---
title: "Mechanical vs heat treatment, tempering"
subject: "Civil Engineering Materials"
chapter: 8
key: "semester_3/civil_engineering_materials:mech-heat-treatment-isothermal-tempering"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/mech-heat-treatment-isothermal-tempering
site: BCT Engineering
---

# Mechanical vs heat treatment, tempering

Civil Engineering Materials · Chapter 8: Metals and Alloys

- Asked in: 2070 Chaitra (old course), 3+5 marks
- Video: [#Heat treatment Process of Steel, metals | Types. Process, Applications](https://www.youtube.com/watch?v=fXrY_BowsmY) — Everything Counts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/mech-heat-treatment-isothermal-tempering
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-8

## Question

Differentiate between mechanical treatment and heat treatment of steel. Explain isothermal annealing and tempering process of heat treatment with neat sketch.

## Short answer

Mechanical treatment applies force to change shape and size (e.g., rolling, forging), while heat treatment uses controlled heating and cooling to alter internal structure without changing shape. Isothermal annealing: heat above upper critical, cool rapidly to just below lower critical (600-700°C), hold for transformation, then air cool. Tempering: reheat hardened steel to 150-650°C, hold, cool in air to reduce brittleness and increase toughness.

## Answer

#### Mechanical treatment versus heat treatment
| Point | Mechanical treatment | Heat treatment |
|---|---|---|
| Method | Applying force: rolling, forging, drawing, extruding | Heating, soaking and cooling at a controlled rate |
| Effect | Changes shape and size; refines grain | Changes internal structure, shape unchanged |
| Purpose | To shape the steel; remove casting defects | To adjust hardness, strength, ductility, toughness |
| Hot/cold | Hot working or cold working | Always uses heating |
| Examples | Hot rolling, cold drawing, forging | Annealing, normalizing, quenching, tempering |

#### Isothermal annealing
**Isothermal annealing** is a variant of full annealing in which steel is heated above its upper critical temperature (austenitised), cooled rapidly to a temperature just below the lower critical point (about 600-700 C), held at this constant temperature until the austenite fully transforms to pearlite/ferrite, and then cooled in air.

```text
 Temp
  ^
  |   ____ austenitise (A3 + 30-50 C)
  |  /    \
  |-/------\------------- A1 (723 C)
  | /       \_________
  |/    rapid cool  | hold (isothermal)
  |                 |_______ air cool
  +--------------------------> Time
```
It saves time compared with full annealing, gives uniform structure and good machinability, and is used for alloy steel forgings and castings.

#### Tempering
**Tempering** is the reheating of hardened (quenched) steel to a temperature below the lower critical point (150-650 C), holding it there, and then cooling in air or oil. It reduces the brittleness and internal stresses of martensite and increases toughness and ductility, with some loss of hardness.

```text
 Temp
  ^
  |        ______ temper temp (150-650 C)
  |       /      \
  |      /        \ cool in air
  |     /          \
  +--------------------------> Time
```
Lower tempering temperature (150-300 C) retains hardness (cutting tools); higher (400-650 C) gives more toughness (springs, shafts).

## Check yourself

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

1. Which statement correctly separates mechanical treatment from heat treatment of steel?
   A. Mechanical treatment applies force to change shape and size; heat treatment uses heating, soaking and controlled cooling to change internal structure
   B. Mechanical treatment changes internal structure without shape change; heat treatment applies force
   C. Both always involve heating, but only heat treatment involves cooling
   D. Heat treatment shapes the steel by rolling and forging
   Answer: A. Rolling, forging and drawing are mechanical treatments that shape the steel. Heat treatment alters the internal structure while the shape stays unchanged. (Key point: Mechanical vs heat treatment)

2. In isothermal annealing, what is done after austenitising?
   A. Cooled slowly in the furnace to room temperature
   B. Quenched in water to form martensite
   C. Cooled rapidly to just below the lower critical point, held at constant temperature until austenite transforms, then air cooled
   D. Reheated to 150-650 C and held
   Answer: C. The steel is cooled quickly to about 600-700 C, held until austenite fully transforms to pearlite/ferrite, then air cooled. (Key point: Isothermal annealing steps)

3. Why is hardened steel tempered?
   A. To convert it back to austenite
   B. To increase hardness further
   C. To make the steel magnetic
   D. To reduce brittleness and internal stress of martensite and increase toughness, with some loss of hardness
   Answer: D. Tempering reheats quenched steel below the lower critical point, trading a little hardness for toughness and ductility. (Key point: Purpose of tempering)

4. Which tempering choice matches the use?
   A. 150-300 C for cutting tools (retains hardness); 400-650 C for springs and shafts (more toughness)
   B. 400-650 C for cutting tools; 150-300 C for springs
   C. Above 723 C for all steels
   D. Tempering temperature does not affect properties
   Answer: A. Low tempering temperatures keep hardness for tools; higher ones give toughness for springs and shafts. (Key point: Tempering temperature choice)

## Related in Civil Engineering Materials

- [Heat treatment, objectives, isothermal annealing](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-isothermal-annealing-s) — Civil Engineering Materials, chapter 8
- [Mechanical vs heat treatment; red shortness](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Amechanical-vs-heat-treatment-red-shortness) — Civil Engineering Materials, chapter 8
- [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
- [Isothermal annealing and quenching](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-isothermal-annealing-quenching) — Civil Engineering Materials, chapter 8
- [Mechanical treatment of steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Amechanical-treatment-steel) — Civil Engineering Materials, chapter 8, asked 2×
- [Heat treatment and three methods](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-three-methods) — Civil Engineering Materials, chapter 8

## 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
- [Heat treatment processes of steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aheat-treatment-of-steels) — Material Science, chapter 6
- [Heat vs work transfer; isothermal work](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aheat-vs-work-isothermal-work) — Fundamental of Thermodynamics & Heat Transfer, chapter 2
- [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