These are the 200-series valves from Supertech. They are made of stainless steel but this valve is marketed as an "entry level" valve from Supertech. Supertech Performance uses SAE EV8 (UNS S63008) for the Intake and Exhaust valve.
Supertech Performance apply's different heat treatments to this material to obtain different hardness and ductility in accordance to application and budget (Street Performance or Heavy Duty Racing).
Sold as a 16pc set (8 intake / 8 exhaust valves)