Question 6

[1 mark] Simple familiar

Topic guidePremiumTopic guide 10 · The steel part of the iron–carbon diagram

Game planPremium4.3 Naming the steel and its structure

The question

A section of an iron–carbon phase diagram is shown.

An iron-carbon phase diagram with temperature from 400 to 1600 degrees Celsius on the vertical axis and percent carbon from 0 to 5 on the horizontal axis. A horizontal line runs across at about 723 degrees and another at about 1147 degrees. A straight line falls from 910 degrees at 0 percent carbon to the eutectoid point at about 0.8 percent carbon on the 723 degree line, and a curved line rises from that point to about 2.1 percent carbon on the 1147 degree line. The letter X sits inside the large region enclosed by these lines and the curve above, at about 0.7 percent carbon and 1100 degrees.

The microstructure of the phase represented by X is solid

Not quite. Cementite is iron carbide, 6.67% carbon, off the right-hand edge of this diagram. Above the 723 °C line it only appears to the right of the curved line, and X is well to the left of it. In 2022, 9.6% of Queensland students chose (A).

That is it. Step through the hints below to see why. In 2022, 69.1% of Queensland students chose (B).

Not quite. Pearlite only forms below the 723 °C line, when austenite breaks down at the eutectoid. X sits at about 1100 °C, far above that line, so there is no pearlite yet. In 2022, 8.4% of Queensland students chose (C).

Not quite. Ferrite is the low-temperature form of iron. In the steel part of the diagram it lives in the thin strip at the left edge, in the small ferrite-plus-austenite triangle just above the 723 °C line, and below that line. X is in none of these. In 2022, 12.5% of Queensland students chose (D).

The QCAA diagram, redrawn with its curves traced off the paper. QCAA put the eutectoid at about 0.78% C, a little left of the 0.83% the topic guides use, so the lever rule here uses 0.78%. The field the point is in is shaded when it is austenite.

Find the landmarks before you look at X. The QCAA diagram has no field labels, so you supply them.

The lower horizontal line is the eutectoid temperature, about 723 °C.

The V on that line is the eutectoid point, 0.83% C in the topic guides. QCAA’s drawing puts it nearer 0.8%.

The upper horizontal line is about 1147 °C, and its left end, about 2.1% C, is where steel stops.

X is at roughly 0.7% carbon and 1100 °C. That is above the 723 °C line, right of the line falling from 910 °C, and left of the curved line rising to 2.1% C.

Those lines fence in one big single-phase field. Above the eutectoid the iron is in its high-temperature form, with all of the carbon dissolved in it. That phase is austenite, (B).

Each wrong option lives somewhere else on the diagram. Move the point to find it.

Pearlite: only below 723 °C.

Ferrite: the thin left strip, the ferrite-plus-austenite triangle just above 723 °C, and below 723 °C.

Cementite: 6.67% C, off the right edge, and the field right of the curved line.

QCAA printed the diagram to 5% carbon. Everything right of about 2.1% C is cast iron, which the 2025 syllabus dropped, so a 2026 diagram question will sit in the steel part.

The steel part is unchanged, and the 2025 syllabus now names hypoeutectoid and hypereutectoid steel, so expect to place a steel either side of the V.

X lies above the 723 °C eutectoid line, inside the single-phase field bounded by the line falling from 910 °C, the curved line rising to about 2.1% C at 1147 °C, and the curve above. That field is austenite, the high-temperature form of iron with the carbon dissolved in it.

(B)

How Queensland students answered in 2022

(A)

9.6%

(B)

69.1%

(C)

8.4%

(D)

12.5%

The most common wrong answer was (D), from treating the left side of the diagram as ferrite. Ferrite only exists in the thin strip at the left edge, in the small triangle just above the 723 °C line, and below that line. A single field that big, above the eutectoid, is always austenite.

Only mark this done when you could do it without help. Reading the solution does not count.

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Question wording and any figures are from the 2022 QCAA Engineering external assessment, © State of Queensland (QCAA) 2022, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.

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