The steel largely recognized as being the “first tool steel” was developed by Robert Forester Mushet, a British metallurgist, in 1868 [1]. Mushet improved the Bessemer steelmaking process through the addition of a small amount of manganese [1]. Later Mushet was experimenting with various additions of elements and discovered that one of his bars of steel had become fully hard despite not being quenched. This was called a “self-hardening” and later “air-hardening” steel because it could be fully hardened in air rather than requiring a water or even oil quench. The reason for the ability to self-harden is due to the property of hardenability, which I have covered in a Bladeforums post [2]. Hardenability is essentially the property of how slowly a steel can be cooled from the hardening temperature while still achieving a hard martensitic microstructure rather than a soft ferrite-cementite microstructure. This steel was high in tungsten and manganese, and it is sometimes erroneously reported that it was the tungsten that gave it the high hardenability; however, it was primarily the manganese that gave it the ability to harden in air, as tungsten adds little to hardenability [3].
