AbstractThe simultaneous decarburising and boronising of high speed steel (HSS) gear cutting hob with 2 kW CO2laser and a four dimensional machine tool is described. First, the HSS gear cutting hob is partially decarburised at the edge, which involves the use of a special decarburising agent. The edge is then partially boronised after coating with a boride based paste. The laser beam is moved across the hob at relative scanning speed of between 400-1800 mm min-1. Under conditions where surface melting does not occur a thin boride layer (FeB and Fe2B) only 0.08 mm deep is produced, but a transformation hardened layer of 0.9 mm is also produced beneath the boride surface. Good keying of the boride layer to the substrate is ensured due to the sawtooth microstructure of the duplex compound layer. Industrial trials have demonstrated a 100% increase in tool life after this treatment.