Question

Q2 What are the phases present at room temperature for steel of 0.76% C? Determine the composition & amounts of ferrite (a) and cementite (Fe3C) at room temperature in pearlite of eutectoid composition (0.76%C). Use attached Fe-Fe3C phase diagram or Fig. 9.24 in your text book.

Answer 1

Steels are alloys of iron (Fe) and Carbon (C). Steel exist as various phases and phase mixtures with equillibrium each other. Pearlite has 0.76% of carbon content and consist of alternate layers of ferrite and cementite.Therfore, It exhibits properties between ferrite and cementite. Generally, carbon occupies interstetial position in the iron and it forms solid solution with alpha, gama and delta phases with iron. There are three imporatant reactions occur in the Fe-C system. They are 1. Eutectic reaction 2. Eutectiod reaction 3. Peritectic reaction.

Maximum solubilty of alpha (α) ferrite in the BCC is maximum 0.025% at 727oC and 0.008% at room temperature. Iron carbide/ Cementite (Fe3C) intermediate compound of Fe and Carbon and has 6.7% of carbon.

Generally Eutectic and Eutectoid reactions are present in the Fe-Fe3C systems

Eutectic:  4.30  wt% C, 1147oC.

Eutectoid: 0.76 wt% C, 727oC.

γ(0.76 wt% C) ↔ α (0.022 wt% C) + Fe3C.