Lakhasly

3.1 Schematic representations of the microstructures for an iron- carbon alloy of hypo eutectoid composition C0 (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature

are determined by constructing a tie line at the temperature Te; the phase will contain 0.022 wt% C, whereas the phase will be of the eutectoid composition, 0.76 wt% C. As the temperature is lowered just below the eutectoid, to point f, all of the phase that was present at temperature Te (and having the eutectoid composition) will transform into pearlite, according to the reaction in Equation g10.76 wt% C2 ?The microstructure at point f will appear as the corresponding schematic inset of Figure 3.1.There will be virtually no change in the phase that existed at point e in crossing the eutectoid temperature--it will normally be present as a continuous matrix phase surrounding the isolated pearlite colonies.cooling heating a10.022 wt% C2 Fe3C 16.70 wt% C2 .

3.1 Schematic representations of the
microstructures for an iron– carbon alloy of hypo eutectoid composition
C0 (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature

are determined by constructing a tie line at the temperature Te; the phase will contain 0.022 wt% C, whereas the phase will be of the eutectoid composition, 0.76 wt% C. As the temperature is lowered just below the eutectoid, to point f, all of the phase that was present at temperature Te (and having the eutectoid composition) will transform into pearlite, according to the reaction in Equation g10.76 wt% C2 Δ cooling heating a10.022 wt% C2 Fe3C 16.70 wt% C2
. There will be virtually no change in the phase that existed at point e in crossing the eutectoid temperature—it will normally be present as a continuous matrix phase surrounding the isolated pearlite colonies. The microstructure at point f will appear as the corresponding schematic inset of Figure 3.1. Thus the ferrite phase will be present both in the pearlite and as the phase that formed while cooling through the phase region. The ferrite that is present in the pearlite is called eutectoid ferrite, whereas the other, that formed above Te, is termed proeutectoid (meaning “pre- or before eutectoid”) ferrite, as labeled in Figure 3.1. Figure 3.2 is a photomicrograph of a 0.38-wt% C steel; large, white regions correspond to the proeutectoid ferrite. For pearlite, the spacing between the and Fe3C layers varies from grain to grain; some of the pearlite appears dark because the many close-spaced layers are unresolved at the magnification of the photomicrograph. Note that two microconstituents are present in this micrograph—proeutectoid ferrite and pearlite which will appear in all hypoeutectoid iron–carbon alloys that are slowly cooled to a temperature below the eutectoid. We use the lever rule in conjunction with a tie line that extends from the ( Fe3C) phase boundary (0.022 wt% C) to the eutectoid composition (0.76 wt% C) inasmuch as pearlite is the transformation product of austenite having this composition. For example, let us consider an alloy of composition C0 in Figure 10.35. The fraction of pearlite, Wp, may be determined according to