Callister - An introduction - 8th edition

11.8 Heat Treatment of Steels • 431
Cooling rate at 700°C (1300°F)
Cooling rate at 700°C (1300°F)
305 125 56 33 16.3 10 7 °F/s
305 125 56 33 16.3 10 7 °F/s
100
170 70 31 18 9 5.6 3.9 °C/s
4
100
170 70 31 18
9 5.6 3.9 °C/s
4
75
Surface
3
4 R
1
2 R
3
75
3
4 R
1
2 R
3
Diameter of bar (mm)
50
Center
3
4 R
Center
2
Diameter of bar (in.)
Diameter of bar (mm)
50
Surface
Center
2
Diameter of bar (in.)
25
1
25
1
Surface
1
2 R
0 0 0 0
0 10 20 30 mm 0 10 20 30 mm
0
1 1 3
in.
4 2 4 1 1 1 4
Equivalent distance from quenched end
(a)
0
1 1 3
in.
4 2 4 1 1 1 4
Equivalent distance from quenched end
Figure 11.17 Cooling rate as a function of diameter at surface, three-quarter radius 1 3 4R2,
midradius 1 1 2R2, and center positions for cylindrical bars quenched in mildly agitated
(a) water and (b) oil. Equivalent Jominy positions are included along the bottom axes.
[Adapted from Metals Handbook: Properties and Selection: Irons and Steels, Vol. 1, 9th
edition, B. Bardes (Editor), American Society for Metals, 1978, p. 492.]
(b)
data are often used in conjunction with hardenability curves. Diagrams similar to
those in Figure 11.17 have also been generated for geometries other than cylindrical
(e.g., flat plates).
One utility of such diagrams is in the prediction of the hardness traverse along
the cross section of a specimen. For example, Figure 11.18a compares the radial
hardness distributions for cylindrical plain carbon (1040) and alloy (4140) steel
specimens; both have a diameter of 50 mm (2 in.) and are water quenched. The
difference in hardenability is evident from these two profiles. Specimen diameter
also influences the hardness distribution, as demonstrated in Figure 11.18b, which
plots the hardness profiles for oil-quenched 4140 cylinders 50 and 75 mm (2 and
3 in.) in diameter. Example Problem 11.1 illustrates how these hardness profiles
are determined.
As far as specimen shape is concerned, because the heat energy is dissipated
to the quenching medium at the specimen surface, the rate of cooling for
a particular quenching treatment depends on the ratio of surface area to the
mass of the specimen. The larger this ratio, the more rapid will be the cooling
rate and, consequently, the deeper the hardening effect. Irregular shapes with
edges and corners have larger surface-to-mass ratios than regular and rounded
shapes (e.g., spheres and cylinders) and are thus more amenable to hardening
by quenching.