Welding of Duct Reducer to Flange

Assuming that by carbon steel you are referring to low carbon steel or mild steel, and also that the stainless is probably 304 or similar; then 'yes' welding mild steel to austenitic stainless is not difficult. It is generally well understood and pretty common.

Tell us more about the 'spool' material intervening?

We don't know what grades of steel you are welding, so expert advice is impossible.

As stated in post #1, I have welded mild steel to 304 using mild steel wire with no rust, outdoors after 15 years.

Are you talking about something like this?

Flexible DUCT CONNECTOR. Minimize Duct noise and vibration.

Use 309 filler rod when welding.

Probably has to do with cathodic reaction, a large stainless steel surface paired with a small galvanized(ms) surface sets up a possible cathodic reaction causing the galvanized steel to corrode....especially if you are in a wet or salt air environment....

https://galvanizeit.org/education-and-resources/resources/technical-faq-dr-galv/stainless-steel-in-contact-with-galvanized-steel

Pardon my confusion but, mild (carbon) steel is NOT galvanized and the word "galvanized" has escaped me in this discussion. Did I miss something?

Also, as I mentioned earlier, from personal experience, joining stainless steel and mild steel does NOT require 309 or necessarily any other grade of filler to effect a reliable weld. It will certainly be good practice, but not absolutely necessarily, depending on the environment, which we do not know.

Shouldn't weld on galvanized anyway. Any portion likely to experience considerable heat from welding must have all galvanuzation removed to aboid crap welds and metal fume fever.

309 is definitely the filler rod to use for mild steel to 300 series stainless. Other fillers often lead to welds susceptable to cracks. As far as corrosion goes, the mild steel base material will rust if precautions aren't taken.

If a weld made joining mild steel and 304 using some e70 filler hasn't rusted, it is likely that enough alloying elements difffused into the pool while welding to confer some corrosion resistance. Depleting alloy content isn't without drawbacks, even if none were evident in a particular case.

Any steel duct for air movement I've ever seen is galvanized....except for stainless....

I'm simply going by the information made available by the OP.

Said the person who never had to grind out and reweld with 309L filler.

If there’s not enough information, which you admit to... it smarter to be either be conservative or close the yapper.

here’s some info to reinforce using 309L for filler...

Do you mean galvanic reaction instead?

If the weld is performed with Monel filler, then no transition piece is needed. The Monel becomes the transition. Typical practice is to butter the carbon steel surface with Monel and then perform the Monel to stainless joint with Monel as well. The only issue is whether galvanic corrosion is an issue, which should not be a problem in what would be the exterior of an air duct. The flanges can be painted.

"... Typical practice is to butter the carbon steel surface with Monel and then perform the Monel to stainless joint with Monel as well...."

Typical? Really and truly? Typical for whom?

It's called a bimetal joint or a transition weld. It is commonly used by the US Navy in seawater and freshwater systems as well as in other industries. It is also used as corrosion resistant cladding on carbon steel parts.

Monel has some interesting properties. Due to the high nickel content, 70% Ni, 30% Cu, it is metallurgically compatible with carbon steels, stainless steels most bronzes and most cast iron varieties. In cast iron, for instance, it has a higher surface bond strength and weld tensile strength that is higher than the cast iron base metal. Useable processes include stick electric arc welding, MIG, TIG and oxyacetylene processes. In cast iron repairs, it works best with oxyacetylene and used as a brazing operation due to reduced residual stresses and reduced thermal shock. It has the benefit of being relatively easy to machine.

You are describing (at least some of it) are well known traits/uses for monel, with which I am familiar.

However, what you described earlier, namely that it was typical to join stainless to (presumably low) carbon steel by first buttering on monel and using it as a transition, is a departure from that with which I am familiar (I suspect others who have worked with monel would also be scratching their heads). It is also a departure from any proceedure I can find doing a quick search.

As a note:

-monel can be highly corrosions resistant, but it does suffer electrolytic corrosions especially in contact with carbon steel.

-monel alloys are notoriously unfriendly to machining. There are a couple highly specialized alloys that are somewhat easier to machine...typically via inclusion of some percentage of sulfur. Most Monel alloys rapidly work harden to a significant degree, which is problematic for maching, as turning something on a lathe goes pretty slowly if you have to anneal after every pass.

300 series stainless is readily weldable to low carbon steel using 309L filler in just one pass. Why would 'buttering' a layer of expensive monel down before actually joining be advantageous except in rare (atypical) scenarios?

Not sure what industry you work in, but the practices I am familiar with were in common usage at Newport News Shipbuilding, Electric Boat, and pretty much all naval shipyards in seawater systems and in transition welds into nuclear plant systems.

Note, in a carbon steel and Monel joint in seawater, the carbon steel is the part that corrodes. The nickel and copper in Monel have higher electronegativity than carbon steel. The carbon steel is anodic and acts as the sacrificial element in a galvanic cell.

In machining Monel, sharp tools are a must. Carbide tooling will usually work with no issues as long as the tooling is sharp and you maintain an adequate depth of cut. I have never had to anneal a Monel workpiece, but then I avoid dull, high speed steel tooling when machining it.

It isn't about the tool not being the proper shape, it is about the piece work hardening. Don't take my word for it. Machinability ratings are available comparing a wide range of metals.

I am open to learning new things and ak curious about what advantages for joining carbon steel to stainless might be worth the extra cost of using monel in the way you describe. If you could poiint me to a proceedure that details such it would be appreciated.

By the way, the 309L weld you suggested is an excellent and cost effective approach to attaching the flanges to the stainless ducting. if corrosion in the area is an issue, the flanges should probably be painted or coated after the weld to keep them in good shape.

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