P9 vs P91 — Why 9Cr-1Mo Is Not the Same as Modified 9Cr-1Mo
P9 (EN 1.7386, X12CrMo9-1) and P91 (EN 1.4903, X10CrMoVNb9-1) are both described as "9Cr-1Mo" steels — but they are completely different materials, incompatible in application and in welding. Confusing them has caused real-world pressure system failures.
The key difference is microalloying. P9 is the original 9Cr-1Mo grade — a ferritic-martensitic steel used in refinery hot piping and reformer tubes since the 1950s. It has no vanadium, no niobium, and no nitrogen additions. Its allowable stress at 600°C is approximately 30 MPa.
P91 (also called Gr.91 or 9Cr-1MoV) was developed in the 1970s by Oak Ridge National Laboratory specifically for supercritical power plant steam systems. The critical additions are: vanadium (0.18–0.25%), niobium (0.06–0.10%), and nitrogen (0.03–0.07%). These elements precipitate as fine MX carbonitrides throughout the martensitic matrix, providing a creep strengthening mechanism that P9 lacks. The result: P91 allowable stress at 600°C is approximately 90 MPa — three times higher than P9.
The ASME P-Number Distinction
The difference is formally captured in ASME IX welding qualification requirements. P9 is ASME P-No.5B. P91 is ASME P-No.15E. A welding procedure qualified for P-No.5B does NOT qualify P-No.15E welds. Separate WPS and PQR are mandatory for P91.
Filler Metal Is Not Interchangeable
P9 uses ER80S-B8 filler (AWS A5.28). P91 requires ER90S-B9 filler exclusively — no substitution is permitted. ER90S-B9 is formulated to match P91's V+Nb+N chemistry in the weld deposit. Using the wrong filler for P91 produces weld metal with inadequate creep strength.
Summary
- P9 (1.7386): older refinery grade, no V/Nb, P-No.5B, ER80S-B8, ~30 MPa at 600°C
- P91 (1.4903): supercritical power plant grade, V+Nb+N, P-No.15E, ER90S-B9 only, ~90 MPa at 600°C
- Never interchangeable — check the EN Werkstoffnummer or ASTM grade designation, not just the Cr-Mo label