dzaver,
without knowing what COC (code of construction) you are working to, and time at temp, and what was the procedure used. i.e. Pre-heat, heating rate, cooling rate, if you used a flux system during welding (i.e. SMAW. FCAW or SAW) then did you perform a bake-out, after bake out did you allow the material to cool to 200 Deg. F. (t=29,36 I'm assuming this is mm).  It  would be difficult to give you any advice. Since it' is P91 material your goal is tempered martensite. There are two ways to determine if you have tempered martensite after PWHT. 1. Taking hardness readings. (typically 190 to 250 HBN) is a good indicator of proper phase transformation. 2. You can take replications and have a metallurgist read the micrographs. Note: This material welded and PWHT correctly is very sustainable.  But, on the other hand, if not welded and PWHT correctly can be very problematic and failure is inevitable. From your post, it does not seem like you have a good grasp on the material or how to achieve good results in joining it together. I would highly recommend that you consult with SME's to assist you in being successful.