21-Mat-B6 Ceramic Materials · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
SAE 1040 (0.40 wt% C) is hypoeutectoid and SAE 1090 (0.90 wt% C) is hypereutectoid; the eutectoid point on the Fe–Fe3C diagram sits at 0.77 wt% C, 727 °C (A1). Which phase field each holding temperature falls in is fixed by comparing it against A1, the hypoeutectoid solvus A3 (for 1040) and the hypereutectoid solvus Acm (for 1090); the standard linear approximations anchored at the diagram's own invariant points give A3(0.40 wt%C) ≈ 816 °C and Acm(0.90 wt%C) ≈ 768 °C.
[Figure not reproduced: Fig. 1.1 — the five circles as printed on the exam, filled in schematically: austenite grains (tan), proeutectoid ferrite network (cream), pearlite lamellae (brown hatch), proeutectoid cementite network (pale line). See the official exam paper.]
1000 °C is well above A3 (≈816 °C) for this composition, so the steel sits entirely inside the single-phase austenite (γ) field. After a long hold, the microstructure is simply a set of equiaxed, polygonal γ grains with plain grain boundaries — no second phase, no substructure.
750 °C lies between A1 (727 °C) and A3 (≈816 °C), i.e. inside the intercritical (α+γ) two-phase field. Proeutectoid ferrite nucleates preferentially at the prior-austenite grain boundaries (the highest-energy, easiest-nucleation sites) and grows inward as a network, leaving the grain interiors as untransformed austenite. A long hold at a FIXED intercritical temperature reaches (and holds at) the equilibrium ferrite/austenite fraction set by the tie line at 750 °C — it does not run to completion the way a continuous slow cool would.
As the steel is cooled slowly on down through the intercritical range, the proeutectoid ferrite network keeps growing (the equilibrium ferrite fraction rises as temperature falls toward A1), while the shrinking pool of austenite is pushed toward the eutectoid composition. The instant the temperature crosses A1 (727 °C), that remaining eutectoid-composition austenite transforms to pearlite (alternating ferrite/cementite lamellae). The room-temperature microstructure is therefore the classic hypoeutectoid equilibrium structure: a continuous proeutectoid ferrite network enclosing pearlite colonies, with the ferrite:pearlite area ratio set by the lever rule at C0 = 0.40 wt%C between the ferrite solvus (≈0.02 wt%C) and the eutectoid composition (0.77 wt%C) — roughly half ferrite, half pearlite by area.
730 °C lies between A1 (727 °C) and Acm (≈768 °C) for this hypereutectoid composition — i.e. just barely inside the (γ+Fe3C) two-phase field, only 3 °C above the eutectoid. Proeutectoid cementite (not ferrite — the hypereutectoid mirror of part (b)) precipitates as a thin, continuous network along the prior-austenite grain boundaries, with the grain interiors remaining austenite. Because the hold temperature is so close to A1, the equilibrium cementite fraction at 730 °C is small and the network stays thin.
Continued slow cooling toward 727 °C thickens the proeutectoid cementite network only slightly further (730 °C to 727 °C is a very narrow interval), after which the remaining eutectoid-composition austenite transforms to pearlite exactly as in part (c). The room-temperature structure is the hypereutectoid mirror image of 1.3: a thin, continuous proeutectoid cementite network outlining the prior austenite grains, enclosing pearlite colonies — here pearlite dominates the area fraction (lever rule at 0.90 wt%C against the 0.77 wt%C eutectoid and the ≈6.7 wt%C cementite composition gives only a few percent proeutectoid cementite by weight).
