Silver solder for SS wire?

[TomP]

Douglas,

First off, you are correct...GTAW is gas tungsten arc welding...better known as TIG.
You are also correct that temperatures as low as 700F can, and do, have an effect on the corrosion resistance capability.
Insofaras using clay dams to limit the heat affected area...no. Clay is a natural insulator, and therefor would not proived a "limiter" to the heat affected zone (HAZ). TIG, due to its very nature, is a precise weld. A skilled TIG welder should produce no more than a 1/16 to an 1/8 of an inch on either side of the weldment itself. Thus limiting the HAZ. There are commercially available "heat pastes" for delicate welding. For the most part, they are an amalgam of tin and aluminum powder suspended in a base...designed to act as a heat sink. I do not have first hand experience with these products, but some welders I know use them. Though I personally feel it is to hide their own short comings as a welder and their failure to control the HAZ.

I co-authored a paper on this...it is quite long, so I will just elaborate on the specifics to this conversation:
True, heat does play into the corrosion resistance of materials. However, it is not necessarily the heat that is the culprit in reduced corrosion resistance, but the process in which it was heated. In this case, let us focus on fabrication, i.e soldering and welding. During fabrication, metal products often come in contact with steel components and tools. Transportation, handling, forming, grinding and welding can all result in physical contact with iron-based structures. During such contact, iron may become embedded into the surface of an alloy component. When stainless steels (e.g. austenitic grades such as AISI 316, 317 and 904L) and superaustenitic grades (e.g. INCOLOY alloy 25-6MO) are
exposed to aggressive acid and acid-halide environments, the embedded iron can accelerate localized attack such as crevice and/or pitting corrosion. Embedded iron has not been shown to affect the corrosion resistance of higher alloyed materials, such
as INCONEL alloys 622, 625, C-276 and 686.

So what can be done? One word: Passivation. Passivation is the formation of a thin, protective film on a metal which makes the metal passive (corrosion-resistant).
While stainless steels require an acid treatment to passivate their surface, nickel and the high-nickel alloys become passive upon exposure to air. The film itself is a chromium-nickel-iron-molybdenum oxide for the chromium-bearing nickel alloys. Chromium is the key constituent giving the film its outstanding corrosion resistance. The film formed in air at the mill is stable. However, during fabrication, the film may be damaged locally when
iron, weld spatter, arc strikes and heat tint scale create local defect sites (i.e. local imperfections in an otherwise passive film). The undamaged surface remains passive
throughout fabrication. Removal of defects, by pickling, electropolishing, or mechanical means, immediately restores the film and passivity so passivating by acid washing is not required. Nitric acid cleaning/passivation treatments for stainless steels are described in ASTM A380. It should be noted that these treatments are not suitable for cleaning structures after fabrication. They are applicable only to passivating chromium-nickel stainless steels and are not applicable for cleaning nickel alloys after fabrication. As already stated, such treatments are not required to optimize the corrosion resistance of the nickel-based alloys.

Whichard made a product called Whichinox (sp) and marketed it as a passivation paste. It essentially was a mild phosphoric/nitric acid pasted with some mild abrasives added to the emulsion. It worked quite well, and from a mettalurgical standpoint, it did, to a degree, passivate the metal.

I hope this was not too much of an overload.

Tom

[Douglas]

Tom , that information you just described, tells me that you are knowledgable about ss and monel fabrication.

There is a piece of information on welding ss , that I hope you can clarify .

I have been told that when welding ss, some of the chromium and nickle are boiled away, from the molton puddle, vaporized as it were, and that changes the alloy balance percentage to somewhat less than the mil spec ,,,,, ie 18 - 8 , might be less, of a percentage, at the finished weld.

I have been also told that using a filler rod, higher in content (richer) of chrome and nickle, likely will replace that boiled away "lost" chrome and nickle , thus keeping the alloy percentages more near the mill spec.

Is there any truth to these ideas of enrichening a weld ?

Douglas

[TomP]

Originally Posted by Douglas

Tom , that information you just described, tells me that you are knowledgable about ss and monel fabrication.

There is a piece of information on welding ss , that I hope you can clarify .

I have been told that when welding ss, some of the chromium and nickle are boiled away, from the molton puddle, vaporized as it were, and that changes the alloy balance percentage to somewhat less than the mil spec ,,,,, ie 18 - 8 , might be less, of a percentage, at the finished weld.

I have been also told that using a filler rod, higher in content (richer) of chrome and nickle, likely will replace that boiled away "lost" chrome and nickle , thus keeping the alloy percentages more near the mill spec.

Is there any truth to these ideas of enrichening a weld ?

Douglas

Douglas,

It is all about the filler that is used. Some filler rods can cover a wide range of specifications, while other are type specific. Generally speaking, one will want to use a filler rod that is exactly, or as close to, specification of the material being welded. Though this does not always hold completely true. Take 4130 tubing for example: With what I just said, one would think to grab a 4130 filler rod and go happily down the road...but this is not the case. Although there are several good filler materials, ER80S-D2, is one you should consider. This filler material is capable of producing welds that approximate the strength of 4130. ER-70S-2 is an acceptable alternative to ER80S-D2, as is ER70S-6, although the weld strength will be slightly lower.
Now, back to your original question of specification. Yes. The filler material, when diluted with the parent material, will typically undermatch the 4130. However, with the proper joint design (such as cluster or gusset, for example), the cross-sectional area and linear inches of weld can compensate for the reduced weld deposit strength.
There is a slight loss...but the bottom line is in the knowledge base and skill of the welder and the joint design.
Though stainless steel is far more forgiving than welding aluminum with it many different alloys, one still needs to determine the base metal alloy type and select the correct filler alloy.
I will refer you to the American Welding Society (AWS) AWS B2.1-BMG:2005 standard and qualifications for base metal grouping for further study.

Tom

[Douglas]

Hi Tom, Thank You for your reply, and taking the time to share your knowledge with all of us, through the forum.

I agree with everything you have said, especially the parts about stainless steel surface contamination with other metals, like steel and iron, proper weld joint design, and fabrication.

I believe that weld filler rod supply companies, like McKay (sp-?) hand out broshures containing information on use applications and guidelines about their filler rods.

I don't think these rod supply companies ever talk about or reccommend using their rods for enrichening a weld, but maybe there are some welding technical bulletins, that do discuss results using various different alloy filler rods.

You also mentioned nitric acid and other acid ss wash, to passivate contaminated ss after fabrication.

I have successfully used SS Pickling Paste to de-contaminate my standing rigging ss 1 X 19 wire .
I had re-rigged my boat using Loos ss 316 1 X 19 wire, and after 6 months, one single outside strand in the wire, went rusty. I suspected that the wire was dragged through a contaminated scrubber of some sort, then later wound or bundled into 1 X 19 .

I removed each shroud, one at a time, and painted on the pickling paste, then after 20 minutes, rinced the paste off with fresh water,,,,, result was no more rusting.

Brion Toss reccommends during his rigging talks, using a hand held 50 power microscope, to inspect SS rigging and fittings, for cracks and flaws. One day I was using my H/H 50 x scope to inspect pitting corrosion on my ss lifeline stancions. I saw iron-rust colored surface staining around each pit. I used the SS pickling paste on the pits and that removed the rust colored stains.

I think the pickling paste contains a blend of nitric, hydroflouric, and phosphoric (sp-?) acids, in a neutral thickening agent.

Have you had successful results in cleaning contaminated ss surfaces with acids or pickling paste, too ?

Douglas