Is it allowed by standard to weld two 45 degrees elbows to get a 90 degrees elbow? and whether the fabricated elbow can withstand pressure and temperature?
Yes it's allowed, or rather isn't prohibited. You might not have the same dimensions as a single 90 elbow and it will look a little bit odd, but if that's all you have then OK.
Why can't you just buy a 90 elbow??
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