---
title: "Surface hardening of metals"
subject: "Civil Engineering Materials"
chapter: 8
key: "semester_3/civil_engineering_materials:surface-hardening"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/surface-hardening
site: BCT Engineering
---

# Surface hardening of metals

Civil Engineering Materials · Chapter 8: Metals and Alloys

- Asked in: 2081 Chaitra (new course), 2 marks; 2068 Chaitra, 3 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/surface-hardening
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-8

## Question

Explain surface hardening as a heat treatment process of metals.

## Short answer

Surface hardening creates a hard, wear-resistant skin on a tough, ductile core. It is achieved by carburising, nitriding, cyaniding, flame or induction heating followed by quenching, giving surface hardness without overall brittleness.

## Answer

**Surface hardening** (case hardening) is a heat treatment that makes the outer layer (case) of a steel part hard and wear resistant, while the inner core remains soft, tough and ductile. It is used for gears, cams, shafts, bearings and tools that need a hard surface to resist wear but a tough core to resist shock.

#### Methods
1. **Carburising:** low carbon steel is heated at about 900-950 C in a carbon-rich medium (charcoal, gas or liquid) so that carbon diffuses into the surface (0.5-2 mm); then quenched and tempered, giving a high-carbon hard case.
2. **Nitriding:** steel containing Al, Cr or Mo is heated at about 500-550 C in ammonia, forming very hard nitrides on the surface without quenching; little distortion.
3. **Cyaniding / carbonitriding:** heated in a molten cyanide salt bath so carbon and nitrogen enter the surface; quenched.
4. **Flame hardening:** the surface is heated with an oxy-acetylene flame and quenched immediately by water spray.
5. **Induction hardening:** the surface is heated by high frequency induced current, then quenched.

```text
   Hard case (high carbon)
   +---------------------+
   |  ...............    |
   |  .  Tough, soft  .  |
   |  .     core      .  |
   |  ...............    |
   +---------------------+
```

## Check yourself

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

1. Surface hardening gives
   A. Hard wear-resistant case and tough core
   B. Hard core
   C. Soft all through
   D. Brittle all through
   Answer: A. Case. (Key point: Purpose)

2. Carburising adds
   A. Nitrogen
   B. Carbon to surface of low carbon steel
   C. Chromium
   D. Oxygen
   Answer: B. Carbon diffuses. (Key point: Carburising)

3. Nitriding uses
   A. Cyanide
   B. Flame
   C. Ammonia at 500-550 C without quenching
   D. Induction
   Answer: C. Nitrides. (Key point: Nitriding)

## Related in Civil Engineering Materials

- [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
- [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
- [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
- [Principle of heat treatment of metal](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-principle) — Civil Engineering Materials, chapter 8, asked 5×
- [Purposes of heat treatment](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aheat-treatment-objectives) — Civil Engineering Materials, chapter 8, asked 5×
- [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

## Related in other subjects

- [Heat treatment processes of steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aheat-treatment-of-steels) — Material Science, chapter 6
- [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
- [Double surface treatment materials and procedure](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Adouble-surface-treatment-material-procedure) — Transportation Engineering II, chapter 3
- [Treatment process; sedimentation factors; coagulants](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Atreatment-impurities-coagulants) — Water Supply Engineering, chapter 6