Engineering Materials, Properties and Heat Treatment
What this domain tests
Material selection, mechanical properties and the heat-treatment cycles that produce them, including common defects.
Core rules
- Plain carbon steels by carbon content: low carbon (up to 0.3 %C) – ductile, weldable, used for structures; medium carbon (0.3–0.6 %C) – shafts, gears after heat treatment; high carbon (0.6–1.4 %C) – springs, files, drills.
- Cast irons: grey iron is brittle but damps vibration and machines well; SG (nodular) iron has higher strength and ductility.
- Non-ferrous: aluminium alloys (light, corrosion-resistant), copper and brass (electrical, machining), bronze (bearings), and white metal (babbitt) for plain bearings.
- Heat treatment: annealing softens and relieves stress (slow cool in furnace); normalising refines grain (air cool); hardening austenitises 30–50 °C above the upper critical temperature and quenches (water for carbon steel, oil for alloy steel); tempering immediately afterwards restores toughness.
- Surface treatments: carburising (low-carbon steel, 0.7–1.1 mm case), nitriding (no quench, very hard thin case), induction hardening (localised), flame hardening.
- Properties: hardness (Brinell for cast irons and soft steels, Rockwell C for hardened steel, Vickers for thin cases and microhardness).
Worked example
A 0.8 % carbon steel is to be hardened for a file. The upper critical temperature is about 750 °C, so austenitise at 780–800 °C, hold until uniform, quench in water with agitation, then temper at 180–200 °C immediately to relieve quenching stress while keeping high hardness.
Traps that cost marks
- Forgetting to temper after hardening, which leaves brittle martensite.
- Quenching alloy steel in water and causing cracks.
- Confusing normalising (air cool) with annealing (furnace cool).
- Using Brinell on a very thin case where the ball penetrates through.