Ship Plate Grades Explained: A, B, D, E and the 36-Class

A two-part naming system

Classification society rules (ABS, BV, CCS, DNV, KR, LR, NK, RINA and others use near-identical systems) name hull steel by strength level + impact grade:

• Ordinary strength (grades A, B, D, E): minimum yield ~235 MPa
• Higher strength (AH32/36/40, DH…, EH…, FH…): minimum yield 315, 355 or 390 MPa

The letter is a temperature

The letter states the temperature at which Charpy impact toughness is guaranteed:

Letter	Impact test temperature
A	No impact test required in most cases*
B	0 °C
D	−20 °C
E	−40 °C
F (higher strength)	−60 °C

*Grade A plate is impact-tested only in certain thickness/product situations; B, D and E always carry a test requirement, and values also vary with thickness.

The logic is brittle fracture: a ship's hull must not crack in the coldest water it sails. Designers assign tougher (later-letter) grades to exposed, highly stressed locations — sheer strakes, hatch corners, ice-going structures — and A/B grades to milder duty.

What AH36 means in full

AH36: higher-strength (H), impact grade A (0 °C), minimum yield 355 MPa (the "36" echoes 36 ksi). DH36 and EH36 are the same strength at −20 °C and −40 °C. Impact energy requirements are of the order of 34 J longitudinally for the 36 class (thickness-dependent) — figures are always read from the society's current rules, not memory.

Buying and certification

Ship plate is bought to a named society's rules, with mill certification (commonly 3.2-type inspection) from that society; a plate is not "AH36" in the abstract. Where welded joints cross plate thickness (T-joints in double bottoms, for example), Z-quality plate (through-thickness ductility, Z15/Z25/Z35) is specified separately to prevent lamellar tearing.

Delivery condition: rolled, normalised or TMCP

The same grade can arrive in different delivery conditions, and societies treat them differently. Older practice normalised higher-strength plate; modern mills commonly supply TMCP (thermo-mechanically controlled processed) plate, which achieves 36- and 40-class strength with lower carbon equivalent — measurably easier welding, a genuine design advantage on thick hull sections. Impact requirements tighten as thickness grows (energy values and sometimes test temperature step with thickness bands in the rules), so a grade letter that suffices at 20 mm may need re-checking at 60 mm. As thickness increases, ordinary-strength grades also acquire impact requirements in some applications — one more reason the society rulebook for the actual order, not the grade nickname, is the specification.

Ice and cold service

Polar and winter-navigation rules push selection toward E and F impact grades for ice-belt plating, and classification notations for polar class vessels add their own material requirements on top of the A–F ladder. The pattern repeats the system's core logic: the colder and more stressed the location, the further down the alphabet the plate must come from.