METHODS OF FABRICATING STAINLESS STEEL ALLOY ARTICLES AND ARTICLES FABRICATED THEREBY

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional U.S. Patent Application No. 63/736,464 filed Dec. 19, 2024, the contents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contracts DE-NE0009193, awarded by U.S. Department of Energy, and 70NANB23H030, awarded by the National Institute of Standards and Technology. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The invention generally relates to methods of fabricating stainless steel alloy articles using laser-based fabrication techniques, and articles fabricated thereby. The invention particularly relates to articles formed of austenitic stainless steel alloys and fabricated by laser based fabrication techniques, and processes to promote the formation of twin-related grain boundary characteristics in the articles, which may be exemplified by twin-related grain boundary networks and optionally other desirable types of coincidence site lattice (CSL) grain boundaries.

Various types of laser-based fabrication techniques have been developed. Laser-based additive manufacturing (AM) methods, such as laser powder bed fusion (LPBF) and directed energy deposition (DED), are layer-by-layer fabrication techniques capable of precisely controlling material deposition, bonding, and solidification of metallic materials, enabling the fabrication of articles having complex geometries and highly tailored microstructures. Laser cladding is a surface modification process in which a focused laser beam is used to melt a metallic powder or wire material, creating a metallurgically-bonded coating (cladding) layer on a substrate, for example, a protective or wear-resistant layer, while causing minimal distortion of the underlying substrate. Laser welding is a joining process in which a laser beam is used to melt and fuse two or more metal components together. The concentrated heat input results in high-quality welds with minimal distortion, even on intricate or thin-walled parts. Due to these and/or other advantages, laser-based fabrication techniques have been employed in the fabrication of articles made of polycrystalline metallic alloys, such as austenitic stainless steels. However, challenges exist when attempting to promote certain mechanical properties of austenitic stainless steel articles using laser-based techniques.

Polycrystalline metallic alloys are made up of individual microscopic crystallites commonly referred to as grains, whose boundaries can be formed and modified through recrystallization and grain growth processes during fabrication and heat treatment. After fabrication, grains typically have highly misoriented and equiaxed grain boundaries that can influence the mechanical properties of the metal, such as high and low cycle fatigue lives, strength, high-temperature creep, corrosion, and oxidation. However, random high-angle grain boundaries can be susceptible to the deterioration of these properties, which leads to damage and premature failures in harsh extreme service environment applications, such as in the oil and gas industry, nuclear industry, power generation, health care devices in the human body, and aircraft engines.

Other types of grain boundaries, such as coincidence site lattice (CSL) grain boundaries, have specially-ordered symmetric lattice structures and exhibit higher damage tolerance, lower diffusivity, and better thermal and radiation stability as compared to random grain boundaries. Among different types of CSL boundaries, Σ3 coherent twin boundaries are considered to be particularly desirable over random boundaries in order to combat a wide range of material degradations in extreme service environments, including corrosion, oxidation, environmentally-assisted cracking (EAC), fatigue damage, thermal aging, and creep damage. Therefore, it can be desirable to fabricate articles that obtain more CSL boundaries, such as Σ3 coherent twin boundaries, in polycrystalline metallic alloys that will be used in extreme service environments.

Grain boundary engineering (GBE) encompasses methods that have been developed to obtain grain boundaries capable of achieving particular material properties. For example, grain boundary engineering has been performed for the purpose of promoting higher percentages of Σ3 coherent twin boundaries and other CSL grain boundaries than random high-angle grain boundaries in polycrystalline materials. For traditional wrought austenitic stainless steels, grain boundary engineering is typically achieved by thermo-mechanical processing techniques, which involve hot or cold rolling and subsequent heat treatments. However, grain boundary engineering based on traditional thermo-mechanical processes is not feasible with articles produced by laser-based fabrication techniques because the rolling operations will change the shape or geometry of the article, which can defeat advantages associated with laser-based fabrication techniques.

Therefore, it would be desirable to have a way to realize the benefits of grain boundary engineering in articles made of polycrystalline metallic alloys, such as austenitic stainless steels fabricated using laser-based fabrication techniques.

BRIEF SUMMARY OF THE INVENTION

The intent of this section of the specification is to briefly indicate the nature and substance of the invention, as opposed to an exhaustive statement of all subject matter and aspects of the invention. Therefore, while this section identifies subject matter recited in the claims, additional subject matter and aspects relating to the invention are set forth in other sections of the specification, particularly the detailed description, as well as any drawings.

The present invention provides, but is not limited to, methods of fabricating stainless steel alloy articles using laser-based fabrication techniques, stainless steel articles fabricated by laser-based fabrication techniques, and methods of grain boundary engineering stainless steel alloy articles fabricated by laser-based fabrication techniques.

According to a nonlimiting aspect of the invention, a method of forming a stainless steel alloy exhibiting twin-related grain boundary characteristics is provided. The method includes fabricating an article from an austenitic stainless steel alloy using a laser-based fabrication technique during which the austenitic stainless steel alloy undergoes solidification and cooling to form a microstructure in the article having a phase constitution, and then heat treating the article. The austenitic stainless steel alloy contains iron, chromium of about 15-20 wt. %, nickel of about 8-14 wt. %, carbon of not more than 0.08 wt. %, nitrogen of not more than 0.10 wt. %, and unavoidable impurities, and is characterized by a balance between ferrite-stabilizing elements and austenite-stabilizing elements that enables a ferrite-to-austenite phase transformation to occur during the solidification and the cooling of the austenitic stainless steel alloy during the laser-based fabrication technique. The solidification and/or the cooling of the austenitic stainless steel alloy include a cooling rate that is sufficient to induce the ferrite-to-austenite phase transformation, and the heat treating modifies the microstructure of the article such that twin-related grain boundary characteristics are formed and stabilized in the article.

Other nonlimiting aspects of the invention include articles produced from the austenitic stainless steel alloy by a method as described above.

Technical aspects of methods and alloys as described above preferably include the ability to promote the formation of twin-related grain boundary characteristics in alloy compositions of austenitic stainless steels made by laser-based fabrication techniques. Such twin-related grain boundary characteristics are preferably exemplified by twin-related grain boundary networks and optionally other desirable types of coincidence site lattice (CSL) grain boundaries.

These and other aspects, arrangements, features, and/or technical effects will become apparent upon detailed inspection of the figures and the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting boundary fraction percentages of Σ3 coherent twin boundaries, Σ9 boundaries, and Σ27 boundaries in two as-built (as-fabricated and not heat treated) stainless steel specimens (“Modified SS 1” and “Modified SS 2”) manufactured by a modified laser-based additive manufacturing (AM) process in accordance with a nonlimiting embodiment of the present invention, and an as-built conventional AISI 316L stainless steel specimen manufactured by a conventional laser-based additive manufacturing process.

FIG. 2 is a graph of the boundary fraction percentages of Σ3 coherent twin boundaries, Σ9 boundaries, and Σ27 boundaries in the Modified SS 1 and Modified SS 2 specimens and the conventional AISI 316L stainless steel specimen of FIG. 1 following heat treatments.

FIG. 3 is a graph showing the microhardness evolution of the Modified SS 1 and Modified SS 2 specimens and the conventional AISI 316L stainless steel specimen of FIG. 2 as a function of the duration of the heat treatment at about 1200° C.

DETAILED DESCRIPTION OF THE INVENTION

The intended purpose of the following detailed description of the invention and the phraseology and terminology employed therein is to describe one or more nonlimiting embodiments of the invention, and to describe certain but not all aspects of what is depicted in the drawings, including the embodiment(s) to which the drawings relate. The following detailed description also describes certain investigations relating to the embodiment(s) depicted in the drawings and identifies certain but not all alternatives of the embodiment(s). As nonlimiting examples, the invention encompasses additional or alternative embodiments in which one or more features or aspects shown and/or described as part of a particular embodiment could be eliminated, and also encompasses additional or alternative embodiments that combine two or more features or aspects shown and/or described as part of different embodiments. Therefore, the appended claims, and not the detailed description, are intended to particularly point out subject matter regarded to be aspects of the invention, including certain but not necessarily all of the aspects and alternatives described in the detailed description.

As used herein the terms “a” and “an” to introduce a feature are used as open-ended, inclusive terms to refer to at least one, or one or more of the features, and are not limited to only one such feature unless otherwise expressly indicated. Similarly, use of the term “the” in reference to a feature previously introduced using the term “a” or “an” does not thereafter limit the feature to only a single instance of such feature unless otherwise expressly indicated.

The present application provides methods that implement laser-based fabrication techniques, heat treatments, and optionally alloy modifications to promote desirable grain boundary formations in articles formed of austenitic stainless steels, more particularly, the formation of twin-related grain boundary networks and optionally other desirable types of coincidence site lattice (CSL) grain boundaries (hereinafter sometimes individually and/or collectively referred to as “twin-related grain boundary characteristics”) in articles formed of austenitic stainless steels. The methods can be implemented in current laser-based fabrication processes without necessitating changes to the laser-based fabrication techniques that would significantly alter the shapes or geometries of the articles, and in doing so might otherwise defeat advantages associated with laser-based fabrication techniques. Suitable laser-based fabrication techniques include, but are not limited to, laser-based additive manufacturing, laser cladding, and laser welding methods. Suitable laser-based additive manufacturing techniques include, but are not limited to, laser powder bed fusion (LPBF) and directed energy deposition (DED). Preferably, although not necessarily, parameters of the laser-based fabrication technique result in an article having a density of greater than 99% of theoretical.

Articles can be formed by a laser-based fabrication technique using a powder and/or wires of an austenitic stainless steel as feedstock, for example, an austenitic stainless steel alloy containing iron (Fe), chromium (Cr) of about 15-20 wt. %, nickel (Ni) of about 8-14 wt. %, carbon (C) of up to about 0.08 wt. %, nitrogen (N) of up to about 0.10 wt. %, with the balance including unavoidable impurities. The austenitic stainless steel alloy composition is further selected to establish a balance between ferrite-stabilizing elements and austenite-stabilizing elements within the alloy, which enables a ferrite-to-austenite phase transformation to occur under the solidification and cooling conditions of the laser-based additive manufacturing process, including a cooling rate during solidification and/or subsequent cooling that is sufficient to induce the ferrite-to-austenite phase transformation and yield a microstructure comprising an austenitic matrix and, prior to heat treatment, optionally a ferritic phase.

Investigations discussed herein were performed with AISI 316L austenitic stainless steel, which has a nominal composition of FeCr18Ni10Mo3, for example, 16.0-18.0 Cr, 10.0-14.0 Ni, 2.0-3.0 Mo, up to 2.00 Mn, up to 1.00 Si, up to 0.10 N, up to 0.045 P, up to 0.030 C, up to 0.030 S, and the balance iron and incidental impurities. For the purpose of achieving the desired twin-related grain boundary characteristics using methods and disclosed herein, critical alloying elements of an austenitic stainless steel are those that control the ferrite-austenite balance under laser-based fabrication conditions, such as those that occur during laser-based additive manufacturing. Nickel and carbon/nitrogen are critical in terms of selecting their upper limits to avoid suppressing the ferrite-to-austenite phase transformation. Chromium is critical in terms of selecting its lower limit to achieve a minimum presence in the alloy as a ferrite-stabilizing element that enables ferrite-to-austenite phase transformation. On this basis, a preferred composition for austenitic stainless steels that are particularly suitable for achieving technical aspects of the invention contains chromium of about 16-18 wt. %, nickel of about 11-13 wt. %, molybdenum of about 0.1-3 wt. %, carbon of not more than 0.06 wt. %, nitrogen of not more than 0.06 wt. %, with the balance comprising or consisting of iron and unavoidable impurities. In the investigations reported herein, the ferrite-to-austenite phase transformation occurred in such a composition during solidification and/or subsequent cooling associated with a laser-based additive manufacturing process.

The invention optionally encompasses alloy modifications to an austenitic stainless steel to promote twin-related grain boundary characteristics through additions of one or more elements from Groups III, IV, and/or V of the periodic table in amounts of about 0.1 to about 3 wt. %., more preferably 1-2 wt. %. Preferred Group III, IV, and V elements include tantalum (Ta), niobium (Nb), vanadium (V), titanium (Ti), zirconium (Zr), hafnium (Hf), yttrium (Y), scandium (Sc), and combinations thereof. While not wishing to be held to any particular theory, such an alloy modification promotes stabilization of the ferrite phase that enables the ferrite-to-austenite phase transformation. The Group III, IV, and/or V elements may be added to the alloy composition using pre-alloying, ball milling, and/or powder mixing techniques.

Heat treatments determined to promote the formation and stabilization of the desired twin-related grain boundary characteristics in articles produced by laser-based fabrication techniques with alloy compositions described above are performed at a temperature of about 1050° C. to 1200° C. for a duration of about eight to twenty-four hours, more preferably for a duration of about ten to eighteen hours. The heat treatment is performed to modify the phase constitution of the microstructure of the article that resulted from the laser-based fabrication technique such that twin-related grain boundary networks are formed and stabilized in the article. The heat treatment may be conducted by various heating processes, such as substrate heating (induction or resistance) and/or laser beam heating.

While not wishing to be held to any particular theory, the desired twin-related grain boundary characteristics are not obtained based on the alloy composition alone, nor by the heat treatment alone, but instead are achieved by the combination of the austenitic stainless steel alloy composition, the laser-based fabrication technique (such as additive manufacturing) characterized by solidification conditions and cooling conditions that cause the ferrite-to-austenite phase transformation within the austenitic stainless steel alloy and produces a microstructure comprising an austenitic matrix and, prior to post-heat treatment, optionally a ferritic phase, followed by an appropriate heat treatment that alters the phase constitution of the microstructure to result in the formation of the desired twin-related grain boundary characteristics, particularly twin-related grain boundary networks within the austenitic matrix of the alloy. Articles produced in this manner are believed to be capable of exhibiting enhanced resistance to stress corrosion cracking (SCC), creep deformation, and/or thermal degradation relative to an additively manufactured AISI 316L stainless steel article that had not been subjected to the heat treatment proscribed above. An alloy composition as disclosed herein may achieve residual strain during the laser-based fabrication process and control grain nucleation and/or abnormal grain growth (AGG)/critical grain growth (CGG) by multiple mechanisms, such as precipitate formation, phase transformation, and boundary interaction during the laser-based fabrication process and/or subsequent heat treatment. A high fraction of CSL boundaries and uniformly distributed precipitates are particularly believed to be favorable for improving resistance to stress corrosion cracking and creep resistance in harsh physical and chemical environments, and promote mechanical wear of the article.

Certain investigations leading to the present invention are discussed below in reference to FIGS. 1, 2, and 3.

A control specimen was produced from AISI 316L austenitic stainless steel alloy having a nominal composition of FeCr18Ni10Mo3, for example, 16.0-18.0 Cr, 10.0-14.0 Ni, 2.0-3.0 Mo, up to 2.00 Mn, up to 0.75 Si, up to 0.10 N, up to 0.045 P, up to 0.030 C, up to 0.030 S, and the balance iron and incidental impurities. Two experimental specimens were also produced, one of which was designated “Modified SS 1” and had a nominal composition of 17.0 wt. % Cr, 12.09 wt. % Ni, 2.5 wt. % Mo, 0.42 wt. % Si, a very low C content of about 0.003 wt. %, and a very low N content of about 0.01 wt. %, with the balance being Fe and unavoidable impurities. The second specimen was designated “Modified SS 2” and had the same composition as Modified SS1 but with the further addition of about 2 wt. % tantalum.

The specimens were produced by a laser powder bed fusion process conducted using laser parameters that included 90 W laser power, a 600 mm/s scan speed, a 25 μm layer thickness, and a 80 μm hatch spacing, which achieved high solidification and cooling rates that are typical of laser-based AM processes, produced a microstructure in each specimen comprising an austenitic matrix and optionally a ferritic phase, and ensured a ferrite-to-austenite phase transformation during the laser-based AM process. The specimens underwent a heat treatment at about 1200° C. for approximately sixteen hours, resulting in full recrystallization, ferrite-to-austenite phase transformation, and formation of twin-related grain boundary characteristics characterized by a highly connected twin-related grain boundary network within their austenitic matrices.

FIG. 1 presents data plotting the fractions of CSL grain boundaries in the control (316L) specimen and the two Modified SS1 and SS2 experimental specimens in the as-built (as-fabricated) condition, in other words, without any thermos-processing performed on the specimens. From FIG. 1 it can be seen that the percentage of CSL grain boundaries in the experimental specimens was significantly than in the control specimen. In particular, it can be seen that that the percentage of twin-related grain boundary networks, including each of Σ3 coherent twin boundaries, Σ9 boundaries, and Σ27 boundaries (twin-related grain boundary characteristics), was significantly increased by the grain boundary engineering technique obtained with the alloy modifications of the experimental specimens.

FIG. 2 presents data plotting the fractions of CSL grain boundaries in the control specimen and the two experimental specimens after further undergoing heat treatment at a temperature of about 1200° C.) for about sixteen hours. From FIG. 2, it can be seen that after the heat treatment the Σ3 coherent twin boundaries became more prominent in all three specimens, with the Σ3 coherent twin boundaries being even more predominant in the experimental specimens, suggesting effective grain boundary engineering occurred was achieved within the experimental specimens during the heat treatment that yielded the desirable twin-related grain boundary characteristics.

Electron backscatter diffraction (EBSD) grain boundary maps were obtained and analyzed for the three specimens in both the as-built and heat-treated conditions.

The EBSD grain boundary map of the control specimen in the as-built condition revealed that the specimen had predominantly random boundary networks with an absence of twin boundaries. After heat treatment, the EBSD grain boundary map of the control specimen revealed the nucleation of small twin-related domains and the formation of low-density twin boundaries.

The EBSD grain boundary map of the Modified SS 1 specimen in the as-built condition revealed a mix of random boundary networks and approximately 30% twin-related grain boundary networks. After the heat treatment, the EBSD grain boundary map of the Modified SS 1 specimen revealed twin-related grain boundary characteristics exemplified by the nucleation of large twin-related domains and the formation of high-density twin boundaries.

The EBSD grain boundary map of the Modified SS 2 specimen in the heat-treated condition revealed twin-related grain boundary characteristics exemplified by the nucleation of large twin-related domains and the formation of high-density twin boundaries.

FIG. 3 plots the microhardness evolution of the control specimen and the Modified SS 1 specimen as a function of heat treatment duration. The graph indicates improved thermal stability of the Modified SS 1 specimen as compared to the control specimen as a result of forming higher-density twin boundaries.

As previously noted above, though the foregoing detailed description describes certain aspects of one or more particular embodiments of the invention, alternatives could be adopted by one skilled in the art. For example, the alloys, methods, and their components could differ in appearance and construction from the embodiments described herein and shown in the drawings, functions of certain components of the alloys and methods, could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function, and various materials could be used in the fabrication of the alloys and/or their components. As such, and again as was previously noted, it should be understood that the invention is not necessarily limited to any particular embodiment described herein or illustrated in the drawings.