CWT (combined water treatment) was introduced in Japan in 1990 and over 50 power generation boilers are now in operation. However, the effect of oxygenated treatment on the steam oxidation of the ferritic-martensitic steels and austenitic stainless steels that are used for superheaters and reheaters is currently far from clear. In this study, laboratory tests were used to examine the effect of the oxygen level of the feed water on the scale growth and the scale exfoliation propensity of T91 ferritic-martensitic steel and 300-series austenitic stainless steels, as represented by TP316H and TP347H (coarse- and fine-grained, respectively). The oxygen level of the feed water had little effect on the steam oxidation rates of all the steels tested. Hematite (Fe 2 O 3 ) formed in the outer layer of the oxide scales on both the ferritic and austenitic steels and is considered to have been encouraged in the simulated CWT atmosphere. The adhesion strength of the oxide scale formed on T91 in the simulated CWT atmosphere, that is, scale in which hematite was present, was lower than that of the oxide scale formed in the simulated AVT (all volatile treatment) atmosphere. The oxidation rate of fine-grained TP347H was confirmed to be slower than that of coarse-grained TP316H. Hematite significantly influenced the scale exfoliation of the austenitic steels and the critical oxide thickness for exfoliation decreased with increasing proportion of hematite in the outer scale.

During commissioning of recently built modern, and highly efficient coal-fired power plants, cracks were detected after very short time of operation within the welds of membrane walls made from alloy T24. The root cause analysis revealed transgranular and mostly intergranular cracks adjacent to the heat affected zone beside weld joints. At that time, the degradation mechanism was rather unclear, which led to an extended root cause analysis for clarification of these failures. The environmentally assisted cracking behavior of alloy T24 in oxygenated high-temperature water was determined by an experimental test program. Hereby, the cracking of 2½% chromium steel T24 and 1% chromium steel T12 were determined in high-temperature water depending on the effect of water chemistry parameters such as dissolved oxygen content, pH, and temperature, but also with respect to the mechanical load component by residual stresses and the microstructure. The results clearly show that the cracking of this low-alloy steel in oxygenated high-temperature water is driven by the dissolved oxygen content and the breakdown of the passive corrosion protective oxide scale on the specimens by mechanical degradation of the oxide scale as fracture due to straining. The results give further evidence that a reduction of the residual stresses by a stress relief heat treatment of the boiler in combination with the strict compliance of the limits for dissolved oxygen content in the feed water according to water chemistry standards are effective countermeasures to prevent environmentally assisted cracking of T24 membrane wall butt welds during plastic strain transients.

Recently, single-phase flow accelerated corrosion (FAC) has been found extensively in Thailand, especially in single shaft combined cycle power plant heat recovery steam generators, the design of which are compact and cannot be easily accessed for service. This takes at least one week for repairing and costs at least half a million dollar per shutdown. In this paper, the investigation of the single-phase FAC in a high-pressure economizer of a combined cycle power plant is demonstrated. Water chemical parameters such as pH and dissolved oxygen are reviewed, the process simulation of the power plant is performed to capture risk areas for the FAC. A computational fluid dynamics study of the flow is done to understand the flow behavior in the damaged tubes next to an inlet header. Some modifications such as flow distributor installation and tube sleeve installation were performed for short-term solutions. Moreover, new economizer headers are designed with low alloy material to mitigate the problem. The installation process of the newly fabricated headers is finally described. The findings in this paper serve as a guideline for FAC risk assessment, FAC investigation and mitigation, and service in compact heat recovery steam generators.

This paper explores the development and qualification of a bainitic-martensitic steel grade and its matching welding consumables for power plants operating under ultra-supercritical steam conditions (605/625°C and 300/80 bar). It provides insights into recent developments and offers practical considerations for handling this material (grade T24) from the perspective of both tubular component manufacturers and welding consumable producers. The paper is structured into three main sections: (1) Development and qualification of the T24 steel base material. (2) Development, qualification, and recommendations for welding consumables compatible with T24 steel. (3) Experiences during manufacturing and installation of components using T24 steel, concluding with key takeaways.

Along with rapid development of thermal power industry in mainland China, problems in metal materials of fossil power units also change quickly. Through efforts, problems such as bursting due to steam side oxide scale exfoliation and blocking of boiler tubes, and finned tube weld cracking of low alloy steel water wall have been solved basically or greatly alleviated. However, with rapid promotion of capacity and parameters of fossil power units, some problems still occur occasionally or have not been properly solved, such as weld cracks of larger-dimension thick-wall components, and water wall high temperature corrosion after low-nitrogen combustion retrofitting.

High chromium HiperFer (High performance ferritic) materials present a promising concept for the development of high temperature creep and corrosion resistant steels. The institute for Microstructure and Properties of Materials (IEK-2) at Forschungszentrum Jülich GmbH, Germany develops high strength, Laves phase forming, fully ferritic steels which feature excellent resistance to steam oxidation and better creep life than state of the art 9-12 Cr steels. Mechanical strength properties of these steels depend not only on chemical composition, but can be adapted to various applications by specialized thermo(mechanical) treatment. The paper will outline the sensitivity of tensile, creep, stress relaxation and impact properties on processing and heat treatment. Furthermore an outlook on future development potentials will be derived.

A combination of creep tests, ex-service blade samples, thermodynamic equilibrium calculations, combined thermodynamic and kinetic calculations, image analysis, chemical composition mapping and heat treatments have been conducted on PWA1483 to determine if microstructural rejuvenation can be achieved when taking the presence of oxidation coatings into account as part of a blade refurbishment strategy. The work has shown that the γ′ morphology changes during creep testing, and that through subsequent heat treatments the γ′ microstructure can be altered to achieve a similar γ′ size and distribution to the original creep test starting condition. Thermodynamic equilibrium calculations have been shown to be helpful in determining the optimum temperatures to be used for the refurbishment heat treatments. The interaction of oxidation resistant coatings with the alloy substrate and refurbishment process have been explored with both experimental measurements and coupled thermodynamic and kinetic calculations. The predictive nature of the coupled thermodynamic and kinetic calculations was evaluated against an ex-service blade sample which had undergone refurbishment and further ageing. In general there was good agreement between the experimental observations and model predictions, and the modelling indicated that there were limited differences expected as a result of two different refurbishment methodologies. However, on closer inspection, there were some discrepancies occurring near the interface location between the coating and the base alloy. This comparison with experimental data provided an opportunity to refine the compositional predictions as a result of both processing methodologies and longer term exposure. The improved model has also been used to consider multiple processing cycles on a sample, and to evaluate the coating degradation between component service intervals and the consequences of rejuvenation of the blade with repeated engine exposure. The results from the experimental work and modelling studies potentially offer an assessment tool when considering a component for refurbishment.

This study examines the corrosion resistance of additively manufactured 316L stainless steel (SS) for nuclear applications across three environments: pressurized water reactor primary water (PWR PW), hot concentrated nitric acid, and seawater. Wire-feed laser additive manufacturing (WLAM) specimens showed oxidation behavior similar to wrought 316L SS in PWR PW, though stress corrosion cracking (SCC) susceptibility varied with heat treatment. In nitric acid testing, laser powder bed fusion (L-PBF) specimens demonstrated superior corrosion resistance compared to conventional SS, primarily due to improved intergranular corrosion resistance resulting from cleaner feedstock powder and rapid solidification rates that minimized grain boundary segregation. Laser metal deposition (LMD) repair studies in seawater environments successfully produced dense, crack-free repairs with good metallurgical bonding that matched the substrate’s mechanical properties while maintaining corrosion resistance. These results emphasize the importance of corrosion testing for additively manufactured components and understanding how their unique microstructures affect performance.

Constricted steam oxidation resistance and finite microstructural stability limits the use of 9 - 12 wt.-% chromium ferritic-martensitic steels to steam temperatures of about 620 °C. Newly developed 12 wt.-% Cr steels are prone to Z-phase precipitation, which occurs at the expense of the strengthening precipitates, and therefore suffer an accelerated decline in strength during longterm operation. While the concept of ferritic-martensitic chromium steels thus seems to hit technological limitations, further improvement in steam power plant efficiency necessitates a further increase of steam pressure and temperature. Furthermore increasing integration of intermitting renewable energy technologies in electrical power generation poses a great challenge for supply security, which has to be ensured on the basis of conventional power plant processes. Besides improved efficiency for resource preservation, load flexibility, thermal cycling capability and downtime corrosion resistance will play key roles in the design of tailored materials for future energy technology. Under these preconditions a paradigm shift in alloy development towards improvement of cyclic steam oxidation and downtime corrosion resistance in combination with adequate creep and thermomechanical fatigue strength seems to be mandatory. The steam oxidation, mechanical and thermomechanical properties of fully ferritic 18 - 24 wt.-% chromium model alloys, strengthened by the precipitation of intermetallic (Fe,Cr,Si)2(Nb,W) Laves phase particles, indicate the potential of this type of alloys as candidate materials for application in highly efficient and highly flexible future supercritical steam power plants.

T24 tube material (7CrMoVTiB10-10), with its combination of high creep strength and potential to be welded without using preheat, is regarded as a candidate waterwall material for Ultra Supercritical (USC) boilers. However, its reputed sensitivity to hydrogen and potential for secondary hardening may have adverse impacts on construction of waterwall panels. Doosan Babcock Ltd have investigated the response of welds made in T24 tubing to secondary hardening via changing hardness in a series of ageing heat treatment trials. Also, the response of the material to hydrogen infusion has been investigated

Starting in 2010 a new generation of coal fired power plants in Europe operating at a steam temperature of up 620°C was commissioned. During that commissioning process many cracks occurred in welds of T24 material which was extensively used as membrane wall material in nearly all of the new boilers. The cracks were caused by stress corrosion cracking (SCC) only occurring in the areas of the wall being in contact to high temperature water during operation. The question which step of the commissioning process really caused the cracking was not answered completely even several years after the damage occurred. To answer this question and to define parameters which will lead to cracking in high temperature water many tests were conducted. Generally it was found that slow tensile tests in controlled environment are well suited to get information about materials SCC sensitivity in the laboratory. In the present paper, first the influence of the cracking of welded T24 material in acidic environment containing well-defined amounts of H2S is investigated to address the question if a chemical cleaning process prior to the testing might lead to hydrogen induced SCC. As a second step, cracking behaviour in high temperature water is being investigated. Here the influence of the temperature, the oxygen concentration of the water, the deformation speed of the sample, the heat treatment and the condition of the material on the SCC is analysed.

A new alloy design concept for creep- and corrosion-resistant, fully ferritic alloys was proposed for high-temperature structural applications in current/future fossil-fired power plants. The alloys, based on the Fe-30Cr-3Al (in weight percent) system with minor alloying additions of Nb, W, Si, Zr and/or Y, were designed for corrosion resistance though high Cr content, steam oxidation resistance through alumina-scale formation, and high-temperature creep performance through fine particle dispersion of Fe 2 (Nb,W)-type Laves phase in the BCC-Fe matrix. Theses alloys are targeted for use in harsh environments such as combustion and/or steam containing atmospheres at 700°C or greater. The alloys, consisting of Fe-30Cr-3Al-1Nb-6W with minor alloying additions, exhibited a successful combination of oxidation, corrosion, and creep resistances comparable or superior to those of commercially available heat resistant austenitic stainless steels. An optimized thermo-mechanical treatment combined with selected minor alloying additions resulted in a refined grain structure with high thermal stability even at 1200°C, which improved room-temperature ductility without sacrificing the creep performance. The mechanism of grain refinement in the alloy system is discussed.

In an earlier paper, preliminary data for HAYNES 282 alloy was presented for potential advanced steam power plant applications. Since then, 282 alloy has continued to be evaluated for a variety of A-USC applications: superheater boiler tubing, large header piping, rotors, casings, etc. Per current practice the alloy achieves its strengthening by a two-step age hardening heat treatment. Given the difficulty of such a procedure, particularly for larger components in the power plant, interest has focused on the development of a single step age hardening treatment. While considerable work on 282 alloy is still going on by a number of investigators, during the preceding years a large amount of data was generated in characterizing the alloy at Haynes International. This paper will briefly review the behavior of 282 alloy in air and water vapor oxidation (10% H 2 O) at 760°C (1400°F), low cycle fatigue properties at 649°C to 871°C (1200°F to 1600°F) and long-term thermal stability at 649°C to 871°C (1200°F to 1600°F). Special focus of the paper will be mechanical behavior: tensile and creep; microstructural analysis, and weldability of 282 alloy as a result of single step age hardening heat treatment: 800°C (1475°F)/8hr/AC.

Main steam control valves are crucial components in power plants, as they are the final elements in the steam piping system before the steam enters the turbine. If any parts of these valves become damaged, they can severely harm the steam turbines. Recently, power plants have been required to operate under cyclical loading, which increases the risk of cracks in the control valve seats. This is due to the different rates of expansion between the Stellite surface and the underlying Grade 91 steel surface when exposed to high temperatures. To ensure a reliable power supply, power plants cannot afford long downtimes, making on-site service essential. This paper presents an on-site technique for post-weld heat treatment (PWHT) of Stellite seats. By using a heating pad arrangement and an induction heater, the required PWHT temperature of 740°C, as specified in the welding specification procedure (WPS), can be achieved. This method allows for on-site valve seat repair and can be applied to other power plants as well.

High efficiency in power generation is not only desirable because of economical reasons but also for enhanced environmental performance meaning reduced quantity of forming ash and emissions. In modern medium to large size plants, improvements require supercritical steam values. Furthermore, in future there will be an increasing share of renewables, such as wind and solar power, which will enhance the fluctuation of supply with the consequence that other power sources will have to compensate by operating in a more demanding cyclic or ramping mode. The next generation plant will need to operate at higher temperatures and pressure cycles coupled with demanding hot corrosion and oxidation environments. Such an operation will significantly influence the performance of materials used for boilers and heat exchanger components by accelerating oxidation rates and lowering mechanical properties like creep resistance. The paper discusses the oxidation behaviour of San25, 800H and alloy 263 in supercritical water at temperatures 650 and 700 °C at 250 bar, and compares the changes of mechanical properties of materials at these temperatures.

The creep-fatigue properties of modified 9Cr-1Mo (grade 91) steel have been investigated for the purpose of design in cyclic service. In this paper test results from creep-fatigue (CF) and low cycle fatigue (LCF) on grade 91 steel are reported. The tests performed on the high precision pneumatic loading system (HIPS) are in the temperature range of 550-600ºC, total strain range of 0.7-0.9% and with hold periods in both tension and compression. Curves of cyclic softening and stress relaxation are presented. The CF test results and results obtained from literature are also analysed using methods described in the assessment and design codes of RCC-MRx, R5 and ASME NH as well as by the recently developed Φ-model. It is shown that the number of cycles to failure for CF data can be accurately predicted by the simple Φ-model. The practicality in using the life fraction rule for presenting the combined damage is discussed and recommendations for alternative approaches are made.

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.

Critical sections of steam plants and heat-recovery steam generators require materials with enhanced properties such as 9Cr-1Mo steel. Ensuring compliance with specifications for heat treatment, chemical composition, contamination limits, and joint design is crucial to prevent premature failures. This study describes the development of a user-friendly, multi-property nondestructive sensor arrangement to qualify heat-treated 9Cr-1Mo steel. Experimental results demonstrate that correlations between thermal heat treatment and electronic, magnetic, and elastic measurements can determine if T91 steel achieves the necessary microstructure and properties for service. Additionally, rejected parts can be assessed for microstructural issues causing unacceptable properties. The techniques utilize a common electronic setup with different sensors, requiring calibration for specific NDE systems and sensor setups, high-speed data acquisition, and frequency analysis (FFT). Further development on crept and welded samples is recommended to enhance NDE practices for in-service T91 steel conditions.

This study evaluates the elevated temperature mechanical performance of 316H stainless steel produced using directed energy deposition (DED) additive manufacturing (AM) from three separate collaborative research programs focused on understanding how AM variables affect creep performance. By combining these studies, a critical assessment of variables was possible including the DED AM method (laser powder and gas metal arc wire), laser power, sample orientation relative to build orientation, chemical composition, and post-processing heat treatment. Detailed microstructure characterization was used to supplement creep and chemistry results to provide insights into potential mechanistic differences in behavior. The study found that sample orientation was a critical variable in determining lower-bound creep behavior, but that in general the lowest creep strength orientation and the lowest creep ductility orientation were not the same. Heat treatment was also an important variable with as-printed materials showing for specific test conditions improved performance and that underlying substructures formed due to inhomogeneous chemical distributions were not completely removed when using standard wrought solution annealing heat-treatments. The chemistry of the final deposited parts differed from the starting stock and may be an important consideration for long-term performance which is not fully appreciated. Overall, the study found that while all the DED materials tested fell within an expected wrought scatter band of performance, the actual creep performance could vary by an order of magnitude due to the many factors described.

The Alloys-by-Design approach, involving large-scale CALPHAD calculations to search a compositional range, has been used to isolate a suitable nickel-based superalloy for additive manufacturing (AM) by optimizing the trade-off between processability and increasing strength. This has been done in response to the limited focus on development of new superalloys designed to overcome the limitations of the AM process, specifically the high defect density of parts made from high-performance alloys. Selected compositions have been made using gas atomization, and laser powder-bed fusion AM trials were performed. The resulting properties were evaluated in the as-processed, heat treated and thermally exposed conditions. The assessment, combined with characterization techniques including scanning electron microscopy and atom probe tomography, rationalizes a temperature capability up to and above 850 °C, and demonstrate the opportunity to develop alloys with properties beyond the current state of the art.