---
title: "Annealing of cold-worked metals"
subject: "Material Science"
chapter: 4
key: "semester_3/material_science:annealing-recovery-recrystallization-grain-growth"
source: https://www.bctengineer.com/q/semester_3/material_science/annealing-recovery-recrystallization-grain-growth
site: BCT Engineering
---

# Annealing of cold-worked metals

Material Science · Chapter 4: Deforming process for materials

- Asked in: year not given, 8 marks
- Video: [Recovery, Recyrstallization, and Grain Growth](https://www.youtube.com/watch?v=a_3fDAg96_Y) — Taylor Sparks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/annealing-recovery-recrystallization-grain-growth
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-4

## Question

Explain the stages of annealing of a cold-worked metal: recovery, recrystallisation and grain growth. Define recrystallisation temperature and list the factors affecting it. What are the changes in properties during annealing?

## Short answer

Annealing stages: recovery relieves stress; recrystallisation nucleates new grains, sharply lowering strength and restoring ductility; grain growth coarsens grains. Recrystallisation temperature (~0.3–0.5 Tm, complete in ~1 h) is lowered by more cold work, smaller initial grain, higher purity, longer time.

## Answer

**Annealing** is heating a cold-worked metal to a suitable temperature, holding, and cooling slowly to remove strain hardening, relieve residual stress and restore ductility.

#### Stages

**1. Recovery** (low temperature, below about $0.3\,T_m$)
- Point defects annihilate and dislocations rearrange into low-energy arrangements (polygonisation).
- Internal stresses are relieved; electrical conductivity partly recovers.
- Hardness, strength and ductility change very little; grain structure stays deformed.

**2. Recrystallisation** (about $0.3$-$0.5\,T_m$)
- New strain-free equiaxed grains nucleate at high-energy sites (grain boundaries, slip bands) and grow, consuming the deformed grains.
- Dislocation density drops sharply; strength and hardness fall, ductility rises to near the annealed values.

**3. Grain growth** (higher temperature or longer time)
- Large grains grow at the expense of small ones to reduce grain-boundary energy.
- Strength and hardness fall further and the surface may look rough (orange peel).

```text
 property
  |\ strength
  | \_______
  |  |       \___
  |  |   |       `--- 
  |  |   | ductility ___--
  |__|___|_____..--'
  +--|---|----------------> temperature
   Rec. Recryst. Grain growth
```

#### Recrystallisation temperature
It is the temperature at which a cold-worked metal completely recrystallises in about one hour. For pure metals it is about $0.3$-$0.5\,T_m$ (absolute), e.g., about 200 °C for copper, 450 °C for iron, near room temperature for lead.

**Factors affecting it:** (i) larger prior cold work gives a lower temperature; (ii) smaller initial grain size lowers it; (iii) higher purity lowers it, since alloying atoms and impurities impede boundary movement; (iv) longer holding time lowers it.

#### Importance
Annealing permits further cold work without cracking, since ductility is restored. Working above the recrystallisation temperature is called hot working; below it, cold working.

## Check yourself

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

1. What happens in recovery?
   A. New grains form
   B. Large grains grow
   C. Internal stresses relieved with little change in strength; structure stays deformed
   D. Ductility restored fully
   Answer: C. Recovery rearranges dislocations and relieves stress with little hardness change. (Key point: Recovery)

2. What happens in recrystallisation?
   A. New strain-free equiaxed grains nucleate and consume deformed grains
   B. Point defects annihilate only
   C. Grains coarsen
   D. Melting
   Answer: A. Dislocation density drops sharply, strength falls, ductility rises. (Key point: Recrystallisation)

3. Which factor lowers recrystallisation temperature?
   A. Smaller prior cold work
   B. Larger grain size
   C. Impurities
   D. Greater cold work
   Answer: D. Greater cold work lowers it; purity and smaller grains also lower it. (Key point: Recrystallisation temperature)

4. Why do large grains grow?
   A. To reduce grain-boundary energy
   B. To increase strength
   C. To add dislocations
   D. Due to cold work
   Answer: A. Growth lowers total boundary energy but further reduces strength. (Key point: Grain growth)

## Related in Material Science

- [Solidification: nucleation and grain growth](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Asolidification-nucleation-and-grain-growth) — Material Science, chapter 5
- [Tensile properties and effect of temperature](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aproperties-from-tensile-test-and-temperature) — Material Science, chapter 3

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