Page 210 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

will have a dramatic effect on the microstructure during annealing. For instance, if
1–2 wt.% Si is present, e-iron carbide particles are stabilized up to temperatures of

400 C, yielding a much harder material at elevated temperatures. Further, transition
metals such as Cr, Mo, V, W, and Ti will form stable carbides with higher enthalpies

of formation than Fe 3 C, typically at temperatures between 500 C and 600 C. A high

temperature is required due to the relatively low diffusivity of the alloying elements
that must substitutionally diffuse through the iron lattice. By contrast, interstitial
dopants such as C, N, and B move between the iron lattice sites with a much greater
diffusivity. It is important to note that the alloy carbides remain as fine suspensions
even after prolonged tempering. This results in substantial strengthening referred to
as secondary hardening.
One serious drawback of the above austenization/rapid quench method for mar-
tensite formation is the possibility of distorting and cracking the metal due to the
rapid cooling event. During the quenching process, thermal stresses arise from the
varying cooling rates experienced by outer and interior areas of the steel. In addition,
there is a volume change when austenite is transformed to martensite. Two methods
that have been used to reduce quenching stresses are martempering and austemper-
ing (Figure 3.26). Martempering allows the transformation of austenite to martensite
to take place at the same time throughout the structure of the metal part. By using
interrupted quench, the cooling is stopped at a point above the martensite transfor-
mation region to allow sufficient time for the center to cool to the same temperature
as the surface. Then cooling is continued through the martensite region, followed by
the usual tempering process. By comparison, in austempering, the austenized steel is
quenched at a rate faster than that required for pearlite formation, but above the
temperature required for martensite growth. Hence, rather than transforming to
martensite, the center and surface are converted to bainite – a strong material that
shares the hardness of martensite with the toughness of pearlite.

Surface hardening
The above changes in the microstructure upon annealing do not only apply to the
bulk material, but also for the surface. If an iron material is placed at high tempera-
ture in the presence of carbon vapor, a procedure known as carburization occurs,
where carbon atoms diffuse into the surface of the steel, increasing the surface
hardness. There must be careful control of the annealing atmosphere; if the steel is
brought into contact with an oxidizing atmosphere, decarburization of the surface
will occur through preferential formation of CO 2 .
Other surface hardening techniques introduce nitrogen to steels containing metals
such as Al, Cr, and V that form stable nitrides (Eq. 19). Surface hardening techni-
ques add a variety of attractive properties to steel components such as increasing
wear and stress resistances, decreasing the odds of fracturing, and increasing
corrosion resistance.

600 C
ð19Þ 4Cr þ 3NH 3ðgÞ ! Cr 4 N 3 þ 9=2H 2ðgÞ
Strengthening of the exterior of a material may be achieved through either
diffusional incorporation of dopants (e.g., B, C, N), or annealing selective portions