High-speed steel (HSS)
High-speed steel (HSS) is a high-alloy tool steel that keeps its hardness far beyond 500 °C. Its trademark is secondary hardening: during tempering, fine carbides form, which harden the steel again.
Physical-Chemical Mechanism of Action
HSS contains a lot of tungsten, molybdenum, and vanadium, often plus cobalt. These elements bind the carbon into very stable that don't soften even at red heat. That's where the comes from, which lets an HSS cutter keep working until the chips fly off blue. During , ultrafine carbides additionally precipitate and add more hardness—that's the . That's why hardening is done at a very high temperature, and tempering is done multiple times.
Metallurgical Relevance & Welding Risks
HSS is one of the most thankless tool steels for the arc. Lots of carbon, lots of alloying: practically any cooling produces , and the weld and its heat-affected zone harden immediately and are extremely crack-sensitive. Plenty of is also left behind, which later transforms and adds more stress. Without heavy and tempering afterward, a repair weld almost always cracks.
Workshop Practice & Action Instructions
Repair only on a well-preheated tool, in short thin passes, and temper right after. Cool properly when sharpening. If the cutting edge turns blue, it exceeded the tempering temperature and lost hardness—check the . Avoid sharp edges and notches on the tool, because on this steel that's exactly where it breaks.