- Practice · 8 marks
Heat treatment processes of steel
Short answer
Full annealing: heat to A₃+30–50°C (hypoeutectoid) or A₁+30–50°C (hypereutectoid), furnace cool → coarse pearlite, softens. Normalising: A₃/Acm+30–50°C, air cool → fine pearlite, refines grain. Hardening: same heating, quench → martensite, high hardness. Tempering: reheat below A₁ → tempered martensite, reduces brittleness.
Explain the following heat-treatment processes of steel with the purpose and the resulting microstructure: (a) full annealing, (b) normalising, (c) hardening, (d) tempering. Give the heating temperatures with respect to the Fe-Fe3C diagram.
Heat treatment is controlled heating and cooling of a solid metal to obtain desired properties.
| Process | Heating temperature | Cooling | Structure | Purpose |
|---|---|---|---|---|
| Full annealing | + 30-50 °C (hypoeutectoid); + 30-50 °C (hypereutectoid) | Very slow, in furnace | Coarse pearlite + ferrite | Soften, improve machinability, relieve stress |
| Normalising | (or ) + 30-50 °C | Air cooling | Fine pearlite + ferrite | Refine grain, uniform structure, moderate strength |
| Hardening | + 30-50 °C (hypoeutectoid); + 30-50 °C (hypereutectoid) | Rapid quench in water or oil | Martensite | High hardness and wear resistance |
| Tempering | Below : 150-650 °C | Any, after hardening | Tempered martensite | Reduce brittleness and quenching stress |
Notes
- Annealing gives the softest, most ductile state. Spheroidise annealing (just below for a long time) turns lamellar cementite into spheres in high-carbon steel.
- Normalised steel is stronger and harder than annealed, because air cooling is faster, giving finer pearlite.
- Hardening needs the cooling rate to exceed the critical cooling rate so that austenite transforms to martensite without diffusion. Hardenability depends on carbon and alloy content. Retained austenite and cracking can occur in high-carbon steels.
- Tempering allows carbon to precipitate as fine carbide. Low tempering (150-250 °C) keeps hardness for tools; medium (350-450 °C) gives spring steel; high (500-650 °C) gives tough, strong shafts and bolts (quench plus high temper is called hardening and tempering).
T
A3 ----- ---- hardening/annealing/normalising
A1 ---------------------
tempering (below A1)
time -->
✓ Answer checked by Claude Opus 5.5 (high effort)
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