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Yes, we love our acetylene torch, but this is more than just a gratuitous fire picture. Excessive hammering, bending, and pulling actually hardens metal, causing it to become brittle. Just like a wire coat hanger will eventually break if you bend it in the same spot over and over, the same can happen with automotive sheetmetal. Heating steel up to a dull red color softens it and makes it more compliant, allowing you to shape it more easily and without cracking or breaking it.