Thank you very much Tim for your feedback. Based on your suggestion on the 4140, I buffed and sand blasted the component and after that I blued it. Unfortunately the discoloration or scar brought about by the welding is still visible. I do not want this visible in the finished product. My process is TIG-weld a “blue hole” or tiny pin hole in the 4140 investment casting material, then file and buff, then anneal the welded portion by means of oxy/acetylene. Then buff and blasting with emery grit # 80, then gun blue for max of 1 hour. After bluing, that's where the discoloration will appear. Sometimes the welded portion is darker than the rest of the work piece, at times the same color with the material but the sides of the welded portion or circling is darker. Any suggestions? Anybody??
Mind you, I know nothing about bluing, but it sounds like the bluing operation is revealing the difference in microstructure between the weld metal, HAZ and base metal. If this is the case, your annealing operation isn't doing what it's supposed to do which is homogenize the materials so they are roughly the same microstructure.
Mike is on the right track, but it might not have to do with the annealing. Minor differences in alloy or microstructure can show up after bluing, it is similar to an acid etch.