For the safe operation of high temperature equipment, it is necessary to ensure creep rupture ductility of the components from the viewpoint of notch weakening. In this study, the effect of heat treatment conditions on creep rupture ductility was evaluated and its underlying metallurgical mechanism was investigated with using a forged Ni-based superalloy Udimet520. In order to improve the creep rupture ductility without lowering the creep rupture strength, it is important to increase both intragranular strength and intergranular strength in a balanced manner. For this purpose, it was clarified that 1) secondary γ' phase within grains should be kept fine and dense, 2) grain boundaries should be sufficiently covered by M 23 C 6 carbide by increasing its phase fraction, and 3) tertiary γ' phase within grains should be redissolved before the start of creep. To obtain such a precipitate state, it is essential to appropriately select the cooling rate after solution treatment, stabilizing treatment and aging treatment conditions.

As many nuclear power plants are in the license renewal operating period and some are entering subsequent license renewal, there is increased probability that repairs will be needed on components that have been exposed to significant neutron fluence. The neutron-driven transmutation of nickel and tramp boron in austenitic materials commonly used in reactor internals can lead to the generation of trapped helium and the associated risk of helium-induced cracking (HeIC) during weld repairs. In the weld heat affected zone, where temperatures are insufficient to allow the helium to diffuse out of the material, the helium can remain trapped. Upon cooling, the residual stresses, combined with weakened grain boundaries due to helium coalescence, can lead to cracking. The current ASME limit for helium content for Code repairs is 0.1 appm. Prior work has demonstrated a strong inverse correlation between helium content and permissible weld heat input for avoidance of HelC. The helium concentration in the material to be repaired is thus a critical input to the development of weld repair processes to be applied to these materials. The reliable measurement of helium in irradiated materials at concentrations relevant for the evaluation of HeIC risk is a specialized process. It is important to demonstrate that the capability is available and can be practically leveraged to support emergent repairs. This paper presents on the execution and results of a multi-laboratory test program aimed at demonstrating the industry capability of acquiring accurate, repeatable, and timely measurements of relatively low concentrations of helium (< ~20 appm) within austenitic materials commonly used in reactor internals. Participating laboratories were supplied with equivalent specimens extracted from boron-doped coupons that were irradiated to drive the boron-to-helium transmutation reaction. The results and lessons learned from the program are expected to support the development of industry guidance for the acquisition of similar measurements supporting nuclear component repairs.

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.

In flexible operation with increased number of startup, shutdown, and load fluctuations, thermal fatigue damage is exacerbated along with existing creep damage in power plant pipe and pressure vessels. Recently, cracks were found in the start-up vent pipe branching from the reheat steam pipe within a heat recovery steam generator(HRSG) of J-class gas turbine, occurring in the P92 base material and repair welds. This pipe has been used at the power plant for about 10 years. Microstructural analysis of the cross-section indicated that the cracks were primarily due to thermal fatigue, growing within the grains without changing direction along the grain boundaries. To identify the damage mechanism and evaluate the remaining life, temperature and strain monitoring were taken from the damaged piping during startup and normal operation.

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.

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.

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.

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.

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.

Microstructure change during creep at 650°C has been examined for a high-B 9%Cr steel by FIB-SEM serial sectioning 3D observation, Nano-SIMS, SEM, EBSD and TEM. The precipitates formed in the steel were M 23 C 6 , Laves phase, and a quite small amount of MX. For as-tempered steel, precipitation of M 23 C 6 on the prior austenite grain boundaries was clearly found, while precipitation of the Laves phase was not confirmed during tempering. The volume fraction of the Laves phase gradually increased with elapsed time, while M 23 C 6 appeared to increase once and decrease afterward, based on the comparison between the 2,754 h ruptured sample and the 15,426 h ruptured sample. Nano-SIMS measurements have revealed that B segregates on the prior austenite grain boundaries during normalizing, and it dissolves into M 23 C 6 .

9-10%Cr-3%Co martensitic steels are the prospective materials for elements of boilers, tubes and pipes for fossil power plants which are able to work at ultra-supercritical parameters of steam (T=620-650°C, P=25-30 MPa). The effect of creep on the microstructure of the 10 wt.%Cr-3Co- 3W-0.2Re martensitic steel was investigated in the condition of 650°C and an applied stress of 140 MPa, time to rupture was more than 8500 h. Previously, this steel was subjected to the normalizing at 1050°C and tempering at 770°C. This heat treatment provided the hierarchical tempered martensite lath structure with the mean size of prior austenite grains of 59 μm and with high dislocation density (2×10 14 m -2 ) within martensitic laths. Boundary M 23 C 6 and M 6 C carbides and randomly distributed within matrix Nb-rich MX carbonitrides were detected after final heat treatment. The addition of Re in the steel studied positively affected creep at 650°C/140 MPa and stabilized the tempered martensite lath structure formed during 770°C-tempering. The formation of the subgrains in the gage section was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase with fine sizes during creep. No depletion of Re and Co from the solid solution during creep was revealed whereas W content decreased from 3 to 1 wt.% for first 500 h of creep. Reasons of improved creep as well as mechanisms of grain boundary pinning by precipitates are discussed.

For last half century the development of creep strength enhanced ferritic steels has been continued and presently ASME grades 91, 92 and 122 extremely stronger than conventional low alloy steels have extensively been used worldwide in high efficient power plants. However the use of these creep strength enhanced 9-12%Cr steels is limited to around 630°C or 650°C at maximum in terms of high temperature strength and oxidation resistance. Consequently the appearance of ferritic steels standing up to higher temperature of around 700°C to substitute of high strength austenitic steels is strongly desired. Under the state, the addition of high nitrogen to ferritic steels is attracting considerable attention because of improving high temperature strength and oxidation resistance of them. This work was done to evaluate the oxidation resistance of high nitrogen steels and to investigate the effect nitrogen and microstructure on oxidation resistance using 9-15%Cr steels with about 0.3% nitrogen manufactured by means of Pressurized Electro- Slag Remelting (PESR) method in comparison with ASME grades 91 and 122. As a result, high nitrogen ferritic steels showed excellent oxidation resistance comparing with nitrogen-free steels and ASME grades 91 and 122. The oxidation resistance of 9%Cr ferritic steels depends on the nitrogen content in the each steel. That is, the weight gain decreases with an increase in nitrogen content. Moreover, the oxide scale of high nitrogen steel contained a high concentration of Cr. It is conjectured that, in high temperature oxidation, nitrogen plays a key role in promoting the formation of the oxide scale which has high concentration of Cr, inhibiting oxidation from proceeding. And also it was found that the oxidation resistance of the high nitrogen steels does not depend greatly on Cr content but on their microstructure. The oxidation resistance of high nitrogen ferritic heat-resistant steels increased as the fraction of martensite structure increased. These results indicate for high nitrogen steels Cr diffusion along grain boundaries is further promoted resulting in the formation of protective oxide scale having high Cr concentration. Furthermore as new findings it was confirmed that the Cr diffusion in substrate of steels to form Cr concentrated oxide scale on the metal surface is accelerated by nitrogen while suppressed by carbon in matrix of steel.

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.

Effects of alloying additions of Ti or Mo to a simplified chemical composition of the γ′′-Ni 3 Nb strengthened type Ni-based alloy 718 on the precipitation mode of δ-Ni 3 Nb phase were investigated to aim at designing grain boundaries using the δ phase for raising temperature capability of the γ′′ strengthened Ni-based wrought alloys. In the base alloy of Ni-22Cr-16Fe-3.5Nb, the δ phase precipitated at the grain boundaries of the matrix phase in a platelet form by continuous precipitation mode at temperatures above 1273K (1000°C) but in a lamellar morphology by discontinuous precipitation mode below that temperature. The boundary temperature where the continuous/discontinuous precipitation mode changes was raised by addition of 1 % Ti and lowered by addition of 5% Mo. The increase in the boundary temperature by Ti addition can be considered to have occurred by an increase in the solvus temperature of γ′′ phase. The decrease in the boundary temperature by Mo addition can be interpreted by the reduction of the strain energy caused by the coherent γ′′ precipitates and/or the volume change by the formation of δ phase from the γ/γ′′ phases, which may promote the continuous precipitation with respect to the discontinuous precipitation.

The behavior of strain-induced abnormal grain growth (AGG) in superalloy 718 has been investigated using compression testing and subsequent heat treatment below the d-phase solvus temperature of 980 °C. The nuclei of AGG grains were slightly newly recrystallized grains by a nucleation because small grains without dislocation was observed in the as- deformed microstructure. AGG was caused by the difference in intragranular misorientation (related to the stored strain energy in a grain) between dynamic recrystallized grains and deformed matrix. The initiation of AGG was retarded with decreasing plastic strain and produced microstructures consisted of larger grains having more complex morphology. It was observed that grain boundary migrated locally in the direction perpendicular to, or mainly in the direction parallel to the S3 {111} twin boundaries along with the formation of high-order twins. As a result of multiple twinning, AGG grains seemed to evolve with the growing directions changed.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

In this work, the effects of phosphorus addition on the creep properties and microstructural changes of wrought γ’-strengthened Ni-based superalloys (Haynes 282) were investigated, focusing on the effects of carbides precipitation. In an alloy with a phosphorus content of 8 ppm, precipitation of M 23 C 6 carbides was observed in both grain boundaries and the grain interior prior to the creep tests. Grain boundary coverage by carbide increased with phosphorus content up to approximately 30 ppm. On the other hand, the amount of M 23 C 6 in the grain interior decreased with phosphorus content. The results of the creep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbides. The microstructure of the crept specimens showed the existence of misorientation at the vicinity of grain boundaries without carbides, as demonstrated via electron backscattered diffraction (EBSD) analysis. These results suggest that the observed improvement in the time to rupture is due to a grain-boundary precipitation strengthening mechanism caused by grain boundary carbides and that phosphorus content affects the precipitation behavior of M 23 C 6 carbides in the grain interior and grain boundaries. These behaviors were different between alloys with the single addition of phosphorus and alloys with the multiple addition of phosphorus and niobium.

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.

Detailed knowledge of the creep and creep crack behavior is essential for a safe operation of thick-walled components in thermal power plants. High mechanical loads and temperatures of more than 700 °C often require the application of nickel-based alloys, e.g. alloy C-263. Unfortunately, manufacturing and non-destructive evaluation (NDE) of thick-walled components (> 50 mm) made of nickel-based alloys are quite challenging. Tolerable critical flaw sizes, experimentally validated for long service durations, play an important role in the quality assurance of such components. It is commonly accepted that manufacturing parameters, e.g. heat treatment procedures, have a significant influence on creep ductility and time-dependent crack behavior. By means of adjusting the process parameters, the ductility and the creep life of notched specimen can be significantly improved in the case of alloy C-263. Essential root cause is the decoration of grain boundaries with carbides which drastically influences creep crack initiation and growth. This results in significant differences for allowable critical flaw sizes and thus, the potential use of the candidate material. On a first generation of alloy C-263 “G1”, a dense population of carbides on the grain boundaries was found, which resulted in an inadmissible creep crack behavior. The resulting critical flaw sizes were only a few tenths of a millimeter. On a second generation “G2”, the grain boundary occupation was positively influenced, so that a satisfactory creep crack behavior could be found. The critical flaw sizes are in the order of one millimeter or more. A critical or impermissible material behavior under creep conditions can be demonstrated by testing smooth and notched round specimens. For example, the first generation “G1” notched round specimens fails earlier than the smooth round specimens, indicating notch sensitivity. On the second generation “G2”, however, a notch insensitivity was found. The critical defect sizes can be determined by a method that takes into account a simultaneous examination of the crack tip situation and the ligament situation.