Patent Application
20250122604
The present invention relates to a method of forming an aluminum oxide scale on the surface of nickel-base (Ni-base) alloys containing aluminum and chromium.
Ni-base alloys have been widely used in industrial applications. Ni-base alloys that contain aluminum (Al) and chromium (Cr) are often chosen for use in extreme high temperature environments because of their excellent high temperature oxidation and corrosion resistance due to a protective aluminum oxide (alumina or Al2O3) scale that forms on these alloys during manufacture or use. In these alumina-forming Ni-base alloys the protection of an aluminum oxide scale is attributed to its dense, stable, and adhesion nature when formed on the surface of a Ni-base alloy. However, the growth rate of aluminum oxide scale is slow, and an alumina-forming Ni-base alloy would suffer corrosion attack in a harsh corrosion environment if a protective aluminum oxide scale was not able to establish rapidly. The types of corrosion that can occur in these harsh corrosion environments include, but are not limited to, high temperature cyclic oxidation, nitridation, carburization, and sulfidation.
For instance in an engine application, an alumina-forming Ni-base alloy could experience premature failure from internal nitride attack because a protective aluminum oxide scale did not establish under severe thermal cycling condition. In another high temperature corrosion environment, an alumina-forming Ni-base alloy could suffer local nitridation attack when exposed to high temperature nitrogen atmosphere. That attack typically occurs on surfaces or portions of a surface where a protective aluminum oxide scale did not form.
Pre-oxidation heat-treatments have been used to produce an oxide scale on an alloy surface to improve oxidation and corrosion resistance of the alloy prior to putting the alloy into service. For this treatment to be successful it is critical to form a protective oxide scale reliably during a pre-oxidation heat-treatment. But there are significant challenges to the formation of a desired aluminum oxide scale on alumina-forming Ni-base alloys because most commercial alumina-forming alloys contain other oxide-forming alloying elements, such as chromium (Cr), nickel (Ni), etc. These alloying elements can form other oxides with less corrosion resistance, such as chromium oxide (Cr2O3), nickel oxide (NiO), and mixed oxides. Consequently, there is a need for a method that will reliably establish a protective aluminum oxide scale and prevent the formation of less-protective oxides on alumina-forming Ni-base alloys.
U.S. Pat. No. 4,439,248 discloses a method to heat treat a NiCrAlY alloy for an effective time in an oxygen potential controlled atmosphere with an oxygen partial pressure and at a temperature of between about 1500° and 2372° F. to provide an essentially aluminum oxide film on the surface of the alloy. The controlled atmosphere is to have oxygen potential, or oxygen partial pressure, located within the range in which aluminum oxide essentially forms, exclusive to formation of other oxides or mixed oxides.
However, we have found that the controlled atmosphere disclosed in U.S. Pat. No. 4,439,248 may not be adequate to form an essentially aluminum oxide scale on a specific Ni-base alumina-forming alloy. For example,
Another aim of a pre-oxidation heat-treatment is to form a protective oxide scale with a sufficient thickness for good oxidation and corrosion protection, and the desired thickness is typically determined by the expected corrosion resistant life of an alloy under specific corrosion conditions. For alumina-forming Ni-base alloys, the formation and growth of an aluminum oxide scale depends on thermodynamic and kinetic factors. The control of oxygen partial pressure to form essentially aluminum oxide as in U.S. Pat. No. 4,439,248 is based on thermodynamic consideration; however, the formation and growth of an aluminum oxide scale also strongly depends on kinetic factors. Sufficient oxygen concentration and oxygen flux are important contributing factors.
We provide a pre-oxidation heat-treatment method for Ni-base alloys containing 1.8 weight (wt.) % minimum aluminum (Al) and 10 weight (wt.) % minimum chromium (Cr) that can produce continuous aluminum oxide (Al2O3) scale having a thickness of at least 100 nanometers (nm) on the alloy surface and this scale contains little or no chromium oxide (Cr2O3), nickel oxide (NiO), titanium oxide (TiO2), silicon oxide (SiO2), and/or mixed oxides. Our method involves heating a workpiece made from the alloy in a furnace containing flowing argon gas or a flowing argon atmosphere containing less than 0.1 volume (vol.) % oxygen at a pressure of at least 500 microns, and preferably of at least 5000 microns (5 torr), at a temperature of 1400° F. to 2150° F. until a desired scale of aluminum oxide is formed on at least one surface of the workpiece. The preferred pre-oxidation heat-treatment time is 0.5 hour minimum from 1700° F. to 2100° F. To form the aluminum oxide scale, gas flux in the method is controlled to be 1 mL/min·cm2 minimum to provide sufficient oxygen supply, and the preferred gas flux is 5 to 25 ml/min·cm2.
Preferably our method is used to heat-treat Ni-base alloys which contain, in weight percent, 1.8 to 5.0% aluminum and 10 to 35% chromium.
We provide a pre-oxidation heat-treatment method that can create essentially aluminum oxide scale on the alloy surface.
We also provide a pre-oxidation heat-treatment method that forms a continuous aluminum oxide scale on the surface of a workpiece made from the alloy.
Our method can provide a protective aluminum oxide scale that has enhanced oxidation and corrosion resistance when compared to that without pre-oxidation treatment.
Our method can be used for workpieces having one or more surfaces that have been fabricated, such as oxidation-annealed and pickled, bright annealed, machined, ground, polished, or abrasive blasted, prior to the pre-oxidation heat-treatment.
Other objects and advantages of the present invention will become apparent from a description of certain preferred embodiments thereof which are described below and shown in the Figures.
A series of heat-treatment experiments were performed on a few commercial Ni-base alloys containing, in weight percent, 0.8 to 5.0% aluminum and 15 to 26% chromium, at selected heat-treatment temperatures with different heat-treatment times in controlled flowing argon atmospheres containing different oxygen and water vapor contents. The argon gas used in the experiments was industrial grade argon. The argon atmosphere was controlled from a minimum vacuum level of 50 microns to one atmosphere pressure (1 atm) at a temperature between 1500° F. and 2100° F. Gas flux was controlled in the range of 6 to 21 mL/min·cm2. These pre-oxidation heat-treatments were discovered to yield a desired oxide scale essentially consisting of aluminum oxide (Al2O3).
The pre-oxidation heat-treatment time required to produce a minimum 100 nm-thick aluminum oxide scale is inversely related to the temperature of heat-treatment; the higher the temperature the shorter the required time. For example, to create an aluminum oxide scale having the minimum 100 nm thickness 24 hours at 1500° F. is needed; while at 2100° F. the required time is much shorter, as little as 30 minutes. Continuing the heat-treatment beyond the required time can increase the thickness of the layer of aluminum oxide.
A Ni-base HAYNES® 233® alloy composition consisting of, in weight percent, 3.3% aluminum, 19% chromium, 19% cobalt, 7.5% molybdenum, 0.5% titanium, 0.5% tantalum, 0.03% zirconium, 0.002% yttrium, was heat-treated for 24 hours at 1700° F. in flowing argon gas. An approximately 200 nm-thick continuous aluminum oxide scale was produced.
Another Ni-base alloy composition, 602CA alloy, consisting of, in weight percent, 2.1% aluminum, 25.3% chromium, 9.5% iron, 0.2% manganese, 0.15% titanium, 0.03% zirconium, 0.08% yttrium, was heat-treated for 12 hours at 2000° F. in flowing argon gas. This treatment yielded a continuous aluminum oxide scale that was approximately 300 nm thick.
As another example a Ni-base HAYNES® 233® alloy composition consisting of, in weight percent, 3.3% aluminum, 19% chromium, 19% cobalt, 7.5% molybdenum, 0.5% titanium, 0.5% tantalum, 0.03% zirconium, 0.002% yttrium, was heat-treated for 8 hours at 2000° F. in a vacuum of 5 torr pressure maintained with flowing argon gas. A continuous aluminum oxide scale approximately 300 nm thick was formed.
As yet another example, a Ni-base HAYNES® 214® alloy composition consisting of, in weight percent, 4.3% aluminum, 16.1% chromium, 3.7% iron, 0.025% Zr, 0.004% yttrium, was heat-treated for 12 hours at 2000° F. by exposure to flowing argon gas. A continuous aluminum oxide scale having an approximate thickness of 500 nm was created.
A cyclic oxidation test performed for 1,000 hours at 1850° F. cycled with 15 minutes at temperature was conducted to determine the effectiveness of the pre-oxidation heat-treatment over no pre-oxidation heat-treatment.
As the other examples, a Ni-base HAYNES® 233® alloy composition consisting of, in weight percent, 3.3% aluminum, 19% chromium, 19% cobalt, 7.5% molybdenum, 0.5% titanium, 0.5% tantalum, 0.03% zirconium, 0.002% yttrium, was heat-treated for 48 hours at 2000° F. in a flowing wet argon atmosphere containing 3 vol. % water vapor or in a flowing argon atmosphere containing 0.1 vol. % oxygen. Chromium oxide (Cr2O) scale formed, along with subsurface aluminum oxide (Al2O3) precipitates, while essential aluminum oxide (Al2O) scale did not establish.
From the pre-oxidation heat-treatment experiments, heat-treatment atmosphere and parameters needed to reliably form the desired aluminum oxide scale were established.
A desired aluminum oxide scale formed between 1500° F. and 2100° F. Because exposure of Ni-base alloys to a temperature above 2100° F. could cause significant grain growth, we did not test at temperatures above 2100° F. We did not test at temperatures below 1500° F. because it would take a relatively long time to form a desired alumina scale, which is not practically favorable for a pre-oxidation heat-treatment.
The pre-oxidation heat-treatment time is also critical to yield aluminum oxide scale with the minimum thickness requirement. Exposure time from 30 minutes to 48 hours was preferred when heat-treated between 2100° F. and 1500° F. To have better oxidation and corrosion resistance, thicker aluminum oxide scale could be expected. Heat-treatment time of 24 hours and longer would form an aluminum oxide scale thicker than 100 nm at the heat-treatment temperature of 1700° F. and below, while an aluminum oxide scale thicker than 500 nm could be received in a shorter time, such as 12 hours or shorter, at the heat-treatment temperature of 2000° F. and above. Next we conducted a series of heat-treatment experiments, involving some non-critical parameters that we believed could improve the formation of a desired aluminum oxide scale. We found that heating rate was not critical, so no control is required on heating rate. We also found that the surface finishing condition of the workpiece prior to heat treatment may affect the aluminum oxide scale that was formed, but the effect was insignificant.
These experiments demonstrate that a continuous oxide scale with essentially aluminum oxide (Al2O3) would form on the Ni-base alloys containing, in weight percent, 1.8 to 5.0 wt. % aluminum and 15 to 26 wt. % chromium through pre-oxidation heat-treatment in a flowing argon atmosphere containing less than 0.1% oxygen (O2) with a minimum 500 micron pressure at a temperature between about 1500° F. and 2100° F. for a time dependent upon the heat-treatment temperature. The achieved aluminum oxide scale would significantly improve oxidation and corrosion resistance of the pre-oxidized Ni-base alloys.
Coupons cut from the mill annealed (MA) HAYNES® 233® alloy sheet were pre-oxidized in 1 atm flowing argon in a tube furnace to show the effect of pre-oxidation temperature and time. The coupons were polished with 120-grit SiC paper and subjected to one of the following pre-oxidation heat-treatments: 1500° F. for 24 hours, 1600° F. for 24 hours, 1700° F. for 24 hours, 1800° F. for 24 hours, 2000° F. for 12 hours, 2000° F. for 24 hours, and 2100° F. for 1 hour. Then the average thickness of the aluminum oxide scale formed on the coupons was measured from SEM examination. Table 1 below contains those measurements.
The 233® alloy coupons showed consistent pre-oxidation results with the formation of an aluminum oxide scale (no internal oxides) after these tests. The aluminum oxide scale formed at 1500° F./24 h was thin (80-100 nm), and longer time exposure would increase the scale thickness to 100 nm and above. In addition, 2000° F./12 h pre-oxidation heat-treatments in the same argon atmosphere also formed ˜500 nm aluminum oxide scale on as-produced mill-annealed HAYNES® 214® (containing 4.3 wt. % Al and 16.1 wt. % Cr) and HR-224® (containing 3.9 wt. % Al and 20.5 wt. % Cr) alloys.
Mill annealed (MA) and bright annealed (BA) HAYNES® 233® alloy coupons with different surface finishing conditions were treated with our pre-oxidation heat-treatment method in 1 atm flowing argon gas in a tube furnace with a gas flux control in the range of 6 to 21 mL/min·cm2 at 2000 and 2100° F. to study the effect of gas flux on the alloy coupons with different finishing conditions. The 233® alloy coupons included as-produced mill annealed (MA), polished using 120-grit SiC paper after having been mill annealed, vapor blasted using glass beads after having been mill annealed, as-produced bright annealed (BA), polished using 120-grit SiC paper after having been bright annealed, and vapor blasted using glass beads after having been bright annealed. Abrasive blasting is used to create a more uniform surface condition with subsurface deformation. Blasting methods other than what we tested, such as grit blasting and sand blasting, could be used. Bright annealing is an annealing process performed in a vacuum or a controlled atmosphere containing inert gases (such as hydrogen). This controlled atmosphere reduces the surface oxidation to a minimum which results in brighter surface. The bright annealed coupons that were used in these tests were annealed in hydrogen. Then the average thickness of the aluminum oxide (alumina) scale formed on the coupons was measured from SEM examination. Table 2 below contains those measurements. Consistent alumina scale of 100 nm and thicker formed in the as-produced, polished, and blasted conditions in the gas flux range of 6 and 21 mL/min·cm2. These experiments also found that 120-grit grinding of alloy surface resulted in the formation of a more uniform aluminum oxide scale because of its more consistent surface conditions, such as surface roughness and subsurface deformation, obtained from the grinding process. However, the benefit was marginal.
Additional testing of MA HAYNES® 233® coupons with the same surface finishing conditions as in Table 2 was performed to determine the effect of different pressures of the argon atmospheres on formation of alumina scale. Table 3 below reports the average thickness of alumina scale formed on the as-produced mill annealed (MA), polished using 120-grit SiC paper after having been mill annealed, vapor blasted using glass beads after having been mill annealed alloy coupons under various heat-treatment conditions of temperature, time, and atmosphere pressure maintained by flowing argon gas.
The data in Table 3 indicates that the atmosphere pressure of 50 microns did not form an alumina scale on the blasted coupon, and 500 microns atmosphere generated a desired alumina scale on as-produced MA condition but the polished and blasted conditions would need longer exposure time, i.e. more than 4 hours at 2000° F., to establish a continuous alumina scale. Accordingly, 500 microns is concluded to be the minimum atmosphere pressure requirement for our pre-oxidation heat-treatment method. When the argon atmosphere pressure was 5 torr (5000 microns) and above, the desired alumina scale could be steadily established on the HAYNES® 233® alloy.
Table 4 below reports the average thickness of alumina scale formed on five commercially available Ni-base alloys in as-produced condition after pre-oxidized in 1 atm flowing argon gas for 12 hours at 2000° F., and the specified and measured aluminum and chromium contents in these alloys are listed in the table. The aluminum content of these alloys ranges from 0.8 to 5.0 wt. %; the chromium content ranges from 15.0 to 26.0 wt. %. An aluminum oxide scale formed on all of these alloys containing 1.8-5.0 wt. % aluminum contents except for 617 alloy which contains 0.8-1.5 wt. % aluminum. The data in Table 4 shows no relationship between the amount of chromium in the alloy and the thickness of the aluminum oxide scale produced by our method. Therefore, we expect that the amount of chromium can be at least 10 wt. % and as high as 35 wt. % or higher provided that there is at least 1.8 wt. % aluminum content in Ni-base alloys.
Referring now to
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a continuous oxide scale (the dark gray layer on the top of the cross-section of the specimen in the SEM image of the
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a continuous oxide scale (the dark gray layer on the top of the cross-section of the specimen in the SEM image of the
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a continuous oxide scale (the dark gray layer on the top of the cross-section of the specimen in the SEM image of the
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a continuous oxide scale (the dark gray layer on the top of the cross-section of the specimen in the SEM image of the
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a continuous oxide scale (the dark gray layer on the top of the cross-section of the specimen in the SEM image of the
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a top chromium oxide (Cr2O3) scale and a continuous subsurface aluminum oxide (Al2O3) layer, along with internal aluminum oxide precipitates. Essential aluminum oxide (Al2O3) scale was not established.
After the pre-oxidation heat-treatment, the surface of the specimen was covered by a top chromium oxide (Cr2O3) scale, along with a subsurface aluminum oxide (Al2O3) layer and precipitates. Essential aluminum oxide (Al2O3) scale was not established.
From the pre-oxidation heat-treatment experiments described above we can conclude that the methods we have disclosed can produce a continuous aluminum oxide scale having an average thickness of at least 100 nm on surfaces of workpieces made from a Ni-base alloy containing 1.8 wt. % minimum aluminum (Al) and 10 wt. % minimum chromium (Cr). This continuous oxide scale can consist essentially of aluminum oxide (Al2O3). Workpieces that have on at least one of their surfaces an aluminum oxide scale having a minimum thickness of 100 nm have enhanced oxidation and corrosion resistance.
Our method can be performed using a flowing argon atmosphere with the control of a minimum vacuum level of 500 microns, preferably at least 5000 microns (5 torr), up to one atmosphere pressure. The argon atmosphere should contain less than 0.1 vol. % oxygen content, and a wet argon gas is not preferred to use. The argon atmosphere flux is determined to be 1 mL/min·cm2 minimum, and the preferred flux is 5 to 25 mL/min·cm2.
The pre-oxidation heat-treatment temperature is between 1400° F. and 2150° F., and preferably at the temperature range of 1700° F. to 2100° F.
The pre-oxidation heat-treatment time is determined by the heat-treatment temperature and desired aluminum oxide scale thickness. An example was 8 hours at 2000° F. to yield a 300 nm-thick aluminum oxide scale. The pre-oxidation heat-treatment time is also alloy dependent.
While we have shown and described certain preset preferred embodiments of our method, it will be understood by those skilled in the art that other modifications may be made therein without departing from the scope of the disclosure and within the following claims.
| Number | Date | Country | |
|---|---|---|---|
| 63589359 | Oct 2023 | US |
