This study investigates the influences of product chemistry and grain size on the high-temperature creep properties of 316 stainless steels by analyzing an extensive range of historical and modern literature data. The investigated 316 stainless steel creep property dataset, including more than 160 heats and 2,400 creep testing data, covers a wide spectrum of elemental compositions and product forms. To perform a prudent analysis of the creep property dataset, a statistical overview was first implemented to understand the data distribution relevant to data sources, chemistries, product forms, testing temperatures, and grain sizes. The creep data of 550°C, 600°C, 650°C, 700°C, and 750°C with ±10°C were grouped together, and the analytical study was performed on each sub dataset to investigate the temperature-specific creep performance. The creep strength was evaluated using the average stress ratio (ASR) between the experimental and predicted creep data of tested 316SS heats. The influence of composition and grain size on the creep strength ratio were evaluated using linear correlation analysis. Effects of specified and non-specified elements including C, N, and B were specifically investigated to understand their impacts on the creep strength with regards to the variation of creep temperature. In addition to the literature data, the most recent EPRI creep data of three commercial heats were used to validate the correlations from the historical creep property dataset.

The creep strength of the base metals and welded joints of ASME Grade 91 type steel under actual service conditions was investigated using long-term used materials in this study. Creep tests were conducted on the materials used for hot reheat or main steam piping at power plants. The remaining creep life of each material under actual service conditions was evaluated using the Larson-Miller parameter for the test result. Then, the creep life of each material under the service condition was estimated as a summation of the service time at the plants and the remaining creep life. The estimation results were useful for examining the validity of the life evaluation formula in the long-term region because it is extremely difficult to obtain creep rupture data under such conditions owing to the long test duration. The estimated creep lives were compared with creep life evaluation curves, which were regulated for Grade 91 type steel in Japan. Regarding the base metals, the estimation results suggest that Grade 91 pipe-type steel tends to exhibit a shorter life than the 99% confidence lower limit of the evaluation curve of the material. This finding indicated that the life evaluation formula for the Grade 91 type steel base metals should be reviewed. On the other hand, the estimation results suggest that the welded joints of Grade 91 type steel generally exhibit a longer life than the 99% confidence lower limit of the evaluation curve of the material, indicating that there is no need to review the life evaluation formula for the Grade 91 type steel welded joints.

Ductility dip cracking (DDC) is known to occur in highly restrained welds and structural overlays made using high chromium (Cr) nickel (Ni) based filler metals in the nuclear power generation industry, resulting in costly repairs and reworks. Previous work explored the role of mechanical energy imposed by the thermo-mechanical cycle of multipass welding on DDC formation in a highly restrained Alloy 52 filler metal weld. It was hypothesized that imposed mechanical energy (IME) in the recrystallization temperature range would induce dynamic recrystallization (DRX), which is known to mitigate DDC formation. It was not shown however that IME in the recrystallization temperature range (IMERT) induced DRX. The objective of the work is to discern if a relationship between IMERT and DRX exists and quantify the amount of DRX observed in a filler metal 52 (FM-52) groove weld. DRX was analyzed and quantified using electron beam scattered diffraction (EBSD) generated inverse poll figures (IPF), grain surface area and grain aspect ratio distribution, grain orientation spread (GOS), kernel average misorientation (KAM), and grain boundary (GB) length density. From the analysis, GOS was determined to be an unsuitable criterion for quantifying DRX in multipass Ni-Cr fusion welds. Based on the observed criteria, higher IMERT regions correlate to smaller grain surface area, larger grain boundary density, and higher grain aspect ratio, which are all symptoms of DRX. High IMERT has a strong correlation with the symptoms DRX, but due to the lack of observable DRX, creating a threshold for DRX grain size, grain aspect ratio, and GB density is not possible. Future work will aim to optimize characterization criteria based on a Ni-Cr weld with large presence of DRX.

A research program has been initiated to develop the first predictive methodology for corrosion fatigue life in steam turbine blades, addressing a critical gap in current understanding despite extensive research into corrosion pitting and fatigue failure. The study focuses initially on dual-certified 403/410 12% Cr stainless steel, utilizing a newly developed test facility capable of conducting high-cycle fatigue tests in simulated steam environments at 90°C with controlled corrosive conditions. This testing platform enables the investigation of various steady and cyclic stress conditions, establishing a foundation for future testing of other blade steels and the development of comprehensive blade life estimation techniques.

Titanium alloys are expected to be used as heat-resisting structural materials in the airplane and automotive industries. In this study, the creep properties of near-α Ti alloys consisting of a lamellar microstructure were studied. Ti–8.5wt%Al–8.0wt%Zr–2wt%Mo–1wt%Nb–0.15wt%Si alloy (alloy code, TKT34) and an alloy with 0.1 wt% of added boron (alloy code, TKT35) were used in this study. An ingot was hot forged at a temperature of 1,403 K and hot rolled (caliberrolling) at a temperature of 1,273 K to a reduction rate of approximately 90%. It then underwent solution treatment in a β single-phase region followed by air cooling. Finally, it was subjected to aging treatment for 28.3 ks at a temperature of 863 K and then air-cooled. Two solution treatment conditions were applied: a time of 1.8 ks at a temperature of 1,323 K (high temperature/short time (HS)) and a time of 3.6 ks at a temperature of 1,223 K (low temperature/long time (LL)). The average grain size of the prior β grains showed a tendency of the solution treatment temperature being low and the boron-added alloys tending to be small. The length and thickness of the lamellar of these alloys shortened or thinned owing to the addition of boron and at a low solution treatment temperature. The creep tests were carried out at an applied stress of 137 MPa and a temperature of 923 K in air. The creep rupture life of these alloys was excellent, in order of TKT35 (LL) < TKT34 (LL) < TKT35 (HS) ≦ TKT34 (HS). Therefore, the creep rupture life of these alloys was shown to be superior under the HS solution treatment condition as compared to the LL solution treatment condition. However, the minimum or steady-state strain rate of these alloys became slower in order of TKT 35 (LL)> TKT34 (LL)> TKT34 (HS) ≧ TKT35 (HS). The creep properties depended on the microstructure of the alloys.

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.

The toughness of girth welds in 9Cr-1Mo-V and 9Cr-0.5Mo-V steel seamless pipe (ASME SA-335 Grades P91 and P92, respectively) made using the flux-cored arc welding (FCAW) process was evaluated. Electrodes from two different suppliers were used for production quality welding of each steel. The welds received post-weld heat-treatment (PWHT) in accordance with the requirements of the ASME Code. The objective of the work was to determine if the fracture toughness of the FCAW welds was acceptable for high-temperature steam piping. Toughness was measured using standard sized Charpy V-notch impact specimens. The specimens were oriented transverse to the weld seam with notch located approximately in the center of the weld metal and parallel to the direction of weld seam. Full-range (lower to upper shelf) Charpy impact energy and shear area curves were developed for each weld joint. These were used to estimate the temperatures corresponding to 30 ft-lb average impact energy. The estimated temperatures were well below the service temperature but were above the typical hydrostatic test temperature.

This study is concerned with the creep damage evaluation for the welded joint of modified 9Cr-1Mo steels. A finite element prediction method based on ductility exhaustion approach has been proposed. Degradation of creep ductility under multi-axial stress state has been formulated from the experimental results of notched bar specimens for the base metal and the fine-grained heat affected zone, and has been taken into the damage model. Creep test of welded joint specimen of modified 9Cr-1Mo steel has been conducted to confirm the accuracy of the damage evaluation method. It has been concluded that the predicted trend of creep damage has good agreement with the experimental results, but the predicted rupture time become longer than the experimental results of rupture time.

A detailed examination has been carried out of the microstructural evolution and mechanical properties of samples of T91 and T92 steels which have been subjected to both a ‘normal’ pre­service heat treatment and an extended stress relief heat treatment at 765°C for up to 16 hours. The samples have subsequently been creep tested to failure at different stresses ranging from 66 to 112 MPa. In each case, a reduction in rupture time was observed of 20-30% in the samples which had experienced the additional stress relief heat treatment compared to those which had not. It is shown that these data, when compared with the mean values expected from European Creep Collaborative Committee (ECCC) Datasheets, result in a reduction in stress of approximately 10% of the mean value predicted from the ECCC data, which is within the allowable scatter band.

The US Advanced Ultra-Supercritical (A-USC) Consortium conducted an extensive program to evaluate available superalloys for use in rotors for steam turbines operating at a nominal temperature of 760 °C (1400 °F). Alloys such as 282, Waspaloy, 740H, 720Li, and 105 were tested in the form of bar supplied from the alloy producers. Ultimately, alloy 282 was down-selected for the turbine rotor based on its combination of creep strength, phase stability, ductility, and fatigue resistance. The next step in development was to produce a full-size rotor forging for testing. A team was established consisting of GE Power (project management and testing), Wyman-Gordon (forging and testing) and Special Metals (melting and billetizing) to pursue the work. A research license to melt the alloy was obtained from Haynes International. The first step of the development was to devise a triple melt (VIM-ESR-VAR) practice to produce 610 mm (24 inch) diameter ingot. Two ingots were made, the first to define the VAR remelting parameters and the second to make the test ingot utilizing optimum conditions. Careful attention was paid to ingot structure to ensure that no solidification segregation occurred. A unique homogenization practice for the alloy was developed by the US Department of Energy (DOE) and National Energy Technology Laboratory (NETL). Billetization was performed on an open die press with three upset and draw stages. This procedure produced an average grain size of ASTM 3. A closed die forging practice was developed based on compressive flow stress data developed by Wyman Gordon Houston for the consortium project. Multiple 18 kg forgings were produced to define the forging parameters that yielded the desired microstructure. The project culminated with a 2.19 metric ton (4830 lb), 1.22 m (48 inch) diameter crack-free pancake forging produced on Wyman Gordon’s 50,000 ton press in Grafton, MA. The forging process produced a disk with an average grain size of ASTM 8 or finer. Forging cut-up, microstructural characterization, and mechanical property testing was performed by GE Power. Fatigue and fracture toughness values of the disk forging exceeded those previously reported for commercially available rolled bar.

The fatigue crack propagation thresholds of SAW weld metal of 25Cr2Ni2MoV simulating product of fossil and nuclear power low pressure turbine rotor at different stress ratios are tested. There is a big dispersity of the test results, even at the same stress ratio. The double logarithm curves of the fatigue crack growth rate and stress intensity factor range are researched. The difference of critical points between stable propagation region and near-threshold region in different specimens is found to be an important cause to the dispersity. Their locations in the specimens can be determined by the method of backward inference. After the observation of the microstructures around the critical points, a good correspondence between the size of prior austenite grain and the maximum size of monotonic plastic zone on the crack tip is confirmed. The difference of the critical points at the same stress ratio is caused by the inhomogeneous microstructures. So the inhomogeneous microstructures in the multi-pass and multi-layer weld metal contribute to the dispersity of the experimental threshold values.

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.

Understanding of the thermomechanical processing that affects microstructures is important to develop new alloys, because the mechanical properties of Ti alloys depend on the microstructures. In our previous study, we found Sn deteriorated the oxidation resistance, while Nb improved the oxidation resistance. Then, we have focused on Ti-Al-Nb-Zr alloys which Nb was added instead of Sn. Zr was added for solid solution strengthening. In this study, the formation of microstructures by thermomechanical processing and the effect of microstructure on the mechanical properties were investigated using the Ti-13Al-2Nb-2Zr (at%) alloy. The samples heat-treated in the β+α phase followed by furnace cooling after processed in the β+α phase formed the equiaxed or the ellipsoid α phase surrounded by the β phase. On the other hand, the sample heat-treated in the β+α phase followed by furnace cooling after processed in the β phase formed the lamellar microstructure. The compression strengths of the equiaxed α structure processed at two temperatures in the β+α phase were almost the same. While creep life of the bi-modal structure was drastically changed by processing temperature.

Modified 9Cr-1Mo alloy steel has been developed over the last few decades and has since gained wide acceptance in the boiler industry for the production of a variety of pressure-critical components, including tubing, piping and headers. The properties of creep-strength enhanced ferritic steels such as grade 91 are critically dependent on manufacturing parameters such as steelmaking, hot deformation, heat treatment and welding. Since the applications for which this material is used impose strict requirements in terms of resistance, corrosion, and creep behavior, poor process control can severely compromise the service behavior. This work discusses the impact of total deformation during the rolling process, and heat treatment parameters on time-independent and time-dependent properties for grade 91. For this study, two heats with similar chemical composition were produced with different reduction ratios: to which, several normalizing and tempering combinations were applied. For each combination, the microstructure was characterized, including evaluation of segregation by metallographic examination, and analysis of secondary phase precipitates by means of X-ray powder diffraction. Mechanical testing and creep testing were performed. A comparison of results is presented, and recommendations on the optimal process parameters are provided to ensure reliable performance of grade 91 material.

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.

The mechanical behavior of a cast form of an advanced austenitic stainless steel, CF8C-Plus, is compared with that of its wrought equivalent in terms of both tensile and creep-rupture properties and estimated allowable stress values for pressurized service at temperatures up to about 850°C. A traditional Larson-Miller parametric model is used to analyze the creep-rupture data and to predict long-term lifetimes for comparison of the two alloy types. The cast CF8C-Plus exhibited lower yield and tensile strengths, but higher creep strength compared to its wrought counterpart. Two welding methods, shielded-metal-arc welding (SMAW) and gas-metal-arc welding, met the weld qualification acceptance criteria in ASME BPVC Section IX for the cast CF8C-Plus. However, for the wrought CF8C-Plus, while SMAW and gas-tungsten-arc welding passed the tensile acceptance criteria, they failed the side bend tests due to lack of fusion or weld metal discontinuities.

The creep degradation/life assessment for high temperature critical component materials is absolutely needed to assure the long-term service operation and there is little experience with the service exposure of the high temperature components made of newly developed Ni-based alloys. In this study, therefore, the creep degradation assessment study on the Ni-based alloys, Alloy 617 and HR6W was conducted based on the hardness method, because the hardness measurement is a useful and simple technique for the materials characterization for any kind of high temperature-serviced steels and alloys. As the result, it was found that the hardness was increased by not only precipitation due to thermal aging but also creep stress/strain, and there existed linear relationship between the applied stress and creep-induced hardness increase. Also the hardness scatter measured was increased along with the progress of creep hardening and damage progressing in terms of creep life consumed. Those findings suggested that the creep life assessment of Ni-based alloys would be possible by means of hardness measurement. The paper also deals with the role and perspective development of non destructive damage detecting techniques, and life assessment issues on Ni-based alloys for A-USC power applications.

For the safe life prediction of components under high cycle fatigue loading at high temperature, such as gas turbine blades and turbocharger components, the behavior of initial defects, which are physically short cracks below the long crack threshold ΔK is of crucial importance. The evolution of different crack closure mechanisms (such as plasticity, roughness and oxide induced crack closure) can lead to crack arrest by a reduction of the effective crack tip loading. To visualize the crack growth behavior of such cracks, cyclic crack resistance curves (cyclic R-curves) are used. The experimental determination of cyclic R-curves is challenging, especially under high temperature conditions due to a lack of optical accessibility. The formation of very short cracks in high strength materials makes it even more complicated to reliably determine these data. Within this study the crack growth behavior of physically short fatigue cracks in three different material states of the nickel alloy IN718 (wrought, cast and PBF-LB/M - processed) is experimentally determined at 650 °C. Based on a load increase procedure applied on Single Edge Notched (SEN) specimens with a compression pre-cracking procedure in advance, crack propagation of physically short cracks is measured with alternating current potential drop systems in air and under vacuum conditions. These examinations are carried out for three different load ratios (R = -1, 0 and 0.5) to investigate the amount of certain crack closure mechanisms active under different loading conditions. Moreover, the formation of a plastic wake along the crack flanks is determined by a finite element simulation. The results determined in air and under vacuum conditions are used to describe the impact of oxide induced crack closure on the behavior of physically short cracks. This allows the evaluation of the behavior of both near-surface and internal defects that are not accessible to the atmosphere.

The extrapolation of short-term laboratory test results to predict long-term high-temperature component failure remains challenging, particularly for P91 steel due to its phase transformation during extended service and susceptibility to type IV cracking. While the NSW model successfully predicts creep crack growth bounds using short to medium-term test data (<10,000 hours), recent literature suggests materials may exhibit more brittle behavior and reduced failure strain in longer-term tests. This study examines whether the NSW model, using short-term uniaxial data, can effectively predict these long-term behavioral changes for more accurate service life assessment.

Ultrasupercritical (USC) steam boiler and heat recovery steam generator (HRSG) technology is constantly evolving to improve efficiency and reduce emissions. Currently, temperatures are pushing beyond the capabilities of even the most advanced ferritic steels with some applications requiring nickel-based superalloys. Cost-effective design of these systems requires the application of a variety of alloys representing a range of cost/property trade-offs. CF8C-Plus is a cast austenitic stainless steel recently developed for application in high temperatures similar to those in power plants (600 - 900 °C) with creep strength comparable to several superalloys. This makes it an attractive alternative for those expensive alloys. EPRI, with assistance from PCC subsidiaries Special Metals and Wyman Gordon Pipes and Fittings, has produced and characterized two pipe extrusions nominally 5.25 inch OD x 0.5 inch wall thickness and 6 inch OD x 0.75 inch wall (13.3 x 1.3 cm and 15.2 x 1.9 cm), each about 1000 lbs, to continue to assess the feasibility of using a wrought version of the alloy in power piping and tubing applications. The mechanical properties from these extrusions show performance in the same population as earlier forging trials demonstrating capability exceeding several austenitic stainless steels common to the industry. Creep-rupture performance in these extrusions continues to be competitive with nickel-based superalloys.