EP0480404B1

EP0480404B1 - Corrosion-resistant and heat-resistant metal composite and method of producing

Info

Publication number: EP0480404B1
Application number: EP91117218A
Authority: EP
Inventors: Shinichiro Yahagi; Hiroshi Yamada; Kikuichi Funao; Fumio Iwane
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 1990-10-09
Filing date: 1991-10-09
Publication date: 1995-07-19
Anticipated expiration: 2011-10-09
Also published as: EP0480404A2; DE69111362D1; EP0480404A3; DE69111362T2

Description

The present invention concerns a metal composite having both corrosion-resistant property and heat-resistant property. The invention also concerns the method of producing the composite.
As the materials for constructing equipments used in a high temperature and corrosive atmosphere, such as liners of reactors, catalyst carriers of automobile exhaust gas cleaners, and electric heaters, there have been used various alloys having corrosion-resistant property and heat-resistant property required for the use.
The requirement for the corrosion-resistant property of this kind of metal materials has been getting severer. Taking the electric heater materials as an example, among the Fe-Cr alloys, 20Cr-5Al-Fe alloy or FCH1 alloy has relatively high corrosion-resistance. On the other hand, "FCH51" (product mark of Daido Steel Co.,Ltd.) containing smaller amounts of Cr and Al, or 15Cr-3Al-Fe alloy, exhibits a little lower high temperature corrosion resistance, but is less expensive and easy to process, and therefore, the alloy has wide use as the heater materials. Thus, there has been a strong demand for the material which exhibits good oxidation resistance and corrosion resistance at high temperatures, and of good processability, and therefore, is suitable for the heater materials.
In view of the facts that corrosion-resistant and heat-resistant alloys are generally hard to process and that the corrosion resistance depends on status of the surface of the material, there has been proposed a variety of composite materials made by combining a substrate having good processability and another surface material of corrosion resistance thereon, as well as materials made by surface treatment for improving the corrosion resistance.
The inventors intended to meet the above demand, and tried to form a surface protecting layer of a corrosion-resistant and heat-resistant substance such as Al₂O₃ by coating it on a heater material made of FCH51 alloy or the like. It was found that, due to the difference in the thermal expansion coefficients of the materials, the coated layer falls off from the substrace after repeated cycles of heating and cooling. Then, they formed protecting layers by vapor-deposition of Al₂O₃ and succeeded in improving durability of the layers. However, the vapor-deposition operation is of low productivity and high costs, and therefore, they concluded that this technology is difficult to be widespread.
With respect to the metal honeycomb material used in the automobile exhaust gas cleaning devices the inventors invented a material of improved corrosion resistance which is made by cladding an Al-foil on an alloy of a basic composition, 15Cr-3Al-Fe (so-called "Elemann's steel"), rolling the cladded sheet, annealing in vacuum and heating in an oxidizing atmosphere to oxidize the surface Al to Al₂O₃. The Elemann's steel has good processability and the surface corrosion resistance is improved by the formation of the Al₂O₃. Thus, it is now possible to produce corrosion-resistant and heat-resistant parts of desired shape. The material was already proposed(Japanese Patent Application Hei 2-192090).
As a matter of fact, it was sometimes observed that, when the above material is used for a long period at a high temperature, pitching occurs. It is considered that the pitching may occur due to the fact that Al₂O₃ at the surface is in the form of whiskers and therefore, though adhesion to the substrate is good, the solidity in the layer direction is not necessarily high throughout the surface.
The inventors also establish the technology for improving high temperature corrosion resistance of electric heater material made of Ni or Ni-Cr alloy by cladding an Al foil thereon and subjecting the cladded sheet to the above mentioned annealing under vacuum and heating in an oxidizing atmosphere to form the Al₂O₃. The technology was also proposed(Japanese Patent Application Hei 2-148158). The heater material exhibits high durability. The reason is considered to be that intermetallic compound, Ni₃Al (partly, NiAl) is formed between the cladded Al layer and the substrate during the heat treatment, and that the intermetallic compound solidly coats the substrate.
US-A-4 535 034 relates to a high-Al heat resistant steel comprising a base steel containing not more than 0.7% C, not more than 3.0% Si, not more than 2.0% Mn, 10 to 40% Ni, 9 to 30% Cr, 2 to 8% Al, with the balance being Fe and unavoidable impurities, and an Al coating formed on the base steel. The Al coating diffuses into the base steel to form an intermediate Al alloyed layer during the Al coating or by heating after the coating, which alloyed layer prevents the Al coating from spalling, thus providing excellent resistance to oxidation, corrosion, carburization, nitrization and sulphurdization at elevated temperatures.
Database WPIL, AN: 84-104408, Derwent Publication Ltd., London, GB; & JP-A-59 047 382 discloses a coated steel sheet with good heat and corrosion resistance which is coated with an intermetallic compound layer composed of FeAl₂ and FeAl and a ceramic layer composed of preferably by weight 15 - 25% SiO₂, 30 - 50% Cr₂O₃ and 35 - 45% Al₂O₃ which ceramic layer is applied by treating the diffusion treated Al hot-dip coated steel sheet with an aqueous solution containing the aforementioned compounds to form the said layer by heating.
EP-A-0 471 505, which is a document within the meaning of Article 54(3) EPC, discloses multilayer coated metallic substrates with a first coating of one or more metals, an optional second coating of a mixture of one or more ceramics and one or more metals or alloys, a third coating of one or more ceramics, and a fourth coating of one or more precious metals or alloys thereof, being substantially non-porous. Preferably the first coating is an alloy of Ni 80 wt.% and Al 20 wt.%. The ceramic of the second and third coatings is preferably alumina and is preferably of 99% purity.
Chemical Abstracts, vol. 90, no. 14, 02.04.1979, Columbus, Ohio, US; abstract no. 108488 discloses Cu tuyeres having an undercoating of a self fluxing alloy and being spray coated with Ni aluminide or Ni-Cr alloys and finally coated with heat-resistant ceramic layers such as Al₂O₃.
The general object of the present invention is to provide a metal composite with high temperature corrosion resistance given by the Al₂O₃ coating, which composite having improved durability by utilizing formation of the above mentioned intermetallic compound such as Ni₃Al and/or NiAl, as well as to provide a method of producing a metal composite, particularly, in the form of a sheet or a wire, having such good corrosion resistance and heat resistance.
A particular object of the invention is to provide a method of producing at a low cost a metal composite used at a high temperature, which consists of a substrate made of an Fe-based alloy and an Al₂O₃ protecting layer solidly adhered to the substrate, and thus, the corrosion resistance, particularly, high temperature oxidation resistance is heightened.
Another particular object of the invention is to provide a method of producing a metal composite used at a high temperature, which consists of a substrate made of an Fe-based alloy and an Al₂O₃ protecting layer solidly adhered to the substrate, and thus, in addition to the corrosion resistance such as high temperature oxidation resistance and chemical resistance are heightened.
Further particular object of the invention is to provide a metal composite used at a high temperature, which consists of a substrate made of Ni or an alloy containing more than 20 weight % of Ni and an Al₂O₃ protecting layer solidly adhered to the substrate, and thus, the corrosion-resistance, particularly, high temperature oxidation resistance as well as salt water-resistance are hightened. To provide a method of producing such a metal composite is also the particular object of the invention.

Fig.1 is a conceptional cross section showing an corrosion-resistant and heat-resistant metal composite in the form of a sheet according to the present invention;
Fig.2 and Fig.3 are cross sections of the material illustrating the steps of producing the metal composite sheet of Fig.1;
Fig.4 is a conceptional cross section showing the structure of an corrosion-resistant and heat-resistant metal composite in the form of a wire according to the present invention corresponding to Fig.1;
Fig.5 and Fig.6 are cross sections of the material illustrating steps of producing the metal composite wire of Fig.4, corresponding to Fig.2 and Fig.3, respectively;
Fig.7 is a cross section showing the structure of a wire other than that of Fig.6;
Fig.8 is a cross section of the material illustrating a step of producing the metal composite sheet, corresponding to Fig.3;
Fig.9 is a cross section of the material illustrating a step of producing the metal composite wire, corresponding to Fig.6;
Fig.10 is a cross section of a metal composite according to the invention in case where the composite is a sheet, similar to Fig.1 but of the embodiment other than that of Fig.1;
Fig.11 is a cross section of a metal composite according to the invention in case where the composite is a sheet, similar to Fig.1 but of the embodiment other than that of Fig.1; and Fig.12 is a conceptional cross section corresponding to Fig.1, illustrating the structure of the metal coposite produced by the step shown in Fig.11;
Fig.13 and Fig.14 are the data of Example 1 of the present invention; Fig.13 showing the results of salt water spray tests, and Fig.14 showing the weight increase by oxidation;
Fig.15 and Fig.16 are the data of Example 2 of the present invention; Fig.15 showing the weight increase at 1100°C in air, and Fig.16 showing the results of salt water spray tests;
Fig.17 and Fig.18 are the data of Example 3 of the present invention; Fig.17 showing the weight increase at 1100°C in air, and Fig.18 showing the results of salt water spray tests;
Fig.19 is the data of Example 5 of the present invention showing the results of salt water spray tests on the metal composite;
Fig.20 is the data of Example 6 of the present invention showing, as in Fig.19, the results of salt water spray tests;
Figs.21-23 are the data of Example 7; Fig.21 being a graph showing the results of tensile and bending tests in relation to the amounts of B added to the substrate; Fig.22 being a graph showing the results of tensile and bending tests in relation to the amounts of Fe and Mn added to the substrate; and Fig.23 being a graph showing the results of tensile and bending tests in relation to the amounts of Re added to the substrate;
Fig.24 is the data of Example 8 of the present invention, which is a graph showing the results of salt water dropping tests on the metal composite;
Fig.25 and Fig.26 are the data of Example 9 of the present invention; Fig.25 being a graph showing the results of salt water dropping tests on the metal composite; and Fig.26 being a graph showing the weight increase at 1100°C in air; and
Fig.27 is the data of Example 10 of the present invention, which is a graph showing the hardness at the part near the surface of the metal composite.

The corrosion-resistant and heat-resistant metal composite of the present invention comprises, in the case of sheets as shown in Fig.1, and in the case of wire as shown in Fig.4, a metal substrate 1, internal layer 5 of at least one of the intermetallic compounds, NiAl, NbAl, FeAl, Ni₃Al, Nb₃Al and Fe₃Al, and surface layer 6 of Al₂O₃. The substrate may be of any metal which has heat resistance required for the respective use, and therefore, can be chosen from a wide range as noted below.
In case where Fe-based alloy (provided that, if Ni is contained in the alloy, the content is up to 20 weight %) is used, the internal layer 5 of the intermetallic compounds will be a mixed layer of Fe₃Al and FeAl.
As the substrate of Fe-based alloy, in case where the intended composite is used as an electric heater material, Fe-Cr alloys of FCH-series, e.g., FCH51-alloy which consists of Cr 14.0 - 15.0 %, Al 2.80 - 3.30 %, C being up to 0.07%, Si up to 0.50% and Mn up to 0.60% and the balance of Fe, are suitable. Other Fe-based alloys may be used as the substrate metal in accordance with the use of the metal composite. They may be Fe-based alloys in which Ni content is up to 20 weight %, for example, various structural steels, ferritic, austenitic and martensitic stainless steels.
The Fe-based alloys to be used as the substrate may further contain a component or components selected from B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr and REMs so as to improve the properties of the alloy and to cause formation of the intermetallic compounds of Al and these components.
Another embodiment of the metal composite of the present invention comprises, as explained above in regard to the first case where an Fe-based alloy is used as the substrate in reference to Fig.1 and Fig.4, a substrate 1 of Ni or an alloy containing more than 20 weight % of Ni, an internal layer 5 in which intermetallic compounds, Ni₃Al and NiAl, coexist, and a surface layer 6 of Al₂O₃.
The substrate may be pure Ni metal or an alloy containing more than 20 weight % of Ni, the latter including various Ni-based alloys and Fe-based alloys. The examples of the alloys are as follows (% is by weight):

a) Ni-Cr alloys consisting of Cr 14.0 - 23.5 %, C 0.15 %, Mn 2.5 % and the balance of Ni,
b) Ni-based alloys consisting of one of B 0.08 - 0.18 % Re 1 - 3 %, Fe 5 - 20 %, and the balance of Ni,
c) Fe-Ni-Cr alloys consisting of Ni 20 - 72 %, Cr 14 - 35 % and the balance of Fe,
d) Ni-Al alloy consisting of Al 1 - 6 % and the balance of Ni, and
e) Ni-Al-Be alloys consisting of Al 1 - 6 %, Be 0.2 - 3.0 % and the balance of Ni.

To the substrate metal of Ni or an alloy containing more than 20 weight % Ni, further addition of one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and Rare Earth Metals (REMs) may be made to improve the properties of the alloy or to cause formation of the intermetallic compound or compounds between these components and Al.
The method of producing the corrosion-resistant and heat-resistant metal composite in the form of a sheet according to the present invention comprises, as shown in Fig.2, cladding an Al sheet 3A and Ni (or Nb) sheet 2A on at least a part of the sheet of the metal substrate 1 (in the illustrated example, on one side of the sheet) in the manner that the latter is inside to form a cladded sheet shown in Fig.3, and annealing the cladded sheet under vacuum and then, heating in an oxidizing atmosphere to form, between the Al layer 3B and the Ni (or Nb) layer 2B, as shown in Fig.1, the internal layer 5 of intermetallic compound or compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl), and to form the Al₂O₃ layer 6 at the surface.
It is convenient to prepare the cladded sheet of Al sheet 3A and Ni (or Nb) sheet 2B as shown in Fig.2 prior to cladding them on the metal substrate 1. In this case, suitable thickness of the cladded sheet is 0.1 - 1.0 mm, in which the thickness of the Al layer is 0.001 - 0.5 mm. Of course it is operable to separately clad Ni and Al or Nb, or to firstly clad Ni (or Nb) sheet 2A on the metal substrate 1, and then, clad Al sheet 3A.
The method of producing the corrosion-resistant and heat-resistant metal composite in the form of a wire according to the present invention comprises, as shown in Fig.5, cladding an Al tube 3E and Ni (or Nb) tube 2E on the wire of the metal substrate 1 by inserting the wire in the tubes in the manner that the latter tube is inside and wire-drawing to form a cladded wire shown in Fig.5, and annealing the cladded wire under vacuum and then, heating in an oxidizing atmosphere to form, between the Al layer 3F and the Ni (or Nb) layer 2F, as shown in Fig.4, the internal layer 5 of intermetallic compound or compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl), and to form the Al₂O₃ layer 6 at the surface.
In a typical instance of producing a wire product, diameter of the metal substrate wire is around 10 mm and the thickness of the covering Al tube and Ni tube (or Nb tube) are, similar to the cases of producing a sheet, 0.1 - 1.0 mm. By wire-drawing operation for several passes of drawing at a reduction of area of 30 - 50 %, a cladded wire of a diameter of 0.5 - 3 mm will be obtained.
The wires may have any profile of section. The section may be of not only the round circle as shown in Fig.4 but also a square, rectangular and so on. For example, a cladded wire of the section profile shown in Fig.7 can be obtained by rolling using profiled rolls, and then, the cladded wire can be further rolled to get the shape near the sheet. This kind of products has a higher corrosion resistance in comparison with the products of Fig.2 and Fig.3, because not only the flat surfaces but also both the sides of the former products are covered with the protecting layers.
The method of producing a metal composite of the present invention using Ni or an alloy containing more than 20 weight % of Ni as the substrate comprises, as shown in Fig.8 and Fig.9, covering the surface of the metal substrate 1, which is made of Ni or an alloy containing more than 20 weight % of Ni, with a layer 3 of Al or Al-alloy; subjecting the covered material to annealing and heating under vecuum or in an inert gas atmosphere to form, as shown in Fig.1 and Fig.4, the layer 5 in which Ni3Al and NiAl coexist , and to form the surface protecting layer of Al2O3.
At practice of the method, in case of sheet products, it is convenient to carry out covering with Al or Al-alloy by clad-rolling. Preferable thickness of Al or Al-alloy foil to be cladded is 0.005 - 0.5 mm, and the thickness after the rolling will be 0.001 - 0.2 mm.
On the other hand, in case of wire products, covering with Al or Al-alloy may be carried out by insering a substrate wire in a tube of a covering material and wire drawing or extrusion processing. In a typical embodiment, diameter of the metal substrate is around 10 mm, and thickness of the Al or Al- alloy tube for covering it may be 0.1 - 1.0 mm as in the case of sheet products. Wire drawing of several passes under a reduction of area of 30 - 50 % will give a cladded wire of a diameter of 0.5 -3 mm.
The other possible ways of covering the substrate with Al or Al-alloy are dipping the substrate in molten Al or Al-alloy, thermal-spray of Al or Al-alloy on the substrate surface, plasma powder welding, chemical plating and vapor-deposition.
In case where an Al-alloy is used for covering the substrate, the alloy may be such ones that further contain one or more of the additional components selected from B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, Zr, Ta, Nb, Sc and REMs so as to enjoy the benefit of improved properties of the Al-alloy depending on the kind of the additional components.
Annealing under vacuum or in an inert gas is carried out by heating to a temperature of 400 - 900°C, preferably 400 - 600 °C for 1 - 10 hours. The subsequent heating in an oxidizing atmosphere is conducted, for example, at a temperature of 400 - 1000°C for 1 - 36 hours.
Further embodiment of the method of producing the metal composite according to the present invention comprises, in cases of sheet products, as shown in Fig.11, compiling the first Al foil 2, Ni or Ni alloy foil 3 and the second Al foil 2' in the order from the outside to the inside on the surface of a substrate 1 in the form of sheet; cladding these foils on the sheet by rolling to obtain a cladded material; annealing the cladded material under vacuum; and then, heating in an oxidizing atmosphere to form, as shown in Fig.12, an internal layer 5 in which intermetallic compounds, NiAl and Ni₃Al and a protecting layer of Al₂O₃ on the surface.
It is recommended to prepare a laminated sheet of the first Al foil 2, Ni foil 3 and the second Al foil 2' prior to cladding them on the substrate. In this case, suitable thickness of the laminated sheet is 0.1 - 1.0 mm, and that of the Al part is 0.005 - 0.5 mm in total of both the sides. It is of course operable to clad the foils one by one, or to prepare a laminate of any two of the three foils and clad it with the remaining one foil. Anyway, sequence of cladding can be chosen arbitrarily.
The substrate 1, which is a sheet or a wire of Fe-based alloy, Ni or Ni-alloy containing more than 20 weight % of Ni, and the layer 2 of Al or Al-alloy adhere intimately during the annealing due to mutual diffusion, and the diffusion further proceeds during the subsequent heating in the oxidizing atmosphere. As the result, Ni3Al and NiAl occur at the places in the direction of depth depending on the concentrations of Ni and Al. If the covering Al layer is thick, concentrations of the formed intermetallic compounds will have a gradient, however, if the layer is thin and the heat treatment is done thoroughly, the concentration gradient substantially disappears and a layer in which Ni₃Al and NiAl uniformly coexist is formed. In case where the substrate is a steel such as a SUS, intermetallic compound, Fe₃Ni, occurs at the interface of the substrate and Ni layer.
In case where the substrate Fe-based metal, Ni or an alloy thereof contains one or more of the above mentioned alloy elements which may form an intermetallic compound, i.e., B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and Rare Earth Metals (REMs), or in case where one or more of such components are contained in the cladded Al-alloy, an intermetallic compound or compounds between these elements and Al will occur instead of the Ni-Al intermetallic compounds.
In any of the above mentioned cases, the Al at the surface is oxidized by heating in an oxidizing atmosphere to form dense Al₂O₃. Crystals of this compound may grow in the form of whiskers to coat the surface. At the same time of the formation of Al₂O₃, Al diffuses into the substrate metal of Ni or an alloy containing more than 20 weight % of Ni to cause good adhesion at the interface of joint, and a portion of the Al is oxidized to form Al₂O₃. As the result, the protecting layer of Al₂O₃ exists on the substrate with ankered roots in the substrate metal, and thus, strongly coats and protects the substrate.
As explained above, Al₂O₃ crystals grow to entwine each other on the surface of the substrate metal and coat the substrate. The coating layer is usually a dense one, but it is inevitable that there are micropores in some places. Corrosive chemical liquid such as salt water may penetrate through the micropores and attack the substrate metal. The metal composite according to the present invention generally has much better corrosion-resistance due to the Ni-Al intermetallic compounds.
In the embodiment in which the substrate 1 is covered by Al-Ni(or Nb)-Al, the interfaces between the substrate and the first Al layer 2, the Al layer 2 and the Ni layer 3, and the Ni layer 3 and the second Al layer 2' adhere solidly due to the diffusions as explained above during the annealing under vacuum, and the diffusions further proceed during the subsequent heating in an oxidizing atmosphere. Thus, at the locations in the direction of depth, layers in which Ni3Al and NiAl coexist depending on the concentrations of Ni and Al components are formed.
In case of using a substrate of Ni alloy containing more than 20 weight % of Ni, certain amounts of B and Fe, and one or more of Mn and Re, the processability of the product metal composite is very good, and therefore, the product can be easily processed even after formation of Al₂O₃ protecting layer.
The present metal composite, which comprises the substrate metal of Fe-based alloy, Ni or Ni alloy containing more than 20 weight % or Ni, and the solid coating layer of Al₂O₃ on the surface of the substrate for protection, exhibits improved corrosion resistance represented by high temperature oxidation resistance and salt water-resistance. In regard to the electric heater material, a typical use of Fe-Cr alloy, the present metal composite can be understood as the product using FCH51-alloy which is less expensive and has good processability as the substrate and improving the high temperature corrosion resistance, which is low in this alloy. The metal composite can find wide use not only for the above mentioned typical use, electric heater, but also various use in industrial apparatus and electric appliances.
The method of producing the metal composite of the present invention enables production of the above described products by employing the easy way of forming Al layer such as cladding and transformation of the Al to Al₂O₃. This method thus gives the product which is protected by an Al₂O₃ layer of a thickness chosen from a wide range and has roots ankered in the substrate for solid adhesion. In the embodiment of adding alloy elements forming intermetallic compounds with Al in the substrate metal or in the Al alloy, it is possible to enjoy the benefits of: improved properties such as better adhesion between the substrate and the intemetallic compound layer, and between the intermetallic compound layer and the Al₂O₃ protecting layer; better processability given by improved ductility of the intermetallic compounds of B, Mn and Al; and higher hardness and higher electric resistance given by addition of Be.
The present invention will now be explained by the following examples. It is a matter of course that, although the description concerns the products in the form of sheet and wire which are easy to produce, as long as a practical way of cladding is used, products of any other form and shape can be produced in accordance with the present invention.

EXAMPLES

Example 1

An Al/Ni laminated sheet and an Al/Nb laminated sheet both having thickness of 0.1 mm, in which Al layer is 0.04 mm thick, were prepared. The laminated sheet were rolled on both sides of thin sheets of SUS430 in the manner that the Ni or Nb was inside, to produce cladded sheets of 0.05 mm thick. Thickness of the Al layers was 0.002 mm (per one side), and that of Ni or Nb layers was 0.004 mm (per one side).
The cladded sheets were slitted to ribbons of 6 mm wide and 200 m long, which were annealed under vacuum at 600°C for 2 minutes, and then heated in air at 600°C for 60 minutes. For comparison, a ribbon of SUS430 alone and a ribbon of SUS430 having Al layers 0.002 mm thick (the same as above) on both sides were prepared and heated in air at 600°C for 60 minutes.
These materials were subjected to the following corrosion tests:

(Salt Water Spray Test)

The test pieces are kept at a temperature of 800°C under a constant tension, and receive spray of 5%-NaCl solution once every 2 minutes, and the times of spraying until the test piece breaks is recorded. The results are shown in Fig.13. The layer structures of the test pieces of the examples and the controls were as follows:

Control 1:: SUS430
Control 2:: Al/SUS430/Al
Example 1:: Al/Ni/SUS430/Ni/Al
Example 2:: Al/Nb/SUS430/Nb/Al

(Weight Increase by Oxidation)

The test pieces are kept in air at a temperature of 1100°C, and the weight increase by passage of time (mg/cm2) is measured. The results are as shown in Fig.14.

Example 2

Al foils 0.1 mm thick were cladded to both the sides of a thin FCH51 alloy of 0.2 mm thickness by rolling. By further rolling a cladded material with a thickness of 0.1 mm in total, in which the thickness of both the Al layers was 0.04 mm.
The cladded material was slitted to ribbons of 30 mm width, and the ribbons were heated in air to 600°C for 1 hour so as to cause the change of Al to Al₂O₃ and to grow the Al₂O₃ layer.
The ribbon of FCH51 alloy having the Al2O3 protecting layers was heated to 1100°C in air and the weight increase by oxidation was observed as the measure of the high temperature oxidation-resistance. For comparison, a ribbon of FCH51 alloy alone and a ribbon of FCH51 alloy on which Al layer was vapor-deposited and subjected to oxidation treatment for formation of Al₂O₃ protecting layer were also tested under the same conditions. The results are shown in Fig.15. From the graph of Fig.15 it can be seen that the Al2O3 protecting layer effectively improved the high temperature oxidation resistance of FCH51 heater material.
The corrosion resistance to the salt water was observed by repetition of the cycles below: Spray of 5%-NaCl water solution (40°C, 4 hours) --- Wetting (50°C, RH 100%, 2 hours) --- Drying (60°C, 2 hours)
Also in this test, a ribbon of FCH51 alloy having no Al2O3 protecting layer was tested, and the evaluation was made by observing the state of the surface. The results are shown in Fig 16. The graph of the Figure proves improvement of corrosion resistance by Al₂O₃ layer.

Example 3

The Fe-based alloys of the alloy compositions (weight %, the balance being Fe) shown in Table 1 were used as the substrates. On both the sides of the sheets of 0.2 mm thickness, Al foils of 0.015 mm thickness were cladded by rolling to give the covered material in the form of sheet, the total thickness of which was 0.05 mm and those of the two Al layers, 0.002 mm.

Table 1

	C	Si	Mn	Cr	Ni	Mo	Ti	V	Al	Others
A	0.005	0.3	2.03	14.20	-	-	0.53	-	3.05	B:0.1
B	0.06	1.23	0.86	18.60	-	-	-	-	3.62	Mg:0.03
C	0.05	0.85	1.58	19.00	9.06	-	-	0.5	-	La:200ppm
D	0.08	0.63	0.88	17.59	-	0.48	-	-	-	Ca:200ppm

The above covered materials were subjected to annealing under vacuum at 600°C for 1 hour followed by heating in air at 800°C for 68 minutes for formation of Al₂O₃.
The product using the substrate of the alloy "A" in Table 1 was tested by bending test and tensile test. The bending test comprises repeated cycles of bending to 90° and flattening, and the number of cycles until the test piece breaks is recorded. The number reached to 5. The elongation was 3 %.

Example 5

On both sides of thin sheets with a thickness of 0.2 mm and a width of 50 mm made of pure Ni or Ni-Cr alloys having the alloy compositions shown in Table 2 (weight %) Al foils with a thickness of 0.15mm were cladded on the sheets by rolling.

Table 2

No.	C	Mn	Cr	Others
1	up to 0.15%	2.0%	20.0%
2	up to 0.10%	0.5%	22.5%	Al: up to 0.40%
				Ti: up to 0.40%
				Nb + Ta: 3.15 - 4.15 %
3	up to 0.15%	0.9%	21.5%	Al: up to 0.20%
3	up to 0.15%	0.9%	21.5%	Ti: 0.90%
4	-	-	-	(pure Ni)

By further rolling cladded materials having total thickness of 0.1 mm, and the thickness of both the Al layers of 0.04 mm was obtained.
The salt water spray test as described in Example 1 was conducted on these samples to determine the high temperature corrosion resistance. The results are shown in Fig.19.

Example 5

Laminated sheets of Al/Ni/Al three layers were prepared. The thickness of the Al foils was 0.008 mm (0.004 mm a side) and the total thickness of the laminate was 0.1 mm. The laminated sheets were rolled on a thin Ni sheet to produce a cladded material with 0.05 mm thickness (No.5). In both the sides, thickness of the Al layers was (both the first and the second layers) 0.002 mm, and that of the Ni layer was 0.05 mm. The material was slitted to ribbons 6 mm in width and 200 m in length, which ribbons were heated under vacuum to 600°C for 60 minutes.
Separately, there were prepared the following two samples:
(No.7) on both the sides of the same thin Ni sheet Al foils were cladded to form the Al layers of 0.002 mm thickness, and the cladded material was then subjected to the same heat treatment as above, and,
(No.6) on one side of the same thin Ni sheet the above mentioned laminated three layer sheet of Al/Ni/Al was cladded and on the other side only an Al foil was cladded, and the cladded material was then subjected to the same heat treatment as above.
The samples, Nos. 5 - 7 were tested by the salt water spray test to determine the high temperature corrosion resistance. The results are shown in Fig.20. The data of the Figure illustrates that the product in which the Al/Ni/Al laminated sheet was cladded to the thin Ni sheet obtained higher corrosion resistance than that of the product in which simply Al foils were cladded to the thin Ni sheet.

Example 6

To the Ni or Ni-Cr alloy of the composition shown in Table 3 (weight %) different amounts of B, Fe or Mn, Re were added, and the resulting alloys were rolled to sheets of 0.2 mm thickness and 50 mm width, which were used as the substrates.

Table 3

No	Cr	Mo	Ni	Others
8	20%	-	balance	Si: 1.13%
9	22.5%	9%	balance	Co:up to 1.0%, Al:up to 0.4%
9	22.5%	9%	balance	Ti:up to 0.4%, Nb+Ta:3.15-4.15%
10	21.5%	3.0%	balance	Cu: 2.5%, Al: up to 0.2%
10	21.5%	3.0%	balance	Ti: 0.9%, Sc: up tp 3.0%
11	-	-	99% or more

On both the sides of these rolled sheets an Al foil of 0.015 mm thickness was cladded, and finally, the thickness of the cladded material was rolled down to 0.1 mm, and that of the Al layers, 0.004 mm.
The cladded material was subjected to annealing under vacuum at 600°C for 30 minutes to form the Ni-Al intermetallic compound, and then, heated in air to about 800°C for 3 hours to form the Al₂O₃ layer at the surface.
Of the obtained metal composite products having Al₂O₃ protecting layer, the product using No.8 metal as the substrate was tested by the bending test and the tensile test. The bending test was described in Example 3. The test results are shown in Figs. 21-23. The plots in the Figures are the averaged data of the bending test on 5 samples of each composition.

Example 7

On both the sides of a thin sheet of NCH2 alloy (Ni: 58 %, Cr: 16 %, Fe: the balance) of 0.4 mm thickness, Al foils of 0.1 mm thickness were rolled to obtain a cladded material of 0.05 mm thickness. In both the sides, thickness of the Al layers are 0.004 mm per layer.
The material was slitted to ribbons 6 mm wide and 200 m long, and a ribbon was heated under vacuum to 600°C for 60 minutes (No.12). For comparison, a ribbon of NCH2 alloy of the same dimensions was prepared (No.13).
The samples were tested by salt water dropping test to determine the high temperature corrosion resistance. The salt water dropping test comprises, as in the salt water spray test, that the test piece is kept at 800°C under a constant tension, and 5%-NaCl water solution 0.5 cm³ is dropped once every 2 minutes on the test piece. The times of dropping the salt water until the break of the test piece is the measure of the corrosion resistance. The test results are shown in Fig.24.

Example 8

Thin sheets with 0.2 mm thickness were prepared using Ni-Al alloys which consist of Al 2 %, 4 %, 6 %, or 8 % and the balance of Ni. On both the sides of these substrates, Al foils 0.015 mm thick were rolled to obtain cladded materials. The thickness was 0.05 mm in total, and that of the Al layers was 0.02 mm.
The cladded materials were subjected to annealing under vacuum at 600°C for 2 minutes followed by heating in air to 600°C for 1 hour to form the Al₂O₃ protecting layer.
To the resulting corrosion-resistant and metal composite products (Nos. 14, 15, 16, 17) the salt water dropping test as described in example 7 was conducted. The results are shown in Fig.25, from which it was observed that the corrosion resistance increases as the addition amount of Al increases, and gradually the effect saturates.
Out of the high temperature corrosion resistant products, No.16, which uses a substrate containing Al 6 %, was subjected to a test of weight increase by oxidation comprising heating the test piece to 1100°C in air. The results are shown in Fig.26 in comparison with the test made on a Ni sheet of the same thickness (No. 18). It is seen from the data that the product according to the present invention exhibits such a high oxidation resistance that it does not get weight even after heating for a long period.

Example 9

Using a thin sheet made of the alloy having the composition of Ni-6%Al-2%Be as the substrate, a metal composite of the present invention was produced by the same operation as those of Example 8.(No. 19)
With respect to the intermetallic layer which is under the Al₂O₃ surface protecting layer, hardness was measured. The values in HV from the surface to the point of 12 »m (micron) deep are shown in Fig.27. The Figure illustrates increase of the hardness at the parts which are 5 »m (micron) deep or shallower, and the mechanism of increasing of the abrasion resistance.

Claims

A corrosion-resistant and heat-resistant metal composite, characterized in that the composite comprises a substrate metal of Ni or an Ni-alloy containing more than 20 wt.% of Ni, an internal layer in which intermetallic compounds Ni₃Al and NiAl coexist, and a surface layer of Al₂O₃.
The metal composite according to claim 1, characterized in that the Ni-alloy containing more than 20 wt.% of Ni is an Ni-Cr-alloy consisting of Cr: 14.0 - 23.5%, C: up to 0.15%, Mn: up to 2.5% and the balance of Ni.
The metal composite according to claim 1, characterized in that the Ni-alloy containing more than 20 wt.% of Ni is an Ni-based alloy consisting one or more of B: 0.08 - 0.18%, Re: 1 - 3%, Fe: 5 - 20% and Mn: 5 - 20%, and the balance of Ni.
The metal composite according to claim 1, characterized in that the Ni-alloy containing more than 20 wt.% of Ni is an Fe-Ni-Cr-alloy consisting of Ni: 20 - 72%, Cr: 14 - 35%, and the balance of Fe.
The metal composite according to claim 1, characterized in that the Ni-alloy containing more than 20 wt.% of Ni is an Ni-Al-alloy consisting of Al: 1 - 6%, and the balance of Ni.
The metal composite according to claim 1, characterized in that the Ni-alloy containing more than 20 wt.% of Ni is an Ni-Al-Be-alloy consisting of Al: 1 - 6%, Be: 0.2 - 3.0%, and the balance of Ni.
The metal composite according to claim 1, characterized in that the Ni-alloy containing more than 20 wt.% of Ni is an alloy of such a composition that one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and Rare Earth Metals (REMs) are added to Ni or the alloys defined in claims 2 to 6.
A method of producing a corrosion-resistant and heat-resistant metal composite, characterized in that the method comprises the steps of covering a metal substrate either made of an Fe-based alloy having, if contained, an Ni-content of up to 20 wt.%, or made of Ni or Ni-alloy containing more than 20 wt.% of Ni, with Al or Al-alloy; annealing the resulting covered material under vacuum or in an inert gas atmosphere by heating at a temperature of 400 to 900°C for 1 to 200 minutes; and heating the material in an oxidizing atmosphere at a temperature of 400 to 1000°C for 1 minute to 36 hours so as to form a layer in which intermetallic compounds, Fe₃Al and FeAl, or Ni₃Al and NiAl, or Nb₃Al and NbAl, respectively, coexist on the substrate and to form an Al₂O₃ layer at the surface.
The method according to claim 8, characterized in that the method comprises the steps of cladding an Al-sheet and one of Ni-sheet and Nb-sheet on at least a part of a metal substrate sheet in the manner that the Ni-sheet or the Nb-sheet is between the Al-sheet and the substrate sheet; annealing the resulting cladded material under vacuum; heating the cladded material in an oxidizing atmosphere so as to form intermetallic compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl) between the Al-layer and the Ni-layer (or Nb-layer) and Al₂O₃ at the surface.
The method according to claim 8, characterized in that the method comprises the steps of cladding an Al-tube and one of Ni-tube and Nb-tube on a metal substrate wire in the manner that the Ni-tube or the Nb-tube is between the Al-tube and the substrate wire; annealing the resulting cladded material under vacuum; heating the cladded material in an oxidizing atmosphere so as to form intermetallic compounds, Ni₃Al (or Nb₃Al) and/or NiAl (or NbAl) between the Al-layer and the Ni-layer (or Nb-layer)and Al₂O₃ at the surface.
The method according to claim 8, characterized in that the method comprises the steps of covering the surface of a substrate made of Ni or an Ni-alloy containing more than 20 wt.% of Ni with, in the direction from the inside to the outside, the first Al-layer, an Ni or Ni-alloy layer and the second Al-layer; annealing the covered material under vacuum or in an inert gas atmosphere; and heating the material in an oxidizing atmosphere so as to form a layer in which intermetallic compounds, Ni₃Al and NiAl coexist on the substrate, and to form an Al₂O₃-layer at the surface.
The method according to claim 8, characterized in that the substrate made of Ni or an Ni-alloy containing more than 20 wt.% of Ni is in the form of sheet, and covering the substrate with Al or Al-alloy is carried out by clad-rolling.
The method according to claim 8, characterized in that the substrate made of Ni or an Ni-alloy containing more than 20 wt.% of Ni is in the form of wire, and covering the substrate with Al or an Al-alloy is carried out by inserting the substrate wire into a tube of the covering material and wire-drawing or extruding.
The method according to claim 8, characterized in that covering the substrate with Al or an Al-alloy is carried out by dipping the substrate in molten Al or Al-alloy, thermal spray of Al or Al-alloy, plasma powder welding, chemical plating, spattering or vapor-deposition.
The method according to claim 8, characterized in that the substrate made of Ni or an Ni-alloy containing more than 20 wt.% of Ni is in a bulky shape, and covering the substrate with Al or Al-alloy is carried out by dipping the substrate in molten Al or Al-alloy, thermal spray of Al or Al-alloy, plasma powder welding, chemical plating, spattering or vapor-deposition.
The method according to claim 8, characterized in that the Al-alloy has such a composition that one or more of B, Si, Mg, Cu, Ca, Mn, Y, Ti, Co, W, V, Zr, Ta, Nb, Sc and Rare Earth Metals (REMs) are added to Al.