Hi There old buddy,
When dealing with boilers piping I visualize you working in accordance with ASME B 31.1 (High pressure) and under ASME section Viii. and maybe ASME B31.3 (ORDINARY REFINERY PIPPING).
Some questions needind clarifications are:
1) What temperature and pressure ranges are we dealing with?

2) Are these water tubes or fire tubes?

3) If we deal with extremely temps we are concerned with metal creep(metal deformation at high temps) Chromium and Moly are alloys that resist creep.They are found in Low Hydrogen rods with chromium moly alloys added to coating.Also,when dealing with CR Moly alloys we are talking stress relief due to high carbon equivalancy of those two alloys

4)high temps you amy choose to TIG root with Chrome Moly filler metal( argon purge) an the hot ,fill & Cap with either 8018 B2 or B3 ( depending on chemical analysis

5) if dealing with SA 106B low carbon pipe ,then you use a routine WPS for low hydrogen rods (F4 filler metal) for hot pass and F3 filler rod (6010) . Post weld heat treat if thickness is over 3/4 ".
As you can tell it depends what your CWP calls for (construction work package),We need to know to what codes we are working to.

6) ASME codes usually calls for hydrotesting ,1.5 times design pressure please check LDT tables( Line designation tables must be provided by design engineer) you should reeust to see them ,especially if you are a QC/QA.

Good Luck and have fun

Regards
Joe Valdez