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.

Because of the problems experienced with steam-side oxide scale exfoliation in commercial power plants, there has been increased interest in understanding the steam oxidation resistance of 300- series stainless steels such as 347H and 304H. Model alloys were used in an attempt to understand the effect of varying Ni (9-12%) and Cr (16-20%) on steam oxidation resistance at 650°C. However, the model alloys generally showed superior oxidation resistance than commercial alloys of similar composition. Several surface engineering solutions also were investigated. The commercially favored solution is shot peening. Laboratory steam testing at 650°C found that annealing temperatures of ≥850°C eliminated the benefit of shot peening and a correlation was observed with starting hardness in the peened region. This effect of annealing has implications for the fabrication of shot peened tubing. Another route to improving oxidation resistance is the use of oxidation resistant diffusion coatings, which can be deposited inexpensively by a vapor slurry process. Uniform coatings were deposited on short tube sections and annealed at 1065°C to retain good 650°C creep properties. The coating was thicker than has been investigated in laboratory processes resulting in increased brittleness when the coating was assessed using 4-point bending.

The mechanisms of recent cracking failures of HR3C super heater pipes of a fossil power plant in the Netherlands were investigated. Initial failure investigations showed that pitting corrosion of the sensitized HR3C initiated subsequent stress corrosion cracking (SCC). It was concluded that magnesium chloride hydrates from condensed seawater had initiated pitting corrosion as well as SCC similar to the standard ASTM G36 SCC test. By experimental application of the ASTM G36 procedure, this tentative mechanism is reproduced and confirmed by a series of laboratory tests with pure magnesium chloride as well as with synthetic seawater. It included the effects of temperature, magnesium chloride concentrations of the evaporating water and applied bending moments on cracking. As a result for the 175h testing period in MgCl2*6H 2 O cracking increases significantly above 100°C up to 120°C but is reduced slightly at temperatures up to 155°C. With increasing bending moments, the U-shaped test pieces revealed increasing crack depths up to total fracture of the 5mm thick sections. Lower magnesium chloride concentrations as in concentrated seawater provided identical cracking, however, to a lower extent. It is therefore concluded that the operational failure of the sensitized HR3C super heater pipes was initiated in presence of condensed seawater and followed the same mechanism as found in the experimental investigation. As a conclusion, the presence of seawater saturated air at temperatures between 100° and 155°C should be avoided.

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.

Welding of collector pipes, flat heads, dished ends and connector pipes performed with high temperature and creep-resistant steels most often has been performed using GTAW process combined with MMA processes. Progress in GMAW process and availability of high quality filler materials (solid wires) enables welding of the above connections also using this method. In order to prove its efficiency, this article presents the results of related tests. The range of tests was similar to that applied during the qualification of welding procedure. The investigation also involved microscopic and fractographic examinations and creep tests. The results reveal that welding with GMAW is by no means inferior to a currently applied SMAW method yet the time of the process is shorter by 50%. The article presents the world’s first known positive results in welding of P92 grade steel using GMAW welding method.

The use of high-nickel superalloys has greatly increased among many industries. This is especially the case for advanced coal-fired boilers, where the latest high temperature designs will require materials capable of withstanding much higher operating temperatures and pressures than current designs. Inconel alloy 740H (UNS N07740) is a new nickel- based alloy that serves as a candidate for steam header pipe and super-heater tubing in coal-fired boilers. Alloy 740H has been shown to be capable of withstanding the extreme operating conditions of an advanced ultra-super-critical (AUSC) boiler, which is the latest boiler design, currently under development. As with all high nickel alloys, welding of alloy 740H can be very challenging, even to an experienced welder. Weldability challenges are compounded when considering that the alloy may be used in steam headers, where critical, thick-section and stub-to-header weld joints are present. This paper is intended to describe the proper procedures developed over years of study that will allow for ASME code quality welds in alloy 740H with matching composition filler metals.

Laves phases are intermetallic phases well known for their excellent strength at high temperatures but also for their pronounced brittleness at low temperatures. Especially in high-alloyed steels, Laves phases were long time regarded as detrimental phases as they were found to embrittle the material. Perusing the more recent literature, it seems the negative opinion about the Laves phases has changed during the last years. It is reported that, if the precipitation morphology is properly controlled, transition metal-based Laves phases can act as effective strengthening phases in heat resistant steels without causing embrittlement. For a targeted materials development, the mechanical properties of pure Laves phases should be known. However, the basic knowledge and understanding of the mechanical behavior of Laves phases is very limited. Here we present an overview of experimental results obtained by micromechanical testing of single-crystalline NbCo 2 Laves phase samples with varying crystal structure, orientation, and composition. For this purpose, diffusion layers with concentration gradients covering the complete homogeneity ranges of the hexagonal C14, cubic C15 and hexagonal C36 NbCo 2 Laves phases were grown by the diffusion couple technique. The hardness and Young's modulus of NbCo 2 were probed by nanoindentation scans along the concentration gradient. Single-phase and single crystalline microcantilevers and micropillars of the NbCo 2 Laves phase with different compositions were cut in the diffusion layers by focused ion beam milling. The fracture toughness and the critical resolved shear stress (CRSS) were measured by in-situ microcantilever bending tests and micropillar compression tests, respectively. The hardness, Young's modulus and CRSS are nearly constant within the extended composition range of the cubic C15 Laves phase, but clearly decrease when the composition approaches the boundaries of the homogeneity range where the C15 structure transforms to the off stoichiometric, hexagonal C36 and C14 structure on the Co-rich and Nb-rich, respectively. In contrast, microcantilever fracture tests do not show this effect but indicate that the fracture toughness is independent of crystal structure and chemical composition of the NbCo 2 Laves phase.

This paper addresses the limitations of P92 steel used in ultra-supercritical power plants, particularly ferrite formation in thick components and its impact on short- and long-term properties. A guideline for determining ferritic content in P92 steel is presented. Furthermore, a novel 12% Cr boiler steel grade, VM12-SHC, is introduced. This new material offers good creep properties and oxidation resistance, overcoming the limitations of P92 steel. Finally, the development of matching filler metals for welding P92 and VM12-SHC steels is presented, ensuring optimal weld compatibility and performance in power plant applications.

A prototype small punch test rig has been developed to extend the range of data output. Through the introduction of a probe, vertical displacements can be measured across a region of the specimen underside. This information provides much greater understanding of the specimen deformation. Having displacement data at a series of measurement points also facilitates the calculation of strains across the sample. The probe can also be used during a test to provide time dependent data from small punch creep tests. The measured displacement data have been used in conjunction with FE analysis to determine a set of calibration curves for inferring strain at any given vertical displacement. Some creep strain data are also presented.

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in grade 91 steel. Failed hot tensile specimens have been sectioned at various positions along the specimen axis, and therefore at different levels of hot tensile strain, to obtain material for creep strength evaluation. Because of the limited amount of material available for creep testing obtained in this way, creep testing has been carried out using the specialised small-scale impression creep testing technique. The grade 91 material has been tested in both the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% Ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant with grade 91 pipework systems.

A failure of the upper casing of the circulation pump led to a big damage in the PP Staudinger unit 5 on 12th of May 2014. According to the §18(2) BetrSichV an extensive root cause analysis (RCA) was started. From the beginning on different lines of activities were initiated to handle the situation with the required diligence. Decisions were made, taking into account safety regulations, possibility of repair and best practice engineering. Following the board decision to repair the unit 5, a lot of detailed work was done. All of the performed work packages were linked in different timelines and needed to meet in the key points. Consequently it was a challenge to achieve the agreed date of unit 5 restart on 15th of January 2015. The unit restart on the targeted date was a proof of the excellent collaboration between all involved parties. The presentation gives a summarizing overview about the damage, the main results of the RCA and the repair activities.

A constitutive equation, with parameters derived from the interpolation of primary and steady state stages of constant load creep curves, has been utilized to estimate the stress relaxation behavior of the martensitic steel X20Cr13, alloy used in many high temperature applications, including heavy duty gas turbines. Creep and stress relaxation tests have been performed at 350°C, close to the negligible creep temperature of the studied alloy for stresses of interest for engineering applications. The creep tests were carried out at stresses below and above the yield stress, whereas, for the relaxation stress tests, the imposed strain was in the range 0.2% to 1.2% with the purpose to have, at the beginning of the tests, the same initial stresses of the performed creep tests. After a stress relaxation period, lasting between 10 to 1000 hours, each specimen was generally reloaded at the initial stress and a new relaxation test, on the same specimen, was carried out. This “reloading procedure”, simulating the re-tightening of bolts, has been repeated several times. The proposed equation has shown to well predict the experimental creep and stress relaxation behavior of the steel under investigation.

The necessity to reduce carbon dioxide emissions of new fossil plant, while increasing net efficiency has lead to the development of not only new steels for potential plant operation of 650°C, but also cast nickel alloys for potential plant operation of up to 700°C and maybe 750°C. This paper discusses the production of prototype MarBN steel castings for potential plant operation up to 650°C, and gamma prime strengthened nickel alloys for advanced super critical plant (A-USC) operation up to 750°C. MarBN steel is a modified 9% Cr steel with chemical concentration of Cobalt and tungsten higher than that of CB2 (GX-13CrMoCoVNbNB9) typically, 2% to 3 Co, 3%W, with controlled B and N additions. The paper will discuss the work undertaken on prototype MarBN steel castings produced in UK funded research projects, and summarise the results achieved. Additionally, within European projects a castable nickel based super alloy has successfully been developed. This innovative alloy is suitable for 700°C+ operation and offers a solution to many of the issues associated with casting precipitation hardened nickel alloys.

In today’s market place power generation plants throughout the world have been trying to reduce their operating costs by extending the service life of their critical machines such as steam turbines and gas turbines beyond the design life criteria. The key ingredient in plant life extension is remaining life assessment technology. This paper will outline remaining life procedures which will incorporate the defect tolerant design concepts applied to the various damage mechanisms such as creep, fatigue, creep-fatigue and stress corrosion cracking. Also other embrittlement mechanisms will also be discussed and how they will influence the life or operation of the component. Application of weld repairs to critical components such as rotors and steam chest casings will be highlighted and how defect tolerant design concept is applied for the repair procedure and the acceptance standard of the nondestructive testing applied. Also highlighted will be various destructive tests such as stress relaxation tests (SRT) which measures creep strength and constant displacement rate test (CDRT) which evaluates fracture resistance or notch ductility. Also shown will be actual life extension examples applied to steam turbine components and weld repairs. Utilization of computer software to calculate fatigue and creep fatigue crack growth will also be presented

Early supercritical units such as American Electric Power (AEP) Philo U6, the world’s first supercritical power plant, and Eddystone U1 successfully operated at ultrasupercritical (USC) levels. However due to the unavailability of metals that could tolerate these extreme temperatures, operation at these levels could not be sustained and units were operated for many years at reduced steam (supercritical) conditions. Today, recently developed creep strength enhanced ferritic (CSEF) steels, advanced austenitic stainless steels, and nickel based alloys are used in the components of the steam generator, turbine and piping systems that are exposed to high temperature steam. These materials can perform under these prolonged high temperature operating conditions, rendering USC no longer a goal, but a practical design basis. This paper identifies the engineering challenges associated with designing, constructing and operating the first USC unit in the United States, AEP’s John W. Turk, Jr. Power Plant (AEP Turk), including fabrication and installation requirements of CSEF alloys, fabrication and operating requirements for stainless steels, and life management of high temperature components

In the test loop HWT II (High Temperature Materials Test Loop) installed in the fossil power plant Grosskraftwerk (GKM) Mannheim in Germany, thick-walled components made of nickel base alloys were operated up to temperature of 725 °C. The operation mode chosen (creep-fatigue) was to simulate a large number of start-ups and shutdowns with high gradients as expected for future high efficient and flexible power plants and to investigate the damage due to thermal fatigue of the used nickel base alloys. In this paper the damage evolution of a header made of the nickel base alloys Alloy 617 B and Alloy C263, which was a part of HWT II test rig, were investigated using nondestructive and destructive techniques. Furthermore, the damage has been considered and evaluated by using numerical methods. In addition, different lifetime assessment methods of standards and recommendations with focus on creep-fatigue damage were used and evaluated. The different lifetime models are applied to the header and the results were compared to the results of metallographic investigations and damage observations.

Nickel-based alloys and stainless steel Super304H, along with various coatings, are undergoing testing in a steam loop at Alabama Power’s Plant Barry. These materials are being evaluated for use in advanced ultra-supercritical (A-USC) fossil-fired power plants at temperatures ranging from 538°C to 815°C. The loop has been operational for over 18 months, with the alloys exceeding 6,300 hours above 538°C. An additional 7,000 hours at high temperatures are planned before the loop’s removal in 2014. Initial inspections show minimal material corrosion, suggesting their suitability for A-USC applications. This paper details the loop’s design, materials, manufacturing, operation, and inspection findings. Additionally, it describes a methodology for predicting steam-side oxidation and fireside corrosion rates and highlights the significance of this testing for A-USC development and commercialization.

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.

Small punch creep testing (SPCT) is a small-scale, accelerated creep test that allows for the determination of creep data using a limited amount of material. The question, however, remains how the data generated by this technique correlate to more established techniques such as uniaxial testing and ultimately to predictions regarding the remaining service life of a plant component. This empirical study investigated the microstructure-to-property relationship of welded 9-12%Cr steels as measured using SPCT. Virgin P91 (X10CrMoVNb9-1) steel was joined to service exposed X20 (X20CrMoV12-1) steel using two different filler materials (X20 and P91) via fusion welding. Site-specific samples were extracted from the parent plates, heat affected zones and weld metals using electro-discharge machining. Small punch creep testing were performed using a 276 N load at a temperature of 625°C. The untested sample microstructures were quantitatively characterized using a range of electron microscopy techniques to determine the precipitate (M 23 C 6 , MX) spacing, subgrain sizes and dislocation densities for each region of the weldments. Multiple linear regression analysis found that the subgrain size (λsg) played the largest contribution to the SPCT rupture life. The heat affected zones had the lowest SPCT rupture times (49-68 hours), which corresponded to the largest subgrain sizes (1.1-1.3 μm). The P91 parent plate material had the longest SPCT rupture time (349 hours), which corresponded to the lowest subgrain size (0.8 μm). The P91 weld metal sample showed lower initial deflection rates during the SPC testing, however the presence of non-metallic SiO 2 inclusions in this zone contributed to accelerated brittle failure.

Steam-side oxidation and the resultant exfoliation of iron-based scales cause unplanned shutdowns at coal-fired power generation plants. Exfoliate removal is currently limited to frequent unit cycling to minimize the volume of exfoliated scale, upgrading a plant with a “blow down” system, or installing a higher alloy. This paper discusses the rate of steam-side oxidation on Type 304H stainless steel (304H) tube after shot peening the internal surface with commercially available techniques. Shot peening the ID of Type 304H austenitic stainless steel superheater tubes has been shown to improve the overall oxidation resistance in steam. Decreasing the oxidation rate directly impacts the volume of exfoliated scale. The adherent spinel scales are thinner and more robust than non-shot peened tubes of the same alloy. Most of the improved oxidation resistance can be attributed to the presence of a spinel oxide layer combined with a continuous chromia layer formed near the steam-touched surfaces. The presence of a continuous chromia layer vastly reduces the outward diffusion of iron and minimizes the formation of iron-based scales that exfoliate. This work showed that a uniform cold-worker layer along the tube ID has a profound effect on oxidation resistance. Incomplete coverage allows oxidation to proceed in the non-hardened regions at a rate comparable to the oxidation rate on unpeened Type 304H.