HEAT TREATMENT

Dr. H. K. KhairaProfessor in MSME

MANIT, Bhopal

Introduction

• Lifting devices in their various forms are constructed from a range of materials that have specific properties for the function they will perform.

• The inherent structure that provides these necessary properties may not be capable of meeting a range of “working demands”.

• It is then necessary to enhance these properties by some means.

Introduction

• Alloying, cold working processes and heat treatment are ways of altering the structure of a material to improve its properties.

• These structure-altering procedures are often employed in combination to achieve the desired results.

• For example, iron alloyed with carbon produces steel that is further enhanced with other metals such as molybdenum, chromium and vanadium.

• The forging process will again change the properties, as will heat treatment.

Tools for Controlling Properties of Metals

• 1. Alloying• 2. Cold Working• 3. Heat treatment

Heat Treatment

• Heat treatment is defined as heating a metal to a specified temperature, keeping it at that temperature for some time followed by cooling at a specified rate.

• It is a tool to get required microstructure and properties in the metal.

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Heat treatment

Heat treatment - controlled heating and cooling basically

The basic steps of heat treatment are:

Heating → Soaking → Cooling

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Important Parameters in Heat treatment

Heating -> Soaking -> Cooling

Temperature Time of soaking Rate of cooling

Medium of cooling

-Different combinations of the above parameters

Give rise to different heat treatments

Cooling Rates

Types of Heat Treatments

• 1. Annealing• 2. Normalizing• 3. Hardening• 4. Tempering• 5. Precipitation Hardening

Annealing• Annealing is a heat treatment in which the metal is heated to a

temperature above its recrystallisation temperature, kept at that temperature some time for homogenization of temperature followed by very slow cooling to develop equilibrium structure in the metal or alloy.

• The steel is heated 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above A1 temperature in case of hyper-eutectoid temperature

• The cooling is done in the furnace itself.

• The aim of annealing is to increase the ductility.

Normalizing

• Normalizing consists of heating a suitable steel to a temperature 50-1000 C above Ae3 temperature in case of hypoeutectoid steels and above Acm temperature in case of hyper-eutectoid steel, soaking for sufficient time and then cooling in still air.

• The aim of normalizing is to increase the toughness.

Hardening

• In hardening heat treatment, the steel is heated 30 to 50oC above Ae3 temperature in case of hypo-eutectoid steels and 30 to 50oC above A1 temperature in case of hyper-eutectoid temperature, held at that temperature for some time followed by cooling at a rate faster than the critical cooling rate to produce martensite which is a hard phase.

• The aim of hardening is to increase the hardness and strength of the steel.

Critical Cooling Rate

Critical cooling rate

Tempering

• Tempering consists of heating a hardened steel to a temperature below eutectoid temperature and keeping it at that temperature for a specified time to reduce brittleness followed by air cooling.

• The aim of tempering is to decrease brittleness of hardened steel.

Precipitation Hardening

• Precipitation Hardening (or Age Hardening) is a heat treatment in which the strength increases due to precipitation of second phase particles in the parent phase.

• The aim of precipitation hardening is to increase the hardness and strength .

Heat Treatment Temperature

The temperature ranges to which the steel has to be heated for different heat treatments

←Acm

A3→

Cooling Rates for Different heat treatments

Tempering