Comparing materials for high-temperature steam piping
The use of X20 and P91 in power stations
Of all the materials used for high-temperature steam piping, X20 (12 percent chromium, 1 percent molybdenum, 1/4 percent vanadium) and P91 (9 percent chromium, 1 percent molybdenum, 1/4 percent vanadium) stand out because of their very high creep rupture properties, even at elevated temperatures.
X20 was introduced in the 1950s in Germany and used in steam lines operating at temperatures of 530 degrees C and higher for fossil fuel-fired power generating sets of 150 megawatts and more. However, two factors limited its use: the extreme care needed for its fabrication and welding and its noninclusion in the American Society of Mechanical Engineers (ASME) Code B31.1.
P91, introduced in the 1980s in the U.S., has both very high strength at elevated temperatures and good fabrication properties. These features have made P91 the material of choice for high-temperature steam and other, similar noncorrosive services.
X20 material was first used in India for high-temperature steam piping around 1970. The next application in India took place nearly two decades later, when the Tata Electric Co. (www.tata.com) selected the material for both the main steam and hot reheat lines of its Trombay power station's 500-MW units.
The use of X20 in preference to P22 (2-1/4 percent chromium, 1 percent molybdenum) in the latter application allowed wall thickness reductions of about 50 percent. These reductions resulted in easier handling; less energy needed for preheating, welding, and postweld heat treatment; and faster start-up, load changes, and shutdown of the unit.
Overall savings in the cost of the piping supplies and their fabrication, including welding, was claimed to be about 40 percent.1
Trombay Unit 6 became operational in 1989. Since then X20 has been used in India for main steam piping in six other power stations.
By the time P91 was included in the ASTM specification A335 in 1984, more than 100,000 metric tons of X20 tube and pipe had been used in power stations worldwide. The cumulative operating time with the material steel had been more than 4 million hours.2Operating behavior in more than 300 high-capacity power stations has been excellent,3and failures have been limited to a few instances occurring when it was introduced because of lack of knowledge about the material's properties.4
Its creep rupture properties have been well-established through laboratory tests and more than 200,000 hours of operation.5
In spite of such strong credentials, X20 has not been included in the ASME Boiler & Pressure Vessel Code. This is likely one of the main reasons that the U.S. remains a notable exception in the long list of countries using this material.
When properly heat-treated, P22, X20, and P91 achieve
these tensile properties at room temperature.
The U.S. had been trying to develop a new material since the middle 1970s to bridge the gap between ferritic P22 and austenitic steels with respect to creep rupture strength for high-temperature service from 540 to 600 degrees C. Development of any new material, especially for high-temperature service, requires many years, because creep rupture strengths are established based on longtime exposure to a range of intended service temperatures.
As a result of these developmental efforts, a new material, designated P91, was introduced in the U.S. in the 1980s by Oak Ridge National Laboratory (ORNL, www.ornl.gov), assisted by Combustion Engineering. This material has proven to have such good strength and fabrication properties that the use of X20 has practically been discontinued in Europe. In fact, even renovations of old power plants are being made with P91 material for steam circuits operating in the creep range.
P91 is a modified form of P9 (9 percent chromium, 1 percent molybdenum) steel. The steel can have low impurity limits, thanks to the development of processes such as argon-oxygen decarburization (AOD) and electroslag remelting (ESR), which make the steel behave consistently during fabrication and resist the effects of aging. When properly heat-treated as specified in ASTM specification A335, the steel acquires room temperature properties as shown in Figure 1.
The steel has high creep rupture strength because of the precipitation of submicroscopic vanadium and niobium carbonitrides. Low carbon content aids its fabrication characteristics. The material responds well to hot and cold bending, as well as to welding.
Comparison of X20 and P91
P91 (top) and X20 (bottom) both are martensitic steels
with similar transformation behavior.
P91 and X20 both are martensitic steels with similar transformation behavior (see Figure 2). Martensite formation temperature for P91 is about 400 degrees C. Welding of P91 steel is, therefore, carried out below this temperature using preheat and interpass temperatures in the range of 200 to 300 degrees C.
The maximum hardness in the weld metal and heat-affected zone in as-welded condition is about 450 HV10, which is lower than that of X20 (greater than 500 HV10). Heavier-wall P91 components may be cooled to room temperature after welding. The joint should, however, be kept dry after welding until postweld heat treatment is complete to avoid stress-corrosion cracking caused by the presence of humidity.6
Martensite formation temperature for X20 is about 300 degrees C, so welding may be carried out either at 250 degrees C (just below the martensite formation temperature) or in the nontransformation range beyond 400 degrees C. The higher temperature helps prevent high hardness values and the attendant risk of cracking during welding. In any case, except for very thin-wall components, X-20 weld deposit must be cooled down to about 100 degrees C and held there for at least one hour for the transformation of austenite into martensite to be complete. The component then is subjected to a tempering treatment at between 730 and 760 degrees C for at least two hours.7
Typical welding consumables recommended for P22, X20, and P91 are shown in Figure 3, and typical nondestructive testing practices recommended for P91 are shown in Figure 4.
Several brands of welding consumables can be used with P22 and P91, while only one brand is recommended for use with X20.
Following are some considerations that influence a choice between P91 and X20:
- Allowable stress, per ASME B31.1 code, is the same for both P91 and X20 at 540 degrees C. The allowable stress is increasingly higher for P91 at higher temperatures. Therefore, any advantages of X20 based on its lower thickness requirement can be obtained by using P91 at 540 degrees C and higher.
- Use of X20 demands extreme care in fabrication and welding of the piping components. Important parameters include induction heating of thicker weld joints; special cooling and storage of bends before heat treatment; low-speed grinding performed intermittently to prevent overheating and cracking; completion of welding and heat treating in one cycle; and extensive NDT for weld joints.
- The thermal expansion coefficient of P91 is comparable to that of X20.
- The thermal conductivity of P91 is higher than that of X20.
- P91 can be readily machined with cutting tools similar to those used for X20.
- P91 has a lower chromium content, which helps to conserve material.
Several types of nondestructive testing
typically are recommended for P91.
Confidence in the use of P91 steel has grown substantially since its first use. ASTM approved the steel under designation A213 Gr. T91 in 1983 and A335 Gr. P91 in 1984. Inclusion of P91 plates, forgings, flanges, and fittings in ASTM standards, and commercial manufacture of such components to these standards, continues to evolve.
A Promising Future
At temperatures higher than 540 degrees C, P91 has increasingly higher allowable stress than X20. It now is possible for fossil fuel-fired power stations to achieve higher pressure and temperature parameters on main steam piping, and thereby realize higher thermal efficiency, using this material. This saves recurring fuel costs and also reduces pollutants, because less fuel is burned.
P91/T91 may be used to replace sections of boiler header and pressure parts that occasionally reach temperatures higher than permissible design limits for P22 or other low-alloy chromium-molybdenum-vanadium steels. T91 also is being applied in superheater and reheater circuits, which used to require austenitic steel because of the design temperatures.
P91 also has been used recently in petrochemical plants for cracking and hydrotreating furnaces that employ higher operating temperatures to increase the yield of unleaded, high-octane fuels.
P91 has a promising future, and its applications are sure to increase until another new material is in a position to challenge it.
P.K. Saha is head of quality and management representative for Bharat Heavy Electricals Limited (BHEL) Power Sector -- Eastern Region, Gillander House, Block B & C, 4th Floor, 8, N.S. Road, Kolkata-700001, India. BHEL is an engineering and manufacturing organization engaged primarily in design, manufacture, supply, installation, and servicing of power plant and industrial equipment.
Reprinted with permission from BHEL Journal, Volume 20, No. 1, March 1999.
1. V.L. Gopalakrishnan, Welding of X20 Pipes for Conveyance of High Pressure Steam for 500 MW Power Plant -- an Indian Experience (The Tata Power Co. Ltd., 1991), pp. 377-383.
2. G. Kalwa, State of the Development and Application Techniques of the Steel X20CrMoV121 (Mannesmann).
3. W. Bendick, V. Harrmann, and M. Zschau, Retrofitting of Exhausted Steamline Components (Mannesmann).
4. K. Niederhoff, G. Wellnitz, M. Zschau, and D. Ziessnitz, Properties and Fabricability of Creep Resistant 9-12% Cr Steels for High Pressure Piping System in Power Plants (Mannesmann, 1991), pp. 221-262.
7. F. Bruhl and H. Musch, Welding of Alloyed Ferritic and Martensitic Steels in Piping Systems for High Temperature Service (Mannesmann).
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