Engineering Materials
What this domain tests
Engineering Materials tests the craftsman's ability to choose and treat materials for workshop jobs: ferrous and non-ferrous metals, cast irons, steels, polymers, ceramics and composites. The exam favours property-to-application reasoning-strength, hardness, toughness, machinability, corrosion resistance and cost-rather than memorised data sheets.
Core rules
- Plain carbon steel is classified by carbon content: low carbon (mild) steel ≈0.05–0.25% C, medium carbon ≈0.25–0.60%, high carbon ≈0.60–1.00%. Higher carbon raises hardness and wear resistance but lowers ductility and weldability.
- Grey cast iron contains free graphite flakes; it has good damping and compressive strength but low tensile strength.
- Heat treatment: annealing softens and relieves stress; normalising refines grain; hardening (austenitise then quench) raises hardness; tempering reduces brittleness after hardening.
- Non-ferrous examples: aluminium alloys are light and corrosion-resistant but lower in stiffness; copper alloys conduct heat and electricity and resist some corrosion; bronze is useful for bearings.
- Material certificates and colour coding should be checked before welding or machining; mixed steel grades are a common workshop failure.
Worked example
A bench vice jaw insert must resist wear but not shatter. A high-carbon steel (≈0.8% C) can be hardened and tempered to give wear resistance, then tempered to a lower hardness to retain toughness. A mild steel would wear quickly; a fully hardened, untempered high-carbon steel could crack under shock. The reasoning is property-to-duty, not hardest is best.
Traps
Equating hardness with toughness; assuming stainless steel is immune to corrosion; ignoring the effect of welding on heat-treated parts; and selecting by colour or scrap-bin guesswork. Exam items often include a property that is irrelevant to the application.