Page 79 - Mechanical Behavior of Materials

78 Chapter 3 A Survey of Engineering Materials

Figure 3.9 Effect of tempering temperature on the yield strength for several steels. These
data are for 13 mm diameter samples machined from material heat treated as 25 mm diameter
bars. (Data from [Boyer 85] p. 4.21.)

of the effects of alloying, sulfur improves machineability, and molybdenum and vanadium promote
grain refinement. The combination of alloys used in the steel AISI 4340 gives improved strength and
toughness—that is, resistance to failure due to a crack or sharp flaw. In this steel, the metallurgical
changes during quenching proceed at a relatively slow rate so that quenching and tempering is effec-
tive in components as thick as 100 mm. Note that the corresponding plain-carbon steel, AISI 1040,
requires very rapid quenching that cannot be achieved except within about 5 mm of the surface.
Various special-purpose low-alloy steels are used that may not fit any of the standard AISI–SAE
designations. Many of these are described in the ASTM Standards, where requirements are placed
on mechanical properties in addition to alloy content. Some of these are classified as high-strength
low-alloy (HSLA) steels, which have a low carbon content and a ferritic-pearlitic structure, with
small amounts of alloying resulting in higher strengths than in other low-carbon steels. Examples
include structural steels as used in buildings and bridges, such as ASTM A242, A441, A572, and
A588. Note that use of the term “high-strength” here can be somewhat misleading, as the strengths
are high for a low-carbon steel, but not nearly as high as for many quenched and tempered steels.
The low-alloy steels used for pressure vessels, such as ASTM A302, A517, and A533, constitute an
additional group of special-purpose steels.

3.3.5 Stainless Steels
Steels containing at least 10% chromium are called stainless steels because they have good corrosion
resistance; that is, they do not rust. These alloys also frequently have improved resistance to high
temperature. A separate system of AISI designations employs a three-digit number, such as AISI
316 and AISI 403, with the first digit indicating a particular class of stainless steels. The correspond-
ing UNS designations often use the same digits, such as S31600 and S40300 for the two just listed.