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| 1.2 Hardenability |
Hardenability is the ability for a material to harden; it refers not to the
highest value of hardness that can be obtained but to the capacity (depth or
thickness over which such high hardness values can be achieved) to harden.
Thus, hardenability is intimately related to the cooling rate that can be
achieved (especially in steels). Typically, hardenability is tested using hardness penetration diagram test (in which, the hardness of a hardened sample
is plotted as a function of depth from the surface), and Jominy end quench test (in which one end of a sample is quenched and the hardness at equal
intervals from the quenched end is measured and plotted). |
| 1.2.1 Effect of alloying elements on hardenability |
In steels, alloying elements are added to increase hardenability; this is achieved
by delaying the time required to transform austenite into ferrite and pearlite
(and thus shifting the nose of the C curve in the TTT diagram to longer
times), which helps in the formation of martensites even with slower cooling
rates.
The alloying elements delay the decomposition of austenite into ferrite and
parlite by reducing either the nucleation or the growth.
The alloying elements in steel are distinguished into two types: namely,
austenite stabilizers (Mn, Ni, Cu) and ferrite stabilizers (Cr, Mo, Si). Even
though both these additions help hardenability, the mechanisms involved are
different. |
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