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Materials selection and design

When steel scrap is remelted in an electric arc furnace for recycling, which elements tend to accumulate in the steel with repeated recycling and cause problems by degrading quality (tramp elements)?

  1. ACopper and tin. They oxidize less readily than iron, so refining cannot remove them, and they cause surface cracking in hot working (hot shortness)
  2. BCarbon. Even oxygen-blowing refining cannot remove it, so repeated recycling always turns the steel into a high-carbon steel
  3. CAluminum. It oxidizes less readily than iron, so it does not pass into the slag during refining and stays in the steel
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Answer: A. Copper and tin. They oxidize less readily than iron, so refining cannot remove them, and they cause surface cracking in hot working (hot shortness)

In steel refining, oxygen is blown in to oxidize elements that oxidize more readily than iron, and they are removed as slag. Copper and tin oxidize less readily than iron, so this method cannot remove them; when copper wire from motors and the like gets mixed into scrap, they become more concentrated in the steel with every recycling. During reheating before hot rolling, iron is oxidized preferentially, so copper becomes enriched at the interface with the scale, and above the melting point of copper (about 1085 °C) it becomes liquid, penetrates the grain boundaries and causes cracking. Tin aggravates this. Carbon can be removed as carbon monoxide by oxygen blowing, so the carbon content can be adjusted after recycling. Aluminum oxidizes far more readily than iron and is in fact used as a deoxidizer; it goes into the slag during refining. Tramp elements are controlled by sorting scrap and by dilution with cleaner iron sources.

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