Welding Tips and Tricks

got a ute here with a broken egr pipe which is probably broken on or next to, the weld like they often do.
it made me wonder about why they break there.

this one has a cast 90 degree fitting on the end, to which a stainless thin wall tube is welded to. the last one broke on the stainless pipe in the HAZ.
can that be rewelded or has the old HAZ stuffed up the stainless so it will just break there again?

its not going to be repaired, i will just block the egr off and take the pipe out. i'm just interested on the cause and if its actually possible to reweld without it breaking again.

You can probably reweld it with something like 309 or 312. If you totally grind out the crack then it’s as good as it ever was. The crack is likely a stress crack from the differing expansion rates of the two metals.