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The Creep Strength Enhanced Ferritic steel grade 91 is widely used for both retrofit applications and primary construction on high temperature power plant. Although to date most structural integrity issues with this material have been associated with welds, as the operating hours of these plants accumulate, there will be a growing need for remanent creep life assessment of the base material. Arguably this is already the case for aberrant grade 91 material entering service in an incorrectly heat treated condition. In these circumstances the strength may fall below the normally accepted lower bound of the creep strength range and some indication of actual strength may be required. One strategy to address potential base material failure is to use small scale sampling of individual components, followed by small scale creep testing, to investigate the current creep strength present. The data can be compared with the equivalent data produced for well characterised material known to be at the lower bound of the creep strength range. This paper describes a methodology for using the impression creep data obtained to provide both creep strength ranking and an estimate of absolute creep strength for individual grade 91 components. This will enable appropriate judgements to be made by plant operators on repair/run decisions. For those components remaining in service, it allows for the weakest items to be given priority for early re-inspection at future outages. The ultimate goal is to identify base material creep damage development at as early a stage as possible and well in advance of failure in service.

Due to their excellent high temperature oxidation resistance, utilities worldwide are adopting advanced austenitic stainless steels (A-ASS) for critical plant components, such as heat exchangers, as they aim to achieve higher operating conditions. However, challenges may be encountered in developing life assessment and life management strategies for such components. This is because conventional methods used for life assessment, such as measuring steam side oxide scale thickness in ferritic and conventional austenitic material to predict tube metal temperature, may not be successfully applied to A-ASS. In such instances, tracking the formation and evolution of microstructural features during service, may offer a possible method to predict the temperature of these steels. For such metallurgy based lifing strategy to be successful, it is essential to develop a good understanding of microstructure evolution in these steels. In this work one heat of Super 304H, that has been creep tested at 600°C, 650°C and 700°C, with applied stress ranging from 110 to 340 MPa, is characterized using a combination of advanced characterization tools and image analysis methods. The amount of sigma phase formed at the gauge and grip sections of the samples is quantified and the methodology used to quantify this phase is presented. From the results, a time-temperature-transformation diagram for sigma formation is developed.

A new method of creep life assessment was developed to consider heat-to-heat variations of welded joints of materials used in power plants. This paper explains a scheme of the assessment method and also describes an actual implementation of the method for Grade 91 steel. In the method, creep properties of the welded joints are related to those of each base metal because the heat-to-heat variations of welded joints strongly depend on the creep properties of the corresponding base metals. To estimate the creep properties of each base metal of the target pipe, microstructure analyses and small punch creep tests were conducted using small samples cut from the base metals in service, and evaluations were done on the basis of material data base obtained using standard test samples of long-term service exposed pipes. It is expected that the precision of the life assessment of pipes will be significantly improved using the developed method because it can consider the heat-to-heat variations of their materials, which are not considered in existing life assessment methods.

The effect of taking miniature sample scoops on the creep life of ASME Grade 91 steel pipes was experimentally and analytically assessed in this work. Internal pressure tests were conducted on tubular specimens having defects on their outer surface, which simulate sampling scoops. The creep life did not decrease until the depth ratio of the defect to the wall thickness of the specimens was about 5%, and the creep life decreased with increasing defect depth when the depth ratio exceeded about 5%. When the depth ratio was about 11%, the creep life decreased to four-fifths of that of a specimen with no defects. In addition, as a result of investigating the stress concentration around a defect with a depth ratio of about 5% by the finite element method, stress concentration was clearly observed around the defect. These results suggest that taking a miniature sample up to a depth of 5% of the thickness of a Grade 91 steel pipe in service has a negligible effect on the creep life of the pipe.

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.

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 aim of this work was to reveal the effects of trace elements on the creep properties of nickel-iron base superalloys, which are the candidate material for the large components of the advanced-ultrasupercritical (A-USC) power generation plants. High temperature tensile and creep properties of forged samples with seven different compositions were examined. No significant differences were observed in the creep rate versus time curves of the samples, of which contents of magnesium, zirconium, manganese and sulfur were varied. In contrast, the curves of phosphorus-added samples showed very small minimum creep rates compared to the other samples. The creep rupture lives of phosphorus-added samples were obviously longer than those of the other samples. Microstructure observation in the vicinity of grain boundaries of phosphorus-added samples after aging heat treatment revealed that there were fine precipitates consisting of phosphorus and niobium at the grain boundaries. The significant suppression of the creep deformation of phosphorus-added sample may be attributed to the grain boundary strengthening caused by the fine grain boundary precipitates.

Nuclear reactor inspections occasionally identify degraded materials in irradiated reactor components. Although mechanical repair options are possible, these repair solutions may be cost prohibitive or impractical to implement due to access restraints and/or the severity of the degradation. Welding repair of reactor components may input excessive heat into these irradiated materials resulting in diffusion of trace amounts of helium within the grain boundaries of the weld heat-affected zone (HAZ). Intergranular HAZ cracking can then result from the combination of this helium diffusion and high localized tensile stresses generated during weld cooling. It is therefore critical to characterize these zones and understand limitations for welding highly irradiated components to prevent helium-induced cracking. To accomplish this, typical reactor structural materials including Types 304L and 316L stainless steels and nickel-based Alloy 600/182 materials irradiated within the High Flux Isotope Reactor facility at Oak Ridge National Laboratory were used in this study for welding and evaluation. A phased array ultrasonic inspection system has been developed to characterize cracking in the weld samples. It provides remote controlled scanning and minimizes handling the samples, minimizing operator dose. The samples are inspected from the side opposite of the welds. The material and weld grain noise were evaluated at 10 MHz and found to be conducive to detecting cracking in the material and welds. Inspection of the samples comprises a 10 MHz phased array probe sweeping a focused longitudinal wave from -60° to 60° while the probe is raster scanned over the sample in small increments. The collected data is analyzed using UltraVision 3. Several of the irradiated samples were inspected prior to welding. Some of the samples had what appear to be small lamination defects in them. One irradiated welded sample has been tested to date with no cracking detected, which has been confirmed by destructive examination.

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.

Creep strength enhanced ferritic (CSEF) steels have shown the potential for creep failure in the weld metal, heat affected zone (HAZ) or fusion line. Details for this behavior have been frequently linked to metallurgical risk factors present in each of these locations which may drive the evolution of damage and subsequent failure. This work is focused on three weld samples fabricated from a commercially sourced Grade 92 steel pipe section. These weld samples were extracted from the same welded section but were reported to exhibit failure in different time frames and failure locations (i.e., HAZ of parent, fusion-line, and weld metal). The only variables that contribute to this observed behavior are the post weld heat treatment (PWHT) cycle and the applied stress (all tests performed at 650 °C). In this work detailed microstructural analysis was undertaken to precisely define the locations of creep damage accumulation and relate them to microstructural features. As part of this an automated inclusion mapping process was developed to quantify the characteristics of the BN particles and other inclusions in the parent material of the samples. It was found that BN particles were only found in the sample that had been subjected to the subcritical PWHT, not those that had received a re-normalizing heat treatment. Such micron sized inclusions are a known potential nucleation site for creep cavities, and this is consistent with the observed failure location in the HAZ of the parent in the sample where these were present. In the absence of BN inclusions, the next most susceptible region to creep cavitation is the weld metal. This has an intrinsically high density of sub-micron sized spherical weld inclusions and this is where most of the creep damage was located, in all the renormalized samples.

The global electric power production is largely dependent on the operation of fossil-fired generation units. Many coal-fired units are exceeding 300,000 hours, which is beyond the expected design life. This has caused a continuous need to inspect steam touched components operating at high temperature and pressure. State-of-the-art coal and combined cycle gas units are specifying ever-greater amounts of the Creep Strength Enhanced Ferritic (CSEF) steels such as Grade 91 or Grade 92. The martensitic 9%Cr CSEF steels were developed to provide greater strength than traditional low alloy power plant steels, such as Grades 11, 12 and 22. The enhanced strength allows for a reduction in overall wall thickness in new or replacement components. Extensive research in both service failures and laboratory testing has shown that time-dependent creep damage can develop differently in Grade 91 steel when compared to low alloy steels. Furthermore, the creep strength in Grade 91 can vary by more than a factor of 10 between different heats. This wide variation of creep strength has led to extensive research in understanding the damage mechanisms and progression of damage in this steel. In this study, large cross weld samples were fabricated from thick wall piping in Grade 91 steel using two different heats of material. One weld was fabricated in a ‘damage tolerant’ heat and another weld was fabricated in a ‘damage intolerant’ heat of material. The samples were subjected to a post-weld heat treatment (PWHT) at a temperature of 745°C (1375°F) for 1.50 hours. Hardness maps were collected on the cross-welds in the as-welded and PWHT condition for both weldments. Cross-weld creep test conditions were selected to develop accelerated damage representative of in-service behavior. The test samples were interrupted at multiple stages and nondestructively evaluated (NDE) with advanced phased-array ultrasonic techniques. Samples were developed to variable levels of damage (50% to 100% life fraction) in both weldments. Metallographic sections were extracted at specific locations to validate the NDE findings using light emitting diode, laser and scanning electron microscopy. This research is being used to help validate the level of damage that can be reliably detected using conventional and advanced NDE techniques.

The relationship between the hot workability and the precipitation morphology of γ′ phase in the Alloy U520 was examined with a focus on the presence of γ′-nodule. To change the morphology of γ’ phase, forged bars of the Alloy U520 were solution treated followed by cooling process with the cooling rates of 5~100 K/h. After the heat treatment, both γ’ phases of intragranular particle and nodule along grain boundaries were observed, and the both sizes increased by slowing down the cooling rate. That is, the area fraction of γ’-nodule increased from about 0.1 % in the sample cooled at 100 K/h to about 70 % at 5 K/h. In Gleeble tension test, the slow-cooled samples basically exhibited higher ductility than water-quenched samples below the γ′-solvus temperature. However, the ductility was maximized in the sample cooled at 20 K/h, and excessive decrease of cooling rate resulted in a drop in ductility. EBSD analysis revealed that dynamic recrystallization (DRX) was often occurred in grain interior but suppressed at γ′-nodule area, indicating that presence of γ′-nodule had a negative influence on hot workability at subsolvus temperature.

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime size and distribution and the degree of rafting has been examined in detail using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM) after high temperature degradation and rejuvenation heat treatments. The relationship between the microstructure, mechanical properties and the applied heat treatment procedures has been investigated. It is shown that there are significant differences in the rafting behaviour, the size of the ‘channels’ between the gamma prime particles, the degree of rafting and the size of the tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical segregation investigations were carried out to establish the cause of reduced mechanical properties of the rejuvenated sample after high temperature degradation compared to an as-received sample after the same degradation procedure. The results indicate that although the microstructure of as-received and rejuvenated samples were similar, the chemical segregation was more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements during rejuvenation was not completely eliminated. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications.

Grade 92 steel has been widely applied in the power generation industry for use as steam pipes, headers, tubes, etc. owing to a good combination of creep and corrosion resistance. For the welding of thick section pipes, a multi-pass submerged arc welding process is typically used to achieve sufficient toughness in the weld. To relieve the internal stress in the welds and to stabilise their microstructures, a post weld heat treatment (PWHT) is commonly applied. The heat treatment conditions used for the PWHT have a significant effect on both the resulting microstructure and the creep behaviour of the welds. In this study, interrupted creep tests were carried out on two identical Grade 92 welds that had been given PWHTs at two different temperatures: 732°C and 760°C. It was found that the weld with the lower PWHT temperature had a significantly reduced stain rate during the creep test. In addition, microstructural examination of the welds revealed that the primary location of creep damage was in the heat affected zone in the sample with the lower PWHT temperature, whereas it was in the weld metal in the sample with the higher PWHT temperature. To understand the effect of the different PWHT temperatures on the microstructure, initially the microstructures in the head portions of the two creep test bars were compared. This comparison was performed quantitatively using a range of electron/ion microscopy based techniques. It was apparent that in the sample subjected to the higher PWHT temperature, larger Laves phase particles occurred and increased matrix recovery was observed compared with the sample subjected to the lower PWHT temperature.

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M 23 C 6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23 C 6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M 23 C 6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M 23 C 6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

A FeCrMnNi concentrated solid-solution alloy was irradiated with a 2 MeV proton beam up to 1 dpa and 6 dpa at temperatures of 400 °C and 600 °C. The microstructural changes induced by irradiation were characterized using Transmission Electron Microscopy (TEM). In samples irradiated at 400 °C, Frank loops were the predominant form of lattice damage at 1 dpa, whereas small defect clusters were more prevalent at 6 dpa. For the sample irradiated to 1 dpa at 600 °C, both Frank loops and small defect clusters were present in similar density. Nanoindentation was employed to assess the changes in mechanical properties (hardness) post-irradiation, revealing significant hardening in all irradiated samples. The results indicated that the hardening effect began to saturate at 1 dpa or earlier. Additionally, nanoindentation creep tests with a 1200-second dwell period produced stress exponents comparable to those obtained from conventional creep testing. The findings suggest a shift in the deformation mechanism from dislocation glide to dislocation climb in the sample irradiated to 6 dpa at 400 °C.

Modified 9Cr-1Mo steel (ASTM Gr.91) is widely used in components of fossil fueled power plants around the world today. This grade of steel has however been shown to exhibit significant variations in creep life and creep ductility, which has led to premature in-service failures. The aim of this work is to define potential metallurgical risk factors that lead to this variation in performance. To achieve this, a set of creep test samples that represent a wide range in this variation of creep behavior in this steel grade have been studied in detail. As a first stage in this characterization the macro-scale chemical homogeneity of the materials were mapped using micro-XRF. Understanding the segregation behavior also allows quantification of microstructural parameters in both segregated and non-segregated areas enabling the variations to be determined. For example this showed a significant increase in the number per unit area of Laves phase particles in high compared with low Mo content areas. To study the effect of MX particles on segregation a methodology combining SEM and TEM was employed. This involved chemically mapping the larger V containing particles using EDS in the SEM in segregated and unsegregated areas and then comparing the results to site-specific TEM analysis. This analysis showed that although the average size of the V containing samples is in the expected 0-50 nm size range, these particles in some samples had a wide size distribution range, which significantly overlaps with the M 23 C 6 size distribution range. This together with the segregation characteristics has important implications for determining meaningful quantitative microstructural data from these microstructurally complex materials.

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

Small punch creep testing (SPCT) is a small-scale, accelerated creep test that allows for the determination of creep data using a limited amount of material. The question, however, remains how the data generated by this technique correlate to more established techniques such as uniaxial testing and ultimately to predictions regarding the remaining service life of a plant component. This empirical study investigated the microstructure-to-property relationship of welded 9-12%Cr steels as measured using SPCT. Virgin P91 (X10CrMoVNb9-1) steel was joined to service exposed X20 (X20CrMoV12-1) steel using two different filler materials (X20 and P91) via fusion welding. Site-specific samples were extracted from the parent plates, heat affected zones and weld metals using electro-discharge machining. Small punch creep testing were performed using a 276 N load at a temperature of 625°C. The untested sample microstructures were quantitatively characterized using a range of electron microscopy techniques to determine the precipitate (M 23 C 6 , MX) spacing, subgrain sizes and dislocation densities for each region of the weldments. Multiple linear regression analysis found that the subgrain size (λsg) played the largest contribution to the SPCT rupture life. The heat affected zones had the lowest SPCT rupture times (49-68 hours), which corresponded to the largest subgrain sizes (1.1-1.3 μm). The P91 parent plate material had the longest SPCT rupture time (349 hours), which corresponded to the lowest subgrain size (0.8 μm). The P91 weld metal sample showed lower initial deflection rates during the SPC testing, however the presence of non-metallic SiO 2 inclusions in this zone contributed to accelerated brittle failure.

A trial weld joint of COST F and COST FB2 steels was produced using the GTAW HOT-WIRE method in conditions used in industry for production of welding steam turbine rotors. Conventional long-term creep tests (CCT) to the rupture of this weldment and the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa (the longest time to rupture was above 52,000 hours). Creep rupture strength was evaluated using Larson-Miller parameter model. Assessment of microstructure was correlated with the creep strength. Precipitation of Laves phase and structure recovery during creep exposures were the main reasons for the failure which occurred in the heat affected zone of steel COST F. The recently developed simulative accelerated creep testing (ACT) on thermal-mechanical simulator allows the microstructural transformation of creep-resisting materials in a relatively short time to a state resembling that of multiyear application under creep conditions. ACT of samples machined from various positions in the weldment was performed at 600 °C under 100 MPa. Changes in the hardness and the microstructures of the samples, which underwent both types of creep tests, were compared. Small sample creep test (SPCT), another alternative method how to obtain information about the creep properties of materials when only a limited amount of test material is at disposal, were performed. It was shown that the same stress-temperature dependence and relationships are valid in the SPCT as in the CCT. Using a simple load-based conversion factor between the SPCT test and the CCT test with the same time to rupture, the results of both test types can be unified.