Search Results for stress relaxation cracking test

Finite element (FE) modeling has been applied to a stress relaxation cracking (SRC) test in order to evaluate the effects of changing sample geometry and material type. This SRC test uses compressive pre-straining to create a tensile residual stress in modified compact-tension specimens and has been used to test 316H stainless steel. The FE model is first used to verify that sample integrity will not be compromised by modifying the geometry. The FE model is then applied to candidate Advanced Ultra Supercritical nickel-base alloys 617, 740H, and 800. It is determined that this stress relaxation test will be appropriate for these alloys.

Advanced power generation systems, including advanced ultrasupercritical (A-USC) steam and supercritical carbon dioxide (sCO 2 ) plants operating above 700°C, are crucial for reducing carbon dioxide emissions through improved efficiency. While nickel superalloys meet these extreme operating conditions, their high cost and poor weldability present significant challenges. This study employs integrated computational materials engineering (ICME) strategies, combining computational thermodynamics and kinetics with multi-objective Bayesian optimization (MOBO), to develop improved nickel superalloy compositions. The novel approach focuses on utilizing Ni 3 Ti (η) phase strengthening instead of conventional Ni 3 (Ti,Al) (γ’) strengthening to enhance weldability and reduce costs while maintaining high-temperature creep strength. Three optimized compositions were produced and experimentally evaluated through casting, forging, and rolling processes, with their microstructures and mechanical properties compared to industry standards Nimonic 263, Waspaloy, and 740H. Weldability assessment included solidification cracking and stress relaxation cracking tests, while hot hardness measurements provided strength screening. The study evaluates both the effectiveness of the ICME design methodology and the practical potential of these cost-effective η-phase strengthened alloys as replacements for traditional nickel superalloys in advanced energy applications.

The susceptibilities of hot cracking and reheat cracking of A-USC candidate Ni-based alloys were evaluated relatively by Trans-Varestraint testing and Slow Strain Rate Tensile (SSRT) testing. In addition, semi-quantitative evaluation of the stress relaxation cracking susceptibility of Alloy 617 was conducted, because stress relaxation cracking in the heat affected zone (HAZ) has actually been reported for repair welds in Alloy 617 steam piping in European A-USC field-testing. Solidification cracking susceptibilities of Alloy 617 were the highest; followed by HR35, Alloy 740 and Alloy 141, which were all high; and then by HR6W and Alloy 263, which were relatively low. In addition, liquation cracking was observed in the HAZ of Alloy 617. The reheat cracking susceptibilities of Alloy 617, Alloy 263, Alloy 740 and Alloy 141 were somewhat higher than those of HR6W and HR35 which have good creep ductility due to the absence of γ’ phase precipitates. A method to evaluate stress relaxation cracking susceptibility was developed by applying a three-point bending test using a specimen with a V-notch and finite element analysis (FEA), and it was shown that stress relaxation cracking of aged Alloy 617 can be experimentally replicated. It was proposed that a larger magnitude of creep strain occurs via stress relaxation during the three-point bending test due to a higher yield strength caused by γ’ phase strengthening, and that low ductility due to grain boundary carbides promoted stress relaxation cracking. The critical creep strain curve of cracking can be created by means of the relationship between the initial strain and the creep strain during the three-point bending tests, which were calculated by FEA. Therefore, the critical conditions to cause cracking could be estimated from the stress relaxation cracking boundary from of the relationship between the initial strain and the creep strain during the three-point bending test.

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

Development of the advanced USC (A-USC) boiler technology has been promoted in recent years, which targets 700°C steam condition. HR6W (Ni-23Cr-7W-Ti-Nb-25Fe) and HR35 (Ni-30Cr-6W-Ti-15Fe) have been developed for A-USC boiler tubes and pipes. The former alloy is mainly strengthened by Fe 2 W type Laves phase. The latter one employs precipitation strengthening of α-Cr phase in addition to Laves phase. Characteristic alloy design of both alloys, which does not use precipitation strengthening of γ′ phase (Ni 3 Al), leads to superior ductility and resistance to stress-relaxation cracking. Stability of creep strength and microstructure has been confirmed by long-term creep rupture tests. The 100,000h average creep rupture strength of HR6W is 85MPa at 700C. That of HR35 is 126MPa at 700°C which is comparable with conventional Alloy617. Tubes of both alloys have been evaluated by the component test in Japanese national A-USC project with γ′ hardened Alloy617 and Alloy263. Detailed creep strength, deformation behavior and microstructural evolution of these alloys are described from the viewpoint of the difference in strengthening mechanisms. Capability of these alloys for A-USC boiler materials has been demonstrated by the component test in the commercial coal fired boiler as the part of the A-USC project.

A longitudinal crack and window opening type failure occurred in neutral zone that is applied to least plastic deformation in the bent TP347H tube during operation. From the analysis of residual stress and plastic deformation during the tube bending, there is low creep strength and high residual stress in neutral zone as compared other regions like intrados and extrados. Therefore, failure occurred in neutral zone due to stress relaxation concentrated in grain boundary during operation.

Single crystal Ni-base superalloys are subjected to tension hold at high temperature in addition to cyclic loading during the operation of gas turbines. Various studies have investigated creep-fatigue crack propagation in superalloys under trapezoidal loadings and evaluated the life time based on parameters such as creep J-integral. However, it is still unclear how damage field and stress-strain condition change at the crack tip during hold time, and how it affects on fatigue crack propagation. In this study, the influence of the tension hold and accompanying creep at crack tip on subsequent fatigue crack propagation behavior was evaluated by introducing single tension holds into pure cyclic loadings. The series of the experiments revealed that because of the tension hold, material degradation and stress relaxation occurred simultaneously ahead of crack tip. In the region where material was degraded, the resistance against crack propagation was reduced, while in the region where stress was relaxed, the crack driving force was lowered.

Inconel alloy 740 is a precipitation-hardenable nickel-chromium-cobalt alloy with niobium, derived from Nimonic 263, and is considered a prime candidate for the demanding conditions of advanced ultrasupercritical boilers. It offers an exceptional combination of stress rupture strength and corrosion resistance under steam conditions of 760°C (1400°F) and 34.5 MPa (5000 psi), surpassing other candidate alloys. Initially, Inconel alloy 740 was prone to liquation cracking in sections thicker than 12.7 mm (0.50 in), but this issue has been resolved through modifications in the chemical composition of both the base and weld metals. Current concerns focus on the weld strength reduction factor for direct-age weldments. This has led to further development in welding Inconel alloy 740 using Haynes 282, which has higher creep strength and may mitigate the weld strength reduction factor. This study details successful efforts to eliminate liquation cracking and compares the properties of Inconel alloy 740 and Haynes 282 filler materials using the gas tungsten arc welding process.

Procedures for assessing components subjected to cyclic loading at high temperatures require material property data that characterize creep-fatigue deformation behavior and resistance to cracking. While several standards and codes define test procedures for acquiring low cycle fatigue (LCF) and creep properties, no formal guidelines exist for determining creep-fatigue data. This paper reviews the results of a global survey conducted by EPRI to support the development of a new draft testing procedure intended for submission to ASTM and, ultimately, ISO standards committees. The survey included a review of relevant national and international standards, as well as responses to a questionnaire distributed to high-temperature testing specialists in Europe, North America, and Japan. Additionally, standards related to the calibration of load, extension, and temperature measurement devices were examined. The questionnaire responses provided insights into test specimen geometry, testing equipment, control and measurement of load, extension, and temperature, and data acquisition practices. This paper outlines the background and considerations for the proposed guidance in the new standard.

The EU NextGenPower-project aims at demonstrating Ni-alloys and coatings for application in high-efficiency power plants. Fireside corrosion lab and plants trials show that A263 and A617 perform similar while A740H outperforms them. Lab tests showed promising results for NiCr, Diamalloy3006 and SHS9172 coatings. Probe trials in six plants are ongoing. A617, A740H and A263 performed equally in steamside oxidation lab test ≤750°C while A617 and A740H outperformed A263 at 800°C; high pressure tests are planned. Slow strain rate testing confirmed relaxation cracking of A263. A creep-fatigue interaction test program for A263 includes LCF tests. Negative creep of A263 is researched with gleeble tests. A263 Ø80 - 500mm trial rotors are forged with optimized composition. Studies for designing and optimizing the forging process were done. Segregation free Ø300 and 1,000mm rotors have been forged. A263 – A263 and A293 – COST F rotor welding show promising results (A263 in precipitation hardened condition). Cast step blocks of A282, A263 and A740H showed volumetric cracking after heat treatment. New ‘as cast’ blocks of optimized composition are without cracks. A 750°C steam cycle has been designed with integrated CO 2 capture at 45% efficiency (LHV). Superheater life at ≤750°C and co-firing is modeled.

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.

Enhanced life assessment methods contribute to the long-term operation of high-temperature components by reducing technical risks and increasing economic benefits. This study investigates creep-fatigue behavior under multi-stage loading, including cold start, warm start, and hot start cycles, as seen in medium-loaded power plants. During hold times, creep and stress relaxation accelerate crack initiation. Creep-fatigue life can be estimated using a modified damage accumulation rule that incorporates the fatigue fraction rule for fatigue damage and the life fraction rule for creep damage while accounting for mean stress effects, internal stress, and creep-fatigue interaction. In addition to generating advanced creep, fatigue, and creep-fatigue data, scatter band analyses are necessary to establish design curves and lower-bound properties. To improve life prediction methods, further advancements in deformation and lifetime modeling are essential. Verification requires complex experiments under variable creep conditions and multi-stage creep-fatigue interactions. A key challenge remains the development of methods to translate uniaxial material properties to multiaxial loading scenarios. Additionally, this study introduces a constitutive material model, implemented as a user subroutine for finite element applications, to simulate start-up and shut-down phases of components. Material parameter identification has been achieved using neural networks.

Based on the research and development of Ni-based alloy of 700°C steam turbine bolts and blades worldwide, the process, microstructure, properties characteristics and strengthening mechanism of typical 700°C steam turbine bolts and blades materials Waspaloy are discussed in this study. The result shows that Waspaloy has higher elevated temperature yield strength, creep rupture strength, anti-stress relaxation property and good microstructure stability. The Waspaloy alloy could meet the design requirements of 700°C steam turbine bolts and blades.

In Europe between 2006 and 2012 several ultra-super-critical (USC) coal-fired power plants were built employing T24 (7CrMoVTiB10-10 / DIN EN 10216-2:2014-03 / VdTÜV sheet 533/2) in membrane walls. During commissioning stress corrosion cracking (SCC) on the tube-to-tube butt welds appeared. The widespread damages required the development of a new patented commissioning procedure to avoid recurring damages. Although this commissioning procedure was employed successfully and the power plants are in operation since then, a debate about the implementation of a hardness limit for such butt welds was initiated. According to the European standards butt welds of T24 boiler tubes with wall thickness < 10 mm (0.3937 in) do not require any post-weld heat treatment (PWHT) and no hardness limits are given. When looking at manufacturing related issues such as an imminent risk of cold cracking after welding of micro-alloyed steels a widely applied but coarse hardness limit is 350 HV. Based on laboratory tests, some authors reallocated this 350 HV hardness limit for addressing SCC susceptibility of low-alloyed steels. This article describes typical hardness levels of T24 boiler tube TIG butt welds and the SCC behavior in high temperature water. Further the effect of the stress relief heat treatment (SRHT) of the boiler membrane walls between 450 °C and 550 °C (842 °F and 1022 °F) on its hardness values and on the SCC behavior is discussed, showing that the hardness values should not be used as an indicator for SCC susceptibility of T24 boiler tube butt welds.

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.

A comprehensive EPRI initiative launched in 2006 has addressed the critical need to better understand creep-fatigue interactions in power plants experiencing cyclic operation. This international collaboration of industry experts has focused on evaluating current test methods, analyzing crack initiation and growth methodologies, examining life prediction approaches for various applications, identifying deficiencies in creep-fatigue damage assessment, and determining future research requirements. This paper presents key findings from the project, with particular attention to the performance of creep-strengthened ferritic steels, specifically Grade 91 and 92 steels, providing essential insights for power plants facing increasingly demanding operational conditions.

In a European ultra-supercritical (USC) power station repaired reheater bundle tubes made out of 25% Chromium stainless steels developed stress relief damages at the tube-to-tube butt welds, leading to leakages after only 8.500 hours of operation. Laboratory investigations of the leakages revealed common features of stress relief cracking (SRC) such as highly localized intergranular cracking in the heat affected zone (HAZ) near the fusion line, creep void formation at the crack tip and around the crack. At that time no other SRC damages were known for the employed 25% Chromium stainless steel boiler tubes. This article briefly describes the SRC damage found on the repaired reheater bundle tubes. It further provides insight on the several laboratory tests employed to assess the SRC behavior of welded joints of different creep resistant stainless steels. Among the selected test methods were Slow-Strain-Rate-Tests (SSRT), static 3-point bending tests derived from the Van Wortel approach and component tests. The results provided by the described tests methods have shown that the SRC behavior of a given material combination must be assessed by different techniques. This is especially the case for the evaluation of potential countermeasures and for the determination of the service conditions leading to the highest susceptibility.

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.

Advanced ultra-supercritical fossil plants operated at 700/725 °C and up to 350 bars are currently planned to be realized in the next decade. Due to the increase of the steam parameters and the use of new materials e.g. 9-11%Cr steels and nickel based alloys the design of highly loaded components is approaching more and more the classical design limits with regard to critical wall thickness and the related tolerable thermal gradients. To make full use of the strength potential of new boiler materials but also taking into account their specific stress-strain relaxation behavior, new methods are required for reliable integrity analyses and lifetime assessment procedures. Numerical Finite Element (FE) simulation using inelastic constitutive equations offers the possibility of “design by analysis” based on state of the art FE codes and user-defined advanced inelastic material laws. Furthermore material specific damage mechanisms must be considered in such assessments. With regard to component behavior beside aspects of multiaxial loading conditions must be considered as well as the behavior of materials and welded joints in the as-built state. Finally an outlook on the capabilities of new multi-scale approaches to describe material and component behavior will be given.

Welding processes and fabrication techniques have been studied in the development of Advanced USC boilers. Advanced 9Cr steels, Fe-Ni alloy (HR6W) and Nickel base alloys (HR35, Alloy 617, Alloy 263, Alloy 740 and Alloy 740H) have been selected as candidate materials for the boiler. The weld joints of these alloys were prepared from plates, small diameter tubes and large pipes, and welding procedure tests were performed. In this study, TIG and SMAW were applied. Both welding process produced good weld joints, and they showed good results in bending tests, tensile tests and the Charpy impact test. To select the annealing conditions for stress relief, stress relaxation tests and hardness tests were conducted on the weld joints after various heat treatments. The microstructure was also evaluated by SEM and TEM. Creep rupture tests are being performed for the weld joints with and without heat treatment. The maximum creep rupture tests are expected to take over 100,000 hours. In the study of fabrication techniques, hot bending tests by high frequency induction heating for large pipes and cold/hot bending tests for small diameter tubes were established. After the bending tests, mechanical property tests such as tensile tests, impact tests and creep rupture tests were conducted. The effect of pre-strain on creep strength was studied to take the creep test results after bending into consideration. The creep rupture test will be continued for specimens from weld joints and bending pipes to show their long term reliability.