US 3293007 A
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United States Patent STEAM CORROSION-RESISTANT IRON-CHROMI- UM-ALUMlNUM-YTTRIUM ALLOYS AND PROC- ESS FOR MAKING SAME Carl S. Wnkusick, Cincinnati, Ohio, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Nov. 29, 1965, Ser. No. 510,476
3 Claims. (Cl. 29-182) The invention described herein was made in the course of, or under a contract with the US. Atomic Enengy Commission.
The invention relates to a selected class of improved steam corrosion and embrittlement resistant alloys of iron, chromium, aluminum, and yttrium.
Alloys of iron, chromium, aluminum, and yttrium were originally developed for their oxidation resistance in air at temperatures over 2000 F. As described in US. Patent 3,027,252, issued March 27, 1962, to James A. 'McGurty and John F. Collins, the useful range of compositions for these alloys was as follows: 20.0 to 95.0 weight percent chromium, 0.5 to 4.0 weight percent alminum, 0.5 to 3.0 weight percent yttrium, and the balance iron.
These alloys have also been found highly resistant to oxidation and corrosion by superheated steam and thus potentially useful in solving a critical problem in nuclear reactor technology. One of the most promising approaches for obtaining increased efTiciency in the generation of power by nuclear reactors is the use of a superheated steam system wherein superheating is effected by direct passage of steam through the reactor core. Development of this type reactor has been hampered by the lack of suitable fuel-element cladding and structural materials.
While satisfactory with regard to oxidation and corrosion by super-heated steam, the mechanical properties of the above-described ironchromium-aluminum-yttriu-m system became adversely affected by prolonged exposure to superheated steam. Specifically, these alloys become severely hardened and embrittled within several hours at temperatures of about 340 C. to 540 C. and prolonged holding at temperatures from 540 C. to 705 C. may also result in embrittlement. Typical superheated steam nuclear reactor systems require an operating steam temperature of about 480 C. to 565 C. and a fuel element temperature up to about 675 C. The high probability of embrittlement and mechanical failure thus precludes use of the 20-95 iron-chromium, 0.54.0 aluminum, 0.5- 3.0 yttrium alloys system for superheated steam environments.
Subsequently, I found that embrittlement in these alloys is reduced by modifying the iron-chromiumalu-minum-yttrium system to a composition consisting of 0-20 weight percent chromium, 0.5l2.0 weight percent aluminum, 0.1-3.0 weight percent yttrium, and the balance iron. This aforementioned low chromium alloy system is described and claimed in my copending application Ser. No. 357,845, assigned to the Atomic Energy Com-mission. While this modified alloy composition amounts to a significant impnovement in terms of lowering embrittlement, it has been shown that the lowered chromium content results in excessive rate of corrosion by superheated steam environmerits, despite the presence of aluminum and yttrium. Further it has also been determined that yttrium is detrimental to the notch impact properties of the iron-chromium-aluminum-yttrium system. The presence of yttrium even in small amounts (i.e., 0.1-0.5 weight percent) increases the ductile-tobrittle transition temperature to 100 C.200 C. as compared to 25 C. for .a yttriumfree ironchromium-aluminum alloy. The structure of the alloy produced by the usual fabrication methods, such as melting, casting, and extrusion consists of an iron chromium-aluminum matrix containing a course dispersion of large particles of a brittle yttrium-iron (YFe compound phase. Impact data indicate that these brittle yttrium-iron particles act as internal notches and thus control or shift the ductile-tobrittle transition temperature.
Where the iron-chromium-aluminum-yttrium system is to be used as a .fuel cladding, it must be drawn to a finished tubing above about 300 C. which is above the ductile-to-brittle transition temperature. It would be more economical to draw tubing at lower temperature; for ex ample, at room temperature. Where the iron-chromiumaluminum-yttrium system is to be used in a superheated steam environment, the steam corrosion rate must be reduced to an acceptably low rate, i.e., to less than a weight gain of 0.5 mg./cm. for at least 1000 hours. It is, therefore, an object of this invention to provide an alloy which has an acceptable level of steam corrosion resistance and which at the same time can be fabricated into tubing at low temperatures.
Among the specific objects of this invention are (1) to provide a steam corrosion resistant iron-chromium-aluminum-yttrium alloy system; (2) to provide an ironchromium-aluminurn-yttrium alloy system with enhanced high temperature strength; (3) to provide a method of increasing the strength and impact resistance of the ironchromium-alummum-yttrium system; (4) to reduce the ductile-to-brittle transition temperature of the iron-chromium-aluminum-yttrium system; and finally (5) to satisfy any one or combination of the aforementioned objects.
Where the objective is to increase steam corrosion resistance of so-called low chromium iron-chromium-aluiminum-yttrium alloys, i.e., those containing 20% by weight chromium or less, my invention comprises pre-oxidizing an alloy consisting of 0-20 weight percent chromium, 0.5- 12.0 weight percent aluminum, and 0.0l3.0 weight percent yttrium in an oxygen-containing atmosphere such as air.
Where the objective is to increase the strength, impact resistance, and is to lower the ductile-to brittle transition temperature of any ironchromium-aluminum-yttriurn alloy, the invention consists in fabricating said alloy in such a manner as to effect dispersion of an insoluble yttriumiron compound phase in an iron-chromium-aluminum matrix such. that the particles are less than about 1 micron in diameter. In the specific embodiment disclosed herein, this is accomplished by atomizing a molten mixture of a given iron-chromiumaluminurm-yttrium composition and then fabricating the resultant alloy powder by a combination of hot extrusion and hot rolling or swaging, followed by cold rolling to a desired finished size.
The iron 25 chromium-4 aluminum-1 yttrium alloy has been found to exhibit excellent resistance to steam corrosion over a wide range of temperatures. However, as previously mentioned, the alloy is subjected to embrittlement especially at temperatures below about 540 C. This embrittlement is caused by precipitation of a chromium-rich ferrite phase. In order to reduce or eliminate the problem, the chromium content must be reduced. But when the chromium content is reduced to below about 20% by weight, it causes a significant lowering of the resistance of the alloy to steam corrosion at temperatures in the range 550 C.-730 C. Increasing the aluminum content of the alloy is effective in improving the steam corrosion resistance but this causes a reduction in ductility. Instead of altering the composition of the low chromium (i.e., 20% chromium) alloy, this invention provides for a pre-oxidation in air at elevated temperatures to efiect a significant improvement in the subsequent steam corrosion resistance. The iron-chromium-aluminum-yttrium alloys containing 15% or less chromium, in particular, exhibit poor corrosion resistance in the temperature range 550 C.730 C. In the pre-oxidized state, however, the corrosion resistance is comparable to the high chromium alloys, i.e., alloys containing more than 20% chromium. The enhanced steam corrosion resistance phenomenon is illustrated in the following example.
Example I The remarkable increase in steam corrosion resistance is brought about by the surface pre-oxidation of the low chromium alloy to produce a continuous protective aluminum oxide coating and is graphically illustrated in Table I below which is a summary of weight gain data for iron-chromium-aluminum-yttrium alloys which were pre-oxidazed in an air atmosphere at temperatures in the range 980 C.1250 C. for the period of time indicated in the table, and exposed to steam at temperatures in the range 550 C.730 C. for 4000 hours at temperature. Optimum pre-oxidation times and temperatures will depend on the degree of protection required. Temperatures from 800 C. to over 1400 C. will be effective because of the formation of protective aluminum oxides by selective oxidation of aluminum from the alloy. At temperatures below 800 C., the oxides formed are not as protective as those formed at higher temperatures. The preferred range is 980 to 1250 C. requiring times from 1 hour to over 100 hours. At temperatures above about 1250 C. it is necessary to carefully support the metal part to prevent distortion. Also incipient melting occurs as temperatures above 1350 C. which is damaging to ductility.
TABLE I Preoxidation Treatment Steam Corrosion Testing in Air (4,000 hours) Alloy Tempera- Time, hr. Tempera- Weight gain, ture, C. ture, 0. mgJcrn.
2541 None 730 0. 11 980 125 730 0. 00 1, 100 125 730 0. 11 1, 260 67 730 0. 08 1541 None 730 0. 52 980 125 730 O. 02 1, 100 125 730 0. 03 1, 260 67 730 0. 03 0501 None 550 0. 79 980 125 550 0. 06 1, 100 125 550 0. 1, 260 67 550 0. 02 0561 None 730 0. 25 980 125 730 0. 05 1,100 125 730 0.08 1, 260 07 7 30 0. 00 1041 None 550 5. 90 980 125 550 0. 04 1,100 125 550 0. 00 1, 260 67 550 0. 15 1041 None 730 4. 48 980 125 730 0. 09 1,100 125 730 0.14 1, 260 07 730 0. 16
11 Alloy compositions are in weight percent:
2541=Fe-25 Cr4 A11 Y 1541=Fe-15 Cr4 Al1 Y 0561=Fe-15 Gr6 A11 Y 1041=Fe- (Jr-4 A11 Y It will be noted that the weight gain in air for all alloys tested was relatively the same, indicating that the air oxidation resistance Was rather independent of the chromium content. On the other hand, those low chromium alloys which were not pro-oxidized exhibited an extremely high weight gain when exposed to steam atmospheres. Note in particular to 10 weight percent alloys which exhibited virtually disastrous weight gain during the 4000 hours steam test. In marked contrast, a pre-oxidation reduced the steam corrosion resistance of the low chromium alloys to the same level as the high, i.e. 20 weight percent chromium-c n ai ing alloys.
As previously noted, embrittlement caused by precipitation of a chromium-rich ferrite phase can be reduced or eliminated by lowering the chromium content, generally to 20% by weight or less. The addition of yttrium to the high or low iron-chromium-aluminum system imparts a significant measure of oxidation resistance to the resultant alloy, but it does so only at the expense of low temperature (room temperature to about 200 C.) notch impact properties.
It has been theorized that if the yttrium could be forced into solution or if the yttrium-iron, YFe precipitated particles were sufficiently small and uniformly dispersed in the iron-aluminum-chrominum matrix, the strength as well as the impact propertie of the resultant iron-aluminum-chromium-yttrium alloy could be improved.
The following example illustrates the manner in which the theory was tested and confirmed.
Example II A mixture of iron, chromium, aluminum, and yttrium containing 3 weight percent yttrium was melted and atomized in an argon atmosphere to form a fine powder of the alloy corresponding to the same composition. The powder particle size was about 100 microns in diameter. The atomized powder was cold pressed in a mild steel casing to 10,000 p.s.i. The casing was sealed and extruded at a reduction ratio of approximately 14:1 at 1000 C.
' This was then followed by hot rolling at 1000 C. and
finally by cold rolling to produce a finished sheet specimen. Stress rupture tests were then conducted on the sheet specimen at temperatures between 650 C.-800 C. after the material was given a preliminary anneal. For purposes of comparison, a 5 chromium, 6 aluminum, 1 yttrium, balance iron alloy was made by casting, extruding, and rolling to a finished size and tested in the same manner. The results are summarized in Table II below.
TABLE II Hr. Rupture Stress at Indicated Temp., kg. em. Alloy 650 C. I 700 C. l 750 C. I 800 0.
0561 Fe-S Cr6 Al-1 Y... 387 253 183 134 0563-.." Fe5 Cr-G A13 Y 775 492 330 211 The results shown above indicate a significant strength improvement of the atomized 3 weight percent yttrium alloy vs. the 1 percent yttrium which had been cast and extruded.
Impact specimens were then machined from these alloys and it was found that the atomized specimen containing 3 weight percent yttrium had a ductile-to-brittle transition temperature of about 50 C. While the cast and extruded 1 weight percent yttrium-containing alloy had a ductile-to-brittle transition temperature of about C. Thus, the decrease in the transition temperature for the 3 Weight percent alloy is over 100 C. in spite of the higher yttrium content. This result indicates that the embrittling effect of yttrium in the basic ironchromium-aluminum alloy is not so much a function of the amount of yttrium added, but is more closely related to the physical form it takes in the resulting metallurgical structure. Thus, the oxidation and steam corrosion resistance of a 1% yttrium-containing alloy is comparable to a 3% yttrium-containing alloy. However, where both alloys are formed by melting, casting, or extrusion, the 3% yttrium alloy will have reduced notch impact properties and will be diflicult to fabricate into sheet form for example. On the other hand, when a 3% yttrium-containing alloy is prepared in such a way as to increase the solubility of yttrium in the matrix phase or alternatively to form a fine dispersion of the yttrium in the ironchromium-aluminum matrix, the adverse embrittling effects Will be considerably ameliorated. Thus, with this occur with increasing yttrium "concentration's.
elaboration of the part played by yttrium addition to a reference iron-chromiumal-uminum system, I am now able to include a higher level of yttrium to obtain simultaneously increased strength and impact resistance, Le, a lower ductile-to-brittle transition temperature.
Thus, the iron-chromiumaluminum-yttrium alloys derived, for example, from any process which either solubilizes the yttrium or allows formation of :a finely divided dispersion of a yttrium-iron phase in an iron-chromiumaluminum matrix permits drawing of tubing at temperatures below 300 C. The atomization technique is particularly advantageous since it allows incorporation of quite small amounts in the range 0.01 to 3 weight percent yttrium to a reference Fe-Cr-Al alloy to obtain oxidation as Well as steam corrosion resistance while reducing or ameliorating the notch or embrittlement effects which In the atomization technique, the yttrium is either retained in solution or precipitated as an extremely fine and fairly evenly distributed dispersion of yttrium-iron particles. In general, if the particles are large, for example 5 to 30 microns in diameter then the notch effect Will operate to increase the ductile-to-brittle transition temperature (as compared to an ironchrominum-aluminum alloy without yttrium) to higher temperatures.
Methods other than atomization may be used to produce the desired dispersion. These include milling or other fragmentation processes. It is not possible to heat treat the alloy to obtain the dispersion because of the very low solubility of yttrium in the solid base metal. Atomization is the preferred process because yttrium is soluble in the liquid metal. Also, the rapid quenching feature involved in the atomizing process prevents large yttriumiron compound particles from forming.
Having thus described my invention, I claim:
1. A dispersion-strengthened alloy of iron-chromiumaluminum and yttrium consisting of an iron-chromiumaluminum matrix and a finely dispersed phase iron and yttrium, the particles of said dispersed phase having a maximum diameter no greater than 5 microns.
2. A method of fabricating an alloy from an alloy system comprising 0-25 chromium, .5-12 aluminum, 0.01-4 yttrium, and the balance iron which comprises melting an alloy of a composition selected from said system and rapidly quenching the melted alloy in such a manner as to produce a precipitated yttrium-containing phase containing a predominant amount of particles having a maximum diameter of less than 5 microns.
3. A method of producing a fine dispersion of a yttrium-containing phase in an alloy having a matrix containing iron, chromium, and aluminum which comprises atomizing a molten composition of iron-chromium-aluminum and from 0.01-4 weight percent yttrium to form an alloy powder whose metallurgical structure consists of a dispersion of yttrium in an iron-chromium-aluminum matrix, the particles of said dispersed phase having a maximum diameter no greater than about 5 microns.
References Cited by the Examiner UNITED STATES PATENTS 2,061,370 11/1936 Rohn -124 2,190,486 2/1940 Schafmeister 75--123 2,864,734 12/1958 Adams et a1 148-6.35 2,890,974 6/1959 Carrigan 148-6.35 2,909,808 10/ 1959 Frehn.
3,027,252 3/1962 McGurty et al 75-124 3,194,658 7/1965 Storcheim 75224 L. DEWAYNE RUTLEDGE, Primary Examiner.
r R. L. GRUDZIECKI, Assistant Examiner.