Callister - An introduction - 8th edition

434 • Chapter 11 / Applications and Processing of Metal Alloys
Surface, midradius, and three-quarter radius hardnesses would be determined
in a similar manner. The complete profile has been included, and the
data that were used are shown in the following table.
Equivalent Distance
from Quenched End
Radial Position [mm (in.)] Hardness (HRC)
Center 9.5 1 3 8 2
28
Midradius 8 1 5 16 2
30
Three-quarters radius 4.8 1 3 16 2
39
Surface 1.6 1 1 16 2
54
DESIGN EXAMPLE 11.1
Steel Alloy and Heat Treatment Selection
It is necessary to select a steel alloy for a gearbox output shaft. The design calls
for a 1-in.-diameter cylindrical shaft having a surface hardness of at least 38 HRC
and a minimum ductility of 12%EL. Specify an alloy and treatment that meet
these criteria.
Solution
First of all, cost is also most likely an important design consideration. This would
probably eliminate relatively expensive steels, such as stainless and those that
are precipitation hardenable. Therefore, let us begin by examining plain carbon
steels and low-alloy steels and what treatments are available to alter their mechanical
characteristics.
It is unlikely that merely cold working one of these steels would produce the
desired combination of hardness and ductility. For example, from Figure 6.19, a
hardness of 38 HRC corresponds to a tensile strength of 1200 MPa (175,000 psi).
The tensile strength as a function of percent cold work for a 1040 steel is represented
in Figure 7.19b. Here it may be noted that at 50%CW, a tensile strength
of only about 900 MPa (130,000 psi) is achieved; furthermore, the corresponding
ductility is approximately 10%EL (Figure 7.19c). Hence, both of these properties
fall short of those specified in the design; furthermore, cold working other
plain-carbon or low-alloy steels would probably not achieve the required minimum
values.
Another possibility is to perform a series of heat treatments in which the
steel is austenitized, quenched (to form martensite), and finally tempered. Let
us now examine the mechanical properties of various plain carbon steels and
low-alloy steels that have been heat-treated in this manner. To begin, the surface
hardness of the quenched material (which ultimately affects the tempered hardness)
will depend on both alloy content and shaft diameter, as discussed in the
previous two sections. For example, the degree to which surface hardness
decreases with diameter is represented in Table 11.11 for a 1060 steel that was oil
quenched. Furthermore, the tempered surface hardness will also depend on tempering
temperature and time.