P11 (1.25Cr-0.5Mo) and P22 (2.25Cr-1Mo) are the two most widely specified chrome-moly alloy pipe grades for elevated-temperature service in power plants, refineries, and petrochemical facilities. They share the same alloying philosophy — chromium for oxidation resistance and molybdenum for creep strength — but their different chromium levels create meaningful performance gaps that determine which grade fits your application.
P11: 1.25Cr-0.5Mo
P11 (ASTM A335 Grade P11, tube equivalent T11 per ASTM A213) provides creep strength adequate for continuous service up to approximately 540°C. The 1.25% chromium content improves oxidation resistance compared to carbon steel and stabilises carbides against the graphitisation that limits carbon steel above 425°C. The 0.5% molybdenum provides solid-solution strengthening and improves creep resistance through carbide precipitation hardening.
P11 is the workhorse grade for superheater and reheater piping in subcritical coal-fired boilers, steam headers operating at 480–540°C, and refinery process piping in moderate-temperature service. Its adequate creep strength at moderate temperatures, good weldability with proper pre-heat (200–250°C) and PWHT (700–750°C), and lower material cost compared to P22 make it the default first choice for service below 540°C.
P22: 2.25Cr-1Mo
P22 (ASTM A335 Grade P22, tube equivalent T22 per ASTM A213) extends the service temperature envelope to approximately 580°C. The higher chromium content (2.25% versus 1.25%) provides better oxidation and sulfidation resistance, and the doubled molybdenum content (1.0% versus 0.5%) delivers approximately 25–30% higher creep rupture strength at 550°C compared to P11.
P22 is specified for main steam piping and headers operating at 540–580°C, hot reheat piping, high-temperature hydroprocessor reactor circuit piping in refineries, and any application where the temperature or required service life exceeds P11’s capabilities. The cost premium over P11 is typically 10–15%, driven by the higher alloy content and the more demanding heat treatment requirements.
Weldability Comparison
Both grades require pre-heat and PWHT for all welds. P11 typically requires 200–250°C pre-heat and PWHT at 700–750°C for 1 hour per 25 mm of wall thickness. P22 requires 200–300°C pre-heat and PWHT at 700–750°C, with a minimum PWHT time that increases with wall thickness. The higher alloy content of P22 makes it slightly more susceptible to reheat cracking during PWHT, requiring careful control of heating and cooling rates.
Welding consumables must be matched: E8018-B2/ER80S-B2 for P11 and E9018-B3/ER90S-B3 for P22. Using the wrong consumables creates a dissimilar metal joint with unpredictable long-term creep behavior at the operating temperature. Consumable selection errors are among the most common root causes of chrome-moly weld failures in power plant service.
Making the Selection
If your design temperature is below 540°C and the allowable stress for P11 at that temperature (from the applicable code’s stress tables) provides adequate wall thickness, P11 is the correct choice. If the temperature exceeds 540°C, or if the allowable stress for P11 results in impractically thick walls, step up to P22. If the temperature exceeds 580°C, move to P91 — neither P11 nor P22 has adequate creep strength for supercritical service.
Need P11 or P22 pipe? Send us your temperature and pressure requirements for a grade recommendation and quotation.