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The background of the NIMS Creep Data Sheet Project, together with the preliminary study and facilities, material selection, and testing method, is summarized. The outcomes from the project are explained, focusing on the long-term creep strength of ferritic and austenitic heat-resistant steels. In some cases, the slope of the stress versus time-to-rupture curve in the long term differed from that in the short term in a manner that was markedly dependent on the type of material. Heat-to-heat variations in creep strength were recognized for ferritic and austenitic steels, even when the chemical compositions of the steels examined were within the range of specifications. The reasons for the heat-to-heat variations were differences in the chemical composition, in the amounts of minor elements, and in the grain size, among others. The existence of inherent creep strength was discovered in the very long term for ferritic heat-resistant steels. The amounts of minor solute elements affect the inherent creep strength, independently of precipitation strengthening or the dislocation structure. An inflection point was observed in the tertiary creep stage for a low-alloy steel and for austenitic stainless steels when precipitation occurred during creep. A region-splitting analysis method was proposed for long-term creep strength evaluation for high-chromium ferritic steels. This method was used to review the allowable stress of high-chromium ferritic steels in Japan. A metallographic atlas, time-temperature-precipitation diagram, and fracture-mode map were proposed for ferritic and austenitic steels on the basis of creep-ruptured specimens.

P92 martensitic heat-resistant steel is widely used in ultra-supercritical (USC) thermal power units due to its excellent creep resistance and high-temperature strength. However, prolonged exposure to high temperatures induces significant microstructural degradation, compromising mechanical properties and operational safety. This study investigates the evolution of microstructure and mechanical properties in P92 steel extracted from main steam pipelines after service durations of 30,000 h, 47,000 h, 56,000 h, 70,000 h, and 93,000 h. Comparative analyses of impact toughness, tensile strength, and creep strength were conducted and advanced characterization of SEM and TEM was used to investigate the microstructural evolution. The results reveal a progressive decline in mechanical properties with increasing service time. Specifically, impact toughness decreased by approximately 66.8%, room-temperature tensile strength reduced by 9.62%, and high-temperature tensile strength at 610 °C declined by 31.6%. Notably, the 105 hour creep rupture strength exhibited a 10.4% decrease compared to as-received material. This decline is attributed to microstructural changes including precipitate coarsening, martensite lath boundary degradation, dislocation reconfiguration, and severe grain coarsening. The coarsening of precipitates weakens their bonding with the matrix, while the widening of martensite laths reduces resistance to crack propagation and dislocation movement, jointly contributing to strength deterioration.

A series of triaxial creep tests were conducted on warm frozen silts extracted from Qinghai–Tibet Plateau at temperature of −1.5 °C under confining pressures of 0.5, 1.0, and 2.0 MPa, respectively. The applied test stress levels were 30, 50, 60, and 70% of triaxial shear strength, respectively. The test results indicate that the creep strain increases with the increase in applied stress level and there is a stress threshold, based on which the test results can be classified into two types of creep strain curves. The creep strain curve only includes primary and secondary creep stages when the stress level is less than the threshold value. When the stress level exceeds the threshold value, the creep strain velocity gradually increases and the specimen quickly fails in tertiary creep stage. Based on the creep test results, a fractional order rheological element model is established for warm frozen silt, which is also generalized from uniaxial stress state to the three-dimensional stress state. From the analysis on the features of the stress threshold, a creep strength criterion is also proposed simultaneously. Comparing the calculated results of the warm frozen silt with the tested ones, it is found that the predicted results of the proposed model are in good agreement with the test results. In the proposed fractional order model, the relationship between the damage factor and time is established to describe the damage degree of the specimen. Compared with the existing creep constitutive model of frozen soil, the proposed fractional order model has advantages of fewer model parameters, higher simulation precision and wider applicability in analyzing the mechanical properties of warm frozen silt.

The present study centres on the effect of post-weld heat treatment (PWHT) on microstructure and mechanical properties of the deep penetration keyhole Tungsten Inert Gas (K-TIG) welded dissimilar joint between creep strength enhanced ferritic (CSEF) steel and austenitic stainless steel (ASS). The as-received normalized and tempered CSEF steel was joined with ASS in a single pass without using any filler materials and edge preparation. Detailed characterization across the welded joint was conducted using stereomicroscope, electron microscopy, energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), hardness test, tensile test and Charpy impact test. Results showed that PWHT had significant effect on the microstructure and mechanical properties of both the weld metal and CSEF steel heat-affected zone (HAZ), while it had little influence on the ASS side. By using proper PWHT, the hardness gradient across the welded joint could be mitigated and toughness in both the weld metal and the CSEF steel HAZ could be restored. 760 °C was considered the most appropriate PWHT temperature for such dissimilar joint in terms of the overall mechanical properties. The tensile properties of K-TIG welded joint after PWHT were comparable to both friction stir welded joint and laser and/or electron beam welded joint, indicating that deep penetration TIG welding technology may provide a good alternative for the nuclear industry. The correlation among welding thermal cycle, various heat treatment, microstructure evolution and mechanical properties was also analysed in detail.

This paper summarizes the results of investigations into heterogeneous P23/P91 welds after long-term creep exposure at temperatures of 500, 550 and 600 °C. Two variants of welds were studied: In Weld A, the filler material corresponded to P91 steel, while in Weld B, the chemical composition of the consumable material matched P23 steel. The creep rupture strength values of Weld A exceeded those of Weld B at all testing temperatures. Most failures in the cross-weld samples occurred in the partially decarburized zones of P23 or WM23 steel. The results of the investigations on the minor phases were in good agreement with kinetic simulations that considered a 0.1 mm fusion zone. Microstructural studies proved that carburization occurred in the P23/P91 weld fusion zones. The partial decarburization of P23 steel or WM23 was accompanied by the dissolution of M7C3 and M23C6 particles, and detailed studies revealed the precipitation of the Fe2 (W, Mo) Laves phase in decarburized areas. Thermodynamic simulations proved that the appearance of this phase in partially decarburized P23 steel or WM23 is related to a reduction in the carbon content in these areas. According to the results of creep tests, the EBSD investigations revealed a better microstructural stability of the partially decarburized P23 steel in Weld A.

The difficulties in using the conventional Norton creep model to rationalise short-term creep data and to subsequently predict long-term creep rupture strengths for creep resistant alloys are presented and analysed. The results of this study show that these difficulties can be resolved if a new tensile creep model that integrates the tensile strength at creep temperature is applied to rationalise the short-term creep data. This is illustrated with the creep and tensile strength data measured for a grade of Ni-based superalloy. Based on this new tensile creep model, the activation energy of creep determined is independent of stress and the stress exponent is not influenced by temperature. Consequently, the model constants obtained from the short-term creep data can be applied together with the Monkman-Grant relationship to make the long-term creep rupture strength predictions at different temperatures. The factors affecting the reliability of the predictions made by this method are also analysed.

Alloys meeting the requirements of “700 °C and above” advanced ultra-super-critical technology, with higher thermal efficiency, have been developed in recent years. Here, a new wrought Ni-based superalloy with excellent high-temperature creep strength based on Waspaloy has been developed and is proposed as a candidate material for application in 700 °C class advanced ultra-super-critical steam turbine blades. In this new alloy, the Molybdenum (Mo) in Waspaloy is partially replaced by Tungsten (W). Creep tests have shown that this new Ni-based alloy has a 70 MPa higher creep-rupture strength than that of Waspaloy at 700 °C by extrapolating the experimental data. Detailed creep-rupture mechanisms have been analyzed by means of scanning electron microscopy, transmission electron microscopy, and chemical phase analysis with a view to devising potential approaches for performance improvements. The results showed that the partial replacement of Mo by W had negligible effect on the composition of carbides precipitated in the alloy. Instead, the amount of the γ′ phase was significantly increased, and mismatch between the γ and γ′ phases was reduced. In this way, the stability of the γ′ phase was increased, its coarsening rate was reduced, and its critical shear stress was increased. As a result, the high-temperature creep-fracture strength of the new alloy was increased.

A mathematical model of the stress-strain state before the onset of high-temperature hydrogen corrosion of a steel hollow cylinder is proposed. Plane deformation of a cross section with cylindrical micropores around which local stresses arise is considered. A criterion is proposed in which the initiation of corrosion cracks is possible when the sum of the circumferential stress under the effect of hydrogen pressure inside the cylinder cavity and the circumferential local stress under the effect of methane pressure inside the micropore reaches the break creep strength.

Integrity assessment of 10Cr ferritic steel/alloy 617M dissimilar metal weld joint (DMWJ) fabricated from hot-wire narrow-gap TIG (NG-TIG) welding process using alloy 617 filler wire (ERNiCrCoMo-1) was carried out under creep testing at 888 K. Microstructural constituents and hardness across the weld joint (10Cr steel-alloy 617 butter layer-alloy 617 weld metal-alloy 617 M) were found to vary significantly. Alloy 617 weld metal and butter layer have possessed higher hardness as compared to base metals of alloy 617M and 10Cr steel. However, lower hardness was observed in the alloy 617 butter layer, which is adjacent to 10Cr steel, and in the outer edge of HAZ in the 10Cr steel. The carbon migration was predominantly observed across the interface between 10Cr steel and alloy 617 butter layer. Creep tests performed on 10Cr steel and DMWJ at 888 K have revealed the lower creep strain accumulation in DMWJ than the 10Cr steel base metal. An early onset of tertiary creep deformation and consequent premature failure of DMWJ were noticed as compared to the 10Cr steel. Creep rupture strength of the DMWJ was about 31% lower than the 10Cr steel. Fracture in the weld joint has occurred at the interface between 10Cr steel and alloy 617 buttered layer with significant reduction in ductility. The formation of coarse M 23 C 6 precipitates and Laves phase (enriched by Mo and W), oxidation, heterogeneity in strength across the interface have facilitated the extensive cavitation at the interface, thereby leading to premature failure of the DMWJ. Weld strength reduction factor of about 0.69 at 888 K for 10 4  h has been obtained for the DMWJ.

The impact resistance and machinability of TiAl alloys, which are used for jet engine turbine blades, are critical for ensuring reliability and reducing manufacturing costs. This study investigated the effects of the microstructure on these properties using Ti–Al–Cr ternary alloys via Charpy impact tests at room temperature and 700 °C and performing cutting tests using a face mill with cemented carbide tools. As a result, it was confirmed that six types of typical microstructures of TiAl alloys, namely, fine FL, coarse FL, L + γ, γ, γ + β, and L + γ + β, could be formed by varying the Al and Cr concentrations and heat-treatment conditions. Impact resistance and machinability are each the exact opposite trends to the other, with coarse FL having the best impact resistance but poor machinability. Meanwhile, γ has the best machinability but the weakest impact resistance. L + γ has no major drawbacks, including creep strength. As the microstructure of TiAl4822, currently used in LEAP (leading edge aviation propulsion) engine blades, is almost a γ single-phase microstructure, we assumed that manufacturers chose this microstructure to improve machinability and thus reduce the cost. However, because the γ microstructure has the lowest impact resistance, caution should be exercised when applying it to other engines with different operating environment. On the other hand, the microstructure containing the β phase is inferior in all aspects, including creep strength. Thus, it is questionable to use TiAl-forged materials with a residual β phase in small-sized products that can be manufactured by casting.