Callister - An introduction - 8th edition

11.8 Heat Treatment of Steels • 429
490
270
Cooling rate at 700°C (1300°F)
305 125 56 33 16.3 10 7 5.1 3.5
170 70 31 18 9 5.6 3.9 2.8 2
°F/s
°C/s
60
Hardness, HRC
50
40
8660
(0.6 wt% C)
8640
(0.4 wt% C)
30
8620
(0.2 wt% C)
8630
(0.3 wt% C)
20
0 10 20 30 40 50 mm
0
1
4
1
2
3
4 1
Distance from quenched end
1 1 3
1 4 1 2 1 4 2 in.
Figure 11.15 Hardenability curves for four 8600 series alloys of indicated carbon content.
(Adapted from figure furnished courtesy Republic Steel Corporation.)
approximate percentage of martensite that is present at various hardnesses for
these alloys.
The hardenability curves also depend on carbon content. This effect is demonstrated
in Figure 11.15 for a series of alloy steels in which only the concentration
of carbon is varied. The hardness at any Jominy position increases with the concentration
of carbon.
Also, during the industrial production of steel, there is always a slight, unavoidable
variation in composition and average grain size from one batch to another.This
variation results in some scatter in measured hardenability data, which
frequently are plotted as a band representing the maximum and minimum values
that would be expected for the particular alloy. Such a hardenability band is
plotted in Figure 11.16 for an 8640 steel. An H following the designation specification
for an alloy (e.g., 8640H) indicates that the composition and characteristics
of the alloy are such that its hardenability curve will lie within a specified
band.
Influence of Quenching Medium, Specimen
Size, and Geometry
The preceding treatment of hardenability discussed the influence of both alloy composition
and cooling or quenching rate on the hardness. The cooling rate of a specimen
depends on the rate of heat energy extraction, which is a function of the
characteristics of the quenching medium in contact with the specimen surface, as
well as the specimen size and geometry.