WC9 is often overlayed with 410 welding wire in the valve seat area, so welding a 410 seat to WC9 with 410 wire should work as well.  PWHT should be described in the code, such as ASME VIII, B31.1, or whatever you are using.  Typical would be about 1275F for 1hr/inch.  We used 300F preheat for overlay.