SE1451283A1 - An article and a method of producing an article - Google Patents

Abstract

ABSTRACT An article (1) comprising a cast iron bulk material (3) and havingan inner surface delimiting an internal passage (2), said innersurface being coated with a coating comprising a corrosionresistant layer (4) comprising MCrAlX, wherein M is one of Feand Ni, and wherein X is an active element, and a ceramic layer(5) provided outside of the corrosion resistant layer. A method offorming such an article comprises forming a core and applying aceramic layer and a corrosion resistant layer comprising MCrAlX,wherein M is one of Fe and Ni, and wherein X is an activeelement, onto the core, assembling the core into a mold, castingiron into the mold at a pouring temperature lower than themelting temperature of the corrosion resistant layer to form said article, and removing the core. (Fig. 1)

Description

An article and a method of producing an article FIELD OF THE INVENTION AND PRIOR ART The invention relates to an article comprising a cast iron bulk material and having an inner surface delimiting an internalpassage, as well as a method of producing such an article. Sucharticles are commonly used in e.g. exhaust system of a vehicle.An example of such an article is an exhaust manifold, which hasa complex geometry including internal passages. Exhaustmanifolds as well as other components in exhaust systems ofinternal combustion engines are subjected to a corrosiveenvironment and to elevated temperatures. However, cast ironarticles are sensitive to corrosion, which reduces the service lifeof the article and impairs the mechanical properties. Cast ironarticles also show relatively high thermal conductivity. Hot exhaust gases flowing through the internal passages of anexhaust manifold made from cast iron therefore lose thermalenergy by heat conduction to the exhaust manifold andsurrounding areas. Large thermal losses are thus present in theexhaust system, which reduce the efficiency of the engine.Moreover, the large temperature variations present in theexhaust system may lead to thermal fatigue of the exhaustCast relatively inexpensive material in comparison with more corrosion manifold, reducing its service life. iron is however a resistant alloys and it is therefore a common choice of material to keep the material costs low.

A way to partly overcome the problem with thermal losses in theexhaust system is to apply a ceramic thermal barrier coating tothe outer surface of articles such as the exhaust manifold withinthe exhaust system. ln this way, thermal conduction from theexhaust manifold to surrounding components is prevented, butthe thermal conduction from the exhaust gases to the exhaustmanifold itself still leads to thermal losses in the exhaust gases.Furthermore, thermal fatigue is not prevented and neither is corrosion of the exhaust manifold.

Alternative materials such as superalloys and stainless steel arerelatively expensive and are therefore not suitable for use inexhaust systems of combustion engines in vehicles such as trucks, buses, cars, etc.

SUMMARY OF THE INVENTION lt is an object of the present invention to provide on one hand anarticle comprising a cast iron bulk material and having an internalpassage, and on the other hand a method of producing such anarticle, which article and method in at least some aspect areimproved with respect to known such articles and methods. lnparticular, it is an object to provide such an article which hasimproved corrosion resistance and thermal insulation propertiesin comparison with standard cast iron articles having an internal passage. ln the case of the article, this object is achieved by means of an article as initially defined, which is characterized in that said inner surface is coated with a coating comprising a corrosionresistant layer comprising MCrAlX, wherein M is one of Fe andNi, and wherein X is an active element, and a ceramic layer provided outside of the corrosion resistant layer.

The article according to the invention has relatively low materialcosts in comparison with e.g. articles having a bulk materialcomprising stainless steel or superalloy. However, thanks to theinternal coating, it has corrosion properties which are superior toplain cast iron articles and it is therefore suitable for use incorrosive environments such as in exhaust systems. The MCrAlXlayer, which is preferably in the form of a distinct non-diffusedlayer, on one hand provides corrosion resistance and on theother hand forms an excellent bond layer for bonding the ceramiclayer to the article. The ceramic layer is preferably a so calledthermal barrier coating, providing good thermal insulationproperties. The article according to the invention is thereforesuitable for use in e.g. exhaust systems of combustion engines,in which exhaust gases of elevated temperature flows throughthe internal passage of the article. The ceramic coating preventsthermal conduction from the exhaust gases to the bulk materialof the article. lt therefore efficiently minimizes power losses inthe exhaust system and thereby increases the efficiency of theengine. lt also increases the corrosion resistance of the bulkhightemperatures. Thereby, also the mechanical properties of the material, since corrosion is generally promoted at article are improved with respect to plain cast iron articles.

According to an embodiment of the invention, the corrosion resistant layer comprises FeCrAlX. By using FeCrAlX, both the corrosion resistant layer and the bulk material are ferritic, whichmeans that the thermal expansion properties of the bulk materialand the corrosion resistant layer are essentially the same. Thestability of the coating during thermal cycling is thereby increased.

According to an embodiment of the invention, the corrosion resistant layer has a thickness in the range 0.05-1 mm,preferably 0.05-O.5 mm, and more preferably O.1-O.5 mm. Thisthickness is sufficient to provide corrosion resistance to thearticle and the layer is also thin enough to keep the material costs low.

According to an embodiment of the invention, the ceramic layerhas a thickness in the range 0.05-1 mm, preferably O.2-O.5 mm.With a ceramic layer of this thickness, sufficient thermalinsulation properties are achieved while the material costs arekept reasonable. A too thick layer may lead to spallation while a too thin layer gives insufficient thermal insulation properties.

According to an embodiment of the invention, the ceramic layercomprises one of a stabilized zirconium oxide, mullite, orforsterite. These materials are all known to have suitable thermal insulation properties.

According to an embodiment of the invention, the ceramic layercomprises yttria-stabilized zirconia. ln this embodiment, the ceramic layer has excellent thermal insulation properties.

According to an embodiment of the invention, the coating furthercomprises an alumina layer formed between the corrosionresistant layer and the cast iron bulk material. The alumina layerreduces the risk of forming chromium carbides in the interfacebetween the cast iron bulk material and the corrosion resistantlayer due to carbon diffusion. Chromium carbides may otherwiseembrittle the corrosion resistant layer material close to the bulkmaterial and thereby reduce the adhesion of the layer. ln thisembodiment, the alumina layer protects the corrosion resistantlayer material from embrittlement and the adhesion of the corrosion resistant layer is improved.

According to an embodiment of the invention, the alumina layerhas a thickness of less than 500 nm, preferably less than 200nm. An alumina layer of this thickness is sufficient to form an efficient barrier for carbon diffusion.

According to an embodiment of the invention, the corrosion resistant layer comprises between 8-15 % Cr by weight, preferably between 10-14 % Cr by weight. By keeping thechromium content of the corrosion resistant layer relatively low,the risk of forming chromium carbides in the interface betweenthe cast iron bulk material and the corrosion resistant layer isreduced. This is thus an alternative way to prevent the bulkmaterial from embrittlement and to improve the adhesion of theOf course, an alumina layer may and the corrosion resistant layer to further reduce the risk of forming corrosion resistant layer. additionally be formed between the bulk material chromium carbides.

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

According to a second aspect of the invention, the abovementioned object is achieved by means of a method of producingan article comprising a cast iron bulk material and having aninner surface delimiting an internal passage, comprising thesteps: forming a core for defining the internal passage, applying a ceramic layer onto the core, applying a corrosion resistant layer comprising MCrAlX onto theceramic layer, wherein M is one of Fe and Ni, and wherein X isan active element, assembling the core into a mold, casting iron into the mold to form said article, and removing the core.

The corrosion resistant layer has a much higher meltingtemperature than the cast iron bulk material, typically about1500°C as compared to a pouring temperature of 1350°C, andMCrAlX therefore forms a distinct and non-diffused layer on theinner surface of the bulk material. By means of the inventivemethod, cast iron articles of complicated geometry can be formedwith a thermally insulating and corrosion resistant coating on aninternal surface of the article. The corrosion resistant layer, apartfrom protecting against corrosion, forms an excellent bond layerfor bonding the ceramic layer onto the article. Therefore, theapplication of a corrosion resistant layer is crucial for the qualityof the ceramic layer and is a prerequisite for obtaining the thermally insulating properties offered by the ceramic layer.

Without the corrosion resistant MCrAlX layer, the adhesion of the ceramic layer to the cast iron bulk material will be too poor.

According to an embodiment of this aspect of the invention, themethod further comprises the step: oxidizing the corrosion resistant layer before casting iron into themold, such that an alumina layer is formed between the corrosionresistant layer and the cast iron bulk material. ln this way, therisk of forming chromium carbides in the interface between theiron bulk materialThereby, the embrittlement and the adhesion of the corrosion resistant layer is cast and the corrosion resistant layer is reduced. bulk material is protected from improved.

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

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

Other advantageous features as well as advantages of the present invention will appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will in the following be further described by means of example with reference to the appended drawings, wherein Fig.1 schematically shows a cross section of an articleaccording to the invention, Fig.2 schematically shows a partial cross section of anarticle according to the invention, and Fig. 3 shows a flow chart of a method according to the invenüon.

DETAILED DESCRIPTION OF EMBODIMENTS OF THEINVENTION An article 1 according to a first embodiment of the invention isschematically shown in cross section in fig. 1. The article 1 isformed with an internal passage 2 and comprises a cast iron bulkmaterial 3, a corrosion resistant layer 4 applied on the cast ironbulk material 3 on its side facing the internal passage 2, and aceramic layer 5 applied on the corrosion resistant layer 4. ln theshown embodiment, the corrosion resistant layer 4 is preferablyin the form of a ferritic FeCrAlX layer, wherein X is one or moreof Hf, Y, Ti, Zr, elements known in the art. The FeCrAlX corrosion resistant layer La, Th, Ce, or one or more other reactive 4 on one hand prevents corrosion of the cast iron bulk material 3 and on one hand increases the adhesion of the ceramic layer 5.The corrosion resistant layer 4 preferably has a thickness in therange 0.05-1 mm, preferably 0.05-O.5 mm, and more preferablyO.1-O.5 mm. Since the FeCrAlX corrosion resistant layer 4 is indirect contact with the cast iron bulk material 3, it is preferable touse a FeCrAlX comprising between 8-15 % Cr by weight,preferably between 10-14 % Cr by weight. Typically, the FeCrAlXlayer may comprise about 10 % Cr by weight. With this relativelylow chromium content, the risk of forming chromium carbides inthe interface between the corrosion resistant layer 4 and the castiron bulk material 3 is reduced, and thereby also the risk ofembrittlement of the corrosion resistant layer 4. The ceramiclayer 5 has approximately the same thickness as the corrosionresistant layer 4 and is a thermal barrier layer in the form of e.g.a stabilized zirconium oxide, preferably yttria-stabilized zirconia, mullite, or forsterite.

A partial cross section of an article 1 according to a secondembodiment is schematically shown in fig. 2. ln this embodiment,the article 1 comprises a cast iron bulk material 3, a corrosionresistant layer 4 in the form of a FeCrAlX layer, wherein X ischosen according to above, and a ceramic layer 5 applied on the4. The article 1 in embodiment differs from the article 1 according to the first corrosion resistant layer this secondembodiment in that between the corrosion resistant layer 4 andthe cast iron bulk material 3, a thin alumina layer 6 is formed.The alumina layer 6 acts as a barrier layer against carbonthe FeCrAlX 4 and embrittlement of the FeCrAlX layer 4 close to the bulk material 3. diffusion into layer thereby prevents Thus, in this embodiment, the chromium content in the corrosion resistant FeCrAlX layer 4 may be increased to around 20-22 %by weight, which improves the corrosion resistance of the article1. The alumina layer 6 has a thickness of less than 500 nm, preferably less than 200 nm. ln a method according to the invention, an article 1 is formed byThe method illustrated as a flow chart in fig. 3. ln a first step S1, a core for means of a casting process. is schematicallydefining an internal passage 2 in the article 1 is formed. The corecan be e.g. a resin bonded sand core or similar. ln a step S2, aceramic layer 5, preferably in the form of an yttria-stabilizedzirconia layer, is applied onto the core, e.g. by thermal sprayingusing any of plasma spraying, flame spraying, wire arc spraying,etc. After applying the ceramic layer 5, a corrosion resistantlayer 4 comprising e.g. FeCrAlX, wherein X is one or more of Hf,Y, Ti, Zr, La, Th, Ce, or one or more other reactive elementsknown in the art, is applied by thermal spraying in a step S3. lnan optional step S4, the corrosion resistant layer 4 is subjectedto an oxidization, whereby a thin alumina layer 6 is formed on topof the corrosion resistant FeCrAlX layer 4. Thereafter, the core isassembled into a mold in a step S5, and iron is cast into the moldat a pouring temperature of approximately 1350°C in a step S6 toform the article 1. The pouring temperature is lower than themelting temperature of the corrosion resistant layer 4, which forFeCrAlX is typically around 1500°C, so the corrosion resistantlayer 4 thereby forms a non-diffused distinct layer on the castiron bulk material 3. ln a step S7, the core is removed and theinternal passage 2 is formed. The article 1 formed according to the method comprises a cast iron bulk material 3, and on its 11 inner surface a coating comprising a thin alumina layer 6, a corrosion resistant layer 4 and a ceramic layer 5 is formed. ln a cast iron exhaust manifold according to the invention, theinternal passages in which exhaust gases flow are covered with aceramic layer acting as a thermal barrier between the exhaustgases and the bulk material, a corrosion resistant layer bondingthe ceramic layer to the bulk material and preventing corrosion ofthe bulk material, and optionally an alumina layer preventing formation of chromium carbides in the corrosion resistant layer.

The invention is of course not in any way restricted to the em-bodiments described above, but many possibilities to modifica-tions thereof would be apparent to a person with skill in the artwithout departing from the scope of the invention as defined in the appended claims.

Claims (14)

1. An article (1) comprising a cast iron bulk material (3) andhaving an inner surface delimiting an internal passage (2), characterized in that said inner surface is coated with a coatingcomprising: a corrosion resistant layer (4) comprising MCrAlX, whereinM is one of Fe and Ni, and wherein X is an active element, and a ceramic layer (5) provided outside of the corrosion resistant layer (4).

2. The article according to claim 1, wherein the corrosion resistant layer (4) comprises FeCrAlX.

3. The article according to claim 1 or 2, wherein the corrosionresistant layer (4) has a thickness in the range 0.05-1 mm, preferably 0.05-O.5 mm, and more preferably O.1-O.5 mm.

4. The article according to any of the preceding claims,wherein the ceramic layer (5) has a thickness in the range 0.05- 1 mm, preferably O.2-O.5 mm.

5. The article according to any of the preceding claims,wherein the ceramic layer (5) comprises one of a stabilized zirconium oxide, mullite, or forsterite.

6. The article according to claim 5, wherein the ceramic layer (5) comprises yttria-stabilized zirconia.

7. The article according to any of the preceding claims, wherein the coating further comprises an alumina layer (6) 13 formed between the corrosion resistant layer (4) and the cast iron bulk material (3).

8. The article according to claim 7, wherein the alumina layer(6) has a thickness of less than 500 nm, preferably less than 200 flm.

9. The article according to any of the preceding claims,wherein the corrosion resistant layer (4) comprises between 8-15% Cr by weight, preferably between 10-14 % Cr by weight.

10. An article according to any of the preceding claims, wherein the article is an exhaust manifold.

11. A method of producing an article (1) comprising a cast iron bulk material (3) and having an inner surface delimiting aninternal passage (2), comprising the steps: forming a core for defining the internal passage (2), applying a ceramic layer (5) onto the core, applying a corrosion resistant layer (4) comprising MCrAlXonto the ceramic layer (5), wherein M is one of Fe and Ni, andwherein X is an active element, assembling the core into a mold, casting iron into the mold at a pouring temperature lowerthan the melting temperature of the corrosion resistant layer (4)to form said article (1), and removing the core.

12. The method according to claim 11, further comprising the step: 14 oxidizing the corrosion resistant layer (4) before castingiron into the mold, such that an alumina layer (6) is formedbetween the corrosion resistant layer (4) and the cast iron bulk material (3).

13. The method according to any of the claims 11-12, wherein thermal spraying is used in the step of applying a ceramic layer (5).

14. The method according to any of the claims 11-13, whereinthermal spraying is used in the step of applying a corrosion resistant layer (4).