Acceptable Ferrite levels post welding.

Date 03-22-2006 15:57

I'm sure you will get plenty of solid advice from some of the posters here.

I am curious about your need for the info. If elevated ferrite, leading to reduced corrosion resistance is that much of a concern, why don't you change to a more corrosion resistant stainless?

Charles (the stainless newb)

By - Date 03-22-2006 17:10

I'm not sure if I understand exactly what you're saying here. I don't see how you can get excessive ferrite in the HAZ when welding a 304L pipe. When welding the austenitic grades of SS, the ferrite tends to decrease in the HAZ, sometimes up to approximately 80%. Normally a 304L base metal has somewhere around 3-6 FN, and if you are using a 308L filler metal the FN is around 3-8FN. But, that would only affect the FN of the weld metal, not the HAZ. Even in the desposited weld metal the FN will tend to decrease due to dillution with the base metal and the possibility of chromium loss through the weld arc. I just cannot see how you can gain ferrite above the equilibrium of the potential for ferrite. Actually, to much heat during welding will have the propensity for lowering the FN, rather than increasing it. If you are using too much heat during welding I would more concerned with sensitization in the HAZ than excessive ferrite levels, which I don't see how that can happen. Long term corrosion problems will be related to sensitization in the HAZ, or if you did not restore the HAZ back to the steel's natural anti-corrosive state by removing the chromium depleted oxide layer in the HAZ.

Chuck

By Martin Forster Date 03-22-2006 18:30

Thanks for the good info here.
We had 304L pipes welded in China (filler 308L in documentation) and the client is rejecting them on weld quality (RT and/or visual), although the quality documents are in order. Excessive penetration indicates too much heat during the process also. The client has cut the welds with a razor disk & will re-weld for good RT & visual without haing to purchse any pipe & fittings, to get job commissioned. However the client states that the pipe may have to be replaced long term due to corrosion concerns due to the possibility of "the ferrite level being too high due to excessive heat" - the HAZ will be the only part of the original pipe in existence. (These reports are coming secondhand from site).
I agree about the sensitization due to too much heat. This has never been mentioned. Further comment would be welcome. Thanks.

By - Date 03-22-2006 20:09

First of all, excessive penetration can be a result of high heat input. I am not doubting that your client said the ferrite levels in the HAZ are attributed to excessive heat input during the weld process. I'm not doubting he is saying that, I just don't agree with it. Actually, there are a couple of issues I'm not understanding. One, why is he attributing the ferrite levels in the HAZ to long term corrosion concerns? Secondly, ferrites in the HAZ will not exceed that of the equilibrium of the base metal and the filler metal. Metallurgically speaking, the FN in the HAZ will be less than the FN of the deposited weld metal. You said this is secondhand reports from the site. I would suggest you get the information firsthand because saying that excessive ferrites in the HAZ will cause long term corrosion concerns makes no sense to me. Seems there is some misinformation being relayed to you.

By Martin Forster Date 03-23-2006 13:46

Comments on excessive ferrite were made by the lab we employed to test that the material was in fact 304L as the fabricator has been unreliable.
The results of their ferrite tests (in the weld metal) were, in the China pipe butt weld 7.5-10FN with small areas of 10-12.5FN, china pipe to flange weld 5-7.5FN.
The repaired weld outside china pipe butt 5-7.5FN, flange to pipe 7.5-10FN.
we have struggled to find a maximum limit on this, but one weld society advise a maximum figure of 12FN.
I agree with you that sensitization could lead to long term intergranular stress corrosion cracking, but this has never been raised. Could carbide solution heat treatment be a possibility if it became an issue? If there is a large enough furnace locally. This could save re-manufacture if it came to it.

By - Date 03-23-2006 14:33

Yes, a full annealing heat treatment is the only one recommended for this grade unless specifically specified otherwise. I know of nowhere where a "maximum" FN is specified for 304L pipe. That would be determined by the FN of the base metal and the filler metal. During the welding process, and due to dillution, the FN in the HAZ will virtually always reduce, as will the as-deposited-weld metall.

By swnorris (****)

Date 03-23-2006 14:48

As chuck said, the method of depositing weld metal can alter ferrite content. Ferrite content is modified by variations in weld metal cooling rates, arc length, and atmospheric contamination. Weld passes that have high dilution with the base metal can have altered ferrite content. Significant variations in ferrite content can exist from weld to weld, and within a weld from the root to the face. For example, production pipe welds have shown that nearly 50% of all welds examined differed by at least 2 FN from procedure qualification ranges. For applications where ferrite content of the weld metal is critical, welding procedures must be rigorously controlled. Nitrogen and chromium variations can significantly influence the ferrite content. Improper welding technique can result in nitrogen contamination because of excessive arc length, and the loss of chromium through oxidation. The result can be a weld with far less ferrite than specified.

By - Date 03-23-2006 15:53

With all respect, I cannot imagine a reputable metallurgical lab stating that a 304L material has "excessive ferrite levels". Ferrite, in a base metal or a filler metal, is determined by the actual chemistry, with the filler metal normally being overalloyed compared to the base metal. I know of no way that one can "gain" ferrite levels when welding a 304L base metal and using a 308L filler metal above the equilibrium potential for the ferrite determined by the chemistry. It will, in virtually every case I've seen, reduce the FN in the HAZ and the deposited weld metal. I would question the lab that made that statement about excessive ferrite levels to present evidence that 12FN for the 304L is determined to be excessive. If the chemistry of the base metal and the filler metal adheres to AWS specifications for chemical requirements and it is within the limits of the Code, the ferrite level will be what it is. If it is determined to be 12FN, who can say it is excessive? If this was a concern, your customer spec should have listed limitations for ferrite levels. What weld society are you referring to that listed the maximum 12FN for a 304L to be excessive? I've never seen that.

By MBSims

Date 03-24-2006 00:54

I agree with Chuck. The base metal has some ferrite-forming ability that can be predicted using the WRC-1999 ferrite diagram, but usually is much lower due to solution annealing. The presence of ferrite in welds or base metal increases resistance to stress corrosion cracking (SCC), but can reduce resistance to microbial induced corrosion (MIC). It seems that MIC is attracted to ferrite. So it depends on the type of "corrosion" that is of concern for the service conditions. If the concern is SCC, then the results of a susceptibility test of ASTM A262 Practice A should be requested. Practice A is simply a screening test, if it passes then it is not susceptible, and if it fails the Practice E test (a much longer duration test) must be performed. If it passes Practice E, then the lab that raised concern with base metal ferrite increase in the HAZ is off base.

By ssbn727 (*****)

Date 03-25-2006 09:09

Hello Martin Forster, Chuck, MBSims, swnorris!!!

I was looking through my favorites and I stumbled across this website.
In it, is a NRC regulatory guide, #1.31- " Control of Ferrite in stainless steel weld metal". Revision 3, dated: April, 1978.

http://www.nrc.gov/reading-rm/doc-collections/reg-guides/power-reactors/active/01-031/

If you look at the last paragraph in "B." heading: "Discussion", just above "C." heading: "Regulatory Position", also in the last paragraph in #4., in "C. Regulatory Position", heading: "Acceptability of Test Results", one will find some interesting comments regarding the initial query and your own opinions for which I believe are well founded... I know it does'nt cover ferrite #'s for the HAZ but, I thought this could cover acceptable F#'s in the welds...
I'll look for the other(HAZ F#'s) info and get back to you!

http://www.osti.gov/bridge/servlets/purl/14580-mGkMzM/webviewable/14580.pdf
http://www.osti.gov/bridge/servlets/purl/11739-cfMpBF/webviewable/11739.pdf

http://scholar.google.com/scholar?hl=en&lr=&q=cache:IuIpuSm6tkQJ:www.atc-ssm.com/PDF/TWR89.pdf+304+Stainless+Ferrite+numbers+in+Heat+Affected+Zone
http://www.doeal.gov/nnsaota/SafetyGuides/SG100Revision2/SG-100_ch9_09-30-05_final.pdf

What do you think Chuck, MBSims, swnorris? I hope this helps...http://sti.srs.gov/fulltext/tr2004456/tr2004456.pdf

Respectfully,

SSBN727
Run Silent... Run Deep!!!

By MBSims

Date 03-25-2006 16:50

These are some good links on the benefits of ferrite in welds to prevent hot cracking. But, I didn't notice any specific reference to the effect of ferrite in the HAZ on corrosion resistance. Here is a link (I hope it works) that provides some good guidance on IGSCC prevention:

http://us.share.geocities.com/welder4956/nureg0313rev2.pdf

By ssbn727 (*****)

Date 03-26-2006 06:45

Hey Marty Chuck!
I was hoping that your link worked but, apparently it does'nt...
I'm not disagreeing with you Marty or Chuck!!!

Personally, the only thing that would remotely make sense here is the very low probability that the chemical composition of the filler metal was such that, 1.an unusually high percentage of ferrite needs to be present,
2. interpass temperature was at least 2-1/2 to 3 times the recommended amount or even higher which as a result, transformed most of the greater than normal amount of ferrite originating from the fller metal, into "delta ferrite" otherwise known as "gamma iron" compared to "alpha iron", resulting in enbrittlement of the welds due to the formation of sigma phase.
3. As a result of excessive dilution, enough of the ferrite that originated from the filler metal, precipitated into the HAZ...
Check these 2 .pdf's... I found them interesting enough...

http://www.arpansa.gov.au/pubs/rrrp/csiro1-10.pdf

http://www.arpansa.gov.au/pubs/rrrp/csiro11-14.pdf

I know it sounds like mush but, under certain conditions it could happen - does cheap, counterfeit filler metal originating from the "far east" ring any alarm bells??? However, if the filler metal came from reputable sources such as Avesta/Outokumpu, the previous hypothesis would not hold any water...

Marty, this one might help shed some light towards your query:

http://www.bbs-systems.com/Pages/PagesGerman/
BegriffeUnDefinitionen/Fachbegriffserkl%E4rungen.html#FachbegriffeInfluenceAnker

I apologize for not keeping the link active in this thread...
I wanted you fellas to be able to read my 2 cents worth regarding the so-called excessive ferrite content in the HAZ or whatever this lab was trying to explain!!
!
Respectfully,
SSBN727
Run Silent... Run Deep!!!

By MBSims

Date 03-26-2006 17:38

The last link is interesting. It suggests that the presence of ferrite contributes to pitting corrosion in concentrated sulfuric acid/sodium chloride solutions. We would not normally select a 304L material for this type of service. A 316L or 317L would be the minimum depending on chloride levels, increasing to a 2205 duplex, AL6XN, C-276 or 254SMO for the higher chloride levels. It would seem either a very low heat input during welding or a solution anneal after welding would be required to reduce the ferrite levels in a 304L to as low as possible.

By ssbn727 (*****)

Date 03-27-2006 00:18

Hi Marty!

I agree wholeheartedly!!!
Here's a link that hopefully will add on to your previous post with regards to weld decay testing... Although it covers ASTM A249 supplmental requirement 7 ( S7 weld decay testing testing ), it does mention that ASTM262 is the wider application choice of the two tests and S7 testing only accounts for a small percentage of conditions where the use of Hydorchloric acid is required for cleaning out tubes that are "Coked" or lined with organic matter. In this situation, Nickel & high nickel alloys are some of the few metallic materials that possess useful long-term resistance to Hydrochloric acid solution...The Austenitic stainless alloys are not suitable for long term exposure to any significant level of Hydrochloric acid.
http://www.rathmfg.com/welddecay.pdf

ASTM262 practices A & E, which test for sensitization or susceptibility to intergranular attack, have good general applicability in a wide range of environments & alloys including 304L, 316L and 317L in both oxidizing and reducing acids.

The article also explains the difference in the ferrite patterns found in both the base metal (isolated & discontinuous) and the weld metal (skeletal, offering a near continuous path for preferential chemical attack)... It also explains what heat treatments actually do to the ferrite in the weld metal (slightly reducing ferrite content but, mostly spheroidizing the the weld ferrite and break up the continuous/skeletal nature)...

It also explains why Laser beam welding of 304L, 316L and 317L has definite advantages over GTA welding of the same material... virtually eliminating the need for heat treatment in order to suppress the weld ferrite through vert fast non-equilibrium solidification ( three times faster than conventional arc processes), high energy density and low total heat input which should result in a very small, almost "invisible" HAZ and very little residual stresses in and and adjacent to the joint.
LBW is becoming more and more applicable to joining a wide variety of metals as the technology improves and will hopefully become one of the most popular welding/joining processes of choice in the very near future because of its clear advantages over conventional arc processes.

Here's another good link about different forms of corrosion types in Stainless Steels. Delta ferrite formation, etc.
http://www.al6xn.com/SSSguide.pdf
Last ones:
http://www.mbaa.com/TechQuarterly/Articles/2001/38_2_67.pdf
http://www.outokumpu.com/files/Group/HR/Documents/STAINLESS20.pdf
Respectfully,
SSBN727
Run Silent... Run Deep!!!

By - Date 03-25-2006 18:59

I agree with Marty. The post confused me becausee someone was confusing "excessive ferrites" will directly cause "long term corrosion problems" in a 304L. I disagree with that.