Dear 02061989, they will actually provide a weld overlay on inside of the top part of the split elbow. The base material is same and of the same thickness. The split was the solution for access to apply this overlay to improve abrasion resistance.
You said: "...radial stress (which is one of the principal stresses) has a strong thermal component in addition to the pressure component.[From equilibrium principles it will also affect the hoop stress.]..." and "...I do not think this issue as important as that radial thermal stress..."
Radial stress is normally caused only by difference between internal and external pressure.
Please elaborate...