The hardening of carbon steel tools
High carbon steels contain between 0.8% and 1.3% of carbon, and tools made of these steels can be hardened. If a tool like a chisel is to be hardened, only the working part which is the edge of the chisel, is hardened. The chisel is first heated to a temperature of about 820°C, until it becomes cherry-red in colour. It is then quenched, by plunging it into cold water or oil. The edge of the chisel will now be very hard, but also very brittle. It is necessary to remove some of this brittleness, or else the chisel will break when in normal use. The process of removing some of this brittleness is called tempering.
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