How much carbon has mild steel?

It depends in the type of Stainless material grade. Each grade has different percentage of elements such as carbon, nickel, chromium,etc. If you look in the ASME Code on materials. I believe Section I, it will tell you about metals and alloys. If you look in Section IX it will tell you about welding procedures etc etc. Another Code that may asssist you is the ASTM. American System of Testing Materials