---
title: "Phases and constituents of steel"
subject: "Material Science"
chapter: 6
key: "semester_3/material_science:ferrite-austenite-pearlite-martensite"
source: https://www.bctengineer.com/q/semester_3/material_science/ferrite-austenite-pearlite-martensite
site: BCT Engineering
---

# Phases and constituents of steel

Material Science · Chapter 6: Iron – Iron Carbide diagram and Heat Treatment of Steels

- Asked in: year not given, 6 marks
- Video: [Microstructure Of Steel - understanding the different phases & metastable phases found in steel.](https://www.youtube.com/watch?v=FE40FawJDkg) — James Sword Engineering 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/ferrite-austenite-pearlite-martensite
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-6

## Question

Write short notes on ferrite, austenite, cementite, pearlite and martensite.

## Short answer

Ferrite: BCC interstitial solid solution of C in Fe, max 0.022% C, soft. Austinite: FCC, up to 2.14% C, ductile. Cementite: Fe3C, very hard and brittle. Pearlite: lamellar eutectoid mixture of ferrite and cementite (0.76% C), forms at 727°C. Martensite: diffusionless shear product, BCT with supersaturate C, extremely hard and brittle.

## Answer

| Constituent | Nature | Structure | Properties |
|---|---|---|---|
| Ferrite ($\alpha$) | Interstitial solid solution of C in $\alpha$-Fe | BCC, max 0.022% C | Soft, ductile, magnetic below 768 °C, tensile strength about 280 MPa |
| Austenite ($\gamma$) | Solid solution of C in $\gamma$-Fe | FCC, up to 2.14% C | Soft, ductile, tough, non-magnetic; stable above 727 °C (retained in austenitic stainless steel) |
| Cementite (Fe$_3$C) | Intermetallic compound, 6.67% C | Orthorhombic | Very hard (about 800 HV), brittle |
| Pearlite | Eutectoid mixture of ferrite and cementite (0.76% C) | Alternate lamellae of ferrite and cementite | Hardness about 200 HB, tensile strength about 800 MPa; fine pearlite is harder than coarse |
| Martensite | Supersaturated solid solution of C in iron | Body-centred tetragonal (BCT), needle/plate shape | Extremely hard (up to 65 HRC) and brittle; formed by rapid quenching without diffusion |

#### Notes
- **Pearlite** forms when austenite of 0.76% C cools slowly through 727 °C: $\gamma \rightarrow \alpha + \text{Fe}_3\text{C}$. Lamellar spacing decreases with faster cooling.
- **Martensite** forms by a diffusionless shear transformation when austenite is cooled faster than the critical cooling rate, below the $M_s$ temperature. Carbon is trapped, distorting the lattice into BCT, which gives hardness. It is tempered to improve toughness.
- Other constituent: **bainite** (fine ferrite + carbide formed between about 250 and 550 °C), **ledeburite** (eutectic of austenite and cementite in cast iron).

## Check yourself

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

1. Which pairing of constituent and crystal structure is correct?
   A. Ferrite - FCC; austenite - BCC
   B. Ferrite - BCC; austenite - FCC
   C. Ferrite - BCT; austenite - BCC
   D. Ferrite - FCC; austenite - BCT
   Answer: B. Ferrite is carbon in BCC alpha-iron (max 0.022% C); austenite is carbon in FCC gamma-iron (up to 2.14% C). (Key point: Crystal structures)

2. What is pearlite?
   A. A supersaturated solid solution of C in iron
   B. A hard iron-carbon compound with 6.67% C
   C. A eutectoid mixture of alternate lamellae of ferrite and cementite (0.76% C)
   D. A eutectic mixture of austenite and cementite
   Answer: C. Pearlite forms when 0.76% C austenite transforms at 727 °C into alternating ferrite and cementite plates. (Key point: Nature of pearlite)

3. Why is martensite extremely hard?
   A. Carbon is trapped by diffusionless quenching, distorting the lattice into BCT
   B. It contains a large amount of cementite
   C. It is a coarse mixture of ferrite and cementite
   D. It is FCC with very high carbon
   Answer: A. Rapid cooling below the Ms temperature prevents carbon diffusion, so a supersaturated, tetragonally distorted lattice results. (Key point: Why martensite is hard)

## Related in Material Science

- [Slow cooling of 1.2% C steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Ahypereutectoid-steel-12-carbon) — Material Science, chapter 6
- [Slow cooling of 0.4% C steel](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Aslow-cooling-04-carbon-steel) — Material Science, chapter 6

## Related in other subjects

- [Ferrite and pearlite in high strength steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aferrite-pearlite-high-strength-steel) — Civil Engineering Materials, chapter 8
- [Phases in 0.80% carbon steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acarbon-percent-0-80-phases) — Civil Engineering Materials, chapter 8
- [Phases in 0.33% carbon steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acarbon-percent-0-33-phases) — Civil Engineering Materials, chapter 8
- [Phases in 0.35% carbon steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acarbon-percent-0-35-phases) — Civil Engineering Materials, chapter 8