Orbital GTAW - recommended FN number

[XL83NL]

I need to have some AISI 321 piping welded, and the current path forward is that it will be done through an automated GTAW machine, thus kinda orbital.
The piping system is acc B31.3, design temperature of 529 deg C. No filler metal to be used, for wall thicknesses up to say 4 mm. Beyond that we'll use filler metal.
I would like to give a range for FN (ferrite number) in which the welds should lie. Ive read some books\articles on recommended values for FN, but cant seem to get a good feeling for this; all books/articles provide different ranges, for cases where details/parameters varie (thus are not the same as for my case).
Any suggestions recommended values? E.g. Api 582 provides some, but to the detail I require

 

[Goahead]

Autogenous welding (without filler metal) needs special attention if primary ferrite solidification is desired to prevent solidification cracking. (ASM Handbook Volume 6, pg 462).
Standard filler metal for 321 is 347. Establishing required FN should include determination method, as wide discrepancies are known for different methods. Minimum recommended FN seems to be between 2 and 3. (As above, pg 464).

http://www.welding-advisers.com/

 

[XL83NL]

Darn! I have a digital copy of the book, but it doesnt include page numbers.
Crtl+f didnt help either. So couldnt find your reference. Which part/chapter/paragraph is it in?
Anyway, thanks for the help Goahaead.

Got any more references on this specific topic (i.e., 321SS piping,
welding with/without filler, high temp service, 3-7 mm wall thickness, hydrogenservice)?
The ASM book of course is good, but a bit too much info to get around right now.
ASME II-C appendix A7 on FSA5.9 sheds some light, but not the desired numbersn or a proposed method ...

Would specifying FN number determination acc ASTM A799 be good?
From what Ive read so far is a little ferrite required to prevent hot cracking,
but too much will give undesirable phases (like sigma)?

Would specifying a range of 4-10 FN be considered 'appropriate'?

 

[Goahead]

Page 462 is the 7th from the start of chapter on Selection of Wrought Austenitic Stainless Steels in ASM Handbook Vol.6.
Transformation of ferrite to sigma occurs in range 600-800 deg C, so that for service temp. of 530 there should not be danger.
You may wish to see an article on :
Solidification cracking in austenitic stainless steel welds (24 pages) at

http://www.ias.ac.in/sadhana/Pdf2003JunAug/Pe1119.pdf

The Welding Journal has a very informative Q&A section on Stainless Steel Welding.

http://www.welding-advisers.com/

 

[XL83NL]

Thanks for the reply Goahaead.
Funny thing is that prior to my reply, I just reviewed the article.
May be useful.

I think we're gonna pass the station of orbital GTAW welding 321 piping, and just do it manually;
using no filler (someone told me) is one of the worst things to do for 321 welding.
Second we have doubts regarding the unit doing the orbital welding.