But the invaluable and rather delicate art of tempering the hardened steel by a very careful and gentle reheating, which removes its extreme brittleness though leaving most of ifs precious hardness, needs such skilful handling that it can hardly have become known until very long after the art of hot-forging.
This brittleness has therefore in general to be mitigated or " tempered," unfortunately at the cost of losing part of the hardness proper, by reheating the hardened steel slightly,
That to which the hardened steel is thus reheated, the more is the molecular rigidity relaxed, the farther on does the transformation go, and the softer does the steel become; so that, if the reheating reaches a dullred heat, the transformation from austenite into ferrite and cementite completes itself slowly, and when now cooled the steel is as soft and ductile as if it had never been hardened.
Hence steel which has been heated very highly, whether for welding, or for greatly softening it so that it can be rolled to the desired shape with but little expenditure of power, ought later to be refined, either by reheating it from below Are to slightly above Ac 3 or by rolling it after it has cooled to a relatively low temperature, i.e.
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