The properties of metals and alloys can be changed as desired by the heat treatment process.
UNIT - 2
Heat Treatment
"I never did anything worth doing by accident nor did any of my inventions come by accident. They come by work."
- Thomas Edison
HEAT TREATMENT PROCESSES
INTRODUCTION
✓ The properties of metals and alloys can be changed as desired by the heat treatment process.
✓ Heat treatment is the controlled heating and cooling of metals voll for the purpose of altering their properties.
✓ Since the heat treatment can greatly alter the mechanical and physical properties of metals and alloys, therefore it is the most important and widely used manufacturing processes.
✓ The heat treatment usually provides a simple and low-cost means of achieving desired properties. At the same time, poor or careless heat treatment may result in more harm than good can result.
✓ Thus the understanding of the heat treatment and correlating it with the other manufacturing processes are very essential for mechanical engineers in order to obtain optimal results.
In this unit, the theory and the various processes of heat treatment will be discussed in detail.
1. Purpose of Heat Treatment
The heat treatment is generally employed for one or more of the following purposes:
1. To relieve internal stresses (which are set up in the metal due to Jns cold working, welding, casting, forging, etc.).
2. To improve machinability.
3. To refine grain size.
4. To soften the metal.
5. To improve hardness of the metal surface.
6. To improve mechanical properties (like tensile strength, hardness, ductility, shock resistance, etc.).
7. To improve magnetic and electrical properties.
8. To increase resistance to wear, heat and corrosion.
9. To improve ductility and toughness.
10. To change the chemical composition.
2. Definition
✓ Heat treatment may be defined as an operation or combination of operations involving heating and cooling of a metal/alloy in solid state to obtain desirable properties.
✓ In other words, heat treatment is a process in which metal/alloy is heated beyond the critical temperature↑ and cooled at 1200-wocontrolled rates to get different microstructures and hence desired mechanical properties.
3. Principles of Heat Treatment
✓ The theory of heat treatment is based on the fact that a change takes place in the internal structure of metal by heating and cooling which induces desired properties in it.
† The temperature at which the structural changes takes place is called the critical temperature.
✓ The rate of cooling is the controlling factor in developing hard or soft structure. Rapid cooling from the critical range results in hard structure, whereas very slow cooling produces the soft structure.
✓ The variety of metal (such as steel) and various metallurgical processes are depending upon :
(i) the method and rate of heating and cooling;
(ii) furnaces used; and
(iii) quenching medium used.
4. Stages of Heat Treatment Process
Any heat treatment process usually undergoes the following three stages:
Stage 1: Heating a metal/alloy beyond the critical temperature.
Stage 2: Holding at that temperature for a sufficient period of time to allow necessary changes to occur.
Stage 3: Cooling the metal/alloy (i.e., quenching) at a rate necessary to obtain the desired properties. That is, cooling at a rate necessary to obtain the desired changes in the nature, form, size, and distribution of micro- constituents.
5. Types of Heat Treatment Processes
(Various Heat Treatment Operations)
The important heat treatment processes are as follows:
1. Annealing
(a) Full annealing,
(b) Process annealing,
(c) Stress relief annealing,
(d) Recrystallisation annealing, and
(e) Spheroidise annealing.
2. Normalising
3. Hardening
4. Tempering
5. Austempering
6. Martempering
7. Case hardening
(a) Carburising,
(b) Nitriding,
(c) Cyaniding,
(d) Carbonitriding,
(e) Flame hardening, and
(f) Induction hardening.
Now we shall discuss the above heat treatment processes in the following sections, in detail.
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