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Which folder? They made a lot of types.
Many are stainless, others are carbon steel.
Welding on a tang and making a fixed blade knife isn't likely to work well at all. There are problems with steel match, weld type, re-heat treatment, and blade thickness....plus you will still have to shape the new tang (without ruining the blade) and handle it.
It would be smarter to use the existing blade as a pattern, and take a piece of 1084 or CPM154 and make a new fixed blade with the exact shape blade. You will have to do the same amount of work either way, but welding on a tang is almost surely doomed to failure.
For those who may think of chiming in with, "No problem, if you heli-arc it with 10Xab97 rod and re-temper at 423.9° it can be done." consider that if he had that ability and equipment, he wouldn't have asked the question.