Page 135 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment

transformation (IT) diagrams, or time-temperature-transformation (TTT) diagrams,
constructed from the data given in Fig. 4.17a. The higher the temperature or the
longer the time, the more austenite that transforms into pearlite. Note that, for each
temperature, there is a minimum time for the transformation to begin. This time
period defines the critical cooling rate; with longer times, austenite begins to trans-
form into pearlite. This transformation can be traced in Figs. 4.17b and c.
The TTT diagrams shown allow metallurgists to design heat treatment sched-
ules to obtain desirable microstructures. For example, consider the TTT curves shown
in Fig. 4.17c. The steel can be raised to a very high temperature (above the eutectic

100 0

§ 75 25 g
I as
5 50 600°C 650 675 50 #5
E 2
Q) U)
3
D_
25 75 <
0 J 100
1 10 102 103
Time (s)
(3)

Q, £2 Transformation
7; E 100 0
g Q A
‘5 Q temperature :Transformation 52
Q, 2 675°C lends 2 800

~5 3 50 ------ f--q 50 E
Eutectoid temperature
`\`
\=~
*E § ;;ag'E;°rmat'°n ‘Q 700'f"\\"""
dl! % O 100 < \\ \\
45 1 101 102 103 104 105 ‘\ \\
Time (S) 500 ' ‘\ \
\ \\ Austenite -»pearlite
9 \
Eutectoid O 500- ‘
l“?T'f‘ii‘?HE)_;43@Qe2Qf@ 3; \ O,
700 _Austenite Q ‘5 \ \01>
A (unstable) ,¢,-- ‘Q 400' M; ‘\%
o " <11 Q,
V I/' Pearlite 2- Critical ) Q \\\
O
_ (D
Q 600 50% Completion curve 2 300 CQZLZQT) \ Martensite
(U l \ ‘\ (start)
:_
co 1 Completion curve [_ 1 \
Q 500 (~100%pear|ite) 200 1 \`
|9 1 \ Martensite
5691" CUVY9 100 _ -li 1 \ + pearlite
400 ' (~0% Peaflltel Martensite : I I Pearlite-
O -1 l
1 10 102 103 104 105 10_1 1 10 102 103 104 105
Time (s) Time (s)
(D) (C)

FIGURE 4.l1 (a) Austenite-to-pearlite transformation of iron-carbon alloy as a function
of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a
transformation temperature of 675°C. (c) Microstructures obtained for a eutectoid
iron-car on a loy as a function of cooling rate.
b
l