The first air-hardening grade tool steel was mushet steel, which was known as air-hardening steel at the time.
Modern air-hardening steels are characterized by low distortion during heat treatment because of their high-chromium content. Their machinability is good and they have a balance of wear resistance and toughness (i.e. between the D- and shock-resistant grades).
This series includes an O1 type, an O2 type, an O6 type and an O7 type. All steels in this group are typically hardened at 800 degrees celsius, oil quenched, then tempered atGrades
steel gets its name from its defining property of having to be water
quenched. W-grade steel is essentially high carbon plain-carbon steel.
This group of tool steel is the most commonly used tool steel because
of its low cost compared to others. They work well for small parts and
applications where high temperatures are not encountered; above 150 degrees Celsius
(302 degrees Fahrenheit) it begins to soften to a noticeable degree. Its hardenability is
low, so W-group tool steels must be subjected to a rapid quenching,
requiring the use of water. These steels can attain high hardness (above
HRC 66) and are rather brittle compared to other tool steels.