Callister - An introduction - 8th edition

430 • Chapter 11 / Applications and Processing of Metal Alloys
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
50
Hardness, HRC
40
30
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 11.16 The hardenability band for an 8640 steel indicating maximum and minimum
limits. (Adapted from figure furnished courtesy Republic Steel Corporation.)
Severity of quench is a term often used to indicate the rate of cooling; the more
rapid the quench, the more severe the quench. Of the three most common quenching
media—water, oil, and air—water produces the most severe quench, followed by
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 11.17a and
11.17b show the quenching rate at 700C (1300F) 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 11.17a) and oil (Figure
11.17b); cooling rate is also expressed as equivalent Jominy distance, because these
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.