Hardened valves?Yeah maybe thats what some guys call them.Exhaust valves are generally made from forged 21-4N stainless steel,with or with out chromed stems, depending on the application.They might be heat treated. Your engine machine shop has catalogs showing the types of valves for your particular engine.Take a look at the specs listed. Like Johnny says its the seats that take the beating from high temperatue erosion. Replacing the valves only is bad information if your engine's seats are plain cast iron.The new exhaust seats are a good idea anyway as usually in older engines the seats are worn and recessed into the head.I also know of unaltered engines run at moderate speeds for many miles without valve problems.The exhaust seats in my old trucks were replaced when I rebuilt the engines.