Titanium frame cracked - again

[gxaustin]

I found a longitudinal crack in the chainstay of my Ti bike. I had it welded but after a while the weld has cracked laterally. I think that it's terminal. My question is "Is there any scrap value in a Ti frame?"

[Brucey]

If you like the frame, I would suggest that you get the entire chainstay replaced, by someone who builds Ti bike frames for a living and who knows exactly what they are doing. Chainstays are often made by rolling sheet and longitudinally welding it on many Ti frames. If there was during manufacture or repair any deficit in cleaning or backside shielding the welds will be no good and the Ti will embrittled in that area; also the rolling process sometimes cracks the material anyway, not all Ti is equally ductile. Either way it is probably best to replace the stay.

A stay replacement will allow the parts to be cleaned and prepared properly.

cheers

[gxaustin]

Hi and sorry for late reply - I had to get a replacement and take it on holiday. I love the frame but I dread to think how much repairs would cost. Luckily I found a suitable replacement bike at a very good price and love it too.
The original repair was carried out by a savvy welder and cyclist who ran argon through the chainstay whilst welding under an argon shield - he warned me it could be brittle depending on the alloy. However, I originally bought the frame at a very favourable price and could replace it for about £600 from China. Therefore I shall scrap it - or make a stool as seen on another thread. I can use the group set to upgrade my winter bike so nothing will be wasted.

[Brucey]

Just to clarify, even if the repair weld was done perfectly, you might have been on a hiding to nothing anyway. The reason is that if the original seam weld in the chainstay was at fault, the material all around the original crack location might already have been embrittled.

Titanium, when heated to welding temperatures, dissolves oxygen and nitrogen like nobody's business and that turns it brittle. In extremis the hardness of the Ti is obviously affected, but the material can be rendered useless even when the hardness is not greatly affected.

IIRC there is a graph in Lancaster's book on welding metallurgy that shows that the ductility of Ti is about halved when the hardness has only been increased by 10-15%. To put it into context 10-15% is the typical range of scatter in hardness test readings, so you could have embrittled Ti and not easily spot it.

This was part of my reasoning behind the suggestion of replacing the faulty chainstay wholesale; chances are that the seam welds in the stays were done using different equipment, less skilled welders, maybe even in a different factory.

cheers

[althebike]

I do not know much about titanium frames, but the one thing I thought I knew until this thread, was the fact they last forever.

[Paulatic]

I do not know much about titanium frames, but the one thing I thought I knew until this thread, was the fact they last forever.

I’ve only know two people with Titanium frames. Both of them had frame breaks and replacements.
That’s 100% failure

[gxaustin]

I’ve only know two people with Titanium frames. Both of them had frame breaks and replacements.
That’s 100% failure

Well at least I'm not alone.
I'm not going to risk a repair - which I doubt will be a cheap operation. I have acquired a nice carbon bike which fits me well (it has woman specific geometry with a shorter frame and taller head tube). Er indoors forgave me for buying it since my other bike is U/S.