Metallurgy
A Tiny Introduction to the World of Metallurgy
At BTR, we’re always on the hunt for ways to improve our products. We leave no stone unturned, especially when it comes to structural parts…
Before I get into this, I need to clear up a few things; 1. I’m not a metallurgist. I research our application, find out what works and what doesn’t. 2. I apologise for this rambling post…if you get to the end, kudos!
Welding can seem like a dark art; some sort of witchcraft whereby intense heat and stunning hand-eye coordination result in two or more pieces of metal being inextricably joined. The physical act of welding still holds this status for me (Tam), so I turn my attention to the other side; the metals we use.
what we have:
Now, the range of materials we have available for bicycle tubing is fairly small (in the grand scheme of things), so our options there are limited. That’s no bad thing, because the options we do have are pretty stunning really! Also unlimited options would definitely be far too much for me to wrap my head around. So we use selected tubes from Reynolds, Columbus and Dedacciai. These companies have been turning out cycle tubing since before I was crawling, and they’ve reached a very high standard already.
what we need:
The ideal combination of properties for a bicycle frame material are high strength (for obvious reasons), and good ductility (so that the frame bends before it breaks, if/when something goes wrong). So we need a high strength steel, we need to weld it, and we need it to still be strong and ductile when we’re finished. Of course you also want a material with good stiffness and low density, but those properties are fairly constant across all grades of steel.
the problem:
The biggest problem with welding is that when you do it, it makes the metal hot…so hot it melts! Obvious, right? The problem with that, is that heating and cooling the vast majority of high strength steels results in a dramatic change in the physical nature of the steel. In general if you heat steel up to about 600°C and above then cool it slowly (over a number of hours; called annealing), the steel will soften greatly. Annealed steel is soft (weak) and ductile (stretchy, like plasticine). If you heat to over 600°C and then cool it quickly (in a few seconds or less), high strength steel will probably harden; becoming strong, but also brittle.
the solution, part 1:
For tubing manufacturers, developing a steel alloy (that’s a mixture of metallic elements, but mainly iron) which can be worked to produce a high strength but also resist drastic changes from heat is the main focus. Where plain steel is just iron with a pinch of carbon, the steel produced for bicycle frames can include iron, carbon, chromium, molybdenum (these two define the familiar ‘chrom-moly’), nickel, manganese, niobium and silicon, to name a few. Many of these elements are added to attain higher strength, but they also maintain the alloy’s properties after a heat cycle.
So the tubes are sorted – great! But we need to add a little bit extra material when we weld them, otherwise the welds would be thinner than the rest of the tube (which is already surprisingly thin, from the pursuit of lighter bikes). We call this material filler, and it’s added into the molten ‘puddle’ during welding. Since this material is also structural, it’s got to be good stuff too. It also mixes with the molten metal from the tubes in the process, so it mustn’t spoil that. But it’s not as simple as just using the same material as the tube.
the solution, part 2:
For example, if you weld 4130 steel tubing with a 4130 steel filler, the end result will be far too brittle, and likely to fail without warning and fatigue quickly. The widely accepted solution is to use an ‘under matched’ filler, which prevents the resulting weld material from hardening excessively. This is usually just a filler material with more iron and less of the alloying elements (carbon, etc).
But even from the beginning we went further than this; we used a filler material which is ‘triple deoxidised’. Big word there. This filler is called ER70S-2, and is actually designed for welding dirty steel. The ‘triple deoxidised’ bit refers to three alloying elements in the material which have a much stronger attraction to impurities in the molten steel, and basically remove them from the equation. Using this filler on (clean) high strength steels results in the deoxidisers snapping up the elements which would normally cause the steel to harden excessively. Funky.
The end result is welds which are slightly weaker than the tube (show me a weld which isn’t, please!), but have sufficient strength and good ductility.
How can we improve?
To do better than ER70S-2, we had to look beyond mild/rustable (seriously, what is the word for a steel which can rust, unlike stainless???) steels, to the realm of stainless. Stainless steels come in many different grades, much like their rustable cousins, and some of them possess utterly hilarious ductility as well as high strength. Again, selection isn’t simple since the interaction between stainless and mild (I’m using that word from now on) steel in the weld puddle can end horribly (brittle welds, cracks, you name it!).
So we needed a stainless filler which plays nice with other steels. Fillers for welding various base metals are usually referred to as being suitable for ‘dissimilar’ joints. It just so happens that a few of these are readily available, and suit applications from welding cast iron, to mild/stainless welds, welding tool steel, and many things in between. The filler we eventually settled on is 312L. It offers 590MPa yield strength alongside 40% elongation at failure; mental metal. To put that into perspective, ER70S-2 offers 450MPa yield and 28% elongation at failure.
That ‘L’ on the end refers to a low carbon filler, but in a range which still lies within the 312 specification. This just helps us be that little bit more certain that the filler won’t mess up the composition of the base metal.
the reality-check:
Great! So a BTR frame will now be 31% stronger!…Wrong, sadly. The reality of the tubing material’s condition in the Heat Affected Zone (H.A.Z. – That’s the area of the tube which got over 600°C, but never melted and didn’t mix with the filler material) mean that it’s still the weakest link in the chain. But, a couple of other factors allow it to perform slightly better than it would otherwise, so all is not lost! The end result is really a frame which is likely to be only about 2% stronger, but that’s still a number in the + column, so we’ll take it.
Oh, that and the fact that it looks epic under our clear powder coat!
Cheers,
Pretend-metallurgist Tam
24 thoughts on “Metallurgy”
Interesting breakdown of why filler choice isn’t just about matching the tube steel—the point that the heat-affected zone remains the weak link even with a stronger filler is a useful reality check.
fngauge.space
Interesting breakdown of why filler choice isn’t just about matching the base metal—the point about under-matched filler and the heat-affected zone still being the weak link is a useful reality check.
xvideodownloader.best
The detail about using an under-matched, triple-deoxidised filler like ER70S-2 to keep welds ductile is a great explanation of how heat-affected zones end up being the real limiting factor. It’s refreshing to see a builder admit that switching to a 312L stainless filler only nets about 2% overall frame strength rather than overstating the gain.
fngauge.space
Shop clips of a welded joint are hard to make engaging. A still photo becomes a short dance video at https://www.rumpelstiltskinai.art (paid credits, not a free download). The metallurgy point stands on its own: the heat-affected zone, not the filler brand, is what limits the frame.
The compact grille-and-lamp idea in fabrication drawings is the same problem as a reversible mark: it has to read after you flip the part. I generate those 180-degree wordmarks at https://www.ambigramsgenerator.org when a bracket label has to work both ways. The 2% frame note is still the headline for me.
The point about the heat-affected zone staying the weak link, even with a stronger 312L filler, is the part I will remember. I kept a short reading break open beside the notes at https://www.randombibleversegenerator.net . Thanks for putting a number on the joint and not only the filler spec.
Interesting that switching to the stronger 312L filler only translates to roughly a 2% stronger frame, since the heat-affected zone remains the weak link. A good reminder that welding metallurgy is about the whole joint, not just the filler specs.
ragdoll-robot-boxing.cfd
Appreciating the honest reality check here—it’s refreshing to see the claim scaled back to roughly 2% stronger because the heat-affected zone remains the weak link, rather than pretending the filler choice changes everything.
gta browser game
Welds being the weakest link in the heat-affected zone makes sense, since the tubing’s own properties change once it exceeds that 600°C threshold regardless of filler choice. Interesting that ER70S-2’s deoxidizers help limit excessive hardening, and that 312L offers such a big jump in yield and elongation yet only nets around 2% frame strength because the H.A.Z. still governs.
ported2browser.sbs
That was a genuinely interesting read. The explanation of why you use an under-matched filler rather than matching the tube composition makes a lot of sense, and the point about the heat affected zone remaining the weakest link is a good reality check against the headline strength numbers of the filler. The jump from ER70S-2 to 312L, with the ductility figures to back it up, is a nice bit of detail for something most people never think about.
worldtophub.com
Tubing selection and filler choice are two different problems, and it’s rare to see the reasoning laid out so clearly. The detail about under-matched filler and triple deoxidised ER70S-2 is useful, and the honest reality-check on the heat affected zone being the limiting factor is refreshing. The point that a filler upgrade only yields a couple of percent in the final frame is a good reminder that weld metal strength isn’t the same as joint strength.
dressmakersteamgame.wiki
Really interesting breakdown of why filler choice matters as much as the tubing itself. The point about the heat affected zone still being the weakest link even with a stronger filler like 312L is a good reality check — a 2% gain sounds small but it’s honest, and the ER70S-2 deoxidisers explanation was genuinely educational.
needleinahaystack.live
This is a genuinely interesting read. The explanation of why welding high-strength steel is tricky, and why you’d use an under-matched filler like ER70S-2 rather than matching the tube material, is really clear for a “pretend-metallurgist” post. The point about the heat-affected zone still being the weakest link, so all that work only nets about a 2% gain, is a nice reality check that keeps things honest. It’s refreshing to see the reasoning behind material choices laid out like this instead of just marketing claims.
aion2rifttimer.online
This is a great read—the explanation of why you use an under-matched, triple-deoxidised filler and the H.A.Z. reality check makes the metallurgy feel practical rather than theoretical. The 312L vs ER70S-2 comparison with yield and elongation numbers is genuinely useful, and it’s refreshing that you’re upfront that the real-world strength gain is closer to 2% than 31%. Nice to see the honest engineering thinking behind the welds.
enshroudedbuilds.buzz
This is a genuinely informative write-up on the practical side of weld metallurgy. The explanation of why 4130 filler on 4130 tubing goes brittle, and the move to an under-matched triple-deoxidised filler like ER70S-2, is refreshingly honest. The 312L comparison is a nice detail, and the reality-check about the H.A.Z. still being the weak link is the kind of perspective you rarely get.
yeatfortniteskin.buzz
That was a genuinely enjoyable read, and the ER70S-2 triple deoxidised detail was the highlight for me since the reasoning behind using a filler designed for dirty steel on clean tubing was explained clearly. The point about the H.A.Z. still being the weakest link even after upgrading to 312L is a useful reality check, and the roughly 2% strength gain being accepted anyway feels honest rather than overhyped.
pokemonwindsandwavesstartertrio.sbs
This is a genuinely fascinating deep-dive into the metallurgy behind frame building. The point about using an under-matched filler like ER70S-2 to avoid excessive hardening in the weld makes a lot of sense, and the move to 312L stainless filler with 590MPa yield and 40% elongation is impressive. The reality check about the heat-affected zone still being the weakest link is a good reminder that welds are only one part of the equation, and a 2% gain is still a gain worth taking.
startbuildingweb.com
The explanation of under-matched filler and the H.A.Z. being the real weak link makes sense, but going from 450MPa/28% to 590MPa/40% and only netting about 2% on the frame is a useful reality check.
startbuildingweb.com
You did a great job explaining the complexities of selecting filler materials, particularly with the comparison between ER70S-2 and 312L. It’s interesting that 312L offers significantly higher yield strength at 590MPa compared to 450MPa for ER70S-2. However, I wonder how the Heat Affected Zone mitigates some of those benefits, and if there are specific practices you recommend to minimize the effects in that area during the welding process.
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interesting reading. So the ‘L ‘ in your 312 filler, is that the same ‘L’ as is on the end of stainless 316 ‘L’ sheet or bar?
We use it for some of our light fittings that go into chlorinated environments or sea side locations.
Have you ever made a frame from 316?
I guess not due to cost.