you built someting out of 4 inch square tube and pressurized it with a flat end on it?..ok that flat end was only 4 inches square now take that flat end and put it on a 36 inch pipe. that pressure is now trying to equalize over a much bigger and weaker area, you are looking for trouble. i will give you the benifit of the doubt (because i don't know you) and say your welds are good and strong enough to pull the ash can right up from hell but if that big flat surface starts to yeild and push outwards in the center, your weld on the flat edge could very well tear from teh direction the metal is going to try and go no matter if it was 3 passes or ten. you ever look at a blank flange on a pipe (you know, the flat end that bolts on) they are usually 4 times thicker than the pipe itself, there is a reason.
take a look at one of them portable air tanks, they are safe to use at over 100 pounds PSI (and likely tested at 5 times that) and are about 1/8 of an inch thick...now you build one the same size and thickness and weld in flat ends, fill it with water and then add air and see how many psi you get in it before the ends buckle or you have complete failure...i would bet (hell i know!) it will be way before 100 psi
flat ends on a pressure vessel are a NO NO and any OWB being tested should be tested with air over water