When I started making valves I didn't know what to make them out of. I first made on out of cold rolled steel. While it looked good to me, I still had doubts about the cold rolled steel (CRS). I sent e-mail to Kurt Bjorn who told me to use stainless steel. I have since discovered that stainless steel is made from nickel and chromium. This gives it a corrosion resistance, and something about the combination gives it an ability to withstand higher temperatures than other steels. This all adds up to a good choice of material for valves.
By the way, I was in a machine shop to purchase some stainless steel and I asked the machinist how many types of stainless steels there are. He told me that there are about 400 different types! WOW!! He also told me that there are some that machine well, and some that don't machine well at all.