Multi layer arc weld

[Drains]

the weld is strong and the pipe is strong but the Heat Affected Zone in between is weakened.

Exactly c/m. So concentrating on faults in the weld is not the point here. I re-iterate, a weld on a silencer doesn't break, it's the HAZ in the pipe which breaks, as the weld is still stronger than everything else. In this case, on George's bike, the weld will be stronger than the fork tube. That will be far more likely to break, at the edge of the weld!

In this case, ironically, the more weld that goes on, the greater the HAZ, and the more chance of a break.

George, I wouldn't put any more weld on if I were you.
All the best
Si

 

[GeorgeB]

This is interesting. I have some spare bicycle tubing. If I weld it up, with plenty of heat, in a similar fashion to the forks, is there are simple test I can carry out to assess any HAZ type impact? Is there a HAZ impact for almost all welding on steel? e.g. if you weld a new panel into a car, does the surrounding steel get degraded to some extent?

 

[Sparkey]

This is interesting. I have some spare bicycle tubing. If I weld it up, with plenty of heat, in a similar fashion to the forks, is there are simple test I can carry out to assess any HAZ type impact? Is there a HAZ impact for almost all welding on steel? e.g. if you weld a new panel into a car, does the surrounding steel get degraded to some extent?

Ofcourse it does Goerge, that's why it's called Heat Affected Zone
In the carpart industry, we are tested all the time...the welds need to be strong enough but welded as cold as possible...(on the lowest setting possible) just to prevent further weakening of the metal...
Which ofcourse is crucial for the parts that have loads of safety rules (brake system, pedals etc)

 

[GeorgeB]

Ofcourse it does Goerge, that's why it's called Heat Affected Zone
In the carpart industry, we are tested all the time...the welds need to be strong enough but welded as cold as possible...(on the lowest setting possible) just to prevent further weakening of the metal...
Which ofcourse is crucial for the parts that have loads of safety rules (brake system, pedals etc)

Thank you for this info. It makes sense but I've never seen it raised as much of a factor on this web site before, i.e. as a downside of welding anything structural. I don't know the type of steel used for my bicycle tubing but is it the case that most bicycle tubing is made of a steel type not well-suited for welding?

 

[oilcooled :)]

i think you have too much weld in george, less is more . my granfather who thought me to weld, used to work as a ironworker on the skyscrapers in chigago, who was coded in their union. anyway he always told me when i was strenghtening something "the strongest point is the weakest point" when you think about it makes sence, the welds you layed are the strongest point and everything above and below is weaker, hence to much weld

 

[Tino]

What a bicycle is made from will be reflected in its price to some extent, the more costly the more exotic the materials. That isn't to say that high strength steels will not be used at all on even fairly lo-cost bikes. There is no way to tell what its made from with out having it analysed. Brazing is often used on high strength tube to get round the HAZ problem (Harris motorcycle frames were bronze welded for this reason) but again that doesn't mean that mild steel isn't brazed. As for the HAZ not being raised as much of a factor, that is because most of the time it isn't. It becomes more critical as joints are stressed more, and more exotic materials are used. It's also more of a consideration on thin material because proportionally the HAZ will affect a greater percentage of the material welded.

 

[cookie munsta]

Thank you for this info. It makes sense but I've never seen it raised as much of a factor on this web site before, i.e. as a downside of welding anything structural. I don't know the type of steel used for my bicycle tubing but is it the case that most bicycle tubing is made of a steel type not well-suited for welding?

structural is not that important though, say for instance you want to weld a 10mm cleat to universal beam, than it is important to get good peno, minimise undercut, over roll, and weld faults. after that if let it cool down on its own it should be fine, but once the steel is alloyed or heat treated/ hardened a little extra effort may be required, and different electrodes could be looked at. most bike tube I've heard of is higher tensile in order to get more strength with less weight this just means they change the metal compisition or carbon equivelent and use heat treating methods to make stronger. some other bikes are aluminium etc. each 1m square 1mm thick of steel weighs around 7.5 kg so it can save a fair bit of weight.

 

[GeorgeB]

I take your concerns seriously about the HAZ but I used a welding approach not dissimilar to the approach suggested here, and elsewhere, for the MIG welding of thin sheet, i.e. a bit at a time, to avoid heat build up, as in blow through or panel warping. The heat build up on that fork tube must have been low from my welding because there was never any sign of the original braze melting away. Neither did I blow through the tube with the low-ish 80A. I suggest the heat must have been far higher in the bicycle factory, in order to complete the braze when the drop plates were first inserted. My guess of the threat level and likelihood of the forks failing, is low, but worth monitoring.

 

[Drains]

I suggest the heat must have been far higher in the bicycle factory, in order to complete the braze when the drop plates were first inserted. .

Doubtful George, as the melting point of steel is around 600 degrees C higher than the melting point of brass...

 

[GeorgeB]

Doubtful George, as the melting point of steel is around 600 degrees C higher than the melting point of brass...

I can't be sure but I assumed that the effect on the surrounduing steel was probably minimal from my short busts of welding, purely because there was ZERO sign of any of the brass melting. Whereas, when the brass was brazed in at the factory, the forks would have been subjected to relatively prolonged heating of the entire joint, to get the brass to flow in.

If the area became dangerously hot with my welding, how come the brass didn't melt or show any sign of change?

 

[Sid]

Put simply George, mild steel is not a steel that can be hardened with heat treatment. Heat treating steels involves you heating them up so the internal crystal structure changes, and then cooling them quickly. Mild steel doesn't care about this cycle, and doesn't harden. As most of us 'play' with mild steel all of the time, we do not need to worry about pre or post heat treatments when welding.

High(er) carbon steels, or high(er) carbon equivalent steels (ie some alloy steels) will harden when you do this. A hard and brittle structure called Martensite is formed. If you weld such a steel it will have melted, and so would be in this upper temperature crystal state when it solidified, and it will have hardened when it cooled. It's not a question of whether you weld in short bursts or prolonged bursts, what matters is that the crystal structure will have changed. If the bike frame is a hardenable steel, you are likely to have a brittle join.

You could possibly de-stress the join by tempering it.

Without knowing what the tube actually is, you could be taking a risk, this is what you are being warned of.

If you have more tube to test, weld it up and when cool bash it with a hammer. If ductile it will bend and deform, if brittle it will break (I imagine). It's your risk and your call to make, or put another way, I'm not taking any responsibility for the validity of this test.

Try looking up the heat treatment of steels. It's a very interesting subject.

 

[GeorgeB]

Sid - many thanks for a particularly helpful and interesting post. I'll set up a test piece and now have some idea what to look out for.

 

[Dogbreath23]

you could do a spark test to see what kind of steel you're using George. Take a piece of the tube your using and a piece of mild steel and take your grinder to them and compare the colour and intensity of the sparks I think carbon steels have larger brighter sparks.look into it anyhow(Im sure you will!