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FERRITIC ALLOY

20170342531 · 2017-11-30

    Inventors

    Cpc classification

    International classification

    Abstract

    A ferritic alloy comprising in weight %: C: 0.01-0.1; N: 0.001-0.1; O: ≦0.2; B: ≦0.01; Cr: 9.0-13.0; Al: 2.5-8.0; Si: ≦0.5; Mn: ≦0.4; Y: ≦2.2; Sc+Ce+La: ≦0.2; Mo+W: ≦4.0; Ti: ≦1.7; Zr: ≦3.3; Nb: ≦3.3; V: ≦1.8; Hf+Ta+Th: ≦6.5; the balance being Fe and unavoidable impurities, wherein, the amounts of Ti+Zr+Nb+V+Hf+Ta+Th and C, N and O are balanced such that:

    [00001] at .Math. .Math. % .Math. .Math. Ti + at .Math. .Math. % .Math. .Math. Zr + at .Math. .Math. % .Math. .Math. Nb + at .Math. .Math. % .Math. .Math. V + at .Math. .Math. % .Math. .Math. Hf + at .Math. .Math. % .Math. .Math. Ta + at .Math. .Math. % .Math. .Math. Th - x * at .Math. .Math. % .Math. .Math. O - at .Math. .Math. % .Math. .Math. N at .Math. .Math. % .Math. .Math. C ≥ 1 wherein x is 0.5 unless the content of Y is more than or equal to 0.01 wt % then x is 0.67.

    Claims

    1. A ferritic alloy comprising in weight %: C: 0.01-0.1; N: 0.001-0.1; O: ≦0.2; B: ≦0.01; Cr: 9.0-13.0; Al: 2.5-8.0; Si: ≦0.5; Mn: ≦0.4; Y: ≦2.2; Sc+Ce+La: ≦0.2; Mo+W: ≦4.0; Ti: ≦1.7; Zr: ≦3.3; Nb: ≦3.3; V: ≦1.8; Hf+Ta+Th: ≦6.5; the balance being Fe and unavoidable impurities, wherein the amounts of Ti+Zr+Nb+V+Hf+Ta+Th and C, N and O are balanced such that: at .Math. .Math. .Math. % .Math. .Math. Ti + at .Math. .Math. % .Math. .Math. Zr + at .Math. .Math. % .Math. .Math. Nb + at .Math. .Math. % .Math. .Math. V + at .Math. .Math. % .Math. .Math. Hf + at .Math. .Math. % .Math. .Math. Ta + at .Math. .Math. % .Math. .Math. Th - x * at .Math. .Math. % .Math. .Math. O - at .Math. .Math. % .Math. .Math. N at .Math. .Math. % .Math. .Math. C ≥ 1 wherein x is 0.5 unless the content of Y is more than or equal to 0.01 wt % then x is 0.67.

    2. The ferritic alloy according to claim 1, wherein the ferritic alloy comprises no added wt % Sc+Ce+La or no added wt % Ce+La.

    3. The ferritic alloy according to claim 1, wherein the ferritic alloy comprises Cr in the range of from 9.0-12.0 wt %.

    4. The ferritic alloy according to claim 1, wherein the ferritic alloy comprises Cr in the range of from 9.0 to 11.5 or in the range of from 9.0 to 11.0 wt %.

    5. The ferritic alloy according to claim 1, wherein 1 ≤ at .Math. .Math. % .Math. .Math. .Math. Ti + at .Math. .Math. % .Math. .Math. Zr + at .Math. .Math. % .Math. .Math. Nb + at .Math. .Math. % .Math. .Math. V + at .Math. .Math. .Math. % .Math. .Math. .Math. Hf + at .Math. .Math. % .Math. .Math. Ta + at .Math. .Math. % .Math. .Math. Th - x * at .Math. .Math. % .Math. .Math. O - at .Math. .Math. % .Math. .Math. N at .Math. .Math. % .Math. .Math. C ≥ 2.3

    6. The ferritic alloy according to claim 1, wherein 1.2 ≤ at .Math. .Math. % .Math. .Math. Ti + at .Math. .Math. .Math. % .Math. .Math. Zr + at .Math. .Math. % .Math. .Math. Nb + at .Math. .Math. % .Math. .Math. .Math. V + at .Math. .Math. % .Math. .Math. Hf + at .Math. .Math. % .Math. .Math. Ta + at .Math. .Math. % .Math. .Math. Th - x * at .Math. .Math. % .Math. .Math. O - at .Math. .Math. % .Math. .Math. N at .Math. .Math. .Math. % .Math. .Math. C ≥ 2.0

    7. The ferritic alloy according to claim 1, wherein C is in the range of from 0.02-0.08 wt %.

    8. The ferritic alloy according to claim 1, wherein N is in the range of from 0.001-0.08 wt %.

    9. The ferritic alloy according to claim 1, wherein O is in the range of from 0.001-0.08 wt %.

    10. The ferritic alloy according to claim 1, wherein O is in the range of from 0.01-0.1 wt %.

    11. The ferritic alloy according to claim 1, wherein Al is in the range of from 3.0-7.0 wt %.

    12. The ferritic alloy according to claim 1, wherein Y is in the range of from 0.01 to 1.2 wt %.

    13. The ferritic alloy according to claim 1, wherein Ti is in the range of from 0.02-1.3 wt %.

    14. The ferritic alloy according to claim 1, wherein Zr is in the range of from 0.04-2.4 wt %.

    15. The ferritic alloy according to claim 1, wherein Nb is in the range of 0.04-2.4 wt %.

    16. A use of the ferritic alloy according to anyone of claim 1 in a temperature range of 300-800° C.

    Description

    BRIEF DESCRIPTION OF DRAWINGS

    [0151] FIG. 1: A schematic drawing showing the purpose of balancing RE and carbon.

    [0152] FIGS. 2a and 2b: Micrographs showing a chromium-rich carbide formed in a comparative alloy.

    [0153] FIGS. 3a and 3b: Micrographs showing oxidation behavior in comparative alloys.

    [0154] FIGS. 4a and 4b: Micrograph showing oxidation behavior in an alloy according to a first alternative of the disclosure.

    [0155] FIGS. 5a and 5b: Micrograph showing oxidation behavior in an alloy according to a second alternative of the disclosure.

    DETAILED DESCRIPTION OF EXAMPLES

    [0156] The disclosure will be described by the following non-limited example:

    [0157] The example aims at investigating Fe-10Cr-4Al alloys, and specifically to investigate the influence of different reactive elements (RE) on the long-term (8,760 h) corrosion resistance to liquid lead at 550° C. In addition, a short-term (1,000 h) test at 450° C. was conducted for comparative reasons.

    [0158] The influence of additions of the reactive elements (RE): Zr, Nb and Y were selected for the example. Nine experimental alloys, with varying RE content, were produced in a vacuum induction furnace. Samples were hot-rolled to 8×1 mm strips and homogenized at 1050° C. for 5 minutes after each step. The analyzed chemical compositions for all studied alloys are presented in table 1.

    TABLE-US-00001 TABLE 1 Chemical composition of studied alloys (wt %) RE RE/C Alloy Fe Cr Al Si Mn C Ti Zr Nb Y (at. %) Zr-0.1 Bal. 10.12 3.98 0.12 0.11 0.04 0.08 0.11 — — 0.89 Zr-0.2 Bal. 10.15 3.95 0.13 0.11 0.03 0.09 0.21 — — 1.67 Zr-0.4 Bal. 10.20 4.06 0.12 0.12 0.03 0.07 0.39 — — 2.30 Nb-0.4 Bal. 10.20 4.10 0.15 0.07 0.03 0.11 — 0.46 — 2.94 Nb-0.8 Bal. 10.17 4.12 0.12 0.12 0.03 0.08 — 0.9  — 4.95 Nb—0.8C Bal. 10.12 4.16 0.12 0.12 0.1 0.06 — 0.85 — 1.32 Y-0.02* Bal. 10.26 4.24 0.07 0.12 0.03 0.07 — — 0.02 0.70 Y-0.1* Bal. 10.21 4.14 0.12 0.13 0.03 0.07 — — 0.09 0.87 Y-0.2* Bal. 10.12 4.05 0.12 0.11 0.03 0.08 — — 0.19 1.45 *Even though the quotient according to the definition herein does not comprise Y, Y was included in quotient for the last three alloys because of comparison. However, as will be shown and which is also discussed in the present disclosure, yttrium will be acting different compared to the RE.

    [0159] The contents of RE and carbon was varied in the nine samples, such that some samples had a deficit of RE in comparison to the amount of carbon (samples Zr-0.1, Y-0.02, Y-0.1), in some samples the amounts of RE and carbon was in balance (samples Zr-0.2, Nb-0.8C, Y-0.2) and in some samples RE was in excess in comparison to carbon (samples Zr-0.4, Nb-0.4, Nb-0.8).

    [0160] Coupons measuring 30×8×1 mm were prepared of each alloy for the oxidation study. The surfaces were polished to a #800 grit finish using SiC abrasive papers, after which the coupons sonicated in ethanol and subsequently placed in alumina crucibles filled with 2 mm 99.9% (metal base) lead shots. The oxidation test was carried out in a tube furnace, where the crucibles were placed inside sealed quartz tubes. The dissolved oxygen content in the liquid lead was controlled by means of a flowing Ar—H.sub.2—H.sub.2O gas mixture. H.sub.2/H.sub.2O ratios of 1.3 and 0.2 were used to achieve a dissolved oxygen concentration of 10.sup.−7 wt % in the liquid lead at 550° C. and 450° C. respectively. After finishing the oxidation tests, 1000 h at 450° C. and 8760 h at 550° C., the samples were air cooled and cleaned from residual lead in a (1:1) solution of acetic acid and hydrogen peroxide. Transmission electron microscopy (TEM) samples were prepared through the standard lift-out using a FEI quanta 3D field emission scanning electron microscope (FEG-SEM). The TEM evaluation was carried out using a JEOL JEM-2100F FEG TEM. Energy dispersive spectroscopy (EDS) elemental analysis was made using an Oxford instruments 80 mm.sup.2 X-Max.sup.N silicon drift detector (SDD). SEM samples were prepared by molding the oxidized sample into a conductive resin followed by fine polishing down to a final 0.25 μm diamond step. A Zeiss Leo 1530 FEG-SEM an Oxford 50 mm.sup.2 X-Max SDD EDS were used for general characterization. Thermodynamic modeling was carried out using ThermoCalc running the TCFE7 and SSOL4 databases.

    [0161] Results from the Investigations

    [0162] The results from the 8,760 h oxidation test at 550° C. showed clear differences in oxidation properties with respect to various RE additions.

    [0163] The three alloys (Zr-0.1, Y-0.02 and Y-0.1) which had a deficit of RE in comparison to carbon formed significant amounts of Cr-rich carbides. FIG. 2a shows a Cr-carbide (1) close to an Al-rich oxide scale (2) formed on the surface (3) of the sample. In FIG. 2a, the Cr-rich carbide may be detected as a white shape inside the encircled area. FIG. 2b is a TEM Cr EDS map of the chrome-rich carbide area (1) encircled in FIG. 2a. Here the shape of the chrome-rich area is clearly visible.

    [0164] The examples showed that nearly all Cr-carbides were formed in contact with the Al-rich oxide at the sample surfaces. This may be explained in that aluminum suppresses carbide formation, i.e. stabilizes graphite. It seems thus likely that the Cr-carbide nucleation is promoted at the Al-depleted metal-oxide phase boundary. Thus, the protective Al-oxide had not been formed properly.

    [0165] Furthermore, the three alloys, Zr-0.1, Y-0.02 and Y-0.1, all showed poor oxidation properties in the oxidation tests. The poor results were consistent both at 550° C. and at 450° C., this shows that it is important to select the correct quotient as described hereinabove or hereinafter.

    [0166] FIG. 3a shows a SEM image of a cross-section of a sample taken from the Zr-0.1 alloy after oxidation at a temperature of 550° C. It is clearly visible in the image an irregularly shaped mixed oxide, which has grown into the bulk of the alloy. The high amount of chromium-rich surface carbides in the metal-oxide interface of the Zr-0.1 sample seemingly leads to a pitting type of accelerated oxidation, displaying inward growing mixed metal oxides measuring up to about 5 μm.

    [0167] At lower temperatures, where a low Cr-content is needed to avoid α-α′ phase-separation, the presence of chromium-rich surface carbides resulted in formation of non-protective oxide scales. This was confirmed by the shorter (1000 h) oxidation test at 450° C. The same three alloys that contained carbides close to the surface (Zr-0.1, Y-0.02, Y-0.1) and that had a deficit of RE in comparison to carbon were completely covered with a three-layered oxide structure, consisting of an outward growing Fe.sub.3O.sub.4 scale and an inward growing FeCrAl mixed oxide, under which an internal oxidation zone was seen. FIG. 3b shows a SEM micrograph of the Y-0.02 sample having the above described structure of oxide layers. The total depth of the corrosion attacks on the Y-0.02 sample was measured to 3-4 μm.

    [0168] Balanced Alloys

    [0169] Hence, by balancing the C and RE content, the corrosion performance of FeCrAl-alloy was improved. Three alloys in the study, Zr-0.2, Y-0.2 and Nb-0.8C, contained RE in near balance with respect to the C-content, displayed significantly different oxidation behavior at 550° C.

    [0170] Zr-0.2

    [0171] The Zr-0.2 alloy showed no signs of oxidation attacks. A TEM evaluation was carried out to study the surface of the Zr-0.2 sample and it showed the presence of a thin, approximately 100 nm thick oxide that had formed during the 8,760 h exposure in liquid lead, see FIG. 4b. The oxide was divided into three layers, an inward growing Al.sub.2O.sub.3 layer and an outward growing FeAl mixed oxide, delimited by a thin Cr-rich oxide layer. FIG. 4b is a TEM EDS line scan showing the chemical composition of the layer in FIG. 3a as a function of the distance from the surface of the layer.

    [0172] At 450° C., the Zr-0.2 alloy displayed favorable oxidation properties, i.e. a protective oxide layer was formed, see FIG. 4a. The thin oxide that was formed on its surfaces was investigated by means of TEM and was measured to about 40 nm. Similar to the TEM results at 550° C., the oxide formed at 450° C. was divided into three zones, an inner layer solely enriched in Al, an outer part rich in Fe, and an intermediate layer rich in Cr (FIG. 4b. FIG. 5b is a TEM EDS line scan showing the chemical composition of the layer in FIG. 5a as a function of the distance from the surface of the layer.

    [0173] Nb-0.8C

    [0174] The Nb-0.8C alloy showed no oxidation attacks at 550° C.

    [0175] Y-0.2

    [0176] Localized oxidation pits were found on the Y-0.2 alloy after treatment at 550° C. despite the slight excess of Y in relation to C. This may be explained by the relatively weak stability of yttrium carbides compared to those of Zr and Nb. The microstructure characterization found Y-rich precipitates, which were enriched in C, O, S and in particular Fe.

    [0177] Large Excess Ratio Alloys

    [0178] The samples having a large excess of RE compared to C, (i.e. samples Zr-0.4, Nb-0.4, Nb-0.8) showed poor oxidation properties at both 550° C. and 450° C. The Zr-0.4 alloy, showed pitting type oxidation attack at 550° C. and Fe and Zr-rich phases measuring up to 2 μm was found throughout the matrix. In addition, the oxide pits on the same alloy were enriched in Zr.

    [0179] The alloys Zr-0.4 and the Y-0.2 preformed in a similar manner at 450° C. The impact of the excess of Y and Zr did not result in a marked decrease in oxidation properties at 450° C., as was the case at 550° C. However, the slower reaction kinetics at 450° C. in comparison to 550° C. and a shorter exposure time may have masked the result.

    [0180] After treatment at 550° C., the Nb-0.8 alloy was almost entirely covered with oxidation pits, which measured up to 5 μm. These precipitates were preferably found to decorate the alloy grain boundaries, similar to the behavior of laves phases, but were also found inside the grains of the Nb-0.8-alloy.

    CONCLUSIONS

    [0181] It has been shown that best oxidation resistance is achieved when the RE-additions are in balance with the carbon content of the alloy, i.e. the RE is in slight excess. A deficit of RE in respect of carbon will lead to the formation of chromium rich carbides close to the surface of the alloy which in turn will lead to poor oxidation properties and reduced pitting corrosion resistance. Further, over-doping of RE will lead to the formation of intermetallics or laves phase, which will also decrease the oxidation resistance of the alloy.