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What is differential tempering?

What is “differential tempering”?

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A planing chisel being subjected to differential heat treatment, going left to right through yellow, red and blue Differential tempering is the process applied to some chisels (such as cold chisels) whereby different parts of the chisel are tempered to different temperatures.
A chisel subjected to differential heat treatment; (from left to right) yellow, red/orange, blue This means that the balance between the chisel’s hardness and toughness is not the same across the tool.
A cold chisel cutting through metal which has been secured in a vice In the case of cold chisels, this allows the cutting edge to be hard, so that it can cut through softer metals…
A mushroomed chisel head …whilst allowing the body to be softer to minimise the effects of mushrooming.

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