Introduction to Basic Manufacturing Processes and ... - always yours

130 Introduction to Basic Manufacturing Processes and Workshop Technology
8
CHAPTER
HEAT TREATMENT
8.1 INTRODUCTION
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.
8.2 HEAT TREATMENT FURNACES
8.2.1 Hearth 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
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