Question 2
A microstructure diagram for steel at room temperature is shown.
What is the composition in the circled area X?
Not quite. The white layers are ferrite, but the circle takes in several layers, white and grey together. A layered mixture of ferrite and cementite is pearlite. In 2025, 12.0% of Queensland students chose (A).
That is it. Step through the hints below to see why. In 2025, 71.0% of Queensland students chose (B).
Not quite. The stem says room temperature. In a plain-carbon steel austenite only exists above 723 °C, so it cannot be in this diagram. In 2025, 7.4% of Queensland students chose (C).
Not quite. The grey layers are cementite, but the circle covers grey and white layers together. Name the whole structure, not one of its layers. In 2025, 9.2% of Queensland students chose (D).
Start with the words “at room temperature”. A plain-carbon steel only holds austenite above 723 °C, so (C) is out before you look at the picture.
That leaves ferrite, cementite and pearlite, the three things a slow-cooled steel can contain.
Look at what the circle holds. It is not one band. It takes in several grey and white layers lying side by side.
One white layer is ferrite and one grey layer is cementite. Mark each on the left. Alternating layers of the two, together, are pearlite. That is (B).
QCAA drew the layers about equal in width. The lever rule at the eutectoid says the cementite is much thinner.
$$\frac{0.83}{6.67}\approx12\%\text{ cementite}$$
So a real ferrite layer is about seven times as thick as the cementite beside it. A diagram is allowed to exaggerate, and the answer does not change.
The circle at X holds several alternating layers, white and grey. White is ferrite and grey is cementite, and a layered mixture of the two is pearlite. Austenite cannot be present at room temperature.
The most common wrong answer was (A), from naming the white background. The white is ferrite, but the circle takes in the grey cementite layers too, and the mixture is pearlite.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2025 QCAA Engineering external assessment, © State of Queensland (QCAA) 2025, 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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