cracked header pipe ???

What would cause a cracked pipe for the head to the muffler? it spider cracked making it very hard for me to try to weld.

does any on have an old one laying around ?
the cart is an 1989 244 pg3
thats the thing , i just had the muffler off and i checked the pipe it is tight and still is tight on the jug and the muffles connection is tight also it is not just a crack [straight line] it spider webed and it is about 6" with a series of spider cracks comming off of it .

I thought i had it to tight and it would not flex, so i dont know will try to get a picture of it tonight .

rusty s