18-8 is the same as 304 stainless. It cannot be hardened by heat treatment, only annealed.
What Dave said. AFAIK the only commonly used hardening techniques for 300 series SS alloys are nitriding and strain hardening
Neither of these are heat treatments; although nitriding does require a certain amount of heat, it is merely to facilate absorption of nitrogen into the surface layer.
Strain hardening (aka work hardening) is very localised and can be tricky to do without making a potential scenario for cracks to start propagating after some time in service, unless the part of the workpiece requiring hardening is really simple.
I suppose something along the lines of shot peening -- perhaps with tungsten balls -- might work in a more generalised way to increase surface hardness somewhat, but we're talking megabucks to set up the media.
Nitriding produces a thin surface layer which is extremely hard. Needs careful research to ensure you don't create the potential for flaking or spalling.