Page 206 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

Figure 3.23. Continued

the host metal. The latter carbides are characteristic of Cr, Mn, and Fe, which have
five or more d electrons and a correspondingly weaker interaction with ligands such
as carbon. Hence, such complex carbides (e.g., Figure 3.23a–d) will have lower
melting points and hardness than analogous interstitial carbides of the 5d and/or
early transition metals (e.g.,W 2 C used as cutting blades).
It is interesting to note that alloying metals that are present in steel will cause stark
changes in TTT diagrams (Figure 3.22b). In particular, the nose of the austenite/
pearlite transformation will be compressed, allowing for pearlite (and martensite)
formation at slower cooling rates. In addition, a second bainitic nose will appear in
the TTT diagram. For a given steel with constant %C, as the concentration of
metallic dopants increase, there will be a decrease in the temperature required for
the onset of martensite formation, M s (Eq. 17):

M s ð CÞ¼ 539 423ð%CÞ 30:4ð%MnÞ 17:7ð%NiÞ
ð17Þ
12:1ð%CrÞ 7:5ð%MoÞ
The greatest effects are seen for the austenite-forming elements of C, Mn, and Ni
where even small concentrations result in a sharp decrease in M s . Whereas pure g-iron

may be converted to martensite at temperatures in excess of 500 C, hypereutectoid
steel is not transformed to martensite until a temperature of ca.160 C is reached during

quenching. For steels with carbon concentrations above 0.7% and/or high dopant