Patent Application
20080019858
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.
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.
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 %.
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.
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 %.
Cobalt is normally not used since it is comparably expensive.
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 %.
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.
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.
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).
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:
The reference sample was sintered in a 100% hydrogen atmosphere for 30 minutes at 1320° C.
The prepared test and reference samples where thereafter ready for the high temperature oxidation test.
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.
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.
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).
As can be seen a Cu-content of 15% provided the best results with regards to high temperature oxidation resistance.
However, during sintering the powder 5 shrank considerably indicating that at Cu-content above around 20 wt % too much liquid phase was formed.
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.
Powder 2 and 3 were further tested at different oxidation temperatures. The following table shows the increase in weight relative to the reference 310B.
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.
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.
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.
Metallic examination was further performed on the tensile test bars. The metallographic pictures, see
| Number | Date | Country | Kind |
|---|---|---|---|
| SE 0601601-8 | Jul 2006 | SE | national |
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.
| Number | Date | Country | |
|---|---|---|---|
| 60840457 | Aug 2006 | US |
