---
title: "Role of Co, Mn, Cr, Ni in steel"
subject: "Civil Engineering Materials"
chapter: 8
key: "semester_3/civil_engineering_materials:alloying-elements-role"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/alloying-elements-role
site: BCT Engineering
---

# Role of Co, Mn, Cr, Ni in steel

Civil Engineering Materials · Chapter 8: Metals and Alloys

- Asked in: 2079 Bhadra, 2 marks
- Video: [EFFECTS OF ALLOYING ELEMENTS ON STEEL](https://www.youtube.com/watch?v=otBA99vM8bo) — MECHANICAL ACADEMY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/alloying-elements-role
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-8

## Question

What is the role of Cobalt, Manganese, Chromium and Nickel in the respective alloy of steel?

## Short answer

Cobalt increases red hardness and heat resistance. Manganese adds strength, toughness, and wear resistance while removing sulphur. Chromium provides hardness, wear resistance, and above 11-12% gives stainless steel. Nickel increases strength, toughness at low temperatures, ductility, and corrosion resistance.

## Answer

| Element | Role in steel |
|---|---|
| **Cobalt (Co)** | Increases hardness at high temperature (red hardness) and heat resistance; improves magnetic properties; used in high-speed tool steel and magnets. |
| **Manganese (Mn)** | Increases strength, hardness, toughness and wear resistance; removes sulphur and oxygen (prevents red shortness); high-Mn steel (12-14%) is used for rail crossings and crushers. |
| **Chromium (Cr)** | Increases hardness, strength, wear and corrosion resistance; above 11-12% Cr gives stainless steel. |
| **Nickel (Ni)** | Increases strength and toughness (also at low temperature), ductility and corrosion resistance; with Cr gives austenitic stainless steel. |

## Check yourself

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

1. Cobalt in steel mainly provides:
   A. Corrosion resistance
   B. Red hardness (hardness at high temperature) and improved magnetic properties
   C. Removal of sulphur
   D. Low-temperature toughness
   Answer: B. Cobalt is used in high-speed tool steel and magnets. (Key point: Role of cobalt)

2. Manganese in steel:
   A. Removes sulphur and oxygen, preventing red shortness, and adds strength and wear resistance
   B. Makes stainless steel at 11%
   C. Lowers hardness
   D. Makes it brittle at all contents
   Answer: A. Mn de-sulphurises and de-oxidises; 12-14% Mn steel is used for rail crossings and crushers. (Key point: Role of manganese)

3. Steel becomes stainless when chromium content is above about:
   A. 1%
   B. 3%
   C. 5%
   D. 11-12%
   Answer: D. Above 11-12% Cr gives stainless steel. (Key point: Role of chromium)

4. Nickel in steel:
   A. Increases strength and toughness (also at low temperature) and corrosion resistance; with Cr gives austenitic stainless steel
   B. Gives red hardness
   C. Removes sulphur
   D. Reduces ductility
   Answer: A. Ni with Cr forms austenitic stainless steel. (Key point: Role of nickel)

## Related in Civil Engineering Materials

- [Plain carbon steel versus alloy steel](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aplain-carbon-vs-alloy-steel) — Civil Engineering Materials, chapter 8

## Related in other subjects

- [Lever rule in Cu-Ni alloy](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Alever-rule-copper-nickel-alloy) — Material Science, chapter 5
- [VBT: [Ni(CN)₄]²⁻ and [Ni(CO)₄]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nicn4-nico4) — Engineering Chemistry, chapter 6
- [Low vs high spin; VBT of [Ni(CO)₄]](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Alow-high-spin-ni-co4) — Engineering Chemistry, chapter 6
- [VBT formation of [Ni(CO)₄] and [Fe(CN)₆]⁴⁻](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Avbt-nico4-fecn6-4) — Engineering Chemistry, chapter 6