Ferrite Number - SS 347

[novice123]

API 582 says in Appendix B, para B3.3 as:
For the overlay alloys listed in Table B.2, ferrite content of the final layer of weld overlay shall be in
the range of 3 FN (Ferrite Number) to 10 FN, except for Type 347 that shall have a range of 5 FN to 11 FN.
Minimum FN for Type 347 may be reduced to 3 FN, provided the fabricator submits data verifying that hot
cracking will not occur using the lower FN consumable to be used in production, and this is approved by the
purchaser.
What is the reason of allowing Type 347 to have acceptable FN as 5?
And which welding consumable should be suitable to get 3 FN while welding 347.

[arcpro]

One part of your question, I will try to answer and that is the recommended consumable for SS347 when targeting low FN is 16-8-2.

[Dlew]

What exactly would you like to have a final layer chemistry, is that 347?
If yes, I would not say that 16-8-2 would get you that.
You need to have ER347 applied to achieve that.

[novice123]

Yes SS347 chemistry, we need to eventually have.
Can't 16-8-2 application provide us that and meeting low FN requirements too?

What exactly would you like to have a final layer chemistry, is that 347?
If yes, I would not say that 16-8-2 would get you that.
You need to have ER347 applied to achieve that.

[mechcolor]

Type 347 weld metal is more sensitive to hot cracking than many “plain” austenitic grades when you let it go too fully-austenitic. The practical mitigation in industry is: keep some delta ferrite present, because delta ferrite helps “soak up” impurities/low-melting constituents that otherwise segregate at austenite grain boundaries during solidification and trigger fissures. Ferrite above a few FN is widely used as the anti-cracking knob in austenitic SS weld metals.
For 347 specifically, multiple welding references and industry discussions note that it commonly needs a higher minimum FN than the generic “≥3 FN” rule to stay away from microfissuring risk in production variability (chemistry scatter, dilution scatter, heat input scatter, ferrite meter scatter).