CAST PRODUCT HAVING ALUMINA BARRIER LAYER

Abstract

The present invention provides a cast product that can further enhance the stability of an alumina barrier layer and can exhibit further superior oxidation resistance, carburization resistance, nitriding resistance, corrosion resistance, and the like when used under a high-temperature atmosphere. The cast product according to the present invention is a cast product having an alumina barrier layer including an aluminum oxide on a surface of a matrix, and the aluminum oxide is (Al.sub.(1-x)M.sub.(x)).sub.2O.sub.3, where M is at least one of Cr, Ni, Si, and Fe, and x satisfies a relationship 0<x<0.5. Furthermore, the cast product according to the present invention is a cast product having an alumina barrier layer including an aluminum oxide on a surface of a matrix, and at least one of Cr, Ni, Si, and Fe forms a solid solution in the aluminum oxide, and at least one of Cr, Ni, Si, and Fe forming the solid solution with Al is contained so as to satisfy a relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio.

Claims

1-12. (canceled)

13. A cast product having an alumina barrier layer including an aluminum oxide on a surface of a matrix, wherein the aluminum oxide is (Al.sub.(1-x)M.sub.(x)).sub.2O.sub.3, where M is at least one of Cr, Ni, Si, and Fe, and x satisfies a relationship 0<x<0.5.

14. The cast product according to claim 13, wherein 80 vol % or more of a crystal structure of the aluminum oxide is a corundum structure.

15. The cast product according to claim 13, wherein at least Cr forms a solid solution in the aluminum oxide, and the Cr forming the solid solution with Al is contained so as to satisfy a relationship Al/Cr≧10 in an atomic % ratio.

16. The cast product according to claim 13, wherein at least one of Ni, Si, and Fe forms a solid solution in the aluminum oxide, and a total atomic % of the at least one of Ni, Si, and Fe forming the solid solution with Al is 10 atm % or less.

17. The cast product according to claim 13, wherein the aluminum oxide has a concentrated-Al layer in which the Al/(Cr+Ni+Si+Fe) on the matrix side is larger than that on the surface side.

18. The cast product according to claim 17, wherein a thickness of the concentrated-Al layer is one fifth or more of a thickness of the alumina barrier layer.

19. The cast product according to claim 13, wherein the alumina barrier layer has a surface roughness (Ra) of 15 μm or less.

20. The cast product according to claim 13, wherein the matrix includes C in an amount of 0.05 to 0.7 mass %, Si in an amount of more than 0 mass % to 2.5 mass % or less, Mn in an amount of more than 0 mass % to 3.0 mass % or less, Cr in an amount of 15.0 to 50.0 mass %, Ni in an amount of 18.0 to 70.0 mass %, Al in an amount of 1.0 to 5.0 mass %, rare earth elements in an amount of 0.005 to 0.4 mass %, and Win an amount of 0.5 to 10.0 mass % and/or Mo in an amount of 0.1 to 5.0 mass %, and the balance includes Fe and inevitable impurities.

21. The cast product according to claim 20, wherein the matrix further includes at least one selected from the group consisting of Ti in an amount of 0.01 to 0.6 mass %, Zr in an amount of 0.01 to 0.6 mass %, and Nb in an amount of 0.1 to 1.8 mass %.

22. The cast product according to claim 20, wherein the matrix further includes B in an amount of more than 0 mass % to 0.1 mass % or less.

23. A reaction tube made of the cast product according to claim 13, wherein the alumina barrier layer is formed on an inner surface of the tube through which hydrocarbon gas as a raw material flows.

24. A cast product having an alumina barrier layer including an aluminum oxide on a surface of a matrix, wherein at least one of Cr, Ni, Si, and Fe forms a solid solution in the aluminum oxide, and the at least one of Cr, Ni, Si, and Fe forming the solid solution with Al is contained so as to satisfy a relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio.

25. The cast product according to claim 24, wherein 80 vol % or more of a crystal structure of the aluminum oxide is a corundum structure.

26. The cast product according to claim 24, wherein at least Cr forms a solid solution in the aluminum oxide, and the Cr forming the solid solution with Al is contained so as to satisfy a relationship Al/Cr≧10 in an atomic % ratio.

27. The cast product according to claim 24, wherein at least one of Ni, Si, and Fe forms a solid solution in the aluminum oxide, and a total atomic % of the at least one of Ni, Si, and Fe forming the solid solution with Al is 10 atm % or less.

28. The cast product according to claim 24, wherein the aluminum oxide has a concentrated-Al layer in which the Al/(Cr+Ni+Si+Fe) on the matrix side is larger than that on the surface side.

29. The cast product according to claim 28, wherein a thickness of the concentrated-Al layer is one fifth or more of a thickness of the alumina barrier layer.

30. The cast product according to claim 24, wherein the alumina barrier layer has a surface roughness (Ra) of 15 μm or less.

31. The cast product according to claim 24, wherein the matrix includes C in an amount of 0.05 to 0.7 mass %, Si in an amount of more than 0 mass % to 2.5 mass % or less, Mn in an amount of more than 0 mass % to 3.0 mass % or less, Cr in an amount of 15.0 to 50.0 mass %, Ni in an amount of 18.0 to 70.0 mass %, Al in an amount of 1.0 to 5.0 mass %, rare earth elements in an amount of 0.005 to 0.4 mass %, and Win an amount of 0.5 to 10.0 mass % and/or Mo in an amount of 0.1 to 5.0 mass %, and the balance includes Fe and inevitable impurities.

32. The cast product according to claim 31, wherein the matrix further includes at least one selected from the group consisting of Ti in an amount of 0.01 to 0.6 mass %, Zr in an amount of 0.01 to 0.6 mass %, and Nb in an amount of 0.1 to 1.8 mass %.

33. The cast product according to claim 31, wherein the matrix further includes B in an amount of more than 0 mass % to 0.1 mass % or less.

34. A reaction tube made of the cast product according to claim 24, the alumina barrier layer being formed on an inner surface of the tube through which hydrocarbon gas as a raw material flows.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a cross-sectional view of a cast product before heat treatment.

[0019] FIG. 2 is a schematic cross-sectional view illustrating a state where a dilute-Al layer is formed by low-temperature heat treatment.

[0020] FIG. 3 is a schematic cross-sectional view illustrating a state where a concentrated-Al layer is formed between the dilute-Al layer and a matrix by high-temperature heat treatment.

[0021] FIG. 4 shows a TEM photograph of a film of Working Example 2 and graphs illustrating the results of an EDX analysis.

[0022] FIG. 5 shows a TEM photograph of a film of Working Example 7 and graphs illustrating the results of an EDX analysis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereinafter, embodiments of the present invention will be described in detail.

[0024] A cast product of the present invention has an alumina barrier layer including an aluminum oxide on the surface of a matrix.

[0025] The aluminum oxide in the alumina barrier layer is (Al.sub.(1-x)M.sub.(x)).sub.2O.sub.3, where M is at least one of Cr, Ni, Si, and Fe, and x is adjusted so as to satisfy the relationship 0<x<0.5.

[0026] Also, at least one of Cr, Ni, Si, and Fe forms a solid solution in the aluminum oxide in the alumina barrier layer, and at least one of Cr, Ni, Si, and Fe forming a solid solution with Al is contained so as to satisfy the relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio.

[0027] Explanation of Reasons for Limiting Components

[0028] As long as the cast product of the present invention is made of a heat-resistant alloy containing Cr in an amount of 15 mass % or more, Ni in an amount of 18 mass % or more, and Al in an amount of 1 to 5 mass %, it is possible to obtain the effects of the present invention. The cast product of the present invention is made of the following components, for example. It should be noted that in the following description, the term “%” refers to “mass %” unless otherwise stated.

[0029] C: 0.05 to 0.7%

[0030] C acts to improve castability and enhance a high-temperature creep rupture strength. Therefore, the content of C is set to at least 0.05%. However, if the content is too large, a primary carbide of Cr.sub.7C.sub.3 is likely to be extensively formed and the movement of Al for forming the alumina barrier layer is suppressed. As a result, Al is insufficiently supplied to the surface portion of a cast body and the alumina barrier layer is locally divided, and thus the continuity of the alumina barrier layer is impaired. Moreover, a secondary carbide excessively deposits to reduce ductility and toughness. Therefore, the upper limit is set to 0.7%. It should be noted that the content of C is more desirably 0.3 to 0.5%.

[0031] Si: more than 0% to 2.5% or less

[0032] Si is contained to serve as a deoxidizer for molten alloy and to enhance the fluidity of molten alloy. If the content is too large, a high-temperature creep rupture strength is reduced, and therefore, the upper limit is set to 2.5%. It should be noted that the content of Si is more desirably 2.0% or less.

[0033] Mn: More than 0% to 3.0% or Less

[0034] Mn is contained to serve as a deoxidizer for molten alloy and to fix S in molten alloy. If the content is too large, high-temperature creep rupture strength is reduced, and therefore, the upper limit is set to 3.0%. It should be noted that the content of Mn is more desirably 1.6% or less.

[0035] Cr: 15.0 to 50.0%

[0036] Cr is contained in an amount of 15.0% or more in order to contribute to the enhancement of high-temperature strength and cyclic oxidation resistance. However, if the content is too large, high-temperature creep rupture strength is reduced, and therefore, the upper limit is set to 50.0%. It should be noted that the content of Cr is more desirably 23.0 to 35.0%.

[0037] Ni: 18.0 to 70.0%

[0038] Ni is an element that is necessary to secure cyclic oxidation resistance and the stability of a metal structure. If the content of Ni is small, the content of Fe relatively becomes large. As a result, a Cr—Fe—Mn oxide is likely to be formed on the surface of the cast body, and thus the formation of the alumina barrier layer is inhibited. Therefore, the content of Ni is set to at least 18.0%. Even if the content of Ni exceeds 70.0%, it is impossible to obtain the efficacy corresponding to the increasing amount, and therefore, the upper limit is set to 70.0%. It should be noted that the content of Ni is more desirably 28.0 to 45.0%.

[0039] Al: 1.0 to 5.0%

[0040] Al is an element that is effective for enhancing carburization resistance and coking resistance. Also, in the present invention, Al is an element that is essential for forming the alumina barrier layer on the surface of the cast body. Therefore, the content of Al is set to at least 1.0%. However, if the content of Al exceeds 5%, the ductility is deteriorated, and therefore, the upper limit is set to 5.0% in the present invention. It should be noted that the content of Al is more desirably 2.5 to 3.8%.

[0041] Rare Earth Elements: 0.005 to 0.4%

[0042] The term “rare earth elements” means 17 elements including 15 elements of the lanthanide series ranging from La to Lu in the periodic table, and Y and Sc. It is preferable that rare earth elements to be contained in the heat-resistant alloy of the present invention include at least one element selected from the group consisting of Ce, La and Nd. Rare earth elements contribute to the formation of the alumina barrier layer and the enhancement of stability thereof.

[0043] When the alumina barrier layer is formed by heat treatment under a high-temperature oxidizing atmosphere, rare earth elements that are contained in an amount of 0.005% or more effectively contribute to the formation of the alumina barrier layer.

[0044] On the other hand, if the content is too large, the ductility and toughness are deteriorated, and therefore, the upper limit is set to 0.4%.

[0045] W: 0.5 to 10.0% and/or Mo: 0.1 to 5.0%

[0046] W and Mo enhance creep rupture strength by forming a solid solution in a matrix and strengthening an austenite phase. At least one of W and Mo is contained in order to achieve this efficacy. The content of W is set to 0.5% or more, and the content of Mo is set to 0.1% or more.

[0047] However, if the contents of W and Mo are too large, ductility is reduced and carburization resistance is deteriorated. Moreover, as in the case where the content of C is large, a primary carbide of (Cr, W, Mo).sub.7C.sub.3 is likely to be extensively formed and the movement of Al for forming the alumina barrier layer is suppressed. As a result, Al is insufficiently supplied to the surface portion of the cast body and the alumina barrier layer is locally divided, and thus the continuity of the alumina barrier layer is likely to be impaired. Furthermore, since W and Mo have a large atomic radius, they suppress the movement of Al and Cr and inhibit the formation of the alumina barrier layer due to the formation of a solid solution in the matrix.

[0048] Therefore, the content of W is set to 10.0% or less, and the content of Mo is set to 5.0% or less. It should be noted that when both elements are contained, the total content is preferably set to 10.0% or less.

[0049] In addition, the following components can be contained.

[0050] At least one selected from the group consisting of Ti in an amount of 0.01 to 0.6%, Zr in an amount of 0.01 to 0.6%, and Nb in an amount of 0.1 to 1.8%

[0051] Ti, Zr and Nb are elements that are likely to form carbides, and form less solid solutions in the matrix than W and Mo. Therefore, Ti, Zr and Nb do not exhibit any particular action of forming the alumina barrier layer, but enhance creep rupture strength. At least one of Ti, Zr and Nb can be contained as needed. The content of Ti or Zr is set to 0.01% or more and the content of Nb is set to 0.1% or more.

[0052] However, if they are excessively added, ductility is reduced.

[0053] Furthermore, Nb reduces the peeling resistance of the alumina barrier layer. Therefore, the upper limit of the content of Ti or Zr is set to 0.6%, and the upper limit of the content of Nb is set to 1.8%.

[0054] B: more than 0% to 0.1% or less

[0055] Since B exhibits an action of strengthening the particle boundaries of the cast body, B can be contained as needed. It should be noted that if the content of B is large, creep rupture strength is reduced, and therefore, the content of B is set to 0.1% or less even in the case where B is added.

[0056] The heat-resistant alloy making up the cast body of the present invention includes the above-described components and Fe as the balance. P, S, and other impurities that are inevitably mixed in the alloy when melting the alloy may be contained as long as such impurities are contained in an amount within a range that is usually allowable to this type of alloy material.

[0057] Cast Product

[0058] Molten metal having a composition including the above-described components is produced and cast by centrifugal casting, static casting, or the like into the cast product of the present invention having the above composition.

[0059] The obtained cast product can be shaped depending on the intended application.

[0060] One example of the cast product is a tube, in particular, a reaction tube used under a high-temperature environment.

[0061] It is particularly preferable to produce the cast product of the present invention by centrifugal casting. This is because when using centrifugal casting, a fine metal structure grows in the radial direction with orientation due to the progress of cooling by a metal mold and thus an alloy structure in which Al easily moves can be obtained.

[0062] Heat treatment, which will be described later, is performed on the cast product. The alumina barrier layer having a stable phase structure is formed by the heat treatment.

[0063] Heat Treatment

[0064] The heat treatment is performed on the cast product of the present invention under an oxidizing atmosphere. The heat treatment can be divided into low-temperature heat treatment and high-temperature heat treatment. It should be noted that the low-temperature heat treatment and the high-temperature heat treatment can be performed in separate steps or the high-temperature heat treatment may be performed subsequently to the low-temperature heat treatment.

[0065] Low-Temperature Heat Treatment

[0066] The low-temperature heat treatment is treatment in which an aluminum oxide layer is formed on the surface of the matrix under an oxidizing atmosphere. One example of a low temperature is a temperature of less than 1050° C. It is desirably 600 to 900° C. It is desirable to perform the low-temperature heat treatment for 5 to 15 hours.

[0067] By performing the low-temperature heat treatment, oxygen comes into contact with a matrix 10 as shown in. FIG. 1 to oxidize Al, Cr, Ni, Si, and Fe that has diffused from the matrix 10 to the surface of the matrix, and an oxide layer 22 is formed as shown in FIG. 2. Since this heat treatment is performed at a low temperature, Al forms oxides prior to Cr, Ni, Si, and Fe. Accordingly, an aluminum oxide layer 22 that contains Al as a main component and in which at least one of Cr, Ni, Si, and Fe, which have similarly diffused from the matrix, forms a solid solution is the oxide layer.

[0068] In the aluminum oxide formed by the low-temperature heat treatment, at least one of Cr, Ni, Si, and Fe forming a solid solution with Al is contained so as to satisfy the relationship Al/(Cr+Ni+Si+Fe)≧2.0 in an atomic % ratio. In addition, it is desirable that the composition thereof is (Al.sub.(1-x)M.sub.(x)).sub.2O.sub.3, where M is at least one of Cr, Ni, Si, and Fe, and x satisfies the relationship 0<x<0.5. Moreover, at least Cr forms a solid solution in the aluminum oxide, and Cr forming a solid solution with Al is more preferably contained so as to satisfy the relationship Al/Cr≧10 in atomic % ratio, and still more preferably the relationship Al/Cr≧15. Furthermore, at least one of Ni, Si, and Fe forms a solid solution, and it is more desirable that the total atomic % of at least one of Ni, Si, and Fe forming a solid solution with Al is 10 atm % or less.

[0069] The aluminum oxide formed by the above-described low-temperature heat treatment has a metastable γ or θ alumina structure, which is a porous structure. Accordingly, the strength is not enough.

[0070] High-Temperature Heat Treatment

[0071] The high-temperature heat treatment is heat treatment that is performed after the low-temperature heat treatment, and in which, as described later, the phase of the aluminum oxide formed by the low-temperature heat treatment is transformed to an α alumina structure (corundum structure), and an aluminum oxide layer containing Al at a high concentration is formed between that aluminum oxide layer and the matrix.

[0072] The high-temperature heat treatment can be performed by heating the cast product on which the low-temperature heat treatment has been performed and the alumina barrier layer having a γ or θ alumina structure has been formed, at a high temperature under an oxidizing atmosphere. One example of a high temperature is a temperature of 1050° C. or more. It is desirable to perform the high-temperature heat treatment for 3 to 15 hours.

[0073] By performing the high-temperature heat treatment, the phase of the aluminum oxide that was formed first and has a γ or θ alumina structure is transformed to a stable a alumina structure (corundum structure). In the present invention, at least one of Cr, Ni, Si, and Fe forms a solid solution in the aluminum oxide layer having a γ or θ alumina structure. This makes it possible to promote the phase transformation from a γ or θ alumina structure to an α alumina structure (corundum structure) compared with the case where the aluminum oxide layer contains Al in a high ratio.

[0074] Then, the high-temperature heat treatment is further continuously performed on the cast product having the aluminum oxide layer in which the phase has been transformed to an α alumina structure (corundum structure), and thus oxygen passes through the aluminum oxide layer 22 as shown in FIG. 3.

[0075] The oxygen that has passed through the aluminum oxide layer 22 oxidizes Al that diffuses from the matrix, and an aluminum oxide layer 24 that contains Al at a high concentration is formed.

[0076] Here, as shown in FIG. 3, the aluminum oxide layer that is formed by the low-temperature heat treatment and in which at least one of Cr, Ni, Si, and Fe forms a solid solution is referred to as “dilute-Al layer”, and the aluminum oxide layer that is formed between the dilute-Al layer and the surface of the matrix and contains Al at a high concentration is referred to as “concentrated-Al layer”. Specifically, in the concentrated-Al layer 24, Al/(Cr+Ni+Si+Fe) is larger than that in the dilute-Al layer 22.

[0077] It is thought that the reason why in terms of the alumina barrier layer, the concentrated-Al layer formed between the matrix and the dilute-Al layer contains Al at a higher concentration than the dilute-Al layer at the surface is as follows.

[0078] The formed dilute-Al layer 22 allows a small amount of oxygen to pass therethrough under an oxidizing atmosphere.

[0079] Al, Cr, Ni, Si, and Fe diffuse from the matrix 10 side to the matrix surface side as shown in FIG. 3. However, since Al needs a smaller amount of energy for binding to oxygen than Cr, Ni, Si, and Fe, Al priorly binds to oxygen, and the aluminum oxide layer (concentrated-Al layer 24) containing aluminum oxide at a high concentration is formed between the matrix 10 and the dilute-Al layer 22.

[0080] The concentrated-Al layer 24 is formed by high-temperature heat treatment, and thus has a stable a alumina structure (corundum structure). It is desirable that 80 vol % or more of a crystal structure of each of the dilute-Al layer 22 and the concentrated-Al layer 24 is an α alumina structure (corundum structure).

[0081] Since the alumina barrier layer 20 is constituted by the dilute-Al layer 22 and the concentrated-Al layer 24 formed between the matrix 10 and the dilute-Al layer 22, both of which have stable a alumina structures (corundum structures), the alumina barrier layer 20 has high denseness, serves as a barrier for preventing oxygen, carbon and nitrogen from entering the base material from the outside during use under a high-temperature atmosphere in the cast product provided therewith, and can maintain superior oxidation resistance, carburization resistance, nitriding resistance, corrosion resistance, and the like for a long period of time.

[0082] It should be noted that the concentrated-Al layer 24 is desirably formed so as to be thicker than the dilute-Al layer 22, and it is preferable to form the concentrated-Al layer 24 such that the thickness of the concentrated-Al layer 24 is one fifth or more of that of the alumina barrier layer 20.

[0083] It is more desirable that the dilute-Al layer 22 has a thickness of 0.04 to 8.0 μm and the concentrated-Al layer 24 has a thickness of 0.01 to 2.0 μm.

[0084] It is desirable to heat the cast product while rotating it in order to preferably form the aluminum oxide layer in the above-described low-temperature heat treatment and high-temperature heat treatment. This makes it possible to heat the cast product uniformly and bring the cast product into contact with oxygen in a good state. As a result, it is possible to reduce the surface roughness (Ra) of the formed alumina barrier layer 20.

[0085] Surface Treatment

[0086] It is possible to perform surface treatment on the alumina barrier layer of the cast product as needed. One example of the surface treatment is polishing. For example, when the cast product is used for a reaction tube, Fe, Ni and the like of the cast product come into contact with a hydrocarbon as a raw material and coke (carbon) is likely to adhere to the inner surface of the tube due to the catalytic action of Fe and Ni, but it is possible to suppress the adhesion of coke by performing the surface treatment to reduce the surface roughness (Ra) of the alumina barrier layer.

[0087] It is desirable to perform the surface treatment such that the surface roughness (Ra) of the alumina barrier layer is 15 μm or less. The surface roughness (Ra) is more desirably 0.05 to 10 μm.

Example 1

[0088] Molten metal was produced by atmospheric melting in a high-frequency induction melting furnace and cast by metal mold centrifugal casting into tube bodies having alloy chemical compositions shown in Table 1 below. The tube body has an inner diameter of 80 mm, an outer diameter of 100 mm, and a length of 250 mm.

TABLE-US-00001 TABLE 1 Alloy chemical composition (the balance includes Fe and inevitable impurities) (mass %) No. C Si Mn Cr Ni Al REM W Mo Ti Zr Nb B Work. 0.33 0.49 0.32 24.5 43.6 1.2 0.25 2.1 0.6 0.7 Ex. 1 Work. 0.45 0.49 0.9 24.3 34.6 1.0 0.16 1.5 0.13 Ex. 2 Work. 0.4 0.33 0.7 23.8 31.5 3.3 0.26 2.8 0.03 Ex. 3 Work. 0.46 1.5 1.2 25.2 35.0 2.8 0.21 4.2 0.09 0.12 Ex. 4 Work. 0.26 0.41 0.5 23.5 34.6 3.1 0.07 0.9 0.07 Ex. 5 Work. 0.31 0.4 0.2 18.3 67.1 4.7 0.01 0.4 1.6 Ex. 6 Work. 0.38 0.26 0.4 23.8 34.4 4.9 0.11 0.95 0.1 Ex. 7 Work. 0.67 1.5 1.1 23.9 40.1 4.9 0.19 2.9 0.03 Ex. 8 Comp. 0.33 1.78 0.17 25.0 33.4 0.0 0.11 0.83 0.12 Ex. 1 Comp. 0.40 1.3 0.9 25.4 15.0 0.9 0.29 2.9 Ex. 2 Comp. 0.27 1.02 0.2 23.8 33.6 1.1 0.19 11.7 Ex. 3 Comp. 0.34 0.6 0.2 25.0 45.4 2.9 0.09 1.5 1.3 Ex. 4 Comp. 0.45 1.43 1.3 22.9 34.7 3.2 0.24 3.15 0.23 Ex. 5 Comp. 0.45 0.54 0.7 23.8 29.7 5.1 0.15 1.5 0.21 Ex. 6

[0089] Two steps of heat treatment that differed in a heating temperature were performed on each of Working Examples 1 to 8, which were obtained examples of the present invention, and Comparative Examples 1 to 6 under an oxidizing atmosphere. First, the low-temperature heat treatment was performed, and the high-temperature heat treatment was subsequently performed. The low-temperature heat treatment was performed for 5 hours, and the high-temperature heat treatment was performed for 5 hours.

TABLE-US-00002 TABLE 2 Low-temperature heat High-temperature heat treatment temperature treatment temperature No. (° C.) (° C.) Work. Ex. 1 700 1050 Work. Ex. 2 900 1100 Work. Ex. 3 800 1050 Work. Ex. 4 800 1100 Work. Ex. 5 900 1100 Work. Ex. 6 600 1050 Work. Ex. 7 700 1100 Work. Ex. 8 600 1150 Comp. Ex. 1 800 900 Comp. Ex. 2 1000 1250 Comp. Ex. 3 1200 1300 Comp. Ex. 4 500 1150 Comp. Ex. 5 900 1000 Comp. Ex. 6 600 800

[0090] The atomic percentages of elements (Al, Cr, Fe, Ni, Si, O) contained in the alumina barrier layer formed on the surface of a sample tube of each of Working Examples 1 to 8 and Comparative Examples 1 to 6 that had been subjected to the heat treatment were measured by an EDX analysis (energy dispersive X-ray spectrometry). Table 3 shows the results.

TABLE-US-00003 TABLE 3 Concentrated-Al Cr + Fe + Al/ layer thickness/ Al Cr Fe Ni Si O Ni + Si (Cr + Fe + Fe + Ni + Si Al barrier layer No. (atm %) (atm %) (atm %) (atm %) (atm %) (atm %) (atm %) Ni + Si) Al/Cr (atm %) thickness Work. 41.66 2.7 4.46 3.52 48.56 10.7 3.9 15.43 7.98 0.50 Ex. 1 Work. 39.2 3.6 1.4 3.2 52.6 8.2 4.8 10.89 4.6 0.30 Ex. 2 Work. 46.6 0.1 0.1 53.2 0.2 233.0 466.0 0.1 0.80 Ex. 3 Work. 40.96 2.23 2.34 2.12 52.35 6.7 6.1 18.37 4.46 0.60 Ex. 4 Work. 38.4 1.5 10.1 6.4 1.2 42.4 19.2 2.0 25.60 17.7 0.40 Ex. 5 Work. 42.49 1.31 2.13 1.95 52.12 5.4 7.9 32.44 4.08 0.75 Ex. 6 Work. 42.69 1.28 55.93 1.3 33.4 33.35 0 0.70 Ex. 7 Work. 41.8 2.4 3.9 6.5 45.4 12.8 3.3 17.42 10.4 0.50 Ex. 8 Comp. 4.61 10.25 7.09 31 47.05 53.0 0.0 0.00 48.34 0.00 Ex. 1 Comp. 11.17 15.25 30.1 25.28 1.47 16.73 72.1 0.2 0.73 56.85 0.10 Ex. 2 Comp. 6.7 35.46 1.65 1.8 1.64 52.75 40.6 0.2 0.19 5.09 0.05 Ex. 3 Comp. 36.7 6.7 1.8 10.3 44.5 18.8 1.95 5.48 12.1 0.15 Ex. 4 Comp. 9.77 21.54 11.34 9.51 1.02 46.82 43.4 0.2 0.45 21.87 0.10 Ex. 5 Comp. 8.35 11 15.18 13.76 51.71 39.9 0.2 0.76 28.94 0.05 Ex. 6

[0091] All of Working Examples 1 to 8, which are examples of the present invention, satisfy the relationship Al/(Cr+Ni+Si+Fe)≧2.0 in atomic % ratio. Furthermore, they satisfy the relationship Al/Cr≧10. On the other hand, Comparative Example 1 contains no Al in the matrix, and therefore, no aluminum oxide is formed and both Al/(Cr+Ni+Si+Fe) and Al/Cr are zero.

[0092] Moreover, all of Comparative Examples 2 to 6 satisfy the relationships Al/(Cr+Ni+Si+Fe)<2.0 and Al/Cr<10.

[0093] Furthermore, Fe+Ni+Si is 10 atm % or less in Working Examples 1 to 4, and 6 and 7, and Comparative Example 3, and more than 10 atm % in other working examples and comparative examples.

[0094] The ratio of the thickness of the concentrated-Al layer with respect to the thickness of the formed alumina barrier layer was measured in each of obtained Working Examples 1 to 8 and Comparative Examples 1 to 6. Table 3 above shows the results.

[0095] Table 3 shows that the ratio of the thickness of the concentrated-Al layer with respect to the thickness of the alumina barrier layer is 0.3 or more, that is, one fifth or more in each working example, but it is 0.15 at most in the comparative examples. It should be noted that since Comparative Example 1 contains no Al, no alumina barrier layer is formed.

[0096] This shows that since the low-temperature heat treatment was performed on the working examples, which were examples of the present invention, at a temperature of less than 1050° C. and the high-temperature heat treatment was performed thereon at a temperature of 1050° C. or more, the dilute-Al layer was formed on the surface of the matrix by the low-temperature heat treatment and then the concentrated-Al layer could be formed between the dilute-Al layer and the matrix by the high-temperature heat treatment.

[0097] On the other hand, it is thought that the ratio of the thickness of the concentrated-Al layer with respect to the thickness of the alumina barrier layer was 0.15 at most in Comparative Examples 2 to 6 on which the alumina barrier layer was formed for the following reasons.

[0098] In Comparative Example 2, the cast body contained Al in a small amount of 0.9%, and Al for forming a film on the surface of the cast body was insufficient. In Comparative Example 3, since the low-temperature heat treatment was performed at a high temperature of 1200° C., oxides containing Cr, Ni, Si, Fe and the like as a main component were formed before the alumina barrier layer having a γ or θ alumina structure was formed. In Comparative Example 4, since the low-temperature heat treatment was performed at a low temperature of 500° C., no alumina barrier layer having a γ or θ alumina structure was formed. In Comparative Examples 5 and 6, the high-temperature heat treatment was performed at a low temperature of 1000° C. As a result, a small amount of oxygen passed through the dilute-Al layer in the high-temperature heat treatment after the dilute-Al layer was formed in the low-temperature heat treatment, and Al did not obtain sufficient energy for binding to the oxygen taken in because the temperature was low.

[0099] Next, a coking test was performed on the obtained sample tubes.

[0100] The coking test was performed by placing the sample tubes in an electric furnace, supplying a hydrocarbon (ethane) to the sample tubes, and then heating them at a high temperature (955° C.) for a predetermined time (12 to 24 hours). After the test, the degrees of carburization of the inner surfaces of the sample tubes were compared, and the weight ratio of coke (carbon) adhering to the inner surface of each sample tube was measured. Table 4 shows the results.

TABLE-US-00004 TABLE 4 Carburization Weight ratio of Surface No. resistance formed coke roughness (Ra) Work. Ex. 1 Good 0.4 0.13 Work. Ex. 2 Fair 0.6 5.51 Work. Ex. 3 Good 0.8 7.63 Work. Ex. 4 Good 0.6 1.53 Work. Ex. 5 Fair 1.0 11.7 Work. Ex. 6 Good 0.6 1.54 Work. Ex. 7 Good 0.7 2.02 Work. Ex. 8 Good 1.1 13.8 Comp. Ex. 1 Poor 0.9 9.17 Comp. Ex. 2 Poor 0.7 4.5 Comp. Ex. 3 Poor 1.5 18.1 Comp. Ex. 4 Poor 0.7 8.32 Comp. Ex. 5 Poor 0.8 6.27 Comp. Ex. 6 Poor 1.4 16.47

[0101] Table 4 shows that all of Working Examples 1 to 8, which were examples of the present invention, had favorable carburization resistance. On the other hand, all of the comparative examples were carburized to the inside of the sample tube.

[0102] It is because the alumina barrier layer having a stable a alumina structure (corundum structure) that was constituted by the concentrated-Al layer and the dilute-Al layer was preferably formed on the surface of the matrix that Working Examples 1 to 8 had superior carburization resistance. In particular, Working Examples 1, 3, 4, and 6 to 8 had extremely superior carburization resistance compared with the other working examples. It is thought that this is because a smaller amount of the concentrated-Al layer was formed in. Working Examples 2 and 5 than in the other working examples.

[0103] Furthermore, the surface roughness (Ra) of each sample tube was measured. Table 4 shows the results as well. Table 4 shows that the weight ratio of the formed coke and the surface roughness (Ra) were substantially in proportion to each other. Accordingly, the surface roughness (Ra) is preferably 15 μm or less, and more preferably 10 μm or less.

[0104] It is possible to adjust the surface roughness (Ra) by performing the heat treatment while rotating the cast product. It is thought that it is because the heat treatment for forming a film was not properly performed and the surface roughness was increased due to the peeling and restoration of the film that the surface roughness of Comparative Examples 3 and 6 exceeded 15 μm.

Example 2

[0105] The alumina barrier layers of Inventive Examples 2 and 7 are observed using a transmission electron microscope (TEM). Furthermore, the EDX analysis was performed on the dilute-Al layer and the concentrated-Al layer of each inventive example. FIG. 4 shows the results from Inventive Example 2, and FIG. 5 shows the results from Inventive Example 7.

[0106] FIG. 4 shows that in Inventive Example 2, small amounts of Cr, Fe, and Ni were detected in the dilute-Al layer 22 formed on the surface side, which contained Al oxides as a main component. On the other hand, no Cr, Fe, Ni, and the like were detected in the concentrated-Al layer 24 but Al. Accordingly, it is found that the concentrated-Al layer 24 was made of aluminum oxide having a very high purity.

[0107] FIG. 5 shows that in Inventive Example 7, a small amount of Cr was detected in the dilute-Al layer 22 formed on the surface side, which mainly included Al oxides. On the other hand, nothing but Al was detected in the concentrated-Al layer 24. Accordingly, it is found that the concentrated-Al layer 24 was made of aluminum oxide having a very high purity.