---
title: "Hot and cold working; temperature and strain rate"
subject: "Manufacturing and Production Processes"
chapter: 5
key: "semester_4/manufacturing_and_production_processes:hot-cold-working-friction-strain-rate"
source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/hot-cold-working-friction-strain-rate
site: BCT Engineering
---

# Hot and cold working; temperature and strain rate

Manufacturing and Production Processes · Chapter 5: Bulk Deformation Process

- Asked in: year not given, 6 marks
- Video: [Recrystallization in Metals | Cold Working and Hot Working | [Explained Easily with Animation]](https://www.youtube.com/watch?v=YkL4U7Rwvlo) — Mech Vidya
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/hot-cold-working-friction-strain-rate
- Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-5

## Question

Differentiate between hot working and cold working. Explain the effect of friction, temperature and strain rate on the flow stress and forming load in bulk deformation processes.

## Short answer

Hot working (above recrystallisation) gives low force, no work hardening; cold working (below) provides strength increase and better finish. Flow stress decreases with rising temperature and increases with higher strain rate (σ = C ε̇^m). Friction raises forming force and causes barrelling; it is controlled by lubricants and die finish.

## Answer

**Hot working** is plastic deformation above the recrystallisation temperature (about $0.5T_m$ in kelvin; hot working is usually done at $0.5$-$0.75T_m$). **Cold working** is deformation below it (usually near room temperature), so the metal strain hardens.

#### Differences

| Point | Hot working | Cold working |
|---|---|---|
| Temperature | Above recrystallisation | Below recrystallisation |
| Strain hardening | None (recrystallisation removes it) | Yes, strength and hardness rise |
| Forming force | Low | High |
| Surface finish, accuracy | Poor, scale forms | Good, close tolerance |
| Grain structure | Refined, porosity closed | Elongated, directional |
| Ductility of starting metal | Not critical (metal is ductile when hot) | Metal must be ductile |
| Annealing | Not needed | Needed between stages |
| Examples | Hot rolling, forging, extrusion | Cold drawing, cold rolling, coining |

#### Effect of temperature

Flow stress falls as temperature rises because of thermal softening and recrystallisation. Above the recrystallisation temperature the strain-hardening exponent is nearly zero, and the forming load drops by a large factor. Too high a temperature causes burning or grain growth.

#### Effect of strain rate

Flow stress depends on strain rate $\dot{\varepsilon}$ as

$$
\sigma = C\,\dot{\varepsilon}^{\,m}
$$

where $m$ is the strain-rate sensitivity exponent: about 0.01 or less for cold working and 0.05-0.4 for hot working. In hot working, a faster press or hammer therefore needs a higher load. The strain rate in forging is $v/h$ (ram velocity divided by the instantaneous height).

#### Effect of friction

- Friction between the work and the die opposes the flow at the interface; it raises the forming force and power.
- In upsetting, it causes **barrelling** and a "friction hill" of pressure that is highest at the centre.
- In rolling, friction pulls the metal into the roll gap, but too much friction raises power and roll wear. The condition for entry is $\mu \ge \tan\alpha$.
- It is controlled by lubricants (graphite, oil, glass) and by the die finish.

Friction therefore has the most influence when the contact area is large compared with the thickness, as in flat rolling of thin sheets and forging of thin discs.

## Check yourself

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

1. Hot working is deformation
   A. Below recrystallisation temperature with strain hardening
   B. Above recrystallisation temperature (about 0.6 T_m), so no strain hardening remains
   C. At room temperature only
   D. Only with lubricants
   Answer: B. Recrystallisation removes strain hardening during hot working, giving low forming force. (Key point: Hot vs cold working definition)

2. Raising temperature in bulk deformation
   A. Raises flow stress
   B. Lowers flow stress and forming load by thermal softening and recrystallisation
   C. Has no effect
   D. Raises strain-hardening exponent
   Answer: B. Flow stress falls with temperature; too high a temperature can cause burning or grain growth. (Key point: Temperature effect)

3. In sigma = C (strain rate)^m, m is
   A. Larger for cold working (0.05-0.4)
   B. About 0.01 or less in cold working and 0.05-0.4 in hot working, so faster hot forming needs higher load
   C. Always zero
   D. The strain-hardening exponent
   Answer: B. m is the strain-rate sensitivity exponent; hot working is strain-rate sensitive. (Key point: Strain rate sensitivity)

4. Friction in upsetting causes
   A. Reduced forming force
   B. Barrelling and a friction hill with pressure highest at the centre
   C. Uniform flow
   D. Lower die heating
   Answer: B. Friction opposes flow at the interface, raising force and power and producing barrelling. (Key point: Friction effect in upsetting)

## Related in Manufacturing and Production Processes

- [Cold upsetting force with friction](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aopen-die-forging-friction-numerical) — Manufacturing and Production Processes, chapter 5
- [Tribology in manufacturing: friction, wear, lubrication](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Atribology-friction-wear-lubrication-manufacturing) — Manufacturing and Production Processes, chapter 2
- [Flow curve constants K and n](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aflow-curve-strength-coefficient-numerical) — Manufacturing and Production Processes, chapter 2
- [Wire drawing stress, force and power](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Awire-drawing-stress-force-numerical) — Manufacturing and Production Processes, chapter 5

## Related in other subjects

- [Cold working and strain hardening](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Acold-working-and-strain-hardening) — Material Science, chapter 4
- [Internal (hot spot) temperature of core](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahot-spot-core-temperature) — Electric Machine Design, chapter 2, asked 6×
- [Hot spot temperature of a plate](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahot-spot-plate) — Electric Machine Design, chapter 2, asked 2×
- [Working stress vs limit state method](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aworking-stress-vs-limit-state-method) — Design of RCC Structure, chapter 1, asked 6×