Heat treatment and material testing

Heat treatment of steels

• Annealing: heat above the upper critical temperature, cool very slowly in the furnace – soft, ductile, relieves stress.
• Normalising: heat above the upper critical temperature, cool in still air – refined grain, tougher than annealed.
• Hardening: heat medium/high-carbon steel above the critical temperature, then quench in water or oil – forms martensite (very hard, brittle).
• Tempering: reheat hardened steel to 200–600 °C – reduces brittleness, increases toughness (colours from pale straw at about 230 °C to blue at about 300 °C).
• Case hardening: carburising mild steel surface, then hardening – hard wear-resistant case, tough core. Also nitriding and induction/flame hardening.

Non-ferrous treatments

Annealing copper and brass (work hardening removed), precipitation (age) hardening of aluminium alloys.

Testing

• Tensile test: stress–strain curve, yield strength, ultimate tensile strength (UTS), elongation, Young's modulus.
• Hardness: Brinell (ball), Vickers (diamond pyramid), Rockwell.
• Impact: Charpy and Izod (toughness, notch sensitivity).
• Non-destructive testing: visual, dye penetrant, magnetic particle, ultrasonic and radiography.

Check yourself

1. What structure forms when steel is quenched from austenite?
2. Why are cutting tools tempered after hardening?
3. Name two non-destructive tests.