Material properties and damage mechanisms exhibit significant variation across weldments. Micro tensile (MT) testing of specimens machined from specific narrow weldment zones is one method to characterize local property variation. Although limited, the literature data on micro-tensile specimen testing reports on low-temperature behavior. However, cross-weld local material data at high service temperatures have not been reported yet. In the present study, MT tests are conducted across similar P22 and P91 steel welds at 550°C and 600°C, respectively. To study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (machined, polished, and electropolished) are tested. Two different loading rates of 0.2 mm/min and 0.5 mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations in weldment material properties are presented as a function of specimen surface conditions and loading speeds. Deformation behavior is studied on the side surfaces of tested micro-tensile specimens using SEM. Deformation is correlated to the microstructural constituent observed on side surfaces.

The efforts of the European Union and Germany in particular to realize the transformation towards a climate-neutral economy over the coming decades have the establishing of a hydrogen economy as a fundamental milestone. This includes production, import, storage, transportation and utilization of great amounts of gaseous hydrogen in existing and new infrastructure. Metallic materials, mainly steels, are the most widely used structural materials in the various components of this supply chain. Therefore, the accelerated use of hydrogen requires the qualification of materials (i.e., ensuring they are hydrogen-ready) to guarantee the sustainable and safe implementation of hydrogen technologies. However, there is currently no easily applicable and standardized method to efficiently determine the impact of gaseous hydrogen on metallic materials. The few existing standards describe procedures that are complex, expensive, and only available to a limited extent globally. This article outlines the key milestones towards standardizing an efficient testing method as part of the TransHyDE flagship project. This new approach enables testing of metallic materials in gaseous hydrogen using tubular specimens. It uses only a fraction of the hydrogen required by the traditional autoclave method, significantly reducing costs associated with technical safety measures. Among the topics to be discussed are the factors influencing the test procedure, including geometrical considerations, surface quality, gas purity and strain rate.

Welding processes and fabrication techniques have been studied in the development of Advanced USC boilers. Advanced 9Cr steels, Fe-Ni alloy (HR6W) and Nickel base alloys (HR35, Alloy 617, Alloy 263, Alloy 740 and Alloy 740H) have been selected as candidate materials for the boiler. The weld joints of these alloys were prepared from plates, small diameter tubes and large pipes, and welding procedure tests were performed. In this study, TIG and SMAW were applied. Both welding process produced good weld joints, and they showed good results in bending tests, tensile tests and the Charpy impact test. To select the annealing conditions for stress relief, stress relaxation tests and hardness tests were conducted on the weld joints after various heat treatments. The microstructure was also evaluated by SEM and TEM. Creep rupture tests are being performed for the weld joints with and without heat treatment. The maximum creep rupture tests are expected to take over 100,000 hours. In the study of fabrication techniques, hot bending tests by high frequency induction heating for large pipes and cold/hot bending tests for small diameter tubes were established. After the bending tests, mechanical property tests such as tensile tests, impact tests and creep rupture tests were conducted. The effect of pre-strain on creep strength was studied to take the creep test results after bending into consideration. The creep rupture test will be continued for specimens from weld joints and bending pipes to show their long term reliability.

Gas turbine efficiency is typically limited by the maximum allowable temperature for components at the inlet side and in the hot gas flow. Refractory alloys and SiC/SiC ceramic-matrix composites (CMCs) are promising candidates for advancing operating temperatures beyond those of Ni-based alloys (>1200 °C). Refractory alloys are more suitable than SiC/SiC CMCs for dynamic components, due to the latter's low toughness and ductility. However, it is well known that refractory alloys suffer from poor oxidation behavior under service lifetimes and conditions, leading to embrittlement concerns. The ARPA-E ULTIMATE program has set out to combine new alloys with advanced coatings to mitigate oxidation/embrittlement effects, while increasing the mechanical performance benefits of refractory materials. Low oxygen (inert gas) or vacuum systems are needed to assess high temperature mechanical performance of developed alloys. To investigate the environmental sensitivity of candidate alloys and develop high temperature testing capabilities, four argon tensile frames were upgraded as well as a single vacuum system at Oak Ridge National Laboratory. Digital image correlation was incorporated into the vacuum frame allowing for surface strain determination and refined insight into thermomechanical response. Creep testing was performed at 1300 °C on two alloys, C-103 and MHC in vacuum and high purity argon environments. The Mo-based alloy showed less sensitivity to oxygen, indicating that testing in well-controlled argon environments may be suitable. The C-103 alloy demonstrated a stronger sensitivity to oxygen in the well-controlled argon environment, illustrating the need for the developed vacuum testing capabilities. “Small” 25 mm and “large” 76 mm MHC specimens showed comparable results in terms of strain rate during creep testing and ultimate tensile strength during tensile testing, suggesting the viability of smaller geometries that use less material of advanced developmental alloys.

This study investigates the impact of adding small amounts of rare earth (RE) elements on the properties and microstructures of SA335P91 steel welds. The RE elements were incorporated into the weld metal using a coating process. The researchers then proposed an optimal RE formula aimed at achieving improved properties and microstructures. To evaluate the effectiveness of this approach, various tests were conducted on both welds with and without RE additions. These tests included tensile testing (both at room and high temperatures), impact testing, metallographic analysis to examine the microstructure, determination of phase transformation points, scanning electron microscopy, and X-ray diffraction. The results revealed that the addition of RE elements has the potential to enhance the properties and modify the microstructure of SA335P91 welds.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

By utilizing computational thermodynamics in a Design of Experiments approach, it was possible to design and manufacture nickel-base superalloys that are strengthened by the eta phase (Ni3Ti), and that contain no gamma prime (Ni3Al,Ti). The compositions are similar to NIMONIC 263, and should be cost-effective, and have more stable microstructures. By varying the aging temperature, the precipitates took on either cellular or Widmanstätten morphologies. The Widmanstätten-based microstructure is thermally stable at high temperatures, and was found to have superior ductility, so development efforts were focused on that microstructure. High temperature tensile test and creep test results indicated that the performance of the new alloys was competitive with NIMONIC 263. SEM and TEM microscopy were utilized to determine the deformation mechanisms during creep.

CrMoV cast steels are widely utilized for steam turbine and valve casings, and are subjected to operating and loading conditions which can promote damage mechanisms such as thermal fatigue, creep, erosion, etc. These components are subjected to variable, and sometimes severe conditions because of flexible operation. Therefore, there is a growing need for weld repair techniques including those which do not mandate post weld heat treatment (PWHT), e.g. so-called ‘temper bead’ weld repair. In this study, a simulated weld repair was performed using a temper bead technique. The maximum hardness in the heat affected zone (HAZ) CrMoV steel was ≤400HV. The integrity of the repair methodology was investigated using destructive testing, including hardness mapping, Charpy impact tests, tensile tests, low cycle fatigue and cross-weld creep, and the microstructure was assessed using light optical microscopy and scanning electron microscopy (SEM).

This study examines the deformation behavior of P92 steel (ferritic, 9% Cr) at high temperatures (600°C to 900°C) using isothermal hot tensile tests. Particular focus is placed on the stress-strain behavior around its alpha-gamma transition temperature (825°C). Additionally, fire accident simulation heating tests were conducted to assess the integrity of P92 beyond 650°C (relevant for short-term creep) and compare it to stainless steel 1.4404 (potential building material). Finally, microstructural analysis was performed on tested samples, revealing that the martensitic structure with characteristic laths was retained at temperatures up to 750°C.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

High temperature strength of a nickel-based superalloy, Alloy 740H, was investigated to evaluate its applicability to advanced ultrasupercritical (A-USC) power plants. A series of tensile, creep and fatigue tests were performed at 700°C, and the high temperature mechanical properties of Alloy 740H was compared with those of other candidate materials such as Alloy 617 and Alloy 263. Although the effect of the strain rate on the 0.2% proof stress was negligible, the ultimate tensile strength and the rupture elongation significantly decreased with decreasing strain rate, and the transgranular fracture at higher strain rate changed to intergranular fracture at lower strain rate. The time to creep rupture of Alloy 740H was longer than those of Alloy 617 and Alloy 263. The fatigue limit of Alloy 740H was about half of the ultimate tensile strength. Further, Alloy 740H showed greater fatigue strength than Alloy 617 and Alloy 263, especially at low strain range.

T24 tube material (7CrMoVTiB10-10), with its combination of high creep strength and potential to be welded without using preheat, is regarded as a candidate waterwall material for Ultra Supercritical (USC) boilers. However, its reputed sensitivity to hydrogen and potential for secondary hardening may have adverse impacts on construction of waterwall panels. Doosan Babcock Ltd have investigated the response of welds made in T24 tubing to secondary hardening via changing hardness in a series of ageing heat treatment trials. Also, the response of the material to hydrogen infusion has been investigated

Carbon Capture and Storage (CCS) has become promising technology to reduce CO 2 emissions. However, as a consequence of CCS installation, the electrical efficiency of coal fired power plant will drop down. This phenomenon requires increase in base efficiency of contemporary power plants. Efficiency of recent generation of power plants is limited mainly by maximum live steam temperature of 620°C. This limitation is driven by maximal allowed working temperatures of modern 9–12% Cr martensitic steels. Live steam temperatures of 750°C are needed to compensate the efficiency loss caused by CCS and achieve a net efficiency of 45%. Increase in the steam temperature up to 750°C requires application of new advanced materials. Precipitation hardened nickel-based superalloys with high creep-rupture strength at elevated temperatures are promising candidates for new generation of steam turbines operating at temperatures up to 750°C. Capability to manufacture full-scale forged rotors and cast turbine casings from nickel-based alloys with sufficient creep-rupture strength at 750°C/105 hours is investigated. Welding of nickel-based alloys in homogeneous or heterogeneous combination with 10% Cr martensitic steel applicable for IP turbine rotors is shown in this paper. Structure and mechanical properties of prepared homogeneous and heterogeneous weld joints are presented.

Metallographic tests, micro-hardness tests, mechanics performance tests and Energy Dispersion Spectrum (EDS) were conducted for a 2.25Cr-1Mo main steam pipe weldment served for more than 32 years. Microstructural evolution of the 2.25Cr-1Mo base metal and weld metal was investigated. Degradation in micro-hardness and tensile properties were also studied. In addition, the tensile properties of subzones in the ex-service weldment were characterized by using miniature specimens. The results show that obvious microstructural changes including carbide coarsening, increasing inter lamella spacing and grain boundary precipitates occurred after long-term service. Degradation in micro-hardness is not obvious. However, the effects of long term service on tensile deformation behavior, ultimate tensile strength and yield stress are remarkable. Based on the yield stress of micro-specimens, the order of different subzones is: WM>HAZ>BM, which is consistent with the order of different subzones based on micro-hardness. However, the ultimate tensile strength and fracture strain of HAZ are lower than BM.

Development of steels used in the power generation industry for the production of boilers characterized by supercritical parameters poses new challenges. The introduction of new combinations of alloying agents aimed at obtaining the best possible mechanical properties, including creep resistance, affects the weldability of new steels. Each of the latter has to undergo many tests, particularly as regards bending and welding, in order to enable the development of technologies ensuring failure-free production and assembly of boiler systems. Martensitic steels containing 9% Cr, used in the manufacturing of steam superheaters, are characterized by excellent creep resistance and, at the same time, low oxidation resistance at a temperature in excess of 600°C. In turn, steels with a 12% Cr content, i.e., VM12-SHC or X20CrMoV12-1 are characterized by significantly higher oxidation resistance but accompanied by lower strength at higher temperatures, which translates to their limited application in the production of boilers operating at the most top parameters.X20CrMoV12-1 was withdrawn from most of the power plants, and VM12-SHC was supposed to replace it, but unfortunately, it failed in regards to creep properties. To fulfill the gap a new creep strength-enhanced ferritic steel for service in supercritical and ultra-supercritical boiler applications was developed by Tenaris and it is designated as Thor115 (Tenaris High Oxidation Resistance). This paper covers the experience gained during the first steps of fabrication, which includes cold bending and welding of homogenous joints.

As flux cored wires for gas metal arc welding offer several technical and economic advantages they are becoming more and more popular. Matching flux cored wires for welding P92 have already been available for several years. A matching flux cored wire for welding the Co-alloyed cast steel CB2, which is used for turbine and valve casings operating at steam temperatures of up to 620°C, was developed recently. To connect casings with P92 pipes, dissimilar welding of CB2 to P92 is necessary. This can be done with filler metal that matches either CB2 or P92. Pre-tests have confirmed that flux cored arc welding (FCAW) can generally be used for dissimilar joint welding of CB2 to P92. To evaluate creep rupture strength dissimilar welds were performed with filler metal matching CB2 and P92, respectively. TIG welding was used for the root and the second pass and FCAW for the intermediate and final passes. Cross-weld tensile tests, side bend tests and impact tests of weld metals and heat-affected zones were carried out at ambient temperatures after two post-weld heat treatments (PWHT), each at 730°C for 12 hours. Creep rupture tests of cross-weld samples were performed at 625°C. This study compares the results of the mechanical tests at ambient temperature and the creep rupture tests, and discusses why P92 filler metals are preferred for such welds.

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.

Hydrogen as a clean fuel is increasingly being used to propel gas turbines and to power combustion engines. Metallic materials including Ni-based alloys are commonly used in conventional gas turbines and combustion engines. However, hydrogen may cause embrittlement in these materials, depending on their chemical composition. In this work, the hydrogen embrittlement behavior of Ni-based alloys containing up to 50 wt.% Fe has been investigated using slow strain rate tensile testing, under cathodic hydrogen charging at room temperature. It was found that the larger the Ni equivalent concentration in an alloy, the more severe the hydrogen embrittlement. It was also found that solid solution alloys have less severe hydrogen embrittlement than precipitation alloys of the same Ni equivalent concentration. In solid solution alloys, hydrogen embrittlement led to cleavage type fracture, which is in line with literature where hydrogen enhanced planar deformation. In precipitation alloys, hydrogen embrittlement resulted in a typical intergranular fracture mode.

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in grade 91 steel. Failed hot tensile specimens have been sectioned at various positions along the specimen axis, and therefore at different levels of hot tensile strain, to obtain material for creep strength evaluation. Because of the limited amount of material available for creep testing obtained in this way, creep testing has been carried out using the specialised small-scale impression creep testing technique. The grade 91 material has been tested in both the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% Ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant with grade 91 pipework systems.

Advanced Ultra-Super-Critical (A-USC) technology is one of the remarkable technologies being developed to reduce CO 2 emissions. The 700°C class A-USC steam turbine project was launched in 2008 to contribute to substantial reductions in CO 2 emissions and major Japanese manufacturers of boilers and turbines joined forces with research institutes to bring the project to reality. The use of Ni-base alloys is necessary for high temperature component of 700°C class AUSC steam turbine, and which is required increasing in size of Ni-base casting alloys to apply inner casing, valve body, nozzle block and so on. Therefore, trial production and verification test of Step block (weight: 1.7 ton) with actual component thickness 100-300mm were firstly performed to investigate basic casting material properties in this study. As candidate alloy, alloy 617 was chosen from a commercially available Ni-base alloy, from the viewpoint of large component castability and balance of mechanical properties stability at 700°C use. Microstructure test, high temperature mechanical test and long-term heating test of each thickness part specimen were carried out and good creep rupture strength was obtained. Next, the nozzle block of alloy 617 was manufactured for the trial casting of the actual machine mock-up component with complex shape (weight: 1.2 ton). For a comparison, alloy 625 was cast at the same time. Both castings of alloy 617 and alloy 625 were able to manufacture without a remarkable defect. Detailed comparisons to microstructures and mechanical properties are included in this paper.