What is the difference between P9 and P11 material
Mar 25, 2025
1. Differences in chemical composition
| composition | P9(ASTM A335 P9) | P11(ASTM A335 P11) |
|---|---|---|
| Cr | 8.0%~9.5% 26 | 1.0%~1.5% 48 |
| Mo | 0.85%~1.05% 26 | 0.44%~0.65% 48 |
| C | 0.08%~0.12% 26 | 0.05%~0.15% 48 |
| V/Nb | V(0.18%~0.25%),Nb 26 | 0 |
P9 is solid solution strengthened by the addition of vanadium (V) and niobium (Nb) to improve high temperature creep strength, while P11 is based on chromium (Cr) and molybdenum (Mo), focusing on basic heat resistance.
2. Comparison of mechanical properties
High temperature strength:
P9 thermal strength at 600 ℃ is 3 times that of P11, and the long-term use temperature can reach 650 ℃;
P11 maximum applicable temperature of 550 ℃, tensile strength ≥ 415 MPa, yield strength ≥ 205 MPa.
Oxidation resistance: P9 is superior to P11 due to its high chromium content (~9%) and the formation of a dense Cr₂O₃ oxide layer, which is superior to P11 in resistance to high-temperature oxidation and sulphide corrosion.
3. Application Scenarios
P9: It is suitable for extreme high temperature and high pressure environment, such as superheater and reheater of supercritical power station boiler, as well as pipeline directly heated by flame in petroleum refining plant (medium temperature >600℃);
P11: mostly used in medium temperature and high pressure pipelines, such as power boiler economiser, petrochemical oil and gas transmission pipelines (medium temperature ≤ 550 ℃).
4. Welding and heat treatment process
Weldability: P11 welding needs to control the interlayer temperature and post-weld heat treatment (heated to 680 ~ 710 ℃ heat preservation and slow cooling), in order to eliminate residual stress; P9 due to the high alloy content, the need for more stringent P9, due to its high alloy content, requires more stringent heat treatment (e.g. normalising + tempering) to avoid cold cracking.
Delivery condition: P11 is usually supplied in normalised + tempered condition, while P9 is mostly quenched + tempered to optimise organisational stability.
