Engineering Materials and Basic Metallurgy
What the exam tests
Composition–property relationships, selection of a material for a given part, and the heat-treatment cycle that produces a required property.
Core rules
- Plain carbon steels: low carbon (to 0.3 % C) is ductile and weldable, used for shafts and bolts; medium (0.3–0.6 % C) for springs and rails; high (0.6–1.4 % C) for tools and cutters.
- On the iron–carbon diagram the eutectoid is 0.8 % C at 723 °C, giving pearlite; slow cooling of a hypoeutectoid steel gives ferrite plus pearlite.
- Cast irons: grey (flake graphite, good damping, used for machine beds), white (cementite, hard and brittle), malleable, and spheroidal graphite (ductile, high strength).
- HSS 18-4-1 means 18 % W, 4 % Cr, 1 % V; it keeps hardness up to about 600 °C, which is why it runs faster than plain carbon tool steel.
- Heat treatment: annealing softens and relieves stress by slow furnace cooling; normalising refines grain in air; hardening austenitises then quenches to martensite, hard but brittle; tempering reheats below the lower critical temperature to restore toughness.
- Case hardening suits low-carbon steel: carburising, cyaniding or nitriding leaves a hard skin over a tough core.
- Non-ferrous: brass is Cu–Zn, bronze is Cu–Sn, duralumin is Al–Cu–Mg, and Babbitt metal is used for bearings.
Worked example
A lathe tool must stay hard at red heat. A 0.8 % C plain carbon steel softens near 250 °C, so it fails. HSS 18-4-1, hardened and triple-tempered, is chosen because its tungsten and vanadium carbides give red hardness - the property the application demands, not room-temperature hardness alone.
Traps
- Saying hardening makes steel tough; it makes it brittle, so tempering must follow.
- Interchanging brass and bronze.
- Treating cast iron as steel - it cannot be quench-hardened and is not forgeable.