Metal materials and properties
This domain examines ferrous and non-ferrous metals, their properties, and how material choice follows from service conditions. You need to define properties precisely and to use simple identification tests rather than guessing from colour.
Ferrous and non-ferrous metals
- Ferrous metals contain iron and are magnetic. Mild steel has about 0.05-0.25% carbon, is ductile and readily welded. Medium-carbon steel has about 0.25-0.6% carbon. High-carbon steel has about 0.6-1.4% carbon, takes a hard temper but is less weldable.
- Cast iron contains roughly 2-4% carbon. It is brittle in tension, strong in compression and difficult to weld without special procedures.
- Stainless steel contains at least 10.5% chromium, which forms a passive oxide film and gives corrosion resistance.
- Aluminium is light, soft, a good conductor and protected by an oxide film. Copper is ductile and highly conductive. Brass is copper-zinc; bronze is copper-tin. Zinc is used for galvanising. Lead is dense, soft and toxic.
Properties
Strength resists stress; hardness resists indentation or scratching; toughness resists impact without fracture; ductility allows drawing into wire; malleability allows shaping by hammering; elasticity allows recovery after loading; brittleness means little plastic deformation before fracture. Machinability, weldability and corrosion resistance are also selection criteria.
Worked example
A rectangular block measures 100 mm x 50 mm x 10 mm and has a mass of 392 g. Volume = 100 x 50 x 10 = 50 000 mm3 = 50 cm3. Density = 392 / 50 = 7.84 g/cm3. This is consistent with steel (about 7.8-7.9 g/cm3), not aluminium (about 2.7 g/cm3). The density test supports, but does not alone prove, the material identification.
Traps that cost marks
- Confusing hardness with strength or toughness.
- Saying aluminium does not corrode; it does, but the oxide film is protective.
- Assuming cast iron is easy to weld.
- Choosing high-carbon steel for a welded bracket without considering crack risk.