GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
584 • Chapter 14 / Synthesis, Fabrication, and Processing of Materials(a) (b)6050403020 40455055Hardness, HRC
Hardness, HRC41404140(^10404140)
50 mm
75 mm
(2 in.)
50 mm
(2 in.)
(3 in.)
Figure 14.12 Radial hardness
profiles for (a) 50 mm (2 in.)
diameter cylindrical 1040 and
4140 steel specimens quenched
in mildly agitated water, and
(b) 50 and 75 mm (2 and 3 in.)
diameter cylindrical specimens
of 4140 steel quenched in
mildly agitated oil.
One utility of such diagrams is in the prediction of the hardness traverse along the
cross section of a specimen. For example, Figure 14.12acompares 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 14.12b, which plots the hardness
profiles for oil-quenched 4140 cylinders 50 and 75 mm (2 and 3 in.) in diameter.
Example Problem 14.1 illustrates how these hardness profiles are determined.
As far as specimen shape is concerned, since 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.
There are a multitude of steels that are responsive to a martensitic heat treat-
ment, and one of the most important criteria in the selection process is hardenability.
Hardenability curves, when utilized in conjunction with plots such as those in Fig-
ure 14.11 for various quenching media, may be used to ascertain the suitability of a
specific steel alloy for a particular application. Or, conversely, the appropriateness
of a quenching procedure for an alloy may be determined. For parts that are to be
involved in relatively high stress applications, a minimum of 80% martensite must
be produced throughout the interior as a consequence of the quenching procedure.
Only a 50% minimum is required for moderately stressed parts.
Concept Check 14.3
Name the three factors that influence the degree to which martensite is formed
throughout the cross section of a steel specimen. For each, tell how the extent of
martensite formation may be increased.
[The answer may be found at http://www.wiley.com/college/callister (Student Companion Site).]