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Up Topic Welding Industry / Technical Discussions / Stainless Steel Sugaring
- - By TimGary (****) Date 01-16-2015 20:31
In non-high purity stainless steel structural applications, and where the completed weldments are to be painted, can acid pickling alone be relied on to remove carbide precipitation, heat tint oxides and sugaring resulting fron no back purge during welding?

Tim
Parent - By js55 (*****) Date 01-16-2015 21:18
Pickling will not help with carbide precipitation or sugaring.
Parent - - By 803056 (*****) Date 01-16-2015 21:19
What is this "sugaring” you speak of?

If you are describing oxides on the backside of the weld that are black and crusty looking, pickling may not be suffici3ent to remove it. The oxide is just that, iron oxide, chrome oxide, nickel oxide resulting from exposure to oxygen while at elevated temperature.

The iridescent film can usually be removed with pickling, but as is the case with any acid, it must be completely neutralized and flushed after being used.

Sanding or grinding is an alternate way of removing the oxides and tint without resorting to harsh chemicals.

Best regards - Al
Parent - - By TimGary (****) Date 01-17-2015 01:19 Edited 01-17-2015 01:22
Thanks Al,
What's the AWS correct term ?  Extreme oxidization? :grin:

So I'm pretty sure I know the answer to this, but I thought I'd poll the masses, maybe start an argument :twisted:

While writing a specification to do this the right way, I got to thinking that I've never tried to pickle off sugar, just rejected or scrapped it.
I'm just wondering that if on a secondary weldment in which the odd configuration of roll formed sheet made backpurging or oxide removal improbable, and if the weldment were to be pickled and "E" coated in a dip tank, would the weld rot out?
Let's leave codes and specs out for the moment, just consider fit, form and function.
Base metal = SAE 210
Weld Process = GMAW Pulse
Filler = ER308L, or ER308L Si
Front side Shielding = 98% Argon / 2% CO2
Thickness = 3mm

Tim
Parent - - By 803056 (*****) Date 01-17-2015 02:09
Is the oxidized surface accessible? Just grind or sand the affected areas. The mechanical properties of the unaffected base metal still meets the mechanical properties as far as the tensile and yield strength goes. However, the oxide does affect the corrosion resistance since it represents a dissimilar metal if it is a wetted environment with an electrolyte present. In which case galvanic action comes into play. If it is a dry environment, no corrosion problem. It is a cosmetic problem unless the weldment is subject to cyclic loading and fatigue is a problem. In that case, it is just another surface condition that can be corrected mechanically.

The alternative is to back gas the backside of the joint so it doesn't oxidize.

Best regards - Al
Parent - - By TimGary (****) Date 01-22-2015 15:51
Thanks Al, appreciate your comments.

So in this hypothetical situation:
The oxidized surface is either inaccessible, or the Production Manager doesn't want to pay someone to remove it.
The weld will essentially be in a dry environment as it will be sealed with primer and paint, so no water, or air can get to it.
The question is, will pickling acids remove heavy black oxide, "Sugar", thus after coating, the cosmetic and galvanic corrosion concerns are not an issue.
Or, will intergranular corrosion or other sub sequential issues cause the weld to rot out from the inside, causing coating failure and eventual visible corrosion?

I know I'm stretching here, but I'm trying to cover my bases before trying to defend expensive requirements to people who don't want to hear it.

Thanks,
Tim
Parent - - By Lawrence (*****) Date 01-22-2015 16:12 Edited 01-22-2015 16:17
Make an unpurged CJP on a plate, or a pipe and cut out the pup.

Go ahead, flip over to the sugared side and try pickling, passivation, acids, rain dances, voodoo, the power of positive prayer and anything else you care to throw on that sugared metal...  It will still be nasty until it is removed mechanically.

I'm pretty unclear though on the whole process...  The backside of the weld is "inaccessible" yet can be accessed in order to place a seal of primer and paint!   but a backing gas (argon or nitrogen) or a heat sink to avoid burn thru (sugar) cannot be placed ?

I've seen miles of sweet sweet austenitic weld melt-thru that was covered with inexpensive nitrogen... Heck, the air separations plants are trying to give the stuff away practically :)   It's not as shiny as argon backed melt-thru... but it sure can be passivized.

Why oh why Tim  ?
Parent - - By TimGary (****) Date 01-22-2015 21:09
" Why Oh Why"...
I hear you Buddy! :grin:

I could sit for the next hour and list very good reasons why not to allow SS to sugar, all based not only on previous experience but technical research as well.
I'm writing a specification to require back purging, even on miscellaneous, non crucial sheet metal welds where purging is going to be impractical and even problematic, because that's the right way to do it.
I'm just trying to cover my bases before hand because inevitably, some hard headed person with well meaning cost reduction ideas in mind will challenge me on this.
That's OK, I'm used to it, but the fact is that I've never actually tried to pickle off sugar before and I don't have the materials in hand yet to run a test, so I'm checking to see if someone on here has.
I've also never coated SS before, so I'm wondering if that will make a difference.
( The primer coat is "E" coat in a dip tank that does a very good job of getting in all the nooks and crannies that are hard to access otherwise.)

Have you ever seen a weld that was done all wrong, but lo and behold, it works anyway?

I understand that an unpurged root will be rough, irregular and full of stress risers, even under the oxide.
In a case where this doesn't matter because the weld will not be stressed or fatigued, and the surface will be sealed from the elements by an instrusive and resilient paint coat, and if any brittle oxide that may cause a coating failure has been removed before coating, is it still worth the money to back purge?

Tim
Attachment: AWSSugaringArticle.pdf - Good article (75k)
Parent - - By Lawrence (*****) Date 01-22-2015 21:17
Ahhh.. More details !

Fit for purpose !    Which is really and engineering level call..

Yes.. 300 series stainless is super tough stuff !   I've seen some horribly ugly welds on 321 that flew millions of miles..  And that's all I'm gonna say  :)

You may have a good point about "will it work and last a long time?"

If you don't need CJP... Or even if you do... Copper backers with just a tiny recess for melt-thru can make the back side look purged sometimes with great fit-up.   A solid flat heat sink without a groove will keep melt-thru/sugar at bay as well.. but may limit CJP.

Is CJP a requirement?   Working to a particular standard?
Parent - By pipewelder_1999 (****) Date 01-27-2015 00:46 Edited 01-27-2015 01:28
I think there may be some issues with mechanical properties if you use copper adjacent to stainless welds if the possibility of contamination exists.

I have used other low melting point alloys to induce cracks in stainless samples for PT.

But as with many things, it may be just fine. I personally would fear the copper more than the sugar!

Have a great day.

Gerald

EDIT: http://products.asminternational.org/fach/data/fullDisplay.do?database=faco&record=384&trim=false
Parent - - By pipewelder_1999 (****) Date 01-27-2015 00:37
If you are in a service environment that requires heat tint and oxidation to be removed, and you do so by chemical means you have t o consider the ability to remove the pickling agent after you are done (faying surfaces that cannot be accessed or internal surfaces that may allow collection.)

You would also normally follow up with a passivation process that restores the chromium oxide layer.

"Sugar" or root oxidation is not a problem in many cases but that is a decision for engineers to put in the specs and not a matter for the lowly inspector. Many codes for piping do not address this "condition" as a defect. As with everything, the service conditions evaluated by engineering professionals should be the deciding factor. 

There are documented research studies  that show the mechanical properties of welds made without purge being equal to or bettter than some welds made without purge.  http://www.aws.org/wj/supplement/WJ_2014_04_s124.pdf

Here is a statement from an abstract of an EPRI article at http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001009717

Moderate to severe surface oxidation (generically referred to as "sugaring") is thought to degrade the mechanical and corrosion properties of SS weldments. This study involved an analysis of surface oxidation on stainless steel weldments.

Test results showed that surface roughness due to severe sugaring could potentially compromise mechanical properties and crack initiation sites if subsurface defects are created. Basic mechanical data (bends, tensile and hardness measurements) showed little affect due to sugaring. Corrosion tests (ASTM Practice A, C and E) also showed no detrimental affects produced by the various degrees of sugaring.


I am not saying sugar is "OK" just that sometimes it just may not matter. Of course its hard to call over all my welding buddies and impress em with the golden wedding band on the inside of my coupon.

Many a superheater pendants have been welded in with no purge and RT.

When I think of "High Purity" the term "paint" never comes to mind however that is based upon a limited exposure to many industries.

Now on to read the rest of the responses :)
Parent - By TimGary (****) Date 01-27-2015 13:08
Thanks Gerald!

Tim
Up Topic Welding Industry / Technical Discussions / Stainless Steel Sugaring

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