Table of Contents
Introduction To Heat Treatment and Objectives Of Heat Treatments
Introduction To Heat Treatment
Heat treatment:It is defined as an operation involving heating and cooling of metals or alloys in its solid state with the purpose of changing the properties of the material.
The physical and mechanical properties of the materials depend upon the size, shape and form of the micro-constituents present. The micro-constituents generally present in steel are ferrite, troostite, sorbite, austenite and cementite.
Steel possesses many properties like strength, cheapness and workability in addition to toughness, stiffness creep resistance, fatigue resistance, impact strength, etc. Proper heat treatment of steel plays an important part in engineering. Heat treatment of all components, whether cast forged or rolled, is necessary before actual use.
During heat-treatment of a metal piece, when it is heated to a definite temperature followed by cooling at a suitable rate, there occur changes in the micro-constituents of the metal. These changes in the microconstituents of the metal may be in their nature, form, size and distribution in the metal piece. Obviously, temperature of heating and rate of cooling are the main controlling factors of changes in micro-constituents.
These changes in micro-constituents then control the changes in physical and mechanical properties of heat treated metal specimen. For various fabrication and manufacturing operations, heat treatment is a very important process.
Factors in Heat Treatment Processes:
Before carrying out any heat treatment operation the following factors need consideration:
1. Chemical composition of the material.
2. Mode of manufacture of the material, i.e. cast, ingot, rolled or forged, etc.
3. Whether any previous heat treatment operation has been carried out on the material and what is its structure.
4. Heat treatment operations to be performed and properties and structure required.
Objectives of Heat Treatment:
When steel is subjected to the heat treatment operations, it undergoes many structural changes due to which the properties of the material change.
In studying the effects of heat treatment therefore, the following points need consideration.
1. Structural condition of the object before heat treatment.
2. Structural changes occurring during the heat treatment.
3. Structural conditions permanently retained.
In addition to the above, the chemical composition of the material plays a vital role in controlling the properties of the materials.
The following are the main objects of the heat treatment of steel.
1. To soften the steel that has been hardened by the previous heat treatment or mechanical working.
2. To harden the steel and increase its strength.
3. To adjust its other mechanical and physical properties like ductility, malleability, permeability corrosion resistance, etc.
4. To stabilize the dimensions of the steel instruments so that they do not expand or contract with time.
5. To refine the grain size of the steel.
6. To reduce the internal stresses. To eliminate gases.
7. To produce a hard surface on a ductile interior.
8. To improve electrical and magnetic properties.
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