High Speed Tool Steel Products: M2 - M4 - M42 - T1 - T15 ..
The main reason cobalt is added in some grades of high speed steel is to increase the hot hardness which increases the cutting efficiency of tools when high temperatures are reached.. There are three types of High Speed Steels: Tungsten-type grades (i..e.. T1), Molybdenum-type grades (i..e.. M2, M4), and Cobalt-type grades (i..e.. T15, M42)..