- Practice · 8 marks
Annealing, normalizing and tempering of steel
Short answer
Annealing: heat 30–50°C above upper critical, furnace cool; softens, refines grain. Normalizing: 40–50°C above upper critical, air cool; refines grain, uniform structure, stronger. Tempering: reheat hardened steel below A1 (150–650°C), air cool; reduce brittleness, improve toughness. Annealing differs from normalizing by slower furnace cooling, giving softer, coarser pearlite vs fine pearlite.
Explain annealing, normalizing and tempering of steel. State the purpose, heating temperature and cooling method for each, and differentiate between annealing and normalizing.
Heat treatment is controlled heating and cooling of a metal in the solid state to change its properties such as hardness, strength, ductility and grain size.
Annealing
- Heat the steel to 30 to 50 °C above the upper critical temperature (A3 for hypoeutectoid; above A1 for hypereutectoid), soak, then cool very slowly in the furnace.
- Purpose: soften the steel, improve machinability and ductility, relieve internal stress, refine grain, and make it ready for further working.
- Structure: coarse pearlite (and ferrite), which is soft.
- Types: full annealing, process (sub-critical) annealing for cold-worked low carbon steel, spheroidising for high carbon steel, stress-relief annealing.
Normalizing
- Heat to 40 to 50 °C above the upper critical temperature, hold briefly, then cool in still air.
- Purpose: refine grain, remove stress left by forging, rolling or casting, give uniform structure and better strength than annealed steel.
- Structure: fine pearlite.
Tempering
- Done after hardening (quenching), which makes steel hard but brittle with residual stress. The steel is reheated below A1 (150 to 650 °C), held and cooled in air.
- Purpose: reduce brittleness and internal stress, improve toughness, with some loss of hardness.
- Higher temper temperature gives lower hardness and greater toughness (150 to 250 °C for cutting tools, 350 to 450 °C for springs, 500 to 650 °C for shafts and gears). Colour changes (straw to blue) indicate temperature on a polished surface.
Annealing versus normalizing
| Annealing | Normalizing |
|---|---|
| Cooling slowly in the furnace | Cooling in still air |
| Softest condition | Harder and stronger |
| Coarse pearlite | Fine pearlite, small grains |
| Better machinability and ductility | Better strength; moderate machinability |
| Longer, more costly cycle | Shorter, cheaper |
✓ Answer checked by Claude Opus 5.5 (high effort)
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