I presume that this was a hand held XRF unit.
What sample prep did you do? Sand paper and solvent wipe at least?
Did you run in double precision mode?
But, if you want to weld then hardenability is the biggest issue so you will need a method that will measure C content also.
Portable OES or a LIBS unit would do that.
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P.E. Metallurgy, consulting work welcomed