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Stainless to Carbon Steel joining?


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I found some stuff on the internet that said for WELDING austentic stainless to carbon steel, basically that you have to:

 

1) Use an ARGON mix with less than 5% CO2

2) Clean the metals with Acetone or alcohol

3) Remove the heat tint with something like a nitric-hydroflouric acid to keep weld corrosion at bay.

 

I'm considering silver brazing the two together as well.

 

The project is to install a stainless fuel trap into a stock steel fuel tank. I won't be able to get at the back of the joint when completed, so removing the heat tint from welding or the flux residue from brazing are an issue.

 

My current thought is to braze with a good silver brazing rod, and use a water based flux that is compatible with silver brazing material to be able to wash the back side of the brazing joint by pouring acetone into the tank and sloshing it around to remove the flux.

 

If I could find a suitable adhesive/caulk, I'd consider gluing it in (probably use a few sheet metal screws as well).

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You can weld 300 series stainless steel to mild or medium carbon steel. Its not that difficult and just requires the proper filler selection and process. For thin wall (under .125" thick) sheet use 308L filler and get a lot of the filler into the weld pool. For thicker sections you first "butter" the mild steel side of the weld using a 309L filler and then weld as normal with a 308L filler. Use pure Argon as the sheilding gas and purge or use a flux on backside of the weld.

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The project is to install a stainless fuel trap into a stock steel fuel tank. I won't be able to get at the back of the joint when completed, so removing the heat tint from welding or the flux residue from brazing are an issue.

 

Then purge the tank with Argon before welding. Remeber, Argon is heavier then air so as you're purging leave an opening on top to let air out. Tape off all the openings except where you're welding, make sure the welding area is at the top, and run the purge for about 1/2 hour at 15cfm through a well sealed side opening. Continue running the purge and start welding.

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When Matman and I were moving all of those Z parts we had a gigantic metal rack for holding Z doors fail in the back of the truck. Luckily, it decided to fail when there were no doors on it. The guy we bought the parts from had a welder and let me use it to weld it back together. He also had some scrap metal under his welding table. I just grabbed a couple pieces to make diagonal braces, and I guess a couple that I grabbed were stainless. Welded right onto the mild steel with his 220V Miller welder. I didn't think you could do that, but it worked fine and the rack held up through the second trip, despite being loaded with parts again. I honestly couldn't tell the difference between welding the mild steel to the mild steel vs the stainless to mild steel. Of course I wasn't going for finesse either. Just slapped them on, didn't even take the paint off of the rack. Anyway, not telling you HOW to do it, just that I was surprised that it could be done.

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Stainelss steel has a 50% greater coefficient of thermal expansion then carbon steel. This trait and the creation of low melting metals (sulpher, phosphorus) in the weld pool can lead to hot cracking as the weld cools and shrinkage stresses build. For most of what we do that's not a problem because of the thin sections we tend to use. For thicker sections and parts that go through heat cycles hot cracking is an issue. Welding a mild steel turbo flange to a stainless steel turbo header will guarantee a failure unless the proper filler is used (308 or 309L).

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