Products Description
P91 is a chrome moly alloy metal that contains excellent strength and temperature resistance. It is designed for enhanced creep strength, making it a creep strength enhanced ferritic (CSEF).
This metal is made by normalizing at 1050 °C, air cooling to 200 °C, and then tempered through heating to 760 °C. This process is the main reason behind P91's creep strength and durability.
P91 stands for the metal's composition of 9% chrome and 1% molybdenum plus vanadium. The chrome increases temperature strength and oxidation resistance. The molybdenum increases the elasticity,
Chemical Composition of ASTM A335 P91 Alloy Steel Pipe
| Element | Content (%) |
|---|---|
| Carbon (C) | 0.08 – 0.12 |
| Manganese (Mn) | 0.30 – 0.60 |
| Phosphorus (P), max | 0.020 |
| Sulfur (S), max | 0.010 |
| Silicon (Si) | 0.20 – 0.50 |
| Chromium (Cr) | 8.00 – 9.50 |
| Molybdenum (Mo) | 0.85 – 1.05 |
| Vanadium (V) | 0.18 – 0.25 |
| Nitrogen (N) | 0.030 – 0.070 |
| Nickel (Ni), max | 0.40 |
| Aluminum (Al), max | 0.02 |
| Columbium (Cb/Nb) | 0.06 – 0.10 |
| Titanium (Ti), max | 0.01 |
| Zirconium (Zr), max | 0.01 |
P91 Welding Procedure
When welding P91, preheating, maintaining inter-pass temperatures and post-weld treatment procedures are essential to maintain the P91 grade. Welding has an important effect on the microstructure and failure to follow the procedures will result in catastrophic failures.
For thick-walled pipes, the use of a modern induction heating system – where the coils do not heat up – is the ideal method. This gives better control, and uniform heating between the inner and outer diameters. Induction heating is ideal for maintaining the inter-pass temperatures and carry out the welding. This is also a more worker-friendly heating process and is ideal for complex shapes like weldolets and tees.
The composition of the welding electrodes used should always be in line with the parent material. The nickel and manganese content of P91, though in smaller percentages, has profound effects on the critical temperatures, which decides the heat treatment temperatures and the cooling rates.
Steel that has not been heat-treated has a great affinity to hydrogen which can cause stress corrosion cracking. Pre-heating must be done properly to remove any moisture. In addition, the post-weld heat treatment must be done as quickly as possible to avoid any contact with moisture from condensation, rainfall, etc. Be sure all joints are post-weld heat treated prior to hydro test.
Dissimilar weld joints – especially at complicated geometries – can result in the heat treatment not having the desired affect throughout the cross sections. This can also lead to failures.
P91 Pipe Applications
A335/SA335 P91 is suitable for bending, flanging (vanstoning), and similar forming operations and for fusion welding. P91 is most commonly used in the power industry and petrochemical plants where fluids and gases are transported at extremely high temperatures and pressures.
P91's strength and high temperature endurance makes it ideally suitable for plants that operate on a cyclic basis - like combined cycle plants. Also, the reduction in thickness suits HRSG designers, in an HRSG the temperature head is limited and locating the coils in the heat transfer path is very critical.
P91 is the correct grade for:
Power generation
Petrochemicals
Gas processing
Petrochemical Plants
Heaters
Oilfield services
