Question 9
Steel containing 0.11% carbon is
That is it. Step through the hints below to see why. In 2023, 79.8% of Queensland students chose (A).
Not quite. Mild carbon steel was the 2019 syllabus band from 0.15% to 0.30%, so 0.11% sits below it even on the old bands. Mild steel is the everyday name for low carbon steel, which is why it tempts you. In 2023, 12.9% of Queensland students chose (B).
Not quite. Medium carbon steel starts at 0.30%, almost three times this carbon content. Placing 0.11% in the middle band treats it as a middling amount instead of checking it against the band edges. In 2023, 4.2% of Queensland students chose (C).
Not quite. High carbon steel starts at 0.60%. If you read 0.11% as 1.1%, you would land here, so check the decimal point. In 2023, 2.9% of Queensland students chose (D).
The 2025 syllabus gives three plain-carbon bands, and it gives them as numbers you have to know. The formula book has none of them.
Put the number against the edges before you read the options.
0.11% is above the bottom edge of the low band and well under the top edge.
$$0.07\%\le0.11\%<0.30\%$$
Medium starts at 0.30% and high at 0.60%, so (C) and (D) are out.
This paper was set under the 2019 syllabus, which split the bottom of the range into low carbon, 0.07% to 0.15%, and mild carbon, 0.15% to 0.30%. Turn on the 2019 bands and 0.11% is still low carbon.
The 2025 syllabus has no mild band. Mild steel now just means low carbon steel. On either syllabus, (B) is wrong.
Check it against the microstructure. Below 0.83% the steel is hypoeutectoid, and the lever rule gives the pearlite. Ferrite is almost pure iron, so take its end of the tie line as 0% carbon.
$$\frac{0.11}{0.83}\approx13\%$$
About 87% ferrite. Mostly soft, ductile ferrite is the low carbon picture.
$$0.07\%\le0.11\%<0.30\%$$
0.11% sits inside the low carbon band on both the 2019 and the 2025 syllabus. Its microstructure is about 87% ferrite and 13% pearlite.
The most common wrong answer was (B), from mild steel being the everyday name for low carbon steel. For 2026 the mild band is gone. If a question says mild steel, answer it as low carbon steel, 0.07% to 0.30%.
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 2023 QCAA Engineering external assessment, © State of Queensland (QCAA) 2023, 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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