What Drives Steel Plate Selection in Marine Service
Selecting plate for a ship, an offshore platform or a subsea structure is a question of matching four properties to the service condition: yield strength, low-temperature toughness, weldability and corrosion resistance. A hull plate is loaded dynamically by wave action and carries fatigue cycles for twenty years or more; a deck plate on a fixed platform has to remain ductile at the lowest ambient temperature of the field; a tank shell has to hold pressure and resist the contents. On top of the metallurgy sits compliance, because the classification society rules and the project specification decide which grades are acceptable in each structural location and what testing must be recorded. Aluminium-killed fine grain steels with controlled carbon equivalent are the basis of most marine plate, and the choice between grades is usually settled by impact test temperature and by the class approval required.
Hull Structure Grades
Hull plating, decks, bulkheads, brackets and primary framing are built from ASTM A131/A131M normal and higher strength hull structural steels. The normal strength grades A, B, D and E share a 235 MPa minimum yield strength and a 400-520 MPa tensile range, separated by impact test temperature, while the higher strength grades AH36, DH36 and EH36 reach 355 MPa minimum yield strength with impact testing at 0 °C, -20 °C and -40 °C respectively. Grade F36 extends the toughness range further for the coldest service. The letters carry the same meaning across the family: the first letter sets the impact temperature and the number sets the strength level. For an ice-class hull or a bow region exposed to freezing spray, the E or F quality is specified even though the strength level is unchanged, because it is toughness and not strength that prevents brittle fracture in cold water.
Offshore Platform and Subsea Grades
| Grade | Standard | Minimum yield strength | Notable property | Typical use |
|---|---|---|---|---|
| API 2H Grade 50 | API Specification 2H | 345 MPa | Offshore structural quality with controlled toughness | Platform decks, legs, topside modules |
| S355G10+M or S355G10+N | EN 10225 | 355 MPa | Weldable fine grain steel with carbon equivalent limits | Subsea structures, jacket nodes |
| A131 AH36, DH36, EH36 | ASTM A131/A131M | 355 MPa | Impact toughness to -40 °C | Hull primary structure |
Offshore structures differ from ships in two respects: they are often welded in thicker sections with higher restraint, and they are designed for a longer life in a more corrosive environment. That combination drives specification towards grades with an explicit carbon equivalent limit and a low-temperature Charpy requirement, which is why EN 10225 structural steels are written with weldability clauses rather than only with strength and impact values. The thicker plate also brings thickness-direction testing into the specification: where a heavily restrained weld transfers load through the plate thickness, Z15, Z25 or Z35 testing is ordered to guard against lamellar tearing.
Storage Tanks and Cryogenic Containment
Fuel tanks, liquefied gas containment and pressure-holding units use ASTM A516/A516M Grade 70, a normalized carbon steel plate with a 485-620 MPa tensile range and a 260 MPa minimum yield strength, which is supplied with optional impact testing for low-temperature service. Where the cargo is liquefied natural gas and the containment is of the membrane or type A or B type, the material changes completely: 9 % nickel steel to ASTM A553/A553M Type I is used for cryogenic service at temperatures approaching -196 °C, with a typical minimum yield strength of 585 MPa and a 690-825 MPa tensile range. The two families are not interchangeable, and the low-temperature design case rather than the strength requirement decides which one applies.
Secondary Structure and General Fabrication
Workshop panels, base plates, access platforms, non-load-bearing partitions and temporary steelwork do not need class approval. These are usually built from ASTM A36 or from EN 10025-2 S235JR and S275JR plate, which combine modest yield strength at 235 MPa and 275 MPa with excellent weldability and forming behaviour. Using a higher grade in these locations adds cost without adding value, and it can complicate welding if the higher strength grade has a higher carbon equivalent. Separating secondary structure from classed structure on the order also keeps the certification cost proportional to the risk.
Certification, Testing and Delivery Practice
Classed plate carries a certificate issued against the rules of the relevant society, together with a mill test certificate showing heat number, chemical analysis, tensile and yield results, elongation and Charpy energy at the specified temperature in both orientations. Typical additional requirements are ultrasonic examination to an agreed acceptance class for primary structure, thickness-direction testing where welds are loaded through the plate thickness, simulated post-weld heat treatment of test coupons where the fabrication sequence demands it, and independent third-party inspection at the works. Plate is normally supplied cut, beveled, shot blasted and primed to suit the block assembly sequence, in widths up to about 4050 mm and lengths up to 15000 mm, packed with edge protection and shipped by container or break bulk to the receiving yard.
Frequently Asked Questions
Q: Which steel plate is used for ship hulls?
A: ASTM A131/A131M hull structural steel is the standard choice, with normal strength grades A, B, D and E at 235 MPa minimum yield strength and higher strength grades such as AH36, DH36 and EH36 at 355 MPa minimum yield strength.
Q: What yield strength is required for offshore platforms?
A: Offshore structural plate is normally specified at a minimum yield strength of 345 MPa or 355 MPa, as in API 2H Grade 50 and EN 10225 S355G10 grades, with low-temperature Charpy requirements written into the same specification.
Q: Which grade is used for LNG containment?
A: 9 % nickel steel to ASTM A553/A553M Type I is used for cryogenic containment at temperatures approaching -196 °C, while ASTM A516 Grade 70 covers ordinary fuel tanks and pressure-holding units.
Q: What is the difference between hull plate and general structural plate?
A: Hull plate is produced to a classification society specification with mandatory impact testing, traceability and class certification. General structural plate such as S235JR or A36 is intended for non-classed secondary structure and is not approved for primary hull use.
Q: Why is low-temperature toughness more important than strength in cold service?
A: Brittle fracture initiates at a stress concentration and propagates without plastic deformation, so a high-strength plate with poor toughness can fail at a lower load than a lower-strength plate with good toughness.
Q: What testing accompanies classed marine plate?
A: Mill test certification with chemical analysis, tensile and yield results and Charpy energy, plus classification society certification, with ultrasonic examination, thickness-direction testing and third-party inspection available as project requirements.
