Metallurgy and the iron–carbon diagram

Crystal structures

Metals are crystalline: body-centred cubic (BCC – ferrite, chromium), face-centred cubic (FCC – austenite, aluminium, copper; more ductile), and close-packed hexagonal (zinc, magnesium). Grains form as metal solidifies; smaller grains give higher strength and toughness. Dislocations moving along slip planes allow plastic deformation; cold working increases dislocation density (work hardening).

The iron–carbon equilibrium diagram

• Ferrite (α-iron, BCC, soft and ductile), austenite (γ-iron, FCC, exists at high temperature), cementite (Fe₃C, hard and brittle), pearlite (laminated ferrite and cementite).
• Eutectoid point: 0.8% carbon at 723 °C – austenite transforms to 100% pearlite on slow cooling.
• Hypo-eutectoid steels (< 0.8% C): ferrite + pearlite. Hyper-eutectoid steels (> 0.8% C): pearlite + cementite.
• Lower critical temperature 723 °C; upper critical temperature varies with carbon content.

Classification of steels

Mild steel (about 0.1–0.3% C), medium carbon (0.3–0.6% C), high carbon/tool steel (0.6–1.4% C); cast iron (2–4% C). Alloy steels: stainless (chromium, nickel), high-speed steel (tungsten, vanadium), manganese steel.

Non-ferrous alloys

Brass (Cu–Zn), bronze (Cu–Sn), duralumin (Al–Cu), solders.

Zimbabwe context

Iron ore and steel (Zisco/Redcliff), ferrochrome smelting from Great Dyke chrome, and platinum group metals.

Check yourself

1. What is the eutectoid composition and temperature of steel?
2. Which structure forms when austenite of 0.8% C cools slowly?
3. Why does cold working make metal harder?