Question 2 of 7: Schematic Microstructures of SAE 1040 and SAE 1090 Steel at Prescribed Holding Temperatures

Question II: Schematic Microstructures of SAE 1040 and SAE 1090 Steel at Prescribed Holding Temperatures (10 marks)

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 A_cm (for 1090); the standard linear approximations anchored at the diagram's own invariant points give A₃(0.40 wt%C) ≈ 816 °C and A_cm(0.90 wt%C) ≈ 768 °C.

[Figure not reproduced: Fig. 2.1 — 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.]

2.1 — (i)(a) SAE 1040 held at 1000 °C

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.

2.2 — (i)(b) SAE 1040 held at 730 °C

730 °C lies between A₁ (727 °C) and A₃ (≈816 °C), only 3 °C above the eutectoid — well inside the intercritical (α+γ) two-phase field but very close to its lower boundary. 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 austenite that has been carbon-enriched almost to the eutectoid composition (≈0.76 wt%C at 730 °C, versus ≈0.67 wt%C at 750 °C). Applying the lever rule at 730 °C (tie line between the ferrite solvus, ≈0.02 wt%C, and that ≈0.76 wt%C austenite composition) gives an equilibrium ferrite fraction of about 48–49% — noticeably MORE ferrite than the ≈42% that the same lever rule gives at 750 °C, and already close to the final room-temperature ferrite fraction reached in part (c), precisely because 730 °C sits so near A₁. A long hold at this FIXED intercritical temperature reaches (and holds at) that equilibrium fraction; it does not run to completion the way a continuous slow cool would.

2.3 — (i)(c) SAE 1040 slow-cooled from 730 °C to 20 °C

Because 730 °C is only 3 °C above A₁, the proeutectoid ferrite network established in part (b) has almost no further growing to do before the temperature crosses A₁ (727 °C): the still-austenite regions, already carbon-enriched to ≈0.76 wt%C at 730 °C, need only the last small step to reach the exact eutectoid composition (0.77 wt%C), and transform to pearlite (alternating ferrite/cementite lamellae) essentially as soon as A₁ is crossed. 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) — about 49% ferrite, 51% pearlite by area, essentially the same fraction already reached at 730 °C in part (b) since so little further transformation was needed to get there.

2.4 — (ii)(a) SAE 1090 held at 730 °C

730 °C lies between A₁ (727 °C) and A_cm (≈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.

2.5 — (ii)(b) SAE 1090 slow-cooled from 730 °C to 20 °C

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).