Hardening and Tempering of Tool Steel: A Comprehensive Guide

Introduction

Tool steels refer to a variety of alloy steels that are specifically engineered for making tools. Their outstanding hardness, strength, and other properties are derived from careful control of alloying elements and heat treatment processes.

The unique properties of tool steels are developed through sequences of hardening and tempering. Hardening transforms the microstructure to very hard martensite, while tempering reduces brittleness and imparts ductility and toughness.

This guide will provide a detailed overview of the theory, methods, procedures, and best practices for successfully hardening and tempering tool steel components of all types.

Why Heat Treat Tool Steels?

Heat treatment is critical for developing the properties needed in tool steel components:

  • Achieve the necessary hardness for cutting, forming, and shaping capability
  • Improve strength, wear resistance, and other mechanical characteristics
  • Enhance dimensional stability during hard machining and grinding
  • Allow proper microstructural development for desired performance

Without proper hardening and tempering, tool steels will not exhibit their intended properties and tool life will suffer dramatically.

The specific objectives of tool steel heat treatment are:

  • Form a very hard martensitic structure via quenching
  • Reduce brittleness and improve toughness through tempering
  • Produce a uniform hardness profile from surface to core
  • Avoid distortion, cracking, or other defects during treatment

With optimized hardening and tempering, tool steels will exhibit maximum effectiveness and longevity in demanding applications.

Tool Steel Metallurgy Fundamentals

To properly heat treat tool steels, it is important to understand some metallurgical fundamentals:

Hardening Mechanism

  • Heating forms austenite which transforms to very hard martensite upon rapid quenching

Tempering Process

  • Tempering reduces brittleness of martensite by allowing the formation of tempered martensite

Alloy Carbides

  • Carbides of vanadium, chromium, molybdenum, tungsten and other elements contribute to high wear resistance

Phases

  • Tool steels contain a mixture of ferrite, carbides, martensite/austenite, and retained austenite phases depending on alloy and condition

TTT Diagrams

  • Time-temperature-transformation diagrams demonstrate the transformations that occur upon heating and cooling

CCT Diagrams

  • Continuous cooling transformation diagrams show microstructural development for a given cooling path

A solid grasp of these metallurgy basics is required to properly select parameters and procedures for effective tool steel heat treatment.

Tool Steel Hardening

Hardening involves heating into the austenite phase field, holding at temperature, then rapidly cooling or quenching to form martensite:

Heating

  • Slow heating to austenitizing temperature above upper critical temperature
  • Protect from oxidation and minimize scaling

Soaking

  • Hold at temperature to allow thorough carbide dissolution into austenite
  • Time depends on alloy composition and section thickness

Quenching

  • Rapid cooling to transform austenite into very hard martensite
  • Quenchant must have sufficient speed and severity to ensure full transformation

Straightening

  • Martensite formation often causes some distortion which should be straightened

The specific temperatures, times, and quenchants depend on the tool steel grade being processed. But the overall goal of hardening remains producing a very hard martensitic microstructure.

Tempering of Tool Steels

Tempering involves reheating quenched martensitic tool steel to intermediate temperatures to impart ductility:

Objectives

  • Reduce brittleness and improve toughness
  • Adjust hardness to final requirements
  • Allow some stress relief from quenching

Temperature

  • Usually 400-1200°F depending on needed hardness
  • Multiple tempers at progressively higher temperatures may be used

Time

  • Soaking time at temperature can range from 1-2 hours depending on alloy

Cooling

  • Air cooling, but some alloys may require faster forced air or oil cooling

Tempering transforms brittle as-quenched martensite into more ductile tempered martensite to achieve the optimal combination of hardness and toughness.

Critical Aspects of Tool Steel Hardening

There are several critical factors that must be controlled to achieve effective tool steel hardening:

Quenchant Selection

  • Each tool steel has an optimum quenching media like oil, water, air, or polymer
  • Quenchant must have sufficient severity and heat transfer rate to properly harden the steel

Avoiding Cracks

  • Some alloys are prone to cracking during quenching from improper quenchant or agitation
  • Precracking before hardening and minimizing constraint helps avoid cracking

Temperature Uniformity

  • Uneven heating causes variation in hardness through cross section
  • Careful preheating, soaking, and use of thermocouples ensures uniformity

Austenite Grain Size

  • Coarse austenite grains reduce toughness – fine grains improve toughness
  • Tight control of temperature minimizes excessive grain growth

Carbide Dissolution

  • Insufficient soaking time can leave undissolved carbides reducing hardness
  • Balance between grain growth and carbide dissolution must be achieved

Decarburization Prevention

  • Loss of surface carbon causes softening which must be prevented
  • Use protective atmosphere or vacuum furnaces when applicable

Critical Aspects of Tool Steel Tempering

Effective tool steel tempering requires control over several key factors:

Temperature Uniformity

  • Uneven heating leads to variability in tempering degree and properties
  • Use of thermocouples and tempering in batches ensures uniformity

Tempering Sequence

  • A sequence of increasing tempers may be required to achieve needed toughness
  • Tempering temperature and number of draws depends on alloy

Cooling Rate After Tempering

  • For some alloys, the cooling rate from the tempering temperature can affect properties
  • Air, forced air, or even oil cooling may be specified

Distortion Control

  • Dimensional changes during tempering must be minimized through process control
  • Fixtures, supports, or restraints help control distortion

Rehardening Prevention

  • Overtempering can result in reaustenitizing and loss of hardness
  • Close monitoring of temperature is critical

Oxidation Resistance

  • Forming a protective oxide before final tempering prevents discoloration
  • Done by tempering at 1200-1500°F to form thin oxide layer

Common Tool Steel Heat Treating Methods

There are several approaches used for heat treating tool steels:

Batch Furnaces

  • Simple box furnaces with protective atmosphere
  • Used for small to medium size tools
  • Tight temperature uniformity must be maintained

Pit Furnaces

  • Large furnaces allowing batch or continuous processing
  • Multiple temperature zones for heating, soaking, quenching
  • Common for larger parts like dies and molds

Fluidized Bed Furnaces

  • Bed of fine media is fluidized by heated gas flow
  • Excellent uniformity and rapid heating/cooling
  • Used for many tool steels

Vacuum Furnaces

  • Heating and quenching performed in evacuated chamber
  • No surface oxidation and very repeatable processes
  • Utilized for premium tooling

Induction Hardening

  • Localized rapid heating via induction field for surface hardening
  • Allows selective hardening of portions of a part
  • Mainly used for areas like tool edges or wear surfaces

Laser/Electron Beam Hardening

  • High intensity laser/electron beam as concentrated heat source
  • Enables very localized hardening of small areas or complex geometries
  • Growing in use for specialty tooling

Best Practices for Optimized Tool Steel Heat Treatment

To consistently achieve proper hardening and tempering of tool steels, some sound practices should be employed:

  • Use certified heat treating facilities with qualified personnel and procedures
  • Ensure all furnace instrumentation and temperature monitoring equipment is properly calibrated
  • Verify actual temperature uniformity across the furnace hot zone during process validation
  • Adhere strictly to prescribed time-temperature cycles for each tool steel grade and condition
  • Closely monitor loads and quenchant circulation and agitation
  • Check representative samples like hardness test coupons to confirm results
  • Maintain detailed records of all process parameters and any deviations
  • Perform metallography periodically to verify proper microstructure
  • Establish a system for correcting any variations from standards

Following these and other good practices helps minimize problems like cracking, distortion, property variation, or other defects during tool steel heat treatment.

Specific Hardening and Tempering Procedures for Common Tool Steel Grades

The following provides more detailed hardening and tempering procedures for some of the most common tool steel types:

O1 Oil Hardening Tool Steel

Hardening:

  • Preheat to 1200-1250°F
  • Austenitize at 1475-1525°F for 30 minutes
  • Quench in warm oil (120°F) with agitation

Tempering:

  • Temper at 350-400°F for 1-2 hours
  • Cool in air after tempering
  • Double temper for maximum toughness

A2 Air Hardening Tool Steel

Hardening:

  • Preheat to 1400-1450°F
  • Austenitize at 1550-1600°F for 30 minutes
  • Air quench or quench in air or gas blast

Tempering:

  • Temper at 900-1000°F for 1 hour
  • Cool in air after tempering
  • Multiple tempers can be used

D2 High Carbon High Chromium Tool Steel

Hardening:

  • Preheat to 950-1100°F
  • Austenitize at 1900-2000°F for 10-20 minutes
  • Quench in warm oil (120-180°F) with agitation

Tempering:

  • Temper at 1000-1025°F for 2 hours, cool in air
  • Draw at 1075-1175°F for 2 hours, cool in air
  • Finish temper at 400°F for 1 hour, air cool

H13 Hot Work Tool Steel

Hardening:

  • Preheat to 1450-1550°F
  • Austenitize at 1950-2050°F for 15-30 minutes
  • Gas, oil, or air quench depending on section size

Tempering:

  • Temper at 1000°F for 2 hours, then air cool
  • Final temper at 1025-1100°F for 1-2 hours

M2 High Speed Tool Steel

Hardening:

  • Preheat to 1200-1250°F
  • Austenitize at 2200-2300°F for 15-30 minutes
  • Air quench or quench in warm oil/salt bath

Tempering:

  • Temper at 1050-1150°F for 1-2 hours, air cool
  • Double temper at same temperature for maximum toughness

As seen, each tool steel grade has unique time, temperature, and quenchant parameters tailored to its individual alloy composition and hardenability.

Common Defects and Remedies for Problematic Tool Steel Heat Treating

Despite best efforts, tool steel heat treatment does not always go as intended. Some potential defects and ways to remedy them include:

Cracking – Increase preheat temp, use more ductile quenchant, reduce severity of quench, precrack before hardening

Distortion – Optimize quench severity, use fixtures or restraints, straighten after quenching

Excessive Grain Growth – Tightly control austenitizing temperature, avoid long soaks

Surface Decarburization – Use protective atmosphere or vacuum furnace, minimize furnace leaks

Non-Uniform Hardness – Improve temperature uniformity, use multiple thermocouples, adjust loading

Overtempering – Carefully follow prescribed tempering temperatures and times, check hardness between tempers

Retained Austenite – Increase hardening temperature or time at temperature, or decrease quench rate from austenitizing temperature

Temper Embrittlement – Keep tempering temperatures below or above susceptible range of 700-850°F

Careful evaluation of all process parameters and metallographic analysis of samples can help diagnose and correct heat treating problems.

Innovations in Tool Steel Heat Treatment Technology

There are ongoing advances that allow for improvements in various aspects of tool steel heat treatment:

  • Computer control and monitoring for enhanced process control and data collection
  • New quenching systems like intensive quenching, marquenching, or IQ technologies
  • Specialty alloy wire quenchants or salt baths for unique cooling requirements
  • Use of vacuum or inert gas environments to minimize decarburization and oxidation
  • Laser and induction heating capabilities for local surface hardening or highly controlled heating
  • Novel polymer quenchants with tailored properties of heat transfer and wetting behavior
  • Additive manufacturing using laser powder bed fusion to allow complex hardened tool geometries

These and other innovations allow heat treaters to develop specialized cycles tailored to the particular nuances of advanced tool steel alloys.

Summary of Tool Steel Hardening and Tempering Key Points

Tool steel heat treatment for developing ideal hardness and toughness involves:

  • Slow heating and soaking in the austenite phase field
  • Rapid quenching using the optimal media for the specific alloy
  • Careful tempering at precise temperatures and times to balance hardness and ductility
  • Close control of all process parameters to achieve uniformity and consistency
  • Adjustments as needed based on hardness testing and microstructural verification
  • Implementation of best practices for temperature monitoring, protective atmospheres, fixturing, and other factors

With a thorough understanding of the fundamentals, phase transformations, critical variables, and potential defects, heat treaters can successfully process a wide range of tool steel grades for outstanding performance.

Frequently Asked Questions About Tool Steel Heat Treating

What is the main purpose of heat treating tool steels?

The primary objective is to develop the necessary hardness, wear resistance, strength, and other properties required for the tooling application through controlled heating and cooling processes.

What happens if a tool steel is not heat treated properly?

Without proper hardening and tempering, tool steels will be too soft, wear too quickly, fail prematurely, and not function as intended. Performance will suffer dramatically.

What are some key advantages of vacuum heat treating?

Vacuum furnaces allow excellent process control, cleanliness, consistency, and minimization of decarburization. They are ideal for high value tooling.

How long does a typical tool steel heat treating cycle take?

A batch process including preheating, austenitizing, quenching, and tempering stages can take 8-12 hours depending on the grade and process parameters.

What are some main difficulties or defects that can arise during heat treating?

Common issues are cracking, distortion, uneven hardness, residual stress, surface oxidation, improper microstructure, or other variances from standards.

What modern innovations are improving tool steel heat treatment?

Additive manufacturing, computer control, new quenchants and heating methods, vacuum processing, and other advances are enhancing capabilities.

I hope this guide provides helpful and practical information on properly heat treating tool steels to achieve optimal microstructures and properties! Please let me know if you have any other questions.