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

132 Introduction to Basic Manufacturing Processes and Workshop Technology
910-1400°C Gamma-Fe (FCC atomic arrangement and austenite structure)
770- 910°C Beta-Fe (Body centered-nonmagnetic)
Up to 770°C Alpha-Fe (BCC atomic arrangement and ferrite structure)
1700
1539°C
1500
Delta
Iron
Body Centered
1404°C A 4
1300
Temperature °C
1100
900
Gamma
Iron
Beta
Iron
Face Centered
910°C A 3
768°C A 2
700
Body Centered
Alpha
Iron
(i)
(ii)
(iii)
(iv)
500
Time
Fig. 8.4 Allotroic changes during cooling of pure iron
First changing occurs at l539°C at which formation of delta iron starts.
Second changing takes place at 1404°C and where delta iron starts changes into
gamma iron or austenite (FCC structure).
Third changing occurs at 910°C and where gamma iron (FCC structure) starts
changes into beta iron (BCC structure) in form of ferrite, leadaburite and austenite.
Fourth changing takes place at 768°C and where beta iron (BCC structure) starts
changes into alpha iron in form of ferrite, pearlite and cementite.
Therefore, the temperature points at which such changing takes place into allotropic
forms are called critical points. The critical points obtained during cooling are slightly lower
than those obtained in heating. The most marked of these range commonly called the point
of recalescence and point of decalescence.
8.5 TRANSFORMATION DURING HEATING AND COOLING OF STEEL
When a steel specimen is heated, its temperature rises unless there is change of state or a
change in structure. Fig. 8.5 shows heating and cooling curve of steel bearing different
structures. Similarly, if heat is extracted, the temperature falls unless there is change in state
or a change in structure. This change of structure does not occur at a constant temperature.
It takes a sufficient time a range of temperature is required for the transformation. This
range is known as transformation range. For example, the portion between the lower critical
temperature line and the upper critical temperature line with hypo and hyper eutectoid