Unit 18.2: Tool Steel

Introduction

Tool steels are a special category of high-carbon alloy steels designed to have high hardness, wear resistance, and toughness, making them ideal for cutting, shaping, and forming materials.

They are widely used in dies, punches, cutting tools, and molds, where the material must retain hardness under stress and heat.

Definition

Tool Steel

Tool steel is an alloy steel containing 0.5–1.5% carbon, along with alloying elements like tungsten, chromium, vanadium, molybdenum, and cobalt, designed for high hardness, wear resistance, and heat resistance in tools and dies.

Core Concept Explanation

High carbon content (0.5–1.5%) increases hardness and wear resistance.
Alloying elements:
- Tungsten (W): Improves red hardness, maintains hardness at high temperatures.
- Chromium (Cr): Improves hardness, wear resistance, and corrosion resistance.
- Vanadium (V): Forms carbides for toughness and wear resistance.
- Cobalt (Co): Enhances hot hardness and strength.
- Molybdenum (Mo): Improves toughness and strength at elevated temperatures.
Tool steels are heat treatable, allowing a balance of hardness and toughness.

Classification of Tool Steels

Type	Key Features	Applications
Water-Hardening (W-Grade)	Hardens in water, moderate wear resistance	Simple cutting tools
Cold-Work (O, A, D Grades)	Used at room temperature, high hardness & wear resistance	Dies, punches, shear blades
Hot-Work (H Grades)	Retains hardness at high temperature	Forging dies, extrusion tools
High-Speed Steel (T, M Grades)	Retains hardness at high cutting speed	Lathe tools, drills, milling cutters
Shock-Resisting (S Grades)	High toughness, absorbs impact	Springs, chisels, hammers

Properties / Characteristics

Property	Description
Hardness	Very high, retains hardness under heat (hot hardness)
Wear Resistance	Excellent due to carbides
Toughness	Moderate to high, depending on type
Heat Resistance	Very good in hot-work and high-speed steels
Ductility	Low, especially in high-carbon grades
Heat Treatability	Excellent (quenching, tempering)

Applications in Engineering

Tool steels are used for manufacturing tools and dies that experience high stress, wear, or heat:

Cutting tools: drills, lathe tools, milling cutters
Dies and punches for forging, stamping, and extrusion
Industrial knives, shear blades
Springs and chisels (shock-resisting types)
Molds for metal and plastic forming

Exam-Focused Points

High carbon content: 0.5–1.5%
Key alloying elements: W, Cr, V, Mo, Co
Properties: High hardness, wear resistance, hot hardness
Heat treatable by quenching and tempering
Types: Water-hardening, cold-work, hot-work, high-speed, shock-resisting
Applications: cutting tools, dies, punches, molds

Common Exam Traps

Confusing high-speed steel with plain carbon steel – remember: HSS retains hardness at high temperatures and cutting speeds
Misidentifying tool steel types – associate cold-work with room temperature tools, hot-work with high-temperature applications
Forgetting alloying elements and their roles

Example Competitive Exam Questions

Question: What is the carbon content range of tool steel?
Answer: 0.5–1.5%

Question: Which alloying element in tool steel improves red hardness?
Answer: Tungsten or Cobalt

Question: Give one application of high-speed tool steel.
Answer: Lathe cutting tools or milling cutters

Question: Name the tool steel type used for forging dies operating at high temperatures.
Answer: Hot-work steel (H-grade)

Quick Revision Summary

Carbon: 0.5–1.5%
Alloying elements: W, Cr, V, Mo, Co
Hardness & Wear Resistance: Very high
Heat Resistance: Excellent in hot-work & high-speed steels
Applications: Cutting tools, dies, punches, molds, industrial knives
Types: Water-hardening, Cold-work, Hot-work, High-speed, Shock-resisting