The answer is, fittings at Lowes aren't compatible with 1 800 psi. AT ALL. I used 1/4" malleable iron fittings at 2 000 psi when I was testing my pressure washer for leaks(because the hydraulic supply was out of stock), but I was scared sh*tless while doing so, and I wouldn't recommend it, because you're getting dangerously close to the burst pressure of 3 000 or so.
I assumed that since you were doing welding, the fittings would be made out of plate with tapped holes, or pipes welded onto plates for male threads. These types of improvised fittings can be made ridiculously strong for almost no extra cost (like the 1.75" thick solid steel plate that forms the manifold for my hybrid's ignition and fueling). Lots of easily available fittings are rated for 3k or 6kpsi and available at McMaster, but plates with tapped threads are cheaper.
Standard malleable iron fittings (rated for 150 psi) are reasonably safe to use at 500 psi, and _Fnord has tested them to 11x hybrid mixes. The fittings are poorly made, but thicker than the pipe.
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