ASTM A335 P91 Type 1 vs Type 2: Key Differences Explained
The main difference between ASTM A335 P91 Type 1 and Type 2 lies in microstructure control and long-term creep performance. Type 2 offers improved toughness, better stability, and enhanced resistance to high-temperature deformation, making it more suitable for critical applications such as power plants and high-pressure steam systems.
What is ASTM A335 P91?
ASTM A335 P91 is a high-strength ferritic alloy steel pipe designed for high-temperature and high-pressure service. It contains chromium, molybdenum, vanadium, and niobium, providing excellent creep resistance and oxidation performance.
Typical Applications:
- Power plant piping systems
- Boilers and superheaters
- Heat exchangers
- High-pressure steam pipelines
Comparison Table: ASTM A335 P91 Type 1 vs Type 2
| Feature | Type 1 | Type 2 |
|---|---|---|
| Microstructure | Standard tempered martensite | Refined and controlled |
| Creep Resistance | High | Enhanced |
| Toughness | Good | Improved |
| Heat Treatment | Standard | Strictly controlled |
| Application | General use | Critical high-temp systems |
Key Differences Between P91 Type 1 and Type 2
1. Microstructure Control
Type 1 uses a conventional tempered martensitic structure.
Type 2 has a more refined microstructure with better carbide distribution, improving long-term stability.
👉 This is the core difference.
2. Creep Strength and High-Temperature Performance
Type 2 provides better creep rupture strength under long-term high-temperature conditions (typically up to 600°C+).
This makes Type 2 more reliable in:
- Supercritical power plants
- Continuous high-pressure systems
3. Chemical Composition Control
Both types follow ASTM A335 requirements, but:
- Type 2 has tighter control of nitrogen and trace elements
- Improves grain stability and performance consistency
| Grade | UNS Designation | Chemical Composition % | |||||||
| P91 Type 1 | K91560 | C | Mn | P | S | Si | Ni | Cr | Mo |
| 0.08-0.12 | 0.30-0.60 | 0.020 | 0.010 | 0.20-0.50 | 0.40 max | 8.00-9.50 | 0.85-1.05 | ||
| V | N | Al | Cb | Ti | Zr | Sn | Sb | ||
| 0.18-0.25 | 0.030-0.070 | 0.02 max | 0.06-010 | 0.01 max | 0.01 max | _ | _ | ||
| P91 Type 2 | K91560 | C | Mn | P | S | Si | Ni | Cr | Mo |
| 0.08-0.12 | 0.30-0.50 | 0.020 | 0.005 | 0.20-0.40 | 0.40 max | 8.00-9.50 | 0.85-1.05 | ||
| V | N | Al | Cb | Ti | Zr | Sn | Sb | ||
| 0.18-0.25 | 0.035-0.070 | 0.02 max | 0.06-0.10 | 0.01 max | 0.01 max | 0.010 max | 0.003 max | ||
ASTM A335 P91 Type 1 & Type 2 have subtle differences mainly in elements Mn, S, Si, N, Sn and Sb content, and Type 2 has stricter requirements on element content than Type 1.
4. Heat Treatment Requirements
Proper heat treatment is critical for P91 performance.
- Type 1: Standard normalizing + tempering
- Type 2: More strictly controlled process
| Grade | Heat Treatment Type | Normalizing Temperature Range F [C] | Subcritical Annealing or Tempering |
| P91 (type 1 and 2) | Normalize and Temper, or | 1900-1975 [1040 – 1080] | 1350-1470 [730 – 800] |
| Quench and Temper | 1900-1975 [1040 – 1080] | 1350-1470 [730 – 800] |
When to Choose Type 1 vs Type 2
Choose Type 1 if:
- Budget-sensitive projects
- Moderate temperature and pressure conditions
- General industrial piping
Choose Type 2 if:
- High-temperature (>600°C) environments
- Long-term service life required
- Critical systems (power plants, refineries)
FAQ: ASTM A335 P91 Type 1 vs Type 2
Q1: What is the main difference between P91 Type 1 and Type 2?
Type 2 has improved microstructure control, resulting in better creep resistance and toughness.
Q2: Is P91 Type 2 better than Type 1?
Yes, Type 2 performs better in high-temperature and high-stress environments.
Q3: Can P91 Type 1 replace Type 2?
Type 1 can be used in less demanding conditions, but Type 2 is recommended for critical applications.
Q4: What industries use P91 Type 2?
Power plants, petrochemical industries, and high-pressure steam systems.
ASTM Standards and Equivalent Specifications
- ASTM A335 P91
- ASME SA335 P91
- EN equivalent: X10CrMoVNb9-1
For Buyers: How to Choose the Right P91 Pipe
When selecting between Type 1 and Type 2, consider:
- Operating temperature
- Pressure conditions
- Required service life
- Project criticality
For long-term reliability, Type 2 is usually the safer choice.
