An innovative additively manufactured gradient composite transition joint (AM-GCTJ) has been designed to join dissimilar metals, to address the pressing issue of premature failure observed in conventional dissimilar metal welds (DMWs) when subjected to increased cyclic operating conditions of fossil fuel power plants. The transition design, guided by computational modeling, developed a gradient composite material distribution, facilitating a smooth transition in material volume fraction and physical properties between different alloys. This innovative design seeks to alleviate structural challenges arising from distinct material properties, including high thermal stress and potential cracking issues resulting from the thermal expansion mismatch typically observed in conventional DMWs. In this study, we investigated the creep properties of transition joints comprising Grade 91 steel and 304 stainless steel through a combination of simulations and creep testing experiments. The implementation of a gradient composite design in the plate transition joint resulted in a significant enhancement of creep resistance when compared to the baseline conventional DMW. For instance, the creep rupture life of the transition joint was improved by > 400% in a wide range of temperature and stress testing conditions. Meanwhile, the failure location shifted to the base material of Grade 91 steel. Such enhancement can be primarily attributed to the strong mechanical constraint facilitated by the gradient composite design, which effectively reduced the stresses on the less creep-resistant alloy in the transition zone. Beyond examining plate joints, it is crucial to assess the deformation response of tubular transition joints under pressure loading and transient temperature conditions to substantiate and demonstrate the effectiveness of the design. The simulation results affirm that the tubular transition joint demonstrates superior resistance compared to its counterpart DMW when subjected to multiaxial stresses in tubular structures. In addition, optimization of the transition joint’s geometry dimensions has been conducted to diminish the accumulated deformation and enhance the service life. Lastly, the scalability and potential of the innovative transition joints for large-diameter pipe applications are addressed.

Dissimilar metal welds between T91 ferritic steels and TP347H austenitic alloys are commonly used in fossil power plants in China. Premature failure of such dissimilar welds can occur, resulting in unplanned plant outages that can cause huge economic losses. In this article, microstructural evolution of T91/TP347H dissimilar welds after different service conditions were studied, mechanical properties before and after service were also analyzed, a full investigation into the failure cause was carried out. The results show, the dissimilar metal welds in the as-welded condition consists of a sharp chemical concentration gradient across the fusion line, failure is attributed to the steep microstructural and mechanical properties gradients, formation of interfacial carbides that promote creep cavity formation.

In this paper, the dissimilar metal welds (DMWs) between 617B nickel-based alloy and 10%Cr martensitic heat-resistant steel filled by 617 filler metal was studied, focused on the high temperature creep rupture properties. The high temperature creep rupture properties of welded joints with different welding processes were tested, and the microstructure of welded joints before and after the creep rupture test was observed by OM and SEM. The results showed that, there were three failure modes: base metal failure, type W failure and interface failure, among which interface failure caused the most serious life reduction. The welded joints using ER NiCr-3 filler metal reduced the strain concentration at the interface, so the fracture location shifted from the interface to HAZ of 10%Cr martensitic heat-resistant steel under high temperature and low stress conditions, and creep rupture life was improved. Similarly, weld cap shifted the creep crack propagation path by changing the groove form, so as to altered the stress state of joint and prolong the creep rupture life.

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.

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.

MoSiBTiC alloy is a promising material for advanced aerospace applications and next generation high pressure turbine blades in jet engines and gas turbines. It mainly consists of Mo solid solution, TiC and Mo 5 SiB 2 phases and has creep strength much stronger than Ni-base superalloys and better than SiC/SiC ceramic matrix composites. Furthermore, the fracture toughness of the alloy is much better (>15 MPa(m) 1/2 ) than Mo-Si-B ternary alloys (<10 MPa(m) 1/2 ) even if the volume fraction of Mo solid solution is less than 50 %. The improvement of fracture toughness would be caused not only by the continuity of Mo solid solution in solidification microstructure but also by TiC phase affecting as a fracture-resistant phase. In order to understand the microstructure evolution during solidification and the effect of TiC phase on the fracture toughness of the MoSiBTiC alloy, Mo-Ti-C ternary model alloys are dealt with in this study. Then, (1) liquidus surface projection and (2) isothermal section and the elastic moduli of TiC phase in equilibrium with Mo solid solution were focused on. The obtained liquidus surface projection suggests that the ternary transition peritectic reaction (L+ Mo 2 C->Mo+TiC) takes place in Mo-rich region. At 1800 °C, TiC phase in equilibrium with Mo phase contains at least 20.2 at% Mo and the Mo/TiC/Mo 2 C three phase region should exist around Mo-15Ti-10C.

Advanced martensitic 9% chromium steels have been identified as the most favored group of materials for high temperature applications in thermal power plants. To extend the temperature range of martensitic steels up to 650°C large effort was put on the development of new alloy concepts. The so-called MARBN concept (Martensitic steel with defined Boron/Nitrogen relation) provides increased creep rupture strength due to higher solid solution strengthening and improved microstructural stability. The major improvement is the reduction of type IV cracking in welded joints, which shifts the focus to the creep rupture strength of the weld metal. This paper illustrates the process experience of the steel foundry for production of heavy cast components in latest state of the art 9-12%Cr-MoCoVNbNB-alloyed cast steel grades and the newest state of development and prototype components in MARBN cast steel grades. Metallurgy, solidification, heat treatment and welding are main items to be considered for development of new, complex steel grades for foundry processing with the help of empiric processing in test programs and thermo-physical simulation. As welding is an essential processing step in the production of heavy steel cast components a good out-of-position weldability is required. Moreover a stress-relieve heat-treatment takes place subsequently after welding for several hours. This contribution also deals with the development of matching welding consumables for the production of heavy cast components and discusses the challenges of defining appropriate welding and heat treatment parameters to meet the requirements of sufficient strength and toughness at ambient temperature. Additionally, first results of creep rupture tests are presented.

9-12%Cr martensitic steels can be applied to the next highest temperature components such as boiler tracts, steam pipelines and turbines of advanced ultra-supercritical power plants with steam temperatures of 650°C. New 10%Cr martensitic steels with high B and low N contents can be a worthy candidate for use in production because it has superior creep resistance. At the same time, resistance to cyclic and dynamic loads is very important. In this work, we studied the low cycle fatigue (LCF) properties at room and elevated (500-650°C) temperatures and Charpy impact toughness at temperatures ranging from -196…100°C of advanced 10% Cr martensitic steel with high B and low N contents. The effect of new alloying scheme and corresponding peculiarities of M 23 C 6 carbides on the low cycle fatigue resistance and impact toughness of the 10%Cr martensitic steel is analyzed. It is revealed that fine and densely distributed carbides has no effect on the fatigue resistance except for the slight improvement of fatigue life at small strain amplitudes and shift the ductile-brittle transition temperature (DBTT) to higher but satisfactory value of +10°C as compared to other high-chromium martensitic steels.

Key components within gas turbines, such as the blades, can be susceptible to a range of degradation mechanisms, including hot corrosion. Hot corrosion type mechanisms describe a sequence of events that include the growth and fluxing of protective oxide scales followed by the degradation of the underlying coating/alloy; this can significantly reduce component lifetimes. To better understand the progress of this type of damage mechanism, a model of hot corrosion progression with both time and corrosive deposit flux is presented for IN738LC and compared to experimental test data collected at 700 °C for four different deposit fluxes. One approach to the interpolation of model parameters between these four fluxes is illustrated. Of particular importance is that the model accounts for the statistical variation in metal loss though the use of Weibull statistics.

The fatigue crack propagation thresholds of SAW weld metal of 25Cr2Ni2MoV simulating product of fossil and nuclear power low pressure turbine rotor at different stress ratios are tested. There is a big dispersity of the test results, even at the same stress ratio. The double logarithm curves of the fatigue crack growth rate and stress intensity factor range are researched. The difference of critical points between stable propagation region and near-threshold region in different specimens is found to be an important cause to the dispersity. Their locations in the specimens can be determined by the method of backward inference. After the observation of the microstructures around the critical points, a good correspondence between the size of prior austenite grain and the maximum size of monotonic plastic zone on the crack tip is confirmed. The difference of the critical points at the same stress ratio is caused by the inhomogeneous microstructures. So the inhomogeneous microstructures in the multi-pass and multi-layer weld metal contribute to the dispersity of the experimental threshold values.

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M 23 C 6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23 C 6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M 23 C 6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M 23 C 6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

Early supercritical units such as American Electric Power (AEP) Philo U6, the world’s first supercritical power plant, and Eddystone U1 successfully operated at ultrasupercritical (USC) levels. However due to the unavailability of metals that could tolerate these extreme temperatures, operation at these levels could not be sustained and units were operated for many years at reduced steam (supercritical) conditions. Today, recently developed creep strength enhanced ferritic (CSEF) steels, advanced austenitic stainless steels, and nickel based alloys are used in the components of the steam generator, turbine and piping systems that are exposed to high temperature steam. These materials can perform under these prolonged high temperature operating conditions, rendering USC no longer a goal, but a practical design basis. This paper identifies the engineering challenges associated with designing, constructing and operating the first USC unit in the United States, AEP’s John W. Turk, Jr. Power Plant (AEP Turk), including fabrication and installation requirements of CSEF alloys, fabrication and operating requirements for stainless steels, and life management of high temperature components

Olefin furnaces contain gravity cast U-bend fittings from Fe-Ni-Cr alloys that can experience premature failures due to a combination of harsh service conditions. The fittings undergo steep temperature variations during startup and shutdown, outer diameter (OD) oxidation from furnace flue gases, and inner diameter (ID) carburization from process fluids. As a result, cracking often occurs along large solidification grain boundaries from interconnected networks of carbides and secondary phases. To address these degradation concerns, Wire Arc Additive Manufacturing (WAAM) is being used to produce a functionally graded fitting that provides increased oxidation, carburization, creep, and thermal fatigue resistance. Three welding wire compositions have been designed based on thermodynamic and kinetic modeling techniques to address the appropriate corrosion resistance and mechanical properties needed in the OD, Core, and ID regions of the U- bend fitting cross-section. A Fe-35Cr-45Ni-0.7Nb solid welding wire is being used for the Core section, and metal-cored welding wires based around this composition with additions of Si or Al are being used for the OD and ID sections, respectively. This study involved weldability evaluation focused on understanding the microstructures and potential additive manufacturing printability challenges associated with graded WAAM structures using these welding wires. To achieve this, Cast Pin Tear Testing (CPTT) was performed to evaluate solidification cracking susceptibility of the welding wires. Additionally, Scheil calculations were performed in Thermo-Calc software to predict solidification microstructures. To validate the results, SEM characterization was conducted on cast buttons of each welding wire to identify phases in the respective microstructures. These unique data will help inform WAAM design parameters needed to produce a Functionally Graded Material (FGM) that improves the lifetime of Fe-Ni-Cr U-bend fittings in olefin furnaces.?

Either at higher temperatures or when a certain alloying element content is exceeded, γ-TiAl alloys contain the β phase (bcc) or its ordered derivate β o (B2). The relatively soft β phase can facilitate hot deformation, but β o is detrimental for creep strength and ductility. Thus, knowledge about β o →β phase transformation is desirable. Surprisingly, for the binary Ti-Al system it is under discussion whether the ordered β o phase exists. Also, the effect of alloying elements on the β phase ordering is still unclear. In the present work the ordering of the β phase in binary Ti-(39,42,45)Al and ternary Ti-42Al-2X alloys (X=Fe, Cr, Nb, Ta, Mo) which was experimentally investigated by neutron and high energy X-ray diffraction is compared with the results of first principles calculations using density functional theory. Except for Cr the experimentally determined and the predicted behavior correspond.

Damage in the grade 91 steel partially transformed zone of weld heat affected zones has historically been associated with many different types of microstructural features. Features described as being responsible for the nucleation of creep damage include particles such as laves phase, coarse M 23 C 6 , inclusions, nitrides, or interactions between creep strong and creep week grains, grain boundaries and potentially other sources. Few studies have attempted to link the observations of damage on scales of increasing detail from macro, to micro, to nano. Similarly, assessments are not made on a statistically relevant basis using 2D or 3D microscopy techniques. In the present paper, 2D assessment using scanning electron microscopy (SEM) and quantification techniques such as energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) are utilized in combination with 3D serial sectioning of large volumes using plasma focused ion beam milling (P-FIB) and simultaneous EDS to evaluate an interrupted cross-weld creep test. Moreover, the sample selected for examination was from a feature cross-weld creep test made using a parent material susceptible to the evolution of creep damage. The test conditions were selected to give creep brittle behaviour and the sample was from a test interrupted at an estimated life fraction of 60%. The findings from these evaluations provide perspective on the features in the microstructure responsible for the nucleation and subsequent growth of the observed damage.

According to ASME Case N-888-3, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW or Machine GTAW Temper Bead Technique, a 48 hr waiting period before conducting the final nondestructive examination (NDE) is required when ferritic filler weld metal is used. The purpose of the 48 hr hold is to confirm the absence of hydrogen-induced cracking in the temper bead heat-affected zone. In previous research, the effect of post-weld heat treatment (PWHT) and temper bead welding (TBW) on the hydrogen-induced cracking (HIC) susceptibility in the coarse-grained heat-affected zone (CGHAZ) in welds of SA-508, P-No. 3 Group 3, pressure vessel steel was investigated using the Delayed Hydrogen Cracking Test (DHCT). In that previous study, the Gleeble thermomechanical simulator was used to generate six CGHAZ microstructural conditions: as-welded (AW), PWHT, and AW with single a TBW reheat at 675, 700, 725, and 735°C. Hydrogen was introduced to the specimen through cathodic charging under in situ constant tensile stress. The HIC susceptibility for these microstructures was ranked by the DHCT at a diffusible hydrogen level significantly exceeding typical GTAW and SMAW processes. The work described in this paper investigates the susceptibility to HIC of these same CGHAZ microstructures with DHCT at variable current densities, further ranking each condition. Test results were analyzed by fracture surface examination of failed tests, and cross-section microstructural analysis under a scanning electron microscope (SEM). Future steps include evaluating critical hydrogen content levels using gas chromatography for each condition. The results from this study will be used to consider potential elimination of the NDE hold time requirement in Case N-888-3 when ferritic weld metal is used.

High gamma prime Ni-based superalloys comprising ≥3.5 % Al are difficult to weld due to high propensity of these materials to weld solidification, heat affected zone liquation, and stress-strain cracking. In this study the root cause analysis of cracking and overview on the developed weldable Ni-based superalloys for repair of turbine engine components manufactured from equiaxed (EA), directionally solidified (DS), and single crystal (SX) materials as well as for 3D AM is provided. It is shown that the problem with the solidification and HAZ liquation cracking of turbine engine components manufactured from EA and DS superalloys was successfully resolved by modification of welding materials with boron and silicon to provide a sufficient amount of eutectic at terminal solidification to promote self-healing of liquation cracks along the weld - base material interface. For crack repair of turbine engine components and 3D AM ductile LW4280, LW7901 and LCT materials were developed. It is shown that LW7901 and LCT welding materials comprising 30 - 32 wt.% Co produced sound welds by GTAW-MA on various SX and DS materials. Welds demonstrated high ductility, desirable combination of strength and oxidation properties for tip repair of turbine blades. Examples of tip repair of turbine blades are provided.

Xcel Energy’s Comanche Unit 3 experienced widespread cracking of T23 membrane wall tubes within the evaporator section, initially occurring during the boiler construction phase, primarily at shop and field tube butt welds. The majority of the tube cracking was attributed to stress-corrosion cracking (SCC), and a lesser number of fabrication-related hydrogen induced cracking (HIC), weld solidification cracking, and brittle cracking within tube swage sections were also experienced. Hundreds of tubes were replaced prior to Unit commissioning, due to both actual tube leaks and those replaced due to weldment cracking and other identified weld defects during radiographic testing. Elevated stress levels and material susceptibility (i.e. hardness in the as-welded condition) were considered the critical factors in the tube cracking.

Supercritical CO 2 (sCO 2 ) is of interest as a working fluid for several concepts including the direct- fired Allam cycle as a low-emission fossil energy power cycle. Over the past 10 years, laboratory exposures at 300 bar sCO 2 have found reasonably good compatibility for Ni-based alloys at <800°C, including an assessment of the sCO 2 impact on room temperature mechanical properties after 750°C exposures. However, initial screening tests at 1 and 20 bar CO 2 at 900°-1100°C showed poor compatibility for Ni-based alloys. In an open cycle, the introduction of 1%O 2 and 0.1- 0.25%H 2 O impurities at 300 bar increased the reaction rates ≥2X at 750°C. At lower temperatures, steels are susceptible to C ingress and embrittlement. Creep-strength enhanced ferritic steels may be limited to <550°C and conventional stainless steels to <600°C. Two strategies to increase those temperatures are higher Ni and Cr alloying additions and Al- or Cr-rich coatings. Alloy 709 (Fe- 20Cr-25Ni) shows some promising results at 650°C in sCO 2 but reaction rates were accelerated with the addition of O 2 and H 2 O impurities. Pack aluminized and chromized Gr.91 (Fe-9Cr-1Mo) and type 316H stainless steel show some promise at 600°-650°C but further coating optimization is needed.

A constitutive equation, with parameters derived from the interpolation of primary and steady state stages of constant load creep curves, has been utilized to estimate the stress relaxation behavior of the martensitic steel X20Cr13, alloy used in many high temperature applications, including heavy duty gas turbines. Creep and stress relaxation tests have been performed at 350°C, close to the negligible creep temperature of the studied alloy for stresses of interest for engineering applications. The creep tests were carried out at stresses below and above the yield stress, whereas, for the relaxation stress tests, the imposed strain was in the range 0.2% to 1.2% with the purpose to have, at the beginning of the tests, the same initial stresses of the performed creep tests. After a stress relaxation period, lasting between 10 to 1000 hours, each specimen was generally reloaded at the initial stress and a new relaxation test, on the same specimen, was carried out. This “reloading procedure”, simulating the re-tightening of bolts, has been repeated several times. The proposed equation has shown to well predict the experimental creep and stress relaxation behavior of the steel under investigation.