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 downstream oil industry applications, high-temperature sulfidation corrosion is generally caused by sulfur species coming from the crude; additionally, naphthenic acids or hydrogen can considerably worsen the corrosivity of the environment. During plant operations, several events may occur that boost the severity of corrosion: high feedstock turnover, with increasing “active” sulfur species; skin temperature rise due to the increasing insulation effect of the scale, generating an over-tempering of the material and possible degeneration into creep conditions. Thor115 is a ferritic steel with 11% chromium content to resist sulfidation. It has excellent creep properties for high temperature environments: higher allowable stresses than grade 91, keeping the same manufacturing and welding procedures. At the same time, it has the characteristics of ferritic steel, ensuring enhanced thermal conductivity and lower thermal expansion compared to austenitic steels. Comparative corrosion tests between Thor115 and other ferritic steels typically used in this industry (e.g., grade T/P5 and grade T/P9) have been carried out to simulate different corrosive conditions, confirming the superior properties of Thor115 relative to other ferritic grades. For these reasons, Thor 115 is a suitable replacement material for piping components that need an upgrade from grade T/P9 or lower, in order to reduce corrosion rate or frequency of maintenance operations.

Modern gas turbines are operated with fuels that are very clean and within the allowances permitted by fuel specifications. However, the fuels that are being considered contain vanadium, sulfur, sodium and calcium species that could significantly contribute to the degradation of components in hot gas flow path. The main potential risk of material degradation from these fuels is “hot corrosion” due to the contaminants listed above combined with alkali metal salts from ambient air. Depending on the temperature regime hot corrosion can damage both TBC coatings and bond coat/substrate materials. Deposit-induced or hot corrosion has been defined as “accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposit”. For the initiation of hot corrosion, deposition of the corrosive species, e.g. vanadates or sulfates, is necessary. In addition to the thermodynamic stability, the condensation of the corrosive species on the blade/vane material is necessary to first initiate and then propagate hot corrosion. Operating temperatures and pressures both influence the hot corrosion damage. The temperature ranges over which the hot corrosion occurs depend strongly on following three factors: deposit chemistry, gas constituents and metal alloy (or bond coating/thermal barrier coating) composition. This paper reports the activities involved in establishing modeling and simulation followed by testing/characterization methodologies in relevant environments to understand the degradation mechanisms essential to assess the localized risk for fuel flexible operation. An assessment of component operating conditions and gas compositions throughout the hot gas paths of the gas turbines, along with statistical materials performance evaluations of metal losses for particular materials and exposure conditions, are being combined to develop and validate life prediction methods to assess component integrity and deposition/oxidation/corrosion kinetics.

A new 9%Cr steel with high boron levels (boron steel) has been developed by optimization studies on steels and alloys that are applicable to advanced ultra-super critical power plants operated at steam conditions of 700°C and 30 MPa and above. The composition and heat treatment condition of boron steel was optimized by the initial hardness, tensile strength, yield strength, and Charpy impact values on the basis of the fundamental investigation with the stability of the long-term creep strength. Creep testing of boron steel was conducted at temperatures between 600 and 700°C. The creep rupture strength at 625°C and 105 h is estimated to be 122 MPa for the present 9% Cr steel with high boron by Larson-Miller parameter method. Furthermore, physical properties as a function of temperature, metallurgical properties, tensile properties, and toughness were examined to evaluate the applicability of the steel for a 625°C USC power plant boiler. It was also confirmed that the steel has good workability for such an application by the flaring and flattening tests with tube specimens having an outer diameter of approximately 55 mm.

This study demonstrates the Electro-Thermal Mechanical Testing (ETMT) system's capability to analyze the thermo-mechanical behavior of Inconel 718 (IN718) at a heating rate of 5 °C/s, achieving temperatures up to 950 °C. The temperature profile peaks at the sample's center and is approximately 25 °C at the extremes. Upon reaching 950 °C, the sample was aged for 30 hours before being rapidly quenched. This process froze the microstructure, preserving the phase transformations that occurred at various temperatures across the temperature parabolic gradient, which resulted in a complex gradient microstructure, providing a comprehensive map of phase transformations in IN718. The integration of thermal measurement, COMSOL modeling, scanning electron microscopy enabled a thorough characterization of the microstructural evolution in IN718, linking observed phases to the specific temperatures which provided a rapid screening of the effect of using different heating treatment routes.

Austenitic stainless steels have been used for boiler tubes in power plants. Since austenitic stainless steels are superior to ferritic steels in high temperature strength and steam oxidation resistance, austenitic stainless steel tubes are used in high temperature parts in boilers. Dissimilar welded joints of austenitic steel and ferritic steel are found in the transition regions between high and low temperature parts. In dissimilar welded parts, there is a large difference in the coefficient of thermal expansion between austenitic and ferritic steel, and thus, thermal stress and strain will occur when the temperature changes. Therefore, the dissimilar welded parts require high durability against the repetition of the thermal stresses. SUPER304H (18Cr-9Ni-3Cu-Nb-N) is an austenitic stainless steel that recently has been used for boiler tubes in power plants. In this study, thermal fatigue properties of a dissimilar welded part of SUPER304H were investigated by conducting thermal fatigue tests and finite element analyses. The test sample was a dissimilar welded tube of SUPER304H and T91 (9Cr-1Mo-V-Nb), which is a typical ferritic heat resistant boiler steel.

In order to enable a compact design for boiler superheaters in modern thermal power plants, cold-worked tube bending is an economical option. For service metal temperatures of 700 °C and above, nickel-based alloys are typically employed. To ensure a safe operation of such cold-worked alloys, their long-term mechanical behavior has to be investigated. In general, superheater tube materials in a cold-worked state are prone to a degradation of their long-term creep behavior. To predict this degradation, sensitive experiments have to be conducted. In this publication, the effects of cold working on the long-term creep behavior of three currently used nickel-based alloys are examined. Creep and creep rupture experiments have been conducted at typical service temperature levels on nickel-based alloys, which have been cold worked to various degrees. As a result, Alloy 263 exhibits no significant influence of cold working on the creep rupture strength. For Alloy 617, an increase of creep strength due to cold working was measured. In contrast, Alloy 740 showed a severe degradation of the creep strength due to cold working. The mechanism causing the sensitivity to cold working is not yet fully understood. Various formations of carbide precipitates at the grain boundaries are believed to have a major influence. Nevertheless, the experimentally observed sensitivity should always be considered in material selection for boiler tube design.

Type IV creep damage of high chromium steel is a problem in thermal power plants and a method of evaluating remaining life is required. Type IV creep damage is characterized by many voids that initiate in the weldment fine grain heat affected zone (FGHAZ), where the stress multiaxiality (expressed by the Triaxiality Factor, TF) is high. As the creep continues, the shape of the grain boundary becomes simple; that is, close to a straight line. It is known that the grain boundary is fractal. The complexity of the fractal is represented by the fractal dimension. Therefore, we considered that the fractal dimension of the grain boundary in FGHAZ could be an indication of creep damage and studied its change as creep proceeded. First, creep tests were conducted to produce damaged materials, and their fractal dimensions were measured. Next, FEM analysis was conducted to obtain the distribution of the principal stress, TF, and creep strain of the observed surface. The distribution of creep damage was obtained by the time fraction rule. The results of this evaluation confirmed that the fractal dimension of the grain boundary decreases with creep time and that the principal stress and TF affect it.

High-temperature corrosion occurs in different sections of energy production plants due to a number of factors: ash deposition, coal impurities, thermal gradients, and low NO x conditions, among others. High-temperature electrochemical corrosion rate (ECR) probes are rarely used at the present time, but if they were more fully understood, corrosion could become a process variable at the control of plant operators. Research is being conducted to understand the effects of probe composition, ash composition, environment chemistry, and measurement technique on the accuracy, response, and longevity of electrochemical corrosion rate probes. The primary goal is to understand when ECR probes accurately measure corrosion rates and when they are simply qualitative indicators of changes in the corrosion processes. Research to date has shown that ECR probe corrosion rates and corrosion rates from mass loss coupons agree within a factor of 2. This good agreement was found to depend on the composition of the sensors, with the best results coming from more highly alloyed materials such as 316L stainless steel and poorer results from carbon steel sensors. Factors being considered to help explain the good or poor agreement between mass loss and ECR probe corrosion rates are: values selected for the Stern-Geary constant, the effect of internal corrosion, and the presence of conductive corrosion scales and ash deposits.

This paper reports on the latest in a series of projects aiming at the qualification of new and proven materials in components under a severe service environment. In the initial stages of the project (HWT I & HWT II), a test loop at Unit 6 of the GKM Power Plant in Mannheim was used to study the behavior of components for advanced ultra-supercritical (A-USC) plants made from nickel alloys at 725 °C under both static and fluctuating conditions. Due to recent changes in the operation modes of existing coal-fired power plants, the test loop was modified to continue operating the existing nickel components in the static section while applying thermal cycles in a different temperature range. HR6W pipes and valves were added to the bypass of the static section, and all components in the cyclic section were replaced with P92, P93, and HR6W components. The test loop achieved approximately 9000 hours of operation and around 800 cycles with holding times of 4 and 6 hours. After dismantling the loop, nondestructive and destructive examinations of selected components were conducted. The accompanying testing program includes results from thermal fatigue, fatigue, thermal shock, and long-term creep tests, focusing on the behavior of base materials and welds, particularly for HR6W, P92, P93, and other nickel-based alloys. Additionally, test results on dissimilar welds between martensitic steel P92 and nickel alloys A617 and HR6W are presented. Numerical assessments using standardized and numerical lifetime estimation methods complement the investigations. This paper provides insights into the test loop design and operational challenges, material behavior, and lifetime, including advanced numerical simulations and operational experiences with valves, armatures, piping, and welds.

Laboratory-scale tests are frequently used to generate understanding of high-temperature oxidation phenomena, to characterise and rank the performance of existing, future materials and coatings. Tests within the laboratory have the advantage of being well controlled, monitored and offer the opportunity of simplification which enables the study of individual parameters through isolating them from other factors, such as temperature transients. The influence of pressure on the oxidation of power plant materials has always been considered to be less significant than the effects of temperature and Cr content, but still remains a subject of differing opinions. Experimental efforts, reported in the literature, to measure the influence of steam pressure on the rate of oxidation have not produced very consistent or conclusive results. To examine this further a series of high pressure steam oxidation exposures have been conducted in a high pressure flowing steam loop, exposing a range of materials to flowing steam at 650 and 700 °C and pressure of 25, 50 and 60 bar. Data is presented for ferritic-martensitic alloys showing the effect of increasing pressure on the mass change and oxide thickness of these alloys in the flowing steam loop. In addition the effect observed on the diffusion of aluminium from an aluminised coating in these alloys is also presented and the differences in the extent of diffusion discussed.

Texas Genco requested Stress Engineering Services to assist in reviewing and assessing a failure that occurred in the cold reheat (CRH) steam line at the W.A. Parish Unit around 12:10 PM on July 15, 2003, resulting in a catastrophic failure scattering components within a 1,200-foot radius. Reliant Resources and Texas Genco conducted their own investigation involving metallographic examinations, fracture surface inspection, review of operating conditions at failure time, and studies related to the CRH line weld profile. Stress Engineering Services' efforts included computational fluid dynamics studies to address how attemperator droplet sizes might impact downstream piping system behavior, followed by mock-up testing and field monitoring using high-temperature strain gauges, accelerometers, and thermocouples. The field monitoring data, along with process data from Texas Genco, were used for finite element analyses calculating static stresses and transient stresses from attemperator cycling (thermal stresses) and line vibration (mechanical stresses). A consulting firm contracted by the Electric Power Research Institute (EPRI) performed a fracture mechanics evaluation of the line, though detailed results are not included. The work by Texas Genco, Stress Engineering Services, and EPRI points to the stress concentration factor associated with the internal weld profile near the failure as the primary cause, with the cyclic thermal shocks from frequent intermittent attemperator use being sufficient to initiate the crack.

This study investigates how temperature affects the plasticity and thermal creep behavior of 347H stainless steel under uniaxial tension. The research combined experimental testing with advanced computational modeling. Two types of experiments were conducted: uniaxial tensile tests at temperatures from 100°C to 750°C using strain rates of ~10⁻⁴ s⁻¹, and creep tests at temperatures between 600°C and 750°C under various stress levels. These experimental results were used to develop and validate a new integrated mechanistic model that can predict material behavior under any loading condition while accounting for both stress and temperature effects. The model was implemented using a polycrystalline microstructure simulation framework based on elasto-viscoplastic Fast Fourier Transform (EVPFFT). It incorporates three key deformation mechanisms: thermally activated dislocation glide, dislocation climb, and vacancy diffusional creep. The model accounts for internal stress distribution within single crystals and considers how precipitates and solute atoms (both interstitial and substitutional) affect dislocation movement. After validation against experimental data, the model was used to generate Ashby-Weertman deformation mechanism maps for 347H steel, providing new insights into how microstructure influences the activation of different creep mechanisms.

An innovative additively manufactured gradient composite transition joint (AM-GCTJ) has been designed to join dissimilar metals, to address the pressing issue of premature failure observed in conventional dissimilar metal welds (DMWs) when subjected to increased cyclic operating conditions of fossil fuel power plants. The transition design, guided by computational modeling, developed a gradient composite material distribution, facilitating a smooth transition in material volume fraction and physical properties between different alloys. This innovative design seeks to alleviate structural challenges arising from distinct material properties, including high thermal stress and potential cracking issues resulting from the thermal expansion mismatch typically observed in conventional DMWs. In this study, we investigated the creep properties of transition joints comprising Grade 91 steel and 304 stainless steel through a combination of simulations and creep testing experiments. The implementation of a gradient composite design in the plate transition joint resulted in a significant enhancement of creep resistance when compared to the baseline conventional DMW. For instance, the creep rupture life of the transition joint was improved by > 400% in a wide range of temperature and stress testing conditions. Meanwhile, the failure location shifted to the base material of Grade 91 steel. Such enhancement can be primarily attributed to the strong mechanical constraint facilitated by the gradient composite design, which effectively reduced the stresses on the less creep-resistant alloy in the transition zone. Beyond examining plate joints, it is crucial to assess the deformation response of tubular transition joints under pressure loading and transient temperature conditions to substantiate and demonstrate the effectiveness of the design. The simulation results affirm that the tubular transition joint demonstrates superior resistance compared to its counterpart DMW when subjected to multiaxial stresses in tubular structures. In addition, optimization of the transition joint’s geometry dimensions has been conducted to diminish the accumulated deformation and enhance the service life. Lastly, the scalability and potential of the innovative transition joints for large-diameter pipe applications are addressed.

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.

The creep degradation/life assessment for high temperature critical component materials is absolutely needed to assure the long-term service operation and there is little experience with the service exposure of the high temperature components made of newly developed Ni-based alloys. In this study, therefore, the creep degradation assessment study on the Ni-based alloys, Alloy 617 and HR6W was conducted based on the hardness method, because the hardness measurement is a useful and simple technique for the materials characterization for any kind of high temperature-serviced steels and alloys. As the result, it was found that the hardness was increased by not only precipitation due to thermal aging but also creep stress/strain, and there existed linear relationship between the applied stress and creep-induced hardness increase. Also the hardness scatter measured was increased along with the progress of creep hardening and damage progressing in terms of creep life consumed. Those findings suggested that the creep life assessment of Ni-based alloys would be possible by means of hardness measurement. The paper also deals with the role and perspective development of non destructive damage detecting techniques, and life assessment issues on Ni-based alloys for A-USC power applications.

Grade 91 steel has been found to be susceptible to Type IV cracking in the base metal heat affected zone (HAZ). In order to better understand this type of failure, a study on the metallurgical reactions occuring within the HAZ was conducted, particularly within the fine grained (FG) and intercritical (IC) regions where Type IV cracking is most commonly found to occur. The course grained (CG), FG and IC regions of the HAZ in Grade 91 steel were simulated using a Gleeble 3800 Thermo-Mechanical Simulator. A dilatometer was used to determine the phase transformations occuring during simulation of weld thermal histories. For the first time, it was shown that ferrite can form in the IC HAZ of Grade 91 steel welds. The magnitude of the ferrite transformation was observed to decrease with faster cooling rates. The presence of ferrite in the simulated IC HAZ microstructure was shown to decrease the high temperature tensile strength and increase the high temperature elongation compared to HAZ regions that did not undergo ferrite transformation. Welding parameters such as heat input, preheat and interpass temperature can be selected to ensure faster cooling rates and reduce or potentially avoid formation of ferrite in the IC HAZ.

A paste, which contains Pt or Pt-xIr (x = 0-30 at%) alloy nano-powder was sprayed on some Ni-based single crystal superalloys. Then the annealing diffusion treatment at 1100 °C for 1 h in flowing Ar atmosphere was conducted to develop Pt and Pt-Ir diffusion coatings. Cyclic oxidation tests were carried out at 1150 °C in still air in order to investigate the thermal stability and oxidation behavior of the coatings and they were compared with electroplated diffusion coatings. It was found that Ir can retard the formation of voids in both the coatings and substrates. In addition, by replacing the electroplating method to the paste coating method, the crack problem due to the brittle feature of electroplated Pt-Ir coatings could be solved. Therefore, the Pt-Ir diffusion coating prepared by the paste- coating method is promising as the bond-coat material due to suppression of voids, cracks and stable Al 2 O 3 on the surface. The Pt-Ir paste diffusion coatings introduced above have several further advantages: they are easy to recoat, cause less damage to substrates, and offer comparable oxidation resistance. Thus, the method can be applicable to the remanufacturing of blades, which may extend the life of components. The future aspect of the paste coating will also be discussed.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

We have reported on the effort being done to develop the A-USC technology in Japan, which features the 700 deg-C steam condition, since the 2007 EPRI conference. Our 9 year project began in 2008. There have been some major changes in the electricity power market in the world recently. At first, the earthquake changed the power system violently in Japan. Almost all nuclear power plants have been shut down and natural gas, oil and coal power plants are working fully to satisfy the market's demands. In the USA, the so called ‘Shale gas revolution’ is going on. In Europe, they are working toward the target of reducing CO 2 emissions by the significant use of renewables with the backup of the fossil fuel power systems and enhancing power grids. A very rapid increase in power generation by coal is being observed in some countries. Despite some major changes in the electric sector in the world and the CO 2 problem, the global need for coal power generation is still high. We can reconfirm that the improvement of the thermal efficiency of coal power plants should be the most fundamental and important measure for the issues we are confronting today, and that continuous effort should be put towards it. Based on the study we showed at the 2007 conference, we developed 700 deg-C class technology mainly focusing on the material and manufacturing technology development and verification tests for key components such as boilers, turbines and valves. Fundamental technology developments have been done during the first half of the project term. Long term material tests such as creep rupture of base materials and welds will be conducted for 100,000hrs continuing after the end of the project with the joint effort of each participating company. Today, we are preparing the plan for the second half of the project, which is made up of boiler components test and the turbine rotating tests. Some boiler superheater panels, large diameter pipes and valves will be tested in a commercially operating boiler from 2015 to 2017. The turbine rotor materials which have the same diameter as commercial rotors will be tested at 700 deg-C and at actual speed.