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