Carbon migration in narrow-gap welding joints of dissimilar steels has been studied using bead-on-plate specimens to determine the factors that influence the formation of a soft ferrite structure in the carbon-depleted zone. Carbon migration was found to occur during tempering, with a ferrite structure formed at the intersection of multiple layers due to severe carbon migration. This was attributed to a steep gradient in Cr content caused by the low fusion penetration at the intersection. Experimental results and the relationship between fusion penetration and weld bead alignment confirmed that low fusion penetration is the main cause of ferrite-structured carbon depleted zones.

This study examines the influence of carbon and austenite stabilizing elements (Ni, Mn, Co, Cu) on Laves phase precipitation, Fe 2 W formation, and creep rupture strength (CRS) in 9-12% Cr steels at 600-700°C. Nickel and manganese had minimal impact on Laves phase and coarse carbide formation up to 1% content. While cobalt increased Laves phase fraction at 650°C, it did not improve long-term CRS and even caused a rapid decrease in short-term CRS. Copper, on the other hand, promoted the precipitation of fine Cu-rich particles that acted as nucleation sites for Laves phase and M 23 C 6 carbide. This resulted in a different needle-like Laves phase morphology compared to the globular type observed in nickel and cobalt alloys, leading to improved CRS in the copper alloy. Increasing carbon content from 0.1% to 0.2% effectively suppressed Laves phase formation, as confirmed by Thermo-Calc calculations. Notably, for cobalt alloys with higher tungsten content, higher carbon content (0.09% to 0.19%) improved CRS at 650°C, whereas the opposite effect was observed in nickel and nickel-manganese alloys. Copper alloys maintained improving CRS trends even with increased carbon, leading to the overall best CRS performance among the tested alloys with 0.2% carbon.

The effect of gas impurities on corrosion behavior of candidate Fe- and Ni-base alloys (SS 316LN, Alloy 800HT, Alloy 600) in high temperature CO 2 environment was investigated in consideration of actual S-CO 2 cycle applications. Preliminary testing in research and industrial grade S-CO 2 at 600 °C (20 MPa) for 1000 h showed that oxidation rates were significantly reduced in industrial-grade S-CO 2 environment. Meanwhile, controlled tests with individual impurity additions such as CH 4 , CO, and O 2 in research-grade CO 2 were performed. The results indicated that CH 4 and CO additions did not seem to significantly affect oxidation rates. On the other hand, O 2 addition resulted in lower weight gains for all alloys, suggesting that O 2 may be primarily affecting corrosion behavior.

Interstitial carbon (C) in β-Ti, α-Ti, α 2 -Ti 3 Al and γ-TiAl phases present in the γ-TiAl alloys with and without substitutional elements (M: transition element) is quantitatively analyzed using soft X-ray emission spectroscopy (SXES), in order to reveal the effect of solute carbon on the phase equilibria. SXES for carbon analysis was used and the peak intensity of the second reflection of carbon Kα is analyzed using the fully homogenized sample having different C content under the optimum condition to make the accurate calibration curves. The obtained calibration curve is in an accuracy of ± 0.07 at. % C. In all heat treated alloys, no carbide is observed. In Ti-Al binary system, the α+γ phase region shifts toward higher Ti side, and the volume fraction of γ phase increases slightly with the carbon addition. In all system, carbon preferentially partitions into the α phase, followed by less partitioning in the γ and β phases in order. The carbon content in the β phase remains unchanged of almost 0.05 at. % regardless of carbon addition in Ti-Al-V system and the partition coefficient of carbon between the α and γ phases becomes larger in Ti-Al-V system than that in TiAl binary system.

For future carbon neutral society, a novel thermal power generation system with no CO 2 emission and with extremely high thermal efficiency (~ 70 %) composed of the oxygen/hydrogen combustion gas turbine combined with steam turbine with the steam temperature of 700°C is needed. The key to realize the thermal power plant is in the developments of new wrought alloys applicable to both gas turbine and steam turbine components under higher temperature operation conditions. In the national project of JST-Mirai program, we have constructed an innovative Integrated Materials Design System , consisting of a series of mechanical property prediction modules (MPM) and microstructure design modules (MDM). Based on the design system, novel austenitic steels strengthened by Laves phase with an allowable stress higher than 100 MPa for 10 5 h at 700°C was developed for the stream turbine components. In addition, for gas turbine components, novel solid-solution type Ni-Cr-W superalloys were designed and found to exhibit superior creep life longer than 10 5 h under 10 MPa at 1000°C. The superior long-term creep strengths of these alloys are attributed to the “grain-boundary precipitation strengthening (GBPS)” effect due to C14 Fe 2 Nb Laves phase and bcc α 2 -W phase precipitated at the grain boundaries, respectively.

The “Cool Earth-Innovative Energy Technology Program,” launched by the Japanese government in March 2008, aims to significantly reduce global greenhouse gas emissions. Among the 21 selected technologies is the Advanced Ultra Super Critical (A-USC) pressure power generation, which targets the commercialization of a 700°C class pulverized coal power system with a power generation efficiency of 46% by around 2015. As of 2004, Japan's pulverized coal power plant capacity reached 35 GW, with the latest plants achieving a steam temperature of 600°C and a net thermal efficiency of approximately 42% (HHV). Older plants from the 1970s and early 1980s, with steam temperatures of 538°C or 566°C, are nearing the need for refurbishment or rebuilding. A case study on retrofitting these older plants with A-USC technology, which uses a 700°C class steam temperature, demonstrated that this technology is suitable for such upgrades and can reduce CO 2 emissions by about 15%. Following this study, a large-scale development of A-USC technology began in August 2008, focusing on developing 700°C class boiler, turbine, and valve technologies, including high-temperature material technology. Candidate materials for boilers and turbine rotor and casing materials are being developed and tested. Two years into the project, useful test results regarding these candidate materials have been obtained, contributing to the advancement of A-USC technology.

Supercritical carbon dioxide cooling during machining has been identified as an effective measure to mitigate the risk of stress corrosion cracking in materials utilized in the primary circuit of light water reactors, particularly in pressure vessel structural steels. This study aims to compare two different cooling methods, the novel supercritical carbon dioxide and conventional high pressure soluble oil, employed during both milling and turning processes for SA508 Grade 3 Class 2 and AISI 316L steels. As the surface conditions of materials are critical to fatigue properties, such as crack initiation and endurance life, the fatigue performance of both cooling methods for each process were then evaluated and the impact on properties determined. To compare the potential benefits of supercritical carbon dioxide cooling against conventional soluble oil cooled machining, fatigue specimens were machined using industry relevant CNC machine tools. Surface finish and machining methods were standardized to produce two different specimen types, possessing dog- bone (milled) and cylindrical (turned) geometries. Force-controlled constant amplitude axial fatigue testing at various stress amplitudes was undertaken on both specimen types in an air environment and at room temperature using a stress ratio of 0.1. The fatigue performance of the supercritical carbon dioxide cooled specimens revealed substantially greater endurance lives for both SA508 and 316L materials, when compared with specimens machined using high pressure soluble oil cooling.

A new 18Cr-9Ni-Nb-V-W-N-low C austenitic boiler tube (XA704) has been developed. Conventional high-strength austenitic stainless steel boiler tubes usually have high susceptibility to intergranular corrosion because of their high carbon content, and require special care for heated sections such as weld joints. Generally, when the carbon content decreases, the intergranular corrosion resistance improves, while the creep strength reduces. However, the creep strength of the developed steel is very high despite lower carbon content in comparison to conventional austenitic boiler tubes. The high temperature strength and the intergranular corrosion resistance of the steel are superior to those of conventional 18Cr steels such as TP347H. This excellent creep strength of XA704 is mainly due to precipitation strengthening by CrVN, and solid solution strengthening by tungsten and nitrogen. Matching welding consumables for the developed steel have also been developed. Thus, newly developed XA704 is a promising material for superheater and reheater tubes for the “600°C generation” of USC boilers. XA704 has already been used in six power plants in Japan. Currently, the steel is being standardized in the ASME Code.

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.

A combined pilot-scale combustion test and long-term laboratory study investigated the impact of oxy-firing on corrosion in coal-fired boilers. Four coals were burned under both air and oxy-firing conditions with identical heat input, with oxy-firing using flue gas recirculation unlike air-firing. Despite higher SO 2 and HCl concentrations in oxy-firing, laboratory tests showed no increase in corrosion rates compared to air-firing. This is attributed to several factors: (1) Reduced diffusion: High CO 2 in oxy-firing densified the gas phase, leading to slower diffusion of corrosive species within the deposit. (2) Lower initial sulfate: Oxy-fired deposits initially contained less sulfate, a key hot corrosion culprit, due to the presence of carbonate. (3) Reduced basicity: CO 2 and HCl reduced the basicity of sulfate melts, leading to decreased dissolution of metal oxides and mitigating hot corrosion. (4) Limited carbonate/chloride formation: The formation of less corrosive carbonate and chloride solutes was restricted by low O 2 and SO 3 near the metal surface. These findings suggest that oxy-firing may not pose a greater corrosion risk than air-firing for boiler materials.

Nickel-base alloys were exposed to flowing supercritical CO 2 (P = 20MPa) at temperatures of 700 to 1000°C for up to 1000 h. For comparison, 316L stainless steel was similarly exposed at 650°C. To simulate likely service conditions, tubular samples of each alloy were internally pressurised by flowing CO 2 , inducing hoop stresses up to 35 MPa in the tube walls. Materials tested were Haynes alloys 188, 230 and 282, plus HR120 and HR160. These alloys developed chromia scales and, to different extents, an internal oxidation zone. In addition, chromium-rich carbides precipitated within the alloys. Air aging experiments enabled a distinction between carburisation reactions and carbide precipitation as a result of alloy equilibration. The stainless steel was much less resistant to CO 2 attack, rapidly entering breakaway corrosion, developing an external iron-rich oxide scale and internal carburisation. Results are discussed with reference to alloy chromium diffusion and carbon permeation of oxide scales.

Unpredictable failures near the phase boundary in Grade 91 dissimilar metal welds (DMW) with nickel based filler metals represent a significant problem for the power generation industry. In order to determine the root cause for these failures, it is necessary to understand the formation of the microstructure in the weld regions around the site of failure. Thermal histories were therefore measured inside the Grade 91 steel heat affected zone (HAZ) of an autogenous weld and of a DMW in the form of bead on plate with Alloy 625 to study the effect of the weld thermal cycle on microstructural formation. It was found that the HAZ in the DMW experienced longer dwell time at high temperatures because of the latent heat of fusion released during Alloy 625 solidification (1350 - 1125 °C). This allowed longer time for carbide dissolution and phase transformations in the DMW than in the autogenous weld. Additionally, the 625 filler metal created a large chemical potential gradient for carbon, which when combined with longer dwell times, yielded carbon depletion in the heat affected zone. Retention of δ ferrite in the coarse grained HAZ (CGHAZ) of DMWs was found to be an indicator for these mechanisms.

Structural alloy corrosion is a major concern for the design and operation of supercritical carbon dioxide (sCO 2 ) power cycles. Looking towards the future of sCO 2 system development, the ability to measure real-time alloy corrosion would be invaluable to informing operation and maintenance of these systems. Sandia has recently explored methods available for in-situ alloy corrosion monitoring. Electrical resistance (ER) was chosen for initial tests due the operational simplicity and commercial availability. A series of long duration (>1000 hours) experiments have recently been completed at a range of temperatures (400-700°C) using ER probes made from four important structural alloys (C1010 Carbon Steel, 410ss, 304L, 316L) being considered for sCO 2 systems. Results from these tests are presented, including correlations between the probe measured corrosion rate to that for witness coupons of the same alloys.

While the water vapor content of the combustion gas in natural gas-fired land based turbines is ~10%, it can be 20-85% with coal-derived (syngas or H 2 ) fuels or innovative turbine concepts for more efficient carbon capture. Additional concepts envisage working fluids with high CO 2 contents to facilitate carbon capture and sequestration. To investigate the effects of changes in the gas composition on thermal barrier coating (TBC) lifetime, furnace cycling tests (1h cycles) were performed in air with 10, 50 and 90 vol.% water vapor and in CO 2 -10%H 2 O and compared to prior results in dry air or O 2 . Two types of TBCs were investigated: (1) diffusion bond coatings (Pt diffusion or simple or Pt-modified aluminide) with commercially vapor-deposited yttria-stabilized zirconia (YSZ) top coatings on second-generation superalloy N5 and N515 substrates and (2) high velocity oxygen fuel (HVOF) sprayed MCrAlYHfSi bond coatings with air-plasma sprayed YSZ top coatings on superalloy X4 or 1483 substrates. In both cases, a 20-50% decrease in coating lifetime was observed with the addition of water vapor for all but the Pt diffusion coatings which were unaffected by the environment. However, the higher water vapor contents in air did not further decrease the coating lifetime. Initial results for similar diffusion bond coatings in CO 2 -10%H 2 O do not show a significant decrease in lifetime due to the addition of CO 2 . Characterization of the failed coating microstructures showed only minor effects of water vapor and CO 2 additions that do not appear to account for the observed changes in lifetime. The current 50°-100°C de-rating of syngas-fired turbines is unlikely to be related to the presence of higher water vapor in the exhaust.

Oxyfuel combustion is considered as one of the most promising technologies to facilitate CO 2 capture from flue gases. In oxyfuel combustion, the fuel is burned in a mixture of oxygen and recirculated flue gas. Flue gas recirculation increases the levels of fireside CO 2 , SO 2 , Cl and moisture, and thus promotes fouling and corrosion. In this paper the corrosion performance of two superheater austenitic stainless steels (UNS S34710 and S31035) and one Ni base alloy (UNS N06617) has been determined in laboratory tests under simulated oxyfuel conditions with and without a synthetic carbonate based deposits (CaCO 3 - 15 wt% CaSO 4 , CaCO 3 - 14wt% CaSO 4 - 1 KCl) at 650 and 720°C up to 1000 hours. No carburization of the metal substrate was observed after exposure to simulated oxyfuel gas atmospheres without deposit, although some carbon enrichment was detected near the oxide metal interface. At 720°C a very thin oxide formed on all alloy surfaces while the weight changes were negative. This negative weight change observed is due to chromium evaporation in the moist testing condition. At the presence of deposits, corrosion accelerated and considerable metal loss of austenitic alloys was observed at 720°C. In addition, clear carburization of austenitic steel UNS S34710 occurred.

Advanced power generation systems, including advanced ultrasupercritical (A-USC) steam and supercritical carbon dioxide (sCO 2 ) plants operating above 700°C, are crucial for reducing carbon dioxide emissions through improved efficiency. While nickel superalloys meet these extreme operating conditions, their high cost and poor weldability present significant challenges. This study employs integrated computational materials engineering (ICME) strategies, combining computational thermodynamics and kinetics with multi-objective Bayesian optimization (MOBO), to develop improved nickel superalloy compositions. The novel approach focuses on utilizing Ni 3 Ti (η) phase strengthening instead of conventional Ni 3 (Ti,Al) (γ’) strengthening to enhance weldability and reduce costs while maintaining high-temperature creep strength. Three optimized compositions were produced and experimentally evaluated through casting, forging, and rolling processes, with their microstructures and mechanical properties compared to industry standards Nimonic 263, Waspaloy, and 740H. Weldability assessment included solidification cracking and stress relaxation cracking tests, while hot hardness measurements provided strength screening. The study evaluates both the effectiveness of the ICME design methodology and the practical potential of these cost-effective η-phase strengthened alloys as replacements for traditional nickel superalloys in advanced energy applications.

Oxyfuel combustion efforts to burn fossil fuels with oxygen, for easier post-combustion CO 2 capture, include schemes to use flue gas to drive turbines for power generation. The environment examined here is 10% CO 2 and 0.2% O 2 , with the balance being steam, with temperatures ranging from 630 to 821 °C. The relatively high C and O 2 activities of this environment, as compared to pure steam, may lead to changes in oxidation behavior and mechanical properties. Oxidation coupons of Ni- and Co-base superalloys, in both bare metal and TBC coated conditions, were exposed to this environment for up to 1000 hours. The results of these exposures, in terms of mass gain and scale morphology, are presented.

Model alloys of Fe-20Cr and Ni-20Cr (all compositions in weight %) and variants containing small amounts of Si or Mn were exposed to Ar-20CO 2 and Ar-20CO 2 -H 2 O (volume %) at 650 or 700°C. Protective Cr 2 O 3 scale was more readily formed on Fe-20Cr than Ni-20Cr, as a result of the different alloy diffusion coefficients. Silicon additions slowed chromia scale growth, promoting passivation of both alloy types. Water vapour accelerated chromia scaling, but slowed NiO growth.

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.

A thorough understanding of interactions between graphite and fluoride fuel salts is crucial, as graphite is a promising candidate for the moderator of molten salt reactors. This study investigates the infiltration of fluoride fuel salts into graphite and the fluorination of graphite by these salts under various pressures and temperatures. A high-pressure salt infiltration test apparatus was developed to examine the infiltration of NaF-KF-UF 4 and NaF-BeF 2 -UF 4 -ZrF 4 fuel salts into two types of graphite at high temperatures. For tests using NaF-BeF 2 -UF 4 -ZrF 4 , two different temperatures were selected to assess the impact of temperature on threshold pressure. The study observed salt infiltration into graphite at pressures exceeding its threshold pressure, and the threshold pressure for infiltration was lower at the higher temperature. In addition, the formation of carbon fluorides on the surface of post-test graphite specimens was identified.