DarryNC, borax started to be used in forge welding when the change over from using real wrought iron to using mild steel came in---the last half of the 19th century after the Bessemer converter was commercialized. "Practical Blacksmithing", Richardson; a collection of articles from 1889, 1890 and 1891, IIRC, discusses the change and the need for a different flux than the old ones.
High alloy tool steels will probably need a more aggressive flux than plain borax, a bit of fluorspar often being added---it's more toxic in it's fumes so beware!
Plain high carbon steels weld quite well with borax.
Some "mild steels" today seem to need a bit more help, which I ascribe to more elements in them than I would like from recycling steel.
Have you read "The Complete Bladesmith, The Master Bladesmith and The Pattern Welded Blade" by James Hrisoulas? He covers fluxes and welding of tool steels in them very well.
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Thomas
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