Laves phases are intermetallic phases well known for their excellent strength at high temperatures but also for their pronounced brittleness at low temperatures. Especially in high-alloyed steels, Laves phases were long time regarded as detrimental phases as they were found to embrittle the material. Perusing the more recent literature, it seems the negative opinion about the Laves phases has changed during the last years. It is reported that, if the precipitation morphology is properly controlled, transition metal-based Laves phases can act as effective strengthening phases in heat resistant steels without causing embrittlement. For a targeted materials development, the mechanical properties of pure Laves phases should be known. However, the basic knowledge and understanding of the mechanical behavior of Laves phases is very limited. Here we present an overview of experimental results obtained by micromechanical testing of single-crystalline NbCo 2 Laves phase samples with varying crystal structure, orientation, and composition. For this purpose, diffusion layers with concentration gradients covering the complete homogeneity ranges of the hexagonal C14, cubic C15 and hexagonal C36 NbCo 2 Laves phases were grown by the diffusion couple technique. The hardness and Young's modulus of NbCo 2 were probed by nanoindentation scans along the concentration gradient. Single-phase and single crystalline microcantilevers and micropillars of the NbCo 2 Laves phase with different compositions were cut in the diffusion layers by focused ion beam milling. The fracture toughness and the critical resolved shear stress (CRSS) were measured by in-situ microcantilever bending tests and micropillar compression tests, respectively. The hardness, Young's modulus and CRSS are nearly constant within the extended composition range of the cubic C15 Laves phase, but clearly decrease when the composition approaches the boundaries of the homogeneity range where the C15 structure transforms to the off stoichiometric, hexagonal C36 and C14 structure on the Co-rich and Nb-rich, respectively. In contrast, microcantilever fracture tests do not show this effect but indicate that the fracture toughness is independent of crystal structure and chemical composition of the NbCo 2 Laves phase.

For VM12-SHC 11-12 wt. % Cr steel, there have been no systematic investigations to define the regions or characterise the microstructures within the heat-affected zone (HAZ) of weldments. In similar steels, these regions relate to the Ac 1 and Ac 3 transformation temperatures and can affect weldment performance. In this study, controlled thermal cycles were applied to VM12-SHC parent metal using a dilatometer and the Ac 1 and Ac 3 temperatures were measured for various heating rates. The Ae 1 and Ae 3 temperatures were also calculated by thermodynamic equilibrium modeling. Through dilatometry, thermal cycles were then applied to simulate the microstructures of the classically defined HAZ regions. The microstructural properties of each simulated material were investigated using advanced electron microscopy techniques and micro-hardness testing. It was found that the simulated HAZ regions could be classified as; (1) the completely transformed (CT) region, with complete dissolution of pre-existing precipitates and complete reaustenitisation; (2) the partially transformed (PT) region, exhibiting co-existing original martensite with nucleating austenite microstructures with partial dissolution of precipitates; and (3) the over tempered (OT) region, with no phase transformation but precipitate coarsening and decreased hardness.

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.

The global electric power production is largely dependent on the operation of fossil-fired generation units. Many coal-fired units are exceeding 300,000 hours, which is beyond the expected design life. This has caused a continuous need to inspect steam touched components operating at high temperature and pressure. State-of-the-art coal and combined cycle gas units are specifying ever-greater amounts of the Creep Strength Enhanced Ferritic (CSEF) steels such as Grade 91 or Grade 92. The martensitic 9%Cr CSEF steels were developed to provide greater strength than traditional low alloy power plant steels, such as Grades 11, 12 and 22. The enhanced strength allows for a reduction in overall wall thickness in new or replacement components. Extensive research in both service failures and laboratory testing has shown that time-dependent creep damage can develop differently in Grade 91 steel when compared to low alloy steels. Furthermore, the creep strength in Grade 91 can vary by more than a factor of 10 between different heats. This wide variation of creep strength has led to extensive research in understanding the damage mechanisms and progression of damage in this steel. In this study, large cross weld samples were fabricated from thick wall piping in Grade 91 steel using two different heats of material. One weld was fabricated in a ‘damage tolerant’ heat and another weld was fabricated in a ‘damage intolerant’ heat of material. The samples were subjected to a post-weld heat treatment (PWHT) at a temperature of 745°C (1375°F) for 1.50 hours. Hardness maps were collected on the cross-welds in the as-welded and PWHT condition for both weldments. Cross-weld creep test conditions were selected to develop accelerated damage representative of in-service behavior. The test samples were interrupted at multiple stages and nondestructively evaluated (NDE) with advanced phased-array ultrasonic techniques. Samples were developed to variable levels of damage (50% to 100% life fraction) in both weldments. Metallographic sections were extracted at specific locations to validate the NDE findings using light emitting diode, laser and scanning electron microscopy. This research is being used to help validate the level of damage that can be reliably detected using conventional and advanced NDE techniques.

Ni-38Cr-3.8Al has high hardness and high corrosion resistance with good hot workability, and therefore, it has been applied on various applications. However, in order to expand further application, it is important to understand the high temperature properties. Then, this study focused on the high temperature properties such as thermal phase stability, hardness, tensile property, creep property and hot corrosion resistance. As the result of studies, we found that the thermal phase stability of (γ/α-Cr) lamellar structure and the high temperature properties were strongly influenced by the temperature. Although the high temperature properties, except for creep property, of Ni-38Cr-3.8Al were superior to those of conventional Ni-based superalloys, the properties were dramatically degraded beyond 973 K. This is because the lamellar structure begins to collapse around 973 K due to the thermal stability of the lamellar structure. The hot corrosion resistance of Ni-38Cr-3.8Al was superior to that of conventional Ni-based superalloys, however, the advantage disappeared around 1073 K. These results indicate that Ni-38Cr-3.8Al is capable as a heat resistant material which is required the hot corrosion resistance rather than a heat resistant material with high strength at high temperature.

In this study, creep rupture behaviors and rupture mechanisms of dissimilar welded joint between Inconel 617B and COST E martensitic steel were investigated. Creep tests were conducted at 600 ℃ in the stress range 140-240 MPa. Scanning electron microscopy (SEM) and micro-hardness were used to examine the creep rupture behaviors and microstructure characteristics of the joint. The results indicated that the rupture positions of crept joints shifted as stress changed. At higher stress level, the rupture position was located in the base metal (BM) of COST E martensitic steel with much plastic deformation and necking. At relatively lower stress level, the rupture positions were located in the fine-grained heat affected zone (FGHAZ) of COST E or at the interface between COST E and WM both identified to be brittle fracture. Rupture in the FGHAZ was caused by type Ⅳ crack due to matrix softening and lack of sufficient precipitates pinning at the grain boundaries (GBs). Rupture at the interface was related to oxide notch forming at the interface.

This study aims to examine the effects of grain boundary oxidation and creep on crack initiation and fracture behaviors in cold worked surface layer, under static tensile stresses in air. To determine these effects in relation to percent cold work and hardness scale, cold-rolled plates with a reduction ratios between 10% and 50% were prepared. Uniaxial constant load (UCL) tests were conducted at elevated temperature in air using smooth round bar specimen. UCL tests with a load of 0.9σy (926MPa) at 550°C show that rupture time for all cold- rolled materials were shorter than that of as-received material. From cross-sectional observation after UCL testing, surface crack at grain boundary and voids were observed in as-received material, whereas creep cracks were also observed in cold-rolled materials. This implied that crack initiation was assisted by cold working. Comparing test results with a load reduced to 0.8σy (823MPa), difference of rupture time was expected as a factor of 5 for as-received material, and measured as 2-3 for cold-rolled materials. It was suggested that cold worked layer was more sensitive to creep than base metal.

The harsh operating conditions of Advanced Ultra-Supercritical (A-USC) power plants, i.e., steam operation conditions up to 760°C (1400°F)/35 MPa (5000 psi), require the use of Ni-based alloys with high temperature performance. Currently, the U.S. Department of Energy Fossil Energy program together with Electric Power Research Institute (EPRI) and Energy Industries of Ohio (EIO) is pursuing a Component Test (Comets) project to address material- and manufacturing-related issues for A-USC applications. Oak Ridge National Laboratory (ORNL) is supporting this project in the areas of mechanical and microstructure characterization, weld evaluation, environmental effect studies, etc. In this work, we present results from these activities on two promising Ni-based alloys and their weldments for A-USC applications, i.e., Haynes 282 and Inconel 740H. Detailed results include microhardness, tensile, air and environmental creep, low cycle fatigue, creep-fatigue, environmental high cycle fatigue, and supporting microstructural characterization.

Creep properties and microstructural changes of 25Cr-20Ni-Nb-N steel (KA-SUS310J1TB) were investigated. Creep tests were performed under 20MPa to 380MPa at 600°C to 800°C. Time to rupture was from 53.5h to 23950h. At 650°C or higher, creep strength degraded in the long-term. Rupture elongation and reduction of area decreased with increasing time to rupture at 600°C to 800°C. The reduction of area was lower than 12% after creep rupture for more than 10000h. Creep voids and cracks were observed on grain boundaries in creep ruptured samples. The hardness of head portion of creep ruptured samples increased with increasing time to rupture at 600°C to 800°C. The hardness of gauge portion of creep ruptured samples was higher than that of as received sample. However, the hardness of gauge portion does not strongly depend on time to rupture. No precipitates were observed in as received sample. On the other hand, a large number of precipitates were confirmed after creep rupture at 600°C to 800°C. M 23 C 6 , sigma phase, eta nitride and Z phase were detected in creep ruptured samples. The precipitation was confirmed on grain boundaries after short-term creep. The precipitates were also formed inside grains after long-term creep. It was confirmed by optical microscope that the grain boundary seemed to have band-like structure after short-term creep exposure. The Cr depletion zone was detected around grain boundary after short-term creep exposure. The Cr depletion zone can be visible when Cr rich precipitates such as M 23 C 6 and sigma phase are formed on grain boundaries. However, the bandlike structure was not observed after long-term creep exposure because the Cr depletion zone became unclear after long-term creep exposure. Creep voids were formed on grain boundaries and at the interface between precipitates such as M 23 C 6 and sigma phase and matrix.

The broken elbow of the final superheater tube (ASME SA213 TP304H) from a coal-fired power plant was evaluated. The root causes were identified by metallographic observation, sensitization evaluation, hardness measurement, and EBSD analysis. The analysis results reached the following conclusions. (1) The tube bending was not performed in accordance with ASME Code requirements—a solid-solution heat treatment was not performed after cold working. (2) The hardness at the elbow is greater than 260 HV, exceeding the ASME code limit. (3) The sensitization was 19%, showing a performance degradation. (4) There are no obvious corrosion elements in the oxide layers of the cracks. (5) Metallographic microstructure analysis shows that there are many intergranular cracks and carbides such as Cr-rich phase and Fe-Cr are precipitated at the grain boundaries, ultimately resulting in strain-induced precipitation hardening damage.

Nimonic 263 alloy was selected for gas turbine combustor transition piece due to its excellent high temperature mechanical performance. In present work, Nimonic 263 alloy plate with thickness of 5mm was welded using 263 filler metal by GTAW, then post weld heat treatment of 800℃/8h/air cool was carried out. The hardness and impact toughness of welded joints were measured, and the microstructure evolution after aging at 750℃ for 3000h was investigated by scanning electron microscopy(SEM). The results show that, during the aging process, the hardness of weld metal increases firstly and then decreases. The impact toughness decreases significantly at first and then increase. Furthermore, some fluctuations can be detected in hardness and impact toughness after long-term thermal exposure. The significant decrease in the impact toughness of the aged welded joints mainly results from the precipitation of η phase around grain boundary and intergranular MC phase. The hardness of weld metal increases due to the precipitation of more carbides and γ′ phase after 1000h aging, then decreases owing to the growth of γ′ phase after 3000h aging.

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).

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.

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.

In this study, 25Cr2Ni2Mo1V filler metal was deposited to weld low pressure steam turbine shafts, which are operated in fossil power plants. A comparison experiment was conducted on the weld metals (WMs) before and after varied various aging duration from 200 hours up to 5000 hours at 350 ℃. Microstructure was characterized by means of scanning electron microscopy (SEM) and electron back-scattered diffraction (EBSD) techniques. In addition, mechanical properties of corresponding specimens were evaluated, e.g. Vickers microhardness, Charpy V impact toughness and tensile strength. It is shown that the tensile strength remained stable while impact energy value decreased with increasing aging duration. Based on the experiment above, it was concluded that the variation of mechanical properties can be attributed to the redissolution of carbides and reduction of bainite lath substructure.

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 efficiency of power plants is depending on the steam temperature and/or the steam pressure. Efficiency increasing from 35% to 42-45% require increasing of the steam temperature over 600°C and the pressure over 26 MPa. According to the designer opinion it is not profitable to use classical low alloy creep resistant steels 16Mo3, 13CrMo4-5 or 10CrMo9-10 for membrane waterwall construction for these service condition. New modified low alloy creep resistance T23 and T24 (7CrMoVTiB10-10) steels were developed for membrane waterwalls. Welding of these steels with small thickness (around 6.3 mm) should be enabled without preheating and post weld heat treatment (PWHT) due to the lower carbon content below 0.1%. High creep rupture strength (CRS) values are achieved by Ti, N and B elements alloyed to T24 steel. The original expectation that the welding small thickness without preheating was early overcome and was wrong. According to the present experience the T24 steel is welded with preheating at 150-250°C depending on the wall thickness and welded joint toughness in order to achieve required hardness and impact toughness values. Opinions on the T24 welded joints post weld heat treatment (PWHT) requirements are still inconsistent. Especially the membrane waterwalls of the supercritical power plants are still produced without PWHT.

In Europe between 2006 and 2012 several ultra-super-critical (USC) coal-fired power plants were built employing T24 (7CrMoVTiB10-10 / DIN EN 10216-2:2014-03 / VdTÜV sheet 533/2) in membrane walls. During commissioning stress corrosion cracking (SCC) on the tube-to-tube butt welds appeared. The widespread damages required the development of a new patented commissioning procedure to avoid recurring damages. Although this commissioning procedure was employed successfully and the power plants are in operation since then, a debate about the implementation of a hardness limit for such butt welds was initiated. According to the European standards butt welds of T24 boiler tubes with wall thickness < 10 mm (0.3937 in) do not require any post-weld heat treatment (PWHT) and no hardness limits are given. When looking at manufacturing related issues such as an imminent risk of cold cracking after welding of micro-alloyed steels a widely applied but coarse hardness limit is 350 HV. Based on laboratory tests, some authors reallocated this 350 HV hardness limit for addressing SCC susceptibility of low-alloyed steels. This article describes typical hardness levels of T24 boiler tube TIG butt welds and the SCC behavior in high temperature water. Further the effect of the stress relief heat treatment (SRHT) of the boiler membrane walls between 450 °C and 550 °C (842 °F and 1022 °F) on its hardness values and on the SCC behavior is discussed, showing that the hardness values should not be used as an indicator for SCC susceptibility of T24 boiler tube butt welds.

Type IV damage is observed in creep-strength-enhanced ferritic (CSEF) steel used in USC plants and the research on the evaluation of such damage has been carried out in the world. Type I failure is recently reported in welded joint of Gr.91 so that the importance of the evaluation of the creep strength of the weld metal is increasing. In this study, the change in hardness with aging and creep strength before and after aging were evaluated to determine the creep strength of the weld metal of Gr.91. The hardness of the weld metal subjected to aging significantly decreased compared with that of the base metal and the heat-affected zone (HAZ). The creep strength of the weld metal was also decreased by aging. From these results, it is suggested that the failure morphology of Gr.91 steel welded joint used for a long term may change from type IV to type I.

The creep degradation/life assessment for high temperature critical component materials is absolutely needed to assure the long-term service operation and there is little experience with the service exposure of the high temperature components made of newly developed Ni-based alloys. In this study, therefore, the creep degradation assessment study on the Ni-based alloys, Alloy 617 and HR6W was conducted based on the hardness method, because the hardness measurement is a useful and simple technique for the materials characterization for any kind of high temperature-serviced steels and alloys. As the result, it was found that the hardness was increased by not only precipitation due to thermal aging but also creep stress/strain, and there existed linear relationship between the applied stress and creep-induced hardness increase. Also the hardness scatter measured was increased along with the progress of creep hardening and damage progressing in terms of creep life consumed. Those findings suggested that the creep life assessment of Ni-based alloys would be possible by means of hardness measurement. The paper also deals with the role and perspective development of non destructive damage detecting techniques, and life assessment issues on Ni-based alloys for A-USC power applications.