What steels are used in shipbuilding, and what is the carbon content of each type?
Short answer
| Steel | Carbon | Use |
|---|---|---|
| Low-carbon (mild) | up to about 0.25% (hull plate 0.15% to 0.23%) | hull and general structure, grades A to E and AH to FH |
| Medium-carbon | 0.25% to 0.50% | shafts, forgings, crankshafts |
| High-carbon | 0.50% to 1.0% | springs, tools, wear parts |
| Cast iron | above about 2% | bedplates, liners, engine components |
| Cryogenic service | 9% nickel steel, stainless, aluminium, Invar (36% nickel) | LNG tanks at about -163°C |
- Why low carbon: more carbon gives strength and hardness but less ductility, toughness and weldability; a welded hull that flexes must not crack.
- By type: low-carbon (mild) up to about 0.25% (hull plate 0.15% to 0.23%); medium-carbon 0.25% to 0.50% (shafts, forgings, crankshafts); high-carbon 0.50% to 1.0% (springs, tools); cast iron above about 2% (bedplates, liners).
- Hull grades (IACS UR W11): normal-strength A, B, D, E graded by Charpy impact temperature (E the toughest when cold) against the ductile-to-brittle transition; higher-strength AH, DH, EH, FH micro-alloyed.
- Proven by: tensile, hardness, Charpy or Izod impact, fatigue, creep and NDT, each grade verified by the mill certificate.
- Cryogenic trap: LNG tanks at about -163°C use 9% nickel steel, austenitic stainless, aluminium or Invar (36% nickel) for brittle-fracture resistance, not thermal expansion.
- As Chief Engineer: diminution allowances are read against the steel grade; a higher-carbon hull would crack.
Full answer
The full answer explains each point step by step, with the regulations and figures behind it and the discussion from other candidates. It is in the MEO Class I section of the Marine One forum, open to members enrolled for the MEO Class I exam.