A friend of mine has a turbo kit, including a manifold, downpipe and exhaust built by a now-non existent fab shop, actually by the guy who taught me to weld, but I didn't know much at the time of the construction. Anyway, some discussion came up about using mild flanges for manifolds/downpipes after I saw it done and I asked him to check his stuff for ***** and giggles. Turns out the flanges AND the welds on the downpipe/manifold are magnetic, both parts are ceramic coated as well. I don't have that much experience with stuff still, so I went out the other day and ran a magnet over a manifold and a couple of downpipes I've made with 304 stainless with both 308 and 309 filler, nothing stuck. What would cause this other than the obvious using carbon steel filler? Would that have anything to do with parts cracking over time and use? Personally I didn't know when I started doing this stuff myself, so I figured it's best to keep similar metals together (flanges, tube, filler) but I've used a bit of 309L filler on 304 when I didn't have 308 available.

 

Most people use mild steel flanges due to the cost difference and less chance of warping that you may see with using stainless flanges. As far as cracking it could be several things... was the manifold packpurged and had efficient enough heat to penetrate? also what wire was used when welding the SS pipes to the mild steel flanges? Any pics of the cracking?

 

What I'm asking about is the wire, I understand that some people use mild flanges.

 

For exhausts and stuff, you will rarely see a problem because of incorrect filler rod. For a manifold it's obviously more important.

I'm not sure what your question is, but if you're asking whether it is possible for the welds to be magnetic, and NOT be regular mild steel filler, then my guess would be not unless it's a 4xx or 5xx series stainless. 409 is often used for exhausts and that is magnetic.

 

You mean magnetic or that a magnet will stick to it (ferous)? there is a difference right?

I can't say I have ever had any steel shavings stick to anything I have welded.

 

This is what may be happening here. Back when I had my shop I would have tons and tons of 304 SS tubing that I would recycle (bad bends and so on). The 1st time I took my stuff in, they checked it with a magnet and it stuck to everything. For the life of me I could not figure it out. After some thinking and testing it turns out that when I would bend the tube, it would get hot at the bend and because of the heat it would become magnetized. So on all my 304 SS bends, the legs where not magnetic, yet the bend area always was.

So maybe it's the same for the welds. The heat made them magnetic.

 

Agtronic:
I know about 409, sorry, I was specifically asking about 304, since I've never seen 409 weld els before, or even 409 filler. I use a lot of 409 for exhausts and sometimes for charge piping when people don't want aluminium. The only reason I use it is simply because I have a local (within a mile) exhaust/repair shop with a mandrel bender that I'm friendly with. They have aluminised and 409 steel, with an occasional piece of 304 here and there. I buy almost all my bends from them for convenience and price since I have a good relationship. Way off topic now Anyway, I've always used 309L filler for 409 tube because I assumed it would be proper... not really sure why it just seemed to make sense like when you use 309 to weld mild to stainless.

Bailhatch:
I meant they are ferrous, the magnet sticks to them, not that they are magnetic themselves.

danielm3:
That's more what I was asking! I'll check some of my bends tomorrow when I'm at the shop. On the manifold it would make sense on the welds I guess if they got hot, but checking my own stuff even the welds on the 304 (with both fillers) it didn't stick. Maybe those welds were a lot hotter? On the manifold the only bends would have been on the weld els.



I guess I'm wondering if I'm doing everything the proper way when I do this stuff. I was never really taught how to make stuff like this, more just how to weld. When I do stainless manifolds, I use 304SS flanges and weld els/pipe, and for the filler I use 309L or 308L, 309 only because that's what I've had since I moved to my shop and bought the minimum at the beginning. Backpurge the root pass, follow it with filler after that, set to max probably 70 amps, that seems to be hot enough to me though.

 

Update! I made a manifold for a Miata yesterday out of 304SS with 308L filler, just checked it and the magnet seems to stick *just slightly* on the welds/HAZ. Maybe it is the heat, did I possibly overheat it?

 

All SS has the capacity to become magnetic - the majority of it is iron. For reasons I can't remember, working (bending) or heating SS can make it magnetic. It has to do with changing the grain structure. Certain SS with less nickel tends to be magnetic regardless.

You'd have to check the charts for your SS alloy that show what it does at different temps. I know mild steel's structure tends to break down around 850c, most SS will do so after 1400c. IIRC, it takes a lot more than 1400c to melt it.

 

I've heard this explained as both die transference and material work hardening. I think you can also make stainless welds magnetic through high carbon precipitation due to no back gas or backing flux. Never really thought to test the precipitation one though.

 

I'm pretty sure his manifold was backpurged, and I know my stuff was for sure.