P5 and P9 CrMo Pipe Fittings: Refinery and Petrochemical High-Temperature Service

Where P5 and P9 Fit in the CrMo Family

The chromium-molybdenum family runs from P11 (1¼Cr-½Mo) to P92 (9Cr-2W), with chromium content determining oxidation and sulphidation resistance and molybdenum providing creep strength. P5 (ASTM A234 WP5 / EN 1.7362, X12CrMo5, 5Cr-½Mo, ASME P-No. 5B Group 1) and P9 (ASTM A234 WP9 / EN 1.7386, X12CrMo9-1, 9Cr-1Mo, ASME P-No. 5B Group 2) occupy the high-chromium end of the traditional CrMo range before the modified grades (P91, P92) with V and Nb additions.

Sulphidation Resistance — The Key Differentiator

The primary reason to specify P5 or P9 over P11 or P22 in refineries is sulphidation resistance. Crude oil and its distillates contain sulphur compounds (H₂S, mercaptans, elemental S) that attack low-alloy steels by forming iron sulphide scale. The "Modified McConomy curves" (API RP 939-C) quantify corrosion rate vs temperature for each Cr level: 5Cr steel has approximately 5× lower sulphidation rate than 1¼Cr at 400°C; 9Cr has approximately 10× lower rate. For hydrotreating, hydrocracking, and crude distillation piping above approximately 260°C in sulphur-containing service, P5 or P9 is the standard specification.

High-Temperature Hydrogen Service (Nelson Curves)

At elevated temperatures and hydrogen partial pressures, low-alloy steels undergo hydrogen attack — methane forms at grain boundaries, decarburising and embrittling the steel. The API 941 Nelson curves define the safe operating limits. P5 and P9 have significantly higher Nelson curve limits than P11 and P22. For hydroprocessing units (hydrotreating, hydrocracking, catalytic reformers), P9 is commonly specified because it sits above the Nelson curve limit for high-temperature, high-pressure hydrogen environments encountered in modern high-pressure hydrotreaters.

PWHT Requirements

• P5 (1.7362): PWHT 720–760°C, hold ≥1h/25mm wall, minimum 2 hours
• P9 (1.7386): PWHT 720–760°C, hold ≥1h/25mm wall, minimum 2 hours

Both P5 and P9 transform to martensite on cooling from welding temperature and require PWHT without exception. The windows are the same, but P9 has a higher chromium content and slightly higher hardness in the as-welded condition — preheat of at least 175–230°C is essential before welding, and the joint must not cool below preheat temperature during the welding sequence.

Welding Consumables

• P5: ER80S-B6 (GTAW) / E8015-B6 (SMAW) — 5Cr-½Mo matching filler
• P9: ER80S-B8 (GTAW) / E8015-B8 (SMAW) — 9Cr-1Mo matching filler

Do not use P11 or P22 fillers (ER80S-B2, ER90S-B3) on P5 or P9 welds — the chromium content is insufficient and the weld deposit will have significantly lower oxidation and sulphidation resistance than the base metal, creating a weak point exactly at the weld.

P9 vs P91 — A Critical Distinction

Both P9 (9Cr-1Mo) and P91 (9Cr-1Mo-V-Nb) contain 9Cr and 1Mo, but they are completely different materials with different ASME P-Numbers (P9 = P-No. 5B Group 2; P91 = P-No. 15E). P91 has vanadium, niobium, and nitrogen additions that dramatically increase creep strength. P91 requires PWHT at 745–775°C — different from P9's 720–760°C window. The two grades require separate WPS qualifications and use different filler metals (ER80S-B8 for P9 vs ER90S-B9 for P91). Substituting one for the other is a serious non-conformance — always verify by PMI and check the full ASTM designation on the certificate.