Heat treatment
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Transcript of Heat treatment
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.
Handouts 2 6
Heat treatment
Heat treatment - controlled heating and cooling basically
The basic steps of heat treatment are:
Heating → Soaking → Cooling
Handouts 2 7
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