Patent Application
20070041862
The invention relates to an iron chromium aluminum alloy manufactured by metallurgic melting and having a long service life.
Such alloys are used for producing electric heating elements and catalyst carriers. These materials form a dense, highly adhesive aluminum oxide layer that protects them against destruction at high temperatures (for example up to 1400° C.). This protection is still improved by adding so called reactive elements, such as for example Ca, Ce, La, Y, Zr, Hf, Ti, Nb, W, which inter alia improve the adhesiveness of the oxide layer and/or reduce the layer growth, as it is for example described in “Ralf Bürgel, Handbuch der Hochtemperatur-Werkstofftechnik, Vieweg-Verlag, Braunschweig 1998” from page 274 onwards.
The aluminum oxide layer protects the material against quick oxidation. Herein, it grows itself, but only very slowly. This growth consumes the aluminum content of the material. If no more aluminum is present, other oxides (chromium and iron oxides) will grow, the metal content of the material will be very quickly consumed and the material will fail due to destructive corrosion. The time period until failing is defined as service life. An increase of the aluminum content increases the service life.
From WO 02/20197 a ferritic rustproof steel alloy is known, in particular for the use as heat conductor element. The alloy is formed by a FeCrAl alloy manufactured by powder metallurgy and comprising (in % by mass) less than 0.02% C, ≦0.5% Si, ≦0.2% Mn, 10.0 to 40.0% Cr, ≦0.6% Ni, ≦0.01% Cu, 2.0 to 10.0% Al, one or more element(s) from the group of the reactive elements, such as Sc, Y, La, Ce, Ti, Zr, Hf, V, Nb, Ta, in contents comprised between 0.1 and 1.0%, the rest being iron as well as unavoidable impurities.
In DE-A 199 28 842 an alloy comprising (in % by mass) 16 to 22% Cr, 6 to 10% Al and additions of 0.02 to 1.0% Si, max. 0.5% Mn, 0.02 to 0.1% Hf, 0.02 to 0.1% Y, 0.001 to 0.01% Mg, max. 0.02% Ti, max. 0.03% Zr, max. 0.02% rare earths, max. 0.1% Sr, max. 0.1% Ca, max. 0.5% Cu, max. 0.1% V, max. 0.1% Ta, max 0.1% Nb, max. 0.03% C, max. 0.01% N, max. 0.01% B, the rest being iron as well as impurities resulting from the melting process is described for the use as carrier foil of exhaust gas catalysts, as heat conductor, as component in the construction of industrial furnaces and in gas ports.
In EP-B 0 387 670 an alloy comprising (in % by mass) 20 to 25% Cr, 5 to 8% Al and additions of 0.03 to 0.08% yttrium, 0.004 to 0.008% nitrogen, 0.020 to 0.040% carbon as well as in about the same portions 0.035 to 0.07% Ti and 0.035 to 0.07% zirconium, and max. 0.01% phosphorous, max. 0.01% magnesium, max. 0.5% manganese, max. 0.005% sulphur, the rest being iron, is described, wherein the sum of the contents of Ti and Zr is 1.75 to 3.5% greater than the percentage sum of the contents of C and N as well as impurities resulting from the melting process. Ti and Zr can be completely or partially replaced by hafnium and/or tantalum or vanadium.
In EP-B 0 290 719 an alloy comprising (in % by mass) 12 to 30% Cr, 3.5 to 8% Al, 0.008 to 0.10% carbon, max. 0.8% silicium, 0.10 to 0.4% manganese, max. 0.035% phosphorous, max. 0.020% sulphur, 0.1 to 1.0% molybdenum, max. 1% nickel and the additions of 0.010 to 1.0% zirconium, 0.003 to 0.3% titanium and 0.003 to 0.3% nitrogen 0.005 to 0.05% calcium plus magnesium as well as 0.003 to 0.80% rare earths, 0.5% niobium, the rest being iron with usual companion elements is described, which is for example used as wire for heating elements of electrically heated furnaces and as construction material for thermally stressed parts as well as foil for the manufacture of catalyst carriers.
In U.S. Pat. No. 4,277,374 an alloy comprising (in % by mass) up to 26% chromium, 1 to 8% aluminum, 0.02 to 2% hafnium, up to 0.3% yttrium, up to 0.1% carbon, up to 2% silicium, the rest being iron, is described which comprises a preferred range of 12 to 22% chromium and 3 to 6% aluminum and which is used as foil for the manufacture of catalyst carriers.
From U.S. Pat. No. 4,414,023 a steel comprising (in % by mass) 8.0 to 25.0% Cr, 3.0 to 8.0% Al, 0.002 to 0.06% rare earth metals, max. 4.0% Si, 0.06 to 1.0% Mn, 0.035 to 0.07% Ti, 0.035 to 0.07% Zr including unavoidable impurities is known.
A detailed model of the service life of iron chromium aluminum alloys is described in the article of I. Gurrappa, S. Weinbruch, D. Naumenko, W. J. Quadakkers, Materials and Corrosions 51 (2000), pages 224 through 235. Here, a model is presented that shows that the service life of iron chromium aluminum alloys is dependent on the aluminum content and the shape of the sample, wherein possible spallings have not been considered yet in this formula.
Considering the spallings, the following formula results for a flat sample of infinite width and length having the thickness d (f=d):
wherein Δm* is the critical change in weight with which spalling starts.
Both formulas express that the service life decreases with reduction of the aluminum content and a high ratio of surface and volume (or small thickness of the sample). In this article, the influence of the thermal cycle has not been taken into account, as it is for example described in J. P. Wilber, M. J. Bennett and J. R. Nicholls “The effect of thermal cycling on the mechanical failure of alumina scales formed on commercial FeCrAl-RE alloys, in Proc. Of Int. Conf. on Cyclic Oxidation of High Temperature Materials”, February 1999, Frankfurt am Main, Germany, Editors M. Schütze and W. J. Quadakkers, p. 133-147 (1999) for cycle times comprised between 1 h and 290 h, wherein in this work the cycle times will only have an effect if spallings occur.
In V. K. Tolpygo, D. R. Clarke “Spalling failure of α-alumina films grown by oxidation: I. Dependence on cooling rate and metal thickness, Materials science and engineering”, A278 p. 142-150 (2000) the influence of the cycle time and the cooling rate is also described. These two articles in particular show that a short heating up period, a short cooling down period and an only short holding time at the high temperature highly reduce the service life.
In the following, the term thermal cycle defines the combination of heating up period, holding time at the temperature, cooling down period, and waiting time until a new heating up. Thermal cycles presenting a short heating up period, a short cooling down period and an only short holding time at the high temperature will be called short and rapid thermal cycles in the following. Among these ones are for example thermal cycles having a total length of time in the range comprised between several seconds and several minutes, wherein total length of time means the sum of heating up period, holding time at the temperature, cooling down period and waiting time until the next heating up period starts.
Heat conductors made of thin films (for example with a thickness of about 30 to 100 μm with a width in the range of one or several millimetres) stand out for a great surface-to-volume ratio. This is advantageous if fast heating up and cooling down times shall be achieved, as they are for example required for the heat conductors used in glass-ceramic cooking zones in order to make the heating up quickly visible and to obtain a fast temperature rise, similar to a gas cooker. But simultaneously the great surface-to-volume ratio is disadvantageous for the service life of the heat conductor (see above). Additionally, the temperature has to be limited below the glass in this application, in order to protect it against deterioration. This can be achieved by switching off the current repeatedly and for short periods of time. Both measures will cause stress for the heat conductor due to short heating up periods and fast cooling down and only short holding times, which further reduces the service life, as described above.
In no one of the above mentioned documents this effect of the thermal cycle is especially treated, i.e. no one of the above mentioned alloys has been developed with respect to this aspect.
It is known from the above described state of the art that little additions of Y, Zr, Ti, Hf, Ce, La, Nb, W highly influence the service life of FeCrAl alloys.
According to J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385, the addition may not be too high, since otherwise a higher oxidation rate will occur which means an increased consumption of aluminum and thus a shortened service life. This higher oxidation rate is for example caused by an addition of only 0.11% hafnium to an iron chromium aluminum alloy comprising 20% Cr, 7% aluminum and 0.01% yttrium. Other examples of a higher oxidation rate caused by a too high addition of a reactive element that are mentioned in the article are an iron chromium aluminum alloy comprising 18.8% Cr, 7% Al and an addition of 0.11% Y or an iron chromium aluminum alloy comprising 20% Cr, 7% Al and additions of 0.04% yttrium, 0.05% Zr and 0.05% Ti. Herein, the range in which a higher oxidation rate is caused by a too high addition of a reactive element varies with the aluminum content. According to J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385, 0.04% Zr in an iron chromium aluminum alloy comprising 20% Cr, 7% Al and 0.05% Y already causes an increased oxidation rate. The same quantity of Zr in an iron chromium aluminum alloy comprising 20% Cr, 5.5% Al and 0.05% Y and 0.05% Hf (J. Klöwer, A. Kolb-Telieps, M. Brede: in Bode, H. (Ed.) Metal-Supported Automotive Catalytic Converters, DGM Informationsgesellschaft, Oberursel, 1997, pages 33 and following) however does not cause an increased oxidation rate. All tests in J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385 and (J. Klöwer, A. Kolb-Telieps, M. Brede: in Bode, H. (Ed.) Metal-Supported Automotive Catalytic Converters, DGM Informationsgesellschaft, Oberursel, 1997, pages 33 and following) were carried out with cycles of 100 h or 96 h in the furnace, which are very long cycles.
It is the object of the invention to provide an iron chromium aluminum alloy that has a longer service life than the hitherto used iron chromium aluminum alloys, in particular for components having great surface-to volume ratios or a small band thickness.
This aim is achieved by an iron chromium aluminum alloy manufactured by metallurgic melting and having a long service life, comprising (in % by mass) 4 to 8% aluminum, 16 to 24% chromium and additions of 0.05 to 1% Si, max. 0.5% Mn, 0.02 to 0.2% yttrium and 0.1 to 0.3% Zr and/or 0.02 to 0.2% Hf, 0.003 to 0.05% C, 0.0002 to 0.05% Mg, 0.0002 to 0.05% Ca, max. 0.04% N, max. 0.04% P, max. 0.01% S, max. 0.5% Cu and the usual impurities resulting from the melting process, the rest being iron.
Furthermore, the element Hf can be completely or partly replaced by at least one of the elements Sc and/or Ti and/or V and/or Nb and/or Ta and/or La and/or cerium, wherein ranges comprised between 0.02 and 0.15% by mass are preferred for a partial substitution.
Advantageously, the alloy according to the invention shall be molten with (in % by mass) max. 0.02% N, max. 0.02% P as well as max. 0.005% S.
In the state of the art according to Corrosion 51 (2000) and DGM Informationsgesellschaft, all tests were carried out with cycles of 100 h or 96 h in the furnace, which are very long cycles.
Surprisingly it has been found in tests with very short cycles that the range of a shortened service life, which simultaneously means a higher oxidation rate, is completely different there. Thus, for the iron chromium aluminum alloy according to the invention, which would show a higher oxidation rate already with 0.1% Zr with min. 0.02% Y during the above mentioned cycles of 100 h or 96 h in the furnace according to J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385 and thus a shorter service life, it has been found in a service life test of wire which has a small surface-to-volume ratio that in a shorter cycle of 2 min “on” and 15 s “off” the alloy shows a service life at the upper limit of the variation range of the service life of the alloy according to the state of the art. This difference becomes even more clear when films of 50 μm thickness that have a very great surface-to-volume ratio and very short cycles of 15 s “on” and 5 s “off” are used in the service life test.
Preferred FeCrAl alloys have the following composition (in % by mass):
Depending on the respective application case, the range of the following elements can be set as follows:
The alloys according to the invention can be preferably used for electric heating elements having short heating up and cooling down periods, short holding times at the temperature and short waiting times until a new heating up period starts.
The alloys according to the invention can also be used for heating elements which require a high dimensional stability or a low sagging.
The alloys according to the invention can also be used for heat conductors made of films having a thickness comprised between 20 and 100 μm.
Also possible is the use of the alloys according to the invention as heat conductors for the use in cooking zones.
Finally it is possible to use the alloy according to the invention in the construction of furnaces.
The details and advantages of the invention are explained in detail in the following examples.
In table 1 the iron chromium aluminum alloys L1 through L8 and E1 through E2 that have been molten in the laboratory and the alloys G1 through G3 that have been molten on big technical scale have been listed. From the alloys molten in the laboratory both wire and 50 μm thick films were manufactured from the material cast in ingots by warm and cold rolling and appropriate intermediate annealing. The film was cut into strips having a width of 6 mm. For the alloys molten on big technical scale a sample of the strip thickness of 50 μm was taken from the industrial production and, if necessary, cut for having the suitable width of about 6 mm.
For heat conductors in form of wire, accelerated service life tests for example under the following conditions are possible and usual for comparing materials to each other:
The service life test of heat conductors is carried out with wires having a diameter of 0.40 mm, the wire coils of which have 12 windings, a coil diameter of 4 mm and a coil length of 50 mm. The wire coils are fixed between two current supplies and heated up to 1200° C. by applying an electric tension. The heating up to 1200° C. is respectively realized for 2 minutes, then, the current supply is interrupted for 15 seconds. At the end of the service life the wire fails in that the remaining cross section fuses thoroughly.
An analogue service life test can be carried out with film strips. Herein, film strips having a thickness of 50 μm and a width of 6 mm are fixed between two current supplies and heated up to 1050° C. by applying an electric tension. The heating up to 1050° C. is respectively realized for 15 seconds, then, the current supply is interrupted for 5 seconds. At the end of the service life the film fails in that the remaining cross section fuses thoroughly.
In both tests the service life indicates the total period of time in which the wire or the film are at the mentioned temperature without interruption times. During the service life test the temperature is measured by an optical pyrometer and, if necessary, corrected to the nominal temperature.
The results of the service life test are indicated in table 1. The mean values indicated in the table are respectively the mean values of at least 3 samples.
In the serve life test of wire, the coils are fixed horizontally at the beginning. In the course of the service life test they start sagging. The smaller the sagging the higher is the dimensional stability of the material. A high dimensional stability is an advantageous technological characteristic, since this means that the parts made of the material present a small modification of their shape when being used at higher temperatures.
The alloys G1 and G2 that have been industrially molten and the alloy L2 molten in the laboratory show an iron chromium aluminum alloy comprising (in % by mass) about 20% Cr, about 5% Al and additions of 0.04 to 0.07% Y, 0.04 to 0.07% Zr and 0.04 to 0.05% Ti and a carbon content of 0.033 to 0.037%, a Si content of 0.15 to 0.34%, a Mn content of about 0.24% and little contents of N, S, Ce, La, Pr, Ne, P, Mg, Ca, as indicated in table 1 according to the state of the art. The service life of a wire made of L2 and having a thickness of 0.4 mm at 1200° C. in a cycle of 120 s “on” and 15 s “off” serves as reference and is indicated as 100%.
The service life of 50 μm thick film at 1050° C. and in a cycle of 15 s “on” and 5 s “off” is comprised between 102 and 124% of the service life of the laboratory batch L1. The industrially molten alloy G3 also shows an iron chromium aluminum alloy comprising about 20% Cr, about 5% Al and additions of 0.06% Y, 0.04% Zr, 0.02% Hf, a carbon content of 0.029%, a Si content of 0.28%, a Mn content of 0.20% and little contents of P, Mg, Ca, as indicated in table 1 according to the state of the art. The service life of 50 μm thick film at 1050° C. and in a cycle of 15 s “on” and 5 s “off” is 148% of the service life of the laboratory batch L1. Thus, the alloys according to the state of the art show values of about 100% to about 150% of L1 in the service life test of 50 μm thick film at 1050° C. and in a cycle of 15 s “on” and 5 s “off”.
In the laboratory batches L1 and L3 through L8 the contents of Si, C, Zr, Ti and Hf have been varied. The Mn content has not been varied and is comprised between 0.24 and 0.28% in all laboratory melts and the little admixtures of P, Mg, Ca, Ce, La, Pr, Ne are as indicated in table 1. Herein, the variant L1 comprising 0.03% Y, 0.04% Zr and 0.02% Hf and a carbon content of 0.007% and a Si content of 0.35% shows a relatively long service life of 116% in a service life test of 0.4 mm thick wire at 1200° C. in a cycle of 120 s “on” and 15 s “off”. The variants L3 and L7 with an addition of Y of only 0.06% or 0.05% and a carbon content of 0.002 or 0.031% and a Si content of 0.34 or 0.35% have a service life of only 41% or 51% in the service life test of wire. The variants L4 and L5 with an addition of 0.04 or 0.05% Y and 0.05 or 0.014% Zr and carbon contents of 0.002 or 0.003% and the Si contents of 0.33 or 0.35% have a service life of 79% or 86%, which is better than the one of L3 and L7, but does not reach the service lives of L2 or L1. The variant L6 with an addition of 0.05% Y and 0.05% Hf and carbon contents of 0.010% and a Si content of 0.36% has a service life of 85%, which is also better than the one of L3 and L7, but does not reach the service lives of L2 or L1. The laboratory batch L8 comprises additions of 0.05% Y, 0.21% Zr and 0.11% Ti and a carbon content of 0.018% and a Si content of only 0.02%. Thus, according to J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385, this alloy, due to the high Zr and Ti content, is already situated in the concentration range of the higher oxidation rate in the service life test with long cycles of for example 100 h or 96 h in the furnace. Nevertheless, it shows a service life of 105% in the heat conductor service life test of wire, which means it is situated between L1 and L2.
The alloys according to the invention E1 comprising 0.05% Y, 0.18% Zr, 0.04% Hf, 0.006% C and 0.35% Si and E2 comprising 0.03% Y, 0.20% Zr, 0.11% Ti instead of hafnium, 0.020% C and 0.61% Si are within the range of the higher oxidation rate in the life service test with long cycles of for example 100 h or 96 h in the furnace. Both alloys have long service lives of 96% for E2 and even 118% for E1 in the heat conductor service life test of wire. Thus, the following ranking of service life results for the laboratory melts (respectively classified according to decreasing service life):
Peak group: E1, L1, L8, L2, E2, characterized by additions of Y and Zr and furthermore by an addition of Ti or Hf.
Medium service life: L5, L6, L4, characterized by additions of Y and Zr or Y and Hf. Short service life: L7, L3, characterized by an addition of only Y.
This corresponds to the knowledge and the experiences of the state of the art. The alloy L2 for example corresponds to the industrially molten alloys G1 and G2 according to the state of the art.
The picture is different, if one looks at the heat conductor service life test of 50 μm thick film at 1050° C. in a cycle of 15 s “on” and 5 s “off”: The alloys L3 and L7, which show a short service life in the test of wire, show a service life of 94% and 110% of L1, which is within the range of the service lives of the alloys according to the state of the art. The alloys L5, L6, L4 which show a medium service life in the test of wire show a service life of 145% or 113% of L1, which is also within the range of the service lives of the alloys according to the state of the art. The alloys L1 and L2 which are in the peak group for the wire test show a service life of 100% or 125% of L1, the alloy L8 shows a service life of 140% of L1, which is only within the range of the service lives of the alloys according to the state of the art.
Surprisingly the mentioned alloys according to the invention E1 and E2 which are within the range of the higher oxidation rate in the service life test with long cycles of for example 100 h or 96 h in the furnace show very long service lives of 256% for E1, which is a value that is highly superior with respect to all other values, and 171% for E2 which is clearly more than the service life range of the alloys according to the state of the art.
Equally surprising and long service lives show the alloys according to the invention E3 comprising 0.05% Y, 0.21% Zr, 0.021% C and 0.19% Si with 201% and E4 comprising 0.07% Y, 0.23% Zr, 0.07% Ti, 0.014% C and 0.19% Si with 227% and E5 comprising 0.07% Y, 0.22% Zr, 0.07% Hf, 0.018% C and 0.20% Si with 249% and E6 comprising 0.05% Y, 0.17% Zr, 0.05% Hf, 0.016% C and 0.19% Si with 283%.
Thus, the following ranking results:
Peak group with service lives of more than 170% of L1: E1 through E6, characterized by the addition of Y and Zr and/or Hf and/or Ti in the range of the higher oxidation rate in the service life test with long cycles of for example 100 h or 96 h in the furnace and a carbon content comprised between 0.003 and 0.025% and Si contents of more than 0.05%.
Group with service lives comprised between about 100% and 150% of L1, which corresponds to the state of the art: G3, L5, L8, L2, G2, L4, L6, G1, L1, L7, L3, characterized by a smaller addition of Y and Zr and/or Hf and/or Ti outside the range of the higher oxidation rate in the service life test with long cycles of for example 100 h or 96 h in the furnace or in the case of L8 by a too low Si content with an addition of Y, Zr and Hf in the range of the higher oxidation rate.
Concerning the dimensional stability that is important for the use and that is measured as sagging of the coils in mm after 50 h burning hours, the alloys according to the invention E1, E2 and L8 show values comprised between 5 and 7 mm and are thus in the peak group in comparison to the other alloys L1 through L7 according to the state of the art which show values comprised between 17 and 19 mm. Thus, the alloys according to the invention also present the advantage of a high dimensional stability.
Thus, the claimed limits of the invention can be justified in detail as follows:
A minimum content of 0.02% Y is necessary in order to maintain the effect of Y to increase the oxidation stability. The upper limit is set to 0.2% by mass for the reason of costs.
A minimum content of 0.1% Zr is required in order to reach the range of high service lives with short and quick temperature cycles. The upper limit is set to 0.3% by mass Zr for the reason of costs.
A minimum content of 0.02% Hf is necessary in order to maintain the effect of Hf to increase the oxidation stability. The upper limit is set to 0.2% by mass Hf for the reason of costs.
A minimum content of 0.02% Ti is necessary in order to maintain the effect of Ti to increase the oxidation stability. The upper limit is set to 0.2% by mass Ti for the reason of costs.
The carbon content should be 0.003% to 0.05% in order to assure the working properties.
The nitrogen content should be maximum 0.04% in order to avoid the formation of nitrides that deteriorate the working properties.
The contents of phosphorous and sulphur should be kept as low as possible, since these surface active elements have a negative effect on the oxidation stability. Therefore, only max. 0.04% P and max. 0.01% S is determined.
Chromium contents comprised between 16 and 24% by mass have no decisive influence on the service life, as it can be read in J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385. However, a certain chromium content is required, since chromium stimulates the formation of the especially stable and protecting α-Al2O3 layer. This is assured from about 16% onwards. Therefore, the lower limit is 16%. Chromium contents of >24% degrade the working properties of the alloy.
The aluminum content of the alloy according to the invention should be comprised between 4 and 8%. According to the “Handbuch der Hochtemperatur-Werkstofftechnik, Ralf Bürgel, Vieweg Verlag, Braunschweig 1998”, page 272 picture 5.13 about 4% aluminum are required in order to form a closed α-Al2O3 layer. Higher aluminum contents than 8% degrade the working properties.
According to J. Klöwer, Materials and Corrosion 51 (2000), pages 373 through 385, additions of silicium increase the service life by improving the adhesiveness of the cover layer. Therefore, a content of minimum 0.05% by mass silicium is required. Too high Si contents have a negative effect on the working properties of the alloy. Therefore, the upper limit is 1%.
Manganese is limited to 0.5%. by mass, since this element reduces the oxidation stability. The same is true for copper.
The contents of magnesium and potassium are set within the range comprised between 0.0002 and 0.05% by mass.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 020 900.6 | Apr 2004 | DE | national |
| 10 2005 016 722.5 | Apr 2005 | DE | national |
