Porosity w/High Nickel Content or What Is That Grey Stuff?

By Kix

Date 02-14-2008 14:11

What base metal are you using for the tests?

By tom cooper (**)

Date 02-14-2008 14:26

Kix-
For the CuNi & 8018 we are using mild steel, A36 is typical. On the 13/8 we would use 13/8 plates expressly for the reason that we have a history of problems on 13/8 and want to duplicate production conditions for the welder.

By js55

Date 02-14-2008 15:07

My first guess may be NiS. Sulpher can be high in A36 and readily bonds with Ni. Let me ask this. Do you see it more towards the top of your multipass application?
S will evolve up through the weldment with each pass similar to Al, Ti, and Zr, as oxides, will with ER70S-6 C-steel wire, which is why you often get the pepper look with multipass welds with that wire when you grind. Which is another question. Do you only see it when you grind?

By tom cooper (**)

Date 02-14-2008 15:47

JS55-
You startled me with you question " Do you see it more towards the top of your multipass application? " because we did kick that around as an odd observation and guessed it may be associated with heat buildup, but we dismissed that because we are paying attention to interpasses of 150 degF. Why are you asking this?

I am not sure of your other question " Do you only see it when you grind?" ; We only see porosity or other voids when we cut and grind. But the grey stuff is their at the completion of the pass, never after we wire wheel or grind it away.

By js55

Date 02-14-2008 16:19

Tom,
Each time you weld another layer the sulpher in the previous layer can evolve up through the layers increasing in concentration to the point that it becomes visibly manifest. And it will be worse with higher currents, greater depth of fusion, and greater dilution. So, its not a question of interpass, its a quesiton of the beginning content of S (you're gonna have high dilutionin your first layer), how much of each layer is liquified, and becomes a part of the subsequent layer (in other words dilution), causing the volume percent to increase with each pass. The thing is, if you're welding Ni on carbon steel you will always have NiS (or maybe NiS2-or Ni2S-or some variant thereof-I don't remember).
The question is, how much and can you see it. If this is the problem (and keep inmind this is only brainstorming) then by going to a cleaner material like 516 the problem would likely disappear.

By js55

Date 02-14-2008 16:21

Let me correct my first post. Not 70S-6. But the triple deox 70S-2. Sorry about that.

By tom cooper (**)

Date 02-14-2008 21:18

Thanks for your ideas JS.
I looked at all the material certs for A36 and the 13/8 that we have and the certs say S is only .05 on the A36 and .0004 on the 13/8. That is pretty small amounts. Do you still think that would be enough create that grey haze in the photo?

By js55

Date 02-14-2008 21:26

Actually I think .05 S is quite a bit. .025/.035 is more common as a max for cleaner materials and as a rule they achieve much less than that. In fact, steel manufacturers got so good at taking out S that they had to put some back in some SS's because too low of S was deleterious to wetting.

By aevald

Date 02-14-2008 21:39

Hello tom cooper, this may be way off base to some extent, but yet it may be relevant in this situation. In many cases where high purity type welding is taking place on carbon steel pipe with the GTAW process, the welders may have it suggested to them to clean all of the corrosion resistant coating off of the various grades of ER70S series fillers. I have seen this done using scotchbrite pads to remove the copper-colored coating immediately prior to using the rod. I have been led to understand that this practice will help to limit some of the SI that is introduced into the welding pool during welding(hopefully this was relayed to me correctly). I am wondering if there are any external coatings or such that are present on the filler wires that you are using that could be having a similar effect to this deposit that you are seeing. I can see where js55 is coming from with asking about whether this manifests itself more so after multiple bead applications. Something like this might be occurring as a result of the accumulative effect of remelting and adding additional impurities with successive passes. Just a little food for thought. Best regards, aevald

By Tommyjoking (****)

Date 02-15-2008 09:37

ok this will cause a lot of laughs....but I always called that "grey stuff" as you put it Tom ....burnt shield gas (laughter inserted here please)...its really a dross of gunk on top of your weld pool. What its origin really is I have no clue...but I doubt its causing your problem inside your weld (here is where I will be proved completely wrong and learn something invaluable).

js55 is on the mark you are drawing something out of the parent material and its coalescing into subsequent layers. The first pic looks like hydrogen bubbles, empty little perfectly round voids. Gentle preheat can take care of that. The second looks like an inclusion...but its a vacant rough hole... You will see the same kind of imperfections in other steels ...4130 in particular, especially in multi pass applications ...but you will only see it if you grind or shoot....these imperfections remain below surface. WHY? My mind always points back to material imperfections NOT process...but hell I am just a welder and I am not qualified not educated enough to answer your questions with serious merit. BUT my welding gut tells me js55 is on the right track.

This is an interesting thread and I posted my weak $.02

Best regards
Tommy

By js55

Date 02-15-2008 13:59

I think Allens got something worth looking at as well. Sorta the same issue, only Si instead of S. Take a look at Si, S, P, contents of Ni fillers and base metals and compare them to carbon steel (especially A36). There's a reason the contents are so low with Ni's.
Having said that I really don't believe that what you are seeing will translate into a metallurgical or mechanical problem. A flawed Ni weld deposit (flawed in the sense of visible inclusions)will still most likely be far superior to A36, or any carbon steel for that matter, in high temp or corrosion service. And I doubt seriously that inclusions of that small extent will even show up on mechanical testing.

By tom cooper (**)

Date 02-15-2008 16:38

I agree that the flaws probably would not grossly affect mechanical tests, but they would certainly be rejectable by x-ray examination.

I will take all advice and go ahead with scrubbing off coatings, at least for our WPT's.

Thankyou all.

By js55

Date 02-15-2008 18:02

tom,
Depends on the size. They would probably be evaluated as slag inclusion. If they showedup at all. And slag inclusion is allowed within a certain tolerance. I don't have a Section V handy but I'm guessing what I saw is acceptable if its called slag.

By spgtti

Date 02-15-2008 16:49

Not knowing much about metallurgy, but having been involved w/ Alloy 600 inconel repairs for the past 5 years I would have to say that the grey haze wouldn't be a variable in your porosity problem. If you could see the 52 inconel overlays we install you would be amazed that these heavy oxides don't affect the quality or mechanical properties of the metal. Once we install the temper bead layers on the P1 and stainless piping the liquid(shiny) portion of the weld puddle is barely visible under a dead gray haze for the balance of layers. The welding engineers at work consider this grey dross oxides. We sometimes power brush or skim grind in between layers but not always. We have very few indications in our work and all the porosity indications we've had have been from gas flow issues.

By 803056

Date 02-15-2008 21:58

I've encountered the haze and oxides on a few nickel alloy piping projects. We solved it by:
1) removing the surface oxides on each weld bead prior to depositing the next weld bead. The trick is to use a mechanical abrasive. Wire brushing only polishes the surface and is not aggressive enough to break up the oxides on the surface of the weld bead. Carbide rotary files or abrasive disks are needed.
2) More thorough shielding. The oxide is reduced if you use a trailing shielding cup and in the case of pipe, better purging of the pipe ID to reduce the heavy gray oxides that can form on the root surface.

3) Porosity is often associated with autogenous welds that have insufficient deoxidizers. Porosity isn't a severe a problem with Ni-Cr-Fe alloys where the Cr can counter the O2.
4) Make sure the surfaces are dry and the moisture content of the shielding gas is very low. I've read literature that mentioned up to 5% hydrogen can be added to the argon or helium to reduce porosity, but I've never tried it. I have used both pure argon for GTAW and mixes of argon and helium to weld nickel alloys with GMAW.

Hot cracking is usually the resultant of S and Si or other low melting point constituents such as lead, phosphorus, tin, etc. can cause hot cracks. Nickel has to be kept very clean. Some cleaning fluids, markers, temperature indicating crayons, etc. can contain elements that form low melting point eutectic with nickel and result in hot cracking problems.

There is literature available from the Nickel Development Institute that may be helpful. ASM also has a book on Nickel and Nickel Alloys that has some information on welding issues. I would expect the AWS Welding Handbook to have some information available as well. You might find what you need in one of the three resources I mentioned.

Best regards - Al

By CWI555

Date 02-16-2008 14:06

Al,

I believe I'll have to go with your explaination as it fits my own experience. As for the added hydrogen theory, I've seen that one attempted with very poor results.

Regards,
Gerald

By 803056

Date 02-17-2008 19:35

Hello Gerald;

I have no personal experience with hydrogen additions. I have seen it "permitted" in some aerospace applications and it is listed in literature available from ASM.

I have tried to use an argon carbon dioxide (0.25%) mix with nickel copper alloy with NO success even though it was recommended by the gas supplier. The weld surface was very rough, irregular, and actually turned green. Fusion defects were prevelent.

I have found that it is important to remove all surface oxides from the weld bead before depositing the next adjacent weld. If the oxide is not removed, incomplete fusion between the weld beads is assured.

However, welding is a funny character. What works for one welder doesn't always work for the next. Even though we try to approach welding from a scientific point of view, the welder as a key component, that can't be overlooked.

Best regards - Al