Question 15
Contrast the suitability of mild and high carbon steel for applications in the manufacture of automotive subframes that experience repeated loads and high impacts, using their microstructure and three relevant mechanical properties.
High-carbon steel is stronger, but mild steel absorbs more energy before it breaks
Two subframe members, one of each steel, take the same repeated loads and then the same impact. Watch what each one does, then watch the area under each stress and strain curve.
The extra cementite in high-carbon steel raises its yield strength but takes away its ductility, so it breaks before it can absorb much energy. A subframe has to survive repeated loads and impacts, which is a job for toughness.
Build the contrast and see which of the six marks it earns
Pick the sentences you would write. Every mark in QCAA’s guide is a comparison with “than” in it, and the last mark is for the decision with the right reason. Three of the sentences look useful and earn nothing.
Work through the solution one mark at a time
Two marks for the microstructure, three for the properties and one for the decision. Every one is a contrast. Try each step yourself before you reveal it.
Contrast the ferrite. Mild steel, about 0.15% to 0.30% carbon, is mostly ferrite. High-carbon steel is mostly pearlite.
QCAA’s sample: “The composition of mild carbon steel includes a higher amount of ferrite, providing ductility.”
Marker: appropriately contrasts mild and high carbon steel and includes wording that indicates the microstructure of mild carbon steel is higher in ferrite than high carbon steel
Contrast the pearlite, as a separate statement. It is a second mark even though it follows from the first.
QCAA’s sample: “The lower amount of pearlite in the composition still provides a high yield strength.”
The guide also accepts the inverse contrast, such as high-carbon steel having more cementite compared to ferrite. Keep QCAA’s comparison and leave out its clause that mild steel’s pearlite “still provides a high yield strength”. It earns no mark, and beside the next mark, which says high-carbon steel’s yield strength is higher, it blurs the contrast.
Marker: the microstructure of mild carbon steel is lower in pearlite than high carbon steel
Give the property that favours high-carbon steel. A contrast is honest about both sides, and this one still earns a mark.
QCAA’s sample: “High carbon steel includes a high amount of cementite compared to ferrite (in pearlite), contributing to a higher yield strength than mild carbon steel.”
Marker: high carbon steel has a higher yield strength than mild carbon steel
QCAA’s sample: “The amount of cementite in the composition provides a less ductile material than mild carbon steel.”
The cementite is the cause. Saying so turns a bare comparison into one the marker can see you understand.
Marker: high carbon steel is less ductile than mild carbon steel
The third property is the one the scenario points at. “Repeated loads and high impacts” is a description of toughness.
QCAA’s sample: “The very high yield strength and the lower ductility of high carbon steel produces lower toughness, which makes the material susceptible to fatigue cracking and less able to absorb the energy of repeated loading and collision impacts.”
Marker: mild carbon steel has better toughness than high carbon steel
Choose, in its own sentence, and give toughness as the reason. The guide’s note is strict: “For marks identifying the more suitable steel to be awarded, the response must include toughness to resist or absorb repeated loads and high impacts or similar wording.” Mild steel because it is cheaper, or easier to weld, earns nothing here.
QCAA’s sample: “The relatively high yield strength and good ductility of mild carbon steel provides high toughness that allows the frame to absorb the repeated loads and impacts. Therefore, mild carbon steel is more suitable than high carbon steel for use in the manufacture of automotive subframes.”
Marker: mild carbon steel is a better material for automotive subframes, because of its higher toughness and ability to resist or absorb the repeated loads and impacts
Each phrase of the question gave you something
The 2022 subject report says the effective responses contrasted the two microstructures, contrasted three relevant properties in light of those microstructures, and justified the choice of material for the stated industrial context. The high-level excerpts it chose argue for mild steel.
You have to hold two steels side by side, pick three properties yourself and read the scenario to find the one that decides the job. The 2022 subject report calls this question complex familiar. The same shape came back as 2025 Question 12, mild against high carbon steel for an industrial knife, so expect it again in 2026, written with low-carbon steel.
If I had to guess where you might lose marks, it would be writing two separate descriptions, mild steel in one paragraph and high-carbon steel in the next, with no “than” anywhere. Every fact can be right and the contrast marks still fall away. The other trap is the decision. Choosing mild steel is not enough on its own. The mark only comes when toughness against the repeated loads and impacts is the reason.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording, any figures and marking-guide steps 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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