Page 204 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

Finally, cooling curve (vi) illustrates very slow cooling (e.g., furnace cooling), which
will result in 100% pearlite.

Solute hardening
The introduction of foreign species into the metallic lattice through alloy formation
introduces alien crystallites that will also impede slip in steel crystals by increasing
the lattice energy in the vicinity of the dopant. Strengthening occurs since more
work is required to propagate a dislocation through these areas. In particular, if the
alloying agent is carbon, hard crystallites of iron carbide may form that changes the
microstructure. By comparison, austenite usually does not contain iron carbide, and
is quite susceptible to slip.
From an analysis of various types of steels, only the following carbides will be
present: Fe 3 C, Mn 3 C, Cr 23 C 6 ,Cr 7 C 3 ,Fe 3 Mo 3 C, Fe 3 W 3 C, Mo 2 C, W 2 C, WC, VC,
TiC, NbC, TaC, Ta 2 C, and ZrC. The occurrence of these species will depend on the
type and concentration of the transition metal dopants within the iron lattice. For
interstitial carbides, the size of the metal atoms will govern the type of carbide
˚
formed. In general, the metal radius must be >1.35 A (e.g., Ti, Zr, Hf, V, Nb, Ta,
Mo, and W) to generate an interstitial vacancy large enough to accommodate C
atoms. Metals with smaller radii (e.g., Cr, Mn, Fe, Co, Ni) do not form MC species,
and form carbides with relatively complex crystal structures (Figure 3.23). It should
be noted that metal carbides do not generally exist as isolated pure species. That is,
carbides of all alloying elements will exist as clusters that also contain iron. Further,
when several carbide-forming dopants are present that share the same crystal
structure, the resultant carbide will be present as a combination of those elements.
As an example, steel containing Cr and Mn dopants will contain particulates of the
complex carbide (Cr, Mn, Fe) 23 C 6 , rather than isolated Cr 23 C 6 and Mn 3 C species.
We have seen that only certain transition metals will form stable carbides; as a
relevant digression, let us consider the chemical rationale behind such reactivity.
The general trend for increasing carbide-forming ability of transition metals is:

Fe < Mn < Cr < Mo < W < V < Nb < Ta < Ti < Zr < Hf
If one follows this sequence using the Periodic Table, this grouping consists of
early transition metals that are relatively electron deficient. As you may recall, the
2
valence shell of zero-valent transition metals in a crystal lattice is [(ns )((n 1)d )].
x
In the bulk solid state, the outer s electrons are completely delocalized, whereas the
wave functions of the d electrons remain localized on the respective metal atoms.
When a carbon atom enters the crystal field it behaves as a ligand toward the metal,
with the ligand and metal electrons electrostatically interacting causing the d orbitals
to lose their original degeneracy. [10] Since this is an electrostatic effect, stronger
metal–carbon bonds will result from more diffuse metal d orbitals (5d vs.4d vs. 3d),
and metals with fewer d electrons (i.e., both corresponding to less electron–electron
repulsions between ligands and the metal).
The transference of electron density from the metal to carbon will result in the
n
formation of a strongly polar covalent bond, between carbide ions (C x ) and