I welded two 10" sch 10 stub ends on yesterday where a new valve will be going.  The prep was die checked, my bead was die checked, then this morning, they die checked the cap.  4" from one of the welds going around the pipe was a large crack.  On the opposite side, where weld #2 is, cracks were found about 1/2" from the weld running away from the weld with the pipe.  Nothing was put on the welds to cool, they were left to cool on there own.  No heat was added today prior die check.  It's 316 to 316.  I am using 316 3/32 wire.  My machine is set lower then normal and I kept the heat the same bead and cap.  MY QUESTION:  what would cause this pipe to crack?  Engineers are leading to just cutting back 10' and doing it again.  I don't want my name on it if in the future it happens again.  The 10" line is coming off a reactor.  Only thing I can come up with is Stress corrosion cracking.  They have the whole line U bolted to the structure with no room to breath.  If you look inside, the previous welds are ugly, ugly, I know somebody wouldn't weld like that.  I am just trying to figure out if I didn't something wrong.  My root passes were blue, red and I did purge with plenty argon.