Hi all,
I have a problem. I done a production test on a weld of a SS pipe tipe 304L welded by GTAW and one the results was that the FN is 10. This pipe will be used for a Hidrogen service (low temperature). It's this FN aceptable? It's true that the FN requeriments for this material is 3 to 5?
Regards,
Cesar
i had the same problem very recently, it is nearly impossible practically to assure deposited material FN 3-5 or tigrod FN 3-5 with commercial products. You may ask to manufacturers also they can not guarantee that much strict limits. In your case it is better to raise a concession request.
Regards
T,
Thanks for your help. Now I more confidence.
Best regards.
Hello Zamorano;
Chuck and some of the other fellows will probably have better insight on this question, but my experience is that the FN is a predictor of the crack sensitivity of the weld during solidification. Once the soldification is complete, the FN isn't going to be a major factor. FN is a measure of the amount of ferrite present. Ferrite has a higher tolerance for low melting point constituants and minimizes the potential for solidification cracking.
The FN or 3 to 5 are minimum values, not a lower and upper boundary. In extream case, excessive ferrite might influence corrosion resistance, but that can be address by passivation if it is a major issue.
Chuck, what do you say?
Best regards - Al
Thanks for your help. Now I more confidence.
Best regards.
The typical FN of base metal 304L is around 3-5FN, but the typical FN of 308L GTAW wire is 8-10, depending on the exact chemistry. It seems that this is quite normal. You said that this is a "low temperature" service. Do you mean this is a cryogenic application? If so, you will need to do the applicable test to see if it meets the lateral expansion test required by Code. If it is not a cryogenic test, I don't see a problem with the 8-10FN results.
Thanks for your help. Now I more confidence.
Best regards.
Chuck touched upon the main point. How low is low. For cryo the ferrite could cause impact issues. Under normal servce the 10FN is fine. As he said, even consistent with 308 expectation. And remember, with SS we're talkin way low. Not just -50 or so. There's ferritics with excellent toughness at that temp. And I think its helpful to add that a ferrite range of 3-5 is far more restrictive than necessary, barring specific service application which may drive a more restricive range. These narorw targets are more difficult to hit in fabrication, and as Chuck said, even more so for 308 fillers.
Zamorano, I've worked with LH tanks before. Your spec for 304L sounds more in the lines with LNG piping rather than 316L which is often used for LH. That be as it may, LH has a boiling temperature of -252 C (-423 F). It's one of coldest liquids produced. In comparison Liquid Helium is -268 C (-452 F) which I believe is pretty close to the coldest that can be contained and produced by current technology. Notch toughness becomes very critical at those temperatures. There have been some very good discussions on this matter, but I missed your post until now. I suggest some reseach into ASME section II, and some calls to Avestas, Lincoln, and others on the matter. What I have seen typically speced out is an FN range of 3-8 being allowed for LNG. I would not be comfortable with a spec of FN 10 for that material and service (speaking of the weld only) as there is an error factor in ferritiscopes. You can search the forum for the specific reference but as I recall Section II gives you a +- 2.2 FN error factor off of a nominal 8. It's further been my experience that when a low number such as the one you've mentioned is specified, its with a potential error factor built in. ASME section VIII will also have reference to Section II and notch toughness for low temp service.
CWI555 really brings it all together. As Chuck said if its not cryo then the 10 is most likely good. However, if its LH (-423F), which is about as cryo as cryo gets, then 10 could be problematic, and testing for LE as Chuck said is certainly justified.
The problem is that you have to cool your samples to way below -423F, if you can even do that. Because, by the time you get it out of the 'freezer', set it up on the anvil and smack it, the temp has increased to way over -423F, or whatever you establish as your withdrawal temp. And in reality you have no way of really knowing what the 'smack' temp is. So emperical evidence is iffy at best. Thats why 0FN's are desired for such low temps, and special 0FN fillers are required. No since on taking chances.
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