... stainless steel 1)The Wire Diameter of this extension spring: 1.00 mm-20.00 mm;2 ... )Raw Mterials of this extension springs:stainles steel 302, hign carbon steel, music ... wire, etc;3)Surface Treatment of this extension springs: powder coating, nickel plating, chrome plating, ... zinc plating,etc;4)This Extension Springs are produced by CNC spring's machines;5 ...