Need some help, cracked 74 Husky CR450 pipe

[Gippslander]

6 years and the pipe's been good, no problems, do the annual checkover -- the exhaust has always leaked a bit at the engine -- so decided to do the job properly -- got some high temp silastic and fitted the exhaust up with a nice neat bead on the so as the pipe goes on it seals but does not get inside the pipe, all goes well.

Start the bike up yesterday, run it for 1 minute only, maybe less just to make sure it runs before going for a ride, starts and runs ok, but, after about 30 seconds starts to sound a little different, not sure but it's loud and I have got muffs on, anyway, have a look at the bike tonight, just sittin' and lookin' and see the line on the exhaust -- cracked both sides wtf....?

Easy to weld it up with the oxy, will do it in the morning -- but cracked in less than a minute not even riding???

Was it waiting to happen and the new sealant allowing almost no movement compared to the old loose and leaky fit put it past it's limit?

any ideas?

Thanks

[Gippslander]

Better put a picture of the bike up 

[PEZBerq]

Most likely just a fatigue crack that started at the toe of the weld for the spring tab and has been growing for several years until you noticed it. I assume the pipe is not paper thin in that area? If not then a weld repair should last a lifetime of vintage racing. Not bad for a 45 year exhaust - good old Swedish steel 

[bigk]

Drill each end of the crack before welding it.
K

[Gippslander]

yep did a small drill hole on each end then when welding when it got red hot other cracks appeared out of the "red", were sort of parallel, made the repair messy but all done, then put the heating tip on and got the whole header pipe red hot to try and relieve it a bit, looked for other cracks as well.
the steel was still good and thick no thin spots
there was a lot of contaminants in the weld made it hard to get a smooth job but then I looked at the original welds, not so smooth either so may be the steel
was using a chrome molly  filler rod so that may have been a problem, don't know about that stuff   

[GMC]

was using a chrome molly  filler rod so that may have been a problem, don't know about that stuff   

Should never have used Cro-Mo filler rod on an exhaust, it would have made the area brittle
It will produce a stronger weld but it would only be beneficial on 4130 and would need to be normalized after welding