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This study presents the results of microstructure and hardness tests of a homogenous welded joint made of Super 304H stainless steel after 1000 h of ageing at the temperature of 700 and 750°C. Super 304H steel is commonly used for high-pressure elements of modern power units for supercritical steam parameters. The article compares hardness with the microstructure images in the initial state and after ageing. The microstructure tests were carried out based on scanning microscopy, while the hardness tests were carried out on a stationary hardness tester in the HV10 scale. An increase in hardness after ageing was observed, which is directly related to the initiated precipitation process, both in the weld and base materials. [ABSTRACT FROM AUTHOR]

P92 steel is a modern martensitic heat-resistant steel currently used for seamless products for pressure equipment operating in supercritical operating parameters. The paper presents the results of a study on the strength properties and structure of a P92 steel welded joint used for pressure components of power units. The paper presents an assessment of the suitability for further operation of both the parent material and a circumferential similar welded joint of finished products in the form of P92 steel pipes after annealing for 3000 hours at 600 and 650°C. Annealing at 650°C results in faster increase in the size of the precipitates and their coagulation along grain boundaries of former austenite and martensite laths. The changes in mechanical properties were compared in relation to the state of the structure of the parent material and the material of the welded joint. Quantitative analysis of M23C6 precipitates was also carried out. [ABSTRACT FROM AUTHOR]

This article presents the results of investigations on HR6W alloy and Sanicro 25 steel and the dissimilar welded joint made of them. The characteristic images of microstructure of the investigated materials in the as-received condition and following the creep test, observed with a scanning electron microscope (SEM), are shown. The X-ray analysis of phase composition of the existing precipitates was carried out. The method for evaluation of creep strength based on abridged creep tests carried out at a temperature higher than the design one is presented. The obtained results do not deviate from the values of creep strength determined in long-term creep tests. The maximum difference in creep strength of the investigated materials is ±20%, which is in compliance with the acceptable scatter band. The methodology presented can be used for verification of creep strength (life time) of the material of finished components to be operated under creep conditions.

This paper presents the characteristics of the performance of P91 (X10CrMoVNb9-1), P92 (X10CrWMoVNb9-2) and VM12 (X12CrCoWVNb12-2-2) steels used for condition assessment of the pressure components of boilers with supercritical steam parameters. Studies on the mechanical properties, microstructure tests using scanning and transmission electron microscopy, and X-ray analysis of the phase composition of precipitates were performed for selected steels in the as-received condition and after long-term annealing. These steel characteristics are used for the evaluation of the microstructural changes and mechanical properties of the material of components after long-term service. The result of this study is the database of material characteristics representing the mechanical properties related to the microstructure analysis and it can be used for diagnosis of the components of pressure parts of power boilers.

The paper shows the degradation process of the modern austenitic Super 304H (X10CrNiCuNb18-9-3) steel which was subjected to long-term ageing for up to 50 000 h at 650 and 700°C. The investigations include microstructure examination (SEM), identification and analysis of the precipitation process, and mechanical properties tests. The Super 304H steel has a structure characteristic of austenitic steels with visible annealing twins and single primary NbX precipitates. Long-term ageing in steel leads to numerous precipitation processes of M23C6, MX carbides, s phase, Z phase, and e-Cu phase. Precipitation processes lead to a decrease in plastic properties and impact energy as well as alloy over ageing. Yield strength and tensile strength values after 50 000 h of ageing were similar to those as delivered. The yield and tensile strength value strongly depend on the applied ageing temperature. [ABSTRACT FROM AUTHOR]

Issues related to the proper design of power boiler elements taking account of thermal loads are still relevant in this era of increasing variability of the boiler power output over time. The aim of this work is to present a thermal and strength analysis for a section of the membrane wall of a steam boiler, designed and manufactured in compliance with international standards, and approved for operation by a pressure equipment supervisory authority. Although the procedures were followed as required, the boiler membrane wall became cyclically damaged. The cause of such periodic damage to the boiler can be explained based on the conducted analyses. [ABSTRACT FROM AUTHOR]

The current work focuses on comparing the effect of tungsten inert gas welding (TIG) and plasma arc welding by keyhole mode (KPAW). Tubes of AISI 304HCu of 45 mm outer diameter and thickness of 9 mm were joined by KPAW autogenously; ER304HCu was used for multipass welding by TIG with V groove edge preparation. Mechanical tests like tensile, impact tests, and microhardness surveys on specimens were carried out and correlated with their metallurgical features. Solidification of microstructure was by ferrite austenite (FA) microstructure. The KPAW weldment had vermicular ferrite morphology, whereas the multipass joint had variations in morphologies. Welded joints exhibited strength close to the base material and failed near the interface between the base metal and the heat-affected zone (HAZ). The weld bead area of the KPAW joint was 40% more minimal than TIG welded joint. [ABSTRACT FROM AUTHOR]

In the present work, the creep strength-enhanced ferritic martensitic P92 steel was welded with 304L austenitic stainless steel. The dissimilar joining of these two different grades of material was performed by using tungsten inert gas welding process. The nickel-based ERNiCr-3 welding consumable was used. The influence of post-weld heat treatment (760 °C, 2h), called as tempering, was also investigated. The secondary electron and optical image confirmed that ERNiCr-3 weld metal is accompanied by an equiaxed austenitic microstructure with Ni weight percentage of 68.29%. The energy-dispersive X-ray spectroscopy and electron probe microanalyzer results of the weld fusion zone revealed the Nb, Cr, and Ti segregation in the inter-dendritic region. The elemental mapping of the carbon-depleted zone, and the interface was performed using an electron probe microanalyzer, which revealed the diffusion of Ni, Cr, and Fe across the interface of P92 steel and ERNiCr-3 filler weld. The tensile test result indicated that part of the dissimilar weld joints (DWJs) with relatively weak ultimate tensile strength was ERNiCr-3 weld metal. The ultimate tensile strength value of the DWJs was observed as 610 MPa and 580 MPa in the as-weld and post-weld heat treatment situations, respectively. The Charpy V-notch impact energy of the ERNiCr-3 weld fusion zone was achieved as 135 ± 2 J and 140 ± 2 J in as-weld and post-weld heat treatment situations, respectively. The low impact toughness of the ERNiCr-3 weld metal was attributed to the presence of niobium carbide (NbC) and titanium carbide (TiC) particles. The peak longitudinal residual stress of 490 MPa and transverse residual stress of 442 MPa were noted in the ERNiCr-3 weld fusion zone at a depth of 3 mm from the top surface. The tempering heat treatment exhibited a significant drop in the residual stresses value for the weld fusion zone and heat affected zone (HAZ). [ABSTRACT FROM AUTHOR]

Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples. [ABSTRACT FROM AUTHOR]

This work aimed to evaluate the effect of a long-term pre-oxidation process in the air (5000 hours at 650 °C) on the steam oxidation resistance of Cr-rich steels with around 18 wt.% Cr (Super 304H) and around 25 wt.% Cr (HR3C) content exposed for the additional 5000 hours at 650 °C. The effect showed a high impact of a long-term pre-oxidation process and a high content of Cr in the metal matrix of the exposed steels. The steel with a lower Cr content showed spallation of the oxide scale under the oxidation process in the air, and only the steel with 25 wt.% Cr showed no spallation of the oxide scale in both conditions: air and steam. The post-exposure analysis in this work was carried out using Scanning Electron Microscopy (SEM) supported by Energy Dispersive X-ray Spectroscopy (EDS) using an X-Ray mapping function. Phase analysis was performed using an XRD diffractometer. [ABSTRACT FROM AUTHOR]

The paper discusses several aspects of degradation of Super 304H steel subjected to long-term aging up to 50,000 h at 650 and 750°C. The study includes microstructure examination by scanning and transmission electron microscopy along with X-ray microanalysis of a wide range of precipitates. The Super 304H steel has a structure characteristic for austenitic steels with annealing twins and single primary NBX precipitates of various sizes. Long-term aging leads to precipitation of several phases such as M23C6, MX carbides, σ phases, Z phase and ε-Cu phase. The precipitation processes lead to changes in the creep strength of the tested steel, the value of which strongly depends on the aging temperature used, which is measurably shown by the creep tests carried out with the elongation measured during the test. [ABSTRACT FROM AUTHOR]

Purpose: The main purpose of the expert system presented in the paper is to support proper decision-making to perform the operation of the complex and crucial technical system in a rational way. Design/methodology/approach: The proposed system was developed using the universal concepts of a semi-Markov process, quality space and a multi-objective analysis. The maintenance and operation processes of a machine were modelled in the form of a semi-Markov process, the quality space was used to exclude the operation and maintenance process of critical quality and finally, thanks to implementation of a multi-objective analysis, the assessment system was build. Findings: By generating each flow of the process, the expert system supports optimization of a technical system operation to choose the best maintenance strategy. Application of the expert system created based on a real industrial system is presented at the end of the paper. Research limitations/implications: The limitations of the proposed approach can be found in the parts of simulation and assessment. As the number of states to be taken into consideration increases, the time of calculation gets longer as well. As regards the assessment, ranges of the criteria argument have to be determined. Unfortunately, in some industrial systems, they are difficult to define or they are infinite and should be artificially limited. Practical implications: The system provides three most important benefits as compared to other solutions. The first benefit is the system ability to make a choice of the best strategy from the perspective of the accepted criteria. The second advantage is the ability to choose the best operation and maintenance strategy from the point of view of a decision-maker. And the third is that the decision-maker can be completely sure that the chosen way of operation is not of critical quality. Originality/value: The novelty of the proposed solution involves the system approach to the expert system design, thanks to the described procedure which is flexible and can be easily implemented in different technical systems which have a crucial impact on reliability and safety of their operation. It is the unique combination of probability-based simulation, multi-dimensional quality considerations and multi-objective analysis. [ABSTRACT FROM AUTHOR]

Two kinds of Fe-18Ni-12Cr-based alumina-forming austenitic (AFA) steels with ultralow carbon content were prepared by vacuum induction melting. Creep rupture tests were carried out with the applied stress range between 80 and 180 MPa at the target application temperature of 700 °C. Microstructure evolution was characterized using glancing X-ray diffraction, scanning electron microscopy equipped with a detector of electron backscatter diffraction and transmission electron microscope. Compared with 18-12-Al, 18-12-AlNb exhibited superior creep properties including ten times longer creep rupture life ( t rup ) and one magnitude smaller minimum creep rate ( ε ˙ min ). For example, the ε ˙ min at the applied stress of 140 MPa was 4.4 × 10–5 and 2.1 × 10–4 h–1 for 18-12-AlNb and 18-12-Al, respectively. The strengthening effect of B2-NiAl would weaken at 700 °C due to its ductile-to-brittle transition. The addition of Nb not only promoted the formation of Laves-Fe2(Mo,Nb), but also resulted in formation of nanoscale NbC. The GB Laves-Fe2(Mo,Nb) did not impair the creep ductility by strengthening GBs, and all the creep ruptured AFA steels exhibited typical ductile fracture mode. The GB Laves-Fe2(Mo,Nb) and NbC precipitates contributed to the superior creep resistance of 18-12-AlNb. [ABSTRACT FROM AUTHOR]

Based on a series of stress rupture tests at 620 °C under 110 to 170 MPa and at 650 °C under 80 to 110 MPa, the relationship between the stress and rupture time was obtained to evaluate the long-term performance of the welded joint (WJ). At 620 °C, the stress rupture occurred in the base metal of 9 pct Cr steel (9 pct Cr-BM), with the stress ranging from 130 to 170 MPa, yet the failure shifted to the heat-affected zone (HAZ) of 9 pct Cr steel (9 pct Cr-HAZ) with the stress ranging from 110 to 120 MPa. This failure behavior was observed at 650 °C with the turning point of 110 MPa. In particular, a ductile-to-brittle transition was determined when the rupture location shifted from 9 pct Cr-BM to 9 pct Cr-HAZ. Moreover, both the Laves phase adjacent to the M23C6 and the independent phases could be detected in the 9 pct Cr-HAZ after the stress rupture test, while only M23C6-type carbides could be found in the 9 pct Cr-BM. The appearance of the microhardness turning point and the formation of the Laves phase in the 9 pct Cr-HAZ are considered as the crucial factors resulting in the transition of the failure mode. [ABSTRACT FROM AUTHOR]

The operating conditions of coal-fired power units are changing because of the current shift in the approach to technologies based on fossil fuels. Due to decarbonization issues, their function is now more often to balance shortages of electricity generated from renewable sources. A substantial rise can be observed nowadays in the significance of the capability of all the installation modules to operate under variable loads. One of the solutions improving the coal-fired power unit flexibility is the twin-boiler concept based on using a system with twin-boilers co-operating with one steam turbine. Such a solution makes it possible to obtain characteristics similar to gas-steam systems and achieve low values of the minimum load on the basic fuel. The solution operating characteristics cannot be determined fully unless issues related to the thermal and stress states of individual elements are discussed first. The subject of the numerical analyses presented in this paper are thermal and stress states arising in the elements of a turbine co-operating with 2 boilers during steady- and unsteady-state operation. Results of numerical simulation of the turbine steady operation, the start-up process and power change are presented. An analysis is conducted of the impact of the rate of changes in power on the level of stresses in the turbine elements and in consequence on the fatigue of the turbine rotors. [ABSTRACT FROM AUTHOR]

Tensile and creep properties of new austenitic steel Sanicro 25 at room temperature and operating temperature 700 °C were investigated by testing on miniature specimens. The results were correlated with testing on conventional specimens. Very good agreement of results was obtained, namely in yield and ultimate strength, as well as short-term creep properties. Although the creep rupture time was found to be systematically shorter and creep ductility lower in the miniature test, the minimum creep rates were comparable. The analysis of the fracture surfaces revealed similar ductile fracture morphology for both specimen geometries. One exception was found in a small area near the miniature specimen edge that was cut by electro discharge machining, where an influence of the steel fracture behavior at elevated temperature was identified. [ABSTRACT FROM AUTHOR]

This paper describes the influence of technological treatments (i.e., bending or welding) on the structural stability of SUPER304H austenitic steel used in reheaters and superheaters in fossil fuel power plants. Although the worldwide trend is transitioning to green power sources, the lifetime of existing power plants has to be prolonged until the transition is complete. Experimental material was tested in as-received state (straight tubes), bends, and homogeneous weld joints. Part of the specimens was solution-annealed after the technological operation. Afterwards, all the samples were thermally aged in furnace (650, 675 and 700 °C) for 7560–20,000 h. For comparison, bent specimens were placed at experimental sites on an operating powerplant for 10,000+ h. The long-term aging causes the formation of Cr-based carbides on the grain boundaries along with the Fe-Cr sigma phase. Combination of elevated temperature and residual stress accelerates formation of the sigma phase. This can be prevented by solution-annealing after bending. Mechanical properties were evaluated by Vickers hardness and tensile tests. The microstructure was observed using light optical microscopy (LOM) and scanning electron microscopy (SEM) with the energy-dispersive X-ray detector (EDXS). Electron backscatter diffraction (EBSD) and X-ray powder diffraction (XRPD) were used to characterize the brittle phases. [ABSTRACT FROM AUTHOR]

In addition to good high-temperature creep resistance and adequate heat resistance, steels for the power industry must have, among other things, good weldability. Weldability of such steels is one of the criteria determining whether or not the material is suitable for applications in the power industry. Therefore, when materials such as martensitic steel Thor 115 (T115) are introduced into the modern power industry, the quality and properties of welded joints must be assessed. The paper presents the results of metallographic and mechanical investigations of T115 martensitic steel welded joints. The analysis was carried out on joints welded with two filler metals: WCrMo91 (No. 1) and EPRI P87 (No. 2). The scope of the investigations included: microstructural investigations carried out using optical, scanning and transmission electron microscopy and mechanical testing, i.e., Vickers microhardness and hardness measurement, static tensile test and impact test. The macro- and microstructural investigations revealed correct structure of the weld, without welding imperfections. The microstructural investigations of joint No. 1 revealed a typical structure of this type of joint, i.e., the martensitic structure with numerous precipitates, while in joint No. 2, the so-called Nernst's layers and δ-ferrite patches were observed in the weld fusion zone as well as the heat affected zone (HAZ). The mechanical properties of the test joints met the requirements for the base material. A slight influence of the δ-ferrite patch on the strength properties of joint No. 2 was observed, and its negative effect on the impact energy of HAZ was visible. [ABSTRACT FROM AUTHOR]

The paper presents a research method concerning the application of statistical prognostic models for assessment of material durability and operational reliability of steel for steam pipelines, whose operation has exceeded the working time of 100,000 h. Decisions on the admission of long-lived materials to work for power industry results from extensive diagnostic examinations are based on the results of tests of mechanical properties, microstructure degradation, and corrosion processes. Considering the economic reasons and available data published in diagnostic reports, the determination of failure-free operating time of steam pipelines is based on the results of static tensile tests—tensile strength (Rm); conventional yield point (Rp); elongation (A) and Vickers hardness (V), correlated with the operating time and the media type (fresh steam and secondarily super-heated steam) for the most sensitive element of a pipeline, namely the elbow. The results of changes in strength properties during operation are presented in the form of graphs of the analyzed material feature vs. operating time in the range from zero hours (for a new material) to 300,000 h, taking into account the impact of random and systematic disturbances within the adopted tolerance limits. It has been found that because of the R2 factor and significance level in the t-Student test for regression and correlation coefficients, exponential, hyperbolic and quadratic models are best fitted to empirical points. Based on the tensile strength results (Rm), it has been found that the forecast time of the steam pipeline ranges from 193,400 to 258,300 h. Taking the yield strength (Rp) into account, it has been ascertained that the time ranges from 225,000 to 293,000 h, and for the working time forecast of steam pipelines based on Vickers hardness results, it ranges from 192,100 to 246,800 h. [ABSTRACT FROM AUTHOR]

Fatigue damage, creep damage, and their interactions are the critical factors in degrading the integrity of most high-temperature engineering structures. A reliable creep-fatigue damage interaction diagram is a crucial issue for the design and assessment of high-temperature components used in power plants. In this paper, a new three-dimensional creep-fatigue-elasticity damage interaction diagram was constructed based on a developed life prediction model for both high-temperature fatigue and creep fatigue. The total tensile strain energy density concept is adopted as a damage parameter for life prediction by using the elastic strain energy density and mean stress concepts. The model was validated by a great deal of data such as P91 steel at 550 °C, Haynes 230 at 850 °C, Alloy 617 at 850 and 950 °C, and Inconel 625 at 815 °C. The estimation values have very high accuracy since nearly all the test data fell into the scatter band of 2.0. [ABSTRACT FROM AUTHOR]

This paper investigates a failure in HP-Mod radiant tubes in a petrochemical plant. Tubes fail after 90,000 h of working at 950 °C. Observed failure is in the form of excessive bulging and longitudinal cracking in reformer tubes. Cracks are also largely branched. The microstructure of service-exposed tubes was evaluated using optical and scanning electron microscopes (SEM). Energy-dispersive X-ray spectroscopy (EDS) was used to analyze and characterize different phases in the microstructure. The results of this study showed that carbides are coarsened at both the inner and the outer surface due to the long exposure to a carburizing environment. Metallography examinations also revealed that there are many creep voids that are nucleated on carbide phases and scattered in between dendrites. Cracks appeared to form as a result of creep void coalescence. Failure is therefore attributed to creep due to a long exposure to a high temperature. [ABSTRACT FROM AUTHOR]

Higher temperatures of the power boiler superheater operation may lead to high strains caused by the creep phenomenon. This paper presents a determination of the maximum allowable operating temperature limited by the creep phenomenon for steam superheater SH3. The calculations are carried out first on the basis of applicable European standards. Then, calculations are performed based on conducted creep tests, a proposed creep model, and a finite element method (FEM) model. A detailed creep phenomenon analysis based on the conducted creep tests shows that stresses that determine the creep process are mainly caused by pressure. Normal stresses resulting from bending are mainly the effect of thermal expansion. These stresses undergo significant relaxation because of creep. The creep phenomenon analysis explains the equations of the European standards. The presented calculations enable estimation of a safe value of the operating temperature which is constant over time. The estimated time of safe operation does not take account of temperature spikes. For this reason, pressure elements working at high temperatures must be inspected regularly to assess their wear state. [ABSTRACT FROM AUTHOR]

Inconel 740H is a nickel-based alloy for pressure components of ultra-supercritical boilers. Its chemical composition and strengthened matrix, as well as corrosion resistance, provide the highest creep resistance among the materials recommended for use in high-performance pressure components of power units. This paper investigates the changes in the microstructure and mechanical properties after ageing at 700 and 750 °C for 1000, 10,000, 20,000, and 30,000 h. Observation of the microstructure was performed using scanning and transmission electron microscopy. The identification of existing precipitates was conducted by X-ray phase analysis. The effects of time and ageing at elevated temperatures on the mechanical properties and precipitation process in the test alloy are discussed. The presented results are part of the material characteristics of the new-generation alloys to be used in the design of pressure equipment for steam boilers, as well as in diagnostic work during operation. [ABSTRACT FROM AUTHOR]

Creep rupture tests of 9Cr-3W-3Co steel were conducted in the range of 120 to 200 MPa at 650 °C. The influence of stress on microstructure evolution was investigated in detail. In the high stress regime, a large density of dislocation was generated and induced precipitation of fine and dispersive particles. However, at lower stresses, a transformation from martensite laths to large size subgrains and a coarsening of precipitates took place due to significant recovery and loss of pinning effect during long term exposure. Thermodynamic results revealed decreasing tungsten content effectively retarded the coarsening behavior of M23C6 and Laves phase, hence further improvement of creep rupture time was achieved experimentally. [ABSTRACT FROM AUTHOR]

The purpose of the present study is to analyze pressure difference changes inside a high-temperature steam generator (HTSG), which produces steam using the heat generated by waste incineration and decreases the pressure of the produced steam while increasing its temperature. The high-temperature, low-pressure steam produced by a HTSG is used for hydrogen production. Therefore, the steam temperature must be at least 700 °C, and the pressure must be lower than 300 kPa; hence, a device is needed to increase the steam temperature in the boiler and decrease the steam pressure. The physical behavior of the device was modeled and experimentally validated. The modeling and experimental results demonstrated good agreement when the steam was not preheated; however, an additional pressure drop required consideration of the opposite case. [ABSTRACT FROM AUTHOR]