SE527176C2 - Stainless steel for use in high temperature applications - Google Patents

Abstract

The present invention relates to a method for producing an alloy comprising of (by weight-%) C <= 0,20 Cr 15,0-25,0 Ni 1,0-20,0 Al 4,5-12,0 Mo+W <= 4,0 Nb <= 2,0 Mn <= 2,0 Si <= 2,0 Zr + Hf <= 0,2 REM <= 0,1 N <= 0,05 with improved oxidation resistance in high temperature applications by depositing an AI-base alloy on a substrate material comprising of (by weight-%): C <= 0,20 Cr 16,0-27,0 Ni 1,2-22,0 Al 0-6,0 Mo + W <= 4,5 Nb <= 2,2 Mn <= 2,0 Si <= 2,0 Zr + Hf <= 0,2 REM <= 0,1 N <= 0,1 balance Fe and normally occurring steelmaking impurities and additions, the use of said alloy and said alloy.

Description

20 25 30 5 2 7 1 7 6 ti; gi; . 'y. -j: = <2: nnnn oo The alloy can be produced by conventional methods, which include, for example, the steps of melting, refining, casting, block rolling or forging followed by hot and cold rolling to produce thin strips with a final thickness of less than 150μm. The alloy can also be made by using a pre-rolled strip with a lower aluminum content than the final desired aluminum content of a catalyst support material and then depositing a layer of an aluminum-rich alloy on the surface of this material.

The deposition can be done in many different ways, for example by dipping the strip in a molten Al alloy, by roll bonding (well plating) an Al alloy on top of a ferritic steel, by coating using PVD technology (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) technique.

In all these examples, the thickness of the strip together with the coating may be the final shape and thickness, or the strip may be rolled down to a smaller thickness after the deposition has been made. The composite material containing the ferritic alloy and the aluminum alloy on one or both sides can be heat treated to produce a homogeneous alloy, or an alloy with an increased concentration of aluminum towards the surface.

The mechanical properties of ferritic Fe-Cr-Al alloys, especially with increased aluminum content, are known to be poor at high temperatures.

Several different ways of improving these properties are known, such as the preparation of fi dispersions of oxide or nitride phases by powder metallurgical processes. These processes involve expensive processes during manufacture and are therefore not suitable for the production of alloys which are to be produced in large quantities at low cost.

Another way to increase the high temperature strength is by separating extremely small particles of aluminum nitrides. This has been described, for example, in US-A-5226984 for a number of Fe-Ni-Cr-Al alloys having a substantially ferritic structure or a mixed austenitic / ferritic structure. At present, such alloys are known only in the rolled or cast state.

If they are to be used in catalysts, it must be possible to shape the alloys by cold rolling down to a final thickness of less than approximately 150 microns.

It is known that the addition of about 10% by weight of nickel to an Fe-Cr-Al alloy further improves the resistance to thermal shock, i.e. extremely rapid heating or cooling, which can cause mechanical collapse of the material; a phenomenon known to cause reduced life in the catalyst application.

The future trend for metallic catalytic steel foils is moving towards thinner thicknesses. There are several serious problems to be solved: - the need for better oxidation properties due to less available aluminum per surface area unit, - materials with increased high temperature strength and resistance to fatigue - production methods with lower costs will be needed.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method for producing a stainless steel alloy with improved oxidation resistance in high temperature applications by depositing an Al-based alloy on a substrate material and having excellent manufacturing properties.

It is another object of the present invention to provide a stainless steel alloy with improved oxidation resistance, increased resistance to cyclic thermal loading and improved mechanical properties such as increased fatigue strength at high temperatures.

It is a further object of the present invention to provide a stainless steel alloy for use as a substrate in exhaust gas purification applications, such as catalysts in internal combustion engines. Another object of the present invention is to provide a production path for the alloy of the invention by depositing an AI alloy on a thin film of a substrate alloy.

To illustrate but not limit, the invention will now be described in more detail with reference to the accompanying figures.

Brief description of the Figur gures Figure 1 Figure 2 Figure 3a Figure 3b Figure 4a Figure 4b Figure 5 Figure 6 shows the elongation at break for alloys according to the present invention compared with a comparative example plotted against the temperature. shows the modulus of elasticity plotted against the temperature of the same samples as in Figure 1. shows the tensile strength plotted against the temperature of the same samples as in the previous samples. shows the tensile strength relative to that of the comparative example at the same temperature plotted against the temperature of the same samples as in the previous samples. shows the yield strength plotted against the temperature of the same samples as in the previous figures. shows the yield strength relative to that of the comparative example at the same temperature plotted against the temperature of the same samples as in the previous samples. shows the result of the oxidation test of alloys of the present invention and a comparative example by plotting the weight change after oxidation at 1100 ° C of the samples plotted against time. shows the result of the oxidation test of alloys of the present invention and a comparative example by plotting the weight change after oxidation at 1200 ° C of the samples plotted against time. Figure 7 shows a part through the Fe-Ni-Cr-Al-Thermo-calc phase diagram at 20% by weight of Cr and 5% by weight of Al.

Figure 8 shows the result of the oxidation test of alloys of the present invention and a comparative example by plotting the change in weight after oxidation at 1100 ° C of the samples plotted against time.

Detailed description of the invention The objects are fulfilled by the method according to claim 1, i.e. a method for producing an alloy containing (in% by weight) C s 0.20 Cr 15.0 - 25.0 Ni 1.0 - 20.0 Al 4.5 - 12.0 Mo + W s 4.0 Nb s 2.0 Mn s 2.0 Si s 2.0 Zr + Hf s 0.2 REM s 0.1 N s 0.05 with improved oxidation resistance in high temperature applications by depositing an Al-based alloy on a substrate material containing ( in% by weight): C s 0.20 Cr 16.0 - 27.0 Ni 1.2 - 22.0 Al 0 - 6.0 Mo + W s 4.5 Nb s 2.2 Mn s 2.0 Si s 2.0 10 15 20 25 30 527 17e 6 Zl '+ Hf S 0.2 REM 5 0.1 NS 0.1 residue Fe and impurities and additives normally present in steel production.

The content of elements in the alloy made by the method of depositing an Al-based alloy on a stainless steel substrate should have the following limitations: the content of Cr should be limited to 15.0 to 25.0% by weight, preferably to 20.0 to 22% by weight. .0% by weight. The content of Ni should be limited to between 1.0 and 20.0% by weight, preferably to 2.5 to 15.0% by weight, most preferably to 5.0 to 12.5% by weight. the content of Al should be limited to 4.5 to 12.0% by weight, preferably to 5.0 to 8.0% by weight, most preferably to 5.0 to 7.0% by weight. The total content of the elements Mo and W should be limited to 4.0% by weight, preferably up to 3.0% by weight.

Another preferred content of the total of Mo and W is more than 1% by weight.

The content of Mn should be limited to 2.0% by weight, preferably up to 0.5% by weight. The content of N will be limited to 0.05% by weight and should be kept as low as possible. The content of C should be limited to 0.20% by weight, preferably up to 0.15% by weight. This alloy can be manufactured by conventional methods such as melting, casting and hot rolling in dimensions down to about 1mm.

The substrate alloy according to the invention can be in the form of, for example, a thin foil or strip, a wire or a sheet. The deposition of an Al alloy on the substrate can be done on a substrate alloy having suitable dimensions for the final product or, if necessary, followed by further cold working and / or diffusion annealing. The composition of the substrate alloy before deposition is as follows (all contents in% by weight): C s 0.20 Cr 16.0 - 27.0 Ni 1.2 - 22.0 10 15 20 25 30 527 176 7 _ Al 0 - 6 .0 Mo + W s 4.5 Nb s 2.2 Mn s 2.0 Si s 2.0 Zr + Hf s 0.2 REM s 0.1 N s 0.1 residue Fe and in steelmaking normally occurring impurities and additives.

The content of Cr in the substrate alloy should be limited to 16.0 to 27.0% by weight, preferably to 20.0 to 24.0% by weight. the content of Ni should be limited to between 1.2 and 22.0% by weight, preferably to 2.5 to 15.0% by weight, most preferably to 5.0 to 14.0% by weight.

The content of Al in the substrate alloy should be limited to 0 to 6% by weight, preferably to 0.0 to 2.0% by weight, more preferably to> 0 to 1.0% by weight. The total content of the elements Mo and W should be limited to 4.0% by weight, preferably up to 3.0% by weight. The content of Mn should be limited up to 2.0% by weight, preferably up to 0.5% by weight. the content of N in the substrate alloy will be limited up to 0.10% by weight, preferably up to 0.05% by weight. the content of C should be limited up to 0.20% by weight, preferably up to 0.15% by weight.

The effect of the various alloy additives is described in the following: Carbon Carbon forms carbides together with, for example, Nb or Cr. These carbides help to increase the mechanical high temperature strength of the alloy and also reduce the tendency for grain growth during use, a phenomenon known to cause embrittlement in ferritic alloys. Coal, on the other hand, can cause embrittlement during cold rolling of the alloy and probably also causes a deterioration of the oxidation resistance. Therefore, the carbon content is limited to a maximum of 0.2% by weight.

Chromium Chromium can form an oxide that protects the alloy from further oxidation. In these alloys at least 15.0% by weight of Cr is necessary to effectively form a protective oxide fi ch. Cr further facilitates the formation of an alumina m lm in alloys containing 4.5% by weight of Al or more, which is necessary if the alloys are to be used at temperatures higher than 1000 ° C.

An excess of chromium in the alloy can cause embrittlement of the alloy during production, and the maximum chromium content of the alloy is therefore limited to 27.0% by weight.

Nickel Nickel is included in the alloy to create reinforcing NiAl particles, the formation of which is considered to be the main reinforcing effect in this alloy.

Aluminum Aluminum, if present in concentrations of 4.5% by weight or more, forms a protective alumina shell on the surface of the alloy when exposed to high temperatures. This oxide protects the alloy from further oxidation. Therefore, the minimum Al content of the final alloy is 4.5% by weight. A low Al content in the substrate alloy is desirable in order to avoid the formation of embrittled phases during production. In contrast, the presence of small amounts of Al in the substrate alloy reduces the amount of Al required to be deposited on the substrate to create a final alloy containing at least 4.5% by weight of Al. Molybdenum and tungsten Molybdenum and tungsten act as a solid solution-reinforcing element, which gives the alloy a higher strength at higher temperatures. As such, the two elements can completely replace each other. If the total content of molybdenum and tungsten is greater than 4.0% by weight, the oxidation properties deteriorate sharply.

Niobium Niobium is known to increase the stability of the NiAl phase and may therefore be useful in effecting an increase in NiAl dissolution temperature.

Thus, Nb can increase the high temperature strength of the alloy according to the invention. Nb also forms Nb (N, C), which adds extra creep strength and resistance to grain growth.

Zr, Hf, REM The effect of these elements is to reduce the oxidation rate of the alloy and increase the resistance to cracking of the alumina and its resistance to peeling during heating and cooling. These elements can be added either in the substrate alloy or in the deposited Al alloy or in both, to optimize the oxidation resistance.

Nitrogen Nitrogen forms embrittled AlN with aluminum and should therefore be present in as small amounts as possible in an alloy made by conventional methods. On the other hand, in an alloy made by depositing Al on a substrate alloy with a low Al content, the nitrogen level can be allowed to be as high as 0.1% by weight. If present, N is known to increase the strength of alloys, both by solid solution reinforcement and by precipitation of nitrides and carbonitrides such as CrgN or Nb (N, C). Description of some preferred embodiments of the present invention The effects of several alloy modifications have been evaluated with respect to oxidation resistance, producibility and mechanical high temperature properties. Some examples of alloy compositions according to the present invention are presented in Table 1.

The alloys were made by induction melting. The ingot was rolled down to billets, which were then hot rolled down to a thickness of 3 mm.

Example 1 High temperature strength tests were performed on all alloys between 600 ° C - 1000 ° C according to Swedish standard SS-EN 10002-5. The modulus of elasticity was measured directly by using strain gauges mounted on the sample.

The oxidation properties of the alloys were evaluated at 1100 ° C and 1200 ° C in a normal atmosphere. The samples were removed from the oven at preselected intervals and weighed to monitor weight gain.

Table 1. Chemical composition Comparative Alloy A Alloy B Alloy C Alloy D Example C 0.009 0.011 0.013 0.015 0.036 Si 0.24 0.21 0.16 0.14 0.17 Mn 0.26 0.28 0.18 0.32 0.1 Cr 20.22 20.4 20.3 20.15 21.96 Ni 0.14 4.99 12.7 4.89 7.04 Mo <0.01 2.01 0.01 0.02 0 .01 Al 5.5 5 5.9 6.2 5.8 Ce 0.026 0.015 0.028 <0.005 0.011 La 0.013 0.008 0.014 0.082 <0.01 Zr <0.005 <0.005 <0.005 0.076 0.06 10 20 25 30 527 176 11 The microstructure of the alloys on a large scale is identical to that of the comparative example. In contrast, SEM and TEM analyzes show that alloys with these compositions contain nickel aluminide particles between 5 nm and 2 μm in size with CsCl-type structure. The particles are formed at regular intervals within the ferrite grains.

The hardness of the material after hot rolling is high: in the range 400-520 HV1.

By annealing, the hardness can be reduced from 490 to 320 HV1. Due to the high hardness of the material, cold rolling was considered impractical.

Figures 1 to 4 show the measured mechanical high temperature properties of the alloys A to D and the comparative example. The modulus of elasticity of the experimental alloys is generally higher than that of the comparative example. An interesting effect is the measured increase in the modulus of elasticity of the two 5.0% Ni alloys above 900 ° C.

In the temperature range below 750 ° C to 800 ° C, the alloys of the present invention have a significantly higher mechanical strength than the comparative example. However, at higher temperatures, the difference between the alloys is within the experimental uncertainty of the equipment used, with two exceptions. The strength of alloy B, and above all, alloy D is significantly higher at 900 ° C and 1000 ° C than it is with the other alloys. The experimental alloys show - consistently less elongation at break than the comparative example as shown in fi g. 1.

Regarding oxidation resistance, this is shown in Figures 5 and 6. At 1100 ° C, alloy A shows a smaller weight gain than the comparative example, as on the other hand alloy D loses weight due to oxide palliation. At 1200 ° C, the experimental alloys behave differently individually: Alloy C is exceptionally good and shows a normal oxidation behavior with a 30.0% lower total weight gain than the comparative example. Alloys A, B and D start with low weight gains but start trellis after a short 10 15 20 25 30 52 7 17 6 12 91337; oooo: O initialization period. The cracking rate of alloy A is comparatively low and only begins to accelerate after 350 hours, as that of alloys B and D is fast from the beginning. Thus, it can be expected that the optimum oxidation resistance can be found in alloys containing approximately 5.0% by weight of Ni. At 1200 ° C, the presence of Mo is clearly detrimental to the oxidation resistance, whereas alloy A with 2.0% by weight Mo is superior to the comparative example at 1100 ° C.

The high hardness of the material is partly due to the presence of Ni-aluminides. A calculated phase diagram section for the Fe-Ni-20Cr-5Al system is shown in fi g. 7.

The phase diagram was calculated with Thermo-calc. This shows that NiAl is probably stable even at low Ni-content alloy.

The dissolution temperature of NiAl is approximately 900 ° C for a 5.0 wt% Ni alloy and 1050 ° C for a 12.5 wt% Ni alloy. No austenite is expected to form below a total Ni content of 14.0% by weight. The difference in lattice parameters between NiAl and ferrlt in equilibrium is expected to be small, and precipitation of NiAl appears to be coherent. The presence of NiAl in alloy B during the hot strength tests above 900 ° C explains the improved yield strength.

The unexpected temperature dependence of the modulus of elasticity between 900 and 1000 ° C for two of the alloys cannot be explained at present, but it can be linked to the dissolution of NiAl. However, the actual figures for the modulus of elasticity are still much higher for the alloy according to the invention than for the comparative example. It must be noted that measurements of the modulus of elasticity are less accurate at high temperatures than at room temperature.

The yield strength is improved for all compositions according to the invention below 800 ° C compared with the average comparative example. At higher temperatures, the effect is less clear. Moz's reinforcing effect appears to be slightly above 600 ° C with respect to the yield strength. The improvement of the yield strength compared to the comparative example is at most at 600 ° C for alloy A. Therefore, this alloy is preferable in catalysts operating comparatively low temperatures. At 700 ° C and above, the improvement of the yield strength is greatest for alloy D. Thus, for an optimum mechanical strength above 700 ° C, a Ni content of 7.0% by weight is preferred.

In order to be able to evaluate the usefulness of these alloys in practical applications, fatigue tests at high temperatures as well as creep tests will probably be necessary. However, the initial tests that have been performed and are described in the present application indicate that these alloys are promising material candidates for catalytic converter bodies in mechanically challenging applications where a combination of high mechanical strength, high temperature properties and oxidation resistance is required.

The oxidation properties of the experimental alloys, as shown in Figures 5 and 6, are unexpectedly good, in some cases superior to that of the comparative example, especially with respect to the high Ni content, which was thought to have a negative effect on the oxidation properties and resistance. In other cases, trellisation is detected, although the trellisation rate is not too severe for possible use of the material in applications other than catalysts.

By adding Ni in amounts of 2.5-15% by weight and Mo + W s 4.0% by weight, it is possible to improve the high temperature strength compared to FeCrAl catalyst steels, without significantly impairing the oxidation resistance.

The alloy may also be useful in other high temperature applications such as heating applications, e.g. in heat treatment furnaces.

Cold rolling tests were performed. However, the material is extremely difficult to produce into thin sections by conventional production routes due to brittleness. Therefore, although this example shows the preferred composition in terms of mechanical properties and corrosion resistance, it does not show the preferred production path for an alloy to be used in the form of thin strips, e.g. of thickness below 150 μm in a catalyst. 10 2 25 30 52 7 176 14 Example 2 Thus, a preferred manufacturing method for the alloy is by coating a low Al alloy with pure Al and / or an aluminum based alloy in one or fl era of the final stages of manufacture.

The coating can be applied by e.g. dipping, plating or a CVD or PVD process. The present example will demonstrate the utility of such a production pathway and further indicate a preferred composition of the substrate alloy. The Al content of the substrate alloy should be below 2.0% by weight, preferably below 1.0% by weight so as not to produce embrittled NiAl precipitates during manufacture.

Strips of the alloy according to Table 2 were made by induction melting.

The ingot was rolled into billets, which were then hot-rolled down to a thickness of 3 mm. The hot-rolled strips were then cold-rolled down to a minimum thickness of 50 μm with intermediate annealing. The data on mechanical properties collected in Table 2 show that the substrate alloy according to the invention made in this way can be annealed to give a soft, machinable alloy, which can be cold worked down to a suitable thickness.

Example 3 A thin film of Al was deposited on strip samples from three of the alloys of Example 2 in an evaporation process, to make samples with a total Al content of 6.0% by weight, as shown in Table 3. Parts of the strips were annealed in hydrogen gas for 10 minutes at 1100 ° C. Oxidation tests and hot strength tests were performed on the annealed samples. The oxidation results for alloy 2C in the annealed state are shown in Figure 8 together with those of alloys A and D and the comparative example. Up to 220h, the oxidation resistance of alloy 2C, which does not consist of any intentional additions of REM, was superior to that of all the other alloys, including alloy A, which have a similar nominal composition. This effect cannot be explained at present. It can be expected that alloy 2C, if additionally alloyed with REM, could exhibit even better oxidation resistance.

Table 2. Compositions of alloys and results from strength tests Element 1A 1B 1C 1D 1E 1F D (weight%) Cr 22.1 21.9 21.8 22.1 22.3 22.0 22 Al 2.1 <0.01 <0.01 <0.01 <0.01 <0.01 5.8 Ni 15.9 7.6 7.6 4.0 4.1 3.0 7.0 Mo <0.1 <0.1 2.0 <0.1 <0.1 <0.1 <0.1 Nb <0.01 0.01 0.01 <0.01 0.47 0.45 <0.01 Mn 0.1 0, 1 0.1 0.1 0.1 0.1 0.1 0.1 Si 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.2 C 0.03 0.03 0.03 0.03 0, 10 0.03 0.11 0.04 N 0.02 0.01 0.01 0.02 0.02 0.02 0.02 0.01 Ti <0.01 <0.01 <0.01 <0.01 < 0.01 <0.01 <0.01 Zr 0.07 <0.01 <0.01 <0.01 <0.01 0.07 0.06 REM <0.005 0.01 <0.005 <0.005 <0.005 < 0.005 0.01 Final 50 50 50 50 50 50 Could not thickness / um cold rolled due to brittleness Mechanical properties in forged rod Rp0,2 335 382 544 484 487 Rm 562 597 891 581 781 A5 44 30 9 6 11 <1 Mechanical properties after annealing: 1100 ° C / 30 min Rp0,2 254 335 301 392 340 Rm 545 587 833 570 718 A5 47 32 23 18 17 10 527 176 16 Table 3. Properties of alloys according to the invention in the deposited in the state Alloy 2C 2E 2F Substrate alloy 1 C 1 E 1 F Band thickness before Al- 50 50 50 deposition / um Nominal thickness of 5 5 5 deposited Al / pm Measured thickness of - 3 - deposited Al Estimated composition (nominal) Cr 21 21 21 Al 6 6 6 Ni 7.2 3.9 2.8 Mo 1.9 <0.1 <0.1 Nb 0 .01 0.44 0.42 Strength properties of Al-deposited bands after annealing for 10 minutes at 1100 ° C in Hz gas: Rm at 700 ° ClMpa 159 Rm at 900 ° C / Mpa 38 The alloys of the present invention are mainly ferritic Fe-Ni-Cr-Al alloys enhanced by the presence of extremely small nickel aluminide particles and if necessary further enhanced by the presence of substitutionally dissolved elements such as Mo or W. Thanks to a high Al content and the presence of reactive elements, the resistance to oxidation at high temperatures is good.

Thus, this is a suitable alloy for use as a support material in metallic catalysts, especially those exposed to a combination of high temperature, cyclic thermal stress and mechanical stress. o o o O I O O I O I I

Claims (1)

Patent claims A method of making an alloy comprising (in% by weight) C s 0.20 Cr 15.0 - 25.0 Ni 1.0 - 20.0 Al 4.5 - 12.0 Mo + W s 4, 0 Nb s 2.0 Mn s 2.0 Si s 2.0 Zr + Hf s 0.2 REM s 0.1 N s 0.05 with improved oxidation resistance in high temperature applications by depositing an At-based alloy on a substrate material comprising (in% by weight): C s 0.20 Cr 16.0 - 27.0 Ni 1.2 - 22.0 Al 0 - 2.0 MO + W s 4.5 Nb s 2.2 Mn s 2, 0 Si s 2.0 Zr + Hf s 0.2 REM s 0.1 N s 0.1 residue Fe and impurities and additives normally present in steel production, and that the substrate with the deposited AI alloy is exposed to a high temperature. 527 176 få '. Method according to claim 1, characterized in that the alloy is manufactured by means of conventional technology in the form of strips and / or foil thinner than 200 μm. . Method according to claims 1 and 2, characterized in that the alloy is manufactured by means of conventional technology in the form of strip and / or foil thinner than 150 μm. . Use of an alloy made according to the method of claim 1 in high temperature applications. . Use of an alloy made according to the method of claim 1 in high temperature applications with cyclic thematic load. Use of an alloy made according to the method of claim 1 in exhaust gas purifying applications such as catalysts.