GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
14.6 Heat Treatment of Steels • 583Diameter of bar (mm) Diameter of bar (in.)43211007550000025Diameter of bar (mm) Diameter of bar (in.)4321100755025Cooling rate at 700°C (1300°F)
305 125 56 33 16.3 10 7 °F/s
170 70 31 18 9 5.6 3.9°C/s(^34) R (^3) R
4
(^34) R
(^12) R
(^12) R
(^12) R
Equivalent distance from quenched end
(a)
Cooling rate at 700°C (1300°F)
305 125 56 33 16.3 10 7 °F/s
170 70 31 18
0 10 20 30 mm 0 10 20 30 mm
9 5.6 3.9°C/s
Equivalent distance from quenched end
(b)
Surface
Surface
Surface
Center
Center
Center
0 41 21 43 1 114 in. 0 14 12 34 1 141 in.
Figure 14.11 Cooling rate as a function of diameter at surface, three-quarters radius (^34 R),
midradius (^12 R), 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
fromMetals Handbook: Properties and Selection: Irons and Steels, Vol. 1, 9th edition, B.
Bardes (Editor), American Society for Metals, 1978, p. 492.]
oil, which is more effective than air.^2 The degree of agitation of each medium also
influences the rate of heat removal. Increasing the velocity of the quenching medium
across the specimen surface enhances the quenching effectiveness. Oil quenches
are suitable for the heat treating of many alloy steels. In fact, for higher-carbon
steels, a water quench is too severe because cracking and warping may be produced.
Air cooling of austenitized plain carbon steels ordinarily produces an almost totally
pearlitic structure.
During the quenching of a steel specimen, heat energy must be transported to
the surface before it can be dissipated into the quenching medium. As a consequence,
the cooling rate within and throughout the interior of a steel structure varies with
position and depends on the geometry and size. Figures 14.11aand 14.11bshow the
quenching rate at 700◦C (1300◦F) as a function of diameter for cylindrical bars at four
radial positions (surface, three-quarters radius, midradius, and center). Quenching
is in mildly agitated water (Figure 14.11a) and oil (Figure 14.11b); cooling rate is
also expressed as equivalent Jominy distance, since these data are often used in
conjunction with hardenability curves. Diagrams similar to those in Figure 14.11
have also been generated for geometries other than cylindrical (e.g., flat plates).
(^2) Aqueous polymer quenchants{solutions composed of water and a polymer [normally
poly(alkylene glycol) or PAG]}have recently been developed that provide quenching rates
between those of water and oil. The quenching rate can be tailored to specific requirements
by changing polymer concentration and quench bath temperature.