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Up Topic Welding Industry / Technical Discussions / Possible ferrite in the HAZ of 316L
- - By jdb 2 Date 06-04-2007 12:25
We have a tube to tube sheet joint currently under investigation.  The tube is 316L and the filler ER316L, GTAW.  The OD of the tube is about 21mm, and two weld passes were used to complete the joint.  The total thickness of the joint is no more than 3mm.  We have observed ferrite stringers in the base metal and ferrite in the weld metal.  My question is related to what we saw in the HAZ.  We saw significant precipitation on the grain boundaries, typically described as sensitization.  This is strange to me since the total welding time could not have been more than 4 minutes, making sensitization of the L grades highly unlikely. 

Can this precipitation possibly be ferrite that formed on the grain boundaries of the HAZ?  I know that some base metals have a fairly high ferrite potential, that helps prevent liquation cracking in the HAZ, but I do not exactly know what it will look like.

Thank you,
Joe
Parent - - By chuck meadows (***) Date 06-04-2007 12:50
Joe,
  As you said, it is unlikely that sensitization occured when less that 4 minutes were in the 800-1500F temperature range. With sensitization, the Cr and C atoms diffuse to the grain boundaries and those carbides grow. It is not the carbides that lower corrosion resistance, but the area immediately next to the carbides. I'm not sure what test you did, but the ferrite will look much different than the Cr/C atoms.
Parent - - By jdb 2 Date 06-04-2007 13:02
At this stage we only did an optical microscope study, where we observed significant precipitation on the grain boundaries.  We also saw what appears to be re-melting of the ferrite stringers where it intersects the weld pool.  These stringers intersecting the weld metal looked different to those we saw in the base metal.  We plan to also do inter granular corrosion tests if we have enough material available and confirm the chemistry.  Unfortunately we do not have access to more high tech equipment to study in more detail the precipitation noted.

Joe 
Parent - By chuck meadows (***) Date 06-04-2007 13:22
Joe,
  Based upon just what you told me, I really doubt that you have any detrimental intermetallic phases, sensitization or sigma. But, without doing more sophisticated testing I'm not sure that you aren't  seeing some precipitation, which can definitely occur. I just don't think it is sensitization. I also believe that the ferrite in the BM HAZ can definitely look somewhat different than that of the weld metal. The ferrite% should be less in the HAZ than in the weld metal. In the HAZ you will lose a small amount of Cr  (Cr is a ferrite former)during the weld process, but the weld metal will incorporate ferrite from the filler metal. Lastly, sensitization is very  hard to detect, so maybe the optical microscope will be unable to detect the Cr/C carbides, espepcially on the grain boundaries of the atoms. 
Parent - By Joseph P. Kane (****) Date 06-04-2007 13:48
Are you sure that your tube is what they say it is?
Parent - By chuck meadows (***) Date 06-04-2007 14:29
Joe,
  Very respectfully speaking, if you are using the base and filler metals you said, and are sure you were not in the sensitization range for any longer than 4 minutes, I can almost guarantee you that you do not have any Cr/C carbides that diffused to the grain boundaries. And, as has been said by more than just  me, I would be very hesitant to base any judgement calls, especially to sensitization, on using an optical microscope. Knowing that the kenitics of sensitization are time, temperature, and carbon content virtually eliminates that possibility. Doing some expensive destructive laboratory tests will, I think, prove that.
Parent - - By chuck meadows (***) Date 06-04-2007 12:57
Joe,
  I'm sorry, I might not have answered your question fully. I'm sure you will observe some ferrite in the HAZ and the weld metal, although it will be significantly less than the amount of austenite. The stringers will normally appear as the dark areas, and the austenite will appear as the lighter areas. So, it is quite possible that you are seeing the ferrite in your test.
Parent - - By js55 (*****) Date 06-04-2007 13:49
If I may add some. If you are looking to match the lower level of ferrite of the HAZ with the weld metal you have required of yourself a difficult task. Impossible if you keep heat inputs and interpass temps where they should be. BM's and WM's are elementaly balanced differently based upon the thermal regimes they will be exposed to. Ferrite, to eliminate cracking is not an issue with BM's.
Also, Chuck is absolutely right, you cannot, from optical mics, to my knowledge, recognize CrC's. You can of course recognize carbides, and if they cluster around the grain boundaries make an educated guess as to them being CrC's, but Mo will also readily bond with C making distinction through optical mics an educated guess.
Parent - By js55 (*****) Date 06-04-2007 13:59
jdb2,
The other thing is, as you seem to have surmised, you cannot, from an optical mic, determine suitability for service, except in the case of extreme indications, it isn't suitably quantitative, imo. You really need the corrosion tests for a definitive result. Really, accept as a go or no go (in a sledge hammer sort of way) optical mics aren't even intended for that.
And even corrosion tests will be just an indication (though inmost cases a very adequate one), since corrosive services are so specific. The only truly reliable way to achieve exacting quantitative results is to test with your exact medium with your exact temps.
The other thing is, of you find that your welding procedure requires prohibitively extreme restriction you may consider a higher alloy.
Parent - By Geese_howard (*) Date 06-06-2007 01:45
Hi Mr. Joe

You can use murakami reagent for etching your metallographic sample, reagent composition is
  10 grs. K3Fe(CN)4
  10 grs KOH o NaOH
  100 ml. Water

Ferrite will be colored yellow, carbides dark, so if you have a good resolution microscope, u can see the difference between ferrite and carbides

Please be warned of the K3Fe(CN)4!!!!!!!!!, very deadly, if u don't have the proper laboratory equipment and technical personal for manipulate hazardous
chemicals.
so ... DON´T TRY THIS AT HOME.
Up Topic Welding Industry / Technical Discussions / Possible ferrite in the HAZ of 316L

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