How the Grade System Works
Shipbuilding steel is specified through classification society rules rather than through a single commercial standard, and the grade letters A, B, D and E describe a rising scale of impact toughness within one strength level. The letter itself is the toughness marker: Grade A carries the least stringent impact requirement, Grade B one step better, Grade D is tested at -20 °C and Grade E at -40 °C. The LR designation identifies the plate as approved by the classification society whose rules the vessel is built to, and equivalent grades exist under the other member societies of the international association of classification societies, which harmonises the technical requirements for hull structural steel.
Grade A and Grade B are both general-strength, or mild, hull steels. They are hot rolled, fully killed and produced with a chemistry and rolling route that suits the ordinary demands of hull construction, deck plating, bulkheads, inner bottoms and superstructures. Neither is a high-strength steel; the strength level is the classic 235 MPa yield class used across the world's shipyards.
Chemical Composition
| Element, max % | LR Grade A | LR Grade B |
|---|---|---|
| C | 0.21 | 0.21 |
| Si | 0.50 | 0.35 |
| Mn | 2.5 x C | 0.80 |
| P | 0.035 | 0.035 |
| S | 0.035 | 0.035 |
| Al, min where fully killed | 0.015 | 0.015 |
Grade A allows manganese to be calculated as a multiple of the carbon content, a legacy formula that lets the producer adjust strength without exceeding a fixed ceiling on carbon. Grade B uses a simpler fixed manganese limit of 0.80% and a tighter silicon limit of 0.35%, which reflects its finer carbon-manganese balance and the need to support a better impact result. Both grades are fully killed for plate above a modest thickness, with aluminum used to bind nitrogen, and both carry the same phosphorus and sulphur ceilings. Where lower sulphur is required for improved through-thickness and weld properties, it is ordered as an additional requirement.
Mechanical Properties and Impact Testing
| Property | LR Grade A | LR Grade B |
|---|---|---|
| Minimum yield strength | 235 MPa | 235 MPa |
| Tensile strength | 400-520 MPa | 400-520 MPa |
| Minimum elongation | 22% | 22% |
| Charpy V-notch test temperature | +20 °C | 0 °C |
| Minimum absorbed energy | 27 J transverse | 27 J transverse |
The mechanical properties of the two grades are the same in every respect except one, and that one item is the reason both exist. Grade A is Charpy tested at +20 °C and Grade B at 0 °C, both to a minimum absorbed energy of 27 J on transverse specimens. The required energy rises with plate thickness: the tables in the classification rules step the minimum energy up for thicker plate, and the longitudinal values are higher than the transverse values on the same thickness. A yard that needs a little more cold-weather margin at moderate cost moves from Grade A to Grade B; a yard that needs a genuinely low service temperature moves to Grade D or Grade E, which are tested at -20 °C and -40 °C and are produced through controlled rolling or heat treatment rather than ordinary rolling.
Applications and Grade Selection
Grade A is used for deck plating, superstructures, internal bulkheads, non-critical secondary structure and general hull areas in vessels operating in warm or moderate climates. It is the cheapest certified hull steel and the easiest to fabricate.
Grade B is specified for shell plating in areas subject to wave impact, for strength deck strakes and for hull locations where a small temperature margin protects against brittle fracture during a cold voyage or a low-temperature loading operation. It is also the usual choice for barges and vessels that trade in northern waters but do not require the full D or E grade. In practice, yards accept a mix of grades on one hull, with Grade A in mild-service areas and Grade B or higher in exposed and highly stressed locations.
Grade selection is governed by the classification rules, which set minimum grade requirements according to plate thickness, structural position and design temperature. Fabrication and repair work on an existing vessel must follow the grade shown in the approved drawings, and any substitution requires agreement with the classification society's surveyor.
Certification, Fabrication and Quality Control
Hull steel is delivered with a classification society certificate confirming that the heat has been tested and approved against the rules, and plates carry the society's brand together with the grade letter, heat number and, for heavier plate, additional identification marks transferred during production. Documentation also follows EN 10204 for the general mill certificate. Impact testing is carried out on specimens taken from the plate, and the surface is inspected for defects such as laminations, scabs and cracks before dispatch.
Both grades are readily welded with ordinary shipyard processes, using low-hydrogen consumables for thick plate and no preheat for moderate thickness at normal ambient temperature. Sequence welding and distortion control matter more than strength considerations on large thin panels. Forming is done cold by press or roll, and any hot forming is followed by normalizing if required by the rules.
For a shipyard, the practical quality points are grade identification, thickness tolerance, flatness, weldability consistency from heat to heat and the availability of the certificate before the plate arrives at the block assembly stage. Steel ordered to a higher grade than necessary increases cost without improving the vessel, while steel ordered to a lower grade than the rules require invalidates the class approval, so the grade matrix should be resolved during detail design rather than in the yard.
Frequently Asked Questions
Q: What is the difference between LR Grade A and LR Grade B steel plate?
A: The impact test temperature. Grade A is Charpy tested at +20 °C and Grade B at 0 °C, with both requiring a minimum of 27 J on transverse specimens and both having the same strength.
Q: Do Grade A and Grade B have the same yield strength?
A: Yes. Both are general-strength hull steels with a 235 MPa minimum yield and a 400-520 MPa tensile range.
Q: Which grade is better for cold climates?
A: Grade B offers a wider temperature margin than Grade A, and for genuinely cold service the rules will normally require Grade D at -20 °C or Grade E at -40 °C.
Q: Are these grades considered high-strength steels?
A: No. They are ordinary general-strength hull steels. High-strength hull grades carry higher yield levels and their own thickness and position rules.
Q: What documentation accompanies the plate?
A: A classification society certificate confirming approval of the heat against the rules, together with the mill inspection certificate and plate marking that allows traceability to the heat.
Q: Can the two grades be welded together?
A: Yes. They are metallurgically the same family, and a procedure suitable for Grade A is generally suitable for Grade B at the same thickness.
