AH36 vs AH40 – Composition, Heat Treatment, Properties, and Applications

Introduction

AH36 and AH40 are marine structural steel grades commonly used for ship hulls, offshore structures, and heavy plate applications. Engineers, procurement managers, and manufacturing planners routinely evaluate these grades when balancing requirements for structural strength, fracture resistance, weldability, manufacturability, and cost. Typical decision contexts include choosing a heavier, more ductile plate to resist brittle fracture in cold environments versus a higher-strength plate to reduce section thickness and weight.

The fundamental practical distinction between AH36 and AH40 lies in their design strength level and the metallurgical measures used to achieve it: AH40 is produced to a higher minimum-strength class than AH36, which influences required alloying, thermo-mechanical processing, and resulting ductility/toughness tradeoffs. These differences make the two grades natural comparators when designers must select between thicker, more ductile steel and thinner, higher-strength steel.

1. Standards and Designations

• Common standards and class rules:
• ABS (American Bureau of Shipping) — AH36, AH40 are ABS hull structural grades.
• ASTM/ASME — ASTM A131 refers to shipbuilding steels with similar grade names.
• JIS (Japan) and EN (Europe) have different nomenclature but comparable HSLA/marine grades.
• GB (China) equivalents exist in national shipbuilding specifications.
• Metallurgical classification:
• Both AH36 and AH40 are carbon-manganese based high-strength low-alloy (HSLA) steels tailored for marine structural applications (not stainless, not tool steel).
• They are designed to provide a balanced combination of strength and toughness rather than hardness or wear resistance.

2. Chemical Composition and Alloying Strategy

Below is a qualitative comparison of typical alloying practices rather than specific guaranteed numbers (actual chemical limits vary by standard, mill, and product form—consult mill certificates for purchase decisions).

How alloying affects properties: - Carbon and manganese increase strength and hardenability but elevated carbon reduces weldability and toughness. Both grades manage carbon to balance properties. - Microalloying elements (Nb, V, Ti) are used in small amounts to refine grain size, enable higher strength through precipitation strengthening, and maintain toughness without requiring large carbon increases. - Silicon is primarily a deoxidizer and has minor strengthening effects. - Alloying for corrosion resistance (Cr, Ni, Mo) is minimal or absent in these non-stainless, marine structural steels; corrosion protection is achieved by coatings and cathodic measures.

3. Microstructure and Heat Treatment Response

• Typical production route:
• Both AH36 and AH40 are most commonly produced by controlled rolling and cooling (thermo-mechanical control processing, TMCP). They are normally supplied in the as-rolled or normalized condition rather than quenched and tempered.
• Microstructure:
• AH36: Fine ferrite-pearlite or ferrite-bainite microstructure with controlled grain size due to TMCP and microalloying. The matrix emphasizes ductility and good low-temperature toughness.
• AH40: Similar family of microstructures but typically has a higher fraction of finer bainitic constituents and/or stronger precipitation strengthening from microalloying. This yields a higher strength level while aiming to maintain acceptable toughness.
• Heat-treatment sensitivity:
• Normalizing (heating above critical and air cooling) can improve toughness and refine grain for both grades but is not always applied for large plates due to cost.
• Quenching and tempering is uncommon for these shipbuilding grades because it increases cost and may reduce weldability; when used, it will significantly raise strength but requires strict process control.
• Thermo-mechanical rolling and accelerated cooling are the preferred means of raising strength while keeping good toughness.

4. Mechanical Properties

The following table provides qualitative comparisons; guaranteed values are thickness- and standard-dependent and should be taken from the applicable specification or mill test certificate.

Explanation: - AH40 achieves higher strength through slightly higher alloying and/or tighter TMCP controls; this typically reduces ductility and may change impact toughness behavior if not processed carefully. - Designers choose AH36 when higher deformation capacity and energy absorption (e.g., for collision scenarios or low-temperature service) are priorities. - AH40 is selected when section reduction and weight savings are prioritized, provided toughness and weldability requirements are met.

5. Weldability

Weldability considerations for these grades hinge on carbon content, effective hardenability, and microalloying content. Two commonly used combined-content indices for weldability are shown below:

$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation (qualitative): - Lower values of $CE_{IIW}$ and $P_{cm}$ indicate easier weldability and lower susceptibility to hydrogen-assisted cold cracking. AH36 typically has lower effective hardenability indices than AH40 because AH40’s chemistry and processing push higher strength. - Microalloying elements (Nb, V) and slightly higher Mn can increase hardenability and the risk of martensitic regions in the heat-affected zone (HAZ) under high cooling rates; consequently, preheat and controlled interpass temperatures are more often required for AH40 in thick sections. - Both grades are weldable with conventional processes (SMAW, GMAW, SAW), but welding procedure specifications must account for plate thickness, joint design, and the grade’s effective hardenability. Post-weld heat treatment is rarely used for shipbuilding plates; instead, preheat and controlled consumables are used.

6. Corrosion and Surface Protection

• These AH grades are non-stainless carbon/alloy steels; intrinsic corrosion resistance is minimal. Corrosion protection relies on coatings, cathodic protection, and design details that avoid crevices or stagnant water.
• Typical protection methods: hot-dip galvanizing (where appropriate for thinner parts or components), industrial coatings (epoxy, polyurethane), and sacrificial anodes for submerged structures.
• PREN (Pitting Resistance Equivalent Number) is not applicable to these non-stainless steels; for reference, PREN is calculated as: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$ but would not be meaningful for AH36/AH40 because Cr and Mo contents are negligible.

7. Fabrication, Machinability, and Formability

• Formability and bending:
• AH36, being more ductile, is easier to form and bend to radii without cracking; springback is lower.
• AH40 requires more careful tooling and bend allowances; smaller bend radii and cold-forming operations must be evaluated against reduced elongation.
• Cutting and machining:
• Higher strength in AH40 can increase cutting forces and tool wear slightly versus AH36; however, both are readily machined with standard practices when using appropriate tooling and feeds.
• Finishing:
• Grinding, shot blasting, and surface prep behave similarly; avoid overheating during cutting or grinding to prevent surface hardening or tempering effects.
• Dimensional control:
• Thinner sections made possible by AH40 should be judged against distortion sensitivity during welding and finishing.

8. Typical Applications

Selection rationale: - Choose AH36 where deformation capacity, low-temperature performance, and simpler fabrication/welding are decisive. - Choose AH40 where material weight reduction, higher allowable stresses, or space constraints justify the higher-strength plate and where manufacturing controls can ensure adequate toughness and weld integrity.

9. Cost and Availability

• Relative cost:
• AH40 is typically priced higher per tonne than AH36 because of tighter processing controls and possible microalloy additions needed to achieve the higher strength class.
• Total component cost may be lower with AH40 if section thickness reductions lead to lower material weight and downstream savings.
• Availability:
• AH36 is widely available in most shipbuilding plate inventories and standard mill product lines.
• AH40 is also common but availability may be more limited for special thicknesses, widths, and grades that require specific TMCP schedules; lead times can be longer for non-standard plate sizes.
• Procurement note:
• Evaluate total life-cycle cost including fabrication, welding prep, coatings, and potential savings from reduced weight when specifying AH40 over AH36.

10. Summary and Recommendation

Recommendation: - Choose AH36 if your primary requirements emphasize ductility, ease of fabrication and welding, robust low-temperature toughness, and straightforward procurement (typical heavy-plate shipbuilding applications). - Choose AH40 if you need a higher design strength to reduce plate thickness and structure weight, and you can accept (and manage) the trade-offs: slightly reduced elongation, tighter processing and welding controls, and potentially higher material unit cost.

Final practical note: Always confirm the applicable standard and review mill test certificates for chemical composition and guaranteed mechanical properties for the specific plate thickness and heat. Welding procedure specifications and qualification tests should be derived from the chosen grade, thickness, and service environment to ensure performance and compliance.