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Heat Treatment: Introduction and Overview

Heat Treatment: Introduction

Heat treatment of steels may be carried out for a number of reasons, for example:

• Inter process thermal treatment to allow further processing, e.g. softening, H2 removal

• To produce specific final properties in the bulk material

• To produce specific surface properties in the final component, e.g. hardness

Different steel microstructures have very different typical mechanical properties, as illustrated below, therefore different types of heat treatment may be used depending on the grade of steel and its desired microstructure and properties. Heat treatments (i.e. defined heating and cooling regimes) are tailored to promote the formation of a specific microstructure, thereby developing the steel’s properties to meet application requirements.

 

Phase     HV   Tensile Strength (MPa)    A (%)    Room T Charpy (J)

Ferrite    130                300                       50                >100

Pearlite   300                950                       15                   40

Bainite    400               1200                     16                    20

Martensite  500            1800                      6                   <10

The alloying content of a steel also affects its microstructure and properties, therefore in each case the specific heat treatment parameters will be dependent on many variables such as the composition, size, processing history of the steel (or steel component) being treated. Examples of heat treatments include:

• Annealing (to soften/increase ductility) e.g. inter process

• Normalizing (to soften, give uniform microstructure)

• Quenching (Hardening) & Tempering (to increase strength, while maintaining toughness & ductility)

• Stress-relieving (to remove residual stresses after a forming operation)

• Surface treatments (to produce specific properties at the component’s surface)

The heat treatment of steels may involve several processing stages, e.g. austenitizing or hardening followed by tempering or surface treatments. Heat treatment processes may be carried out in various types of furnace, which differ in characteristics such as pressure, atmosphere, number of chambers, continuous or batch etc.

With modern applications becoming increasingly challenging, heat treatment, which can lead to improved mechanical properties or allow additional processing of steels, is an important part of the production process. This module will give you examples of different types of heat treatment and where they might be used. It will also introduce some of the many types of steel which rely on specific heat treatments to give them very specific microstructures and properties to meet the requirements of demanding applications.

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