Page 76 - Mechanical Behavior of Materials

Section 3.3 Irons and Steels 77

Figure 3.8 Steel microstructures: ferrite-pearlite structure in normalized AISI 1045 steel (left),
with ferrite being the light-colored areas, and pearlite the striated regions; quenched and
tempered structure in AISI 4340 steel (right). (Left photo courtesy of Deere and Co., Moline, IL.)

vehicles. High-carbon steels are limited to uses where their high hardness is beneficial and the low
ductility is not a serious disadvantage, as in cutting tools and springs.
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In quenching and tempering, the steel is first heated to about 850 C so that the iron changes to
the FCC phase known as γ -iron or austenite, with carbon being in solid solution. A supersaturated
solution of carbon in BCC iron is then formed by rapid cooling, called quenching, which can be
accomplished by immersing the hot metal into water or oil. After quenching, a structure called
martensite is present, which has a BCC lattice distorted by interstitial carbon atoms. The martensite
exists either as groupings of parallel thin crystals (laths) or as more randomly oriented thin plates,
surrounded by regions of austenite.
As-quenched steel is very hard and brittle due to the two phases present, the distorted crystal
structure, and a high dislocation density. To obtain a useful material, it must be subjected to a second
stage of heat treatment at a lower temperature, called tempering. This causes removal of some of
the carbon from the martensite and the formation of dispersed particles of Fe 3 C. Tempering lowers
the strength, but increases the ductility. The effect is greater for higher tempering temperatures and
varies with carbon content and alloying, as illustrated in Fig. 3.9. The microstructure of a quenched
and tempered steel is shown in Fig. 3.8 (right).

3.3.4 Low-Alloy Steels
In low-alloy steels, also often called simply alloy steels, small amounts of alloying elements totaling
no more than about 5% are added to improve various properties or the response to processing.
Percentages of the principal alloying elements are given for some of these in Table 3.5. As examples