Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59

586 • Chapter 14 / Synthesis, Fabrication, and Processing of Materials

rate at the center is equivalent to that approximately 9.5 mm (^38 in.) from the

Jominy specimen quenched end (Figure 14.13a). This corresponds to a hardness

of about 28 HRC, as noted from the hardenability plot for the 1040 steel alloy

(Figure 14.13b). Finally, this data point is plotted on the hardness profile in

Figure 14.13c.

Surface, midradius, and three-quarter radius hardnesses would be deter-

mined in a similar manner. The complete profile has been included, and the

data that were used are tabulated below.

Equivalent Distance from

Radial Position Quenched End[mm(in.)] Hardness(HRC)

Center 9.5 (^38 )2 8

Midradius 8 ( 165 )3 0

Three-quarters radius 4.8 ( 163 )3 9

Surface 1.6 ( 161 )5 4

DESIGN EXAMPLE 14.1

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

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 7.31, 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 repre-

sented in Figure 8.19b. 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 8.19c). 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 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.