High-Speed Steel (HSS)
High-alloy tool steel containing tungsten, molybdenum, vanadium, and cobalt, designed to retain high hardness at cutting temperatures up to 600 °C.
HSS is a high-alloy tool steel that gets its hardness mainly from tungsten, molybdenum, and vanadium. Cobalt can be present, but it's more for hot hardness, not corrosion resistance – that's a different topic. The problem when welding is the high hardness and the associated risk of cracking. If you don't work cleanly, the part will crack quickly. After welding, things don't get easier because the heat can cause uncontrolled changes in hardness in the weld area. In practice, this means you have to properly preheat the material and cool it down slowly and controlled, otherwise you'll get stress cracks. Just welding away and letting it cool quickly doesn't work with HSS.