Syllabus
Governing: Applied Marine Engineering syllabus section 1 (Materials technology: iron and steel). Builds on: AEC workshop materials.
Why materials matter
Every failure investigation starts with "what is it made of and what was it asked to do". The AME syllabus asks you to know the types of iron and steel, what an alloy is, the effects of alloying elements, and the stainless steel families, specifically grade 304 and marine grade 316.
Iron and steel
| Material | Character | Typical uses aboard |
|---|---|---|
| Cast iron | High carbon, graphite flakes; strong in compression, damps vibration, brittle | Engine blocks, pump casings, some valve bodies |
| Wrought iron | Almost pure iron with slag fibres; tough, corrosion-tolerant, now rare | Old fittings, chain on classic yachts |
| Mild steel | Low carbon; ductile, easily welded and formed | Hull plate, frames, brackets |
| Cast steel | Steel poured into moulds; tougher than cast iron | Valve bodies, rudder components |
| Alloy steels | Deliberate additions for strength or toughness | Shafts, bolts, gears |
Alloys and alloying elements
An alloy is a metal with one or more elements added deliberately to change its properties.
| Element | Main effect in steel |
|---|---|
| Carbon | Strength and hardness up, ductility and weldability down |
| Manganese | Strength, deoxidation, counters sulphur brittleness |
| Nickel | Toughness, especially at low temperature; stabilises austenite |
| Chromium | Hardenability, and above a threshold a passive oxide film (stainless) |
| Molybdenum | Pitting resistance, high-temperature strength |
| Silicon | Deoxidiser; magnetic properties |
Stainless steels
Stainless relies on a thin chromium oxide film that reforms if scratched, provided oxygen is available.
| Family | Properties | Examples |
|---|---|---|
| Ferritic | Magnetic, moderate corrosion resistance, not hardenable | Trim, exhaust parts |
| Martensitic | Magnetic, hardenable by heat treatment | Knife blades, some pump shafts |
| Austenitic | Non-magnetic when annealed, tough, weldable | 304, 316 |
304 v 316. ASSDA gives both around 18 % chromium; 316 adds 2 % molybdenum. ASSDA says 304 suits fittings frequently washed with fresh water, while 316 ("marine grade") covers most marine applications such as deck fittings above the waterline. In sea water, 316 performs well up to around 30 °C for pitting, and crevice corrosion appears 15–20 °C lower. For immersed service in warm water, ASSDA points to duplex 2205, with much higher pitting and crevice resistance and roughly double the yield strength of 316.
WARNING — Stainless is not stain-proof below the waterline Stagnant sea water starves the oxide film. Shaft sections under cutless bearings, fasteners in wet timber and bolts under gaskets are classic crevice sites.
The yacht angle
- Guardrail stanchions and deck hardware: 316, polished to reduce tea-staining.
- Propeller shafts: often duplex or proprietary high-strength stainless.
- Exhaust: 316L, because the low carbon limits weld decay.
- Fasteners: never mix grades blindly; a 304 bolt in a 316 fitting will be the one that rusts.
TIP — The magnet test Austenitic stainless is essentially non-magnetic when annealed but can become slightly magnetic after cold work. A magnet separates ferritic and martensitic grades from austenitic, not 304 from 316.