U.S. patent number 3,883,944 [Application Number 05/426,867] was granted by the patent office on 1975-05-20 for method of preparing oxidation resistant materials and structures.
This patent grant is currently assigned to Chrysler Corporation. Invention is credited to Gordon E. Allardyee, Claude Belleau, Royal E. Davis, Amedee Roy.
United States Patent |
3,883,944 |
Davis , et al. |
May 20, 1975 |
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.
Inventors: |
Davis; Royal E. (Farmington,
MI), Roy; Amedee (Troy, MI), Belleau; Claude (Troy,
MI), Allardyee; Gordon E. (Dearborn Heights, MI) |
Assignee: |
Chrysler Corporation (Highland
Park, MI)
|
Family
ID: |
26981668 |
Appl.
No.: |
05/426,867 |
Filed: |
December 20, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
318785 |
Dec 27, 1972 |
3807030 |
|
|
|
Current U.S.
Class: |
29/460; 29/527.2;
228/220; 228/219; 427/377 |
Current CPC
Class: |
C23C
10/30 (20130101); F28F 21/084 (20130101); B23K
20/2275 (20130101); Y10T 29/49888 (20150115); Y10T
29/49982 (20150115) |
Current International
Class: |
C23C
10/30 (20060101); C23C 10/00 (20060101); F28F
21/00 (20060101); B23K 20/22 (20060101); B23K
20/227 (20060101); F28F 21/08 (20060101); B23p
003/00 (); B23p 019/04 () |
Field of
Search: |
;29/470,494,498,502,460,527.2 ;117/131,17.2P,22 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Smith; Al Lawrence
Assistant Examiner: Joyce; Margaret M.
Attorney, Agent or Firm: Talburtt & Baldwin
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.
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.
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 ______________________________________
* * * * *