Unit 17.5: Cobalt Steel

Introduction

Cobalt steel is an alloy steel containing cobalt as the main alloying element.
Cobalt increases hardness, strength, and red hardness, making it ideal for cutting tools, high-speed tools, and parts operating at high temperatures.

It is extensively used in mechanical engineering applications where materials must maintain hardness under high stress and temperature.

Definition

Cobalt Steel

Cobalt steel is an alloy steel containing carbon (0.5–1.5%) and cobalt (5–12%), often with chromium and tungsten, designed to provide high hardness, wear resistance, and stability at elevated temperatures.

Core Concept Explanation

• Cobalt improves hardness and red hardness, allowing tools to cut metals at high speeds without losing hardness.

• Cobalt steel is heat treatable, giving a balance between hardness and toughness.

• It is mainly used in high-speed cutting tools, drill bits, and dies.

• Compared to tungsten steel, cobalt steel has higher hot hardness and better temperature stability.

Composition

Cobalt acts as a hardness stabilizer at high temperatures, improving cutting efficiency.

Properties / Characteristics

Applications in Engineering

Cobalt steel is used in high-speed and high-temperature applications:

• High-speed cutting tools: drills, lathe tools, milling cutters

• Dies and punches for metal forming

• Aircraft components exposed to high stress and temperature

• Industrial knives and shear blades

• Components requiring red hardness and wear resistance

Exam-Focused Points

• Cobalt content: 5–12%

• Carbon content: 0.5–1.5%

• Properties: Very high hardness, excellent red hardness, wear resistance

• Heat treatable by quenching and tempering

• Applications: high-speed cutting tools, dies, aircraft components

Common Exam Traps

• Confusing cobalt steel with tungsten steel – remember: cobalt → higher red hardness and hot hardness, tungsten → wear resistance at high temperature

• Forgetting red hardness property, key for cutting tools

• Assuming good ductility – cobalt steel is brittle if not tempered properly

Example Competitive Exam Questions

Question: What is the typical cobalt content in cobalt steel?
Answer: 5–12%

Question: Which property makes cobalt steel ideal for high-speed tools?
Answer: Retains hardness at high temperatures (red hardness)

Question: Give one common application of cobalt steel.
Answer: High-speed cutting tools or drill bits

Question: Can cobalt steel be heat treated?
Answer: Yes, by quenching and tempering

Quick Revision Summary

• Carbon: 0.5–1.5%

• Cobalt: 5–12%

• Hardness & Red Hardness: Very high

• Wear Resistance: Very good

• Ductility: Low

• Heat Treatment: Quenching & tempering

• Applications: High-speed cutting tools, dies, drill bits, aircraft components