Engineering Materials and Basic Metallurgy
The exam checks classification of metals, carbon percentages, properties, simple heat treatment and the purpose of basic tests.
Ferrous metals
- Plain carbon steel: low carbon up to 0.3% C (soft, ductile, weldable, used for bolts and sheet); medium carbon 0.3-0.6% C (axles, gears after treatment); high carbon 0.6-1.5% C (springs, files, drills, punches).
- Cast iron: 2-4% C, strong in compression, brittle, excellent damping, used for machine beds and vice bodies.
- Wrought iron: almost no carbon, fibrous and corrosion resistant, now rare.
- Alloy steels: stainless 18/8 means about 18% chromium and 8% nickel (type 304); high speed steel contains tungsten, chromium, vanadium and molybdenum and keeps hardness at red heat.
Non-ferrous metals
Aluminium is light and corrosion resistant; copper is the best commercial conductor; brass is copper plus zinc; bronze is copper plus tin. Properties to know: hardness (resistance to indentation), toughness (absorbs shock), ductility (drawn into wire), malleability (hammered into sheet), brittleness, elasticity.
Heat treatment
Annealing softens and relieves stress with slow furnace cooling. Normalising refines grain with air cooling. Hardening heats and quenches in water, brine or oil. Tempering reheats below the critical range to remove quenching brittleness while keeping hardness. Carburising adds carbon to a low-carbon steel surface to give a hard case with a tough core.
Worked example
A gear needs a hard wearing surface but must survive shock. Choose low-carbon steel near 0.2% C, carburise at about 900-950 C, then harden and temper. Reasoning: the low-carbon core stays tough, the carburised skin becomes hard on quenching, and tempering removes the internal stress that would otherwise cause cracking.
Traps that cost marks
- Confusing hardness with toughness.
- Swapping brass and bronze compositions.
- Mixing up annealing and normalising cooling.
- Quenching without tempering.