Search Results for test specimens

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

The efforts of the European Union and Germany in particular to realize the transformation towards a climate-neutral economy over the coming decades have the establishing of a hydrogen economy as a fundamental milestone. This includes production, import, storage, transportation and utilization of great amounts of gaseous hydrogen in existing and new infrastructure. Metallic materials, mainly steels, are the most widely used structural materials in the various components of this supply chain. Therefore, the accelerated use of hydrogen requires the qualification of materials (i.e., ensuring they are hydrogen-ready) to guarantee the sustainable and safe implementation of hydrogen technologies. However, there is currently no easily applicable and standardized method to efficiently determine the impact of gaseous hydrogen on metallic materials. The few existing standards describe procedures that are complex, expensive, and only available to a limited extent globally. This article outlines the key milestones towards standardizing an efficient testing method as part of the TransHyDE flagship project. This new approach enables testing of metallic materials in gaseous hydrogen using tubular specimens. It uses only a fraction of the hydrogen required by the traditional autoclave method, significantly reducing costs associated with technical safety measures. Among the topics to be discussed are the factors influencing the test procedure, including geometrical considerations, surface quality, gas purity and strain rate.

The use of the bainitic creep strength enhanced ferritic steel T/P23 has increased over the last decade in a wide range of applications including headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in weldments of this material, such as hydrogen induced cracking, reheat cracking and stress corrosion cracking. In order to help characterize high temperature cracking phenomena, including reheat cracking, a limited number of laboratory creep crack growth tests are being conducted as part of an ongoing project. Tests were run on as-welded sections with the test specimen crack-tip located in select zones of the weldment. Test temperatures are intended to bookend the range of applications from a waterwall condition of ~482°C (900°F) to the superheat/reheat condition of 565°C (1050°F). This paper describes the results of some early testing at 482°C (900°F). The tests provided useful insight into the cracking susceptibility of the material at this temperature with respect to not only time-dependent cracking, but also fatigue crack growth and fracture toughness. The paper includes details of the test method and results, as well as findings from post-test metallographic examinations of the tested specimens.

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.

Two materials with different purity of 2.25Cr-1Mo steel thick weld joint were prepared and creep rupture behavior was investigated by large sized specimens. For high purity material, two types of challenging heat treatment was tried to modify the original microstructural conditions. Weld joints were made and large sized creep test specimens were machined. Creep tests were performed at 903K, 40MPa. Specimen made from low purity material fractured at fine grained heat affected zone (FGHAZ) and showed so-called Type IV cracking. On the other hand, specimen made from high purity material showed maximum creep damage at weld metal. In the case of specimens applied challenging heat treatment, remarkably high ductility were observed at fracture. Regarding 2.25Cr-1Mo steel, it was confirmed that the suppression of Type IV cracking had been basically achieved by past improvement on purity level. At the same time, improvement of heat treatment condition was found to have further effect. Because of improved creep properties of high purity material, properties of weld metal had rose up to be the next issue to be examined. At least, taking care on layout design of weld beads to avoid creating wide spread fine grained portion is desired.

This research compares creep crack growth behavior of two heats of creep strength enhanced ferritic (CSEF) steel, grade 91. These heats represent extremes of creep damage susceptibility, one heat exhibiting low creep ductility and the other high creep ductility. Creep crack growth tests were performed with compact tension specimens and were monitored with direct current potential drop and optical surface measurements. Load line displacement was measured throughout the duration of the tests. Specimens were sectioned, mounted, and analyzed using optical and scanning electron microscopy to assess the presence of oxidation, micro-cracking, creep damage, and void density. Tests were performed over a range of initial stress intensities on the low ductility material to investigate the impact of creep ductility. Metallurgical evidence and test data for each crack growth test was assessed to evaluate crack growth behavior linked to creep crack growth parameter (C*) and stress/creep damage distribution in the vicinity of the crack.

The creep strength and ductility of Grade P22 steel (2¼ Cr) was measured at 600°C under standard uniaxial tensile conditions at 150MPa. Test specimens were prepared by solution heat treatment at austenitization temperatures ranging from 900°C - 1200°C followed by normalization at 900°C before continuous air cooling to room temperature. In addition to specimens tested in the solution treated state, creep tests were also performed after tempering. The variable austenitization temperatures gave rise to different prior austenite grain (PAG) sizes, which in turn influenced the crystallographic packet and block boundary misorientation angle distribution. The latter parameters were measured using electron backscattered diffraction which also allowed partial reconstruction of the PAG boundaries. The time to creep failure at 600°C increased as function of PAG size up to approximately 70µm, but significantly decreased when the average prior austenite grain size measured approximately 108 µm. However, the minimum creep rate decreased even up to the largest PAG size with corresponding decrease in creep ductility. The stability of the crystallographic packet and block boundaries influences the high strength-low ductility for the large PAGs in comparison to the dominant effect of PAG boundaries at the smallest grain size where extensive recovery and recrystallization reduces creep strength.

Long-term creep tested specimens of the advanced austenitic stainless steel Super 304H were subjected to detailed metallographic analysis with an emphasis on the relationship between creep induced cavities (voids) and microstructural features. The creep specimens were tested between 873 and 973 K (600 and 700°C) at stresses between 110 and 340 MPa, with rupture times up to ~1.8 x 10 8 s (50,000 hours). To characterize damage, the distributions of creep cavities along the length of the gage section were determined and microstructural features associated with the cavities were investigated using optical microscopy and scanning electron microscopy.

The creep behavior of a γ-TiAl based alloy at 1073 K was investigated, examining three different microstructures: equiaxed γ (Eγ), γ/γ fully lamellar (FLγ), and equiaxed γ with α 2 phase on grain boundaries (Eγα 2 ). The aim was to understand the influence of lamellar interfaces and grain boundary α 2 phase on creep behavior. Initially, creep rates were consistent across all specimens upon loading. However, Eγ exhibited a gradual decrease in creep rate compared to Eγα 2 and FLγ. Notably, the minimum creep rate of Eγ was one order of magnitude lower than that of Eγα 2 and FLγ. Conversely, Eγα 2 and FLγ displayed a slight acceleration and the longest rupture strain, albeit with the shortest rupture time compared to Eγ. Upon microstructural analysis of of the creep-test specimens, it was observed that numerous dynamic recrystallized grains (DXGs) and sub-grains formed along grain boundaries and interiors in Eγ, whereas they were limited to the region along grain boundaries in FLγ. In contrast, very few DXGs were formed in Eγα 2 . These findings indicate that γ/γ interfaces inhibit the extension of DXGs into grain interiors, suggesting that the grain boundary α 2 phase effectively suppresses the formation of DXGs.

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.

Creep properties of 9Cr heat resistant steels can be improved by the addition of boron and nitrogen to produce martensitic boron-nitrogen strengthened steels (MarBN). The joining of this material is a crucial consideration in the material design since welds can introduce relatively weak points in the structural material. In the present study, creep tests of a number of MarBN weld filler metals have been carried out to determine the effect of chemistry on the creep life of weld metal. The creep life of the weld metals was analysed, and the evolution of creep damage was investigated. Significant differences in the rupture life during creep have been observed as a function of boron, nitrogen and molybdenum concentrations in the weld consumable composition. Although the creep lives differed, the particle size and number in the failed creep tested specimens were similar, which indicates that there is a possible critical point for MarBN weld filler metal creep failure.

In 9~12% Cr containing martensitic stainless steels, Laves phase usually occurs after long term high temperature exposure, while in the present work, some sparse relatively large particles of (Fe,Cr)2Mo type Laves phase were observed in virgin FB2 steel. It is speculated that the large Laves phase particles formed in casting process due to dendritic segregation. Then the evolutionary behavior of Laves phase during welding thermal cycle was studied and constitutional liquation of Laves phase was found, suggesting a liquation crack tendency in FB2 steel. At last, the hot ductility tests showed that the area where constitutional liquation occurred would act as crack initiation site, and the tested specimen fractured without any obvious plastic deformation. This work provided some guidance for the practical production of welded turbine rotors made of FB2 steel.

Grade 91 creep strength-enhanced ferritic steel is a critical material in power generation, widely used for high-temperature, high-pressure tubing and piping applications. Its superior elevated-temperature strength derives from a distinctive microstructure of tempered martensite with uniformly dispersed secondary phases (carbides and carbo-nitrides). This microstructure, crucial for reliable service performance, is achieved through precise control of the manufacturing process, including steelmaking, hot forming, and final heat treatment. This investigation builds upon earlier research into the relationship between manufacturing parameters and short-term creep-rupture properties in T91 tubes, and a recent update that included test results exceeding 30,000 hours. This study presents a comprehensive metallurgical analysis of ruptured test specimens. The investigation focuses on correlating manufacturing parameters with not only creep strength but also material ductility and microstructural evolution during long-term exposure, providing valuable insights into the material’s behavior under extended service conditions.

The effects of pre-strain on creep properties of Alloy 740 have been investigated. Tensile strain was 7.5% and introduced by room temperature tensile test. Creep tests were conducted under 750 degree C, 275-350MPa. Creep rupture life of pre-strained sample decreased by half compared with as-heat treated sample. Creep behaviors of both samples were almost similar in primary creep stage, but onset of creep rate acceleration of pre-strained sample was faster than those of as-heat treated sample. As a result, minimum creep rate of pre-strained sample were two times larger than that of as-heat treated sample. From the observation of ruptured specimen, pre-strained sample had much more sub cracks than as-heat treated sample. On the other hand, microstructure of both samples was also different. There were MC precipitates on grain boundary in both ruptured specimens, but both size and number of MC precipitates were larger in pre-strained sample although creep life of pre-strained sample was shorter than that of as-heat treated sample. In this paper, the difference of creep behavior will be discussed in terms of both the microstructural change and mechanical damage.

In order to establish a creep damage assessment method for 47Ni-23Cr-23Fe-7W (HR6W), which is a candidate material of A-USC, microstructure observation of creep interrupted specimens and ruptured specimen was conducted, and the creep damage process was examined. Creep tests were conducted under conditions of 800°C, 70 MPa, 700°C, and 100 MPa. For creep damage assessment, an optical microscope was used for replicas sampled from the outer surface of specimens, and crack ratio at grain boundaries was assessed. The results indicated that creep voids and cracks were initiated at grain boundaries from about 0.35 of creep life ratio, and crack ratio increased drastically after creep life ratio of 0.65. This crack ratio was almost the same regardless of the specimen shape Therefore, the method to assess crack ratio using replicas is considered to be an effective method for creep damage assessment of HR6W. An increase in the crack ratio due to an increase in creep life ratio showed the same trend as the change in elongation of creep interrupted specimens. Microstructure observations were conducted with interrupted specimens using SEM-ECCI (Electron Channeling Contrast Imaging) in order to clarify the cause of acceleration creep. The results showed that sub-boundary developed significantly near grain boundaries, which indicates that sub-boundary development may cause acceleration.

Theoretical and experimental investigations, including fracture tests, acoustic emission (AE) studies, fractography, micro-sclerometric analyses, and spectral/chemical analyses of specimens, have established the possibility of revealing, recognizing in-service acquired, age-related, and prefabricated flaws based solely on AE data. Results show a linear dependence between AE and mechanical deformation power of steel specimens in original and creep stage 3a-3b conditions, decreasing fracture load and J1c value for aging steel, creep processes at stage 3a-3b having J-integral value below 0.05J1c, possibility of assessing and distinguishing different flaw development stages with ≥87% accuracy, revealing zones of tough and brittle fracture, and recognizing inclusions/pre-fabricated flaws and assessing individual/interacting flaws. Experiments confirmed the absence of the Kaiser effect under repeated loading of flawed specimens and demonstrated using AE for defect revelation. Analysis showed that creep-associated AE is mainly continuous, with repeated loading decreasing burst AE contribution during plastic deformation development.

An internal pressure creep test has been carried out on a Gr. 91 steel longitudinal welded pipe at 650°C to examine the type IV failure behavior of actual pipes, using a large-scale experiment facility “BIPress”, which can load internal pressure and bending force on large diameter pipes at high temperatures. The creep test was also interrupted three times to measure hardness and voids density in the HAZ region of the outer surface of the test pipe. Results of the measurement of the hardness and voids density at the interruption did not indicate creep damage accumulation. The welded pipe suddenly ruptured with large deformation, which caused crushing damage to the surrounding facility. Type IV cracking occurred in the longitudinal welded portion of the test pipe, and the length of the crack reached 5000mm. SEM observation was carried out at the cross section of the welded portion of the test pipe and voids density was measured along the thickness direction in the HAZ region. To clarify the stress/strain distribution in the welded portion, creep analysis was conducted on the test pipe, where the materials are assumed to consist of base metal, weld metal and HAZ. After stress redistribution due to creep deformation, stress and strain concentrations were observed inside the HAZ region. Then, the authors' creep life prediction model was applied to the creep test result to examine its validity to actual size pipes. It was demonstrated that the life prediction model can evaluate damage of the Gr. 91 steel longitudinal welded pipe with sound accuracy.

Supercritical carbon dioxide cooling during machining has been identified as an effective measure to mitigate the risk of stress corrosion cracking in materials utilized in the primary circuit of light water reactors, particularly in pressure vessel structural steels. This study aims to compare two different cooling methods, the novel supercritical carbon dioxide and conventional high pressure soluble oil, employed during both milling and turning processes for SA508 Grade 3 Class 2 and AISI 316L steels. As the surface conditions of materials are critical to fatigue properties, such as crack initiation and endurance life, the fatigue performance of both cooling methods for each process were then evaluated and the impact on properties determined. To compare the potential benefits of supercritical carbon dioxide cooling against conventional soluble oil cooled machining, fatigue specimens were machined using industry relevant CNC machine tools. Surface finish and machining methods were standardized to produce two different specimen types, possessing dog- bone (milled) and cylindrical (turned) geometries. Force-controlled constant amplitude axial fatigue testing at various stress amplitudes was undertaken on both specimen types in an air environment and at room temperature using a stress ratio of 0.1. The fatigue performance of the supercritical carbon dioxide cooled specimens revealed substantially greater endurance lives for both SA508 and 316L materials, when compared with specimens machined using high pressure soluble oil cooling.

Effect of thermomechanical and magnetic treatment on creep characteristics of advanced heat resistant ferritic steels for USC power plants has been investigated to explore fundamental guiding principles for improving creep rupture strength at elevated temperatures over 600°C. A model steel with a composition of Fe-0.08C-9Cr-3.3W-3Co-0.2V-0.05Nb-0.05N-0.005B-0.3Si-0.5Mn (in mass%) has been prepared by vacuum induction furnace. Creep tests at 650 °C and microstructural observations were performed on the thermomechanical and magnetic treated specimens after tempering. New thermomechanical treated samples without magnetic field showed some improvement in creep strength comparing with ordinarily normalized and tempered specimens. Further improvement was observed in the specimen that had been exposed to a magnetic field during transformation into the martensite. From the result of microstructural observation, it was found that the finely distributed precipitates such as MX and M 23 C 6 caused this improvement. And it was suggested that the magnetic treatment at martensitic transformation increase the precipitation sites during tempering, resulting in increasing the amount and preventing the growth of the precipitates.

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.