I watched a smith do a forge weld the other day. The first time he stuck the weld but then tried to reshape the edge and it broke the weld, the second time it took the weld nicely. He used a borax flux. He was making a nail header. First he made the loop to form the head which required a weld, then he used a piece of spring steel to make the face of the header. He showed me that he put the hot piece (the handle and looped end of the header) on the bottom, then the colder spring steel face he put on top, with borax in between, and then set it in the fire to get both pieces up to the same color then pulled it out and smacked it together. The top piece slid off center slightly, but stuck. He hammered it back to shape then set it aside to cool so he could finish file the nail header the following day.