Search Results for boilers

The article gives a brief overview of the newly developed austenitic material “Power Austenite”. The microstructure of the Power Austenite is characterized by grain boundary strengthening with boron stabilized M23(C,B)6 and secondary Nb(C,N) in combination with sigma phase and Nb(C,N) as the major grain strengthening precipitates. The material shows a significant creep strength at 700 °C (1292 °F) and 650 °C (1202 °F) as well as fireside corrosion resistance which makes it a possible candidate for 700 °C (1292 °F) power plants.

Seeking to reduce CO 2 emissions and improve power generation efficiency, a project to develop a 700°C A-USC (advanced ultra super critical) power plant has been under way in Japan since 2008. HR6W (44Ni-23Cr-7W) is a candidate material for application in the maximum temperature areas of A-USC boilers. In this study, the creep rupture properties of plastic deformed material were investigated in comparison with those of solution treated material, in order to clarify the capability of HR6W as a material for use in A-USC plants. The creep strength of 20% pre-strained HR6W was found to increase substantially as compared with the solution treated material. 20% pre-strained material is in a state where high dislocation density has been introduced by plastic forming strain, with M 23 C 6 and Laves phase precipitating preferentially by dislocation diffusion from the early stages of creep. In particular, since high dislocation density is accumulated in connection with creep deformation near the grain boundaries, precipitation is accelerated and the grain boundaries are covered with M 23 C 6 from the early stages of creep. Then, even though the intragranular precipitate density decreases, given that the fraction of grain boundaries affected by precipitation is maintained in a high state, it is presumed that a high density of dislocation is maintained in the long-term region. This was considered to be the reason why the creep rupture strength of the 20% pre-strained material increased so remarkably in comparison with the solution treated material.

High temperature regions in the upper sections of the advanced ultrasupercritical (AUSC) boilers are exposed to temperatures higher than traditional supercritical (SC) boilers and require high strength materials. Use of modified 9-12% Cr materials such as T91 and T92, while meeting the strength requirements, are still under research stage for large-scale fabrication of the membrane walls for several reasons, such as required post weld heat treatment PWHT (ASME Code) or hardness limits on as-welded structures (European codes). The main objective of this paper is to explore alternate tubing materials that do not require a PWHT in the high temperature sections of the AUSC boiler membrane walls. Composite bimetallic tubing with high strength cladding, applied by weld overlay or co-extrusion that may meet the requirement of high operating temperature and high overall strength, is addressed through an alternate design criterion. Bimetallic tubes can replace the single metal tubes made from 9-12% Cr materials. The bimetallic tube is assumed to be fabricated from Grade 23 steel (base tubes) with Alloy 617 overlaid. The alternate design method is based on an iterative analytical solution for the through-wall heat transfer and stresses in a composite tube with temperatures and strength variations of both the materials considered in detail. A number of different analyses were performed using the proposed analytical approach, methodology verified through benchmark solutions and then applied to the membrane wall configurations.

Inconel alloy 740H was specifically developed for use in coal-fired AUSC boilers. This alloy displays a unique combination of steam and coal-ash corrosion resistance, microstructure stability, creep strength and heavy section weldability. During the past two years Special Metals and Wyman-Gordon have undertaken an intense effort to demonstrate their capability to manufacture full-size boiler components, characterize their properties and simulate field assembly welds. This work was performed according to the requirements of ASME Boiler Code Case 2702 that was recently issued. This paper covers manufacturing of tube and pipe products and property characterization including recent data on the effect of long time exposure on impact toughness of base and weld metal. New data will also be reported on coal ash corrosion of base metal and weld metal. An overview of welding studies focused on integrity of circumferential pipe joints and a discussion of remaining technical issues will be presented.

A model based on a concept of “fraction of exfoliated area” as a function of oxide scale strain energy was developed to predict the extent of exfoliation of steam-side scale from boiler tube superheater loops. As compared with the Armitt diagram, which can be used to predict when scale damage and exfoliation would be likely to occur, a “fraction of exfoliated area” approach provides an estimation of mass of scale released and the fraction of tube likely to be blocked by the exfoliation. This paper gives results for the extent of blockage expected in a single bend of a superheater loop was predicted as a function of operating time, bend geometry, and outlet steam temperature under realistic service conditions that include outages. The deposits of exfoliated scale were assumed to be distributed horizontally the tubes bends. Three types of bends were considered: regular bends, short bends, and hairpin bends. The progressive increase in steam and tube temperatures along a single loop of superheater tubing and the ensuing variation of oxide scale thickness are considered. Numerical simulation results for a superheater loop made of TP347H austenitic steel indicated that tube blockage fractions larger than 50% are likely to occur within the first two years of boiler operation (with regularly scheduled outages) for outlet tube temperatures of 540-570°C, which is consistent with practical experience. Higher blockage fractions were predicted for tubes with hairpin bends than for tubes with regular bends, of length that are larger than five internal tube diameters. Finally, the blockage model presented can be used with some confidence to devise operating schedules for managing the consequences of oxide scale exfoliation based on projections of time to some critical blockage fraction for specific boiler operating conditions.

Inconel Filler Metal 72 (FM 72) and Incoclad 671/800H co-extruded tubing have been successfully used for over 20 years to protect boiler tubing from high-temperature degradation. A newer alloy, FM 72M, offers superior weldability and the lowest corrosion rate in simulated low NOx environments. Both FM 72 and 72M show promise in addressing challenges like circumferential cracking and corrosion fatigue in waterwall tubing overlays. Additionally, 72M’s superior wear resistance makes it ideal for replacing erosion shields in superheater and reheater tubing. Beyond improved protection, these alloys exhibit increased hardness and thermal conductivity over time, leading to reduced temperature difference across the tube wall and consequently, enhanced boiler efficiency and lower maintenance costs. This paper discusses the historical selection of optimal alloys for waterwall and upper boiler tubing overlays, analyzes past failure mechanisms, and highlights the key properties of successful choices like FM 72 and 72M.

In recent years continuous and extensive research and development activities have been being done worldwide on 700°C A-USC (Advanced Ultra Super Critical) power plants to achieve higher efficiency and reduce the CO 2 emission. Increasing steam temperature and pressure of such A-USC boilers under consideration require the adoption of Ni based alloys. In the Japanese national project launched in 2008, Ni based alloy HR6W (45Ni-23Cr-7W-Ti, ASME Code Case 2684) is one of the candidate materials for boiler tube and pipe as well as Alloy617, Alloy263 and Alloy740H. The most important issues in A-USC boiler fabrication are the establishment of proper welding process for thick wall components of these alloys and verification of the long term reliability of their weldments. In our previous study, the weldability of HR6W was investigated and the welding process for Ni based thick wall pipe was established with the narrow gap HST (Hot wire Switching TIG) welding procedure originally developed by Babcock-Hitachi K.K. In this paper, creep rupture strengths of HR6W weldment were verified by the long term test up to 60,000 hours for tube and 40,000 hours for pipe. In Japanese national project, narrow gap HST welding process was also applied to the welding test for the other Ni based candidate pipe materials. Furthermore, as the practical A-USC boiler manufacturing trials, header mockup test was conducted and qualified for HR6W.

The addition of boron without the formation of any boron nitrides during normalizing heat treatment at high temperature minimizes the degradation in creep strength of both base metal and welded joints of 9Cr steel at 650 °C and long times. The enrichment of soluble boron near prior austenite grain boundaries (PAGBs) by the segregation is essential for the reduction of coarsening rate of M 23 C 6 carbides in the vicinity of PAGBs, enhancing boundary and subboundary hardening, and also for the production of same microstructure between the base metal and heat-affected-zone (HAZ) in welded joints, indicating no Type IV fracture in HAZ. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalizing, which reduces the soluble boron concentration and accelerates the degradation in creep rupture ductility at long times. 9Cr- 3W-3Co-VNb steel with 120 - 150 ppm boron and 60 - 90 ppm nitrogen (MARBN) exhibits not only much higher creep strength of base metal than Gr.92 but also substantially no degradation in creep strength due to Type IV fracture at 650 °C. The pre-oxidation treatment in Ar gas promotes the formation of protective Cr 2 O 3 scale on the surface of 9Cr steel, which significantly improves the oxidation resistance in steam at 650 °C.

Recently, boiler waterwall tube damage such as fireside corrosion and circumferential cracking in low NOx environments has become a serious issue in Japan, despite the typical use of relatively lower sulfur content coal is typically being used than in US. Thermal spray coating has been the most popular method for tube protection in Japan, and thermal spray coated tubes have been used for this purpose. However, extensive damage to thermal spray coating tubes from cracking and exfoliation has been recently experienced. It has been reported that the thermal fluctuations occurring due to operational changes create alternating stress, leading to cracking and exfoliation of the thermal sprayed thin coating. Corrosion-resistant weld overlays, such as Type 309 stainless steel (in sub-critical boilers) and Alloy 622 (in sub-critical and super-critical boilers), are commonly used to protect boiler tubes from corrosion in low NOx coal fired boilers in U.S. In order to develop a fundamental understanding of the high temperature corrosive behavior of Alloy 622 weld overlay, gaseous corrosion testing and certain mechanical tests for consideration of long-term aging were undertaken. After four years of service in the low NOx combustion environment of a coal fired supercritical boiler, field tests on Alloy 622 weld overlay panels are in continuation. This paper describes the field test behavior of Alloy 622 weld overlay panels installed in a Japanese supercritical boiler, the laboratory results of weight loss corrosion testing, and the results of cyclic bend tests with overlay welded tubes related to aging.

Japanese USC-PC units successfully operate with steam temperatures of 610°C (1130°F). IHI is developing the next generation of USC-PC boilers, targeting 620°C (1150°F) for improved efficiency. Investigations are underway for even higher 700°C (1290°F) steam temperatures. IHI collaborates with turbine manufacturers to create reliable and efficient boiler-turbine packages for both new and existing power plants.

In Europe, the development of boilers and steam turbines for operation above 700°C is part of the EU-supported AD700 project. This collaborative effort includes major European power plant manufacturers, utilities, and research institutes. The project began in 1998 and was extended to 2003, with a second phase running from 2002 to 2005, potentially extending further for long-term creep tests. The goal is to develop the necessary technology for constructing and operating such plants. This paper outlines the development of high-temperature materials crucial for the AD700 project. It covers factors influencing alloy design and selection, the scope and results of investigations on candidate alloys, and the ongoing program for full-scale prototype component manufacturing. These prototypes undergo extensive long-term testing. Additionally, the development of joining procedures for these materials is discussed.

This paper focuses on the key properties of newly developed high-strength, heat-resistant steels for application in ultra-supercritical (USC) boilers. For some ferritic steels, improvements made to enhance their resistance to steam oxidation are highlighted. The latest welding techniques employed for these steels are introduced. Additionally, the high-temperature strength and weldability of Alloy 617 (52Ni-22Cr-13Co-9Mo-Ti-Al), a potential candidate material for the next generation of 700°C USC boilers, are described. The paper provides insights into the materials and welding technologies crucial for the development of advanced USC boilers operating at higher temperatures.

Utilities worldwide are facing increased demand for additional electricity, reduced plant emissions and greater efficiency. Part of the solution is achieved by increasing boiler temperature, pressure and coal ash corrosion resistance of the materials of boiler construction. In this paper, a new nickel-base tube alloy, INCONEL alloy 740, meeting this challenge is characterized with emphasis on mechanical properties, coal ash and steam corrosion resistance as well as weldability. Microstructural stability as a function of temperature and time is addressed as well as some of the early methodology employed to arrive at the current chemical composition. Brief mention is made of certain current and future alloy characterization efforts and potential environmental benefits to be expected should the boiler technology utilizing INCONEL alloy 740 be adopted.

The power generation industry worldwide aims to develop coal-fired boilers operating at much higher efficiencies than current supercritical plants. This increased efficiency is expected through ultrasupercritical steam conditions, requiring new materials for critical components. To limit the use of expensive alloying materials, it is necessary to maximize the strength and corrosion capabilities across the material spectrum from ferritic to austenitic and nickel-based alloys. Sandvik Materials Technology has developed an austenitic alloy, Sanicro 25, with excellent high-temperature strength and corrosion resistance using an economical alloy composition. The alloy is designed for use within 700°C (1300°F)/300 bar (4500 psi) steam conditions and is a leading candidate material for such high-temperature applications. This paper introduces Sanicro 25, its development status, and properties.

Ultrasupercritical (USC) coal-fired boilers, currently under study, will be required to utilize a variety of new, high strength alloys. These alloys will have improved mechanical properties compared with more traditional boiler materials and so will be suitable for operation in higher temperature service. However, environmental resistance, i.e. internal steam oxidation and external coal-ash corrosion, will be a factor limiting application of some materials under consideration. In those cases, the operating range of lower-cost alloys can be significantly extended by the use of surface modification techniques. This paper will review potential surface modification techniques and report on early test results of some laboratory evaluations.

The demand for higher efficiency and reduced emissions in coal-fired power boilers will result in the use of higher steam temperatures and pressures. A significant materials effort is required to reach a target steam condition of 760°C/35MPa. These new Ultrasupercritical (USC) units will require the use of nickel-based superalloys. Long-term creep strength will be a determining factor in achieving the highest possible steam conditions. To this end, the creep strength of commercially available (Haynes 230), modified/controlled chemistry (CCA617/Maгco 617), and new (INCONEL 740) alloys, including weldments, are being investigated at Oak Ridge National Laboratory (ORNL). Creep tests at ORNL show that the CCA617 provides a significant improvement in strength over the standard alloy 617 at 650°C to possibly 750°C. The strength of alloy 230 is well characterized, thus the testing on 230 has focused on specific specimen configurations for evaluating the high temperature behavior of weldments. Creep testing on INCONEL alloy 740 has shown good strengths (higher than 230 or CCA617) that may meet the target steam conditions. Microstructural analysis by electron microscopy on aged and tested material is being used to further understand the structure-properties relationship in these materials and determine long-term stability of the microstructures.

Recent advances in materials technology for boilers materials in the advanced USC (A-USC) power plants have been reviewed based on the experiences from the strengthening and degradation of long term creep properties and the relevant microstructural evolution in the advanced high Cr ferritic steels. P122 and P92 type steels are considered to exhibit the long term creep strength degradation over 600°C, which is mainly due to the instability of the martensitic microstructure strengthened too much by MX carbonitrides. This can be modified by reducing the precipitation of VN nitride and by optimizing the Cr content of the steels. An Fe-Ni based alloy, HR6W strengthened by the Fe2W type Laves phase is found to be a marginal strength level material with good ductility at high temperatures over 700°C and to be used for a large diameter heavy wall thick piping such as main steam pipe and hot reheat pipe in A-USC plants, while Ni-Co based alloys such as Alloys 617 and 263 strengthened by a large amount of the y’ phase are found to be the high strength candidate materials for superheater and reheater tubes, although they are prone to relaxation cracking after welding and to grain boundary embrittlement during long term creep exposure. A new Ni based alloy, HR35 strengthened by a-Cr phase and other intermetallic phases has been proposed for piping application, which is specially designed for a good resistance to relaxation cracking as well as high strength and a good resistance to steam oxidation and fire-side corrosion at high temperatures over 700°C.

Inconel alloy 740 was initially developed to enable the design of coal-fired boilers capable of operating at 700°C steam temperature and high pressure. The alloy successfully met the European program's targets, including 100,000-hour rupture life at 750°C and 100 MPa stress, and less than 2 mm metal loss in 200,000 hours of superheater service. However, thick section fabrication revealed weldability challenges, specifically grain boundary microfissuring in the heat affected zone (HAZ) of the base metal. This paper describes the development of a modified variant with significantly improved resistance to HAZ microfissuring and enhanced thermal stability, while maintaining desirable properties. The formulation process is detailed, and properties of materials produced within the new composition range are presented. Additionally, the microstructural stability of the original and modified alloy compositions is compared, demonstrating the advancements achieved in this critical material for next-generation power plants.

The creep enhanced low alloy steel with 2.25Cr-1.6W-V-Nb (HCM2S; Gr.23, ASME CC2199) has been originally developed by Mitsubishi Heavy Industries, Ltd. and Sumitomo Metal Industries, Ltd. The steel tubes and pipe (T23/P23) are now widely used for fossil fired power plants all over the world. Recently, the chemical composition requirements for ASME Code of the steel have been changed and a new Code Case 2199-4 has been issued with the additional restriction regarding Ti, B, N and Ni, and the Ti/N ratio incorporated. In this study, the effects of additional elements of Ti, N and B on the mechanical properties and microstructure of T23/P23 steels have been evaluated. It is found that N decreases the hardenability of the steel by forming BN type nitride and thus consuming the effective B, which is a key element for hardening of the steel. The addition of Ti, on the other hand, enhances the hardenability of the steel by precipitating TiN and thus increasing the effective B. It is also found that too much addition of Ti degrades the Charpy impact property and creep ductility of the steel to a great extent. This phenomenon might affect the steel's long-term creep rupture properties, although a steel with the original chemical composition has demonstrated high creep strength at temperatures up to 600°C for more than 110,000 h.

In the “Boiler Materials for Ultrasupercritical Coal Power Plants” program, sponsored by the U.S. Department of Energy and the Ohio Coal Development Office, various materials are being assessed for their suitability in the high-temperature, high-pressure environment of advanced ultrasupercritical (A-USC) boilers. Beyond mechanical properties and fireside corrosion resistance, these materials must also exhibit adequate steamside oxidation and exfoliation resistance. A comprehensive database of steamside oxidation test results at temperatures relevant to A-USC conditions has been compiled over recent years. These tests have been conducted on ferritic and austenitic materials with chromium content ranging from 2 to 26%. The specimens were evaluated for oxidation kinetics and oxide morphology. The findings indicate that steamside oxidation behavior is significantly affected by temperature, the chromium content of the material, and the ability of chromium to diffuse through the material's crystallographic lattice structure. Additionally, surface treatments have been applied to enhance the steamside oxidation resistance of certain materials. While these treatments have shown potential, their effectiveness can be limited by the operational temperatures.