JP2012206169A - Casting process, material and apparatus, and castings produced therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a casting process for producing metal alloy castings; facecoats; apparatuses suitable for carrying out the process; and cast products produced by the process.SOLUTION: The casting process entails the use of a mold 10 having a cavity 12 with a continuous aluminum-containing solid facecoat 16 on its surface 14. A molten quantity of a metal alloy is introduced into the mold cavity 12 so that the molten alloy contacts the solid facecoat 16. The mold 10 is then allowed to cool to solidify the molten alloy and form a cast product 18. During the casting process, aluminum in the solid facecoat 16 diffuses into the cast product 18 to form an aluminum-containing surface region 20 on the casting surface 22. Following removal of the cast product 18 from the mold 10, an alumina-containing scale grows on the casting surface 22 as a result of oxidation of the casting surface region 20.

Description

  The present invention broadly relates to a casting method and apparatus and materials used in the casting method. In particular, the present invention relates to a metal casting method suitable for simultaneously forming an oxidation-resistant surface region on a cast product manufactured by the method, and the method is not particularly limited, but is an iron-based alloy. Conventional casting methods such as casting and sand mold casting may be used.

  The internal components of gas turbines, including gas turbine engines used in the power generation and aircraft industries, must be capable of operating at high temperatures (often above 1000 ° F. (about 540 ° C.)). Since the efficiency of a gas turbine depends on its operating temperature, there is a continuing need for gas turbine components with improved temperature performance. With increasing material requirements for gas turbine components, various processing methods, alloy components and coatings have been developed to enhance the mechanical, physical and environmental properties of gas turbine components.

In order to increase oxidation resistance and operating life, internal components of gas turbine engines are often manufactured as castings of high alloy stainless steel, nickel and cobalt alloy steels, cobalt base alloys, nickel base alloys. These alloys rely on chromium and / or nickel as metal additives to form an oxidation resistant scale as a surface layer that can protect the alloy from high temperatures and oxidation conditions inside the gas turbine. The scales can have a chromium and nickel content of these steels (for example, about 18% or more chromium and about 8% or more nickel, nickel and cobalt alloy steels, nickel-base alloys and cobalt-base alloys by about 18% or more by weight). Since chromium is 20% by weight or more, and nickel base alloy is about 50% by weight or more nickel, chromium oxide (chromia; Cr ₂ O ₃ ) and nickel oxide (NiO) are the main components. The scale effectively passivates the part surface, prevents further corrosion of the part surface, and inhibits subsurface corrosion. These alloys are superior in terms of oxidation resistance, but less than low alloy steels and irons (which typically contain less than 10 wt.% Chromium and very little nickel, if any). Is also much more expensive.

It is well known that aluminum oxide (alumina; Al ₂ O ₃ ) scale can provide better protection than chromium oxide and nickel oxide in many high temperature applications. Certain nickel-base superalloys, such as Rene N5 (US Pat. No. 6,074,602), contain a sufficient amount of aluminum to promote stable alumina scale growth. However, it is difficult to add aluminum to molten alloy steel during the initial manufacturing process, which is often infeasible or uneconomical.

  Alternatively, aluminum can be diffused onto the surface of the part using diffusion methods such as pack cementation, vapor phase (vapor phase) aluminizing (VPA) or chemical vapor deposition (CVD). In such diffusion processes, the surface of the component is often reacted with aluminum-containing vapor to form an additional layer on the surface of the component, and a diffusion layer is formed below the additional layer. The additional layer typically includes an environmentally resistant intermetallic phase MAl (M is iron, nickel or cobalt, depending on the substrate material), and a diffusion layer is typically present on the aluminum and the substrate. It includes various intermetallic phases and metastable phases of the elements that it produces. Although the diffusion aluminizing method is effective, it requires additional processing steps such as heat treatment after casting, so that it cannot be performed depending on the application, and in any case, additional cost and time are required.

US Pat. No. 6,074,602

  In view of the above, an iron-based alloy casting product that exhibits sufficient oxidation resistance for use in a high-temperature oxidizing environment, as in the case of internal components of a gas turbine engine, even when the alloy component content is relatively low. There remains a desire to produce.

  The present invention provides a casting method for producing a cast product that forms an aluminum-containing surface region of the cast product that is capable of forming an oxidation resistant alumina-containing scale on the product surface. The present invention further provides a facecoat and apparatus suitable for carrying out the method and a cast product produced by the method.

  In a first aspect of the invention, the casting method includes providing a mold having a cavity and a continuous solid facecoat on the mold surface within the cavity. The solid face coat contains at least 15% by weight of metal aluminum-containing powder and an optional active agent. A molten amount of metal alloy is introduced into the mold cavity to bring the molten alloy into contact with the solid facecoat. The mold is then cooled to solidify the molten amount of alloy and form a cast alloy product. During the casting process, at least a portion of the metallic aluminum in the solid facecoat diffuses to the surface to form an aluminum-containing surface area of the cast product. If an activator is present in the solid facecoat, the activator reacts with the powder to produce a volatile aluminum compound that promotes the diffusion of metallic aluminum to the surface, but the volatile aluminum compound then becomes a solid facecoat. It reacts on the surface of the molten alloy and / or cast product in contact with the metallic aluminum, and metal aluminum enters the surface. When the cast product is removed from the mold, the scale grows on the surface of the cast product due to oxidation of the surface area of the cast product. The scale contains alumina due to the oxidation of aluminum introduced into the surface area of the cast product during the casting process.

  Additional aspects of the invention include face coat gels used to form the solid face coat described above, as well as cast products made by the method described above. The alumina scale is present in sufficient quantity to be sufficiently continuous on the surface of the cast product and to act as a passivation layer that inhibits oxidation and corrosion of the cast product surface.

  Yet another aspect of the present invention is a solid facecoat used in the casting method described above and a casting apparatus that uses a continuous solid facecoat on the mold and the mold cavity surface of the mold. The solid face coat contains at least 15% by weight of metal aluminum-containing powder and an optional active agent. The mold cavity is adapted to receive a molten amount of the metal alloy so that the molten alloy contacts the solid facecoat.

  Alloys that can be used in the present invention include iron-base alloys, particularly low-alloy iron-base alloys and steels, which may contain chromium, but typically contain little or no nickel. As such, it does not exhibit adequate oxidation resistance when exposed to a gas turbine engine environment. The present invention is also applicable to other alloys, including nickel-based and cobalt-based alloys, where the alumina-containing scale passivation layer formed by the present method may be useful. A significant advantage of the present invention is that the solid facecoat contacts the molten alloy during the casting process, so that during melting and / or solidification, metallic aluminum diffuses from the solid facecoat into the alloy, resulting in a cast product. An aluminum-enriched surface region capable of forming a continuous adhesion protective alumina scale is formed on the surface.

1 is a partial cross-sectional view of a mold assembly showing a facecoat gel applied to an internal mold cavity surface according to one embodiment of the present invention. FIG. 2 is a partial cross-sectional view of the mold assembly of FIG. 1 showing a cast product in contact with a solid facecoat formed of the facecoat gel of FIG.

  Other aspects and advantages of this invention will be better appreciated from the following detailed description.

  1 and 2 schematically show parts of a casting apparatus applicable to the present invention. The apparatus and the following description of the apparatus is considered as a non-limiting description of the mold 10, such as a permanent mold or sand mold, which can be used in a casting method for producing a metal casting (18 in FIG. 2). . As is well known in the art, the mold 10 is preferably formed of a refractory material whose surface can be protected with a ceramic facecoat. The mold 10 forms an internal mold cavity 12 having a surface 14 having a cast product 18 of a desired shape, such as an internal component of a gas turbine engine. Specific and non-limiting examples of such parts are diaphragms, shrouds and other stationary gas turbine parts. As shown in FIG. 2, the mold cavity surface 14 is provided with a solid facecoat 16, which will be described in detail below.

  The mold 10, its cavity 12 and cavity surface 14 may be manufactured or formed in any manner and will not be discussed in detail herein. The mold 10 and its cavity 12 may be used in combination with one or more cores (not shown) to form an internal cavity or passage within the cast product 18. Further, the mold 10 may be adapted so that it can be cooled according to the target casting product 18. In addition, depending on the particular alloy being cast, the casting method using the mold 10 may be performed in air, in a vacuum, or in an inert atmosphere. In use, the mold 10 is preheated to a temperature of, for example, about 200 to about 1000 ° F. (about 90 to about 540 ° C.), and then a molten amount of alloy is introduced into the preheated mold 10 and then normally The mold 10 can be cooled in this manner to initiate and complete the solidification of the alloy.

  Various alloys can be cast using a mold of the type shown in FIGS. The present invention is particularly directed to the formation of a continuous protective scale, such as a continuous chromium oxide or nickel oxide scale, in an amount that can achieve a desired level of oxidation resistance in an iron-based alloy, particularly a cast product 18. Cast iron and low alloy steels that do not contain any chromium, nickel or other oxide-forming elements. As an example, a specific iron-based alloy contains 10% by weight or less of the alloy components, and the balance is often iron and inevitable impurities. Such alloy components typically include one or more of chromium, manganese, and carbon. Of particular interest are iron-base alloys containing up to 10% by weight chromium and even if intentionally added (less than 1% by weight) nickel, eg 8-10% by weight chromium and 1 Contains some low alloy grades containing less than 1 wt% nickel, 1-8 wt% chromium and other low alloy grades containing less than 1 wt% nickel, and less than 1 wt% chromium and nickel Yet another low alloy rope. These chromium and nickel levels are of interest for high alloy stainless steels and nickel alloy steels commonly used in the formation of gas turbine components and are often over 18% by weight due to high temperature operating requirements. Chromium and / or 8% by weight or more of nickel. This content is often sufficient to form a protective surface oxide. Notable examples of such alloys include stainless steel grades such as 304, 316, 310, and nickel grades such as 800, 625, and 738.

  It should be noted that the benefits of this method can be extended to the casting of other iron-based alloys such as ferritic steel (general chromium content up to 17% by weight). The benefit of this method is that a sufficient amount of chromium, nickel or other oxide-forming elements to form a continuous protective scale in an amount capable of achieving a desired level of oxidation resistance in a metal alloy other than an iron-based alloy, particularly a cast product 18. It can also be expanded to cast metal alloys that do not contain. Notable examples of such alloys include certain nickel-based and cobalt-based alloys of the type often used in the manufacture of gas turbine components.

  A particular aspect of the present invention compensates for the lack of oxide-forming elements in the alloy by diffusing a certain amount of metallic aluminum into the surface region 20 (FIG. 2) of the cast product 18, and the oxidation resistance of the cast product 18. In particular, the formation of an oxide scale (not shown) on the surface 22 that is mostly alumina (in other words, the scale contains a higher weight of alumina than any other individual oxidizing component). Is to promote. As described above, FIG. 2 shows a fragment of the wall section of the mold 10 with the solid facecoat 16 applied to the internal cavity surface 14 so that the molten alloy is introduced into the mold cavity 12 and contacts the solid facecoat 16. To express. The solid facecoat 16 is shown as containing a dispersoid of metallic aluminum-containing particles 24, wherein the metallic aluminum-containing particles are 15% by weight or more of the solid facecoat 16, such as 50% by weight or more of the solid facecoat 16. Preferably, it constitutes 65% by weight to about 90% by weight of the solid face coat 16. The components of the solid facecoat 16 are adapted so that its aluminum content is sufficient to provide a source of metallic aluminum that diffuses into the cast product 18 during the casting process to form the surface region 20 shown in FIG. Is done. Specifically, the aluminum content in the solid facecoat 16 and the temperature of the molten iron-base alloy and cast product 18 during the casting process is such that the diffusion of metallic aluminum from the solid facecoat 16 to the surface region 20 of the cast product 18. Is a combination that is sufficient to produce The minimum temperature sufficient to produce this effect depends in part on the composition of the particles 24. If particles 24 are essentially aluminum (with impurities), a temperature of about 1300 ° F. (about 705 ° C.) is considered appropriate, and particles 24 are essentially composed of a majority of aluminum. A temperature of 1400 ° F. (about 760 ° C.) is considered suitable for non-limiting aluminum alloys such as CrAl, CoAl, NiAl, FeAl, MoAl, MnAl, AlZr and Includes AlNb alloy. As will be described in more detail below, the diffusion of aluminum into the surface region 20 can be further facilitated by the presence of an active agent in the solid facecoat 16, which is preferably up to about 35 of the solid facecoat 16. It comprises, for example, about 10 to about 35% by weight of the solid facecoat 16, more preferably about 10 to about 20% by weight of the solid facecoat 16. The balance (if any) of the solid facecoat 16 may be a solid that is inert to the casting process.

  The surface region 20 can also be referred to as the aluminum-enriched surface region 20 of the cast product 18 in that the aluminum content resulting from the diffusion method exceeds the aluminum content (if any) of the remainder of the cast product 18. The aluminum content in the surface region 20 is preferably such that the casting surface 22 defined by the surface region 20 can form a continuous oxide scale containing an amount of alumina sufficient to promote the oxidation resistance of the cast product 18. Is more than 10% by weight, more preferably more than 12% by weight. A particularly suitable range is believed to be 14% to about 40% aluminum by weight of the surface region 20. The aluminum content in the surface region 20 is generally in the form of one or more intermetallic compounds, and its configuration depends on the composition of the casting surface 22. For example, an iron-based casting surface forms a FeAl intermetallic compound, a nickel-based casting surface forms a NiAl intermetallic compound, and a cobalt-based casting surface forms a CoAl intermetallic compound. The surface region 20 preferably extends at least 50 μm below the surface 22 of the cast product 18, for example in the range of about 50 to about 300 μm of the casting surface 22, and possibly even below (eg about 400 μm). Due to the diffusion method, the aluminum content does not become uniform throughout the thickness of the region 20 but tends to decrease away from the casting surface 22 and the aluminum at the interface between the surface region 20 and the bulk casting. The content is adapted to be essentially equal to the aluminum content (if any) in the alloy used to form the cast product 18.

The volume of aluminum in the solid face coat 16 is in the form of individual particles 24 due to the particular method by which the solid face coat 16 is formed. Specifically, FIG. 1 shows a face coat gel 26 applied to the surface 14 of the mold 10, and the face coat gel 26 is then heated to form the solid face coat 16 shown in FIG. The facecoat gel 26 contains aluminum-containing particles 24 and an active agent, both of which cause the particles 24 to be dispersed in the solid facecoat 16 of FIG. 2 by the solid binder phase formed by the active agent. 1 is suspended in the face coat gel 26 of FIG. In order to promote the dispersion of the particles 24 in the solid face coat 16 and the diffusion of the aluminum into the surface region 20, the particles 24 and the active agent are preferably in the form of a powder. Suitable particle size ranges for the aluminum-containing particles 24 range up to about 100 mesh (about 149 μm), more preferably about 200 to about 325 mesh (about 74 to about 44 μm). Suitable activators for the application of the present invention are well-known halide activators, in particular sodium fluoride (NaF), potassium fluoride (KF), ammonium chloride (NH ₄ Cl), ammonium fluoride (NH ₄ F), Fluoride or chloride salts such as ammonium bromide (NH ₄ Br), ammonium iodide (NH ₄ I), and ammonium bifluoride (NH ₄ HF ₂ ) are included. When the solid facecoat 16 is exposed to the high heat of the molten alloy, the activator reacts with the aluminum in the solid facecoat 16 to form volatile aluminum halide (eg, AlF ₃ ), after which the casting surface 22 and / or Alternatively, it reacts with the surface of the molten alloy in contact with the solid facecoat 16 to deposit and diffuse aluminum to form the aluminum-containing surface region 20.

  In addition to the particles 24 and the active agent, the composition of the face coat gel 26 comprises an organic polymer binder. A suitable binder should be burned out without leaving any residue so that the solid facecoat 16 is not contaminated by the binder residue. A specific example of a suitable binder that meets these criteria is the aqueous organic polymer binder VITTA GEL®, commercially available from Vita. Other organic polymer binders could be used. The solid facecoat 16 is essentially aluminum or aluminum alloy particles 24 in the solid binder phase formed by the activator because the binder is completely burned out when the facecoat gel 26 is heated sufficiently. The organic polymer binder preferably comprises no more than 70% by weight of the facecoat gel 26, more preferably 20 to 50% by weight, with the remainder preferably being particles 24 and an activator (eg to obtain the preferred ranges described above). About 40 to about 60% by weight of particles 24 and about 10 to about 20% by weight of active agent). The face coat gel 26 contains a sufficient amount of an organic polymer binder that facilitates application of the face coat gel 26 to the mold surface 14. For this reason, the amount of polymer binder in the face coat gel 26 depends on the particle size and amount of the powder particles 24 and the form and amount of the active agent. It is within the scope of the present invention to form the face coat gel 26 using only the particles 24 and the organic polymer binder, so that the solid face coat 16 is essentially a layer of metallic aluminum or aluminum alloy. May be.

  The face coat gel 26 can be prepared by standard techniques using conventional mixers, and then conventional to form the solid face coat 16 on the mold cavity surface 14 by, for example, dipping, spraying, or other suitable techniques. You can receive the method. The facecoat gel 26 can be applied as a single layer or multiple layers, for example, to achieve the desired thickness of the facecoat gel 26 on the cavity surface 14. The viscosity of the facecoat gel 26 is preferably adapted such that the facecoat gel 26 can be applied to the mold surface 14 to achieve a final thickness of at least 50 μm, more preferably in the range of about 100 to about 300 μm. . The preferred thickness of the facecoat gel 26 depends on a variety of factors, including its aluminum content and the aluminum content desired for the surface region 20.

  Prior to introducing the molten alloy into the mold cavity 12, the facecoat gel 26 is preferably heated to burn out the polymer binder to form the solid facecoat 16 shown in FIG. Face coat gel 26 having the composition described above results in a solid face coat 16 comprising aluminum-containing particles 24 dispersed in a solid binder phase formed by the active agent. Heating the facecoat gel 26 to form the solid facecoat 16 is performed at a temperature in the range of about 200 to about 500 ° F. (about 90 to about 260 ° C.) prior to introducing the molten alloy into the mold cavity 12. Can be implemented. This treatment can be performed during the preheating of the mold 10 prior to the introduction of the molten alloy into the mold cavity 12. When the molten alloy comes into contact with the solid facecoat 16, the composition of particles 24, particularly the metal aluminum content, diffuses from the solid facecoat 16 into the alloy during melting and solidification to form the aluminum-enriched surface region 20. . If there is an active agent in the solid binder phase of the solid facecoat 16, it reacts with the aluminum in the solid facecoat 16 to further promote the deposition and diffusion of aluminum into the surface region 20 and be volatile. The aluminum halide is formed and then reacts with the solid face coat 16 as well as the surface of the molten alloy in contact with the casting surface 22 as solidification proceeds.

  As mentioned above, the aluminum content of the surface region 20 is sufficient to form a continuous adherent alumina-containing oxide scale on the surface 22 of the cast product 18. The oxide scale is continuous in that the oxide scale is present and covers the entire aluminum enriched surface region 20. Such oxide scale grows thermally on the casting surface 22 defined by the surface region 20 as a result of exposing the cast product 18 to high temperatures in the presence of oxygen. This exposure may be the result of the operating environment of the part produced from the cast product 18 or the result of a heat treatment specifically aimed at thermally growing the oxide scale. Alternatively, or in addition, oxide scale growth can occur during heat treatment of the cast product 18 that can be performed to produce the desired mechanical properties of the alloy. In order to provide sufficient oxidation resistance, the scale preferably contains at least 12 wt% alumina, and more preferably at least 15 wt% alumina, with the balance being other oxides such as chromium and nickel. An oxide formed as a result of an iron-based alloy containing a forming element. In the present method, the use of a low alloy iron-base alloy is preferred, so that the scale forming the cast product 18 has a chromium oxide and nickel oxide content less than alumina, more typically about 10% by weight or less. Contains chromium oxide and little or no nickel oxide.

  Thus, the present invention allows the use of low alloy iron-base alloys in place of the more expensive chromium-nickel alloy steels in various high temperature applications such as various internal components of gas turbine engines. The present invention achieves this aspect by improving the oxidation resistance of the low alloy iron-base alloy using a more protective oxide scale, the majority of which is alumina.

  Although the present invention has been described with respect to particular embodiments, other forms may be adopted by those skilled in the art. Accordingly, the scope of the invention should be limited only by the attached claims.

DESCRIPTION OF SYMBOLS 10 Mold 12 Cavity 14 Surface 16 Facecoat 18 Casting product 20 Area | region 22 Surface 24 Particle | grain 26 Gel

Claims (10)

A method for producing a cast product (18) from a metal alloy comprising:
Preparing a mold (10) having a cavity (12) and a continuous solid facecoat (16) on the mold surface (14) inside the cavity, wherein the solid facecoat (16) is 15% by weight or more of metallic aluminum Including a powder containing and an optional active agent;
Introducing a molten amount of metal alloy into the cavity (12) of the mold (10) to contact the molten metal alloy with the solid facecoat (16);
Cooling the mold (10) to cool and solidify the molten metal alloy to form a cast product (18) of the metal alloy, wherein during cooling, the metal aluminum in the solid facecoat (16) At least partially diffusing into the surface (22) of the cast product (18) to form an aluminum-containing surface region (20) in the cast product (18);
Removing the cast product (18) from the mold (10);
Growing scale on the surface (22) of the cast product (18) by oxidation of the surface region (20) of the cast product (18), wherein the scale is aluminum in the surface region (20) of the cast product (18). Including the step of containing alumina by oxidation of.   The method of claim 1, wherein the metal aluminum-containing powder in the solid facecoat (16) comprises aluminum and inevitable impurities.   The method of claim 1, wherein the metallic aluminum in the solid facecoat (16) is in the form of an aluminum alloy selected from the group consisting of CrAl, CoAl, NiAl, FeAl, MoAl, MnAl, AlZr and AlNb alloys.   The metal aluminum-containing powder is dispersed in a solid binder phase containing an activator, and the activator is formed by the formation of volatile aluminum halide by reaction with the metal aluminum in the solid facecoat (16). A halide activator that promotes the diffusion of metallic aluminum into the surface region (20), the volatile aluminum halide reacts with the surface of the molten alloy in contact with the solid facecoat (16) in the introduction step, and is cooled 4. A method according to any one of claims 1 to 3, wherein the step reacts with the surface (22) of the cast product (18). Forming a face coat gel (26) comprising a metallic aluminum-containing powder and an optional active agent in an organic polymer binder;
Applying a face coat gel (26) to the mold surface (14);
The method of any preceding claim, further comprising heating the facecoat gel (26) to burn out the organic polymer binder to form a solid facecoat (16).   The method according to any one of claims 1 to 5, wherein the metal alloy contains 10% by weight or less of an alloy component, and the balance is iron and inevitable impurities.   The method according to any one of claims 1 to 6, wherein the metal alloy contains 10 wt% or less chromium and 1 wt% or less nickel.   The method according to claim 1, wherein the scale contains 12% by weight or more of alumina.   The surface region (20) of the cast product (18) contains 10% by weight or more of aluminum and extends 50 μm or more below the surface (22) of the cast product (18). The method according to claim 1.   The method according to any of the preceding claims, wherein the cast product (18) is a gas turbine engine component.