Patent Description:
This application is based on and claims priority to <CIT> and to <CIT>,.

The present disclosure relates to a weldment comprising a first metallic body and a second metallic body joined by a weld material. The present disclosure further relates to a method of fusion welding.

In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.

Materials have chemical, microstructural, and mechanical properties that are a function of composition and processing. While materials will have a composition within specified ranges and/or have a specified processing method, the specification does not always state or put limits on specific interstitial constituents (such as C or N) or specific processes that influence every desired property of the material. Furthermore, even when a material is within a specification, there is source-to-source and heat-to-heat variability in the composition of the material, which may lead to variability in properties to be outside of acceptable limits and/or tolerance bands.

This variability applies, for example, to the materials and methods used in welding. As an example, ERNiCr-<NUM> (a grade of welding wire also known as Filler Metal <NUM>) is widely used for joining dissimilar metals via fusion welding and is commonly found in, for example, the nuclear power industry for dissimilar metal welds in pressurized water reactor components. ERNiCr-<NUM> can be sourced from various manufactures and, while ERNiCr-<NUM> from each manufacturer will be compositional within specification, the material specifications and manufacturing methods for these materials do not control nitrogen content or nitride formation except through the control of bulk concentrations of constituent elements and there is variability in material content and material performance due to localized variability in composition whereby the ERNiCr-<NUM> material still exhibits a wide range of chemical, microstructural, and mechanical properties.

These unspecified compositional variations can impact the performance of the weld material joining the two metallic bodies forming a weldment, as well as the weldment overall. For example, variations in constituents not covered by the specification alone or in combination with narrower ranges for constituents covered by the specification for various fusion welding processes and materials can influence formation of precipitates within inter-dendritic and inter-granular sites of the weld microstructure and can have an influence on the material properties (among other things, weld crack susceptibility) and result in performance variability.

<CIT> discloses a nickel base alloy for cladding by welding. The alloy comprises ≤<NUM>% C, ≤<NUM>% Si, <NUM>-<NUM>% Mn, ≤<NUM>% S, <NUM>-<NUM>% Cr, <NUM>-<NUM>% Mo, <NUM>-<NUM>% Nb, <NUM>-<NUM>% Ti, ≤<NUM>% Fe, <NUM>-<NUM>%N, and Ni as balance.

<CIT> discloses an example of an inert gas mixture and a method for welding using said gas mixture.

<CIT> discloses TIG welding of Hastelloy XR using shielding gas comprising nitrogen gas.

The scope of protection is defined by the appended independent claims. Generally, Applicants have investigated the above-discussed variations and their effect on composition - structure - property relationships in fusion welding and propose solutions applying these relationships to improvements in fusion welding processes and materials with attendant improvements in performance of weld materials joining the two metallic bodies forming a weldment, as well as the weldment overall and other processes and structures associated with, for example, overlays, buttering or cladding of non-corrosion resistant base materials with corrosion resistant weld metals. In particular, Applicants have observed that variability in material content and performance among weld materials, such as those formed using ERNiCr-<NUM> materials, often relates to the nitrogen content and the carbon content, which influences metal carbide type (MC-type), nitride (e.g. MN-type), and/or complex carbide/nitride type (e.g. MX-type) precipitation. It has been determined that early formation and presence of high temperature nitrides have a prominent effect on the volume (via enhanced nucleation) and morphology of carbide or nitride type phases and the properties/characteristics of accompanying primary/secondary phases within both partially- and fully-solidified materials and in both ferrous and non-ferrous materials. Relatedly, control of nitrogen and/or carbon content, in particular the amount of high temperature nitride available during joining, such as by welding, and/or carbide formation within weld material can provide a method to regulate carbide/nitride precipitation and growth and desired material effect(s). Thus, Applicants have developed weld materials and processes for fusion welding that modifies one or more of the nitrogen content and carbon content and precipitates in weld materials by one or more of the following: (i) adjusting the shield gas composition to increase nitrogen gas and nitride species, (ii) adjusting the shield gas composition to decrease nitrogen gas and nitride species, (iii) adjusting the composition of nitrogen, nitride forming and nitride solubilizing constituents in the materials to obtain a desired concentration of nitrogen and nitrides, and (iv) using other processes, such as use of fluxes and filler materials, to introduce nitrogen or nitrides to the molten metal forming the weld material, overlay, buttering or cladding.

In general, exemplary embodiments of a weldment comprises a first metallic body and a second metallic body joined by a weld material, wherein the weld material has a composition including <NUM> to <NUM> wt. % Cr, <NUM> to <NUM> wt. % Mn, up to <NUM> wt. % Fe, <NUM> ppm to <NUM> ppm N, and equal to or greater than <NUM> wt. In each of the above embodiments, other elements may be present as follows: Nb+Ta <NUM> to <NUM> wt. %, max <NUM> wt. % Ti, and other elements (total) max <NUM> wt. In the case of overlays and cladding, exemplary embodiments may utilize one metallic body, and may utilize a second metallic body that is a base material component or a weld deposit. In further embodiments, the weldment can include multiple base material components and one or multiple weld material components.

An exemplary method of fusion welding comprises forming a region of molten material between a first metallic body and a second metallic body, wherein the molten material includes molten base metal from the first metallic body, molten base metal from the second metallic body, weld metal from a welding alloy, and, optionally, one or more molten additive material, modifying at least one of a nitrogen content of the molten material and a nitride content of the molten material, coalescing the molten material, and solidifying the molten material to form a weld material, wherein the solidified weld material joins the first metallic body to the second metallic body to form a weldment, and wherein the solidified weld material has a composition including <NUM> ppm to <NUM> ppm nitrogen.

Also, in exemplary embodiments, a microstructure of the weld material includes a plurality of precipitates, wherein the plurality of precipitates include one or more of a plurality of metal carbide precipitates and a plurality of metal carbide/nitride precipitates. In exemplary embodiments, a volume fraction of the plurality of precipitates is <NUM> or less for nitrogen in the range of <NUM> ppm to <NUM> ppm (alternately, <NUM> or less for nitrogen in the range of <NUM> ppm to <NUM> ppm). The above precipitate volume fractions were observed for ERNiCr-<NUM>/EN82 wire with a fixed carbon content of nominally <NUM> wt. % (for example ranging from <NUM> to <NUM> as reported in Table <NUM>).

All values used in the discussion of embodiments herein are reported as nominal (whether or not that term is used in the text) and all values in examples and tests are reported as actual.

The following detailed description of preferred embodiments can be read in connection with the accompanying drawings in which like numerals designate like elements and in which:.

Fusion welding joins a first metallic body to a second metallic body to form a weldment. Because metals must be heated to the melting point for fusion welds to be produced, the first metallic body and the second metallic body are sufficiently heated to form molten base material from each and, with any added material such as weld wire and filler material, to form a region of molten material. The region of molten material coalesces by which the different molten constituents mix together to form a molten mass. There is often a high degree of homogeneity among the component metals that have been melted, but localized compositional variation can occur. The coalescence typically occurs by convection, but oscillation and even evaporation can also play a role. The coalesced molten region then solidifies to form weld material that joins the first metallic body to the second metallic body to form the weldment.

<FIG> shows a simplified schematic cross-section of a portion of a weldment <NUM> at the fusion weld joint. The weldment comprises a first metallic body <NUM> and a second metallic body <NUM> joined by a weld material <NUM>. The weld material <NUM> is a mixture of base materials from the first metallic body <NUM> and the second metallic body <NUM> that have completely melted and any added material such as weld wire and filler material. At a distance from and outside the weld material <NUM> and separated therefrom by an interface/partially melted zone <NUM>, there is a heat affected zone (HAZ) <NUM>. The heat affected zone is a volume of material in the first metallic body <NUM> and in the second metallic body <NUM> in which the base metal, while not melted, still has had its microstructural or mechanical properties altered by high temperature heat from the welding process. At a further distance from the weld material <NUM> and outside the heat affected zone <NUM>, the base metal of the first metallic body <NUM> and the second metallic body <NUM> is essentially unaffected. In the illustrated portion of weldment <NUM>, the regions in the metallic bodies <NUM>, <NUM> where the base metal is essentially unaffected are indicated by <NUM> and <NUM>, respectively.

The disclosed fusion welding also extends to other applications, such as a weld overlay, buttering, and cladding. <FIG> shows a simplified schematic cross-section of a portion of a weld overlay. The weld overlay <NUM> applies one or more metals with specific characteristics to a base metal or base structure to improve desirable properties or to restore the original dimension of the component. As shown in <FIG>, the weld overlay <NUM> comprises a weld overlay material <NUM> to a base structure <NUM> (in <FIG>, the base structure is a pipe). A weld material <NUM> is located at a joint or contact point between two sections of the base material <NUM> and the weld material <NUM> extends along a surface of the base structure <NUM> past a defect <NUM> (in <FIG>, the defect is a crack). <FIG> shows a simplified schematic cross-section of a portion of weld buttering <NUM>, in which a layer of weld metal <NUM> has been sequentially applied over a base metal <NUM> to form multiple layers of weld metal. Although <FIG> illustrates weld deposits applied parallel to the weldment surface, weld deposits may also be applied perpendicular to the weldment surface.

In exemplary embodiments, the weld material <NUM>, <NUM>, <NUM> (and optionally the weld overlay material <NUM>) is based on ERNiCr-<NUM> weld metal. ERNiCr-<NUM> weld metal provides excellent corrosion resistance and high temperature mechanical properties while providing a means to bridge the differences in coefficient of thermal expansion between the first metallic body and the second metallic body when using dissimilar metals, such as ferritic and austenitic materials. Although ERNiCr-<NUM> weld metal is generally considered to be resistant to mechanical flaws and defects such as solidification cracking, solidification cracking has been observed in high restraint situations. Examples of high restraint situations include thick section weld deposits, highly restrained metallic bodies, and metallic bodies with high yield strength. Furthermore, it has been observed that considerable heat-to-heat variability in susceptibility to solidification cracking occurs despite only slight variations in chemical composition within the specification range of conventional ERNiCr-<NUM> weld metal. One test by which such solidification cracking can be observed and compared between samples is the cast pin tear test (CPTT), which is disclosed and described in <NPL>, the entire contents of which is incorporated herein by reference.

One or more of the nitrogen content and the carbon content and the precipitate properties (number, size and/or distribution) of the weld material influence the material strength of the weld material as well as certain types of crack resistance of the weld material. Applicants have determined that control of nitrogen and nitride content in the weld material can be achieved by control of nitrogen sources and nitride sources to the weld environment and can be used to achieve desired properties in the weld material, including a reduction in certain cracking susceptibility and improved mechanical strength. Similarly, control of carbon and carbon content in the weld material can be achieved by control of carbon sources and carbide sources to the weld environment and can be used to achieve desired properties in the weld material, including a reduction in certain cracking susceptibility and improved mechanical strength. Additionally, combining both nitrogen and carbon control can be used to achieve such desirable properties.

For example, in a first embodiment, a range of <NUM> ppm to <NUM> ppm nitrogen appears to provide resistance to hot cracking mechanisms, such as solidification cracking.

Nitrogen content in the range of <NUM> ppm to <NUM> ppm, alternatively <NUM> ppm to <NUM> ppm and a low volume fraction of precipitates (e.g., less than <NUM>) have little impact on mechanical properties such as tensile strength and <NUM>% yield strength, and improves resistance of the weld material to hot cracking mechanisms, such as solidification cracking.

Considering the above, a specific embodiment of a weld material has a composition that includes nitrogen content in a range of <NUM> ppm to <NUM> ppm. In addition, a microstructure of the weld material includes one or more of a plurality of metal carbide precipitates and a plurality of metal carbide/nitride precipitates. When the nitrogen content is in the range of <NUM> ppm to <NUM> ppm, the microstructure of the weld material includes a plurality of precipitates having a volume fraction of <NUM> or less. The volume fraction of precipitates can be determined by quantitative image analysis with SEM (as discussed further below).

The following Table <NUM> summarizes compositions of ERNiCr-<NUM> weld material where the carbon content and the nitrogen content are controlled to obtain the specified compositions and with attendant effects on hot cracking mechanisms, such as solidification cracking and solid state crack mechanisms such as ductility dip cracking (DDC).

The volume fraction of precipitates can be determined by quantitative image analysis with SEM (as discussed further below) and provides a metric to correlate with the known resistance to hot cracking mechanisms, such as solidification cracking, in high restraint applications.

The amount of precipitates in the weld material can be influenced by the presence and amount of nitride and carbide forming constituents. Dominate carbide or nitride forming constituents for ERNiCR-<NUM> are niobium and titanium. Niobium is the primary carbide forming element in ERNiCr-<NUM>. The traditional phase transformation sequence for ERNiCr-<NUM> only results in gamma phase (i.e., austenite) and NbC. Therefore, in a specific example, the composition of the weld material can include <NUM>-<NUM> wt. % Nb, and up to <NUM> wt. % Ti, alternatively, <NUM> to <NUM> wt.

In other respects, the weld material can be any weld material suitable for joining the first metallic body and the second metallic body to form a weldment. In a further specific embodiment, the first metallic body and the second metallic body are formed of similar metals or dissimilar metals(alternatively are ferrous and non-ferrous metals), and the weld material is an ERNiCr-<NUM> based weld material.

In a first embodiment, the ERNiCr-<NUM> based weld material has a composition that includes:.

In each of the above, M is a carbide or nitride or complex carbide/nitride forming constituent such as Nb or Ti. When Nb is present, its limit is <NUM>-<NUM> wt. %; when Ti is present, its limit is up to <NUM> wt. %, alternatively, <NUM> to <NUM> wt. Nb and Ta are primary carbide forming elements in ERNiCr-<NUM> and can be substituted for each other; when present, the traditional phase transformation sequence for ERNiCr-<NUM> results in gamma phase and NbC and/or TaC.

Carbon is also be present in the weld composition and can contribute to the precipitate properties (number, size and/or distribution) of the weld material and has a prominent influence on precipitation volume.

Within a single weldment, specific regions can employ more or less nitrogen and/or precipitates to obtain desired properties. For example, in a weldment with a multi-layer weld deposit, different regions or layers of the weldment can have weld material with different amounts of nitrogen in the weld material composition. In regions of the weldment with a higher susceptibility to solidification cracking (which typically occurs towards the end of weld solidification), the nitrogen content of the weld material can be adjusted to be <NUM> ppm to <NUM> ppm (approximately <NUM> wt. % to <NUM> wt. %) to provide increased resistance to solidification cracking. Within the same weldment but at a different layer, some regions have a lower susceptibility to solidification cracking but a higher susceptibility to solid state crack mechanisms such as ductility dip cracking (for example, regions containing migrated weld grain boundaries or insufficient MC-type carbide precipitates). It is also contemplated that in some instances, the nitrogen and/or carbon content of weld material within the same weldment but at a different layer can be unadjusted.

Without being bound to any particular theory, it is currently understood that solidification of weld material in the welding process occurs sufficiently rapidly that metal element-based (such as Nb-based) or carbon-based routes to forming carbide (e.g. MC-type), nitride and/or complex carbide/nitride (e.g. MX-type) type particulates in the weld material have only limited time to nucleate, grow and form precipitates before such precipitate forming processes are essentially stopped by the solidification of the weld material. Under these conditions, there is little to no ability to influence variations in the final precipitate properties (number, size and/or distribution) in the weld material. However, increasing the presence of nitrogen (or nitrides) in the molten weld material increases the liquidus temperature, allowing for more mobility of constituents of the weld material composition and a longer solidification process, both of which promote increased precipitate properties (number, size and/or distribution) as compared to weld material with lower amounts of nitrogen, e.g. less than about <NUM> ppm nitrogen. These increased precipitate properties (number, size and/or distribution) have been correlated qualitatively such that an increase in nitrogen/nitride concentrations will increase precipitate properties and a decrease in nitrogen/nitride concentrations will decrease precipitate properties.

Additionally, weld materials having low nitrogen content (<NUM> ppm to <NUM> ppm) and low volume fraction of precipitates (less than <NUM>) have little to no improvement in mechanical properties as compared to weld material in which the nitrogen and precipitates have not be modified, but do display improved resistance to hot cracking mechanisms.

Turning to a method of fusion welding in which a weld material joins a first metallic body to a second metallic body to form a weldment, an exemplary method includes forming a region of molten material between a first metallic body and the second metallic body. This molten material includes molten base metal from the first metallic body, molten base metal from the second metallic body, weld metal from a welding alloy, and, optionally, one or more molten filler metals. At least one of the nitrogen and the carbon content of the molten material is modified, as discussed further below, and the molten material, coalesces, by which the different constituents of the molten base metal from the first metallic body, molten base metal from the second metallic body, weld metal from a welding alloy, and, optionally, one or more molten filler metals join together and become a mixture. The coalesced molten material then solidifies to form a weld material that has a composition, depending on the disclosed embodiment used, that includes <NUM> ppm to <NUM> ppm.

In addition, modifying the nitrogen and nitride content of the molten material modifies the microstructure of the weld material to include one or more of a plurality of metal carbide precipitates and a plurality of metal carbide/nitride precipitates. In exemplary embodiments, the microstructure of the weld material includes a plurality of NbC precipitates and/or TiN precipitates at a low volume fraction of precipitates (less than <NUM>) to improve resistance to hot cracking mechanisms.

Methods disclosed herein modify the nitrogen content and or the carbon content of the molten material by controlling the presence of nitrogen (and nitrides) and/or carbon (and carbides) in the molten weld material. Controlling the presence of nitrogen (and nitrides) and/or carbon (and carbides) in the molten weld material also influences and modifies the presence of nitrogen and carbide (e.g. MC-type) type precipitates and/or complex carbide/nitride (e.g. MX-type) type precipitates and their content in the weld material. For example, a welding process can be used which includes one or more of the following steps to modify one or more of the nitrogen content and the carbon content of the molten material: (i) the shield gas composition is adjusted to increase nitrogen gas and nitride species, (ii) the shield gas composition is adjusted to decrease nitrogen gas and nitride species, (iii) the composition of nitrogen and nitride forming and nitride solubilizing constituents in the materials is adjusted to obtain a desired concentration of nitrogen and nitrides, and (iv) the use of other processes, such as the use of fluxes and filler materials, are used to introduce nitrogen or nitrides to the molten metal forming the weld material.

After preparing the workpieces (e.g., the metallic bodies that will be joined by the weld material to form the weldment) and materials for welding, for example by shaping and/or cleaning, the welding process can commence and proceed to the point at which a region of molten material is formed between the first metallic body and the second metallic body. This region of molten material includes molten base metal from the first metallic body, molten base metal from the second metallic body, molten weld metal from a welding alloy, and, optionally, one or more molten filler metals.

If a shield gas is present, such as in gas metal arc welding and gas tungsten arc welding, the shield gas is supplied to the region of the molten material and forms a protective gaseous barrier to oxygen, water vapor and other impurities that can reduce the quality of the weld. The shield gas can be supplied by any suitable means. For example, the shield gas can be supplied through a shield gas line leading to a gas nozzle at the end of the welding line where the shield gas is expelled to the welding work zone around the welding arc or the shield gas can be supplied from a separate source and device and be applied to the welding work zone around the welding arc.

As an example of a shield gas, the shield gas can be an inert or semi-inert gas (such as argon or helium) whose composition has been modified to include one or more of from <NUM> to <NUM> mol. % nitrogen and from <NUM> to <NUM> mol. % CO<NUM> (whether individual gases or a mixed gas).

In the welding work zone around the welding arc, the protective gaseous barrier of the shield gas is in contact with the molten material and nitrogen (either in the form of elemental nitrogen or, due to the high temperatures used in welding, a nitride) or carbon (either in the form of elemental carbon or, due to the high temperatures used in welding, a carbide) from the gaseous barrier is introduced into the molten material. In the molten material and during solidification, the nitrogen (or nitride) and/or carbon (or carbide) remains as nitrogen/carbon or reacts with nitride/carbide forming species in the molten metal to form nitrides/carbides, which themselves then promote carbide (e.g. MC-type) and/or complex carbide/nitride (e.g. MX-type) type particulate as well as contribute to the nitrogen content of the weld material.

Alternatively or concurrently, the amount of nitrogen (or nitride) or carbon (or carbide) material in the weld material can be controlled by the composition of nitrogen (and/or carbon) forming and nitrogen (and/or carbon) solubilizing constituents in the materials of one or more of the first metallic body and the second metallic body and the weld metal of the welding alloy. The amount of nitrogen and nitrides (and/or carbon and carbides) can be influenced by adding nitrogen/nitride (and/or carbon/carbide) forming elements or adding elements that increase or impede solubility of nitrogen or nitride promoting elements (and/or carbon or carbide promoting elements). As examples, Ti and Al can be used as nitrogen/nitride forming elements, and Mn can be used as an element to promote the formation of nitrogen or nitrides. These two types of additions can be used singly or in combination to balance the desired effect on the amount and distribution of nitrogen and the carbide (e.g. MC-type), nitride and/or complex carbide/nitride (e.g. MX-type) type particulate, as well as contribute to the nitrogen content of the weld material. For example, one or both of the first metallic body and the second metallic body can have a composition in which the nitrogen content and/or the nitride content has been modified. As another example, one can use measured additions of nitrogen/nitrides during primary/secondary melt processing, such as during VIM melting or a pressurized electroslag remelting process (P-ESR). The analogous situation can be arrived at for carbon content and/or carbide content by using carbon/carbide forming elements and/or processes. For example, carbon/carbide forming elements can be incorporated into welding supplies, such as weld wire, during the VIM melting of the material.

As a further example, the nitrogen content in weld metal of the welding consumable, for example ERNiCr-<NUM> weld metal, can be modified to include nitrogen and/or nitrides by, for example, nitriding the weld metal or by coating the weld metal. Carbon content can be similarly adjusted by, for example, carbiding the weld metal or by coating the weld metal. To nitride or carbide the weld metal, one can expose conventional weld metal to a nitrogen/carbon containing atmosphere at temperature and/or pressure to nitride/carbide the weld metal and form a nitrogen enriched/carbon enriched surface zone in the weld metal. This nitrided/carbided weld metal can then be used as a welding consumable in a welding process by which the nitride/carbide material of the weld metal is added to the region of molten material. Varying the amount of nitriding/carbiding of the weld metal thereby modifies the amount of nitrogen/carbon in the composition of the molten material. Certain flux-based or powder core products could also be modified/created to similar effect.

Alternatively or concurrently, the amount of nitrogen (or nitride) material in the weld material can be controlled by the composition of nitrogen, nitride forming and nitride solubilizing constituents in the materials of the welding aides used in the welding process. Similarly, the amount of carbon (or carbide) material in the weld material can be controlled by the composition of carbon, carbide forming and carbide solubilizing constituents in the materials of the welding aides used in the welding process. Examples of welding aides include fluxes and fillers. For example, in shielded metal arc welding (SMAW) with an ERNiCrFe-<NUM> welding consumable, when the flux is heated and flows it contacts the region of molten material and forms a protective barrier to oxygen, water vapor and other impurities that can reduce the quality of the weld. Although the flux does not mix with the molten material, at the interface of flux and molten material the constituents from the flux can enter into the molten composition. This can occur when there is sufficient driving force for such constituents to cross the flux-molten material interface, such as the high temperatures used in welding or the chemical potentials of the molten material and the flux. If one or more additive is used, such as nitride powders, carbon powders, nitride forming elements, carbide forming elements, and coatings that generate nitrogen gas and/or carbon gas during welding, the additives melt and their content contributes to the content of the molten material. Modifying the nitrogen, nitrides, nitride forming and nitride solubilizing, carbon, carbides, carbon forming and carbide solubilizing content of the fillers, by for example modifying the composition, nitriding, carbiding, or coating, varies the amount of nitrogen, carbon, nitrides, carbides, nitride forming and nitride solubilizing and carbide forming and carbide solubilizing content in the material and, when the material is used as a consumable in a welding process, modifies the amount of nitrogen and/or carbon in the composition of the molten material.

EXAMPLES: Tables <NUM> and <NUM> report results from compositional analysis and mechanical testing on samples with varying amounts of nitrogen in the weld material. The samples were prepared by manufacturing a weldment using AGTA-CW welding and a metallic body of Alloy <NUM> (UNS06600), a nickel-chromium-iron alloy. The weld beads were deposited in flat position on the Alloy <NUM> test coupon; producing a multi-layer weld buildup with an approximate geometry of <NUM> inch (width) x <NUM> inch (length) x <NUM> inch (height). The weld metal of the samples in Tables <NUM> and <NUM> was an ERNiCr-<NUM> based weld metal. No additive or flux was used. During the welding process for the samples in Table <NUM> and <NUM>, argon-mixed shield gas was used that had a nitrogen content that varied between the four samples as follows: a <NUM> mole percent (mol. %) nitrogen shield gas was used for sample YT0159-0NC, a <NUM> mol. % nitrogen shield gas was used for sample YT0159-1NC, a <NUM> mol. % nitrogen shield gas was used for sample YT0159-5NC, and a <NUM> mol. % nitrogen shield gas was used for sample YT0159-20NC).

The ERNiCr-<NUM> weld metal of the samples in Tables <NUM> and <NUM> had been prepared by vacuum melting technique in a three-step melt process including vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR), which results in nitrogen content in the range of <NUM> ppm to <NUM> ppm. ERNiCr-<NUM> that has been prepared by vacuum melting techniques is a preferred weld consumable as compared to air melt ERNiCr-<NUM> because the nitrogen and nitride content in vacuum melt ERNiCr-<NUM> is more consistent between samples and also the nitrogen and nitride content is <NUM> ppm to <NUM> ppm, which is sufficiently low that control of the nitrogen and nitride content in the weld material can be essentially completely dependent on and controlled by the amount of nitrogen and nitrides added by the user during the welding process, e.g., through control of one or more of the shield gas composition, the composition of nitrogen (and nitride) forming and nitrogen (and nitride) solubilizing constituents in the materials, and other processes such as use of fluxes and additive materials.

The results for the compositional analysis reported in Table <NUM> were obtained using standard analytical techniques, such as inductively coupled plasma mass spectrometry (ICP-MS) and combustion techniques in either induction or resistance furnaces. The combustion techniques were used for carbon, hydrogen, nitrogen, oxygen and sulfur. The reported nickel content is based on the balance of the as-measured contents of the other constituents.

Sample YT0159-0NC was then tested for mechanical properties. Table <NUM> contains the testing results and, for each sample, reports tensile strength (in ksi), <NUM> % yield strength (in ksi), elongation (in % in 4D) (meaning the % elongation was measured in a specimen whose gage length is <NUM> times its gage diameter) and reduction of area (in %) in both the longitudinal direction and the transverse direction relative to the weld travel direction.

More specifically, the four samples were tested following AWS B4. <NUM> (Standard Methods for Mechanical Testing of Welds) with reference to ASTM E8 - 15a (Standard Test Methods for Tension Testing of Metallic Materials). The testing was conducted at room temperature at <NUM> in/in/min strain rate until <NUM>% total strain and at <NUM> in/in/min crosshead rate thereafter until failure. Each sample consisted of all weld metal (corresponding to the compositions in Table <NUM>) and the test blanks for each sample were <NUM>/<NUM> in x <NUM>/<NUM> in x <NUM> in, machined to <NUM> in diameter round tensile specimens.

Also for comparison, per Annex A of ASME IIC, SFA-<NUM>, typical mechanical properties of conventional ERNiCr-<NUM> weld metal include: tensile strength of <NUM> ksi. Also for comparison, MIL spec equivalent weld material (EN82H), MIL-E-<NUM>, has a minimum tensile strength of <NUM> ksi and a minimum elongation of <NUM>%, and annealed UNS N06600 (ASTM B166/<NUM>), which has tensile strength and elongation values consistent with EN82H, has a minimum yield strength (<NUM>% offset) of <NUM> ksi. Comparing properties of the as-tested inventive samples to the properties of the comparative conventional sample, the as-tested inventive samples displayed an improvement in tensile strength of <NUM> ksi to <NUM> ksi (corresponding to an improvement of <NUM> % to <NUM> %), in <NUM>% yield strength of <NUM> ksi to <NUM> ksi (corresponding to an improvement of <NUM> % to <NUM>%), and a change in elongation of -<NUM>% to +<NUM>% (corresponding to a decrease of <NUM> % to an increase of <NUM> %).

<FIG> contains a series of scanning electron micrographs taken at <NUM> kV and 1500X magnification showing the microstructure of samples of weld material prepared using a shield gas (i) containing <NUM> mol. % nitrogen at two locations (<FIG>); (ii) containing <NUM> mol. % nitrogen at two locations (<FIG>); (iii) containing <NUM> mol. % nitrogen at two locations (<FIG>); and (iv) containing <NUM> mol. % nitrogen at two locations (<FIG>). <FIG> show features similar to those shown and described with respect to <FIG>, although with differing phase volume of precipitates as the amount of nitrogen present during the welding process was varied. As evident in the SEM micrographs, the increased nitrogen content of the weld shielding gas and the resultant weld material (see Table <NUM>) resulted in noticeable increases in MC- and MX-phase volume; both from increased distribution and size. The corresponding improvements to material strength can also be attributed to these precipitation increases, where such secondary phases are known to impede the motion of dislocations through crystallographic structure of the material. Only subtle differences in mechanical properties and volume fraction precipitates were noted between the <NUM> mol. % and <NUM> mol. % (in the mixed gas) nitrogen samples, however, likely due to the solubility limit of the ERNiCr-<NUM> weld material (i.e., bulk nitrogen content of <NUM> - <NUM> ppm (in the weld metal).

In another example, welds formed of alloys of ERNiCr-<NUM>/EN82H were investigated in which the nitrogen and carbon contents varied as follows: <NUM>-<NUM> ppm nitrogen and nominally <NUM> wt. % carbon or nominally <NUM> wt. The welds materials were investigated for tensile properties in both the longitudinal direction and the transverse direction relative to the weld travel direction. Table <NUM> presents the results of these investigations and, for each sample, reports tensile strength (in ksi) and <NUM> % yield strength (in ksi),.

In the above results in Table <NUM>, the carbon content of the samples did not vary the carbon outside of selecting two wire heats with a different carbon contents - a high carbon content of <NUM> wt. % and low carbon content of <NUM> wt. Comparing data from samples with equivalent or nearly equivalent nitrogen content can be used to isolate carbon effects in these samples. In this case and using the data from Table <NUM>, comparing values for mechanical properties of sample YT0159 1N to sample B8142 1N shows the effect of varying carbon content (<NUM> wt. % C for YT0159 1N; <NUM> wt. % C for B8142 1N) with nominally constant nitrogen content in the shield gas or <NUM> mol% N in mixed gas. For these samples, testing reported in Table <NUM> shows a reduction in longitudinal mechanical properties between the samples, but the transverse mechanical properties are nominally constant. In another case and using the data from Table <NUM>, comparing values for mechanical properties of sample YT0159 <NUM>. 4N to sample B8142 <NUM>. 4N shows the effect of varying carbon content (<NUM> wt. % C for YT0159 <NUM>. 4N; <NUM> wt. % C for B8142 <NUM>. 4N) with nominally constant nitrogen content in the shield gas or <NUM> mol% N in mixed gas. For these samples, testing reported in Table <NUM> shows a nominal constant value in longitudinal mechanical properties between the samples, but the transverse mechanical properties are show a reduction in values for the low carbon sample (<NUM> wt. % C) as compared to the high carbon sample (<NUM> wt.

One conclusion from the above investigation is that nitrogen has a minimal effect on tensile and yield strength and that carbon has a stronger effect than nitrogen on tensile and yield strength. Additionally, the difference in solubility of nitrogen in the low carbon sample B8142 5N as compared to the sample YT0159 5N for the same amount of nitrogen in the shield gas is noteworthy and suggests/confirms that starting material content, most notably of carbon (and to a lesser extent Ti and Nb), will have a direct influence on nitrogen solubility during and after the welding/melting process(es). These effects are also anticipated via non-equilibrium computational thermodynamic modeling and are expected to be observable during primary/secondary melting processes of bulk material production or product manufacturing. This provides insight to the interdependence of carbon and nitrogen content and the consideration required to achieve target material composition; both in original product form or throughout manufacturing.

Also, one can compare the mechanical properties of various samples to observe the effect of varying nitrogen content in the shield gas for both nominal <NUM> wt. % C and nominal <NUM> wt. % C samples using a shield gas with <NUM> mol. % nitrogen (the 5N samples) to the corresponding samples prepared using a shield gas with <NUM> mol. % nitrogen, one can observe the change in mechanical properties as the amount of nitrogen in the shield gas (and, by proxy the amount of nitrogen in the weld material, increase. The changes in tensile strength (absolute and in percentage) are shown in Table <NUM>.

Claim 1:
A weldment, comprising:
a first metallic body and a second metallic body joined by a weld material,
wherein the weld material has a composition including:

<TAB>

wherein Nb and/or Ta are present in an amount of <NUM> to <NUM> wt.% in total, and wherein Ti is optionally present in a limit of up to <NUM> wt.%,
wherein a microstructure of the weld material includes a plurality of precipitates, wherein the plurality of precipitates include one or more of a plurality of metal carbide precipitates and a plurality of metal carbide/nitride precipitates, and
wherein, for a carbon content of <NUM> wt.%, a volume fraction of the plurality of precipitates is <NUM> or less, measured by quantitative image analysis with SEM according to the method provided in the description.