George, the numbers are for the pipe itself. The thread would obviously weaken the pipe, but a properly made up screwed joint will have the male thread pretty much buried within the female thread with the exception of the last couple threads which aren't cut deep due to the taper on the pitch line. The portion of the pipe within the fitting is reinforced by the fitting itself. Very little screwed pipe is run in sizes above 2", and the smaller sizes have the greatest margin of safety, so the minor weakening in these sizes shouldn't be a big concern. The majority of pipe sold today meets A-53 Grade B, so the 15,000 psi safe working stress is a good number. Of course, one should always pay attention to the markings on the pipe itself to verify the spec, particularly in higher pressure applications.
Run of the mill CI and malleable fittings are rated at 150 psi. 300 psi fittings are also readily available. I mentioned forged steel fittings earlier because they're available in 2000 and 3000psi ratings and the quality on them is consistently high as compared to CI and malleable fittings. They're available both in screwed pattern and socket weld pattern. The fitting itself is identical in both patterns with the only difference being one is threaded and the other is bored such that the pipe will slide into it and make a place for a fillet weld. 3/4 pipe can easily be gas welded, and probably the cheapest high quality installation would be achieved by using plain end pipe and butt weld fittings, thereby staying away from any concerns with reduced wall due to thread cutting. (It would also be good because I despise running screwed pipe
) We had a guy who worked for us for a long time when we still did contracting work who could run 1/2 and 3/4 pipe for air lines and such, and form the bends and offsets as he went by heating and bending the pipe. About the only places he would have a weld was at the end of a joint of pipe or wherever he had to put in a tee. It made a real nice looking installation. Although he was an excellent pipe welder using stick, he always did this small stuff with oxy-acetylene, because he said it was too hard to keep from running slag into a position weld on pipe with such small diameters when running a stick. TIGing the joints gives the ultimate in appearance, but is too slow and therefore too costly for this type work.
Copper would also work fine for air @ 400psi, as long as the joints are made with a hard solder such as sil-phos. The condenser side of HVAC systems often run at pressures in this range and at elevated temperatures, all done in hard soldered copper, and subject to loads of vibration. A copper system costs more for material up front, but is far faster to install and stays much cleaner over time. The downside to the copper is that it can't stand the physical beating that steel pipe can take in a lot of industrial applications.