---
title: "Heat treatment processes of steel"
subject: "Material Science"
chapter: 6
key: "semester_3/material_science:heat-treatment-of-steels"
source: https://www.bctengineer.com/q/semester_3/material_science/heat-treatment-of-steels
site: BCT Engineering
---

# Heat treatment processes of steel

Material Science · Chapter 6: Iron – Iron Carbide diagram and Heat Treatment of Steels

- Asked in: year not given, 8 marks
- Video: [Heat Treatment of Steel](https://www.youtube.com/watch?v=fLvZkZxiXnE) — FlinnScientific
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/heat-treatment-of-steels
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-6

## Question

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.

## 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.

## Answer

**Heat treatment** is controlled heating and cooling of a solid metal to obtain desired properties.

| Process | Heating temperature | Cooling | Structure | Purpose |
|---|---|---|---|---|
| Full annealing | $A_3$ + 30-50 °C (hypoeutectoid); $A_1$ + 30-50 °C (hypereutectoid) | Very slow, in furnace | Coarse pearlite + ferrite | Soften, improve machinability, relieve stress |
| Normalising | $A_3$ (or $A_{cm}$) + 30-50 °C | Air cooling | Fine pearlite + ferrite | Refine grain, uniform structure, moderate strength |
| Hardening | $A_3$ + 30-50 °C (hypoeutectoid); $A_1$ + 30-50 °C (hypereutectoid) | Rapid quench in water or oil | Martensite | High hardness and wear resistance |
| Tempering | Below $A_1$: 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 $A_1$ 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*).

```text
 T
 A3 ----- ---- hardening/annealing/normalising
 A1 ---------------------
         tempering (below A1)
 time -->
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is needed for hardening to give martensite?
   A. Slow furnace cooling
   B. Air cooling from above A3
   C. Cooling faster than the critical cooling rate (quenching)
   D. Holding just below A1
   Answer: C. Austenite becomes martensite by diffusionless transformation only if cooling exceeds the critical rate. (Key point: Hardening cooling rate)

2. Why is normalised steel stronger and harder than annealed steel?
   A. Air cooling is faster, giving finer pearlite
   B. It is heated to a lower temperature
   C. It is quenched in water
   D. It contains martensite
   Answer: A. Annealing is furnace cooling (coarse pearlite); normalising is air cooling, giving finer pearlite and refined grain. (Key point: Annealing vs normalising)

3. Where does tempering take place and what is its purpose?
   A. Above A3, to soften the steel
   B. Below A1 (150-650 °C), to reduce brittleness and quenching stress of hardened steel
   C. At A3 + 50 °C, to refine grain
   D. Just above A1, to form martensite
   Answer: B. Tempering reheats martensite below A1 so carbon precipitates as fine carbide, trading some hardness for toughness. (Key point: Tempering temperature)

4. To what temperature are hypereutectoid steels heated for hardening?
   A. A3 + 30-50 °C
   B. Below A1
   C. A1 + 30-50 °C
   D. Above Acm by 100 °C
   Answer: C. Hypereutectoid steels are heated just above A1, leaving some cementite which adds wear resistance. (Key point: Hardening temperature of hypereutectoid steel)

## Related in Material Science

- [Precipitation hardening of Al-Cu](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aage-hardening-of-aluminium-copper-alloy) — Material Science, chapter 9
- [Slow cooling of 1.2% C steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Ahypereutectoid-steel-12-carbon) — Material Science, chapter 6
- [Iron-iron carbide phase diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Airon-iron-carbide-phase-diagram) — Material Science, chapter 6
- [Tensile test and stress-strain diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Atensile-test-engineering-stress-strain) — Material Science, chapter 3

## Related in other subjects

- [Annealing, normalizing and tempering of steel](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aheat-treatment-annealing-normalizing-tempering) — Workshop Technology, chapter 7
- [Mechanical vs heat treatment, tempering](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Amech-heat-treatment-isothermal-tempering) — Civil Engineering Materials, chapter 8
- [Notes on powder metallurgy and heat treatment](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Apowder-metallurgy-heat-treatment-notes) — Machine Design II, chapter 3
- [Heat treatment, tempering and quenching](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-tempering-quenching) — Civil Engineering Materials, chapter 8