Patent Description:
Since <NUM> and as part of the United States Department of Energy's (DOE) Coal Power Program, National Energy Technology Laboratory (NETL) has launched a research program entitled "Development of Advanced Materials for Ultra-supercritical Boiler Systems," to identify and develop next generation materials in advanced ultra-supercritical (A-USC) boilers, and turbine systems. In these boilers and turbine systems, a target steam temperature and pressure of about <NUM> (<NUM>°F) and about <NUM> MPa (<NUM> psi), respectively, may reduce all emissions including carbon dioxide (CO<NUM>), by about <NUM>% or greater compared to some boilers and turbine systems.

A challenge in coal-fired A-USC systems has been in the area of materials and manufacturing technologies. As a critical materials component in the boiler, superheater tubes encounter severe service conditions and should meet stringent requirements with respect to fireside coal-ash corrosion/erosion, steam side oxidation and spallation, creep strength, thermal fatigue strength, and weldability. During a DOE A-USC program, alloys <NUM> and <NUM>, have been identified as candidates for components for A-USC systems. Therefore, acceptable manufacturing and welding processes, especially for materials in dissimilar metal welds (DMW), are desired for applications and success of these systems.

Within the disclosure of the present document, all aspects, examples and features mentioned below can be combined in any technically possible way. The invention as herein claimed, however, is defined in the appended claims.

As an initial matter, in order to clearly describe the subject matter of the current disclosure, it will become necessary to select certain terminology when referring to and describing relevant characteristics, properties, and components within an additively manufactured and graded composite transition joint. To the extent possible, common industry terminology will be used and employed in a manner consistent with its accepted meaning. Unless otherwise stated, such terminology should be given a broad interpretation consistent with the context of the present application and the scope of the appended claims. Those of ordinary skill in the art will appreciate that often a particular component may be referred to using several different or overlapping terms. What may be described herein as being a single part may include and be referenced in another context as consisting of multiple components. Alternatively, what may be described herein as including multiple components may be referred to elsewhere as a single part.

It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur or that the subsequently describe component or element may or may not be present, and that the description includes instances where the event occurs or the component is present and instances where it does not or is not present.

Where an element or layer is referred to as being "on," "engaged to," "connected to" or "coupled to" another element or layer, it may be directly on, engaged to, connected to, or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to" or "directly coupled to" another element or layer, there may be no intervening elements or layers present.

Robust dissimilar metal welds (DMWs) can enable advanced ultra-supercritical (A-USC) systems. Robust DMWs can also be employed in and enable enhancement via refits and renovations of existing A-USC power plants. As renewable energies have become affordable and incorporated into electric grids, the electric power utility industry may need to retrofit and renovate fossil coal and natural gas power plants to operate in a flexible operational mode with renewable energy. A flexible operational mode enables use of intermittent and unstable power generation that may be inherent to renewable energy sources. Under increased cyclic operating conditions in existing fossil fuel power plants, failures at a DMW may occur. Cyclic operation places DMWs in stress conditions. In some cyclic operation, stress conditions may shift from creep dominant stress to thermal creep-fatigue domination. Weakened microstructures at a DMW's fusion or bond line interface can be exposed to high cyclic stresses caused by a coefficient of thermal expansion (CTE) mismatch of two different materials that are fused together at the fusion or bond line. As a result, service life and performance of DMWs may be reduced.

Accordingly, as embodied by the disclosure, functionally gradient composite transition joints are formed by additive manufacturing processes. Additive manufactured dissimilar metal weldments (DMWs), as embodied by the disclosure, can provide gradient composite transition joints. Gradient composite transition joints, as embodied by the disclosure, can meet compositional physical property requirements at DMWs. Compositional physical property requirements, as embodied by the disclosure, include but are not limited to room temperature and high temperature strengths, creep resistance, and corrosion resistance. Further, as embodied by the disclosure, gradient composite transition joints can reduce stress concentrations due to the transition of CTE between both sides of additive manufactured dissimilar metal welds. Accordingly, as embodied by aspects of the disclosure, additive manufactured dissimilar metal weldments will improve the cracking and thermal creep fatigue (TCF) resistances DMWs.

A-USC systems may use superalloys for parts given advantageous compositional physical property requirements. As embodied by the disclosure, superalloys can include, but are not limited, to nickel (Ni) based superalloys, or iron (Fe) based superalloys, or cobalt (Co) based superalloys, or combinations thereof. Further, A-USC materials may include austenitic stainless steel (ASS), creep strength enhanced ferritic steel (CSEFS), alone or possibly combined together or combined with other materials. These materials can be provided at different DMW regions depending on temperature and corrosion resistance requirements of a DMW in the A-USC structure. As a result, DMWs between materials may mitigate differences in CTE adverse effects, and may facilitate design, development, and manufacturing of enhanced A-USCs.

For example, and not intending to limit embodiments of the disclosure in any manner, the average linear CTE for an alloy steel is in a temperature range between about <NUM> (<NUM> °F) to about <NUM> (<NUM> °F) is on the order of approximately <NUM>µ/°C ( 9µ/°F), while austenitic stainless steels are on the order of approximately <NUM>µ/°C (<NUM>µ/°F). A temperature change from room temperature to approximately <NUM> (<NUM>°F) would generate a thermal strain of approximately <NUM>%, resulting in thermal stress in the range of approximately <NUM> MegaPascals (MPa), which is much higher than the typical amount allowed operational stresses. The thermal stress would be expected to be relaxed over time (in months or years) so it would only play a secondary role compared to the applied stresses in DMW performance under the steady operating condition at approximately <NUM> (<NUM>°F) (creep dominant), which has been the norm of fossil power plants over the past decades. On the other hand, under thermal cyclic operation conditions, such thermal strains would continue to generate and accumulate before they are relaxed. The considerable thermal strain accumulation and damage may have a larger role under thermal creep fatigue (TCF) associated cyclic operation, possibly leading to the premature failure of DMW. Therefore, the thermal stresses in the DMW joints poses a new challenge to the safe operation of fossil energy plants under the increased cyclic operation mode. Understanding and managing thermal stresses in DMWs for future operation may enhance new installations and life extension of DMWs.

Eliminating compositional changes across the fusion or bond line of a DMW can prevent DMW failures.

In DMWs that include nickel-based fillers, failure through creep and/or creep fatigue cracks have been attributed to morphology of carbides. DMWs, including stainless steel fillers, exhibit failure through creep and/or creep fatigue cracks that may possibly be formed along austenite grain boundaries close to the fusion or bond line boundary.

Embodiments of the disclosure provide a method for producing a gradient (or graded) composite transition joint (GCTJ) part. In one aspect of the embodiments, a GCTJ can be used to connect and join dissimilar metals where the gradient characteristics meet the differing properties, such as differing physical and thermal properties, of metals of components which the GCTJ will connect. In another aspect of the embodiments, GCTJ can cost effectively remedy premature failure of the conventional dissimilar metal welds (DMW) under increased cyclic operating conditions of fossil power plants.

Additively manufactured GCTJs (AM-GCTJ) can be suitable for next generation advanced ultrasupercritial (A-USC) power plants, as embodied by the disclosure. In certain aspects of the embodiments, AM-GCTJs can be provided for enhancement via retrofit or replacement of conventional DMWs in existing fossil power plants. This retrofit or replacement may enable safe and economical operation in cycling modes beyond their original intended design life.

For instance, welding dissimilar metals including Creep Strength Enhanced Ferritic Steels (CSEFS) or Austenitic Stainless Steel (ASS) to superalloys are two types of important DMWs finding possible application in the next generation of A-USC systems.

In one non-limiting illustrative aspect of the embodiments, Austenitic Stainless Steel (ASS) is based on <NUM>%Cr-<NUM>%Ni. As embodied by the disclosure, ASS can include, but is not limited to, Super <NUM> or Sanicro25, and their equivalents.

In another non-limiting aspect of the embodiments, CSEFS, also known as martensitic or super ferritic steels can include but are not limited to, Grades <NUM> and <NUM> steels. Additionally, superalloys can include but are not limited to Ni-based, Fe-based, and/or Co-based superalloys.

In an A-USC system, a DMW may be provided in the system between features formed from different materials, such as but not limited to, CSEFS (in one aspect of the embodiments, Grade <NUM>/<NUM>) and ASS (in one aspect of the embodiments, Super <NUM>). In <FIG>, a tube <NUM> is connected to a header <NUM> at a nipple weld <NUM> that may be positioned at an inlet of a final stage superheater/reheater. The superheater design condition can be approximately <NUM> bar/<NUM> in a non-limiting embodiment of the disclosure. DMW dimension has an outer diameter (OD) approximately <NUM> and approximately <NUM> thick in a non-limiting embodiment of the disclosure. DMW of ASS (in one aspect of the embodiments, Sanicro25) to Ni-base superalloys (in one aspect of the embodiments, <NUM> or H0282) may be at a tube <NUM> to a header <NUM> at a nipple weld <NUM> at a final stage superheater/reheater outlet. The superheater design condition may be approximately <NUM> bar/<NUM>. DMW dimension may have an outer diameter (OD) approximately <NUM> millimeters (mm) in OD and approximately <NUM> in thickness.

Potentially undesirable DMW traits may be mitigated through an additively manufactured graded composite transition joints (AM-GCTJ) <NUM>, as embodied by the disclosure. One aspect of an AM-GCTJ <NUM>, as embodied by the disclosure, is illustrated in <FIG>.

The change in microstructure across a fusion or bond line of DMWs in the as-welded condition may be due to a steep chemical concentration gradient. <FIG> illustrates a heat affected zone (HAZ) between Grade <NUM> steel and a Ni-based filler metal. <FIG> illustrates creep and/or creep fatigue crack failures in a DMW fusion or bond line. Creep and/or creep fatigue cracks can occur along the fusion or bond line boundary and HAZ between different alloys of the DMW. Creep and/or creep fatigue cracks can be attributed to, at least one of residual, external, and thermal stresses caused by CTE mismatch. Thermal stresses due to CTE mismatch may be significant. Failure may be accompanied with damages in the HAZ.

<FIG> illustrates an additive manufacturing process for forming an additively manufactured graded composite transition piece (AM-GCTJ) <NUM>. First, the additively manufactured graded composite transition piece (AM-GCTP) is formed. AM-GCTP <NUM> includes a first alloy or Alloy A <NUM> and second alloy or Alloy B <NUM>. The GCTP is formed with a gradual, gradient change in mixture of Alloy A <NUM> and Alloy B <NUM> along its structure. The composition of the mixture of Alloy A <NUM> and Alloy B <NUM> is <NUM>% Alloy A <NUM> at a first (one) end to <NUM>% Alloy B <NUM> at the second (second end) so the area between the ends transitions in a controlled concentration from substantially all Alloy B <NUM> at first end to all Alloy A <NUM> at the second end. Alloy A <NUM> and Alloy B <NUM> thus constitute a purposely formulated composite that includes a mixture of Alloy A <NUM> and Alloy B <NUM>. However, the mixture may optionally include other additives in between the ends.

Transition of Alloy A <NUM> and Alloy B <NUM> in the composite or more specifically, a ratio or concentration of Alloy A <NUM> and Alloy B <NUM> is gradually changed from first end of the AM-GCTP to the second end of the AM-GCTP. The gradual concentration change is provided to mitigate and reduce sharp changes in chemistry and thermal stresses in DMWs, such as but not limited to those by differences in CTEs.

After the AM-GCTP is formed, AM-GCTP may be placed between and welded to the two structural members, such as those shown in <FIG> to form an AM-GCTJ <NUM>. In certain aspects of the embodiments, AM-GCTJ <NUM> can be formed between a Ni-based superalloy and CSEFS in the DMW. Similarly, AM-GCTJ <NUM> can be formed between an ASS and CSEFS in a DMW.

Since ends of the AM-GCTJ <NUM> have substantially a similar chemistry to the materials of structural members to which the AM-GCTJ is to be welded, the welds connecting AM-GCTJ <NUM> to the two structural members are essentially made between two materials with matching or compatible chemistry, including but not limited to matching CTEs. This same material welding can eliminate factors causing premature failures of DMWs, such as but not limited to transition of chemistry and CTE mismatch-induced thermal stresses at a DMW fusion or bond line.

Another aspect of the embodiments includes producing AM-GCTP via an additive manufacturing (AM) process to produce the AM-GCTJ <NUM>. As illustrated in <FIG>, a first process step includes producing grating or lattice patterns <NUM> from Alloy A <NUM>. (See also <FIG> which illustrates an illustrative and non-limiting grating or lattice patterns <NUM>, as embodied by the disclosure). In an illustrative aspect of the embodiments, Alloy A <NUM> can include, but is not limited to, ASS. The grating or lattice patterns <NUM> from Alloy A <NUM> is produced with additive manufacturing processes. Such additive manufacturing processes include, but are not limited to, at least one of selective laser melting (SLM) and selective laser sintering (SLS).

The grating or lattice patterns <NUM> formed from Alloy A <NUM> include pores <NUM>. Sizes of the pores <NUM> in grating or lattice patterns <NUM> can vary. The cross-sectional size of pores <NUM> can be in a range between about tens of micrometers to about sub millimeters. Grating or lattice patterns <NUM> can be formed denser on the grating or lattice pattern's first end and less dense at second end of the grating or lattice pattern <NUM>. The density or concentration can be expressed as a volumetric ratio. The density/volumetric ratio gradually reduces the volumetric ratio from about <NUM>% to about <NUM>% in the grating or lattice pattern <NUM>. This gradual reduction in volumetric ratio of the grating or lattice pattern <NUM> can be achieved by increasing at least one of the pore <NUM> size and reducing the grating or lattice <NUM> density as layers are additively manufactured and built towards a top of the grating or lattice patterns <NUM>.

After grating or lattice patterns <NUM> are formed via an additive manufacturing process, Alloy B <NUM> powders can be provided in to grating or lattice patterns <NUM>. Alloy B <NUM> powders include but are not limited to Grade <NUM> steel powders. The provision of Alloy B <NUM> powders is at first end of grating or lattice patterns <NUM>, the end with less density of Alloy A <NUM>(<FIG>). Alloy B <NUM> powders include but are not limited to, steel powders, for example Grade <NUM> steel powder. After Alloy B <NUM> powders are added, ultrasonic energy may be applied to the grating or lattice pattern <NUM> to fill the grating or lattice pattern <NUM> and Alloy B <NUM> powder therein, wherein the filling includes vibrating Alloy B <NUM> powder. The ultrasonic energy ultrasonically vibrates and may cause Alloy B <NUM> powders to fill by falling (shaking) through pores <NUM> in the grating or lattice patterns <NUM> towards the end with highest density of Alloy A <NUM> of grating or lattice patterns <NUM>.

Next, the grating or lattice patterns with Alloy A <NUM> and Alloy B <NUM> is subjected to hot isotropic pressing (HIP). Hiping (<FIG>) is applied to densify Alloy A <NUM> and Alloy B <NUM> in grating or lattice patterns <NUM>. Hiping can achieve approximately <NUM>% density of Alloy A <NUM> and Alloy B <NUM> with the grating or lattice patterns, and form a composite. After the hiping, a graded composite transition of Alloy A <NUM> and Alloy B <NUM> in the grating or lattice patterns is achieved from first end of grating or lattice patterns <NUM> to the second end of grating or lattice patterns <NUM>. The graded composite transition, as embodied by the disclosure, will be from approximately <NUM>-<NUM>% Alloy A <NUM> to Alloy B <NUM> from an end to the second end of grating or lattice patterns <NUM>.

An Integrated Computational Weld Engineering (ICWE) modeling tool from Oak Ridge National Laboratory (ORNL) can be used to design an AM-GCTJ <NUM>. ICWE can assist in evaluating the thermal stresses caused by thermal cyclic loading in operation conditions of the dissimilar metal welds (DMW). Certain benefits of AM-GCTJ <NUM> are summarized below with an example. An illustrative, non-limiting application was welding and preparing a piping system with a DMW formed from a Grade <NUM> steel against a Stainless Steel <NUM> (See <FIG>). A straight section of the piping system with DMW to join the two pipes was used, with an outer diameter (OD) of the pipes about <NUM> (<NUM> inches), and the pipe thickness was about <NUM> (<NUM>/<NUM> inches) (dimensions often found in fossil power plants). A thermal cyclic loading profile used in the simulation is illustrated in <FIG>. The temperature was raised from the ambient temperature to the approximate operating temperature <NUM> in about two hours. The modeling system then held at the operating temperature for <NUM> days, followed by cooling to the ambient in <NUM> hours. Such thermal loading cycle modeling was repeated to simulate operation and calculate an accumulative effect of thermal cyclic loading.

<FIG> illustrates an illustrative and non-limiting grating or lattice patterns <NUM>. The lattice of alloy A <NUM> and the pores <NUM> can be provided in any configuration, size, cross-section, and dimensions, with the density and pore size provided as embodied by the disclosure. Therefore, as long as the density is more dense on the first end than the second end of the grating or lattice pattern (<NUM>) and the density is gradually reduced by at least one of increasing the pore size from the first end to the second end and reducing the density of the grating or lattice pattern (<NUM>) as the grating or lattice pattern (<NUM>) is additively manufactured, the configuration, size, cross-section, and dimensions of the grating or lattice of alloy A <NUM> and the pores <NUM> may vary.

It is noted that graded transition joints (GTJs) can be produced with a variety of manufacturing techniques that are readily scalable for mass fabrication. Compared with the conventional GTJ technologies, the AM-GCTJ <NUM> approach as embodied by the disclosure, has several technical effects:.

AM-GCTJ <NUM>, as embodied by the disclosure, produces composites that preserve the features, such as thermal and physical characteristics, of Alloy A <NUM> and Alloy B <NUM>. Conventional GTJ technologies, either using wire or powders, melt both Alloy A <NUM> and Alloy B <NUM> together to build a transition joint layer-by-layer. Melting and mixing the two different alloys in different ratios is time and resource intensive, and may create a complex and irregular microstructure, which may have unpredictable microstructure and thus unknown or unpredictable properties, such as thermal and physical characteristics. As an example of such an unpredictable microstructure, melting a <NUM>/<NUM> ratio of Grade <NUM> and <NUM> would create a "new" material with unproven microstructure stability and high temperature performance, as well as potential solidification defect issues.

Further, as embodied by the disclosure, Alloys A and Alloy B <NUM> are not melted together compared to a traditional weld operation. In accordance with the disclosure, Alloy A <NUM> and Alloy B <NUM> are bonded together through the solid-state HIP process. The solid-state HIP process (hiping) forms a "composite" material of Alloy A <NUM> and Alloy B <NUM>. Accordingly, the hip'ed composite material of Alloy A <NUM> and Alloy B <NUM> does not introduce metallurgical complications of a melted "new" A+B alloy.

After suitable heat treatment, AM-GCTJ <NUM> can be used as the intermediate connection between two structural members that include properties similar to Alloy A <NUM> and Alloy B <NUM>. If needed, fusion welding can be provided at both ends of a DMW. Thus, fusion welding can be applied at interfaces, or fusion or bond lines between two structural members that include properties similar to Alloy A <NUM> and Alloy B <NUM>, which will eliminate previous issues encountered with DMW at fusion or bond lines, such as differing CTEs.

A further technical effect, as embodied by the disclosure, provides a smooth transition between Alloy A <NUM> and Alloy B <NUM>. The smooth transition is graded and should reduce and mitigate issues related to CTE mismatch. Reduced CTE mismatching may improve turbomachine component life including during cycling operation(s).

Another technical effect of the instant embodiments is that the process and weld can provide enhanced control of DMW compositions. Further, as embodied by the disclosure, the process, including additively manufacturing gratings and lattice patterns <NUM> can enable differing DMW geometry, which may not be feasible with other welding processes. Accordingly, as embodied by the disclosure, scale-up issues may be reduced in the manufacture of the large quantities of GTJs needed for A-USC systems. Moreover, life extension for existing fleets may be realized with the AM-GCTJ <NUM> embodiments herein.

The present embodiments provide an additively manufactured, graded composite transition joint for dissimilar metal weldments in advanced ultra-supercritical power plants. In another embodiment of this disclosure, a method is provided for producing an additive manufactured- graded composite transition joint ("AM-GCTJ <NUM>").

It will be appreciated that the above described invention is described with respect to an advanced ultrasupercritical (A-USC) power plant and related alloys. However, it is an aspect of the disclosure that the concepts and embodiments may be used in other dissimilar metal welds (DMWs). Moreover, it is a further aspect of the disclosure that the concepts and embodiments may be used in other applications or industries other than power plants.

It will be apparent to those persons skilled in the art that numerous modifications and variations of the described examples and embodiments set forth herein are possible in light of the above teachings of this disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the scope of this invention.

Claim 1:
A method for producing an additively manufactured, graded composite transition joint AM-GCTJ (<NUM>), the method comprising:
preparing a grating or lattice pattern (<NUM>) from a first alloy A (<NUM>), wherein the grating or lattice pattern (<NUM>) includes pores (<NUM>) in the grating or lattice pattern;
building the grating or lattice pattern (<NUM>) from a first end to a second end of the grating or lattice pattern, the grating or lattice pattern (<NUM>) having a density being more dense on the first end than the second end, and wherein the grating or lattice pattern (<NUM>) gradually reduces the density by at least one of increasing the pore size from the first end to the second end and reducing the density of the grating or lattice pattern (<NUM>) as the grating or lattice pattern (<NUM>) is additively manufactured;
adding a second alloy B (<NUM>) powder to the first end of the grating or lattice pattern (<NUM>);
filling the second alloy B (<NUM>) powder from the first end towards the second end of the grating or lattice pattern (<NUM>);
forming a composite of the first alloy A (<NUM>) and the second alloy B (<NUM>) powder in the AM-GCTJ (<NUM>); and
subjecting the composite to hot isotropic pressing HIP to densify the composite, wherein, the second alloy B (<NUM>) powder has a graduated concentration from the first end to the second end of the AM-GCTJ (<NUM>).