P11 and P22 CrMo Pipe Fittings: Boiler and Steam Service Workhorses

Grade Profiles

P11 (ASTM A234 WP11 / EN 1.7337, 13CrMoSi5-5, 1¼Cr-½Mo-Si, ASME P-No. 4): 1.00–1.50% Cr, 0.44–0.65% Mo. Rated for steam service to approximately 530°C. The most cost-effective CrMo grade for elevated-temperature service above the limit of carbon steel. Used for main steam and hot water piping in subcritical industrial boilers, heat exchanger shells, and low-temperature superheater headers.

P22 (ASTM A234 WP22 / EN 1.7380, 10CrMo9-10, 2¼Cr-1Mo, ASME P-No. 5A): 1.90–2.60% Cr, 0.87–1.13% Mo. Rated to approximately 565°C. Higher Cr and Mo give better creep strength and higher allowable stress than P11 at the same temperature. Standard for main steam lines in subcritical utility power plants and for high-temperature reactor piping in refineries.

Creep Strength Comparison

At 550°C, the ASME Section II allowable stress for P22 is approximately 60–70 MPa vs approximately 40–50 MPa for P11. This ~40% advantage in allowable stress translates directly into thinner walls at the same design pressure — an important economic factor for large-bore, high-temperature headers. For temperatures below approximately 480°C, P11 is typically the economic choice; above 500°C, P22's higher allowable stress usually justifies the small premium in material cost.

PWHT Windows

• P11: 690–730°C, hold ≥1h/25mm wall, minimum 1 hour. ASME P-No. 4.
• P22: 700–750°C, hold ≥1h/25mm wall, minimum 1 hour. ASME P-No. 5A.

The PWHT temperature windows differ by 10°C — a procedure qualified for P22 at 710°C is within the P22 window but below the P11 minimum of 690°C. This matters when a site has a mixed piping system with both grades. A separate PWHT procedure and thermocouple record is required for each grade.

Welding Consumables

• P11: ER80S-B2 (GTAW) / E8015-B2 (SMAW) — 1¼Cr-½Mo filler
• P22: ER90S-B3 (GTAW) / E9015-B3 (SMAW) — 2¼Cr-1Mo filler

The fillers are not interchangeable. Using ER80S-B2 on P22 base metal produces an under-alloyed weld with insufficient creep strength. B2 filler is qualified only for P-No. 4 (P11). B3 filler is qualified for P-No. 5A (P22) and is also acceptable for P11 in some code configurations (over-alloyed but not harmful). In practice, filler metal should always match the base metal to avoid tracking complexity across a multi-grade project.

Preheat Requirements

Both P11 and P22 require preheat before any welding, including repair welds:

• P11: Minimum 150°C preheat; maintain inter-pass ≥150°C
• P22: Minimum 200°C preheat; maintain inter-pass ≥200°C

The joint must not be allowed to cool below the preheat temperature during the welding sequence. Failure to maintain preheat is the single most common cause of HAZ cracking in CrMo pipe fittings discovered during post-weld NDE.

IBR / CCOE Approval

Both P11 (WP11, EN 1.7335) and P22 (WP22, EN 1.7380) fittings are supplied by Arshya with IBR Form III-B for Indian boiler service. The IBR certificate covers the fitting type, size range, and design pressure. For Indian utility power plants and industrial boilers under the Indian Boilers Act, IBR-approved P11 and P22 fittings are mandatory for steam piping above the IBR notification thresholds.