What pressures are created by black powder?
Pressures are determined by a number of factors. In the case of black powder muzzle loading cannons, shot weight, charge weight, bore diameter, and powder granulation are major factors. For any given bore size, more powder increases pressure, heavier shot weight increases pressure, finer granulation increases pressure. For a given charge weight and granulation, smaller bore size increases pressure. The factors that increase pressure work together so that heavier charge weight and heavier shot weight increase pressure more than just one or the other. I guess what I am trying to convey is that one cannot quote some number as "black powder pressure" for all or even many circumstances. This is the same as smokeless powder cartridge reloads; pressure is not a factor of smokeless powder but a function of all the factors combined. All that being said, I use 20,000 PSI as chamber pressure when doing calculations.
There have been experiments made with heavy wall cylinders plugged at both ends and filled with black powder and touched off. The results of this experiment were reported as the tube acted like a ground bloom (firework) with the fire coming out of the touchhole. With thinner wall tubing (like ordinary water pipe), you would have a pipe bomb with this combination. If you want to understand why you will never see anyone recommending water pipe as cannon material, look up water pipe specifications on the Internet. The strongest is the smallest and it is less than 1000 PSI.
Can anyone be more specific than "strong enough" in describing 1018? What is it's yield strength?
1018 has a yield strength of 57,000 PSI according to at least one source on the Internet. There are many sources and I recommend you look it up yourself and evaluate the source as well as the number. Keep in mind that the numbers are general and any given piece of steel may be stronger or weaker, although it shouldn't be significantly weaker if it really meets the standard for 1018. This uncertainty is one reason why real world engineering projects are designed with safety factors.