LR AH36 marine steel plate is a normal-strength hull structural steel certified under the rules of Lloyd's Register for the construction of seagoing vessels. The designation combines the classification society approval with the grade itself: AH36 delivers a minimum yield strength of 355 MPa with guaranteed Charpy V-notch impact toughness at zero degrees Celsius, making it a workhorse material for hull shells, decks and internal framing.
The letter A in the grade indicates the impact test temperature group, zero degrees Celsius in this case, while 36 denotes the minimum yield strength in kilogram-force per square millimetre, equivalent to 355 MPa. Plates are supplied in the as-rolled or controlled-rolled condition and are approved for welded construction of hulls, superstructures, watertight bulkheads and other primary structures of ships and offshore units.
Chemical Composition of LR AH36
Maximum values for the main elements are controlled to secure weldability together with strength. Grain-refining additions are limited individually and in total so that the welding procedure remains simple.
| Element | Content, % |
|---|---|
| C | 0.18 max |
| Si | 0.50 max |
| Mn | 0.90 to 1.60 |
| P | 0.035 max |
| S | 0.035 max |
| Al | 0.015 min |
| N | 0.020 max |
| Nb | 0.02 to 0.05 |
| V | 0.03 to 0.10 |
| Ti | 0.02 max |
| Cu | 0.35 max |
| Cr | 0.20 max |
| Ni | 0.40 max |
| Mo | 0.08 max |
Aluminium is added for fully killed steel and to fix nitrogen, which protects toughness in the heat-affected zone. The combined micro-alloying content of niobium, vanadium and titanium is normally kept at or below 0.12 percent so that the carbon equivalent remains low enough for unrestricted welding.
Carbon Equivalent and Crack Sensitivity
Two indices are used to judge welding risk on hull plates. Carbon equivalent is calculated as Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, while crack sensitivity is expressed as Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B. For AH36 plate, Ceq is generally held below 0.40 percent for thicknesses up to 50 mm, which allows welding with conventional consumables and modest preheat.
Mechanical Properties and Impact Toughness
| Thickness, mm | Yield, min MPa | Tensile, MPa | Elongation, min % | Impact at 0 C, transverse, J | Impact at 0 C, longitudinal, J |
|---|---|---|---|---|---|
| Up to 50 | 355 | 490 to 630 | 21 | 24 | 34 |
| Over 50 to 70 | 355 | 490 to 630 | 21 | 27 | 41 |
| Over 70 to 100 | 355 | 490 to 630 | 21 | 34 | 50 |
Impact energy requirements increase with thickness because section size raises the constraint on the notch. Energy 1 values refer to transverse specimens and Energy 2 to longitudinal specimens, and both are taken on Charpy V-notch test pieces in accordance with classification society requirements.
Heat Treatment and Delivery State
LR AH36 is usually supplied in the hot-rolled or controlled-rolled condition, which provides a good balance of hardness, ductility and formability. Normalising or thermo-mechanical controlled processing can be ordered when a finer grain structure or improved toughness is required. Surface protection is achieved by shop priming, coating or shot blasting rather than by alloying, and plates are checked for flatness and internal soundness before dispatch.
Applications in Shipbuilding and Offshore Work
Hull shell plating and deck plating of cargo ships, bulk carriers and tankers.
Watertight bulkheads, cabin structures, hatch covers and cargo hold linings.
Internal framing, stiffeners and brackets of primary hull structures.
Bridges over waterways, dock structures, pontoons and floating platforms.
Deck machinery foundations and other welded assemblies exposed to seawater.
Because the grade tolerates complex stress states and repeated wave loading, it supports the fatigue life required over a vessel's design service period while remaining economical to fabricate.
Frequently Asked Questions
Q: What does the 36 in LR AH36 stand for?
It indicates a minimum yield strength of 36 kilogram-force per square millimetre, equal to 355 MPa.
Q: At what temperature is LR AH36 impact tested?
Grade A plates are impact tested at zero degrees Celsius with a minimum Charpy V-notch energy requirement.
Q: Is LR AH36 the same as ASTM AH36?
The grade strength designation is the same, but certification, marking and testing follow the rules of the issuing classification society, so the two approvals are not identical documents.
Q: Can LR AH36 be welded in cold weather?
Welding is possible at low ambient temperature if the joint is preheated and the qualified welding procedure is followed, keeping the surface free of moisture and frost.
Q: Which thicknesses are commonly supplied?
Plates from 6 mm to 100 mm are standard, with thickness above 70 mm requiring higher impact energy as shown in the property table.
Q: What delivery condition is standard?
As-rolled or controlled-rolled plate is standard, while normalised or thermo-mechanically controlled rolled plate is supplied when extra toughness is specified.
