Carbon steels only contain trace amounts of elements besides carbon and iron. This group is the most common, amounting up to 90% of steel production systems due to the versatile uses of iron-based structural alloys. One example is the Akashi Kaikyo Bridge, completed in 1998, which connects Kobe with Awaji Island. It is the world's longest suspension bridge, with a span between the towers of 1.9 km.

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 (i) Cite one remarkable engineering design or building (except Akashi Kaikyo Bridge) that used plain carbon steel as a structural part. Explain why the engineer choose plain carbon steel instead of alloy steel? Justify your answer with suitable references regarding the advantage of plain carbon steel mechanical properties and other suitable properties related to your chosen design/building.

(ii) Using the Fe-C phase diagram, explain how the phase diagram is used to produce various types of cast iron. Discuss why and how the microstructures of cast iron are changed and modified the mechanical properties of cast iron.
 

Composition (at% Be) 10 15 20 Liquid 1000 a +L 866C 800 a +Y1 -620C 600 a + Y2 400 1 (Cu) Composition (wt% Be) Figure 1 Cu - Be Phase Diagram Temperature (C)