Page 209 - Materials Chemistry, Second Edition

196 3 Metals

Figure 3.25. Crystallographic representation of the phase transformation from austenite to martensite.
Two neighboring fcc unit cells of austenite associate, resulting in a body-centered tetragonal (bct) unit
cell. A postanneal known as tempering converts the bct structure to a-Fe. Also shown is the placement of
an interstitial carbon atom, remaining in an octahedral site among lattice iron atoms.

microstructure at the expense of the e-Fe x C particles. At temperatures
approaching 350 C, the bct lattice is transformed to ferritic bcc.

(iii) 350–700 C: The cementite particles undergo a coarsening process (between

300 C and 400 C), and spheroidization (near 700 C). These processes drasti-
cally reduce the hardness of the material, but improve overall ductility and
brittleness characteristics (more desirable for particular applications).
Hence, to obtain high-strength tempered steels, it is essential to anneal at low
temperatures (<350 C). As one might expect, the presence of alloying elements