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.

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

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.

700°C advanced ultra-supercritical system and supercritical CO 2 turbine system are developed for high efficiency turbine systems for next generation. This study covered the feasibility of creep life assessment of γ’-Ni 3 (Al,Ti) precipitation strengthened Ni-based superalloy rotor material, TOS1X-2, a modified alloy of UNS N06617 for these systems, based on hardness measurement method. It was found that the hardness of TOS1X-2 was governed by the change in precipitation strengthening and strain hardening during creep. The clear relationship between hardness increase in crept portion and macroscopic creep strain was observed, suggesting that it might be possible to estimate the creep strain or initiation of acceleration from hardness measurement. Microstructure inhomogeneity and microstructure evolutions during creep especially focused on dispersion of creep strain were characterized by EBSD quantitative analysis. It was found that creep strain was accumulated along the grain boundary, while it was relatively absent in coarse grains with low Schmid factor of {111} <110> slip system in fcc structure. The upper limit of hardness scatter band is thought to be important, since it represents the local and critical creep damage of the alloy.

The nucleation and growth of precipitates such as Laves phases, carbides and nitrides reduce fracture toughness and high-temperature strength of high chromium steels used in thermal power plants. For this reason, to ensure a long-term plant reliability, it is important to estimate material deterioration by aging. The study presented in this paper involves micro structural evolution by thermal aging of COST-E, F, and FB2 steels, all turbine materials. The results indicate that the Laves phases and other precipitates can be separately detected and quantified by the electrochemical technique. The results also clarify the correlation between the amount of Laves phases precipitated and electrochemical polarization parameters.

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.

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

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

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.

9-12%Cr martensitic steels can be applied to the next highest temperature components such as boiler tracts, steam pipelines and turbines of advanced ultra-supercritical power plants with steam temperatures of 650°C. New 10%Cr martensitic steels with high B and low N contents can be a worthy candidate for use in production because it has superior creep resistance. At the same time, resistance to cyclic and dynamic loads is very important. In this work, we studied the low cycle fatigue (LCF) properties at room and elevated (500-650°C) temperatures and Charpy impact toughness at temperatures ranging from -196…100°C of advanced 10% Cr martensitic steel with high B and low N contents. The effect of new alloying scheme and corresponding peculiarities of M 23 C 6 carbides on the low cycle fatigue resistance and impact toughness of the 10%Cr martensitic steel is analyzed. It is revealed that fine and densely distributed carbides has no effect on the fatigue resistance except for the slight improvement of fatigue life at small strain amplitudes and shift the ductile-brittle transition temperature (DBTT) to higher but satisfactory value of +10°C as compared to other high-chromium martensitic steels.