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

Heat Treatment 131
(b)
Movable type
It consists of two types
(1) The car bottom type
(2) The rotary type
8.2.2 Bath Furnaces
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
8.3 CONSTITUENTS OF IRON AND STEEL
Fig. 8.1 shows micro structure of mild steel (0.2-0.3% C). White constituent in this figure is very
pure iron or having very low free carbon in iron in form of ferrite and dark patches contain
carbon in iron is chemically combined form known as carbide (Cementite). Cementite is very
hard and brittle. Now if the dark patches of the above figure are further observed, a substance
built up of alternate layer of light and dark patches is reflected in Fig. 8.2. These layers are
alternatively of ferrite and cementite. This substance is called as pearlite and is made up of 87%
ferrite and 13% cementite. But with increase of carbon content in steel portion of pearlite
increases up to 0.8% C. The structure of steel at 0.8% C is entirely of pearlite. However if carbon
content in steel is further increased as free constituent up to 1.5% C, such steel will be called
as high carbon steel. The micro structure of high carbon steel is depicted in Fig. 8.3.
Cementite
Areas
Ferrite
Crystals
Fig. 8.1 Micro structure of mild steel Fig. 8.2 Micro structure of Fig. 8.3 Micro structure of
pearlitic eutectoid steel of high carbon steel
8.4 ALLOTROPY OF IRON
In actual practice it is very difficult to trace the cooling of iron from 1600°C to ambient
temperature because particular cooling rate is not known. Particular curve can be traced
from temperature, time and transformation (TTT) curve. However allotropic changes observed
during cooling of pure iron are depicted in Fig. 8.4. When iron is cooled from molten condition
up to the solid state, the major allotropic changing occurs which are:
1539-1600°C Molten-Fe (Liquid state of iron)
1400-1539°C Delta-Fe (Body centered)