Callister - An introduction - 8th edition

11.8 Heat Treatment of Steels • 427
Figure 11.13
Correlation of
hardenability and
continuous cooling
information for an
iron–carbon alloy of
eutectoid
composition.
[Adapted from H.
Boyer (Editor), Atlas
of Isothermal
Transformation and
Cooling
Transformation
Diagrams, American
Society for Metals,
1977, p. 376.]
Temperature (°C)
800
600
400
200
Hardness, HRC
70 End-quench hardenability
60
50
40
30
20
0 0.5 1.0 1.5 2.0 2.5 3.0
Distance from quenched end (in.)
A B C D
M (start)
Austenite
Pearlite
1400
1200
1000
800
600
400
Temperature (°F)
Austenite
Martensite
200
0
A
B C D
0
Martensite Martensite and Fine Pearlite
pearlite pearlite
Cooling transformation diagram
Cooling curves
Transformation during cooling
0.1 1 10 10 2 10 3
Time (s)
here. This correlation between position and cooling rate is the same for plain
carbon steels and many alloy steels because the rate of heat transfer is nearly
independent of composition. On occasion, cooling rate or position from the
quenched end is specified in terms of Jominy distance, one Jominy distance unit
being 1.6 mm 1 1
16 in.2.
A correlation may be drawn between position along the Jominy specimen and
continuous cooling transformations. For example, Figure 11.13 is a continuous cooling
transformation diagram for a eutectoid iron–carbon alloy onto which are superimposed
the cooling curves at four different Jominy positions and corresponding
microstructures that result for each. The hardenability curve for this alloy is
also included.
The hardenability curves for five different steel alloys all having 0.40 wt% C,
yet differing amounts of other alloying elements, are shown in Figure 11.14. One
specimen is a plain carbon steel (1040); the other four (4140, 4340, 5140, and 8640)
are alloy steels. The compositions of the four alloy steels are included with the figure.
The significance of the alloy designation numbers (e.g., 1040) is explained in
Section 11.2. Several details are worth noting from this figure. First, all five alloys
have identical hardnesses at the quenched end (57 HRC); this hardness is a function
of carbon content only, which is the same for all these alloys.