Hypoeutectoid alloys

As we have seen, the alloys studied contain from 88 % to 92 % of copper, and in that range consist of the solid solution a plus the eutectoid (a+y). At 88 % of copper, the alloy consists of almost pure eutectoid, below that value the y constituent makes its appearance. We have, therefore, only to consider the hypoeutectoid alloys, and to study the solution a and the eutectoid (a+y). 

Mass fractions of a proeutectoid phase (ferrite or cementite) and pearlite may be computed using the lever rule and a tie line that extends to the eutectoid composition (0.76 wt% C) [e.g., Equations 9.20 and 9.21 (for hypoeutectoid alloys) and Equations 9.22 and 9.23 (for hypereutectoid alloys)]. 

Ci Composition of a hypoeutectoid alloy (in weight percent carbon)... 

Figures 11.2-11.6 show how the room temperature microstructure of carbon steels depends on the carbon content. The limiting case of pure iron (Fig. 11.2) is straightforward when yiron cools below 914°C a grains nucleate at y grain boundaries and the microstructure transforms to a. If we cool a steel of eutectoid composition (0.80 wt% C) below 723°C pearlite nodules nucleate at grain boundaries (Fig. 11.3) and the microstructure transforms to pearlite. If the steel contains less than 0.80% C (a hypoeutectoid steel) then the ystarts to transform as soon as the alloy enters the a+ yfield (Fig. 11.4). "Primary" a nucleates at y grain boundaries and grows as the steel is cooled from A3...

I99OG0I] Goldenstein, H., Aaronson, H.I., Overall Reaction Kinetics and Morphology of Austenite Decomposition Between the Upper Nose and the Ms of a Hypoeutectoid Fe-C-Cr Alloy , Metall Trans. A, 21A(6), 1465-1478 (1990) (Morphology, Phase Relations, Experimental, Kinetics, 65)... 

Bow] Bowman, F.E., Partition of Molybdenum in Hypoeutectoid Iron-Carbon-Molybdenum Alloys , Trans. Am. Soc. Met., 36, 61-80 (1946) (Experimental, Phase Relations, 10) [1950Kra] Krainer, H., (in German), X-Ray Study of Carbides in W-, Mo-, and V-Steels , Arch. 

Iroii alloys with caibon content of less than 2% are known as steels and those with more than 2% are known as cast iron. Steels are further divided into those with carbon content of more than 0.8%, called hypereutectoid steels, and those with caihon content of less than 0.8%, known as hypoeutectoid steels. Most steels used in pressure vessel applications have a carbon content of less than 0.4%. Steels with carbon content of over 0.4% are very brittle and hard to weld. 

Microstructures for iron-iron carbide alloys having other than the eutectoid composition are now explored these are analogous to the fourth case described in Section 9.12 and illustrated in Figure 9.16 for the eutectic system. Consider a composition Cq to the left of the eutectoid, between 0.022 and 0.76 wt% C this is termed a hypoeutectoid ( less than eutectoid ) alloy. Coohng an alloy of this composition is represented by moving down the vertical hne yy in Figure 9.29. At about 875°C, point c, the microstructure consists... 

Figure 9.29 Schematic representations of the microstructures for an iron-carbon alloy of hypoeutectoid composition Cq (containing less than 0.76 wt% C) as it is cooled from within the austenite phase region to below the eutectoid temperature.

Relative amounts of both pearlite and proeutectoid FcjC microconstituents may be computed for hypereutectoid steel alloys in a manner analogous to that for hypoeutectoid materials the appropriate tie line extends between 0.76 and 6.70... 

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