Deslagging tools for ss welds

By Niekie3 (***)

Date 02-13-2002 18:09

The issue you raise here is one that is often poorly understood by many fabrication and inspection personnel. Sometimes there seems to be an unreasonable expectation wrt. the tools used during the welding operation, while scant regard is paid to the materials handling during fabrication.

What is the cetral problem with C/Steel coming into contact with the S/Steel? It is that "bits" of the CS can become embedded into the SS, through the SS's passive oxide layer. It is this layer that gives SS it's corrosion resistant properties. When this bit of CS is embedded into the SS, it obviously corrodes preferentially at a very high rate. This corrosion results in the initiation of a pit in the SS. Pits are the biggest enemies of SS because they do not become re-passivated by themselves. Once a pit forms, it can (depending on the corrosive medium) corrode through the material in a very short space of time.

So, where does this leave us regarding the tools to use during welding? We must remember that your typical 300 series SS contains around 70% Fe. The extra Fe that could therefore "contaminate" the SS weld metal when using CS tools is negligible. Also, unless you are using cast iron tools, the C that comes from the CS is negligible. (This is not the case with rubber bonded abrasives, but that is another story.) We must remember that any contamination from the CS tools within the "weld prep" will be melted and will not result in the pitting described above. Contamination alongside the weld is however another story. Unless removed, it will stay there and could result in the pitting described.

To work with SS to ensure that you overcome the abovementioned problems, there are a number of different strategies. These strategies are generally a function of your workshop's particular facilities and the preferance of your customer. The stategies are as follows:

1)Use no particular care (excepting the bare minimum like SS or wooden racks) when handling the SS or working with it. When you are finished building your component, you pickle and passivate the entire component.
2)Handle and store your SS very carefully. (e.g. Use SS rolls etc. to perform the rolling of the material; Do not let it lie on the floor where people can walk over it; Do not work it in the same workshop as CS; Use only SS or plastic slings when lifting; etc) But the tools that you use around the weld itself, is CS. In other words, the chipping hammers, blades etc. are CS. (Always use SS brushes though.) When you have completed the fabrication, you pickle and passivate the welds, but include an area wide enough to ensure that any "stray" hammer hits etc. is also covered.
3)Handle and store your SS as in point (2) above. In addition, only use "non contaminating" tools in the weld area. This would typically be SS tools. Remember that you can also get martensitic SS alloys from which you can make these tools. (e.g. 410) Following the fabrication, only a very narrow band around the weld is pickled and passivated. (To remove the "burned" oxide scale.)

If your application is critical (from a corrosion point of view) then it is a good idea to paint it with a special solution that you can purchase from welding suppliers. (Forgotten what it is called.) It will make purplish marks on the component where there is any surface contamination present. You can then again ensure that the contamination is removed.

Regarding the low temperature performance of your material, the surface contamination from the use of CS tools will not affect this. Your typical 304 SS will easily handle temperatures down to liquid nitrogen temps (-198°C)without becoming brittle.

Hope this helps

Regards
Niekie Jooste

By Kulkarni (*)

Date 02-14-2002 05:52

Niekie3, thanks a lot for your detailed reply. Incidentally, the vessels I mentioned, will undergo complete pickling and passivation. Also, I believe the choice of stainless steel is on account of the low temperatures.
As you say martensitic stainless steel chipping hammers etc can be used. I was curious about their actual need and whether it is the standard/norm in the pressure vessel industry.

By DGXL

Date 02-14-2002 15:43

K,
I was waiting to see who else would respond to your post. I thought there would be many hits on this subject.

In the eighties I was GTAW my first nuclear-application manifold of some sort using 347 SS. When I put a piece of rod (filler metal) on the steel bench, then the inspector would walk up and toss it. Many TIG welders hold filler rod in their mouth's when positioning parts, etc. He would discard these as well. I finally figured out he could toss as much rod as I could chew, and he would always be there when I did. Had to learn my table manners the hard way. Had no idea at the time why he kept taking my rod.

Who knew...

By Seldom

Date 02-14-2002 23:22

Kulkarni,
Upon Niekie3’s answer to your question, you mentioned in your reply-
“I was curious about their actual need and whether it is the standard/norm in the pressure vessel industry.”

To answer your question from my experience with my former company as an Owners, QA Rep. and as a vessel fabricator/welder, the answer is no! It is not the standard/norm in the pressure vessel industry. The plant site where I worked contained hundreds (possibly more) 300L series pressure vessels built by outside vendors as well as internally fabricated. Few required passivation/pickling and fewer (that I observed) were built with specialized fabrication tooling (SS hand tools as an example). Few specs require, likewise vendor and company shops voluntarily institute, clad rolls, special shears, SS platens, SS stock racks, SS clad power head jaws, SS clad rollers, etc. unless specified/required to do so. Many (not all), out-of-hand will use SS clad ½ clamps, SS nuts & key-plates, SS shim stock, and nylon lifting straps as a few examples.

As Niekie pointed out, the severity/criticality of the intended service and the environment in which the vessel is set, is what governs the engineering design specifications. The specifications are what are supposed to govern the inspector as well! It’s been my experience managing inspectors that it’s easy for an inspector with the best intentions, to apply requirements (or his personal interpretation of industry standards) that he possibly used on another project to the one at hand without conferring with the design engineer. This is all too common of a problem that causes considerable conflict and loss of productivity, which translates to a higher cost to the customer, and possibly a loss of a client to the vendor.

As an example, I had a new (just left my orientation), contracted inspector provided by a noted international inspection agency, literally jump out of my truck, rush up to a fitter who had a SS elbow clamped in a cast iron/steel jawed vise. He made a heck of a scene and rejected the fitting because of not having brass jaws in the vise and was in the process of rejecting the entire project. He’d never seen the project’s specs, he just assumed they were the same as some other company’s he’d worked for sometime in the past!

Regardless of what we as construction participants think is right or wrong, specifications are specifications and the documented history of a company’s specific operations speak volumes of what works and when it doesn’t!

By BP Maas (**)

Date 02-14-2002 16:51

It would seem to me the first item to try and elminate or minimize is the undercutting etc. Clean up would be faster and easier and the wire brushes should be able to remove the heat tint and slag from the surface in between passes. There are companies that manufacture stainless steel tools i.e. chipping hammers, descalers, cold chisels etc.
The price will probably give you sticker shock though, it is one way to passify the inspectors concerns. I am confident the "tramp iron" introduced by the hacksaw blade will not affect the performance of the vessel, as long as you pickle or electro-polish before putting into service.
You may want to do a simple ferroxyl test to confirm "tramp iron" has been removed.
Wire brushes: I have done extensive salt spray testing on the effect of stainless steel powered wire brushing on structural stainless profiles 304/304L.
Quick summary, type 302 and 304 are poison unless you pickle or electro-polish after fab and welding, a coupon cleaned with 316L brushes
will pass a 24hr salt spray test as long as the base metal is not contaminated with iron. Just a little tip to consider.

Best regards, Brad

By TimGary (****)

Date 02-14-2002 20:24

The folks at MECO in New Orleans who make 317L SS Water Purifiers for the Pharmaceutical Industry are fanatical about preventing Carbide Precipitation (contaminating SS wit CS). They go through outrageous steps to assure that no CS comes into contact with their material all the way from the mill to the customer, even though they mechanically polish, electroplolish, and passivate.
Anyway, I was just wondering which process you are using on your vessels. It sounds like slag is a problem. Surely you're not stick welding as SS SMAW slag is almost impossible to remove...you must be usin Flux-Core. I suggest doing a little bit of operator training to make sure that your Welders understand how to set and operate their machines to their advantage. Also, is you joint design built for efficient clean up? Welds buried down deep with no room to get at are always a problem. FCAW will flow pretty good so that the slag peels off very easily if some prior planning/effort is done. The best way to remove slag from undercut/overlap is to not put it there to begin with. I don't mean to sound like a smart ass, but it's true. Have you considered GMAW or GTAW? You may find that these cleaner processes will save you time in the long run.
Good Luck!

By BP Maas (**)

Date 02-14-2002 21:27

I just thought it might be good to clarify the difference between "carbide precipitation" and "carbon steel contamination".
Carbide precipitation is a "time at temperature phenomena", from 850F to 1500F, I don't have the book out so these temps may be a little rough, carbon will combine with chrome and precipitate as chromium carbide, this leaves a less corrosion resistent area adjacent to the weld zone. The only way to remove this is by heat treatment. This is why when putting equipment into service as welded you opt for ELC .03% or less carbon. It dosen't eliminate the problem, but minimizes it. Since it is a time at temperature phenomena, the thicker the material the more succeptable to chromium carbide precipitation, this is why people who weld sheet, 7 guage and under don't have much of a problem, usually.

Carbon steel contamination comes from abrasion, impact or any similar action, of this type material, contacting stainless steel. It leaves iron rich deposits on the surface which will rust and promote pitting corrosion in a lot of circumstances. Chemical removal or electro-polishing are the best ways to remove the damage. Most commonly used forms of abrasive blasting will not get all of it off. This a short explanation of a deep subject, hope it helps...Brad

By RonG

Date 02-14-2002 21:28

I find this all very interresting to say the least.

In my line of work we weld a very large variety of materials and one thing we do a lot of is inlay 309 in areas that come in to hard contact with CS for years.
These inlays and SS strips serve as seating surfaces sometimes with a gasket sometimes without, also as aligning (crush pads).

These items come back to us on a regular basis (about 3 years with out a wreck) and I have never notice any damage like described here.

Can it be because of elevated operating temperatures (any where from 300 F up to 1000 F)

2 reasons for using the 309 is no PWHT and erosion resistance.

By BP Maas (**)

Date 02-15-2002 19:14

Don't know what the application is, but pitting usually occurs in the presence of chlorine and chlorides, even with ideal surface condition these will rapidly cause pitting when the level of their presence is out of 304/304L stainless steels performance range. At the temps you refer to SCC stress corrosion cracking is another concern. Main point is, there are different corrosion mechanisms; and contamination in whatever form, will accelerate the corrosion. I'm guessing you have surfaces that are rusty in appearance, the ones with direct contact, if not it must be really dry where the equipment is. One last thing, the higher the chrome/nickel level the more resistent to corrosion mechanisms, in most cases. I've tried generalize because corrosion problems, or lack of, have many variables...Brad

By Niekie3 (***)

Date 02-15-2002 20:56

There are many reasons to potentially use any particular material. In the case of your 309 application I guess it has more to do with its performance as an erosion resistant material and ease of application. It does not sound like a highly corrosive environment.

We need to keep in mind that for corrosion to occur, one needs an electrolyte. Without an electrolyte, no ion / anion transfer occurs which means no corrosion. (Of the chemical type. - Not talking about oxidation etc.) At elevated temperatures, there is often a lack of electrolyte. The contamination then becomes irrelevant.

Just an additional comment to this whole discussion. I have often seen the "rusting" of contaminated SS when exposed to plain old rain water. This just shows you how serious the iron contamination issue can be.

Hope this sheds some light on your circumstances.

Regards
Niekie Jooste

By roybuck (*)

Date 02-15-2002 16:34

Whenever I see stainless with carbon steel weld spatter (from welding carbon too close to it without protecting it), I ask that it be cleaned off. A stainless wire wheel, a flapper, or some emery cloth will do the trick, at least to visual standards. In some cases, I will notice a blush of rust on and around a ss weld where apparently someone used a CS brush after welding. I always ask that this be removed also.
I feel like this is a good idea; without addressing pressure, temperature, or service medium. Some of these SS pipes are fairly thin wall anyway, and any corrosion or pitting is bound to lessen the life of the pipe.