Callister - An introduction - 8th edition

432 • Chapter 11 / Applications and Processing of Metal Alloys
Hardness, HRC
60
50
40
30
1040
4140
Hardness, HRC
55
50
45
4140
4140
Figure 11.18 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.
20 40
50 mm
(2 in.)
50 mm
(2 in.)
75 mm
(3 in.)
(a)
(b)
A multitude of steels are responsive to a martensitic heat treatment, 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 Figure
11.17 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 11.8
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 www.wiley.com/college/callister (Student Companion Site).]
EXAMPLE PROBLEM 11.1
Determination of Hardness Profile for Heat-Treated
1040 Steel
Determine the radial hardness profile for a 50-mm- (2-in.-) diameter cylindrical
specimen of 1040 steel that has been quenched in moderately agitated
water.