Abstract:
An atomised pre-alloyed iron-based powder which comprises by weight-% 
                                       10.5-30   Cr           3-15   Al           5-20   Cu         max 0.1   C         max 0.2   N         max 3.0   Mn         max 2.5   Si         max 3.0   Mo                                      
balance essentially only iron and unavoidable impurities.

Description:
[0001]    This application claims priority to U.S. Patent Application No. 60/840,457, filed 28 Aug. 2007, and to Swedish Patent Application No. 0601601-8, filed 21 Jul. 2006, the complete disclosures of which are incorporated herein by reference. 
     
     TECHNICAL FIELD 
       [0002]    The invention concerns atomised iron based powders having good high temperature oxidation resistance, more particular powders which are pre-alloyed with chromium and aluminium. 
       BACKGROUND 
       [0003]    Conventional iron based alloys containing typically Fe and 10-30% Cr and 1-10% Al, so-called FeCrAl-alloys, have been found highly useful in various high temperature applications, due to their good oxidation resistance and can be used at temperatures as high as 1200-1400° C. Thus, such materials have been used in the production of electrical resistance elements and as carrier materials in motor vehicle catalysts. As a result of its aluminium content, the alloy is able to form at high temperatures and in the majority of atmospheres an impervious and adhesive surface oxide consisting substantially of Al2O3. This oxide protects the metal against further oxidation and also against many other forms of corrosion, such as carburization, sulphuration etc. 
         [0004]    A problem in powder metallurgy however is that that FeCrAl-powders are difficult to sinter, due to the fact that the aluminium oxide is harder to reduce than the chromium oxide. The formation of sintering necks through diffusion is impaired by the aluminium oxide layer and it would be advantageous if this problem could be overcome. 
         [0005]    U.S. Pat. No. 5,970,306 describes a method for manufacturing high temperature resistant shaped parts from a FeCrAl-powder by hot isostatically pressing (HIP). 
         [0006]    DE4235141 describes a method of producing a part made from hot pressed powder based on a FeCrAl-alloy in which the powder is initially exposed to an oxygen-contg. atmos. to produce an chromium oxide protective layer around the particles. 
         [0007]    U.S. Pat. No. 6,761,751 describes a method of producing an FeCrAl material by gas atomization, wherein in addition to containing iron (Fe), chromium (Cr) and aluminium (Al) the material also contains minor fractions of one or more of the materials molybdenum (Mo), hafnium (Hf), zirconium (Zr), yttrium (Y), nitrogen (N), carbon (C) and oxygen (O). 
         [0008]    U.S. Pat. No. 6,569,221 describes a powder metallurgical FeCrAl alloy comprising, in weight %, less than 0.02% carbon; greater than 0.0 and &lt;=0.5% silicon; greater than 0.0 and &lt;=0.2% manganese; 10.0-40.0% chromium; &lt;=0.6% nickel; &lt;=0.01% copper; 2.0-10.0% aluminum; one or more of Sc, Y, La, Ce, Ti, Zr, Hf, V, Nb and Ta in an amount of 0.1-1.0; remainder iron and unavoidable impurities. 
       OBJECTS OF THE INVENTION 
       [0009]    It is an object of the invention to provide a metal powder which can provide a good sintered structure when sintered in conventional sintering processes, e.g. loose sintering, cold compaction or warm compaction, and which sintered components have good high temperature oxidation properties. More specific it is an object of the invention to provide a stainless iron powder comprising more than 10.5 wt % chromium and 3-15 wt % aluminium, but which powder is easier to sinter than what is known in the prior art. 
       SUMMARY OF THE INVENTION 
       [0010]    This object is solved by an atomised iron based powder pre-alloyed with 10.5-30 wt % Cr, 3-15 wt % Al and 5-20 wt % Cu. By pre-alloying the powder with Cu it is possible to sinter a component in conventional sintering processes and maintaining satisfactory material properties of the sintered component which component also have excellent high temperature oxidation resistance. 
         [0011]    Further in a second embodiment an iron based powder pre-alloyed with 10.5-30 wt % Cr, 3-15 wt % All, 5-20 wt % Cu and 8-20 wt % Ni is proposed. 
         [0012]    The powders of the invention are preferably produced by providing a melt of iron and the alloying elements, water atomizing the melt whereby the powder forms from atomized droplets upon solidification. 
         [0013]    A sintered component can be produced from the powders of the invention by a) providing a sintering material comprising the powder of the invention; b) forming a green body from the sintering material; and c) sintering the green body in a reducing or neutral atmosphere, at an atmospheric pressure or below, and at a temperature above 1100° C. 
         [0014]    The sintering material could e.g. be loose sintered, cold compacted or warm compacted. 
         [0015]    Concerning cold compaction or warm compaction the sintering material is a mixture between a binder and/or a lubricant with the powder of the invention. 
         [0016]    Cold compaction is performed at temperatures below 100° C., preferably at a compaction pressure within the range of 100-1000 MPa. 
         [0017]    Warm compaction is performed at temperatures within the range of 100-200° C., preferably at a compaction pressure within the range of 300-1000 MPa. 
         [0018]    Loose sintering is performed without compacting the green body. Here the sintering material could be a mixture between a binder and/or a lubricant with the powder of the invention, but also the powder it self i.e. without mixing the powder with a binder and/or a lubricant. E.g. when not using a binder the sintering material could be poured into a form where after the form containing the sintering material is inserted into the sintering furnace. For instance filters having excellent high temperature oxidation resistance can be produced by loose sintering the powder of the invention. 
         [0019]    Further, it has been shown that a sintered component which exhibits excellent high temperature oxidation resistance may be produced from the powder of the invention which sintered component has a sintered density above 6.5 g/cm 3 , a tensile strength above 500 MPa and a yield strength above 400 MPa. 
     
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  shows a Fe—Cu phase diagram, and 
           [0021]      FIG. 2A  shows metallographic picture of a test bar comprising Cr, Al, Cu and Fe, and 
           [0022]      FIG. 2B  shows metallographic picture of a test bar comprising Cr, Al and Fe, and 
           [0023]      FIG. 3A  shows metallographic picture of a test bar comprising Cr, Al, Cu, Ni and Fe, and 
           [0024]      FIG. 3B  shows metallographic picture of a test bar comprising Cr, Al, Ni and Fe. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0025]    The invention concerns pre-alloyed iron based powders comprising more than 10.5 wt % chromium, as well as certain amounts of aluminium and copper. As described above FeCrAl-alloys have been shown to exhibit excellent oxidation resistance at high temperatures, but are unfortunately difficult to sinter under atmospheric pressure or below (vacuum). That is the reason why compounds based on FeCrAl powders are produced by the HIP-process (as described in e.g. U.S. Pat. No. 5,970,306). By also pre-alloying with copper the problems with the sintering was reduced with an improved sintered structure as the outcome—compared to a reference material without copper. The copper content is shown to facilitate the formation of sintering necks as can be seen from the accompanying metallographic pictures. We believe that this effect occurs due to a break-up of the aluminium oxide layer by liquidised copper. Admixing copper and a FeCrAl-powder were also tested but sintering did not significantly improve in that case. 
         [0026]    The powders of the invention are made by making a melt of iron and the desired alloying elements. The melt is thereafter atomised whereby the powder is formed from the atomized droplets upon solidification. The atomization is performed according to conventional technology, e.g. gas or water atomization. In fact it is highly preferred that the melt blend is water atomized, since a water atomised powder is easier to compact than a gas atomized powder. When the powder forms due to the water atomization the powder is oxidized and thin chromium and aluminium oxide layers forms on the surface of the powder particles. 
         [0027]    The effective range of the aluminium content was tested, as described below, an it was concluded that the aluminium content should be above 3%, preferably the aluminium content should be above 5%, in order to obtain the desired oxidation resistance. However, if the aluminium content becomes too high the melting point is depressed and the material looses strength at elevated temperatures. Further it can be assumed that above a certain amount of aluminium the oxidation resistance is not drastically increased and further increase of the aluminium content would only slightly improve the oxidation resistance. Therefore according to the invention the upper limit for the aluminium content is set to 15 wt %, and in fact it is preferred to have the aluminium content below 12 wt %. 
         [0028]    The boundaries for the copper content were derived from the tests described below. Accordingly it the copper content should be above 5 wt % to facilitate the formation of sintering necks and providing a sintered component having good high temperature oxidation resistance. Further the Cu-content should be below 20 wt %, powders having higher Cu-content may very well be useful for certain applications, but they are not within the scope of the present invention. 
         [0029]      FIG. 1  shows the Fe—Cu phase diagram, but it is believed that that Cu will influence a system in a similar way. To reduce/break-up the aluminium oxide layer it is believed that a certain amount of liquid phase must be formed, i.e. the area of (γFe+L) is of interest. Since the diagram is for the pure Fe—Cu system the information retrieved from it can only be used as a guideline. Of particular interest is the amount of liquid phase formed during the sintering. Formation of liquid phase is required to break up the aluminium oxides but excess amounts of liquid phase collapses the structure during sintering. The amount of liquid phase formed is related to the chemical composition and the sintering temperature. The element having the strongest influence of the formation of liquid is copper. That is why different sintering temperatures depending of copper content of the samples were applied before the oxidation test. 
         [0030]    Of course other alloying elements could also be of interest. In particular if an austenitic structure is desired the powder can also be pre-alloyed with austenite-forming elements in particular nickel, but also the nickel equivalent manganese. Besides being an austenite forming element nickel is also known to have a beneficial effect on the oxidation resistance which of course is desirable in the applications intended for the powders of the invention. If nickel is to be included in the powder it is preferred that the nickel content is in the interval of 8-20 wt %. Manganese can also be an additional austenite forming alloying element, preferably the manganese content is below 3 wt %. 
         [0031]    Cobalt is normally not used since it is comparably expensive. 
         [0032]    It is further preferred to keep the carbon content low, since carbon has a tendency to cause intergranular corrosion why preferably the carbon content should be less than 0.1 wt % carbon. In the tested specimens the carbon content was about 0.02 wt % or lower. It is also preferred to keep the nitrogen content as low as possible, preferably the nitrogen content is below 0.2 wt %. 
       EXAMPLE 1 
       [0033]    Seven different water atomised powders having the compositions of Table 1 were made by making a melt of iron and the desired alloying elements. The melt was thereafter water atomised whereby the powder formed from the atomized droplets upon solidification. The atomization was performed according to conventional water atomization technology. The resulting powders were extracted through a grid providing a maximum diameter of 75 μm. 
         [0034]    For each powder sintered test samples were prepared. The sintered test samples and a reference sample having a 310B composition (25 wt % Cr+20 wt % Ni+2.5 wt % Si+bal. Fe) were subjected to a high temperature oxidation test described below. The material 310B was chosen as reference since it is known to possess good high temperature oxidation resistance. 
         [0035]    The test samples and the reference sample were produced by filling a form (10 mm diameter and 2 mm thickness) with the powder of interest, followed by smoothing out the surface without compacting the powder. This procedure provides samples with high specific area (ca 45% porosity). 
         [0036]    The test samples were sintered in a 100% hydrogene atmosphere for 30 minutes at a temperature depending of the Cu content according to the following table: 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 a. 
                  5% Cu 
                 1150° C. 
               
               
                 b. 
                 10% Cu 
                 1320° C. 
               
               
                 c. 
                 15% Cu 
                 1350° C. 
               
               
                 d. 
                 20% Cu 
                 1320° C. 
               
               
                   
               
             
          
         
       
     
         [0037]    The reference sample was sintered in a 100% hydrogen atmosphere for 30 minutes at 1320° C. 
         [0038]    The prepared test and reference samples where thereafter ready for the high temperature oxidation test. 
         [0039]    The oxidation tests were carried out in a laboratory furnace, a Lenton 12/50/300, at a temperature of 800° C. in air. A scale, Mettler Toledo AE260, was linked to a computer in order to save the data automatically. Six samples could be tested at the same time by placing them on a sample holder and at each test run two of the samples were reference samples. 
         [0040]    The samples were weighted before they were introduced in the furnace. Short term cycles were performed, each cycle consisting of 2 min heating and 30 sec cooling, which is sufficient for the samples to cool down below 150° C. This cycle was repeated 15 times, resulting in 30 minutes in the furnace. After every 30 minutes in the heating zone, the samples were weighted and the gain-in-weight for each of them was saved. The tests were stopped after 20 hours in the heating zone. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 chemical 
                   
                   
                   
               
               
                   
                 comp. 
               
               
                   
                 [wt %], 
               
               
                 Powder 
                 bal. Fe 
                 Weight 
                 Weight gain 
                 Increase in weight 
               
             
          
           
               
                 No 
                 Al 
                 Cu 
                 Cr 
                 gain [g] 
                 ref. [g] 
                 relative to reference [%] 
               
               
                   
               
             
          
           
               
                 1 
                 10 
                 15 
                 22 
                 0.3 
                 1.25 
                 24 
               
               
                 2 
                 5.5 
                 15 
                 22 
                 0.3 
                 1.15 
                 26 
               
               
                 3 
                 10 
                 10 
                 22 
                 0.6 
                 1.75 
                 34 
               
               
                 4 
                 5.5 
                 10 
                 22 
                 0.7 
                 1.75 
                 40 
               
               
                 5 
                 5.5 
                 20 
                 22 
                 0.5 
                 1.25 
                 40 
               
               
                 6 
                 5.5 
                 5 
                 22 
                 1.3 
                 1.15 
                 113 
               
               
                 7 
                 1 
                 10 
                 22 
                 1.9 
                 1.3 
                 146 
               
               
                   
               
             
          
         
       
     
         [0041]    The results show that the oxidation resistance of powder 6 and 7 were worse than the reference powder 8. Looking at the samples having an Al content of 5.5 wt %, i.e. powder 2, 4, 5 and 6, it can be see that increasing the Cu content from 5 wt % (sample 6) to 10 wt %, (powder 4) drastically improved the oxidation resistance and at a Cu content of 15 wt %, (powder 2) the highest oxidation resistance was achieved. Increasing the Cu content further to 20 wt % (powder 5); the oxidation resistance results was as of the powder having 10 wt % Cu (powder 4). 
         [0042]    As can be seen a Cu-content of 15% provided the best results with regards to high temperature oxidation resistance. 
         [0043]    However, during sintering the powder 5 shrank considerably indicating that at Cu-content above around 20 wt % too much liquid phase was formed. 
         [0044]    Comparing powder 4 to powder 3 and powder 2 to powder 1 it can be seen that increasing the Al content from 5.5 wt % increases the oxidation resistance slightly. 
       EXAMPLE 2 
       [0045]    Powder 2 and 3 were further tested at different oxidation temperatures. The following table shows the increase in weight relative to the reference 310B. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Powder 3 
                 Powder 2 
                   
               
               
                 Test 
                 Increase in weight 
                 Increase in weight 
               
               
                 temperature 
                 relative reference 
                 relative reference 
               
               
                 [° C.] 
                 (%) 
                 (%) 
                 REMARKS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 800 
                 46 
                 24 
                   
               
               
                 850 
                 43 
                 22 
               
               
                 900 
                 21 
                 21 
               
               
                 950 
                 14 
                 14 
               
               
                 1000 
                 20 
                 13 
                 Terminated after 16 
               
               
                   
                   
                   
                 hours 
               
               
                   
               
             
          
         
       
     
         [0046]    Table 2 shows that difference in oxidation resistance between samples containing Cu and Al and reference samples is further pronounced at temperatures above 800° Celcius. Furthermore, the composition having a Al content of 5.5% and a Cu content of 15% seems to have better oxidation resistance compared to the composition having 10 Al and 10% Cu. 
       EXAMPLE 3 
       [0047]    In order to evaluate the effect of added Cu-content with regard to sintered density, tensile strength and yield strength, four different powders having were compared. The powders were as in example 1 and 2 water atomized powders. The powders were mixed with 1% of Acrawax®. The mixes were compacted at a compacting pressure of 600 MPa into tensile test bars. The test bars were sintered for 30 minutes at 1320° Celsius in an atmosphere of 100% hydrogen. Sintered density, tensile strength and yield strength were measured. The results are shown in table 3. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Chemical 
                 Sintered 
                 Tensile 
                 Yield 
               
               
                   
                 composition wt %, 
                 density 
                 Strength 
                 strength 
               
               
                   
                 balance Fe 
                 [g/cm3] 
                 [MPa] 
                 [MPa] 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 22Cr + 5.5Al + 10Cu 
                 6.87 
                 582 
                 522 
               
               
                   
                 22Cr + 5.5Al (ref.) 
                 5.74 
                 295 
                 259 
               
               
                   
                 22Cr + 18Ni + 5.5Al + 8Cu 
                 6.70 
                 507 
                 412 
               
               
                   
                 22Cr + 18Ni + 5.5Al (ref.) 
                 4.96 
                 87 
                 69 
               
               
                   
                   
               
             
          
         
       
     
         [0048]    The table 3 shows that the density and the mechanical properties of Al-containing Cr or Cr—Ni stainless steel powders increases considerably if the powder are pre-alloyed with Cu. This indicates much improved sintering activity. 
         [0049]    Metallic examination was further performed on the tensile test bars. The metallographic pictures, see  FIGS. 2A ,  2 B and  FIGS. 3A ,  3 B, clearly show that incorporation of Cu to Al-containing Cr— or Cr—Ni based stainless steel powders considerably enhance the sintering of the material.  FIG. 2A  shows metallographic picture of a test bar comprising 22Cr+5.5Al+10Cu+bal. Fe and  FIG. 2B  shows metallographic picture of a corresponding reference test bar comprising 22Cr+5.5Al+bal. Fe.  FIG. 3A  shows metallographic picture of a test bar comprising 22Cr+5.5Al+18Ni+8Cu+ bal. Fe and  FIG. 2B  shows metallographic picture of a corresponding reference test bar comprising 22Cr+5.5Al+18Ni+bal. Fe.