Creep-fatigue lives of nickel-based Alloy 617 and Alloy 740H were investigated to evaluate their applicability to advanced ultrasupercritical (A-USC) power plants. Strain controlled push-pull creep-fatigue tests were performed using solid bar specimen under triangular and trapezoidal waveforms at 700°C. The number of cycles to failure was experimentally obtained for both alloys and the applicability of three representative life prediction methods was studied.

Creep-fatigue tests strain-controlled with different strain amplitudes and different hold times at 725 were done on nickel-based alloy 617 as a typical candidate material for turbine rotor of advanced ultra-supercritical power plant. Stress relaxes during the hold time when the strain remains at the tensile peak. The analysis of the stress relaxation during different strain hold times shows that the ratio of the relaxation stress and the maximum stresses has strong correlation with strain amplitude and hold time. The failure life also has a certain dependence on the relaxation stress ratio. The failure life decreases and the relaxation stress ratio increases as the strain amplitude increases. The failure life decreases and the relaxation stress ratio increases as the hold time increases. Therefore the stress relaxation ratio was used as an intermediate variable to obtain the corresponding relationship model by establishing the relationship between the relaxation stress ratio and the strain and the relationship between the relaxation stress ratio and the failure life. This model can be used to predict the creep-fatigue interaction life more simply and directly.

In power plants operated at elevated temperatures, the operating life of structural materials increases. Therefore, it is very important to be able to predict creep strength in long term above 100,000 h. Furthermore, it has been reported that in the long term, the actual creep strength is lower than the predicted life. Although this problem has been analysed, the reasons remain unclear. In this study, a fracture energy model is used to evaluate the mechanisms of the creep strength reduction for martensitic steels. In the model, changes in fracture energy with rupture time are expressed by a power law. The energy density rate is calculated using stress, rupture elongation, and rupture time. The model indicates two mechanisms of creep strength reduction. One is the increase in rupture elongation, which leads to reduction in creep strength with ductility; the other is the decrease in reduction of area, which leads to reduction in creep strength with brittleness. Difference between the two mechanisms affects creep-fatigue strength. The study also shows that the equation based on the fracture energy model for creep-fatigue life can be obtained by a parallel translation of that for creep.

To enhance power plant efficiency, global projects aim to increase operating temperatures to 700 °C (1292 °F) and beyond, surpassing the capabilities of conventional ferritic and austenitic steel alloys and necessitating the use of nickel-based alloys like Alloy 617. This study evaluated the fatigue and creep-fatigue performance of Alloy 617, including both parent metal and welds, at 650 °C (1202 °F). Tests were conducted on virgin material, service-aged samples (up to 25,000 hours), and material over-aged at 800 °C (1472 °F) for 1,000 hours. Results indicated that service aging only slightly reduced the pure fatigue properties of Alloy 617, but significantly decreased its life under creep-fatigue conditions. The creep-fatigue life of ex-service welds was reduced to less than one-third of that of virgin parent metal. The data suggests that the introduction of a tensile hold period impacts Alloy 617's life more than Alloy 263 but less than Alloy 740, potentially linked to the cyclic strength of the alloys. The reduction in life for Alloy 617 is notably greater than that observed in conventional ferritic alloys.

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.

Grade 92 steel, a creep strength-enhanced ferritic (CSEF) steel, is used in supercritical steam fossil power plants for boilers and piping systems. While its creep strength is crucial, understanding the interaction between creep and fatigue damage is also vital for assessing component integrity under cyclic loading. Despite existing studies on its creep-fatigue behavior, additional data under creep-dominant conditions relevant to plant evaluations are needed. Girth welds, critical to piping system integrity, are particularly important in this context. EPRI and CRIEPI initiated a project to develop a comprehensive database on the creep-fatigue behavior of Grade 92 steel's base metal and welded joints and to establish a suitable life estimation procedure. Key findings include: (i) a thick pipe with submerged arc welding (SAW) was manufactured for testing; (ii) base metal and cross-weld specimens showed similar behavior under short-term creep and cyclic loading; (iii) these specimens had lower creep strengths than average literature values for this steel class in the short time regime, with differences decreasing as stress decreased; and (iv) the fatigue and creep-fatigue behavior of these specimens were similar to those of Grade 91 and 122 steels, with common characteristics in creep-fatigue failure prediction models across the three CSEF steels.

Creep rupture strength is the principal material property prioritized in designing power generation plants against the steady-state stress due to internal pressure. Increasingly plants must cycle so there is a possibility of life reduction due to creep-fatigue interaction. Grade 92 steel is one of the creep strength enhanced ferritic (CSEF) steels which has superior creep strength compared to other CSEFs. It is expected to be widely used in coal-fired ultra-super critical plants as well as in LNG-fired combined cycle plants. However, at present there is insufficient information regarding the creep-fatigue behavior of this material. A joint study has been conducted to understand the behavior of this steel under creep-fatigue condition and see how accurate the failure life can be estimated. Three kinds of base materials as well as two kinds of welded joints have been tested under strain-controlled cyclic loading with or without hold times as well as under constant load creep condition. Continued decrease in the number of cycles to failure was observed with the extension of hold time in all the base metals and cross-weld specimens. It was found that the modified ductility exhaustion approach based on inelastic strain, as well as its extension employing the inelastic strain energy density, made reasonably accurate predictions of failure lives under a wide range of test conditions. Temperature- and rate-dependencies of fracture limits in terms of inelastic strain and energy density were able to be uniquely expressed using simple thermal activation energy parameters.

In response to the strong needs for the life assessment of various components in fossil power plants, studies on Grade 91 and Grade 92 steels have been jointly performed by EPRI and CRIEPI for a last decade. These studies have been covering the effects of load variation (creep- fatigue) and stress multiaxiality as well as the behavior under uniaxial creep conditions. Based on abundant test data accumulated in this period and associated analytical evaluation, approaches based on inelastic strain energy have been developed for accurately assessing creep damage and failure lives under various conditions. The essence of these efforts is presented in this paper.

The harsh operating conditions of Advanced Ultra-Supercritical (A-USC) power plants, i.e., steam operation conditions up to 760°C (1400°F)/35 MPa (5000 psi), require the use of Ni-based alloys with high temperature performance. Currently, the U.S. Department of Energy Fossil Energy program together with Electric Power Research Institute (EPRI) and Energy Industries of Ohio (EIO) is pursuing a Component Test (Comets) project to address material- and manufacturing-related issues for A-USC applications. Oak Ridge National Laboratory (ORNL) is supporting this project in the areas of mechanical and microstructure characterization, weld evaluation, environmental effect studies, etc. In this work, we present results from these activities on two promising Ni-based alloys and their weldments for A-USC applications, i.e., Haynes 282 and Inconel 740H. Detailed results include microhardness, tensile, air and environmental creep, low cycle fatigue, creep-fatigue, environmental high cycle fatigue, and supporting microstructural characterization.

Gas turbine blades are operated in a high temperature and a high pressure. In order to cope with that harsh condition, the blades are made of Nickel based superalloys which show excellent performance in such environment. Manufacturers of the blades usually provide the standards for the blade inspection and replacement. According to their guide, the blades are replaced after 3 times of operations and 2 times of refurbishments. Howsoever, purchase the new blades is always costly and burdensome to the power plant owners hence, the assessment of the blade lifespan and the rejuvenation of the degraded blades are indeed crucial to them. In this study, the optimal rejuvenation conditions for gas turbine blades were derived and verified. In addition to that, the creep durability was evaluated based on the actual blade inspection interval. LCF tests have been carried out on the rejuvenated blade and the result was compared with the fatigue life of the new blades. In order to secure the safety of the rejuvenated blade during operation, a heat flow analysis was performed to simulate the operating conditions of the gas turbine during operation, and the main stress and strain areas were investigated through the analysis results. And then LCF and creep considering the actual operating conditions were evaluated. The calculated life of fatigue and creep life is compared to the hot gas path inspection interval. For the rejuvenated blades, the creep life and the LCF interval were reviewed based on the temperature, stress, and strain acquired by computational analysis. The creep life was calculated as 59,363 hours by LMP curve, and the LCF was calculated as 2,560 cycles by the Manson Coffin graph.

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

Significant development is being carried out worldwide for establishing advanced ultra supercritical power plant technology which aims enhancement of plant efficiency and reduction of emissions, through increased inlet steam temperature of 750°C and pressure of 350 bar. Nickel base superalloy, 50Ni-24Cr-20Co-0.6Mo-1Al-1.6Ti-2Nb alloy, is being considered as a promising material for superheater tubes and turbine rotors operating at ultra supercritical steam conditions. Thermal fluctuations impose low cycle fatigue loading in creep regime of this material and there is limited published fatigue and creep-fatigue characteristics data available. The scope of the present study includes behavior of the alloy under cyclic loading at operating temperature. Strain controlled low cycle fatigue tests, carried out within the strain range of 0.2%-1%, indicate substantial hardening at all temperatures. It becomes more evident with increasing strain amplitude which is attributed to the cumulative effects of increased dislocation density and immobilization of dislocation by γ′ precipitates. Deformation mechanism which influences fatigue life at 750°C as a function of strain rate is identified. Hold times up to 500 seconds are introduced at 750°C to evaluate the effect of creep fatigue interaction on fatigue crack growth, considered as one of the primary damage mode. The macroscopic performance is correlated with microscopic deformation characteristics.

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. The blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified (DS) or cast as single crystals (SX). Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic orientation in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature. Comparisons between the more complex TMF tests and simpler isothermal low cycle fatigue (LCF) tests with hold times as cyclic test methods for qualifying alternative repair, rejuvenation, and heat-treatment procedures are discussed. Using the extensive set of DS and SX data gathered from the open literature, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, temperature, and several other cycling and loading parameters.

High temperature strength of a nickel-based superalloy, Alloy 740H, was investigated to evaluate its applicability to advanced ultrasupercritical (A-USC) power plants. A series of tensile, creep and fatigue tests were performed at 700°C, and the high temperature mechanical properties of Alloy 740H was compared with those of other candidate materials such as Alloy 617 and Alloy 263. Although the effect of the strain rate on the 0.2% proof stress was negligible, the ultimate tensile strength and the rupture elongation significantly decreased with decreasing strain rate, and the transgranular fracture at higher strain rate changed to intergranular fracture at lower strain rate. The time to creep rupture of Alloy 740H was longer than those of Alloy 617 and Alloy 263. The fatigue limit of Alloy 740H was about half of the ultimate tensile strength. Further, Alloy 740H showed greater fatigue strength than Alloy 617 and Alloy 263, especially at low strain range.

There are main drivers for the design and assessment of steam turbine components of today such as demands for improved materials, higher plant cycling operation, and reduced life-cycle costs. New materials have been developed over the last decades resulting in advanced martensitic 9-10CrMoV steels already applied in different types of turbines successfully. Heavy cyclic loading getting more importance than in the past results in utilization of the fatigue capabilities at high and low temperatures which might lead to crack initiation and subsequent crack propagation. Fracture mechanics methods and evaluation concepts have demonstrated their applicability to assess the integrity of components with defects or crack-like outage findings. Based on realistic modelling of the failure mechanism, accurate prediction of crack sizes at failure state can be improved defining the appropriate damage criteria. Ductility is a main aspect for robust design but its value definition can depend on component type, design rules, real loading conditions, service experience, and material characteristics. The question which direct material parameter is able to serve as limit value in design and how it can be determined has to be solved. Examples of advanced analysis methods for creep crack growth and fatigue interaction involving the crack initiation time show that the reserves of new martensitic 9-10Cr steels in high temperature application can be well quantified. The creep rupture elongation A u and the loading conditions in the crack far field are main factors. If the A u value is sufficient high also after long-time service, the material remains robust against cracks. Investigations into the influence of stress gradients on life time under fatigue and creep fatigue conditions show that e.g. for 10CrMoWV rotor steel crack growth involvement offers further reserves. The consideration of constraint effect in fracture mechanics applied to suitable materials allows for further potentials to utilize margin resulting from classical design. The new gained knowledge enables a more precise determination of component life time via an adapted material exploitation and close interaction with advanced design rules.

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.

Future, flexible thermal energy conversion systems require new, demand-optimized high-performance materials. In order to provide a basis for the targeted development of fatigue-resistant, cost-effective steel grades, the microstructural damage to materials and the failure of conventional and novel steels were investigated in thermo-mechanical fatigue and fatigue crack propagation experiments. Based on the results, improved, ferritic “HiperFer” (High performance Ferrite) steels were designed, produced and characterized. A brief description of the current state of development is given.

To improve efficiency and flexibility and reduce CO 2 emissions, advanced ultra super critical (AUSC) power plants are under development, worldwide. Material development and its selection are critical to the success of these efforts. In several research and development programs / projects the selection of materials is based on stress rupture, oxidation and corrosion tests. Without doubt, these criteria are important. To improve the operational flexibility of modern power plants the fatigue properties are of increased importance. Furthermore, for a safe operation and integrity issues the knowledge about the crack behavior is essential. Crack initiation and crack growth may be caused by natural flaws or cracks induced by component operation. In order to develop new materials, properties like tensile strength and creep strength are an important part of qualification and subsequent approval by notified bodies. Consequently short term properties as well as time-temperature dependent properties are generated and taken into considerations. In the case of high strength γ'-strengthening nickel-base alloys investigating the creep crack behavior is also strongly recommended. This article shows results of currently investigated nickel-based alloys for newly developed headers, pipes and other high temperature boiler applications and their critical creep crack propagation behavior.

This study aims to elucidate the chemical compositions and microstructural factors that affect longterm creep rupture strength and creep rupture ductility using multiple heats of Gr.92 steel. Evaluating the reduction behavior in long-term creep rupture strength, we propose a relative creep rupture strength value, which is expressed as the logarithmic ratio of the estimated creep strength for each rupture time exceeding 10,000 hours, with 10,000 hours as the reference. Higher initial hardness correlates with greater pronounced strength reduction in the long-term regime. While smaller prior austenite grain sizes lead to greater reductions in creep rupture strength, this effect diminishes above 30 μm. However, no clear correlation was observed between Cr content and creep strength reduction in this study. Brittle creep ruptures with smooth test specimens were observed just below the extensometer ridge in the parallel section of test specimen, indicating notch weakening. Even in heats with excellent creep ductility, the amount of inclusions tended to be higher than in heats with lower creep ductility. Factors other than inclusions also seem to influence long-term creep ductility.

A material test loop has been installed at GKM Mannheim, which enables thick-walled components of future highly-efficient power plants to be exposed to steam temperatures of up to 725 °C. The project goal was to demonstrate the feasibility of a 700 °C power plant.