Page 73 - Mechanical Behavior of Materials

72 Chapter 3 A Survey of Engineering Materials

phase situations involve needlelike or layered microstructural features, or crystal grains of more
than one type. For example, some titanium alloys have a two-phase structure involving grains
of both alpha (HCP) and beta (BCC) crystal structures. Multiple phases increase strength,
because the discontinuities in the crystal structure at the phase boundaries make dislocation
motion more difficult, and also because one phase may be resistant to deformation. The two-
phase (alpha–beta) structure just mentioned for titanium provides a portion of the strengthen-
ing for the highest strength titanium alloys. Also, the processing of steels by quenching and
tempering, which will be discussed in the next section, owes its benefits to multiple phase
effects.

3.3 IRONS AND STEELS

Iron-based alloys, also called ferrous alloys, include cast irons and steels and are the most widely
used structural metals. Steels consist primarily of iron and contain some carbon and manganese, and
often additional alloying elements. They are distinguished from nearly pure iron, which is called
ingot iron, and also from cast irons, which contain carbon in excess of 2% and from 1 to 3% silicon.
Irons and steels can be divided into various classes, depending on their alloy compositions and other
characteristics, as indicated in Table 3.3. Some examples of particular irons or steels and their alloy
compositions are given in Table 3.4.
A wide variation in properties exists for various steels, as illustrated in Fig. 3.6. Pure iron
is quite weak, but is strengthened considerably by the addition of small amounts of carbon.
Additional alloying with small amounts of niobium, vanadium, copper, or other elements permits
strengthening by grain refinement, precipitation, or solid solution effects. If sufficient carbon is

Table 3.3 Commonly Encountered Classes of Irons and Steels
Class Distinguishing Features Typical Uses Source of Strengthening
Cast iron More than 2% C Pipes, valves, gears, Ferrite-pearlite structure
and 1 to 3% Si engine blocks as affected by free graphite
Plain-carbon Principal alloying Structural and machine Ferrite-pearlite structure if
steel element is carbon parts low carbon; quenching and
up to 1% tempering if medium to
high carbon
Low-alloy Metallic elements High-strength Grain refinement, precipitation,
steel totaling up to 5% structural and and solid solution if low
machine parts carbon; otherwise quenching
and tempering
Stainless At least 10% Cr; Corrosion resistant Quenching and tempering if
steel does not rust piping and nuts and < 15% Cr and low Ni; otherwise
bolts; turbine blades cold work or precipitation
Tool steel Heat treatable to Cutters, drill bits, Quenching and tempering, etc.
high hardness dies
and wear resistance