Method of preparing oxidation resistant materials and structures

Abstract

Preparation of iron-base alloys, particularly in the form of regenerator cores and other similar matrices, by the codiffusion of aluminum and chromium, using aluminum-iron alloy powder and chromium, aluminum-iron alloy powder and chromium-iron powder or Al--Cr alloy powder as sources of the aluminum and chromium and an atmosphere of mixed H.sub.2 and HF to accomplish in situ formation of the aluminum and chromium and their diffusion, and alloying with the iron-base alloy. Assemblies may be bonded to form an integrated structure along with the heating for diffusion of aluminum and chromium.

Parent Case Text

This is a division of application Ser. No. 318,785 filed Dec. 27, 1972, now U.S. Pat. No. 3,807,030.

Description

BACKGROUND

This invention relates generally to materials and to matrix structures of oxidation resistant iron-base alloys. The term iron-base alloy is used herein to define low carbon mild steel and similar iron-base alloys. This invention relates to a method of diffusing aluminum and chromium into iron-base alloys and iron-base matrix assemblies and the simultaneous bonding of iron-base alloy assemblies to form integral structures. The invention is specifically directed to regenerator cores for turbine engines although it is applicable to similar matrix structures wherein low carbon, mild steel and iron parts form various passageways, the walls of which are to be diffusion alloyed with chromium and aluminum and the parts of which are to be bonded together. The term mild steel or low carbon steel is commonly used and is used herein to describe well-known steels, particularly commercial steels, containing less than about 0.25 percent by weight carbon, balance iron and the usual impurities. Examples of some commercial low carbon irons are Armco Supersoft (Armco Steel Co.), Bethnamel (Bethlehem Steel Corp.) and Vitrenamel (United States Steel Corp.). An example of a low carbon mild steel is USS Steel Foil (United States Steel Corp.).

Numerous methods have been investigated as a means of producing oxidation resistant iron-base alloys by the diffusion of chromium and/or aluminum. Unfortunately, most of these techniques have not been too successful, particularly in developing satisfactory matrix constructions, such as gas turbine regenerator assemblies. The long narrow passages of such regenerators and similar matrix constructions promote an uneven distribution of the alloying elements resulting in unsatisfactory heat resistant structures. Furthermore, a vapor phase diffusion process is unacceptable because of the high pressure drop across such honeycomb type matrix structures. Metallic vapors are found to deposit preferentially on the entering surfaces resulting in eventual plugging of the passage and poor distribution of the metals carried by the vapor.

Other difficulties in accomplishing diffusion exist due to the nature of the specific materials used, i.e., the aluminum and chromium. For example, providing oxidation resistant iron-base alloys by chromium diffusion requires high chromium levels which ordinarily results in sigma formation and in the formation of other brittle Fe--Cr compounds when the material is used in high temperature environments as are turbine engine regenerators. On the other hand, aluminum tends to form brittle alloys when diffused alone into iron.

Attempting to simultaneously diffuse metallic aluminum and metallic chromium has been unsatisfactory heretofore also. Chromium requires high temperatures in excess of about 1,200.degree. F. to initiate diffusion. At such a temperature, metallic aluminum wets the workpiece surface and prevents the diffusion of the chromium into it.

In addressing itself to these problems, the present invention uses source materials for the aluminum and chromium which in combination with a certain atmosphere form proper amounts of aluminum and chromium in situ for codiffusion thereby overcoming many of the problems typically associated with the diffusion of these elements.

SUMMARY OF THE INVENTION

This invention makes use of a novel approach in order to codiffuse aluminum and chromium and thereby provide oxidation resistant material. In such an approach, the source of the diffusing metals (aluminum and chromium) is placed in close proximity to the substrate. A slurry technique has been found to be very successful in this invention as a means of distributing the source of materials, directly on a substrate, such as the surfaces of a matrix assembly in the form of a regenerator core and in producing good alloying and bonding of the parts thereof by the diffusion of the aluminum and chromium as provided herein. Both chromium and aluminum are formed in situ at the substrate and diffused into the substrate material during a heating cycle in the presence of hydrogen and HF gas. The source of the aluminum is an iron-aluminum or chromium-aluminum alloy while the source of chromium may be chromium per se, an aluminum-chromium alloy or an iron-chromium alloy. Hereinafter when reference is made to "chromium" or Cr as a source material it should be taken to include not only the metal per se but Cr--Fe and Cr--Al alloys as well as mixtures of Cr and Cr--Fe or Cr--Al alloys.

It is a general object of this invention to diffuse aluminum and chromium into iron-base alloys, such as low carbon mild steel and low carbon iron materials, by a new and improved method wherein the actual diffusion elements, aluminum and chromium, are formed in situ.

It is also an object to thereby provide oxidation resistant materials and matrix structures from relatively low cost materials, namely, low carbon iron and mild steels, by such codiffusion.

It is a specific object to provide low cost oxidation resistant regenerator cores for turbine engines.

It is an object to provide a method wherein processing techniques of reasonable cost may be used for making oxidation resistant materials and matrix structures from low carbon iron or mild steel.

It is also an object to simultaneously bond a matrix assembly into an integral structure during diffusion.

It is still another object to use iron-aluminum or chromium aluminum alloys as a source of aluminum and chromium, chromium aluminum alloys or iron-chromium alloys as source materials of chromium for diffusion into low carbon iron or mild steel to provide oxidation resistant materials and structures.

It is another object to provide useful oxidation resistant materials from low carbon iron or mild steel.

It is also an object to provide regenerator cores of a novel relatively inexpensive material.

These and other objects of the invention will become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a regenerator core for a turbine engine and indicating the matrix structure thereof;

FIG. 2 is an end view of FIG. 1;

FIG. 3 is a fragmentary enlarged plan view of a portion of the matrix illustrated in FIG. 1 showing the bonded joints thereof;

FIG. 4 is a graph illustrating the effect of atmosphere flow rate on the method of the invention;

FIG. 5 is a graph illustrating the effect of temperature and time on the method of the invention;

FIG. 6 is a graph illustrating the effect of slurry composition on the method of the invention with Fe--Al + Cr as the source material;

FIG. 7 is a graph illustrating the effects of the diffusion atmosphere composition on the method of the invention with Fe--Al + Cr as the source material;

FIG. 8 illustrates and classifies the oxidation resistance of various Al--Cr materials at 1,400.degree. F. in circulating air, the results being expressed in terms of weight gain due to oxidation;

FIG. 9 is a graph illustrating the oxidation resistance of various portions of a specific regenerator matrix sample, "93.5 percent Recovery" meaning that 93.5 percent of the slurry materials diffused and alloyed;

FIG. 10 is a graph illustrating variations in slurry retention during dipping in terms of withdrawal rate, "SWG" meaning "slurry weight gain" as a result of dipping;

FIGS. 11 and 12 are graphs illustrating the variations in slurry retention with changes in viscosity for several binder compositions, P & S meaning Pierce and Stevens Co;

FIGS. 13 and 14 illustrate slurry distribution through the cross-section of a core sample resulting from dipping and its effect on resultant composition therethrough;

FIG. 15 is a graph illustrating oxidation resistance of the cold and hot faces of a regenerator core sample according to the invention having compositional variation, the alloy distribution curves being plotted on the lower ordinate, the centered curve being an oxidation weight gain curve plotted on the upper ordinate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While this invention is applicable to oxidation resistant materials comprised basically of iron-base alloys and to the preparation of such materials, particularly the fabrication of matrix structures of such materials, it will be described in connection with one type of such matrix structure, the preferred embodiment herein, to which it is particularly applicable i.e., turbine engine regenerator cores of the type shown in FIGS. 1, 2 and 3. Such a core, when complete, consists of a rim 10, a hub 12 and a matrix portion 14, which is best shown in detail in FIG. 3. As can be seen from FIGS. 1 and 2, the regenerator is a relatively flat, round structure with a plurality of passageways extending through the matrix for the flow of gases. The passageways in the particular design shown are formed by alternately positioned corrugated layers of low carbon iron stock and flat layers of low carbon iron stock. Other variations and designs are known. To form an integral structure these parts are bonded together and lastly, the rim and hub are attached. The method of the invention is preferably performed on the matrix of the core prior to the attachment of the rim and hub.

Referring specifically to the method of the invention, the codiffusion of aluminum and chromium into low-carbon iron material is most efficient at high temperatures when the metals to be diffused are in close proximity or immediate physical contact with the iron base workpiece. In the method of this invention the source materials are aluminum-iron and either chromium or chromium-iron or the source materials are aluminum-chromium alloys, which are placed in contact with the workpiece and heated at high temperatures in a certain reducing atmosphere, herein also termed a diffusion. These source materials are believed converted in the atmosphere to fluoride salts and then reduced to the metallic state at the surface of the iron workpiece. The diffusion atmosphere consists essentially of hydrogen (H.sub.2) and hydrogen fluoride (HF) gases which together promote the requisite fluoride salt formation and subsequent in situ reduction to the respective metallic constituents, i.e., aluminum and chromium.

During the high temperature heating in the diffusion atmosphere, the following reactions, among others, are believed to take place more or less concurrently.

Reactions

2 Cr + 6HF .fwdarw. 2CrF.sub.3 + 3H.sub.2

2 al + 6HF .fwdarw. 2AlF.sub.3 + 3H.sub.2

2crF.sub.3 + Cr .fwdarw. 3CrF.sub.2

CrF.sub.3 + Al .fwdarw. CrF.sub.2 + AlF

AlF.sub.3 + 2Al .fwdarw. 3AlF

2crF.sub.3 + H.sub.2 .fwdarw. 2CrF.sub.2 + 2HF

3crF.sub.2 + 3H.sub.2 .fwdarw. 3Cr + 6HF

CrF.sub.2 + 2Al .fwdarw. Cr + 2AlF

2alF + H.sub.2 .fwdarw. Al + 2HF

The first two reactions promote the formation of fluorides. The other reactions indicate the reduction of these fluorides and intermediate fluorides by either hydrogen or the metals. Furthermore, the last three reactions show the actual deposition of chromium and aluminum on the surface of the workpiece. The regeneration of hydrogen and hydrogen fluoride and the deposition of chromium and aluminum on the workpiece surface favor completion of these reactions. With the use of a static atmosphere during the heating cycle, the volatile constituents are better retained within the passages of a matrix, such as the preferred embodiment, to allow reaction with the low carbon mild steel stock. In this connection reference is made to FIG. 4 which demonstrates that the amount of materials diffused increases as flow rate decreases and is best when the atmosphere is static. The amount of diffused material is expressed in terms of the approximate percent reacted and may include minor amounts trapped in some passages but not actually reacted. This is also referred to as the percent recovered.

Pure aluminum is an active reducing agent. If used in its elemental form, it will result timewise in the premature reduction of the chromium fluorides to metallic chromium and monoaluminum fluoride at too low a temperature for the effective diffusion of aluminum into the iron. For this reason, among others, this invention substitutes aluminum alloys for pure aluminum as a source material. The preferred iron-aluminum alloy, preferably 1:1, for example, is much less reactive and has a much higher melting point than aluminum alone. With iron-aluminum, premature reaction at low temperature is delayed until a more favorable temperature is reached and consequently a higher aluminum and chromium alloy content is produced during heating and diffusion according to the method of this invention.

It has been determined by testing that the preferred method for contacting the source materials and the workpiece comprises dipping the workpiece into a slurry containing the suspended source materials. The following procedure is typical in the preparation of matrix assemblies according to this invention.

Procedure

1. Decarburizing -- In the case of materials and assemblies using mild steel, carbon removal is usually necessary. This may be accomplished by placing the material or assembly into a suitable heat resistant container. Exposure to wet H.sub.2 for about 11/2 hour at about 1,600.degree. F. usually accomplishes decarburization. In the case of an assembly, an additional advantage is provided due to the bonding (referred to hereinbelow as a prebonding simple diffusion heat treatment) which occurs under these conditions. When iron is used this step is not necessary.

2. A preferred slurry consists, for example, of iron-aluminum (1:1 alloy composition, by weight) powder and chromium powder mixed in the 4:5 ratio by weight and suspended in a binder such as Pierce & Stevens Binder No. 9658, which is a solution of an acrylic resin in toluene. Additions of aluminum palmitate may be used to control the viscosity.

3. The matrix assembly is put together and clamped, brazed, decarburized, or the like, for temporarily holding it together. It may be cleaned and then coated with the above slurry, preferably by dipping and preferably to obtain a weight gain of about 20 - 30 percent. Regenerator core samples made from 0.002 inch stock exhibiting a weight gain of about 5 mg/cm.sup.2 of surface area were found to be acceptable for turbine engine use.

4. The assembly is next sealed in a suitable container which is placed in a furnace and heated up to about 700.degree.-800.degree. F. under a flow of argon substantially to remove the binder vehicle.

5. Thereafter, a diffusion atmosphere of hydrogen and hydrogen fluoride (about 1 percent hydrogen fluoride by volume, balance hydrogen, is preferred although about 1-5 percent is acceptable) is introduced into the container. This can be achieved by long time purging or by evacuating the container and refilling with the H.sub.2 --HF atmosphere.

6. A positive pressure (about 4 to 6 inches of oil on an oil manometer) is preferably maintained in the container during the heating and diffusing-bonding cycle, which is, for example, preferably about 2 hours at a holding temperature of about 2,000.degree. F. for stock having a thickness on the order of 0.002 inches.

7. After cooling to below about 1,000.degree. F., the container is purged with argon until room temperature is reached.

8. The assembly may be weighed upon removal from the container, after loose residue has been blown out, to determine the amount of source materials used. This was the basis for the data in the graphs of FIGS. 4, 6 and 7 for percent reacted or recovered.

9. Fluoride residues may subsequently be eliminated by heating the assembly for approximately 1 hour at temperatures above 1,700.degree. F. in a wet hydrogen atmosphere.

This procedure has been found to effectively alloy low carbon iron with aluminum and chromium by diffusion and also to bond matrix assemblies together into integral structures as shown in FIG. 3, for example.

Source Materials

Cr--Al alloy of 30% Cr, balance Al has been used. Commercially available alloys such as 15Cr-- 85Al, 20Cr--80 Al and 66Cr-- 34Al may be used also. If the alloy is to be prepared as a powder for use in a slurry the 15-60% Cr, balance Al should be used because it is brittle and easily powdered.

Fe--Cr alloy of 67.2% Cr -- balance Fe, a commercial alloy has been used. Low carbon, low silicon ferrochromes are desirable in which the carbon is less than about 0.10 percent, the silicon is less than 2 percent and the chromium runs about 64-75 percent.

Fe--Al alloy of 50% Fe -- 50% Al has been used. It is a commercially available material. Use of an alloy of about 45 - 70% Al provides one which is rather brittle and easily powdered for use in a slurry. The 50--50 alloy typically sold for use in permanent magnets is satisfactory.

Evaluation of Samples

Oxidation resistance at 1,400.degree. F. is the principal quality criterion of workpieces treated by the method of this invention. Samples were tested at that temperature in a circulating air furnace. Their weight gain, in milligrams per square centimeter, was recorded for 2, 24, 48, 100, 500, 1,000 and 2,000 hours. They were also examined metallographically to determine their condition and mode of failure if any. Cycling from room temperature to about 1,400.degree. F. was obtained by withdrawing all the samples from the oxidation testing furnace when some of them were to be weighed. In the case of uniform oxidation, a weight gain rate of 0.5 mg/cm.sup.2 /100 hours appeared to be an acceptable maximum limit.

In the case of regenerator core samples, bonding of the corrugated and flat stock forming the regenerator core assemblies was evaluated qualitatively, under a low power microscope, by mechanical "prodding" at the joints with a suitable tool, such as a pick. Metallographic examination was also used to determine the depth of diffusion, detect any anomaly of the microstructure and confirm the quality of the bonds. The actual chromium and aluminum content of representative samples was determined both by wet chemical analysis and X-ray fluorescence analysis.

Results

The following results list a selection of those typically obtained under different conditions for sample regenerator cores of the type shown in FIGS. 1, 2 and 3. Of course, the method is applicable to other matrix structures such as those shown in Hubble U.S. Pat. No. 3,532,157 and Topouzian U.S. Pat. No. 3,391,727 and the so called "sunburst" core. The diffusion treatment of this invention is believed to result in the sintering of any unreacted metal powder to the inside of the matrix passages. The calculated "percent reacted" figures, therefore, include residual source materials, which do not contribute to the oxidation resistance of the substrate. However, these figures, as the weight percent of slurry constituents retained in the regenerator after the diffusion cycle, were found to be a good indication of the efficiency of the process. When percent reacted or "recovered" is over 90 percent, a slurry weight gain of 23 percent has been found to yield satisfactory oxidation results. Because of the sintering of residual powder, analysis results are only a measure of total chromium and aluminum and not of the effective amount of alloying elements.

Since the strength of diffusion bonds is a function of many variables including the pressure at the points of contact, evaluation of bonding on regenerator core samples, when segments of wound cores are used, is only partially representative of the bonding of a full-size unit. In wound cores, the flat and corrugated strips, forming the matrix assembly, tend to maintain a uniform contact pressure between the successive layers. In addition, diffusion of chromium and aluminum into the core matrix sufficient to make it oxidation resistant results in a physical growth of 0.75 to 1.0 percent in the assembly size. This increase in volume promotes a higher contact pressure of the stock at processing temperature and favors more effective bonding.

The samples referred to herein were segments cut from 10 inch diameter cores previously diffusion bonded (without Al--Cr diffusion) to prevent the various layers of flat and corrugated stock from coming apart during sectioning and subsequent processing. Samples were approximately 1 by 1 inch in section and the length was the full thickness of an actual regenerator core, i.e., 3.5 inches.

The terms percent Al Top and Bottom and percent Cr Top and Bottom refer to the actual Cr and Al contents by analysis of the top or bottom one-half inch of the samples. Top and bottom refer to the position of the sample when dipped in the slurry.

TABLE I __________________________________________________________________________ % % % % 100 hrs. Oxid. at 1400.degree.F. Sample Source % Cr Cr Al Al Wt. gain mg/cm.sup.2 No. Conditions Materials SWG* Recovery** Top Bottom Top Bottom Top Bottom __________________________________________________________________________ 9721 2 hrs/2000.degree. FeAl + Cr 26.8 89.8 6.4 5.3 -- -- 0.107 0.108 static 1% HF, (3:2 by balance H.sub.2 weight) 9722 2 hrs/2000.degree.F. FeAl + Cr 27.2 94.0 7.8 6.1 0.085 0.086 static 1% HF, (1:1 by balance H.sub.2 weight) 9921 2 hrs/2000.degree.F. FeAl + Cr 30.4 94.2 8.6 11.0 2.8 2.8 0.175 0.098 static 1% HF, (4:5 by balance H.sub.2 weight) 9924 2 hrs/2000.degree.F. FeAl + Cr 29.0 93.6 11.6 12.9 3.5 4.4 0.140 0.014 static 1% HF, (4:5 by balance H.sub.2 weight) __________________________________________________________________________ *SLURRY WEIGHT GAIN i.e., gain in wt. of sample after dipping in slurry. **% Recovery-increase in sample weight divided by SWG and indicates the amount of aluminum and chromium retained in sample

TABLE II __________________________________________________________________________ % % % % 100 hrs. Oxid. Sample Source % % Cr Cr Al Al wt. mg/cm.sup.2 No. Conditions Material SWG Recovery Top Bottom Top Bottom Top Bottom __________________________________________________________________________ 9995 2 hrs/2000.degree.F. FeAl + Cr 29.5 94.1 9.5 7.4 3.9 2.9 0.079 0.084 static 2% HF, (4:5 by balance H.sub.2 weight) 9996 2 hrs/2000.degree.F. FeAl + Cr 28.8 95.1 8.6 10.1 2.4 2.9 0.074 0.091 static 2% HF (4:5 by balance H.sub.2 weight) 10180 2 hrs/2000.degree.F. FeAl + Cr 29.8 96.3 13.0 11.4 0.171 0.079 static 2.6% (4:5 by HF, balance H.sub.2 weight) 10181 2 hrs/2000.degree.F. FeAl + Cr 27.3 95.9 8.8 9.7 0.088 0.083 static 2.6% HF, (4:5 by balance H.sub.2 weight) 10361 2 Hrs/2000.degree.F. FeAl + Cr 22.6 94.3 6.000 0.471 static 0.5% HF, (4:5 by balance H.sub.2 weight) 10250 2 hrs/2000.degree.F. FeAl + Cr 19.1 93.5 9.0 5.3 3.3 2.1 0.062 0.142 static 1% HF, (4:5 by balance H.sub.2 weight) 10471 2 hrs/2000.degree.F. FeAl + Cr 28.8 95.3 9.5 6.8 0.062 0.079 static 1% HF, (4.5 by balance H.sub.2 weight) __________________________________________________________________________

TABLE III __________________________________________________________________________ 100 hrs. Oxid. Sample Source % % % % wt. mg/cm.sup.2 No. Condition Materials SWG Recovery Cr Al Bottom __________________________________________________________________________ 8576 4 hrs/1900.degree.F flowing FeAl+ Cr 24.1 35.6 3.7 6.0 0.189 H.sub.2 + 60 sec. pure HF (1:3 by at temp. weight) 8436 4 hrs./1900.degree.F. flowing FeAl + Cr 49.9 70.0 13.6 2.6 0.065 2% HF + 2 sec. pure HF (1:3 by at temp weight) 8425 4 hrs/1900.degree.F. FeAl + Cr 40.0 51.5 11.2 4.0 0.079 flowing 2% HF + 5 sec. (1:2 by pure HF at temp. weight) 8713 4 hrs/2000.degree.F. flowing FeAl + Cr 28.7 79.2 8.7 1.2 0.097 2% HF + 10 min. 53 sec. (1:4 by pure HF weight) __________________________________________________________________________

TABLE IV __________________________________________________________________________ % % % % 100 hrs. oxid. Sample Source % % Cr Cr Al Al wt. mg/Cm.sup.2 No. Conditions Materials SWG Recovery Top Bottom Top Bottom Top Bottom __________________________________________________________________________ 9632 4 hrs/2000.degree.F. FeAl + Cr 28.9 92.3 10.4 8.4 4.0 4.3 0.190 0.115 static 5% HF, (4:5 by balance H.sub.2 weight) 9637 4 hrs./2000.degree.F. FeAl + Cr 30.6 93.8 13.9 7.2 4.2 4.2 0.145 0.082 static 1% HF, (4:5 by balance H.sub.2 weight) 9641 4 hrs./2000.degree.F. FeAl + Cr 28.1 87.7 9.1 8.3 0.335 0.350 static 1% HF, (3:5 by balance H.sub.2 weight) 9672 2 hrs./2000.degree.F. FeAl + Cr 31.1 92.4 9.7 7.7 0.083 0.086 static 1% HF, (4:5 by balance H.sub.2 weight) __________________________________________________________________________

TABLE V __________________________________________________________________________ % % % % 100 hrs. Oxidation Sample Source % % Cr Cr Al Al wt. mg/cm.sup.2 No. Conditions Materials SWG Recovery Top Bottom Top Bottom Top Bottom __________________________________________________________________________ 10806 2 hrs./2000.degree.F. CrAl 22.2 74.3 3.2 2.9 6.2 6.5 0.042 0.013 static 1% HF (30-70) + Bal H.sub.2 10807 2 hrs./2000.degree.F. CrAl 16.0 79.3 0.015 0.014 static 1% HF (30-70) + Bal H.sub.2 10808 2 hrs./2000.degree.F. CrAl 20.8 88.3 0.012 0.014 static 1% HF (30-70) + Bal H.sub.2 10809 2 hrs/2000.degree.F. CrAl 22.7 84.7 5.5 4.1 4.9 9.1 0.015 0.020 Static 1% HF + (30-70) Bal H.sub.2 10893* 2 hrs./2000.degree.F. CrAl 14.6 79.4 5 5 10 10 0.010 0.033 static 1% HF (30-70) + Bal H.sub.2 __________________________________________________________________________ Note: Resulting alloy may be found too brittle for some uses due to large amount of aluminum relative to the chromium

TABLE VI __________________________________________________________________________ 100 hr. oxidation Sample Source % % wt. mg/cm.sup.2 No. Conditions Materials SWG Recovery Top Bottom __________________________________________________________________________ 10339 2 hrs./1900.degree.F. FeAl+FeCr* 28.6 92.1 static 1% HF (4:7.15) +Bal H.sub.2 10363 2 hrs./2000.degree.F. FeAl+FeCr static 0.5% HF (4:7.15) 26.8 90.8 0.81 +Bal H.sub.2 10376 2 hrs./2000.degree.F. FeAl+FeCr 29.6 89.1 0.862 0.233 static 1% HF (4:7.15) +Bal H.sub.2 10387 2 hrs./2000.degree.F. FeAl+FeCr 30.5 81.9 0.300 0.232 static 1% HF (4:7.15) +Bal H.sub.2 10434 2 hrs./2000.degree.F. FeAl+FeCr 29.4 76.6 static 1% HF (4:7.15) +Bal H.sub.2 10439 3 hrs./2000.degree.F. FeAl+FeCr 28.5 88.1 static 1% HF (4:7.15) +Bal H.sub.2 __________________________________________________________________________ *Note: FeCr (Low Carbon Ferrochrome 67.2% Cr) is preferred as a chromium source because it is less expensive than pure chromium.

Some Variables

With reference to FIGS. 5, 6 and 7, it can be seen that the variables of (1) time and temperature, (2) slurry composition and (3) atmosphere composition are all interrelated and should be considered insofar as optimizing the subject invention for any particular assembly or material concerned.

FIG. 5 illustrates that the depth of diffusion may be obtained in shorter time intervals at higher temperatures with the converse also being true. FIG. 6 illustrates how diffusion appears to be best at a ratio of Fe--Al to Cr of about 4:5 as the relative amount of chromium increases, up to about 4:5, then dropping off.

FIG. 7 indicates that a mix of about 1% HF with H.sub.2 is optimum in the case of source materials of Al--Fe + Cr.

Materials

FIG. 8 demonstrates a preferred composition best suited to the purpose of this invention as can be seen from the parts of the graph which fall into the central square area thereof labeled "best." The area labeled "good" is useful but is more expensive in that larger amounts of chrome are required without any substantial gain in oxidation resistance. Weight gain is of course an indication of the amount of oxidation.

FIG. 9 shows the relative oxidation resistance of the top and bottom faces of a sample regenerator core 4 inches in diameter prepared by codiffusing Al and Cr at 2,000.degree. F. for 2 hours according to the subject invention, the source of Aluminum being Al--Fe.

Slurrying Operations

In the case of regenerator cores, different results may be obtained depending on the slurry operation. Using sample cores of the type shown in FIGS. 1 and 2, wherein one face is a hot face, the other being a cold face because of the difference in temperature to which the faces are exposed in actual use, certain variables were found to provide certain results.

For example, controlled withdrawal of the core from the slurry during dipping was found to affect the amount of slurry retained by the matrix and its distribution therein. Amount gained is shown in FIG. 10, a graph which demonstrates that the percent SWG, i.e., slurry weight gain resulting from dipping, was found to increase with withdrawal rate from the slurry.

Referring to FIGS. 11 and 12, the graphs thereof show that SWG increases with the viscosity of the slurry. Of course, other variables such as corrugation spacing, fold radius and passage uniformity in the matrix along with Al and Cr source particle size also affect SWG. A particle size of 325 mesh has been found to be acceptable in most cases, although this can vary a great deal.

Slurry distribution in the matrix of a core of the type shown in FIGS. 1 and 2 will not be completely uniform and will result in a compositional gradient from top to bottom across the matrix i.e., from one face to the other. This is acceptable and even desirable from an economical standpoint since the regenerator face containing the greater relative amounts of aluminum and chromium is used as the regenerator "hot face" while the other face is used as the "cold face," the cold face not requiring the high temperature properties of the hot face.

FIGS. 13, 14 and 15 and Table VII below demonstrates distribution of chromium and aluminum by incremental segmentational analysis before and after diffusion processing.

Table VII ______________________________________ X-RAY FLUORESCENT ANALYSIS OF Cr-Al REGENERATOR CORES SAMPLE NO. % Cr %Al ______________________________________ (Hot Face) Top 361-1 10.4 3.1 361-2 10.9 3.4 361-3 8.4 3.4 361-4 10.0 4.2 361-5 8.7 3.1 361-6 9.7 4.2 361-7 7.1 3.0 (Cold Face) 361-8 7.8 4.2 ______________________________________ Preferred Procedure for Processing a Full-Size Regenerator Core (17 inche in diameter and 3.5 inches thick)

A slurry is prepared consisting of iron-aluminum and chromium powders in a 4:5 ratio (by weight) suspended in a vehicle of acrylic binder, hexane and toluene in a ratio of 6:5:1 with the addition of 0.25 percent aluminum palmitate. Viscosity is adjusted to about 230 (at 78.degree. F.) centipoises, as measured with a Brookfield Viscometer.

A low carbon iron regenerator matrix assembly, preferably pre-bonded by a simple diffusion heat treatment as described above at "decarburizing" is coated with the above slurry by dipping, involving a controlled withdrawal rate according to FIG. 10 of about 6-8 inches/minute. A cleaning procedure using low pressure compressed air, and blotting is used to remove the resultant drip edge. Drying with warm low pressure air follows. A preferred slurry weight gain of about 25 to 30 percent should be obtained. Adjustment of viscosity may be used to influence retention. After cleaning up excess slurry, any component, such as hub, rim, etc. to be brazed to the core is assembled using a copper flake slurry. The slurried core is then placed in the diffusion container on an Inconel screen coated with "stop-off" and supported on an Inconel grid. Control samples are located at the periphery of the core and a cover made of 316 stainless steel (0.015 inches) is welded on the container which may be Inconel also.

After testing for leaks, the container is purged with argon while being heated to about 700.degree. to 800.degree. F. for a minimum of 2 hours to remove the acrylic binder from the dried slurry. The cooled container is then evacuated to a few millimeters of mercury. The preferred diffusion atmosphere consisting of hydrogen and hydrogen fluoride (1 percent by volume balance substantially H.sub.2) is bled in until internal pressure is back to atmospheric, preferably slightly higher, then the retort is further purged by flowing the gas mixture through it for an additional 15 minutes. The container gas outlet is then connected to an oil manometer to establish a static atmosphere and monitor pressure during the process.

Heating to about 2,000.degree. F. (2,050.degree. F. if copper brazing is to be done at the same time) then proceeds as an average heat-up rate of 400.degree. to 500.degree. F. per hour. Final temperature of about 2,000.degree. F. is held for 2 hours. The static atmosphere is maintained by manipulation of the pressure regulator on the gas mixture cylinder so as to maintain a preferred height of about 4 to 6 inches in the oil manometer. After the holding period, the furnace is turned off and the container is cooled at the highest practical rate. Upon reaching a temperature of approximately 1,000.degree. F, the hydrogen-hydrogen fluoride atmosphere is purged from the container with an inert gas, preferably argon. After cooling to room temperature, the processed core is cleaned of loose residue by blowing with compressed air.

TABLE VIII ______________________________________ STAGES OF A HEATING CYCLE Time Heating Rate Final (Minutes) .degree.F./min. Temp..degree.F. Notes ______________________________________ 0-30 25-30 1000 Predominant action in this portion of 30-60 Rate changes 1400 cycle includes the from 25-30 aluminum reactions, down to formation and dif- about 6 fusion. 60-180 6 2050 Predominant action in this portion of cycle 180-420 0 2050(K) includes the aluminum reactions, formation and diffusion. Cooling 420-510 7 1400 510 16-20 RT ______________________________________

Claims

Having described the invention, the exclusive rights and privileges claimed are defined as follows:

1. The method of preparing oxidation resistant matrix structures alloyed at least in portions thereof with chromium and aluminum, comprising the steps of:

providing an iron-base alloy matrix assembly,

providing a mixture of aluminum source material and chromium source material, the aluminum source material being selected from the group consisting of Al--Fe alloys, Al--Cr alloys, and mixtures thereof the chromium source material being selected from the group consisting of Cr--Fe alloys, Al--Cr alloys, Cr and mixtures thereof at various surfaces of the matrix assembly, and

heating the assembly in an atmosphere of H.sub.2 and HF to an elevated temperature for a time sufficient to effect the diffusion of Al and Cr into the iron-base alloy.

2. The method according to claim 1 wherein the source materials are in the form of powdered metals.

3. The method according to claim 1 wherein the aluminum source material is Al--Fe alloy of about a 1:1 type.

4. The method according to claim 1 wherein the atmosphere is about 1-5% HF by volume, balance essentially H.sub.2.

5. The method according to claim 1 wherein the aluminum source material is Al--Fe alloy and the chromium source material is Cr, the two being present in a ratio of about 1:1.

6. The method according to claim 5 wherein the ratio of Al--Fe to Cr is about 4:5.

7. The method of simultaneously diffusion alloying and bonding a matrix assembly made up for the most part of low carbon steel foil by the codiffusion of aluminum and chromium into the assembled steel parts, comprising the steps of:

dipping the assembly into a slurry comprised of about 1:1 Al--Fe and Cr powders present in about a 4:5 ratio and suspended in a viscous binder vehicle;

removing excess slurry material from the assembly after withdrawing it from the slurry;

placing the assembly in a sealed diffusion container;

purging the container with argon at a temperature of about 700.degree.-800.degree. F. for a period of about 2 hours to substantially remove the binder vehicle;

cooling the container and contents;

introducing a mixed atmosphere of H.sub.2 and about 1% HF by volume into the container;

causing the gas mixture to circulate through the container for about 15 minutes;

establishing a substantially static atmosphere of the mixture in the container;

heating the container contents to about 2,000.degree. F. at an average rate of about 400.degree.-500.degree. F. per hour and holding the 2,000.degree. F. temperature for about 2 hours after it is reached;

cooling the container and contents to room temperature, purging the container with argon at about 1,000.degree. F. during cool-down, and finally removing loose residue remaining on the core.

8. The method of simultaneously diffusing aluminum and chromium comprising:

applying a mixture of aluminum source material and chromium source material to a metal surface, the aluminum source being selected from the group consisting of Al--Fe alloys, Al--Cr alloys and mixtures thereof, the chromium source being selected from the group consisting of Cr--Fe alloys, Al--Cr alloys, Cr and mixtures thereof,

establishing a diffusion atmosphere predominantly of H.sub.2 mixed with minor amounts of HF,

heating the surface in the diffusion atmosphere for a time at a temperature sufficient to form fluoride salts of the aluminum and chromium,

continuing the heating to reduce the salts and form aluminum and chromium metal at the surface and

further continuing the heating to cause the diffusion of the aluminum and chromium into the surface and alloying therewith.