DE112015004365T5 - Cast iron article with a corrosion resistant layer and method of making the article - Google Patents

Abstract

An article (1) comprising a cast iron base material (3) and an inner surface defining an inner passage (2), the inner surface being coated with a coating comprising a corrosion resistant layer (4) comprising MCrAIX, wherein M Fe and wherein X is an active element, and a ceramic layer (5) provided outside the corrosion-resistant layer. One method of forming such an article involves forming a core and applying a ceramic layer and a corrosion resistant layer comprising MCrAIX, where M is one of Fe and Ni and where X is an active element on the core, inserting the core into a mold, pouring iron into the mold at a casting temperature lower than the melting temperature of the corrosion-resistant layer to form the article, and removing the core.

Description

FIELD OF THE INVENTION AND PRIOR ART

The invention relates to an article having a cast iron base material and an inner surface that defines an inner passage, and a method of manufacturing such an article. Such articles are commonly used, for example, in the exhaust system of a vehicle. An example of such an article is an exhaust manifold that has a complex geometry with internal passages. Exhaust manifolds as well as other components in exhaust systems of internal combustion engines are exposed to a corrosive environment and elevated temperatures. However, cast iron articles are susceptible to corrosion which reduces the life of the article and degrades mechanical properties. Cast iron articles also have a relatively high thermal conductivity. Hot exhaust gases flowing through the inner passages of an exhaust manifold made of cast iron therefore lose heat energy by conduction to the exhaust manifold and surrounding areas. For this reason, large heat losses occur in the exhaust system, reducing the efficiency of the engine. In addition, the high temperature fluctuations present in the exhaust system can lead to fatigue due to thermal stress of the exhaust manifold, thereby shortening its life. However, cast iron is a relatively inexpensive material compared to more corrosion resistant alloys, and for this reason is commonly chosen as the material to keep material costs low.

One way in which this problem of heat losses in the exhaust system can be partially overcome is to apply a ceramic thermal barrier coating to the outer surface of articles such as the exhaust manifold in the exhaust system. In this way, heat conduction from the exhaust manifold to surrounding components is prevented, but the heat conduction from the exhaust gases to the exhaust manifold itself still results in heat losses in the exhaust gases. In addition, neither the fatigue due to thermal stress nor the corrosion of the exhaust manifold is prevented.

Alternative materials, such as superalloys and stainless steels, are relatively expensive and, for that reason, unsuitable for use in exhaust systems of internal combustion engines in vehicles such as trucks, buses, cars, etc.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide on the one hand an article with a cast iron base material and an inner passage and on the other hand a method for producing such an article, which article and method are improved in at least some aspects compared to known such articles and methods. In particular, it is an object to provide such an article which has improved corrosion resistance and improved thermal insulation properties as compared to standard cast iron internal passage articles.

In the case of the article, the object is achieved with an article as initially defined, which is characterized in that the inner surface is coated with a coating comprising a corrosion resistant layer comprising MCrAIX, where M is one of Fe and Ni and wherein X is an active element and a ceramic layer provided outside the corrosion-resistant layer.

The article of the invention has relatively low material costs compared to, for example, articles having a base material comprising stainless steel or a superalloy. However, thanks to the inner coating, it has corrosion properties superior to simple cast iron articles, and for this reason is suitable for use in corrosive environments, such as in exhaust systems. The MCrAIX layer, which preferably has the form of a special non-diffusing layer, on the one hand provides corrosion resistance and on the other hand forms an excellent bonding layer for bonding the ceramic layer to the article. The ceramic layer is preferably a so-called thermal barrier coating that provides good thermal insulation properties. The article according to the invention is therefore suitable, for example, for use in exhaust systems of internal combustion engines, in which exhaust gases with elevated temperature flow through the internal passage of the article. The ceramic coating prevents heat conduction from the exhaust gases to the base material of the article. Thus, it effectively minimizes energy losses in the exhaust system and thus increases the efficiency of the engine. It also increases the corrosion resistance of the base material, as corrosion is generally promoted by high temperatures. This also increases the mechanical properties of the article relative to simple cast iron articles.

According to the invention, the corrosion resistant layer comprises FeCrAIX. By using FeCrAIX are both the corrosion resistant Layer as well as the base material ferritic, that is, the thermal expansion properties of the base material and the corrosion resistant layer are substantially the same. The stability of the coating during the thermal cycling is thereby increased.

According to the invention, the corrosion resistant layer comprises between 8-15% by weight Cr, preferably between 10-14% by weight Cr. By keeping the chromium content of the corrosion resistant layer relatively low, the risk of forming chromium carbides in the interface between the cast iron base material and the corrosion resistant layer is reduced. This thus represents an alternative way of preventing the base material from becoming brittle and improving the adhesion of the corrosion-resistant layer. Of course, an aluminum oxide layer may also be formed between the base material and the corrosion resistant layer to further reduce the risk of forming chromium carbides.

According to one embodiment of the invention, the corrosion-resistant layer has a thickness in the range of 0.05-1 mm, preferably 0.05-0.5 mm, and more preferably 0.1-0.5 mm. This thickness is sufficient to impart corrosion resistance to the article, and the layer is also thin enough to minimize material costs.

According to one embodiment of the invention, the ceramic layer has a thickness in the range of 0.05-1 mm, preferably 0.2-0.5 mm. With a ceramic layer of this thickness sufficient thermal insulation properties are achieved while keeping material costs under control. Too thick a layer can lead to chipping, while too thin a layer will result in insufficient heat insulating properties.

According to one embodiment of the invention, the ceramic layer comprises one of a doped zirconia, mullite or forsterite. These materials are known to have all suitable thermal insulation properties.

According to one embodiment of the invention, the ceramic layer comprises yttrium-doped zirconium. In this embodiment, the ceramic layer has excellent heat insulating properties.

According to one embodiment of the invention, the coating further comprises an aluminum oxide layer formed between the corrosion resistant layer and the cast iron base material. The aluminum oxide layer reduces the risk of forming chromium carbides in the interface between the cast iron base material and the corrosion resistant layer due to carbon diffusion. Chromium carbides may otherwise brittle the material of the corrosion resistant layer in the vicinity of the base material and thereby reduce the adhesion of the layer. In this embodiment, the aluminum oxide layer protects the material of the corrosion-resistant layer from becoming brittle, and the adhesion of the corrosion-resistant layer is improved.

According to one embodiment of the invention, the aluminum oxide layer has a thickness of less than 500 nm, preferably less than 200 nm. An aluminum oxide layer of this thickness is sufficient to provide an efficient barrier to carbon diffusion.

According to one embodiment of the invention, the article is an exhaust manifold. Advantages and advantageous features of such exhaust manifold are listed above.

• – Bilden eines Kerns zum Definieren der inneren Passage,
• – Aufbringen einer Keramikschicht auf den Kern,
• – Aufbringen einer korrosionsbeständigen Schicht, die MCrAIX umfasst, auf den Kern, wobei M eines von Fe und Ni ist und wobei X ein aktives Element ist,
• – Einsetzen des Kerns in eine Gussform,
• – Gießen von Eisen in die Gussform, um den Artikel zu bilden, und Entfernen des Kerns.

According to a second aspect of the invention, the above-described object is achieved by a method for producing an article with a cast iron base material and an inner surface that delimits an inner passage, comprising the following steps:

• Forming a core for defining the inner passage,
• Applying a ceramic layer to the core,
• Applying a corrosion resistant layer comprising MCrAIX to the core, wherein M is one of Fe and Ni, and wherein X is an active element,
• Inserting the core into a mold,
• - Pour iron into the mold to form the article and remove the core.

The corrosion resistant layer has a much higher melting temperature than the cast iron base material, typically around 1500 ° C, compared to a casting temperature of 1350 ° C, and MCrAIX thus forms a special and non-diffusing layer on the inner surface of the base material. By means of the method according to the invention, cast iron articles having a complicated geometry with a heat-insulating and corrosion-resistant coating can be formed on an inner surface of the article. In addition to protecting against corrosion, the corrosion resistant layer forms an excellent bonding layer for bonding the ceramic layer to the article. Thus, the application of a corrosion resistant layer is critical to the quality of the ceramic layer and is a prerequisite for obtaining the thermal insulating properties provided by the ceramic layer. Without the MCrAIX corrosion resistant layer, adhesion is the ceramic layer on the cast iron base material bad.

According to one embodiment of this aspect of the invention, the method further comprises the following step:
Oxidizing the corrosion resistant layer prior to pouring iron into the mold such that an aluminum oxide layer is formed between the corrosion resistant layer and the cast iron base material. In this way, the risk of forming chromium carbides in the interface between the cast iron base material and the corrosion resistant layer is reduced. This protects the base material from becoming brittle and improves the adhesion of the corrosion-resistant layer.

According to one embodiment of this aspect of the invention, thermal spraying is used in the step of applying a ceramic layer. Thermal spraying includes, for example, plasma spraying, flame spraying, arc spraying, etc. Thermal spraying is an efficient way of applying a coating in the form of a ceramic layer having a desired thickness to the core, which may be, for example, a sand core or the like.

According to one embodiment of this aspect of the invention, thermal spraying is used in the step of applying a corrosion resistant layer. This is an efficient way of applying a coating in the form of a corrosion resistant layer having a desired thickness on top of the ceramic layer.

Further advantageous features as well as advantages of the present invention will become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of example with reference to the accompanying drawings. In which show

1 schematically a cross section of an article according to the invention,

2 schematically a partial cross-section of an article according to the invention, and

3 a flow diagram of a method according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

An article 1 according to a first embodiment of the invention is shown schematically in cross section in FIG 1 shown. The item 1 is with an inner passage 2 formed and includes a cast iron base material 3 , a corrosion-resistant layer 4 pointing to the cast iron base material 3 on its side is applied, that of the inner passage 2 is arranged facing, and one on the corrosion-resistant layer 4 applied ceramic layer 5 , In the embodiment shown, the corrosion resistant layer has 4 preferably the form of a FeCrAIX ferritic layer, wherein X is one or more of Hf, Y, Ti, Zr, La, Th, Ce, or one or more other reactive elements known in the art. The corrosion-resistant FeCrAIX coating 4 prevents on the one hand the corrosion of the cast iron base material 3 and on the other hand, increases the adhesion of the ceramic layer 5 , The corrosion resistant layer 4 preferably has a thickness in the range of 0.05-1 mm, preferably 0.05-0.5 mm, and more preferably 0.1-0.5 mm. Because the corrosion resistant FeCrAIX layer 4 in direct contact with the cast iron base material 3 Preferably, a FeCrAIX comprising between 8-15% by weight of Cr, preferably between 10-14% by weight of Cr is preferably used.

Typically, the FeCrAIX layer may comprise about 10 weight percent Cr. With this relatively low chromium content, there is the risk of forming chromium carbides in the interface between the corrosion resistant layer 4 and the cast iron base material 3 reduces and thereby also the risk that the corrosion resistant layer 4 becomes brittle. The ceramic layer 5 has about the same thickness as the corrosion resistant layer 4 and is a thermal barrier coating in the form of, for example, a doped zirconia, preferably yttrium-doped zirconium, mullite or forsterite.

A partial cross-section of an article 1 according to a second embodiment is shown schematically in FIG 2 shown. In this embodiment, the article comprises 1 a cast iron base material 3 , a corrosion-resistant layer 4 in the form of a FeCrAIX layer, wherein X is selected as described above, and is a ceramic layer 5 on the corrosion resistant layer 4 applied. The item 1 in this second embodiment differs from the article 1 according to the first embodiment, that between the corrosion-resistant layer 4 and the cast iron base material 3 a thin aluminum oxide layer 6 is formed. The aluminum oxide layer 6 serves as a barrier layer against carbon diffusion into the FeCrAIX layer 4 and thereby prevents the FeCrAIX layer 4 near the base material 3 becomes brittle. Thus, in this embodiment, the chromium content in the corrosion resistant FeCrAIX layer 4 increased to about 20-22 weight percent, reflecting the corrosion resistance of the article 1 improved. The aluminum oxide layer 6 has a thickness of less than 500 nm, preferably less than 200 nm.

In a method according to the invention becomes an article 1 formed by means of a casting process. The method is shown schematically as a flowchart in FIG 3 shown. In a first step S1, a core for defining an inner passage is formed 2 in the article 1 educated. The core may be, for example, a resin-bonded sand core or the like. In a step S2, a ceramic layer is formed 5 , preferably in the form of a layer of yttrium-doped zirconium, applied to the core, for example by thermal spraying using one of plasma spraying, flame spraying, arc spraying, etc., after application of the ceramic layer 5 becomes a corrosion resistant layer 4 which comprises, for example, FeCrAIX, wherein X is one or more of Hf, Y, Ti, Zr, La, Th, Ce, or one or more other reactive elements known in the art, applied by thermal spraying in a step S3. In an optional step S4, the corrosion resistant layer becomes 4 Subjected to oxidation, creating a thin aluminum oxide layer 6 on top of the corrosion-resistant FeCrAIX layer 4 is formed. Thereafter, the core is inserted into a mold in a step S5 and, in a step S6, iron is poured into the mold at a pouring temperature of about 1350 ° C to form the article 1 to build. The casting temperature is lower than the melting temperature of the corrosion resistant layer 4 , which is typically around 1500 ° C for FeCrAIX, so that the corrosion resistant layer 4 thereby a non-diffusing special layer on the cast iron base material 3 forms. In a step S7, the core is removed and the inner passage 2 educated. The article formed according to the method 1 includes a cast iron base material 3 and on its inside surface is a coating that is a thin alumina layer 6 , a corrosion-resistant layer 4 and a ceramic layer 5 includes, formed.

In a cast iron exhaust manifold according to the invention, the inner passages in which the gases flow are provided with a ceramic layer serving as a thermal barrier between the exhaust gases and the base material, a corrosion resistant layer which binds the ceramic layer to the base material and prevents corrosion of the base material, and optionally an alumina layer which prevents the formation of chromium carbides in the corrosion resistant layer.

Of course, the invention is by no means limited to the embodiments described above, but many possibilities for modifications will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims (13)

Items ( 1 ), which is a cast iron base material ( 3 ) and an inner surface having an inner passage ( 2 ), wherein the inner surface is coated with a coating comprising: a corrosion resistant layer ( 4 ) comprising MCrAIX, where M is one of Fe and Ni, and where X is an active element and one outside the corrosion resistant layer ( 4 ) provided ceramic layer 5 , characterized in that the corrosion resistant layer ( 4 ) FeCrAIX and that the corrosion-resistant layer ( 4 ) comprises between 8-15% by weight of Cr. Article according to claim 1, wherein the corrosion resistant layer ( 4 ) has a thickness in the range of 0.05-1 mm, preferably 0.05-0.5 mm, and more preferably 0.1-0.5 mm. Article according to claim 1 or 2, wherein the ceramic layer ( 5 ) has a thickness in the range of 0.05-1 mm, preferably 0.2-0.5 mm. Article according to one of the preceding claims, wherein the ceramic layer ( 5 ) comprises one of a doped zirconia, mullite or forsterite. Article according to claim 4, wherein the ceramic layer ( 5 ) Comprises yttrium-doped zirconium. Article according to one of the preceding claims, wherein the coating further comprises an aluminum oxide layer ( 6 ) between the corrosion resistant layer ( 4 ) and the cast iron base material ( 3 ) is formed. Article according to claim 6, wherein the aluminum oxide layer ( 6 ) has a thickness of less than 500 nm, preferably less than 200 nm. Article according to one of the preceding claims, wherein the corrosion-resistant layer ( 4 ) comprises between 10-14% by weight of Cr. An article according to any one of the preceding claims, wherein the article is an exhaust manifold. Method for producing an article ( 1 ), which is a cast iron base material ( 3 ) and an inner one Surface includes an inner passage ( 2 ), comprising the steps of: forming a core for defining the inner passage ( 2 ), Applying a ceramic layer ( 5 ) on the core, applying a corrosion-resistant layer ( 4 ), which comprises MCrAIX, onto the ceramic layer ( 5 where M is one of Fe and Ni and where X is an active element, inserting the core into a mold, pouring iron into the mold at a casting temperature lower than the melting temperature of the corrosion resistant layer ( 4 ) to the article ( 1 ) and removing the core. The method of claim 10, further comprising the step of: oxidizing the corrosion resistant layer ( 4 ) prior to pouring iron into the mold such that an alumina layer ( 6 ) between the corrosion resistant layer ( 4 ) and the cast iron base material ( 3 ) is formed. A method according to any one of claims 10-11, wherein in the step of applying a ceramic layer ( 5 ) thermal spraying is used. A method according to any one of claims 10-12, wherein in the step of applying a corrosion resistant layer ( 4 ) thermal spraying is used.

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