Why is 312 a worse choice than 880, if the composition is almost the same?
It seems like the majority of 4130 failures are at the edge of the haz due to fatigue, so I’m trying to understand how the filler material choice affects the failure mode. For a bike frames failure mode, why would 70s2, 70s6, 880, 312, or 309 result in a different failure if all else is equal?
The tricky thing is we have no idea exactly what Weldmold 880 is. They don’t give us the chemical composition of it. To make it more confusing, I’ve heard its closer to 309. But the source I heard that from doesn’t really have any evidence to back that up. 
To be pedantic (or at least more specific) most failures I’ve seen start at the toe of the weld. Any small discontinuity in the boundary between the weld, and base metal will create a stress riser. And the toe of the weld is one of the worst spots to have discontinuities for fillet welds.
That can include things like undercut, overlap, or even internally, like carbide precipitation.
The reason I’ll always recommend more ductile weld filler is it will help prevent a small discontinuity in the weld from turning into a failure.
Extreme example, but think of it like a piece of glass. Glass is super strong, but one tiny crack and the whole thing explodes.
Something softer can take some abuse before failing.
In the case of 4130 to itself, having a undermatched (but similar) filler like 70s-2 is ideal. The filler, and base metal mix together and dilute at the boundary of weld metal to base metal.
Using a stainless variant for 4130 to itself is adding unnecessary chromium to the mix. Chromium mixing with the carbon from 4130 is how carbide precipitation can occur.
But in the case of 4130 to Stainless, we need the filler to sort-of match both metals. So we just have to do the best we can, because it’s never ideal welding those two together.
The discussion about the filler to use seems to be quite the can of worms to dive into.
Don’t really have anything to contribute other than there seems to be a lot of “historic” choices that don’t really make sense in a metallurgic way. To me it really is quite confusing as I myself do not have a lot of experience to go by, and all my eingineering textbooks say the historic choice is wrong…
The only thing that I’m very certain is that whenever you weld stainless (=very low carbon but lots of other stuff including nickel and chromium steel usually) to 4130/mild steel/something with a decent amount of carbon, you should use a filler material that has a lower carbon content than the stainless. Otherwise the carbon from the 4130 diffuses through the puddle over to the stainless and leads to intercristalline carbide and martensitic structures, which are very brittle.
In German welding terminology this joint is called a “black-white-joint” (lots of C to not so much C), not sure if this name exists in English as well. General recommendation for a filler rod in that case would be E309
This morning, I needed to answer a customer’s question and found my answer here! Thank you for this thread!
On the topic of amperage, what’s the amperage when joining a thicker metal to a thinner one. For example bb to dt joint. From what I read, dial it to the thicker material, and also aim more toward the thicker material. Can someone please go into a little more detail about the process, torch angle, etc. Thanks.
Besides the excellent description Ben shared at the beginning of this topic, Kristofer Henry of 44 Bikes has written an excellent description on how he tig welds his bikes. I’ve read it atleast 100 times and get something new every time.
This is great , thanks for sharing
You’re correct with that (at least how I do it). Dial up the amperage more towards the thicker piece, and keep the tungsten angle pointed mostly to the thicker piece as well.
I like to think of it as creating the puddle on the thicker piece, and letting it grow until the edge of it meets the thinner member. Typically joining the two into the puddle without melting the thinner tube away is the hardest part. Once you have a weld or tack to start on, continuing the weld is much easier.
Hi and thanks for this fantastic information. What would you recommend for finish amps and down slope time if tig welding without a foot pedal? For example, if peak amps are 50A, would something like 10A finish over a 2 second slope be a suitable end setting? Or would you cool over a longer duration? Thanks.
Generally speaking, you want to taper off nice and slowly. So a 2-second down slope time would likely be just fine, but it also doesn’t hurt to increase that time even longer.
I would have to guess my down slope time is in the 3 to 4 second range just from habit. Longer at higher amps.