What is Heat Treatment?
Heat treatment is a heating and cooling process of a metal or an alloy in the solid state with the purpose of changing their properties. It can also be said as a process of heating and cooling of ferrous metals especially various kinds of steels in which some special properties like softness, hardness, tensile-strength, toughness etc, are induced in these metals for achieving the special function objective. It consists of three main phases namely;
(i) heating of the metal
(ii) soaking of the metal and
(iii) cooling of the metal.
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 rate of cooling is the major controlling factor. Rapid cooling the metal from above the critical range, results in hard structure. Whereas very slow cooling produces the opposite affect i.e. soft structure. In any heat treatment operation, the rate of heating and cooling is important. A hard material is difficult to shape by cutting, forming, etc. During machining in machine shop, one requires machineable properties in job piece hence the properties of the job piece may requires heat treatment such as annealing for inducing softness and machineability property in workpiece. Many types of furnaces are used for heating heat treatment purposes. The classification of such heat treatment furnaces is given as under.
Heat Treatment Furnaces
These furnaces are heated by fuel which may be coke, coal, gas (town, blast or natural) and fuel oil. They can also be operated electrically. They are generally of two types.
(a) Stationary type: It consists of four types
(1) Direct fuel fired furnace
(2) Indirect fuel fired furnace
(3) Multiple furnace
(4) Re-circulation furnace
(b) Movable type: It consists of two types
(1) The car bottom type
(2) The rotary type
In bath type furnaces, heating may be done using by gas, oil or electricity. These furnaces are further classified as:
(1) Liquid bath type
(2) Salt bath type
(3) Lead bath type
(4) Oil bath type