The P91 designation is considered a "Modified Chrome". It is the same as the normal 9Cr-1Mo steel form except the P91 has added vanadium+niobium+nitrogen. The precautions are the preheat and PWHT between the two grades. The 9Cr-1Mo require a preheat of 350F for thicknesses up to 0.5". Above the 0.5" the recommended preheat is 400F. This applies to the 9Cr-1Mo and the P-91. The PWHT for the 9Cr-1Mo is 1300-1400 and the P-91 is 1350-1400F. This is normally for 1 hr. per inch of thickness. Naturally, a low hydrogen filler metal is beneficial. Your 5,7, and 9 chromes, along with the P-91, all require the same preheat based upon metal thickness. The 5,7, and 9 Chromes require the 1300-1400 PWHT, but the P-91 requires the higher minimum temperature, 1350F. One good reference is the AWS Welding Handbook, Volume 4, Materials and Applications, 8th Edition.