Supertech PEIVN-28692 black nitrided valve Peugeot 306 XU10J4R intake +0.00 mm

STAINLESS STEEL ALLOYS:
Made as a one piece forged valve for higher strength and reliability.
We use mostly EV8 (21-4N) for Intake and Exhaust valves but there are other alloys that we use along with different heat treatments to manufacture valves for different applications. 
Other hard metal alloys may be welded to harden certain areas like the tips or seats.

We use mainly two different type of coatings:
CHROME COATING: Widely used in high performance valves.
BLACK NITRIDE: This is a hardening treatment that even though it is not widely used as the chrome coating, it has several benefits and is becoming very popular, mainly for European and Japanese applications.

The microhardness is higher than the stainless steel base material, keeping good ductility beneath the hard nitrided layer (microhardness is 800HV minimum).
Surface finish is smoother than with chromed stems, having less friction between stem and guide.

Valve seat surface is harder with the nitrided layer, lasting longer with lead free fuels, alcohols, nitro or other "explosive" mixes. Matches any kind of seats (nodular iron seats, steel powder metal seats, hard aluminum-copper seats or beryllium copper seats).

Higher rpm's engines take full advantage of all previous features. The higher the engine speed the bigger the power loss due to internal engine friction! The black nitride layer reduces this power loss due to less valve-guide  friction.