Let’s suppose you’ve drilled a hole in a valuable steel part, and then discover it’s not quite where it needs to be. If you have access to a TIG welder, you can quickly and neatly fill the hole. All you need is a brass block.
Jig the hole over the block so the surfaces are in contact. Clamp if necessary.
You’ll fill the hole in the usual manner; start at an edge and progress around the hole in a spiral pattern, working in toward the center and adding filler rod as you go.
The upper weld surface will be fairly flat, assuming you don’t go crazy with the filler rod.
The beauty of the brass block method is found on the back side of the part; you get a clean flat surface. The brass block is not harmed and does not alloy into the steel. After a minimum of surface finishing, you’ll be ready to mark and drill a new hole.
A final note: If the part is made of 4130 steel, allow it to cool slowly. 4130 is an air-hardening steel; rapid cooling will make it difficult to drill.