Ferrite number for 317L weld

[GRoberts]

We have some 317L SMAW electrode that per the manufacturers chemical analysis has a 5.9 FN (WRC), and we welded up a "weld button" (deposit 5-6 layers on a sst block), and got a 5.5 FN (good correlation with WRC) using a Fisher Ferritscope. Our customer wants results from an actual joint also, so we welded up a sample. (Base material is ASTM A351 CG3M). Chemical analysis on the weld reveals a FN or 6.8 on WRC, but Ferritscope test is Root FN = 15.9, Fill FN = 10.8, and Cap FN = 13.3. Why is the actual weld FN as tested with a ferritscope so far different than the WRC diagram in the second case? The Ferritscope calibration was also found to be very close to ideal. I need to keep the FN under 10 for this project.