Thick-walled valves, steam chests, and casings suffer service damage from thermal stresses due to the significant through-thickness temperature gradients that occur during operating transients. Fatigue is the primary damage mechanism, but recent examination of turbine casings has revealed extensive sub-surface creep cavitation. The low primary stress levels for these components are unlikely to cause creep damage, so detailed inelastic analysis was performed to understand the complex stress state that evolves in these components. This illustrates that fatigue cycles can cause elevated stresses during steady operation that cause creep damage. This paper will explore a case study for a 1CrMoV turbine casing where the stress-strain history during operating transients will be related to damage in samples from the turbine casing. This will also highlight how service affects the mechanical properties of 1CrMoV, highlighting the need for service- exposed property data to perform mechanical integrity assessments. Finally, the consequences for repair of damage will be discussed, illustrating how analysis can guide volume of material for excavation and selection of weld filler metal to maximize the life of the repair. This, in turn, will identify opportunities for future weld repair research and material property data development.

This study investigates how temperature affects the plasticity and thermal creep behavior of 347H stainless steel under uniaxial tension. The research combined experimental testing with advanced computational modeling. Two types of experiments were conducted: uniaxial tensile tests at temperatures from 100°C to 750°C using strain rates of ~10⁻⁴ s⁻¹, and creep tests at temperatures between 600°C and 750°C under various stress levels. These experimental results were used to develop and validate a new integrated mechanistic model that can predict material behavior under any loading condition while accounting for both stress and temperature effects. The model was implemented using a polycrystalline microstructure simulation framework based on elasto-viscoplastic Fast Fourier Transform (EVPFFT). It incorporates three key deformation mechanisms: thermally activated dislocation glide, dislocation climb, and vacancy diffusional creep. The model accounts for internal stress distribution within single crystals and considers how precipitates and solute atoms (both interstitial and substitutional) affect dislocation movement. After validation against experimental data, the model was used to generate Ashby-Weertman deformation mechanism maps for 347H steel, providing new insights into how microstructure influences the activation of different creep mechanisms.

The current research adopts a novel approach by integrating correlative microscopy and machine learning in order to study creep cavitation in an ex-service 9%Cr 1%Mo Grade 91 ferritic steel. This method allows for a detailed investigation of the early stages of the creep life, enabling identification of features most prone to damage such as precipitates and the ferritic crystal structure. The microscopy techniques encompass Scanning Electron Microscopy (SEM) imaging and Electron Back-scattered Diffraction (EBSD) imaging, providing insights into the two-dimensional distribution of cavitation. A methodology for acquiring and analysing serial sectioning data employing a Plasma Focused Ion Beam (PFIB) microscope is outlined, complemented by 3D reconstruction of backscattered electron (BSE) images. Subsequently, cavity and precipitate segmentation was performed with the use of the image recognition software, DragonFly and the results were combined with the 3D reconstruction of the material microstructure, elucidating the decoration of grain boundaries with precipitation, as well as the high correlation of precipitates and grain boundaries with the initiation of creep cavitation. Comparison between the 2D and 3D results is discussed.

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.

Alloy 718 is an important class of Nb-bearing Ni-based superalloys for high-temperature applications, such as compressor disks/blades and turbine disks in gas turbine systems. The service temperature of this alloy is, however, limited below 650 °C probably due to the degradation of its strengthening phase γ"-Ni3Nb. Aiming at understanding and improving creep properties of 718-type alloys, we investigated creep behaviors of alloy 718 and alloy Ta-718 where different types of γ" phases, Ni3Nb and Ni3Ta, were precipitated, respectively. Creep tests were conducted at 700 °C under stress conditions of 400 and 500 MPa for the two alloys in aged conditions. It was found that while the minimum creep rates were comparable in the two alloys, the creep rate acceleration was lower in alloy Ta-718 than in alloy 718 under the creep conditions studied. Microstructural observations on the specimens before and after the creep tests suggested that the γ" precipitates were distinguishably finer in alloy Ta-718 than in alloy 718 throughout the creep tests. The formation of planar defects and shearing of γ" precipitates occurred frequently in the alloy 718 specimen. The observed creep deformations were discussed in terms of the critical resolved shear stress due to shearing of γ" particles by strongly paired dislocations.

This study investigates the influence of build orientation on the high-temperature mechanical properties of IN738LC manufactured via metal laser powder bed fusion (PBF-LB/M). Since the PBF-LB/M layer-wise manufacturing process significantly affects grain morphology and orientation—ranging from equiaxed to textured grains—mechanical properties typically exhibit anisotropic behavior. Samples were manufactured in three build orientations (0°, 45°, and 90°) and subjected to hot tensile and creep testing at 850°C following DIN EN ISO 6892-2 and DIN EN ISO 204 standards. While tensile properties of the 45° orientation predictably fell between those of 0° and 90° orientations, creep behavior over 100-10,000 hours revealed unexpected results: the 45° orientation demonstrated significantly shorter rupture times and faster creep rates compared to other orientations. Microstructural analysis revealed distinct creep deformation mechanisms active within different build orientations, with the accelerated creep rate in 45° specimens attributed to multiple phenomena, particularly η-phase formation and twinning. These findings provide crucial insights into the orientation-dependent creep behavior of PBF-LB/M-manufactured IN738LC components.

Laser additive manufacturing (AM) is being considered by the nuclear industry to manufacture net- shape components for advanced reactors and micro reactors. Part-to-part and vendor-to-vendor variations in part quality, microstructure, and mechanical properties are common for additively manufactured components, attributing to the different processing conditions. This work demonstrates the use of microstructurally graded specimen as a high throughput means to establish the relationship between process-microstructure-creep properties. Through graded specimen manufacturing, multiple microstructures, correlated to the processing conditions, can be produced in a single specimen. The effects of a solution annealing heat treatment on the microstructure and creep properties of AM 316H are investigated in this work. Using digital image correlation (DIC), the creep strain can be calculated in these graded regions, allowing for multiple microstructures to be probed in a single creep test. The solution annealing heat treatment was not sufficient in recrystallization of the large, elongated grains in the AM material; however, it was sufficient in removing the cellular structure commonly found in AM processed alloys creating a network of subgrains in their place. The resulting changes in microstructure and mechanical properties are presented. The heat treatment was found to generally increase the minimum creep rate, reduce the minimum creep rate, and reduce the ductility. Significant amounts of grain boundary carbides and cavitation were observed.

This paper presents three recent example cases of cracking in Grade 91 steel welds in longer-term service in high temperature steam piping systems: two girth butt welds and one trunnion attachment weld. All the cases were in larger diameter hot reheat piping, with the service exposure of the welds ranging from approximately 85,000 to 150,000 hours. Cracking in all cases occurred by creep damage (cavitation and microcracking) in the partially transformed heat-affected zone (PTZ, aka Type IV zone) in the base metal adjacent to the welds. The location and morphology of the cracking are presented for each case along with operating conditions and potential contributors to the cracking, such as system loading, base metal chemical composition, and base metal microstructure.

In this work, two unique heats of 9Cr creep strength enhanced ferritic (CSEF) steels extracted from a retired superheat outlet header after 141,000 hours of service were evaluated. These two CSEF steels were a forging manufactured to SA-182 F91 (F91) reducer and a seamless pipe produced to SA-335 P91 (P91) pipe. Their creep deformation and fracture behavior were assessed using a lever arm creep frame integrated with in-situ high-temperature digital image correlation (DIC) system. Critical metallurgical and microstructure factors, including composition, service damage, grain matrix degradation, precipitates, and inclusions were quantitatively characterized to link the performance of the two service aged F91 and P91 CSEF steels. The creep test results show the F91 and P91 steels exhibit a large variation in creep strength and creep ductility. The F91 steel fractured at 572 hours while P91 steel fractured at 1,901 hours when subjected to a test condition of 650 °C and 100 MPa. The nominal creep strains at fracture were 12.5% (F91) and 14.5% (P91), respectively. The high-resolution DIC strain measurements reveal the local creep strain in F91 was about 50% while the local creep strain in P91 was >80%. The characterization results show that the F91 steel possessed pre-existing creep damage from its time in service, a higher fraction of inclusions, and a faster matrix grain coarsening rate. These features contribute to the observed reduction in performance for the F91 steel. The context for these findings, and the importance of metallurgical risk in an integrated life management approach will be emphasized.

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 effects of chemical composition and heat treatment on the creep properties of ASME Grade 91 type steel were experimentally investigated using materials whose chemical compositions and heat treatment conditions in the steel making process were completely controlled. Regarding chemical composition, only the Al, Cr, and Ni contents were systematically varied while keeping the contents of the other elements and heat treatment conditions constant. Regarding heat treatment, the normalizing and tempering temperatures were varied while keeping the contents of chemical components constant. The creep tests of materials were performed for approximately up to 50,000 h at 650°C. The creep strength of Grade 91 type steel decreased with increasing Al content under the test conditions of short-term to long-term range. On the other hand, the effect of Cr content on the creep life of Grade 91 type steel depended on the stress or time range, and the creep strength of the steel decreased at high Cr contents under test conditions of only the longterm range. No effect of Ni content on the creep life of the materials was observed in the test data obtained in this study. As creep tests are currently being conducted at 625°C and 60 MPa, which are conditions closer to the actual service conditions of main steam piping at ultra-super critical power plants, the creep deformation data at present indicate that the above trends hold true in the long-term range. Regarding the effect of heat treatment, the creep life of the materials tended to increase with increasing normalizing temperature or decreasing tempering temperature. The results obtained in this work indicate that within the scope of the material standards for Grade 91 type steel, the effect of chemical composition on creep life is greater than that of heat treatment.

This study investigates the role of grain-boundary precipitates in enhancing creep rupture strength of Ni-based alloys through analysis of Ni-15Cr-15Mo and Ni-15Cr-17Mo (at.%) model alloys. The investigation focused on the “Grain-boundary Precipitation Strengthening (GBPS)” effect from the thermally stable TCP phase, a phenomenon previously observed in Fe-Cr-Ni-Nb austenitic heat-resistant steels. Through multi-step heat treatments, specimens were prepared with varying grain boundary coverage ratios (ρ) of TCP P phase (oP56) and consistent grain-interior hardness from GCP Ni2(Cr, Mo) phase (oP6). In the 15 at.% Mo alloy, specimens with a higher coverage ratio (~80%) demonstrated significantly improved creep performance, achieving nearly four times longer rupture time (3793 h vs. 1090 h) at 300 MPa and 973 K compared to specimens with lower coverage (~35%). However, the 17 at.% Mo alloy showed unexpectedly lower performance despite high coverage ratios, attributed to preferential cavity formation at bare grain boundaries. These findings confirm that GBPS via thermally stable TCP phase effectively enhances creep properties in Ni-based alloys, with grain boundary coverage ratio being more crucial than intragranular precipitation density.

Creep deformation and rupture properties of several long-term used Super 304H steel boiler tubes were presented in this paper. The aged superheater tubes that have been in service for about 140,000 hours at the approximate metal temperature ranged from 550°C to 640°C, were investigated. Creep tests were conducted at 650°C and 700°C using standard and miniature specimens taken from the axial and circumferential directions of tubes, and effects of specimen size, sampling direction and position on creep properties were discussed. Creep deformation of long-term used materials with significant microstructural evolution accelerated earlier than that of virgin material, and the time to creep rupture and the fracture ductility were also smaller. The degradation of rupture properties of the long-term used material was discussed in relation with microstructural evolution. In addition, there was little effects of specimen size and sampling direction on creep deformation and rupture time, whereas the time to creep rupture changed significantly due to the sampling position.

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.

A newly developed ferritic heat-resistant steel; 9Cr-3W-3Co-Nd-B steel has higher creep rupture strength both in the base metal and welded joints than the conventional high-Cr ferritic heat-resistant steels. The creep rupture strengths of 9Cr-3W-3Co-Nd-B steel welded joints were below the lower limit of the base metal in long-term creep stage more than 20,000 hours. The creep rupture position was heat-affected zone (HAZ) from 1.0 to 1.5 mm apart from the fusion line on the welded joint specimen ruptured at 34,966 hours. The equiaxed subgrains and coarsened precipitates were observed in HAZ of the ruptured specimen. In order to clarify the creep fracture mechanism of the welded joints, the microstructures of HAZ were simulated by heat cycle of weld, then observed by EBSD analysis. Fine austenite grains formed along the prior austenite grain boundaries in the material heated just above A C3 transformation temperature, however there were no fine grains such as conventional steel welded joints. The prior austenite grain boundaries were unclear in the material heated at 1050 °C. The creep rupture life of the material heated at just above A C3 transformation temperature exceeded the lower limit of base metal and there was no remarkable degradation, although it was shorter than the other simulated materials. It is, therefore, concluded that the creep fracture of 9Cr-3W-3Co-Nd-B steel welded joint in long-term stage occurred at HAZ heated at from just above A C3 transformation temperature to 1050 °C. It is speculated that the fine austenite grains formed along the prior austenite grain boundaries and inhomogeneous microstructures cause the coarsening precipitates and recovery of lath structure during long-term creep deformation.

This study presents a characterization of the microstructural evolutions taking place in a 9%Cr martensitic cast steel subjected to fatigue and creep-fatigue loading. Basis for this study of investigation is an extensive testing program performed on a sample heat of this type of steel by conducting a series of service-like high temperature creep-fatigue tests. The major goal here was to systematically vary specific effects in order to isolate and describe relevant damage contributing mechanisms. Furthermore, some of the tests have been interrupted at several percentages of damage to investigate not only the final microstructure but also their evolution. After performing those tests, the samples were examined using transmission electron microscopy (TEM) to characterize and quantify the microstructural evolutions. The size distribution of subgrains and the dislocation density were determined by using thin metal foils in TEM. A recovery process consisting of the coarsening of the subgrains and a decrease of the dislocation density was observed in different form. This coarsening is heterogeneous and depends on the applied temperature, strain amplitude and hold time. These microstructural observations are consistent with the very fast deterioration of creep properties due to cyclic loading.

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 effect of boron nitrides (BN) and aluminum nitrides (AlN) on long-term creep life and rupture ductility has been investigated for martensitic 9 to 12Cr steels at 550 to 700 °C. The BN particles form in 9 to 12Cr steels during normalizing heat treatment at 1050 to 1150 °C, suggesting no change in the amount of BN particles during creep. On the other hand, the AlN particles gradually form during creep at 550 to 700 °C, decreasing the concentration of nitrogen free from the AlN particles. The degradation in creep life takes place more significantly with test duration in the steels containing high Al but not in those containing the BN particles. The rupture ductility is evaluated by using a semi-logarithmic diagram of the RA and total elongation, showing the necking dominant and void swelling dominant regions. The BN and AlN particles are responsible for the degradation in RA at low stresses and long times by accelerating the formation of creep voids at interfaces between the BN and AlN particles and alloy matrix.

This work deals with the potential of microstructurally based modeling of the creep deformation of martensitic steels. The motivation for the work stems from the ever increasing demand for higher efficiency and better reliability of modern thermal power plants. Service temperatures of 600°C and stress levels up to 100 MPa are currently the typical requirements on critical components. High creep and oxidation resistance are the main challenges for a lifetime 10+ years in steam atmosphere. New materials may fulfill these requirements; however, the save prediction of the creep resistance is a difficult challenge. The model presented in this work takes into consideration the initial microstructure of the material, its evolution during thermal and mechanical exposure and the link between microstructural evolution and creep deformation rate. The model includes the interaction between the relevant microstructural constituents such as precipitates, grain- lath- and subgrain boundaries and dislocations. In addition, the material damage is included into the model. The applicability of the model is then demonstrated on standard creep resistant alloys. Contrary to phenomenological models, this approach can be tested against microstructural data of creep loaded samples and thus provides higher reliability. Nevertheless, potential improvements are discussed and future developments are outlined.

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