The steam leaving the superheater of a modern large capacity boiler is in the order or 570 °C to 600 °C and at pressures ranging from 170 bar to 230 bar. This means the last stages of the superheater and the pipes carrying the steam to the turbine should withstand these extreme conditions. This requires this material should have very high strength properties, which do not deteriorate with time, and should be creep resistant.
Advantages of P91
SA 213 T91 or SA 335 P91 is such a ferritic alloy steel that meets this condition. This material has been in use for the last two decades successfully in power plant service. It is also called 9 Cr 1 Mo steel based on its composition.
Compared to its predecessor, the T22 or P22 grade, grade 91 exhibits high strength up to temperatures in the range of 600 °C. Also the oxidation temperature limits are higher. This allows the power plant designers to engineer components, superheater coils, headers and steam piping, with less thickness. This contributes to a higher thermal fatigue life of almost ten times. This allows them to increase the operating temperature to a higher level, increasing the efficiency of the power plant.
This makes it ideally suitable for plants that operate on a cyclic basis like combined cycle plants. Also the reduction in thickness suits HRSG designers since in an HRSG the temperature head is limited and locating the coils in the heat transfer path is very critical.
Why is P91 different ?
What makes this steel different is the addition of a high amount of Chromium. Grade 91 contains 9 % Chromium and 1 % Molybdenum compared to 2.5 % Chromium in the next best P22 grade. Chromium improves high temperature strength and increases oxidation resistance. Molybdenum increases the creep resistance. Also present are smaller quantities of Nickel and Manganese which increase the hardenability of the steel.
More important than the alloying elements is the formation of this alloy steel. The steel is formed by normalizing at 1050 °C, air cooling down to 200 °C. It is then tempered by heating to 760 °C. The temperatures and the cooling rates are very important. This produces the microstructure that results in the high creep strength properties.
This steel is not tolerant to variations in its microstructure, unlike P22 grade or other grades.
The steel has to be from manufacturers who strictly and precisely follow the heat treatment requirements. Many cases have been reported of failures of the base materials in the early stages of usage.
After the steel is worked, proper and precise heat treatment is required to reinstate the microstructure back to its original conditions. If this is not done the steel has properties that are much lower than its predecessor P22. Many failures have resulted because of this. In the case of P22 and other low alloy steels, the effect of variations in heat treatment on the properties is not as vehement as in P91.
During the fabrication and construction phase, any process that affects the micro structure has to be reversed by a precise heat treatment. This brings back the microstructure back to original.
Welding is one process that is widely used during the construction. This affects the microstructure. Preheating, maintaining inter-pass temperatures, and post-weld heat treatment procedures are very critical for P91 grade. Failure to follow the procedures will result in catastrophic failures.
For thick walled pipes, the use of an induction heating system is the ideal method. This gives better control, and uniform heating between the inner and outer diameters. In induction heating the coils themselves do not heat up. This is ideal for maintaining the inter-pass temperatures and carry out the welding. This is a more worker friendly heating process. This is also ideal for complex shapes likes weldolets and tees.
The Nickel and Manganese content, even though in smaller percentages, have profound effects on the critical temperatures, which decides the heat treatment temperatures and the cooling rates. Because of this, the composition of the welding electrodes used should be in line with the parent material.
Effect of Water
The un-heat treated steel has great affinity to Hydrogen. Hydrogen can cause stress corrosion cracking. Pre-heating has to be done properly to remove any moisture. The post weld heat treatment has to be done as quickly as possible to avoid any contact with water likely from moisture condensation, rainfall, etc. Great care has to be taken to see that 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 effect throughout the cross sections. This can also lead to failures. Great care has to be taken to avoid such design flaws.
As the industry accepts these practices of constructions, the use of 91 grade steel continues to its successful journey.