Differential tempering is the process applied to some chisels (such as cold chisels) whereby different parts of the chisel are tempered to different temperatures.
This means that the balance between the chisel’s hardness and toughness is not the same across the tool.
In the case of cold chisels, this allows the cutting edge to be hard, so that it can cut through softer metals…
…whilst allowing the body to be softer to minimise the effects of mushrooming.
