Patent Application
20080038143
These objects are attained by means of an austenitic product that is manufactured by coating an austenitic substrate alloy with the following composition (in % by weight): 20-70% of Ni, 15-27% of Cr, 0-5% of Al, 0-4% of Mo and/or W, 0-2% of Si, 0-3% of Mn, 0-2% of Nb, 0-0/5% of Y, Zr and/or Hf, 0-0.5% of Ti, 0-0.1% of one or more rare earth metals (REM) such as, e.g., Ce, La, Sm, 0-0.2% of C, 0-0.1% of N, balance Fe and normally occurring impurities, with an aluminium composition such as aluminium or an aluminium-based alloy such as is described below.
A preferred composition of the substrate material is (in % by weight) 25-70% of Ni, 18-25% of Cr, 1-4 % of Al, 0-4 % of Mo and/or W, 0-2% of Si, 0-3% of Mn, 0-2% of Nb, 0-0.5% of Y, Zr and/or Hf, 0-0.5% of Ti, 0-0.1% of one or more rare earth metals (REM) such as, e.g., Ce, La, Sm, 0-0.1% of C, 0-0.05% of N, balance Fe and normally occurring impurities.
By a two-stage process, the content of aluminium of the final product as well as its mechanical properties and oxidation resistance can be optimized independently of each other.
After coating the substrate material with aluminium or an aluminium-based alloy, the final alloy has a composition consisting of (in % by weight) 25-70% of Ni, 15-25% of Cr, 4.5-12% of Al, 0-4% of Mo and/or W, 0-4% of Si, 0-3% of Mn, 0-2% of Nb, 0-0.5% of Ti, 0-0.5% of Y, Sc, Zr and/or Hf, 0-0.2% of one or more rare earth metals (REM) such as, e.g., Ce, La, Sm, 0-0.2% of C, 0-0.1% of N, balance Fe and normally occurring impurities.
The austenitic substrate material has in itself a good high-temperature strength, which is increased by the presence of precipitations of Ni (Nb, Al) and, if required, also by Mo and/or W in solid solution. Additionally increased mechanical stability and resistance to grain growth may be given by the presence of precipitations of carbides and/or nitrides of any one or some of the elements Ti, Nb, Zr, Hf.
Carbon in solid solution or as carbides contributes to an increased mechanical strength at high temperatures. Simultaneously, higher contents of carbon in the substrate material imply deteriorated properties upon cold working. Therefore, the maximal content of carbon in the substrate should be limited to 0.2% by weight.
Nitrogen in solid solution or as nitrides contributes to an increased mechanical strength at high temperatures. Simultaneously, higher contents of nitrogen in the substrate material imply that embrittling aluminium nitride may be formed in the production of the substrate or after coating with aluminium or an aluminium-based alloy. Therefore, the maximal content of nitrogen in the substrate should be limited to 0.1% by weight.
The austenitic alloy manufactured according to the invention is used in a coated and not heat-treated state or after a diffusion annealing. The most favourable compositions for the substrate alloy are obtained if it contains 1-4% by weight of Al. This content of aluminium gives the finished alloy an improved oxidation resistance and an improved production economy without entailing an increased risk of production disturbances in comparison with the manufacture of a material of low content of aluminium. After coating with aluminium or an aluminium-based alloy, the material should in total contain more than 4.5% by weight of Al.
According to the invention, the coating with aluminium or an aluminium-based alloy should take place within a temperature range of the substrate that is lower than the melting point of the aluminium, i.e., at a temperature between 100° C. and 600° C., preferably 150° C.-450° C.
Addition of Zr and/or Hf and REM and/or Y and/or Sc gives an increased resistance to peeling and flaking of the formed oxide. The finished product's contents of said elements may be supplied by addition in the substrate alloy and/or in the aluminium-based alloy that are used in the coating.
Certain compositions of the alloy according to the invention could be manufactured by conventional metallurgy. However, unlike this, in production by means of the process according to the present invention, a material can be obtained, the microstructure of which is controlled and the oxidation properties and mechanical properties of which are optimal. It is an additional advantage of the process according to the present invention that the total content of aluminium of the final product is not limited by the embrittling effect that contents of aluminium above approx. 4.5% by weight may give upon later cold and/or hot working. Furthermore, the method to coat a substrate material with aluminium or an aluminium-based alloy according to the invention gives a final product, the contents of which of, e.g., Mo, C, Nb can be considerably higher than in a conventionally manufactured material without the presence of said elements resulting in any noticeable deterioration of the oxidation properties.
The proper coating of the substrate alloy with aluminium or an aluminium-based alloy may be effected by processes such as, e.g., dipping in melt, electrolytic coating, rolling together strips of aluminium or an aluminium alloy from a gas phase by so-called CVD or PVD technique. The coating with aluminium or aluminium-based alloy can be carried out after the substrate alloy has been rolled or in another way been machined to desired product dimension. During this process, a diffusion annealing may be carried out in order to provide a homogenization of the material and then plastic machining in one or more steps may be carried out in order to provide the final product. Plastic machining, such as, e.g., rolling or drawing may also be effected directly on a coated product of larger dimensions than the desired final dimension. In this case, the plastic machining may be followed by annealing.
The content of aluminium in the final product can be varied by means of different factors: the thickness of the substrate material in relation to the thickness of the coating, the content of aluminium in the substrate material as well as the content of aluminium of the coating.
However, as has been described above, the total content of aluminium in the finished product always has to be at least 4.5% by weight in order to secure sufficient properties. The product may be used in the form of an annealed, homogeneous material or a laminate or a material having a concentration gradient of Al with the Al content being higher at the surface than in the centre of the material.
Depending on the coating process used, various compositions of the applied Al alloy are more suitable than others. The aluminium alloy contains 0-25% of Si and/or 0-2% by weight of one or more of the elements Ce, La, Sc, Y, Zr, Hf and/or 0-5% by weight of one or more of the elements Mg, Ti, Cr, Mn, Fe, Ni, Co and/or 0-1% by weight of one or more of the elements B, Ge, preferably the aluminium alloy should contain at least 90% of Al, 0-10% of Si and/or 0-2% by weight of one or more of the elements Ce, La, Sc, Y, Zr, Hf, more preferably the aluminium alloy should contain at least 95% of Al, 0-5% of Si and/or 0-2% by weight of one or more of the elements Ce, La, Sc, Y, Zr, Hf.
In the following, it is shown how the requirements on strength and oxidation resistance are met by an austenitic Al alloyed material manufactured according to the method described in the present invention. Furthermore, it is shown that a material manufactured according to the same method is superior to a material that has the same composition but has been manufactured according to conventional methods, in respect of high-temperature strength, oxidation resistance and workability.
Table 1 indicates examples of compositions of examined alloys. The alloys according to examples A and B as well as the Comparative examples 1, 2 and 3 were manufactured in the conventional way by pyrometallurgy and hot working.
Comparison example 1 is an alloy that today is used as supporting material in catalytic converters and that has acceptable oxidation resistance for this use. Comparison example 2 is an austenitic alloy of high Al content, manufactured by conventional methods. The yield upon hot working of said alloy was only approximately 10%, i.e., 90% of the material had such internal defects in the form of, e.g., cracks that it could not be used for further working.
The alloys according to examples A and B have compositions that are suitable to be used as substrate materials in a coating process where a thin layer of aluminium or an aluminium-based alloy is deposited on said substrate. From the alloys according to Examples A and B as well as comparison example 1, 50 μm thick strips were manufactured via hot rolling and cold rolling. The yield in the production of the alloys in examples A and B was the same as comparison example 1.
In order to avoid formation of aluminium nitride, the content of nitrogen in the substrate materials is low. In order to limit this tendency further, Ti, Nb and/or Zr and/or Hf were added. Addition of these elements results in the formation of nitrides that are more stable than AIN, which entails a reduced formation of the latter. Furthermore, the compositions are chosen in order to enable efficient production of thin strips of the substrate material. For instance, the content of carbon is below 0.10%, which allows satisfactory material yields in cold working processes. By the relatively high Al content in the substrates, the necessary amount of Al that has to be deposited on the substrate is decreased with the purpose of achieving sufficient Al content in the finished product.
In table 2, it is shown that the substrate alloys have a very good high-temperature strength; e.g., at 700° C. the ultimate strength of the alloys according to examples A and B is up to 3 times larger than of the conventional material in comparison example 1, and at this temperature the yield point in tension is 2.8 to 5 times larger than of comparison example 1. At 900° C., the yield point in tension of the alloy according to examples A and B is approximately 5 times larger of for comparison example 1, while the ultimate strength is at least 3.5 times higher than of comparison example 1.
Thus, the two substrate alloys used in Examples A and B meet the requirements of sufficient mechanical strength and manufacturability as thin foil.
From
The alloys in examples C and D were manufactured by coating the two surfaces of cold-rolled, 50 μm thick, strips of the alloy according to examples A and B, respectively, by vaporization or sputtering with Al in such an amount that the total Al content corresponded to 5.5-6% (see table 3). The coating was effected by a certain heating of the substrate material, however not to such a high temperature that melted Al was present on the substrate. The coating with Al or Al alloy according to the invention should accordingly be effected within a temperature range of the substrate of 100° C.-660° C., preferably in the temperature range of 150-450° C.
The alloy according to comparison example 2, which has approximately the same total composition as the alloy in example C, could, as has been mentioned previously, be forged, but only with a very low material yield. Thus, the limited hot ductility entails that this alloy hardly can be manufactured in the form of thin strips. However, the same alloy has, as is seen in table 2, a very good heat resistance; e.g., the ultimate strength at both 700° C. and 900° C. is 3 to 4 times larger than of the conventional material in comparison example 1, and the yield point in tension is more than 4 times as large at both test temperatures.
The thickness of the Al layer obtained on 50 μm thick strip according to example C was measured by GDOES (Glow Discharge Optical Emission Spectroscopy), a method that enables accurate measuring of compositions and thicknesses of thin surface layers. The analysis showed that the sample had a total Al content of 5-6% by weight. These samples were oxidized in air at 1000° C. for up to 620 h. The results are shown in
The alloy according to Example C was oxidation tested at 1100° C. together with comparison example 1, which is shown in
Examples E and F are the alloys according to examples C and D, respectively, that have been annealed at 1200° C. for 20 min with the purpose of providing an equalising of the Al content in the material (see table 4). The ductility of the material was assessed by means of a bending test where the smallest bending radius that the material could be bent to without fracturing was determined (see table 4). The narrowest radius that the material was tested at was 0.38 mm. None of the materials exhibited any damage after this bending. The radius is smaller than the one used in the production of catalytic converters. Thereby, strips manufactured according to the invention have a fully sufficient ductility to allow the use thereof in catalytic converters. At 900° C., example E has an ultimate strength of 166 MPa (see table 2), which is more than four times larger than the material according to comparison example 1 used at present, and furthermore somewhat higher than a conventionally manufactured material according to comparison example 2, having a similar composition as alloys that have been manufactured in accordance with the invention.
The alloys according to Examples E and F were oxidation tested at 1100° C. together with the alloy of Example C according to the invention as well as comparison examples 1 and 2. The results are shown in
It is evident from
The alloys according to Examples C and E have almost the same composition as the alloy in comparison example 2, and also here, a similar effect of different sample thickness would be expected, as for comparison example 1. However, the relative improvement in oxidation resistance with decreasing sample thickness of the alloy according to the invention is considerably larger than it is of comparison example 1 (see table 5). This may be regarded to be a highly unexpected and valuable effect of the method according to the invention.
Furthermore, the diffusion annealing that differs between Examples C and E has turned out to give an unexpectedly large additional improvement of the oxidation resistance (see table 5).
To start with, the alloy according to Example F has equally good oxidation resistance as Example C or comparison example 1 in the form of foil. Testing was interrupted after 220 h for the alloy according to Example F. However, comparison between the increase in weight up to 220 h at 1100° C. of examples E and F shows that the alloy according to Example E has the most suitable combination of composition and way of production as regards oxidation resistance.
A 50 μm thick strip of the alloy according to example A was coated with Al by means of vaporization. Various samples were annealed for different times at 1050° C. in Ar gas. Concentration profiles of Al in the material were determined by GDOES. The results are shown in
A 50 μm thick strip of the alloy according to example A was coated with Al by means of vaporization. A sample was annealed for 50 min at 1150° C. in Ar gas. The micro structure was analysed by means of SEM (scanning electron microscopy).
Catalytic conversion is since a number of years a requirement in most industrialised countries. The catalytically active material is carried mechanically by a supporting material. The requirements on the supporting material are, among other things, that it should have a large surface, withstand temperature variations and have sufficient mechanical strength and oxidation resistance at the operating temperature of the catalytic converter.
Two main types of supporting materials are used today: ceramic and metallic. The ceramic supporting materials, which frequently are manufactured from cordierit, are not affected by oxidation, however their brittleness means that the resistance to impacts and other mechanical stresses as well as to temperature variations such as fast changes of temperature is very limited. Today, metallic supporting materials generally are based on thin strips of ferritic Fe—Cr—Al alloys with additions of small amounts of reactive elements such as rare earth metals (REM) or Zr or Hf. In order to give the monolith a maximum active surface, the supporting material should be as thin as possible, usually between 10 μm and 200 μm. Today, a common strip thickness is 50 μm, but considerably enhanced efficiency of the catalytic converter by virtue of an increased surface/volume-ratio and/or decreased fall of pressure over the catalytic converter can be expected upon a reduction of the strip thickness to 30 μm or 20 μm. The high ductility of the metal gives a good resistance to mechanical and thermal fatigue. Aluminium in contents above approx. 4.5% by weight gives, together with the reactive elements, the material the possibility of forming a thin, protective, aluminium oxide upon heating. Furthermore, the reactive elements make the oxide getting a considerably reduced tendency to peel, i.e., come loose from the metal upon cooling or mechanical deformation. However, conventional Fe—Cr—Al alloys have a large disadvantage: they are mechanically very weak at high temperatures, and therefore tend to be greatly deformed also upon small stresses by virtue of, e.g., acceleration, changes of pressure, mechanical impacts or changes of temperature.
The invention is not limited to products in small dimensions, such as thin strips or thin wire. Since an austenitic material having a content of aluminium that is larger than 4.5% by weight cannot be produced with sufficient productivity and material yield by hot working, it is valuable to be able to manufacture such an alloy in thicker dimensions by the coating method described in the present invention. This may be effected, e.g., by manufacturing a product in the form of, e.g., sheet-metal plate, strip, foil or a seamless tube, in a substrate alloy, and then said product is coated, on one or both surfaces with an aluminium alloy in such an amount that the total content of aluminium of the material exceeds 4.5% by weight. For instance, a seamless tube having the composition according to example A may be manufactured by means of conventional methods to the following dimensions: outer diameter 60.33 mm, wall thickness 3.91 mm. In order to be able to achieve a total content of aluminium of at least 4.5% by weight in such a tube, it needs to be coated with aluminium on the inner and outer surface with a thickness of at least 0.1 mm. Such an amount may be applied to the surfaces of the tube by conventional methods, e.g., by dipping in a melt of an aluminium alloy. If a homogeneous material is desired, a longer heat treatment at high temperature is required, suitably at least 1000° C. Therefore, the finished product should suitably be manufactured in a partly homogenized form, where the material has an aluminium gradient that increases towards the surfaces, e.g., by a heat treatment where the material slowly is heated to 1100° C. and is heat-treated at this temperature for between 5 min and 10 h, depending on the desired aluminium distribution. It is evident to a person skilled in the art that, if this product should be possible to be manufactured with satisfactory productivity, the content of aluminium in the substrate material should be as high as possible, without causing production disturbances in the manufacture of the substrate. In this case, a suitable content of aluminium in the substrate material is 2-4% by weight. This method can be used to manufacture a finished product or to manufacture a starting material for continued plastic machining at low temperature, e.g., a tubular blank for pilgrim step rolling.
In industrial furnaces and in consumer goods including resistive heating, such as hotplates, radiant heaters, flat irons, ovens, toasters, hairdryers, tumble-dryers, drying cupboards, electric kettles, car seat heaters, underfloor heating equipment, radiators and other similar products, there is also a need for using strip, wire or foil having the above-described properties. Availability of a material having this product specification results in the development of more efficient heat sources having longer service life and/or higher operation temperature and efficiency.
The alloy produced according to the invention may also be used in other high temperature applications, such as applications requiring a high oxidation resistance and good mechanical properties. For example, it could be used in heat exchangers or as protective plates. Also, it could be used in other environments such as in reducing atmosphere. In this latter case it could be advantageous in some cases to pre-oxidise the product before use in order to assure a stable and dense Al-containing oxide on the surface.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0303608-4 | Dec 2003 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE04/02017 | 12/15/2004 | WO | 00 | 5/3/2007 |
