Problem $4$: TTT Diagrams $($Callister$6$th $11\mathrm{.}19)$

Using the isothermal transformation diagram for an iron$-$carbon alloy of eutectoid

composition $($Figure $11\mathrm{.}23),$ specify the nature of the final microstructure $($in terms

of microconstituents present and approximate percentages of each$)$ of a small

specimen that has been subjected to the following time$-$temperature treatments. In

each case assume that the specimen begins at $760C(1400F)$ and that it has been

held at this temperature long enough to have achieved a complete and

homogeneous austenitic structure.

$($a$)$ Cool rapidly to $700C(1290F),$ hold for ${10}^{4}s,$ then quench to room

temperature.

$($b$)$ Reheat the specimen in part $($a$)$ to $700C(1290F)$ for $20$ h $.$

$($c$)$ Rapidly cool to $600C(1110F),$ hold for $4$ s $,$ rapidly cool to $450C(840F),$

hold for $10$ s $,$ then quench to room temperature.

$($d$)$ Cool rapidly to $400C(750F),$ hold for $2$ s $,$ then quench to room

temperature.

$($e$)$ Cool rapidly to $400C(750F),$ hold for $20$ s $,$ then quench to room

temperature.

$($f$)$ Cool rapidly to $400C(750F),$ hold for $200$ s $,$ then quench to room

temperature.

$($g$)$ Rapidly cool to $575C(1065F),$ hold for $20$ s $,$ rapidly cool to $350C(660F),$

hold for $100$ s $,$ then quench to room temperature.

Figure $11\mathrm{.}23$: The complete isothermal transformation diagram for an iron$-$

carbon alloy of eutectoid composition: A$,$ austenite $(-Fe$; B$,$ Bainite $(-Fe+$

$\{$:$F{e}_{3}C)$; M$,$ martensite $)$; P $,$ pearlite $(-Fe+F{e}_{3}C).$