Method for increasing oxidation resistance of Fe-Cr-Al alloy

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for increasing the oxidation resistance of a Fe-Cr-Al alloy by forming a protective film of excellent oxidation resistance on the surface of said alloy.
2. Description of the Prior Art
In order to increase the oxidation resistance of a metal, it is generally known to form an oxidation-resistant protective film on the surface of the metal.
For example, an article "Oxide Structure of Stainless Steels under Controlled Oxygen Atmospheres" (Toshiyuki Yashiro, "Heat Treatment", Vol. 31, No. 4, pp. 205-211, 1991) and an article "Surface Modification of Stainless Steels Using Thermal Passivation" (Toshiyuki Yashiro, Keiichi Terashima, Taketomo Yamazaki, "Surface Technology", Vol. 41, No. 3, pp. 41-48, 1990) report a method for increasing the corrosion resistance of stainless steel or the like, which comprises heat-treating stainless steel or the like in a low-pressure oxygen or a controlled atmosphere to form a passive oxide film on the surface.
Also, Japanese Patent Publication No. 63148/1992 discloses a method for forming an alumina film on the surface of a TiAl intermetallic compound, which comprises placing said compound in an atmosphere having an oxygen partial pressure of 1.times.10.sup.-2 to 1.times.10.sup.-5 Pa at 900.degree.-1,050.degree. C. for 30 minutes to 100 hours to oxidize only Al selectively.
The above prior art is used for stainless steel or the like, or a TiAl intermetallic compound. When used for a Fe-Cr-Al alloy, however, the prior art has been unable to form a homogeneous protective film (an alumina film) because the pressure employed during film formation is too low (-10.sup.-2 Pa or lower).
SUMMARY OF THE INVENTION
Under such a situation, the present invention has been made in order to provide a method capable of forming a homogeneous protective film of excellent oxidation resistance even on a metal having a non-homogeneous composition, such as Fe-Cr-Al alloy or the like.
The present invention provides a method for increasing the oxidation resistance of a Fe-Cr-Al alloy, which comprises placing said Fe-Cr-Al alloy in an atmosphere having an oxygen partial atmosphere of 0.02-2 Pa at a temperature of 950.degree.-1,200.degree. C. to form, on the surface of said alloy, an alumina-based protective film having excellent oxidation resistance.
The present invention further provides a method for increasing the oxidation resistance of a Fe-Cr-Al alloy, which comprises placing said Fe-Cr-Al alloy in an air having a pressure of 0.1-10 Pa at a temperature of 950.degree.-1,200.degree. C. to form, on the surface of said alloy, an alumina-based protective film having excellent oxidation resistance.
The present invention furthermore provides a Fe-Cr-Al alloy having excellent oxidation resistance, which has an alumina-based dense protective film on the surface and wherein an yttrium component is enriched in said protective film.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an electron micrograph (a secondary electron image) of the surface of the sample after heat treatment under reduced pressure, of Example 5.
FIG. 2 is an electron micrograph (a secondary electron image) of the surface of the sample after heat treatment in air, of Comparative Example 8.
FIG. 3 is an electron micrograph (a back scattered electron image) of the surface of the sample after heat treatment under reduced pressure, of Example 4.
FIG. 4 is an electron micrograph (a back scattered electron image) of the surface of the sample after heat treatment under reduced pressure, of Comparative Example 1.

DETAILED DESCRIPTION OF THE INVENTION
In the method of the present invention, a Fe-Cr-Al alloy is heat-treated in an atmosphere having an oxygen partial pressure of 0.02-2 Pa at a temperature of 950.degree.-1,200.degree. C. to form an oxidation-resistant protective film on the surface of said alloy. The heat treatment is conducted, for example, in an air having a reduced pressure of 0.1-10 Pa at 950.degree.-1,200.degree. C. By conducting such a heat treatment under reduced pressure, a dense, homogeneous, crack-free protective film (an alumina film) can be formed without being contaminated by undesirable components, and the resulting Fe-Cr-Al alloy having said protective film on the surface has increased oxidation resistance. When the present method is applied to an yttrium-containing Fe-Cr-Al alloy, it has been found that in the resulting alloy having a protective film on the surface, yttrium is enriched in or near said protective film. Yttrium imparts improved adhesivity to the protective film and is therefore presumed to give a favorable effect to the increased oxidation resistance of Fe-Cr-Al alloy. The above-mentioned oxygen partial pressure is preferably achieved by making the system vacuum, but it may be obtained by allowing an inert gas (e.g. argon or nitrogen) to contain a small amount of oxygen.
In the present invention, the pressure of the atmosphere is 0.1-10 Pa, for the following reasons. When the pressure is lower than 0.1Pa, Cr vaporizes in a large amount, making difficult the formation of an alumina protective film; when the pressure is higher than 10 Pa, the alumina protective film formed has a number of cracks and, when the Fe-Cr-Al alloy as starting material contains yttrium, the enrichment of yttrium in or near the surface protective film is insufficient and the protective film has low adhesivity as compared with when the yttrium enrichment is sufficient. The pressure of the atmosphere is preferably 0.1-7 Pa because a homogeneous film is obtainable. These results were obtained from the experiments in an air of reduced pressure, etc. The pressure range of the atmosphere in the present invention is 0.02-2 Pa in terms of oxygen partial pressure.
In the present invention, the temperature of the heat treatment is 950.degree.-1,200.degree. C. for the following reasons. When the temperature is lower than 950.degree. C., the rate of alumina film formation is small and the formation of a homogeneous film is difficult; when the temperature is higher than 1,200.degree. C., film formation is easily affected by the vaporization of alloy components and the formation of a homogeneous film is difficult as well.
When the temperature is lower than 1,060.degree. C., contamination by the tungsten, etc. contained in the furnace used, etc. occurs easily during the heat treatment under reduced pressure, and this may give an adverse effect on the oxidation resistance of the resulting Fe-Cr-Al alloy. Hence, the temperature of the heat treatment is preferably 1,060.degree.-1,200.degree. C.
The time for which the Fe-Cr-Al alloy is heat-treated under reduced pressure, varies depending upon the temperature employed, etc. but about 5-15 hours is preferred generally. Satisfactory increase in oxidation resistance is obtained by determining the time for heat treatment under reduced pressure so that the weight increase per unit surface area (hereinafter referred to as "pre oxidation amount") by heat treatment under reduced pressure becomes 0.20 mg/cm.sup.2 or less, preferably 0.06-0.15 mg/cm.sup.2.
When a Fe-Cr-Al alloy is subjected to the abovementioned heat treatment under reduced pressure, the resulting alloy has an alumina-based dense protective film on the surface and has increased oxidation resistance. When an yttrium-containing Fe-Cr-Al alloy is subjected to the same treatment, the resulting alloy contains yttrium in the formed protective film in an enriched state and has even higher oxidation resistance because yttrium imparts higher adhesivity to the protective film.
The present invention is described in more detail below by way of Examples. However, the present invention is not restricted to these Examples.
In the following Examples, the test items were measured as follows.
[Pressure (Pa)]
Was measured using a Pirani gage or an ionization gage.
[Temperature (.degree.C.)]
Was measured using an R thermocouple thermometer specified by JIS.
[Pre oxidation amount (mg/cm.sup.2)]
Weight increase per unit surface area, of a sample after heat treatment under reduced pressure was calculated using the following formula (1):
(W.sub.1 -W.sub.0)/S (1)
wherein W.sub.1 =weight of sample after heat treatment under reduced pressure,
W.sub.0 =weight of sample before heat treatment under reduced pressure, and
S=surface area of sample.
[Total oxidation amount (mg/cm.sup.2)]
Was calculated using the following formula (2) after conducting an oxidation test of placing a sample in air at 1,100.degree. C. for 150 hours:
(W.sub.2 -W.sub.0)/S (2)
wherein W.sub.2 =weight of sample after oxidation test,
W.sub.0 =weight of sample before heat treatment under reduced pressure, and
S=surface area of sample.
[Homogeneity of oxide film (presence of non-homogeneous portions in oxide film)]
The surface of an oxide film formed by heat treatment under reduced pressure was observed using a scanning type electron microscope, and the homogeneity of the film was evaluated according to the density of the back scattered electron image obtained and the presence of non-homogeneous portions (portions of high density) was examined. (It is known that heavy elements such as Fe, Cr and the like, as compared with light elements such as Al and the like, give a back scattered electron of higher intensity. Therefore, when a non-homogeneous alumina film is formed, the back scattered electron image of said film has different densities, whereby the homogeneity of the film can be evaluated.)
[Cracks in oxide film]
The surface of an oxide film formed by heat treatment under reduced pressure was observed using a scanning type electron microscope, and the presence of the cracks having a length of 5 .mu.m or more seen in the secondary electron image was examined.
[Yttrium amount in oxide film]
The surface of a sample after heat treatment under reduced pressure and the inside of said sample exposed by argon etching (100 minutes) were measured for respective yttrium amounts, using the spectral peak intensity (counts per second, CPS) of Y 3 d electrons obtained by electron spectroscopy for chemical analysis. When the ratio of the yttrium amount(C) in or near the film and the yttrium amount (B) of the surface, i.e. (C/B) was 1.5 or more, it was judged that yttrium was enriched in or near film.
[Tungsten peak in oxide film]
The surface of a sample after heat treatment under reduced pressure was measured for the spectral peak intensity of W 4 d electrons by electron spectroscopy for chemical analysis. The peak intensity was rated in the three scales of n (not present), w (weak) and s (strong).
EXAMPLES 1-6 AND COMPARATIVE EXAMPLES 1-6
A pure Fe powder, a pure Cr powder, a Fe-Al (Al:50% by weight) alloy powder, a Fe-B (B:20% by weight) alloy powder and a Y.sub.2 O.sub.3 powder were mixed so as to give a composition A shown in Table 1. The mixture was mixed with an organic binder and water. The resulting mixture was kneaded and passed through an extrusion die to form a honeycomb structure of 100 mm in diameter, 100 .mu.m in rib thickness and 500 cells/in..sup.2 in cell density. The honeycomb structure was dried and then sintered in a hydrogen atmosphere at 1,350.degree. C. for 2 hours to obtain a sintered honeycomb material. The shrinkage factor on firing was 17%. The sintered honeycomb material was subjected to chemical analysis, which gave a carbon content of 0.21% by weight.
Cubic samples (5 cells.times.5 cells.times.8 mm) were cut out from the sintered honeycomb material and subjected to a heat treatment under reduced pressure under the conditions shown in Table 2. In the heat treatment under reduced pressure, the heating was conducted by using an electric furnace using a tungsten mesh as a heater or by using an induction heating furnace, and the reduced pressure was produced by degassing the furnace inside using a vacuum pump or a diffusion pump, to keep the pressure inside the furnace at a constant vacuum. Each sample after the heat treatment under reduced pressure was examined for pre oxidation amount and oxide film properties. Also, each sample after the heat treatment under reduced pressure was subjected to an oxidation test of keeping the sample in air in an electric furnace of 1,100.degree. C. for 150 hours, to measure the total oxidation amount. The results are shown in Table 2. For reference, the electron micrograph (secondary electron image) of the sample after the heat treatment under reduced pressure, of Example 5 is shown in FIG. 1, and the electron micrographs (back scattered electron images) of the samples after the heat treatment under reduced pressure, of Example 4 and Comparative Example 1 are shown in FIG. 3 and FIG. 4, respectively.
COMPARATIVE EXAMPLE 7
The same sample as used in Examples 1-6 and Comparative Examples 1-6 was subjected to the same oxidation test as in Examples 1-6 and Comparative Examples 1-6, without being subjected to any heat treatment under reduced pressure, to measure the total oxidation amount. The results are shown in Table 2.
COMPARATIVE EXAMPLE 8
The same sample as used in Examples 1-6 and Comparative Examples 1-6 was subjected to a heat treatment of placing it in air in an electric furnace using SiC as a heater, at 1,150.degree. C. for 1 hour. The sample after heat treatment was examined for pre oxidation amount and oxide film properties. Also, the sample after heat treatment was subjected to the same oxidation test as in Examples 1-6 and Comparative Examples 1-6 to measure the total oxidation amount. The results are shown in Table 2. For reference, the electron micrograph (secondary electron image) of the sample after heat treatment is shown in FIG. 2.
TABLE 1______________________________________ Composition (wt. %)Symbol Fe Cr Al Si B Y.sub.2 O.sub.3______________________________________A Remainder 12 10 0 0.03 0.3B Remainder 20 5 2 0.03 0.3C Remainder 10 10 0 0.03 0.3D Remainder 20 5 2 0.03 0E Remainder 10 10 0 0.03 0______________________________________
TABLE 2__________________________________________________________________________ Properties of oxide film formed by heat treatment Conditions for heat treatment under reduced pressure Pre oxidation Total oxidation under reduced pressure Oxygen partial Total pressure Temperature Time amount amount Non-homogeneous portions pressure (Pa) (Pa) (.degree.C.) (hr) (mg/cm.sup.2) (mg/cm.sup.2) in oxide__________________________________________________________________________ filmExample 1 0.08 0.4 950 15 0.07 0.72 Not present2 1.1 5.3 1060 15 0.13 0.68 Not present3 1.3 6.7 1150 15 0.11 0.60 Not present4 0.02 0.1 1060 15 0.06 0.64 Not present5 0.06 0.3 1150 15 0.15 0.55 Not present6 1.9 9.5 1150 15 0.19 0.72 Partially presentComparative 1.1 5.3 1250 15 0.40 2.33 Easily seenexample 12 0.02 0.1 1250 15 -3.90 7.00 Easily seen3 0.001 0.005 1060 5 0.01 0.82 Easily seen4 0.001 0.007 1150 5 0.04 0.97 Easily seen5 0.002 0.009 1250 5 0.18 1.20 Easily seen6 0.1 0.5 900 15 0.05 0.80 Easily seen7 (Not treated) -- -- -- -- 0.75 --8 20000 100000 1150 1 0.11 0.75 Partially present (in air)__________________________________________________________________________ Properties of oxide film formed by heat treatment under reduced pressure Cracks in Y amount (CPS) Y concentration W peak in oxide film surface (B) Inside (C) ratio (C/B) oxide__________________________________________________________________________ film Example 1 None 80 260 3.3 w(week) 2 None 100 180 1.8 n(not present) 3 None 100 170 1.7 n 4 None 80 230 2.9 n 5 None 60 210 3.5 n 6 None 90 160 1.8 n Comparative None 110 120 1.1 n example 1 2 None 140 140 1.0 n 3 None 160 110 0.7 n 4 None 90 110 1.2 n 5 None 130 120 0.9 n 6 None 110 150 1.4 s(strong) 7 -- 100 70 0.7 n 8 Present 80 100 1.3 n__________________________________________________________________________
As is clear from Table 2, each of the samples of Examples 1-6 had a satisfactory protective film after the heat treatment under reduced pressure conducted under the conditions specified by the present invention, and showed excellent oxidation resistance. In contrast, the samples of Comparative Examples 1 and 2 heat-treated at too high a temperature, the sample of Comparative Example 6 heat-treated at too low a temperature, the samples of Comparative Examples 3 and 4 heat-treated at too low a pressure, and the sample of Comparative Example 5 heat-treated at a low pressure and at too high a temperature, distinctly contained non-homogeneous portions in respective protective films and were inferior in oxidation resistance. The sample of Comparative Example 7 subjected to no heat treatment under reduced pressure and the sample of Comparative Example 8 heat-treated in air were inferior in oxidation resistance as well. As seen in FIG. 2, the sample of Comparative Example 8 after heat treatment had a large number of cracks in the protective film.
The yttrium concentration ratios in the samples of Examples 1-6 after heat treatment under reduced pressure, as compared with those in the samples of Comparative Examples 1-6, are greatly high and it is presumed that the enrichment of yttrium in or near film contributes to the increase in oxidation resistance in some form. From the fact that the sample of Comparative Example 6 shows a strong tungsten peak, it is presumed that when the treatment temperature is low, a sample is contaminated and its oxidation resistance is adversely affected thereby.
EXAMPLE 7
A sintered honeycomb material was obtained in the same manner as in Examples 1-6 and Comparative Examples 1-7 except that the honeycomb structure before drying and sintering had dimensions of 50 mm in diameter, 100 .mu.m in rib thickness and 400 cells/in..sup.2 in cell density. The sintered honeycomb material had a shrinkage factor on firing, of 19% and a porosity of 6%. The material had a carbon content of 0.08% by weight when subjected to chemical analysis. A cubic sample (5 cells.times.5 cells.times.8 mm) was cut out from the material and subjected to a heat treatment under reduced pressure under the conditions shown in Table 3. In the heat treatment under reduced pressure, the heating was conducted using an electric furnace using a tungsten mesh as a heater, and the reduced pressure was produced by degassing the furnace inside using a diffusion pump, to keep the pressure inside the furnace at a constant vacuum. The sample after the heat treatment under reduced pressure was examined for pre oxidation amount and oxide film properties. Also, the sample after the heat treatment under reduced pressure was subjected to the same oxidation test as in Examples 1-6 and Comparative Examples 1-6, to measure the total oxidation amount. The results are shown in Table 3.
EXAMPLE 8
A sintered honeycomb material was obtained in the same manner as in Example 7 except that a pure Fe powder, a pure Cr powder, a Fe-Al (Al:50% by weight) alloy powder, a Fe-Si (Si: 75% by weight) alloy powder, a Fe-B (B:20% by weight) alloy powder and a Y.sub.2 O.sub.3 powder were mixed so as to give a composition B shown in Table 1. The sintered honeycomb material had a shrinkage factor on firing, of 20% and a porosity of 9%. The material had a carbon content of 0.14% by weight when subjected to chemical analysis. A cubic sample (5 cells.times.5 cells.times.8 mm) was cut out from the materiral and subjected to the same heat treatment under reduced pressure as in Example 7 and the same oxidation test as in Example 7, to measure various test items. The results are shown in Table 3.
EXAMPLE 9
A sintered honeycomb material was obtained in the same manner as in Example 7 except that a pure Fe powder, a pure Cr powder, a Fe-Al (Al:50% by weight) alloy powder, a Fe-B (B: 20% by weight) alloy powder and a Y.sub.2 O.sub.3 powder were mixed so as to give a composition C shown in Table 1. The sintered honeycomb material had a shrinkage factor on firing, of 18% and a porosity of 8%. The material had a carbon content of 0.08% by weight when subjected to chemical analysis. A cubic sample (5 cells.times.5 cells.times.8 mm) was cut out from the materiral and subjected to the same heat treatment under reduced pressure as in Example 7 and the same oxidation test as in Example 7, to measure various test items. The results are shown in Table 3.
EXAMPLE 10
A sintered honeycomb material was obtained in the same manner as in Example 7 except that a pure Fe powder, a pure Cr powder, a Fe-Al (Al:50% by weight) alloy powder, a Fe-Si (Si:75% by weight) alloy powder and a Fe-B (B:20% by weight) alloy powder were mixed so as to give a composition D shown in Table 1. The sintered honeycomb material had a shrinkage factor on firing, of 19% and a porosity of 10%. The material had a carbon content of 0.13% by weight when subjected to chemical analysis. A cubic sample (5 cells.times.5 cells.times.8 mm) was cut out from the materiral and subjected to the same heat treatment under reduced pressure as in Example 7 and the same oxidation test as in Example 7, to measure various test items. The results are shown in Table 3.
EXAMPLE 11
A sintered honeycomb material was obtained in the same manner as in Example 7 except that a pure Fe powder, a pure Cr powder, a Fe-Al (Al:50% by weight) alloy powder and a Fe-B (B:20% by weight) alloy powder were mixed so as to give a composition E shown in Table 1. The sintered honeycomb material had a shrinkage factor on firing, of 20% and a porosity of 8%. The material had a carbon content of 0.07% by weight when subjected to chemical analysis. A cubic sample (5 cells.times.5 cells.times.8 mm) was cut out from the material and subjected to the same heat treatment under reduced pressure as in Example 7 and the same oxidation test as in Example 7, to measure various test items. The results are shown in Table 3.
COMPARATIVE EXAMPLES 9-13
The same samples as used in Examples 7-11 were subjected to the same oxidation test as in Examples 7-11 without being subjected to the same heat treatment under reduced pressure as in Examples 7-11, to measure their total oxidation amounts. The results are shown in Table 3.
TABLE 3__________________________________________________________________________ Conditions for heat treatment under reduced Pre oxidation Oxygen partial Total pressure Temperature Time amount Composition pressure (Pa) (Pa) (.degree.C.) (hr) (mg/cm.sup.2)__________________________________________________________________________Example 7 A 0.2 1.0 1150 15 0.128 B 0.2 1.0 1150 15 0.099 C 0.2 1.0 1150 15 0.2010 D 0.2 1.0 1150 15 0.0111 E 0.2 1.0 1150 15 0.20Comparative A (Not treated) -- -- -- --example 910 B (Not treated) -- -- -- --11 C (Not treated) -- -- -- --12 D (Not treated) -- -- -- --13 E (Not treated) -- -- -- --__________________________________________________________________________ Properties of oxide film formed by heat treatment Total oxidation under reduced pressure amount Non-homogeneous portions Cracks (mg/cm.sup.2) in oxide film in oxide film__________________________________________________________________________ Example 7 0.34 Not present None 8 0.59 Not present None 9 0.34 Not present None 10 0.62 Not present None 11 0.56 Not present None Comparative 0.66 -- -- example 9 10 1.39 -- -- 11 0.62 -- -- 12 1.19 -- -- 13 1.18 -- --__________________________________________________________________________
As is clear from Table 3, the Fe-Cr-Al alloys having the compositions A to E, when subjected to the heat treatment under reduced pressure according to the present invention to form a protective film as in Examples 7-11, as compared with when subjected to no heat treatment under reduced pressure as in Comparative Examples 9-13, have excellent oxidation resistance.
EXAMPLE 12
Each of the same samples as used in Examples 1-6 and Comparative Examples 1-6 was placed in an alumina pipe (SSA-S) of 15 mm in inside diameter. The alumina pipe was covered, at the both (upper and lower) ends, with a sapphire plate of 27 mm in diameter and 1 mm in thickness. The resulting set was placed in an alumina crucible (SSA-S) with a cover, and a heat treatment under reduced pressure was conducted under the conditions of 1.1Pa (total pressure), 1,150.degree. C. (temperature) and 5 hours (time). In the treatment, the heating was conducted using an electric furnace using a tungsten mesh as a heater, and the reduced pressure was produced by degassing the furnace inside using a diffusion pump to keep the furnace inside at a constant vacuum. After the heat treatment under reduced pressure, the deposit on the lower side of the upper sapphire plate was subjected to elemental analysis using a scanning type electron microscope. As a result, slight amounts of Cr and Fe were detected.
COMPARATIVE EXAMPLE 14
Each of the same samples as used in Example 12 was subjected to the same heat treatment under reduced pressure except that the conditions for the treatment were 0.5 Pa (total pressure), 1,250.degree. C. (temperature) and 15 hours (time). After the heat treatment under reduced pressure, the deposit on the lower side of the upper sapphire plate was subjected to elemental analysis using a scanning type electron microscope. As a result, large amounts of Cr and Fe were detected.
It is appreciated from Example 12 and Comparative Example 14 that when the heat treatment under reduced pressure is conducted at too high a temperature or at too low a pressure, the vaporization of alloy components occurs in large amounts.
As stated above, the method of the present invention enables the formation of a homogeneous protective film of excellent oxidation resistance even on metals of non-homogeneous composition such as Fe-Cr-Al alloy and the like and is very effective for increasing the oxidation resistance of Fe-Cr-Al alloy.

Number	Name	Date
2269601	Perrin	Jan 1942
4439248	Herchenoeder	Mar 1984
4588449	Sigler	May 1986

Number	Date	Country
0393581A2	Oct 1990	EPX
1226734	Jul 1960	FRX
4-63148	Oct 1992	JPX
2001677	Feb 1979	GBX
2094656	Sep 1982	GBX
2160892	Jan 1986	GBX

Method for increasing oxidation resistance of Fe-Cr-Al alloy

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Patent Abstracts of Japan, vol. 12, No. 438, JP-A 63-162502 (A), Jul. 5, 1988.
Patent Abstracts of Japan, vol. 8, No. 67, JP-A 58-217677 (A), Dec. 17, 1983.
"Oxide Structure of Stainless Steels under Controlled Oxygen Atmospheres", Toshiyuki Yashiro, Heat Treatment, vol. 31, No. 4, pp. 205-211, 1991.
"Surface Modification of Stainless Steels Using Thermal Passivation", Toshiyuki Yashiro et al., Surface Technology vol. 41, No. 3, pp. 41-48, 1990.