AH32 vs AH36 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
AH32 and AH36 are widely used high‑strength structural steels in shipbuilding and heavy fabrication. Engineers, procurement managers, and manufacturing planners commonly face a selection dilemma between lower-cost, easier-to-fabricate grades and higher‑strength grades that permit thinner sections or lower weight. Typical decision contexts include choosing between weldability and forming ease versus the benefits of higher yield for reduced plate thickness, or comparing life‑cycle cost when corrosion protection and repairs are considered.
The central practical distinction between the two is that AH36 is specified to provide higher minimum yield strength than AH32, enabling greater load capacity or reduced section thickness for the same design stress. Because both grades are designed for maritime structural applications, they are often compared when optimizing hull plating, deck structures, brackets, and other primary structural members.
1. Standards and Designations
- Classification societies and standards bodies that commonly cover AH grades: ABS (American Bureau of Shipping), DNV‑GL / DNV, Lloyd’s Register, Bureau Veritas.
- International and national standards that reference equivalent shipbuilding grades or structural steel rules: ASTM A131 (Hull structural steel), and various national specifications and supplier datasheets. Equivalent or similar steels may be found under EN and JIS systems but exact grade names differ.
- Material classification: AH32 and AH36 are carbon–manganese, high‑strength low‑alloy (HSLA) structural steels intended for shipbuilding and marine construction (non‑stainless, non‑tool steel).
2. Chemical Composition and Alloying Strategy
The AH family is engineered to balance strength, toughness at low service temperatures, and weldability. Microalloying (Ti, Nb, V) and controlled additions of Mn and Si are commonly used to produce the target mechanical properties without excessive carbon.
Table: Typical composition ranges (wt%) — indicative and representative for common mill practice; actual guaranteed limits are set by specification or classification society.
| Element | AH32 (typical range) | AH36 (typical range) |
|---|---|---|
| C (carbon) | ~0.10 – 0.20 | ~0.10 – 0.22 |
| Mn (manganese) | ~0.50 – 1.60 | ~0.50 – 1.60 |
| Si (silicon) | ~0.10 – 0.50 | ~0.10 – 0.50 |
| P (phosphorus) | ≤ ~0.035 (max) | ≤ ~0.035 (max) |
| S (sulfur) | ≤ ~0.035 (max) | ≤ ~0.035 (max) |
| Cr (chromium) | trace – ~0.30 | trace – ~0.30 |
| Ni (nickel) | trace – ~0.50 | trace – ~0.50 |
| Mo (molybdenum) | trace – ~0.10 | trace – ~0.10 |
| V (vanadium) | trace (microalloy) | trace (microalloy) |
| Nb (niobium) | trace (microalloy) | trace (microalloy) |
| Ti (titanium) | trace (microalloy) | trace (microalloy) |
| B (boron) | trace (occasionally) | trace (occasionally) |
| N (nitrogen) | trace (~0.010–0.015) | trace (~0.010–0.015) |
Notes: - These ranges are indicative. Exact compositions depend on mill practice, thickness, purchaser requirements, and classification society rules. - AH36 achieves higher specified yield strength typically by tighter control of rolling/thermo‑mechanical processing and, in some cases, modest differences in microalloying or cooling strategy rather than large step‑changes in carbon content.
How alloying affects properties: - Carbon and manganese raise strength and hardenability but can reduce weldability and ductility if increased excessively. - Microalloying elements (Nb, V, Ti) refine grain size and provide precipitation strengthening, enabling higher yield for modest carbon levels—this is a key route to higher strength without large losses in toughness. - Si and trace Cu/Ni additions can improve atmospheric corrosion resistance modestly and can assist strength when combined with TMCP (thermo‑mechanical controlled processing).
3. Microstructure and Heat Treatment Response
Typical microstructure - As‑rolled and normalized AH32/AH36: predominantly fine ferrite and pearlite with entrained bainitic or acicular ferrite in TMCP product forms. The microstructure aims for a fine ferritic grain to preserve low‑temperature toughness. - TMCP (thermo‑mechanical controlled processing) is widely used to produce both grades at higher strength levels while retaining good toughness. TMCP produces fine polygonal ferrite and dispersions of bainitic/upper‑bainite features, and encourages acicular ferrite nucleation in the weld heat‑affected zone (HAZ).
Heat treatment response - These grades are normally supplied in the as‑rolled condition; post‑weld heat treatment is uncommon for large ship structures. - Normalizing (reheating above A3 and air cooling) can refine coarse as‑rolled grain and restore toughness after heavy processing but is not typically performed on large plates in service. - Quenching and tempering are not standard production/repair processes for AH grades and are generally reserved for specialized higher‑strength steel classes; quench & temper would change classification and hydrogen embrittlement/weldability considerations. - TMCP and accelerated cooling strategies allow increased yield without the need to increase carbon content—this is the common route used to reach AH36 performance levels.
4. Mechanical Properties
Table: Typical mechanical property ranges (indicative; exact guaranteed values are determined by specification, thickness, and testing).
| Property | AH32 (typical) | AH36 (typical) |
|---|---|---|
| Minimum Yield Strength (MPa) | ~315 MPa | ~355 MPa |
| Tensile Strength (MPa) | ~430 – 570 MPa | ~460 – 610 MPa |
| Elongation (A, % in 200 mm or specified gauge) | ~20 – 22% | ~18 – 22% |
| Charpy Impact Toughness (J) | Specified at low temperatures (e.g., -20 to -40°C); depends on thickness | Specified at low temperatures (e.g., -20 to -40°C); comparable but may require process control |
| Hardness (HB) | Typical range depends on condition; moderate (lower than quenched steels) | Slightly higher on average due to higher yield |
Interpretation: - AH36 is specified to a higher minimum yield, making it the stronger grade in terms of elastic/plastic design limit and enabling thinner sections or higher load capacity. - Toughness is controlled by processing and chemistry; both grades can be supplied with good low‑temperature impact properties, but higher strength targets (AH36) require tighter control of rolling and cooling to avoid loss of toughness. - Ductility (elongation) is similar in both grades, though very thick plates and higher yield requirements can reduce ductility if processing is not managed.
5. Weldability
Weldability considerations center on carbon equivalent and microalloying. Lower carbon and controlled alloying favor welding; higher hardenability increases risk for HAZ martensite and cold cracking.
Common weldability formulas (interpret qualitatively for AH steels): - Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - Pcm (more detailed): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Both AH32 and AH36 are designed for welding; their carbon equivalents are kept modest by limiting carbon and controlling Mn/Cr/Ni levels. - AH36, because of its higher yield/higher strength target, may have a slightly higher carbon equivalent or microalloying content and therefore a marginally greater sensitivity to HAZ hardening and hydrogen‑induced cracking. This can require more careful pre‑ and post‑weld procedures (e.g., preheat, controlled interpass temperatures, and hydrogen control) for thicker sections. - Use of low‑hydrogen consumables, proper joint design, and control of restraint and preheat typically manage weld cracking risks in both grades.
6. Corrosion and Surface Protection
- Neither AH32 nor AH36 is stainless; atmospheric or seawater corrosion protection is achieved by coatings, cathodic protection, or metallurgical corrosion‑resistant claddings.
- Typical protection systems: zinc-rich primers, epoxy and polyurethane topcoats, hot‑dip galvanizing (for smaller components), and specialized marine coatings for hulls.
- PREN (pitting resistance equivalent number) is not applicable for these carbon steels because PREN is designed for stainless alloy ranking: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For ship hulls and exposed structures, selection is driven by coating system life and repairability rather than metallurgical corrosion resistance of the base AH steel.
7. Fabrication, Machinability, and Formability
- Cutting: Plasma, oxy‑fuel, and laser cutting are used; AH36’s slightly higher strength and hardness can increase cutting power and wear on consumables.
- Forming and bending: Both grades can be formed in common plate bending presses and rolls; AH36 may require larger bend radii or higher forming forces for the same thickness due to increased yield.
- Machinability: AH grades are not optimized for machining—higher strength (AH36) increases tool wear and required cutting forces.
- Finishing: Surface preparation for coating is similar for both grades; weld grinding and dressing are slightly more demanding on AH36 when achieving smooth fillet profiles.
8. Typical Applications
Table: Common uses by grade
| AH32 (common uses) | AH36 (common uses) |
|---|---|
| General hull plating for moderate strength design | Primary hull plating and structural members where higher yield is required |
| Deck and superstructure plates where cost and formability are prioritized | High‑load brackets, web frames, bulkheads designed for reduced thickness |
| Internal stiffeners, brackets, non-critical fixtures | Heavy frames, girders, collision bulkhead elements, areas needing greater margin to yield |
| Fabricated components where extensive forming/bending is required | Ship sections where weight saving via thinner plate is pursued |
Selection rationale: - Choose AH32 when forming, bending, or cost is more critical and structural loads permit the lower yield. Its slightly better ease of fabrication and marginally lower material cost can reduce production time and outlay. - Choose AH36 when design loads, section reduction (weight savings), or regulatory/membership requirements demand the higher specified yield. In many modern designs, AH36 permits thinner plating to meet the same structural criteria.
9. Cost and Availability
- Relative cost: AH36 typically commands a modest premium over AH32 because of tighter process controls, potentially higher alloying or TMCP processing requirements, and its higher performance classification.
- Availability: Both grades are commonly available worldwide in plate, cut lengths, and sometimes in profiled sections. Very thick plates or specialty thicknesses may have longer lead times, and availability of specific tempers or impact‑tested plate at extreme low temperatures may be more limited.
- Procurement tip: Specify thickness, required impact testing temperature, and supply condition (e.g., TMCP) to receive accurate quotes and lead time estimates.
10. Summary and Recommendation
Table: comparative snapshot
| Category | AH32 | AH36 |
|---|---|---|
| Weldability | Excellent (very good) | Very good (requires slightly more HAZ/control in thick sections) |
| Strength–Toughness balance | Good | Higher yield; requires process control to equal toughness |
| Cost | Lower (typically) | Higher (typically) |
Recommendation: - Choose AH32 if your design prioritizes forming, bending, and lower purchase cost, and if the structural design allows the lower yield strength (suitable for many secondary structural components and non‑critical hull areas). - Choose AH36 if you need higher minimum yield strength to reduce section thickness or meet stricter structural requirements (appropriate for main hull plating, primary load‑carrying members, and when weight savings or increased design margin are the chief drivers).
Concluding note: The practical difference between AH32 and AH36 is mainly an increase in specified yield strength for AH36, achieved through controlled chemistry and thermomechanical processing rather than radical composition changes. Selection should balance fabrication processability, weld procedure capability, impact‑testing requirements, and life‑cycle cost. In procurement and design specifications, always reference the relevant classification society rules and material certificates to ensure compliance with thickness‑dependent property requirements.