7. The mass fraction of eutectoid ferrite in an iron-carbon alloy is 0.82. On th

Question

7. The mass fraction of eutectoid ferrite in an iron-carbon alloy is 0.82. On the basis of this information, is it possible to determine the composition of the alloy? If so what is its composition? If this is not possible, explain why. 8, A steel alloy contains 97.5 wt% Fe, 2.0 wt% Mo, and 0.5 wt% C. (a) What is the eutectoid temperature of this alloy? (b) What is the eutectoid composition? (c) What is the proeutectoid phase? Assume that there are no changes in the positions of other phase boundaries with the addition of Mo ates)

Explanation / Answer

7. Yes, it is possible to determine the composition of the alloy. This is done in following way:

Eutectoid reaction: ?(0.76 wt. % C) <=> ?(0.022 wt. % C) + Fe3C(7.6 wt. % C)

Mass fraction of eutectoid ferrite + Mass fraction of proeutectoid ferrite = Mass fraction of ferrite

Weutectoid ? + Wproutectoid ? = W?

=> 0.82 + (C? – C0)/ (C? – C?) = (CFe3C – C0)/ (CFe3C – C?)

=> 0.82 + (0.76 – C0)/ (0.75 – 0.022) = (6.7 – C0)/ (6.7 – 0.022)

=> 0.82 + (0.76 – C0)/ (0.728) = (6.7 – C0)/ (6.678)

=> C0 = 0.703

So, carbon % = 0.703% and Fe% = 99.297%.

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