Ferrite percent and Ferrite Number ?? 2

[Flareman]

We design equipment and have it manufactured in 300 series alloys. With fully austenitic materials, we have traditionally indicated a desire for a weld consumable which would deposit at least 10 percent delta ferrite as a guard against hot cracking, which gave us a lot of problems some years ago. This was on the advice of a metallurgist to whom we now don't have access.
Currently we have a client requirement to work in the range 3 to 10 FN (Ferrite Number) and we're not really sure how the % compares to FN. Can anyone jump in with a comparison table because I've a feeling that they're not exactly the same?

Whilst we're at it, my fabricator is reporting that the original test button gave 8FN whereas the final job is showing some numbers as high as 16FN. What techniques do I tell him to use to either
- fix the existing deposits (I can't anneal the job) or
- control the next deposits so that he won't have the same problems.

Is 16FN off the charts for AISI-310 equipment which might see temps as high as 400 degC?

Looking forward to some great suggestions !!!

 

[PAN]

For high temperature service or PWHT of austenitic stainless steel weldment is inevitable. I understand that ferrite content less than 10% should be controlled.

 

[mcguire]

Nearly all austenitic stainless steel has its composition balanced so that it has 3 to 10% delta ferrite according to the Delong diagram or its subsequent refinements. The annealing the flat rolled product gets dissolves this ferrite, so you don't see it unless you melt and refreeze it as you do with a weld.
This amount is enough to prevent weld cracking which happens if the material freezes in an mixed austenitic/ferritic mode. If the composition registers zero delta ferrite, by the diagram, it will solidify wholly austenitically. Then the sulfur which is insoluble in austenite will segregate to the grain boundaries making them hot short, causing cracks with a little shrinkage.
So, besides controlling ferrite percent( or number, used interchangably) make sure to specify LOW sulfur for high alloy grades like 310, say 0.002% max.
My experience with ferrite is that over 10% is extraneous for hot cracking prevention and leads to other problems in mechanical properties through the formation of undesirable phases at 375C and above.

 

[Flareman]

Thanks for your interest guys.
In the event, we find that treating the high FN welds with a wash using the Tig arc, but no wire after the weld is completed, acts like a heat treatment and gets the FN16 down to about FN7-8.
I don't see anything wrong with this technique myself but I'm open to horrified shouts of amazement if anyone does think its a no-go.

David

 

[mqreis]

Ferrite generally has been expressed as the volume percent in the weld metal bead. FOR A GIVEN LOCATION USING STANDARDIZED WELDING PROCEDURE AND MEASUREMENT TECHNIQUES (INCLUDING INSTRUMENT CALIBRATION) THE PERCENTAGE WERE RELIABLE AND REPRODUCIBLE WITHIN RATHER NARROW LIMITS. On the other hand, it was virtually worthless to try to compare results obtained from various testing sites using a variety of welding procedures and measuring equipment.
In accordance with the Welding Research Council recommendation, percentage no longer will be used to express ferrite content. Ferrite percentage values was replaced by a “Ferrite Number” (FN). Elimination of the percentage value, in effect, recognizes the inability to measure absolute ferrite content by substituting a series of values arrived at using standardized procedures and measured using calibrated equipment. However, percentage ferrite content is still supplied only on request basis.

 

[TEV]

I agree with McGuire on ferrite number and the effects of sulfur, but be aware that sulfur has a very strong effect on welding. 300 series austenitic stainlesses designed for fabrication by machining will have typical sulfur levels of about 0.025%, whereas some products out there will have sulfur levels very low, of the order that McGuire recommends. Variation of the sulfur level over this range will reduce TIG welding penetration AT THE SAME POWER INPUT setting by a factor of about 2, and trying to weld two pieces together with substantially different sulfur levels will cause a perfect devil of a time for setup, as the weld penetration will be asymmetrical. The austenitic stainless tubing manufacturers have a tighter specification of 0.005 to 0.012% sulfur to minimize this effect. There is some controversy about the minimum. Some welders say they cannot weld material with less than about 0.005% sulfur, particularly in tubing with wall thicknesses greater than about 0.080 inch, although we do it routinely.

I think McGuire's advise about FN is appropriate and it should protect you from hot cracking if you use tubing with prevailing sulfur specs. But be aware that it is VERY desirable to match sulfur levels of pieces to be welded within about 0.007 percentage points to avoid weld penetration asymmetry, and if you go to the very low sulfur levels that McGuire recommends (and we use, also) you may have some unhappy welders until they get used to it! You may also want to use laser or other welding processes to mitigate this effect, or welding wire with appropriate composition.

 

[mcguire]

TEV is very correct about the difficulties of welding together tubes of different sulfur levels. We need the orbital welding equipment makers to address this issue so that corrosion resistance doesn't need to be compromised.