Question 10
The paper prints a black-and-white micrograph of a carbon steel here. It shows dark, mottled grains, each one outlined by a continuous white network along the grain boundaries. A large white arrow points at a stretch of that white network.
The micrograph is by Dr R F Cochrane, University of Leeds, and the QCAA used it with permission. It is third-party material, so it is described here instead of reproduced.
What is the hypereutectoid formation indicated by the arrow in the carbon steel microstructure?
That is it. Step through the hints below to see why. In 2021, 42.8% of Queensland students chose (A).
Not quite. A plain-carbon steel has no austenite at room temperature. The network traces the old austenite grain boundaries, because that is where the cementite formed, but the austenite itself turned to pearlite at 723 °C. In 2021, 14.4% of Queensland students chose (B).
Not quite. Pearlite is the dark, mottled grains inside the network. The arrow touches the white boundary between them. In 2021, 19.7% of Queensland students chose (C).
Not quite. In a hypoeutectoid steel a white grain-boundary network would be ferrite, and it looks the same. This steel is hypereutectoid, above 0.83% carbon, so the extra constituent is cementite. In 2021, 22.9% of Queensland students chose (D).
The stem does half the work. Hypereutectoid means more carbon than the eutectoid, 0.83%.
Slow cooled, a hypereutectoid steel is pearlite plus extra cementite. So the arrowed formation is one of those two, which leaves (A) or (C).
Follow the steel as it cools. Above the curved line on the iron-carbon diagram it is all austenite. Crossing that line, the austenite cannot hold all its carbon, so cementite comes out along the austenite grain boundaries.
At 723 °C the rest of the austenite turns to pearlite. That leaves pearlite grains wrapped in a cementite network, which is exactly the picture. The arrow is on the network, so it is cementite.
Slide the carbon below 0.83% and the network does not go away. It turns into ferrite, which comes out first in a hypoeutectoid steel, and in a micrograph it looks just as white.
So the picture alone cannot tell you cementite from ferrite. The word hypereutectoid can, and that is why it is in the stem.
The lever rule sizes the network. For a 1.4% carbon steel:
$$\frac{1.4-0.83}{6.67-0.83}\approx10\%\text{ cementite}$$
A thin network, but continuous, so a crack can run along it. That is why hypereutectoid steel is hard but brittle when slow cooled.
A hypereutectoid steel has more than 0.83% carbon. As it cools, the extra carbon comes out of the austenite as cementite along the austenite grain boundaries, before the rest turns to pearlite at 723 °C. The white network the arrow points at is that cementite, and the dark grains it surrounds are pearlite.
The most common wrong answer was (D). In a hypoeutectoid steel the grain-boundary network is ferrite and looks just as white, so the picture alone cannot settle it. The word hypereutectoid in the stem does.
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 2021 QCAA Engineering external assessment, © State of Queensland (QCAA) 2021, 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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