Mushet steel, also known as Robert Mushet's Special Steel (RMS), self-hardening steel and air-hardening steel, is considered the first tool steel and air-hardening steel. It was invented in 1868 by Robert Forester Mushet. Prior to Mushet steel, steel had to be quenched to harden it. It later led to the discovery of high speed steel.
The chemical composition of Mushet steel varied; tungsten was the main alloying constituent, which ranged between 4 and 12%, while manganese (2–4%) and carbon (1.5–2.5%) were the secondary alloying constituents. A typical sample contained 9% tungsten, 2.5% manganese, and 1.85% carbon.
Mushet steel was primarily used in machine tools due to its ability to retain its hardness at high temperatures. In 1894, Frederick Winslow Taylor conducted machining comparison tests between Mushet steel and high carbon tool steel. He found that it could cut 41 to 47% faster on hard tire steel forgings and approximately 90% faster on mild steels. He also found that if a stream of water was used as a cutting fluid the cutting speed could be increased 30%. After Taylor's tests results were published Mushet and other self-hardening steels became popular in machine tools. Prior to Taylor's tests Mushet steel was often just used to increase the time between regrinds, take larger cuts, or machine harder materials.
In 1899 and 1900, Taylor and Maunsel White were experimenting with hardening processes for Mushet steel and other self-hardening steels. They discovered if the steel is heated to near its melting point it creates a more durable metal. The metal will retain its hardness up to a red heat. This type of hardened self-hardening steel was the first high speed steel.
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