH40 style HSLA steels. These are representative ranges used by mills and found in classification-documents; actual certified chemistry must be taken from the mill certificate for a supplied lot.

Element	EH40 (typical range, wt%)	FH40 (typical range, wt%)
C	0.08 – 0.16	0.06 – 0.14
Mn	0.6 – 1.5	0.6 – 1.8
Si	0.02 – 0.50	0.02 – 0.50
P (max)	≤ 0.03 – 0.04	≤ 0.03 – 0.04
S (max)	≤ 0.010 – 0.025	≤ 0.010 – 0.025
Cr	trace – 0.4	trace – 0.6
Ni	trace – 0.6	trace – 0.8
Mo	trace – 0.05	trace – 0.08
V	0.00 – 0.08	0.00 – 0.10
Nb (Nb/Ta)	≤ 0.05	≤ 0.06
Ti	trace	trace
B	trace (ppm)	trace (ppm)
N	control-levels (ppm)	control-levels (ppm)

Explanatory notes: - These grades are HSLA steels where strength comes from a combination of carbon, manganese, and microalloying elements (V, Nb, Ti) plus processing (thermo-mechanical control). - FH40-style chemistry may show slightly lower carbon and somewhat higher microalloy additions to promote fine carbide/nitride precipitation and better through-thickness toughness in very thick sections. - Alloying increases hardenability and strength (Mn, Cr, Mo) but also raises the risk of cold cracking in welds; microalloying (Nb, V, Ti) enables high strength with lower carbon by precipitation strengthening and grain refinement.

3. Microstructure and Heat Treatment Response

Typical microstructures for EH40 and FH40 grades depend strongly on processing:

• EH40 (standard heavy plate, TMCP or normalized):
• Typical microstructure: fine-grained ferrite–pearlite or ferrite with dispersed bainite depending on cooling rate and alloying.
• Thermomechanical controlled processing (TMCP) produces a refined ferrite matrix with controlled amounts of lower bainite or degenerate pearlite for increased yield strength and good toughness.

• Normalizing can be used to homogenize structure in thicker plates; quench-and-temper is not typical for large structural plates due to distortion and cost.

• FH40 (very thick-plate optimized):

• Emphasis on through-thickness properties: tighter control of cooling and microalloy precipitates, often with lower carbon and more microalloying to maintain toughness through the centerline of very thick plates.
• Microstructure is engineered to reduce banding and to promote acicular ferrite or fine polygonal ferrite with distributed carbides and nitrides.
• Thermo-mechanical rolling schedules, accelerated cooling, and controlled reheating are used to achieve uniform grain refinement through thickness.

Heat treatment response: - Normalizing improves uniformity and toughness but may be impractical for extreme thicknesses. - Controlled rolling and accelerated cooling are the industrial route to obtain the necessary combination of strength and ductility without full quench-and-temper. - FH40 may require more stringent process control and additional nondestructive testing for very thick plates to assure through-thickness toughness.

4. Mechanical Properties

Below are representative mechanical property ranges commonly specified for EH40 and FH40 style HSLA plates. Values vary with thickness, processing, and specification limits—consult the contract requirements.

Property	EH40 (typical)	FH40 (typical)
Tensile strength (MPa)	490 – 650	480 – 640
Yield strength (MPa)	355 – 485	320 – 460
Elongation (% on 50 mm or as-specified)	18 – 26	18 – 26
Charpy impact (J)	Specified at low temperatures; typical 27 J at −20 °C to −40 °C	Stricter through-thickness requirements; 27 J at lower temperatures and/or higher-thickness testing
Hardness (HB)	160 – 250	150 – 240

Interpretation: - EH40 and FH40 overlap in nominal strength ranges; EH40 is often specified for slightly higher yield targets in standard thicknesses. - FH40 is commonly configured to emphasize through-thickness toughness rather than marginally higher yield strength — this can translate to slightly lower nominal yield but superior crack arresting and fracture toughness in thick plates. - Ductility (elongation) is comparable when each grade is produced to its specification; toughness performance, particularly through-thickness and at low temperature, is the differentiator.

5. Weldability

Weldability of these HSLA steels is determined by carbon content, carbon equivalent, and microalloying content. Common weldability indices used to qualitatively assess susceptibility to hydrogen-assisted cold cracking include:

• Carbon equivalent (IIW form): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• 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 carbon and controlled Mn favor weldability; microalloying elements (Nb, V, Ti) increase hardenability, which can increase the risk of hard zones in the HAZ and cold-cracking if hydrogen and restraint are not controlled. - FH40-style chemistries (with lower C and more microalloying) are often selected to balance hardenability and toughness; preheat, controlled interpass temperature, low-hydrogen procedures, and post-weld heat treatment considerations must be addressed depending on plate thickness and specification. - For very thick plates, controlling heat input, preheat, and HAZ cooling rate is critical; welding procedure qualification (WPS/PQR) and hydrogen control become more demanding.

6. Corrosion and Surface Protection

• Both EH40 and FH40 are non-stainless low-alloy steels and require surface protection in corrosive environments (marine sea water, atmospheric exposure).
• Common protection methods: hot-dip galvanizing (where applicable), multi-coat epoxy systems, polyurethane topcoats, metallizing (thermal spray), and sacrificial anodes for submerged applications.
• Stainlessness indices (e.g., PREN) are not applicable for these carbon/alloy steels: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This formula is meaningful only for stainless steels with significant Cr, Mo, and N; for EH40/FH40 the Cr and Mo levels are too low for PREN-based corrosion classification.
• Selection of coatings and cathodic protection must consider the design life, maintenance access, and intended environment (splash zone, submerged, atmospheric).

7. Fabrication, Machinability, and Formability

• Machinability: Both grades are moderate in machinability; lower-carbon variants (typical of FH40) can be slightly easier to machine. Carbide or coated tooling and appropriate feeds/speeds are recommended for heavy-section cuts.
• Formability/bending: EH40 with higher yield aims may reduce allowable forming strain compared with lower-carbon FH40. Cold bending of thick plates is limited and often requires heating or roll bending; forming limits must be validated with bend tests per specification.
• Cutting and thermal cutting: Plasma and oxy-fuel cutting are standard for thick plates. Preheating and controlled post-cut cleaning reduce residual stresses and heat-affected microstructures.
• Surface preparation for welding and coatings must be carefully controlled on very thick plates to avoid lamination or centerline defects becoming failure initiators.

8. Typical Applications

EH40 (common uses)	FH40 (common uses)
Ship hull and deck plating where higher nominal yield is desired for standard heavy plates	Very thick hull or deck plates where through-thickness toughness is critical (e.g., ice-class sections, large offshore platform base plates)
Structural members (girders, brackets) in heavy fabrication	Deep-section welded structures and thick transition plates requiring minimized centerline embrittlement
Pressure-retaining non-pressurized components where strength and cost balance is important	Critical welded joints with large thickness where crack arrest and fracture toughness across thickness must be assured

Selection rationale: - EH40: chosen for high strength in standard production thickness ranges where conventional TMCP yields the required properties. - FH40: chosen when plate thickness exceeds normal TMCP window or when more severe through-thickness toughness demands exist; processing and chemistry optimized to maintain properties deep into the plate.

9. Cost and Availability

• Cost: Generally both grades are in the HSLA price band; FH40 may command a premium because of stricter process control, more stringent testing, and possible special rolling schedules for very thick plates.
• Availability: EH40-style plates are common from many mills in standard thickness ranges. FH40-style plates can be available but may require special ordering, longer lead times, and certification of through-thickness testing for very thick sections.
• Product forms: Plate, cut-to-size, and pre-fabricated assemblies. Very thick FH40 plates may be produced by fewer mills and so lead times and minimum order quantities should be discussed early in procurement.

10. Summary and Recommendation

Attribute	EH40	FH40
Weldability	Good with standard controls; watch HAZ hardenability	Good but requires stricter WPS for very thick sections
Strength–Toughness balance	High nominal strength in standard thickness	Optimized through-thickness toughness in very thick plates
Cost	Typically lower for standard production	Potential premium for special processing and testing

Recommendations: - Choose EH40 if you need high-strength HSLA plate in conventional heavy-plate thicknesses where standard TMCP or normalizing gives adequate through-thickness toughness, and you prioritize strength-to-cost for common structural applications. - Choose FH40 if you are specifying very thick plates (extreme thickness) or if design requires assured through-thickness fracture toughness and minimal centerline embrittlement; FH40-style chemistry and processing help maintain uniform properties through large cross-sections, though at a likely higher procurement and processing cost.

Final note: For any critical application, specify required thickness-dependent mechanical and toughness criteria, welding procedure qualifications, and nondestructive examination in the contract. Confirm mill certificates and conduct independent testing where necessary to ensure the selected grade meets project-specific through-thickness and welding performance requirements.