You may want to track down ASTM B167 Standard Specification for Nickel-Chromium-Iron Alloys (UNS N06600, N06601, N06603, N06690, N06693, N06025, N06045, and N06696).
Maybe double check your material spec, I could find B167-N06600 in ASME B31.3, but not B167-N06601.
Maybe instead of "UN6601", it may be UNS N06601.
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Regards,
Dave Clark