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Why is steel heated above the critical temperature during normalizing?
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steel
Molten steel being poured into a ladle from an electric arc furnace, 1940s.
During the normalizing process, steel is heated to a temperature 20° to 40° C above the G-S-K line to hold the material within the gamma zone. This step is essential because it transforms the existing as-rolled or as-cast microstructure into austenite, which effectively dissolves carbides.1
By reaching this state, the subsequent cooling in still air allows for the formation of a very uniform grain structure, consisting of either pearlite and ferrite or pearlite and cementite, depending on the carbon content of the steel.1