---
title: "Cold working and strain hardening"
subject: "Material Science"
chapter: 4
key: "semester_3/material_science:cold-working-and-strain-hardening"
source: https://www.bctengineer.com/q/semester_3/material_science/cold-working-and-strain-hardening
site: BCT Engineering
---

# Cold working and strain hardening

Material Science · Chapter 4: Deforming process for materials

- Asked in: year not given, 6 marks
- Video: [Strain Hardening](https://www.youtube.com/watch?v=UiYogITTl9w) — Torbjörn Carlberg
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/cold-working-and-strain-hardening
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-4

## Question

What is cold working? Name its common types. Explain strain hardening and the effect of increasing cold work on strength, hardness and ductility.

## Short answer

Cold working plastically deforms metal below recrystallization temperature, increasing dislocation density. Common types include rolling, drawing, extrusion, bending. Strain hardening raises strength and hardness but reduces ductility. As % cold work increases, yield and tensile strengths rise, while ductility (elongation) decreases.

## Answer

**Cold working** is plastic deformation of a metal below its recrystallisation temperature, so that the grains are deformed and strengthened without recrystallising.

#### Types of cold working
- Cold rolling (sheets, strips)
- Wire/bar drawing and tube drawing
- Cold forging and extrusion
- Bending, deep drawing and spinning of sheet
- Shot peening (surface cold work)

#### Strain hardening
During cold work, the dislocation density rises from about $10^{6}$-$10^{8}$ cm$^{-2}$ to $10^{10}$-$10^{12}$ cm$^{-2}$. Dislocations multiply (Frank-Read sources), tangle and block each other, so a higher stress is needed to move them. Hence the metal becomes stronger and harder as it is deformed. This is also called **work hardening**. The flow curve is $\sigma_T = K\varepsilon_T^{\,n}$.

#### Effect of % cold work
$$
\%\,CW = \frac{A_0 - A_d}{A_0}\times 100
$$

```text
 property
  |         tensile strength
  |       _..---
  |   _.-'
  |  /       yield strength lower than UTS
  |  \_
  |    `-._ ductility (% EL)
  +----------------------> % cold work
```

- Yield strength, tensile strength and hardness **increase** (yield strength rises faster, approaching the UTS).
- Ductility (% elongation, % RA) **decreases**.
- Electrical conductivity falls slightly; corrosion resistance is reduced; grains elongate along the working direction and residual stress is introduced.

A heavily cold-worked metal can be restored by annealing.

## Check yourself

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

1. Cold working is deformation:
   A. Above the recrystallisation temperature
   B. Below the recrystallisation temperature
   C. At the melting point
   D. Only at room temperature
   Answer: B. It is below the recrystallisation temperature so grains are deformed, not recrystallised. (Key point: Cold work definition)

2. Why does strain hardening occur?
   A. Dislocations multiply and tangle, blocking each other
   B. Dislocations disappear
   C. Grains grow
   D. Carbon dissolves
   Answer: A. Higher dislocation density needs higher stress to move dislocations. (Key point: Mechanism)

3. Effect of increasing cold work?
   A. Strength up, ductility up
   B. Strength down, ductility up
   C. Strength up, ductility down
   D. Both down
   Answer: C. Strength and hardness rise, ductility falls. (Key point: Property changes)

4. How is a cold-worked metal restored?
   A. More cold work
   B. Quenching
   C. Shot peening
   D. Annealing
   Answer: D. Annealing removes strain hardening and restores ductility. (Key point: Restoring)

## Related in Material Science

- [Percent cold work of a drawn wire](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Apercent-cold-work-numerical) — Material Science, chapter 4
- [Strain-hardening exponent and true stress](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Astrain-hardening-exponent-numerical) — Material Science, chapter 3
- [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
- [Ductile and brittle stress-strain behaviour](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aductile-vs-brittle-stress-strain) — Material Science, chapter 3
- [True stress and true strain relations](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Atrue-stress-strain-relations) — Material Science, chapter 3

## Related in other subjects

- [Hot and cold working; temperature and strain rate](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Ahot-cold-working-friction-strain-rate) — Manufacturing and Production Processes, chapter 5
- [Yield, strain hardening, stress-strain diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Ayield-strain-hardening-stress-strain) — Civil Engineering Materials, chapter 1, asked 2×
- [Define strength and ductility](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aproperty-strength-ductility) — Civil Engineering Materials, chapter 1
- [Earthquake design philosophy, strength and ductility](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aearthquake-design-philosophy-strength-and-ductility) — Design of RCC Structure, chapter 12