Page 200 - Materials Chemistry, Second Edition

187
3.2. Metallic Structures and Properties

not familiar with the microcrystalline changes they were imposing, they knew
through experience that heating/cooling regimes were effective means to improve
many properties of steel such as hardness, toughness, ductility, machinability, and
wear/stress resistance. The softening of steel at elevated temperatures is due to the
formation of large iron carbide crystallites that may undergo facile slip deforma-
tions. The prolonged high temperature environment affects the microstructure
through interrupting Fe–Fe and Fe–C bonds of either pearlitic or bainitic steel.
This allows for cementite (Fe 3 C) regions to agglomerate into spheres, which are
dispersed within a ferrite matrix – aptly referred to as spheroidite (Figure 3.20). This
process is an example of precipitation hardening, which induces an increase in
hardness through the formation of homogeneous dispersions that reduce slippage
between grains. The formation of homogeneous suspensions of small, finely dis-
persed particles in a matrix prevents grain slippage (Figure 3.21). Since this process
occurs as the alloy ages, this is also referred to as age hardening. Examples of alloys
that are hardened via precipitation formation include Al/Cu, Cu/Be, Cu/Sn, and
Mg/Al. It should be noted that the heating of iron surfaces is now often achieved by
more energetic sources such as electron beams or lasers. Such a focused thermal
treatment allows for localized surface hardening to improve its wear resistance.

Figure 3.20. Optical micrograph of a pearlitic steel (note the lamellar regions) that has been partially
transformed to spheroidite. Image taken at 2,000 resolution. Photograph courtesy of US Steel
Corporation.