Discussion
Preheating is essential in welding for......
Answer: C
Preheating is essential when welding cast iron.
Cast iron is brittle and has a high carbon content. If the weld area heats and cools quickly, the uneven contraction cracks it, and the rapid cooling forms hard, brittle white iron and martensite near the weld. Preheating the casting (often to a few hundred degrees) and cooling it slowly reduces these thermal stresses.
Some tool steels such as high speed steel are also preheated, but cast iron is the standard case where preheating is essential. Stainless steel and German silver (a copper-nickel-zinc alloy) are normally joined without it.
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