That kind of forge welding is process welding, much different than firing up the Mk I hammer and going after the anvil. I would think it went fairly quickly with the proper setup, particularly once compressed gas heat was available. With coal-fired heat, you'd need longer heating time and much more qualified smiths. I wonder more about modern safe-ending and whether commercial steam shops have adopted or looked into spin welding technology. Seems like it might be more economical.
Reference. Kelly - did yours?