The improvement of thermal efficiency by increasing the operating temperature and pressure of boilers has recently led to the development of new creep-resistant steels. For 9–12% Cr steels, grade T/P91, developed at the end of the seventies by CE (Combustion Engineering) and ORNL (Oak Ridge National Laboratories) [1] in the USA, marked the starting point for these developments. With its excellent mechanical characteristics at elevated temperatures and good workability, it was rapidly adopted worldwide for erecting new power generators. The tube design temperature is limited to around 610°C (1130°F) inside the combustion chamber. This limit depends on such factors as heat flow and corrosion. Steam lines in P91 are generally operated at temperatures below or slightly above 600°C (1112°F). More recent developments to produce new grades such as T/P911 and T/P92, have improved mechanical properties at high temperatures, in particular an increase in creep strength of 10 to 30% in 100,000 hrs at 600°C (1112°F). This makes it possible to reduce the wall thickness of the pipes and consequently improve their behavior to thermal fatigue. The new grades T/P23 and T/P24 are well suited for boiler components working at lower temperatures. While these grades were initially developed for manufacturing the water wall panels of Ultra Super Critical Boilers (USCB), they are also used for the superheaters and reheaters of conventional boilers and Heat Recovery Steam Generators (HRSG) in combined cycles. In addition to their excellent workability, they have the advantage to be used without post-weld heat treatment in case of thin wall tubes. Furthermore, owing to the good creep properties, they can be used to replace P22 and for some applications even P91, with the advantage of lower costs. This paper sets out the characteristics of these new grades and gives the main recommendations for processing them.
Abstract A new 12%Cr steel, VM12, has been developed with the combined strength and oxidation resistance characteristics desired for high-temperature applications. The steel increases chromium content by around 0.2% to improve oxidation properties while alloying with other elements such as cobalt, tungsten, and boron to meet a range of requirements, including extending fatigue life. The steel is designed to have the same creep strength as T/P92 but with better oxidation resistance due to the higher chromium content. It has about a 0.2% increase in mechanical properties compared to T/P92 steel. Results are presented for tubes and pipes cast with a variety of surface conditions. In addition, detailed results are provided on the effects of alloying elements on creep and oxidation resistance.
The application of new heat resistant steels in power plants requires reliable long term creep rupture strength values as basis for design. A major task of ECCC (European Creep Collaborative Committee) is to supply such values by making assessments of large creep data sets. Different assessment methods have been used in the past involving different extrapolation techniques. The modern martensitic 9% Cr-steels E911 (T/P911) and T/P92 have complex microstructures changing with service exposure. Since the microstructural changes have an effect on strength, an extrapolation of long term strength properties becomes most difficult. Now that long term creep test results are available it has been shown that a revision of earlier assessments on E911 and T/P92 is necessary. New assessments are presented using different methods. The advantages of a heat by heat preassessment are discussed.
Improvement of boiler efficiency is reached by increasing the pressure and temperature of newly designed boilers. Recently developed steel grades such as T/P91 and T/P92 are used in the advanced power plants thanks to their high creep rupture strength resistance which enables maintaining acceptable thickness of the tubes and pipes. Nevertheless their operating temperature range is limited by their oxidation behavior which is lower than classical 12%Cr steels or austenitic steels. For these reasons we have developed a new steel grade which combines good creep resistance and high steam-side oxidation behavior. This new steel, based on chromium content of 12% and with other elements such as cobalt, tungsten and boron, is named VM12. Manufacturing of this grade has been proved by production of several laboratory and industrial heats and rolling of tubes and pipes in several dimensions by different rolling processes. In addition to base metal property investigations — including creep tests and high temperature oxidation behavior — welding, cold bending and hot induction bending qualifications took place. This paper summarizes the results of the investigations and presents the first findings for processing.
Today, newly developed 9% Cr-steels, such as T/P911 and T/P92, are used for power plants with advanced steam parameters, thanks to their high creep rupture strength values. They show an increase of creep values by around 10 to 25 %, compared with T/P91. Nevertheless, their range of use is limited for high temperature because their oxidation resistance is lower than that of the classical 12%Cr-steels, such as X20CrMoV12-1 or the austenitic steels. In order to meet higher design parameters, a new steel, named VM12, has been developed by Vallourec and Mannesmann Tubes (V&M). The aim was to keep the high creep level of the T/P92 together with improvement of the steam oxidation resistance to fit with design steam temperature up to 650°C.
The application of new heat resistant steels in power plants requires reliable long term creep rupture strength values as basis for design. Modern martensitic 9% Cr-steels have complex microstructures that change with service exposure. That is why extrapolations of long term strength properties will be most difficult. Due to new long term test results, re-assessments became necessary for grades 911 and 92. Different methods have been used. Good agreement was obtained between a graphical and the numerical ISO 6303 method. In both cases a two-step assessment procedure was used. First the raw data was prepared in a suitable way, which was followed by mathematical averaging procedures. For comparison a Larson-Miller analysis on the raw data was performed, too. The results turned out to be too optimistic at temperatures higher than 575°C (1050°F). It is shown that a suitable preparation of data can improve the Larson-Miller assessment. As a result of the new assessments the design values had to be reduced for both grades. With respect to previous assessments the new values are up to almost 10% lower. In the case of grade 92 the difference from the former ASME values are even higher. Consequences concerning design and service operation are discussed.
Creep strength loss of T92 steel after long-term creep exposure at 600°C and 650°C is partially due to a thermal aging of the steel during the first part of the test. In order to quantify the effect of long-term aging on the creep strength loss, creep tests were conducted at 600 and 650°C on T92 steel thermally aged for 10,000h at the same temperature and on as-received T92 steel. Laves phases precipitates were found after thermal aging at 600°C and 650°C with an average equivalent diameter of about 200nm and of about 350nm, respectively. No significant change in hardness and in the matrix substructure as revealed by electron backscatter diffraction occurred during aging. For stresses higher than 170MPa at 600°C and higher than 110MPa at 650°C the time to rupture is four times lower in the aged steels compared to the as-received steel, this is correlated to a secondary creep rate four times higher for the aged specimens compared to that of the as-received steel. Creep tests conducted at 650°C under lower stresses revealed a creep lifetime only twice lower after aging.
Abstract In fossil-fired boilers, combustion-generated thermal energy transfers to the working fluid via exchanger tubes, where an internal oxide layer forms over time, reducing thermal conductivity and raising metal temperatures. This self-activating process accelerates creep damage, significantly shortening component lifespan. Boiler design codes set Maximum Allowable Stresses based on mechanical properties, primarily creep resistance, but oxidation effects are only indirectly considered through “design temperature” selection—an approach inadequate for next-generation high-performance boilers with increasingly severe steam conditions. This paper highlights the need to integrate oxidation behavior into the design of advanced heat-exchanging components by examining the impact of steam oxidation on tube lifespan, including oxide layer growth, metal loss, temperature rise, and reduced creep rupture time, with thermal flux effects illustrated through examples. It also compares the behavior of two 9-12Cr% steels: Grade 92, known for strong creep resistance, and VM12, which offers superior oxidation resistance. Additionally, it proposes a revised “design temperature” expression incorporating oxidation resistance performance indices and exchanger thermal characteristics. The study concludes by emphasizing the need for further research into oxidation kinetics, thermal properties, and oxide layer exfoliation mechanisms.
Abstract The T/P91 and T/P92 steel grades were developed as a result of a demand of high creep strength for advanced power plants. Nevertheless, their operating temperature range is limited by their oxidation performance which is lower compared with usual 12%Cr steels or austenitic steels. Moreover, the new designed power plants require higher pressure and temperature in order to improve efficiency and reduce harmful emissions. For these reasons, Vallourec and Mannesmann have recently developed a new 12%Cr steel which combines good creep resistance and high steam-side oxidation resistance. This new steel, with a chromium content of 12% and with other additional elements such as cobalt, tungsten and boron, is named VM12. Manufacturing of this grade has been successfully demonstrated by production of several laboratory and industrial heats and rolling of tubes and pipes in several sizes using different rolling processes. This paper summarizes the results of the investigations on base material, including creep tests and high temperature oxidation behavior, but also presents mechanical properties after welding, cold bending and hot induction bending.
