Seeking to reduce CO 2 emissions and improve power generation efficiency, a project to develop a 700°C A-USC (advanced ultra super critical) power plant has been under way in Japan since 2008. HR6W (44Ni-23Cr-7W) is a candidate material for application in the maximum temperature areas of A-USC boilers. In this study, the creep rupture properties of plastic deformed material were investigated in comparison with those of solution treated material, in order to clarify the capability of HR6W as a material for use in A-USC plants. The creep strength of 20% pre-strained HR6W was found to increase substantially as compared with the solution treated material. 20% pre-strained material is in a state where high dislocation density has been introduced by plastic forming strain, with M 23 C 6 and Laves phase precipitating preferentially by dislocation diffusion from the early stages of creep. In particular, since high dislocation density is accumulated in connection with creep deformation near the grain boundaries, precipitation is accelerated and the grain boundaries are covered with M 23 C 6 from the early stages of creep. Then, even though the intragranular precipitate density decreases, given that the fraction of grain boundaries affected by precipitation is maintained in a high state, it is presumed that a high density of dislocation is maintained in the long-term region. This was considered to be the reason why the creep rupture strength of the 20% pre-strained material increased so remarkably in comparison with the solution treated material.

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.

In order to clarify the effect of stress and strain on microstructural changes during creep for T91 steel, creep interrupted tests were performed at 600°C for 10000h, 20000h, 30000h, 50000h and 70000h. The steel studied was T91 steel with high Ni content (0.28mass%) in the range of specification. Changes of dislocation structure and precipitates distributions were observed for the grip and gauge portions of creep interrupted samples. The subgrain size gradually increased with increasing creep time up to 50000h in both the grip and gauge portions. However, the subgrain size abruptly increased after 50000h in the gauge portion as compared with the grip portion. Decrease in dislocation density inside subgrain was promoted in the gauge portion as compared with the grip portion. The size of M 23 C 6 gradually increased with increasing creep time up to 50000h in both the grip and gauge portions. The increase in M 23 C 6 size was accelerated after 50000h in the gauge portion as compared with the grip portion. The Z phase formation was promoted in the gauge portion as compared with the grip portion. The number density of all kinds of particles gradually decreased with increasing creep time in the gauge and grip potions. After 50000h, the number density rapidly decreased in the gauge portion as compared with the grip portion.

Significant development is being carried out worldwide for establishing advanced ultra supercritical power plant technology which aims enhancement of plant efficiency and reduction of emissions, through increased inlet steam temperature of 750°C and pressure of 350 bar. Nickel base superalloy, 50Ni-24Cr-20Co-0.6Mo-1Al-1.6Ti-2Nb alloy, is being considered as a promising material for superheater tubes and turbine rotors operating at ultra supercritical steam conditions. Thermal fluctuations impose low cycle fatigue loading in creep regime of this material and there is limited published fatigue and creep-fatigue characteristics data available. The scope of the present study includes behavior of the alloy under cyclic loading at operating temperature. Strain controlled low cycle fatigue tests, carried out within the strain range of 0.2%-1%, indicate substantial hardening at all temperatures. It becomes more evident with increasing strain amplitude which is attributed to the cumulative effects of increased dislocation density and immobilization of dislocation by γ′ precipitates. Deformation mechanism which influences fatigue life at 750°C as a function of strain rate is identified. Hold times up to 500 seconds are introduced at 750°C to evaluate the effect of creep fatigue interaction on fatigue crack growth, considered as one of the primary damage mode. The macroscopic performance is correlated with microscopic deformation characteristics.

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.

The microstructures and mechanical properties of T122 steel used for superheater tube of the boiler in a 1000 MW ultra supercritical power plant after service for 83,000h at 590℃ were investigated, and compared with data of that served for 56,000h in previous studies. The results show that compared with T122 tube sample service for 56,000h, the tensile properties at room temperature and the size of precipitated phase exhibit few differences, but the lath martensites features are apparent, and the Brinell hardness value are obviously higher. SEM and TEM experiments show that the substructure is still dominated by lath martensite. A few lath martensites recover, subgrains appear and equiaxe, and the dislocation density in grains is relatively low. A large number of second-phase particles precipitated at boundaries of original austenite grains and lath martensite phases, which are mainly M 23 C 6 and Laves phases.

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.

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.

In the present study a creep resistant, ferritic steel, based on the chemical composition of Crofer 22 H, was analysed regarding microstructure and particle evolution. Because of the preceding hot-rolling process formation of sub-grain structures was observed, which disappears over time. Additionally formation of particle-free zones close to high angle grain boundaries was observed. These zones are considered to be responsible for long-term material failure by lacking particle hardening and thus a concentration of deformation. Therefore in-depth analyses by transmission and scanning electron microscopy were performed to investigate dislocation behaviour in these areas

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.

Microstructure in the gage sections of ruptured GX12CrMoWVNbN10-1-1 cast steel specimens was examined after creep tests under applied stresses ranging from 120 to 160 MPa at T=893 K. The microstructure after tempering consisted of laths with an average thickness of 332 nm. The tempered martensite lath structure was characterized by M 23 C 6 -type carbide particles with an average size of about 105 nm, and MX carbonitrides with an average size of about 45 nm. Precipitation of Laves phase occurred during creep test. The structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. MX carbonitrides and M 23 C 6 -type carbide particles increase in size slightly under long-term creep. Microstructural degradation mechanisms during creep in GX12CrMoWVNbN10-1-1 cast steel are discussed.

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.

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

The effect of multiple hot rolling in the temperature interval of 700-1000°C (1290-1830°F) on microstructures and tensile behavior of an S304H-type austenitic stainless steel was studied. The structural changes during hot working are characterized by the elongation of original grains towards the rolling axis and the development of new fine grains. The fraction of fine grains and the average grain size increase with increasing the rolling temperature. The multiple hot rolling results in significant strengthening. The offset yield strength approaches 1080 MPa in the sample processed at 700°C (1290°F), while that of 390 MPa is obtained after rolling at 1000°C (1830°F). On the other hand, the tensile strength at elevated temperatures of 600-700°C (1110-1290°F) decreases with a decrease in the rolling temperature. The relationship between the deformation structures and the tensile behavior is considered in some detail.

Large heat-to-heat variation of creep rupture strength in weldments of mod.9Cr-1Mo steels was observed in the creep rupture tests conducted for two different heats at 600°C and 650°C. One heat showed consistently lower time-to-rupture than the other for 130-60MPa at 600°C. Detailed microstructural investigations revealed that the number density of precipitates in the weaker heat was remarkably lower than that associated with the stronger heat through most of the creep region. Accordingly, heat-to-heat variation of creep rupture strength was attributed to the difference in the precipitate strengthening effects throughout creep. Equilibrium calculation predicted that the smaller phase fraction of M 23 C 6 and VN precipitates due to the lower content of chromium and lower ratio of nitrogen/aluminum in the weaker heat. However, given that long-term creep rupture strength at 650°C converged for the two heats, the microstructure including precipitates may settle into a similar level for subsequent longer hours even at 600°C.

In order to clarify the effect of stress state on microstructural changes during creep, the microstructure was observed in the central part of the cross section of the fine-grained heat-affected zone (FGHAZ) and in the surface region of the FGHAZ in Gr.92 steel welded joint. Creep tests were performed under constant load in air at 650°C, using cross-weld specimens. The creep strength of welded joint was lower than that of base metal. Type IV fracture occurred in the long-term. Creep voids were detected in the FGHAZ after the fracture. Number of creep voids was higher in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. It was checked the multiaxiality of stress during creep was higher in the central part of the cross section of the FGHAZ than in the surface region of the FGHAZ. The recovery of dislocation structure occurred after creep in the base metal and the FGHAZ. Mean subgrain size increased with increasing time to rupture. However, there was no difference of change of subgrain size during creep in the central part of the cross section of the FGHAZ and in the surface region of the FGHAZ. The growth of M 23 C 6 carbide and MX carbonitrides was observed during creep in the base metal and the FGHAZ. Laves phase precipitation occurred during creep. There was no difference of the change of mean diameter of MX carbonitrides in the central part of the cross section of the FGHAZ and in the surface region of the FGHAZ after creep. However, the growth rate of M 23 C 6 carbide in the FGHAZ was much higher in the central part of the cross section than in the surface region.

Structural changes in P92-type steel after creep at temperature of 600°C under a stress of 140 MPa were investigated. The steel was solution treated at 1050°C and tempered at 780°C. The structure in the grip portion of the creep specimen changed scarcely after creep exposure for 6876 h. In contrast, the structural changes in the gage and neck sections were characterized by transformation of the tempered martensite lath structure into relatively coarse subgrain structure. The formation of a well-defined subgrain structure in the gage and neck sections was accompanied by the coarsening of M 23 C 6 carbides and precipitations of Laves phase during creep. Mechanisms of grain boundary pinning by precipitates are discussed.

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.

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700 °C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700 °C, for example, 19MPa at 700 °C after 10 5 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.

Prediction of long-term creep strength is an important issue for industrial plants operated at elevated temperatures, although the creep strength of high Cr ferritic steels depends on their microstructural evolution during creep. The state of microstructure in metallic materials can be expressed as numerical values based on a concept of system free energy. In this study, in order to evaluate long term creep strength of 9Cr ferritic steel containing B, change in the system free energy during creep of the steel is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. The system free energy decreases with creep time. Change in the energy is expressed quantitatively as a numerical formula using the rate constants which depend on applied stress. On the basis of these facts, long term creep strength of the steel can be evaluated at both 948K(675°C) and 973K(700 °C).