Search Results for creep void initiation

Type IV creep damage is a problem in high-temperature steam piping made of high chromium steel at thermal power plants, and a method for evaluating the remaining life is required. In this study, we considered that void’s initiation and growth can be expressed by initiation rate f, growth rate h, and initiation start time t 1 , and that stress and TF affect f, h and t 1 . We also proposed the method to estimate f, h and t 1 by measuring the change of the distribution of radius of voids during creep test. The creep test conditions are (1) test temperature of 650 C, maximum principal stress σ 1 of 79.5MPa, and TF of 2.5 ~ 3.0, and (2) test temperature of 650C, maximum principal stress of 71.5MPa, and TF of 2.5 ~ 3.0. The influence of σ 1 to f, h and t 1 was quantified by comparing the result of test (1) and that of test (2).

Metallurgical factors affecting the fusion boundary failure and damage mechanism of DMWs (Dissimilar Metal Welds) between the CSEF (Creep Strength Enhanced Ferritic) steels and austenitic steels were experimentally and theoretically investigated and discussed. Long-term exservice DMWs up to 123,000 hours were investigated; the precipitates near the fusion boundary were identified and quantitatively evaluated. Comparing with the other generic Ni-based weld material, MHPS original filler metal HIG370 (Ni bal.-16Cr-8Fe-2Nb-1Mo) showed superior suppression effect on fusion boundary damage of DMWs, which was verified by both of the microstructure observation and thermodynamic calculation. Based on the microstructure observation of crept specimen and ex-service samples of DMWs, temperature, time and stress dependence of fusion boundary damage of DMWs were clarified. Furthermore, fusion boundary damage morphology and mechanism due to precipitation and local constituent depletion was discussed and proposed from metallurgical viewpoints.

This study aims to examine the effects of grain boundary oxidation and creep on crack initiation and fracture behaviors in cold worked surface layer, under static tensile stresses in air. To determine these effects in relation to percent cold work and hardness scale, cold-rolled plates with a reduction ratios between 10% and 50% were prepared. Uniaxial constant load (UCL) tests were conducted at elevated temperature in air using smooth round bar specimen. UCL tests with a load of 0.9σy (926MPa) at 550°C show that rupture time for all cold- rolled materials were shorter than that of as-received material. From cross-sectional observation after UCL testing, surface crack at grain boundary and voids were observed in as-received material, whereas creep cracks were also observed in cold-rolled materials. This implied that crack initiation was assisted by cold working. Comparing test results with a load reduced to 0.8σy (823MPa), difference of rupture time was expected as a factor of 5 for as-received material, and measured as 2-3 for cold-rolled materials. It was suggested that cold worked layer was more sensitive to creep than base metal.

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.

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.

Type IV creep damage of high chromium steel is a problem in thermal power plants and a method of evaluating remaining life is required. Type IV creep damage is characterized by many voids that initiate in the weldment fine grain heat affected zone (FGHAZ), where the stress multiaxiality (expressed by the Triaxiality Factor, TF) is high. As the creep continues, the shape of the grain boundary becomes simple; that is, close to a straight line. It is known that the grain boundary is fractal. The complexity of the fractal is represented by the fractal dimension. Therefore, we considered that the fractal dimension of the grain boundary in FGHAZ could be an indication of creep damage and studied its change as creep proceeded. First, creep tests were conducted to produce damaged materials, and their fractal dimensions were measured. Next, FEM analysis was conducted to obtain the distribution of the principal stress, TF, and creep strain of the observed surface. The distribution of creep damage was obtained by the time fraction rule. The results of this evaluation confirmed that the fractal dimension of the grain boundary decreases with creep time and that the principal stress and TF affect it.

As long term laboratory creep data became available the original estimates of the allowable stresses for creep strength enhanced ferritic steels (CSEF) had to be reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength is below that originally predicted from a simple extrapolation of short term data. One of the microstructural degradation mechanisms responsible for the reduction in strength is the development of creep voids. Nucleation, growth and inter linkage of voids also result in a significant loss of creep ductility. Indeed, elongations to rupture of around 5% in 100,000 hours are now considered normal for long term creep tests on many CSEF steels. This relatively brittle behaviour, and the associated creep void development, promotes burst rather than leak type fracture in components. Moreover, the existence of significant densities of voids further complicates in-service assessment of condition and weld repair of these steels. The present paper examines background on the nucleation and development of creep voids in 9 to 12%Cr martensitic steels and discusses factors affecting brittle behavior.

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.

Recovery of microstructure and void formation were investigated in creep-ruptured specimens of ASME Gr. T91 steels to understand the cause of loss of creep rupture ductility in the long-term creep condition and its heat-to-heat variation. The specimens studied were two heats (MGA, MGC) of Gr. T91 steels creep-ruptured at 600 °C under the stress conditions of 160-80 MPa. The reduction of area at rupture (RA) was 55% for MGA, but 83% for MGC in the long-term condition (under the creep stress of 80 MPa), while RA was higher than 80 % for the two heats in the short-term conditions (under the creep stresses above 100 MPa). In both heats, equiaxed grains were observed in the vicinity of ruptured surface in the long-term condition, indicating that recovery and recrystallization occurred extensively in the creep condition, while grains were elongated in the short-term conditions. In the uniformly deformed regions with a small area reduction in the long-term crept specimens, recovered and recrystallized grains were observed in the limited region close to high angle grain boundaries in MGA, while they were extended into grain interiors in MGC. In the long-term creep conditions two types of voids were observed: fine ones with a diameter below 1 μm and coarse ones with a diameter from 2 μm up to 50 μm. Fine creep voids were found to grow with necking in MGA while they neither nucleated nor grew with necking in MGC. Coarse creep voids increased in size and in number with necking in both heats and were larger and denser in MGA than in MGC.

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.

23Cr-45Ni-7W alloy (HR6W) is a material being considered for use in the high temperature parts of A-USC boilers in Japan. In order to establish an assessment method of creep damage for welded components made using HR6W, two types of internal pressure creep tests were conducted. One is for straight tubes including the circumferential weld and the other is for welded branch connections. The test results for the circumferential welds ensured that the creep rupture location within the area of the base metal, as well as the time of rupture, can be assessed by mean diameter hoop stress. On the other hand, the creep rupture area was observed in the weld metal of the branch connections, although the creep strength of Inconel filler metal 617 was higher than that of HR6W. FE analyses were conducted using individual creep strain rates of the base metal, the heat affected zone and the weld metal to clarify this difference in the failures of these two specimens. Significant stress was only produced in the weld metal as opposed to the base metal, due to the difference in creep strain rates between the welded branch connections and creep crack were initiated in the weld metal. The differences between the two failure types were assessed using the ductility exhaustion method.

Creep brittle behaviour in tempered martensitic, creep strength enhanced ferritic (CSEF) steels is linked to the formation of micro voids. Details of the number of voids formed, and the tendency for reductions in creep strain to fracture are different for the different CSEF steels. However, it appears that the susceptibility for void nucleation is related to the presence of trace elements and hard non-metallic inclusions in the base steel. A key factor in determining whether the inclusions present will nucleate voids is the particle size. Thus, only inclusions of a sufficient size (the critical inclusion size is directly linked to the creep stress) will act directly as nucleation sites. This paper compares results from traditional uniaxial laboratory creep testing with data obtained under multiaxial conditions. The need to understand and quantify how metallurgical and structural factors interact to influence creep damage and cracking is discussed and the significant benefits available through the use of high quality steel making and fabrication procedures are highlighted. Details of component behaviour are considered as part of well-engineered, Damage Tolerant, design methods.

Inconel 718 is a nickel-based superalloy known for its excellent combination of high-temperature strength, corrosion resistance, and weldability. Additive Manufacturing (AM) has revolutionized traditional manufacturing processes by enabling the creation of complex and customized components. In this work, three prominent AM techniques: Laser-Based Powder Bed Fusion (PBF), Wire Direct Energy Deposition (DED), and Binder Jet (BJ) processes were explored. A thorough metallographic analysis and comparison of samples was conducted after short-term creep testing originating from each of the three aforementioned techniques in addition to wrought material. Detailed electron microscopy unveiled equiaxed grains in both BJ and wrought samples while PBF samples displayed elongated finer grain structures in the build direction, characteristic of PBF. The DED samples revealed a more bimodal grain distribution with a combination of smaller equiaxed grains accompanied by larger more elongated grains. When assessing the three processes, the average grain size was found to be larger in the BJ samples, while the PBF samples exhibited the most significant variation in grain and sub-grain size. Number density, size, and shape of porosity varied between all three techniques. Post-creep test observations in PBF samples revealed the occurrence of wedge cracking at the failure point, accompanied by a preference for grain boundary creep void formation while BJ samples exhibited grain boundary creep void coalescence and cracking at the failure location. In the DED samples, void formation was minimal however, it seemed to be more prevalent in areas with precipitates. In contrast, the wrought sample showed void formation at the failure site with a preference for areas with primary carbide formation. Despite BJ samples demonstrating similar or even superior rupture life compared to other AM techniques, a noteworthy reduction in rupture ductility was observed. While a coarse, uniform grain size is generally linked to enhanced creep resistance and rupture life, the combination of pre-existing voids along grain boundaries and the formation of new voids is hypothesized to accelerate rapid fracture, resulting in diminished ductility. This research shows careful consideration is needed when selecting an AM technology for high- temperature applications as creep behavior is sensitive to the large microstructural variations AM can introduce.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of these simple rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. This is the second part of a two-part paper; Part I reviewed the selected tests and discussed them from a mechanical perspective. The association of performance with specific microstructural features is briefly reviewed in this paper and the remaining gaps are highlighted for consideration among the international community.

Continuous and extensive works have been going to develop 700°C A-USC (Advanced Ultra Super Critical) power plants worldwide. Since Japanese national project launched in 2008, Ni based alloy HR6W (45Ni-24Fe-23Cr-7W-Ti, ASME Code Case 2684) was selected as one of the promising candidate materials of A-USC boiler tube and pipe for long-term creep strength evaluation and field exposure test. In the present study, to establish the creep damage and life assessment method for Ni based alloy component, long-term creep rupture properties, microstructural stability, and creep damage morphology of HR6W weldment were experimentally investigated. Creep tests of HR6W weldment were conducted at temperature range of 700 to 800°C for durations up to 70,000 hours. Failure behavior of creep void formation and creep crack growth was identified, and damage mechanism of weldment during creep were discussed and characterized. Furthermore, uniaxial interrupted creep tests were carried out, the creep damage evaluation was conducted and life assessment approach was proposed based on the metallographic quantification evaluation of creep void and microstructure evolution. It demonstrated the possibility and validity to evaluate creep damage of Ni based alloy component with creep void and microstructure parameters.

The present study presents a detailed investigation on the evolution of the microstructure during welding on virgin and long-term service exposed (creep aged 1 = 535°C; 16.1 MPa; 156 kh and creep aged 2 = 555°C; 17.0 MPa; 130 kh) 12% Cr (X20CrMoV11-1) martensitic steel. This study was carried out in order to understand the impact of welding on prior creep exposed Tempered martensite ferritic (TMF) steel and to explain the preferential failure of weldments in the fine grained heat affected zone (FGHAZ) of the creep aged material side instead of the new material side. Gleeble simulation (Tp = 980°C; heating rate = 200 °C/s; holding time = 4 seconds) of the FGHAZ was performed on the materials to create homogeneous microstructures for the investigation. Quantitative microstructural investigations were conducted on the parent plate and simulated FGHAZ materials using advanced electron microscopy to quantify: a) voids, b) dislocation density, c) sub-grains, and d) precipitates (M 23 C 6 , MX, Laves, Z-phase) in the materials. Semi-automated image analysis was performed using the image analysis software MIPARTM. The pre-existing creep voids in the creep aged parent material and the large M 23 C 6 carbides (Ø > 300 nm) in the FGHAZ after welding are proposed as the main microstructural contributions that could accelerate Type IV failure on the creep aged side of TMF steel weldments.

Creep properties and microstructural changes of 25Cr-20Ni-Nb-N steel (KA-SUS310J1TB) were investigated. Creep tests were performed under 20MPa to 380MPa at 600°C to 800°C. Time to rupture was from 53.5h to 23950h. At 650°C or higher, creep strength degraded in the long-term. Rupture elongation and reduction of area decreased with increasing time to rupture at 600°C to 800°C. The reduction of area was lower than 12% after creep rupture for more than 10000h. Creep voids and cracks were observed on grain boundaries in creep ruptured samples. The hardness of head portion of creep ruptured samples increased with increasing time to rupture at 600°C to 800°C. The hardness of gauge portion of creep ruptured samples was higher than that of as received sample. However, the hardness of gauge portion does not strongly depend on time to rupture. No precipitates were observed in as received sample. On the other hand, a large number of precipitates were confirmed after creep rupture at 600°C to 800°C. M 23 C 6 , sigma phase, eta nitride and Z phase were detected in creep ruptured samples. The precipitation was confirmed on grain boundaries after short-term creep. The precipitates were also formed inside grains after long-term creep. It was confirmed by optical microscope that the grain boundary seemed to have band-like structure after short-term creep exposure. The Cr depletion zone was detected around grain boundary after short-term creep exposure. The Cr depletion zone can be visible when Cr rich precipitates such as M 23 C 6 and sigma phase are formed on grain boundaries. However, the bandlike structure was not observed after long-term creep exposure because the Cr depletion zone became unclear after long-term creep exposure. Creep voids were formed on grain boundaries and at the interface between precipitates such as M 23 C 6 and sigma phase and matrix.

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.

The localized creep failure in the heat-affected zone (HAZ) of Grade 91 steel weldments has been identified as one of the most important factors causing significantly shortened service lifetime and structural integrity issues of welded components in advanced fossil and nuclear power plants. To conduct a reliable creep lifetime assessment, a new engineering assessment approach has been developed by incorporating the experimentally determined local properties of the heterogeneous HAZ. By creep testing a purposely simulated HAZ specimen with in situ digital image correlation (DIC) technique, the highly gradient creep properties across the HAZ of Grade 91 steel was quantitatively measured. A physical creep cavitation constitutive model was proposed to investigate the local creep deformation and damage accumulation within the heterogeneous HAZ, which takes into account the nucleation of creep cavities and their growth by both grain boundary diffusion and creep deformation. The relationship among the local material property, creep strain accumulation, and evolution characteristic of creep cavities was established. The approach was then utilized to investigate the creep response and subsequent life for an ex-service 9% Cr steel weldment by incorporating the effects of pre-existing damages which developed and accumulated during long-term services. The predicted results exhibited quantitative agreement with the DIC measurement in terms of both nominal/local creep deformation as well as the subsequent life under the test conditions at 650 and 80 MPa.

Boron and nitride additions are emerging as a promising design concept for stabilizing the microstructure of creep-resistant martensitic high-chromium steels. This approach, known as MarBN steel (martensitic steel strengthened by boron and nitrogen), combines the benefits of solid solution strengthening from boron with precipitation strengthening from nitrides. However, initial welding trials revealed challenges in achieving a uniform fine-grained region in the heat-affected zone (HAZ), which is crucial for mitigating Type IV cracking and ensuring creep strength. Despite these initial hurdles, preliminary creep test results for welded joints have been encouraging. This study presents an improved MarBN steel formulation and its investigation through uniaxial creep tests. Base material and welded joints were subjected to creep tests at 650°C for up to 25,000 hours under varying stress levels. The analysis focused not only on the creep strength of both the base material and welded joints but also on the evolution of damage. Advanced techniques like synchrotron micro-tomography and electron backscatter diffraction were employed to understand the underlying creep damage mechanisms. By combining long-term creep testing data with 3D damage investigation using synchrotron micro-tomography, this work offers a novel perspective on the fundamental failure mechanisms occurring at elevated temperatures within the HAZ of welded joints in these advanced steels.