Two-step infiltration in a single furnace run

There is provided a process for infiltrating a compacted ferrous powder metal body with copper or a copper alloy which process is characterized by presintering the ferrous metal body at a temperature of from about 1875.degree. F. to a temperature below the melting point of the infiltrant, and then in the same furnace, raising the temperature above the melting point of the copper or copper alloy infiltrant for a period sufficient to infiltrate the powder metal body. This process is more economical than the prior double run infiltration processes and provides excellent impact strengths and tensile strengths.

This invention relates, as indicated, to an improved infiltration procedure 
for ferrous bodies formed of a powdered iron. Infiltration of the ferrous 
bodies with copper or a copper alloy improves the mechanical properties of 
the ferrous bodies, especially the Charpy impact strength and the tensile 
strength. 
RELATED PATENTS AND APPLICATIONS 
This application is related to commonly owned U.S. Pat. No. 4,606,768 dated 
Aug. 19, 1986 to Svilar et al and to our copending application Ser. No. 
935,854 filed Nov. 28, 1986. These references are incorporated herein by 
reference thereto. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
Conventional ferrous powder metal bodies, or parts, produced by simple 
pressing and sintering, are known to have rather inferior mechanical 
properties; e.g., impact and fatigue strength, because of the presence of 
pores in such bodies. Methods for overcoming these inferior properties are 
based upon achieving full or nearly full density. One method for obtaining 
nearly full density is to infiltrate the bodies with copper or a copper 
alloy, a process which has been in common practice since the 1940's. 
Inspite of the ability to achieve substantially full density by 
infiltration with a suitable infiltrant, only a small improvement in 
dynamic properties has been achieved over uninfiltrated ferrous powder 
bodies. 
The impact strength of ferrous powder metal parts is important for many 
applications, e.g., gears wherein a critical region is at the root of the 
gear teeth with weakness at that point leading to gear failure; and in 
hammers for use in hammer type mill wherein a critical area is the area 
between the head and the shank. Imperfection in this area can lead to 
failure. 
A conventional method for determining impact strength of specimens in the 
Charpy impact test procedure is described in the Metal Powder Industries 
Federation (MPIF) Standard 40, 1974 Metal Powder Industries Federation 
P.O. Box 2054, Princeton, N.J. 08540. In this test, unnotched specimens 
are formed into a defined rectangular shape having specified dimensions 
and are placed in a pendulum-type impact machine with a capacity of at 
least 110 foot pounds (15.2 m-kg). The impact strength is the average of 
three tests reported to the nearest foot pound. Standard 40 is 
incorporated herein by reference thereto. For purposes of the present 
application, the term impact strength, where used, shall mean unless 
otherwise noted, the strength values obtained following the Charpy-type 
test procedure outlined in Standard 40. 
Another mechanical property of interest in the preparation of many ferrous 
powder metal parts is the tensile strength. This property, and the test 
for determining it, are described in MPIF Standard 10, also incorporated 
herein by reference thereto. An aspect of the tensile strength of a powder 
metal part is the elongation of the part that occurs prior to failure. In 
the present application, the tensile strength and elongation shall be 
given (unless otherwise stated) in terms of kips per square inch (ksi or 
thousands of pounds per square inch) and percent elongation (E%), 
respectively, following the procedure of Standard 10. 
Parts made according to state of the art powder metallurgy technology, 
i.e., pressed and sintered or infiltrated, have very low impact 
strengths--typically only 3 to 20 ft. lbs measured by the unnotched Charpy 
Test. Higher impact strength would enable these low cost methods to be 
used for higher performance parts that are now made by alternative 
technologies that are more expensive, i.e., powder metal forging, hot 
pressing, injection molding, etc. 
Copper in iron is known to enable the iron to precipitation harden. Iron 
also can be hardened by adding carbon and heat treating. The use of carbon 
and heat treatment is least expensive and virtually the most common way 
the strengh and toughness of steel is improved. 
Prior patent application Ser. No. 755,282 filed July 15, 1985, now U.S. 
Pat. No. 4,606,768 dated Aug. 19, 1986, assigned to assignee of the 
present application, describes how to improve significantly impact 
strength of copper infiltrated steel by assuring the absence of erosion 
and local porosity (defined statistically in terms of pore volume and 
maximum pore size). Unnotched Charpy impact strengths as high as 130 ft. 
lbs at an ultimate tensile strength of 103 ksi have been obtained. 
Combinations of high impact and high ultimate tensile strength are sought 
in many engineering applications. The disclosure of U.S. Pat. No. 
4,606,768 is incorporated by reference herein. 
State of the art copper infiltration of iron and steel parts uses long 
infiltration times to ensure the most complete infiltration possible and 
improve tensile strength. Typically, the times range from 30 minutes to 90 
minutes, although shorter infiltration times have been reported. For these 
times there is partial alloying of the copper with the iron due to 
dissolving and reprecipitation of the iron because of liquid phase 
sintering as well as solid state diffusion of the copper into the iron. In 
the areas where copper and iron are both present, optimizing heat 
treatment for impact toughness is complicated by both carbon and copper 
hardening mechanisms operating at the same time. 
Several investigators (see U.S. Pat. No. 4,606,768) have attempted to 
obtain higher combinations of impact strength and tensile strength. Some 
of these investigators (i.e., Kuroki et al in 1973, Impact Properties of 
Copper Infiltrated Sintered Iron; Journal Japan Society Powder Metallurgy, 
July, 1973, Vol. 20., pages 71-79) have employed short infiltration times 
but were unable to obtain the desirable and large improvements reported 
herein. 
In application Ser. No. 935,854, supra, the applicants found that it was 
possible to obtain even better combinations of impact strength and tensile 
strength by controlling the microstructure of the infiltrated steel in 
such a way that the diffusion of copper into the steel matrix is kept 
within a certain range. Control of the cleanliness of the steel matrix 
affords an additional improvement. Through combination of the improvements 
of the U.S. Pat. No. 4,606,768 and those disclosed in Ser. No. 935,854 it 
was possible to obtain impact strengths (unnotched Charpy) of over 240 ft. 
pounds, and ultimate tensile strengths of over 100 ksi. Also, with the 
improvement of Ser. No. 935,854, it was found possible to obtain unnotched 
Charpy impact strengths of 50 ft. pounds at a tensile strength of over 100 
ksi at a low overall density of about 7.55 g/cm.sup.3. At such low overall 
density, conventional processing typically gives an unnotched Chapy impact 
strength of less than 20 ft. lbs. 
The invention of Ser. No. 935,854 also provides an infiltrated ferrous 
powder metal body infiltrated with copper or a copper alloy characterized 
as having after infiltration an overall density of at least 7.5 g/cm.sup.3 
and a diffusion depth of copper into the steel matrix of less than about 4 
micrometers as determined by chemical etching or less than about 8 
micrometers as determined by electron dispersive X-ray analysis (EDXA). 
An important aspect of the invention of Ser. No. 935,854 conducive to 
staying within the diffusion depth parameters stated above is employing as 
the powder metal an iron powder having a carbon content in the range of 
about 0.3 to about 1.4%, based on the weight of the copper-free iron 
skeleton. The percent carbon is the amount by weight added to the iron 
powder for preparing a so-called "green part". During sintering and 
infiltrating, a portion of this carbon is lost due to the formation of 
carbon oxides, the oxygen content of the iron powder being the source of 
the oxygen. Carbon may also be lost through the formation of hydrocarbons 
with any hydrogen used in the sintering atmosphere. Typical losses amount 
to about 0.1 to 0.2% based on the copper-free steel skeleton. 
In U.S. Pat. No. 4,606,768 as well as Ser. No. 935,854, we have taught how 
to obtain excellent combinations of impact and tensile strengths by 
various infiltration processes that produce specific structures. Examples 
in Ser. No. 935,854 show that a "two-step" process (sintering followed by 
infiltrating) gave properties significantly better than the "single-step" 
(sintering and infiltrating performed simultaneously) process. For 
example, Examples 13 and 15 in Ser. No. 935,854 gave impact strengths of 
64 and 90 ft. lbs. and tensile strengths 100 to 110 ksi whereas two-step 
processing gave impact strengths of 200 ft lbs. or more at similar tensile 
strengths. 
In this application, for purposes of clarity, reference will henceforth be 
made to "single run" and "double run" to indicate whether one or two 
separate furnace operations are used. The term "step" will be reserved to 
indicate whether one or more distinct temperature plateaus are used in any 
individual furnace run. 
BRIEF STATEMENT OF THE INVENTION 
Briefly, stated, the present invention is in a process for infiltrating a 
compacted ferrous powder metal body, compacted to a density of at least 
about 70% of theoretical density, with copper or a copper alloy at an 
infiltrating slug-to-metal matrix ratio of from 14% to about 55% by 
weight, said powder metal body after infiltration having a carbon content 
of from about 0.15% to about 1.25% by weight, and an infiltrated density 
of from about 7.5 g/cm.sup.3 to about 8.2 g/cm.sup.3, the improvement 
which comprises the steps, carried out in a single furnace run, of 
presintering the ferrous powder metal body at a temperature of from about 
1875.degree. F., to below the melting point of the infiltrant for a period 
of from about 5 to about 60 minutes, and then infiltrating the presintered 
body with copper or a copper alloy by raising the temperature in said 
furnace to a temperature above the melting point of the copper or copper 
alloy infiltrant for a period of from about 5 to about 90 minutes. This 
procedure results in a substantial reduction in cost and the attainment of 
similar or better Charpy impact values and tensile strength properties. 
DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EXAMPLES 
Conventional double run infiltration consists of sintering the powder metal 
(ferrous) matrix in a furnace at a temperature of about 2050.degree. F. to 
2150.degree. F. and then infiltrating during a second furnace run. 
Sintering the ferrous powder body before infiltration produces an only 
slightly stronger body because of the stronger bonding of the iron 
particles. Thus, this double run process of sintering and then 
infiltrating has been virtually replaced by a single, single step or 
"sintration" process because of reduced cost and similarity of properties 
achieved by the two processes. In the ordinary single run infiltration, 
the ferrous powder body is infiltrated without prior high temperature 
sintering. 
The applicants have found a means for improving the impact strength of 
single run and double run infiltrated parts. Single run "sintrated" parts 
in the past gave unnotched Charpy impact strengths of 60 to 90 ft. lbs., 
while double run infiltrated parts have impact strengths greater than 240 
ft. lbs. There is thus a clear incentive to use the more expensive double 
run process. 
As indicated above, the applicants have now found that infiltration can be 
made less costly by sintering and infiltrating in a two-step single 
furnace run. This can be achieved by sintering the matrix below the 
melting point of the infiltrant to form strong bonds between the ferrous 
particles, and then, without removing the part from the furnace, raising 
the temperature in such furnace to the infiltrating temperature. Although 
the bonds between the matrix particles do not form as readily because the 
sintering temperature used is lower than typical, surprisingly high impact 
values were found when this invention was used in connection with high 
performance infiltration technology. 
Further improvements in the process are obtained by adding sintering 
agents, e.g., boron and phosphorus, to the matrix material in amounts 
ranging from about 0.01% to about 0.1% by weight, which have the effect of 
enabling more complete sintering at temperatures below about 1875.degree. 
F. Also, better results are achieved by adjusting the composition of the 
infiltrating alloy so that the infiltrating temperature can be raised to 
allow for more complete sintering to occur. 
Residual porosity after infiltration and maximum pore size of the 
uninfiltrated pores is an important aspect in attaining high impact 
strength. A powder metal iron or steel part infiltrated with a copper or 
copper alloy desirably has, after infiltration an overall density of at 
least 7.5 g/cm.sup.3, a residual uninfiltrated porosity of less than about 
10% by volume, and a maximum pore size of the uninfiltrated pores of less 
than about 120 micrometers, wherein both values are taken from a worst 
field of view of a functionally critical area. Within these parameters, 
Charpy impact strengths greater than 50 foot pounds and a high ultimate 
tensile strength of more than 49,000 pounds per square inch are obtained. 
These values are obtained in the as-infiltrated condition prior to any 
heat treatment. 
For purposes of the present invention, the critical area is defined as that 
area adjacent a fractured surface of an infiltrated part subjected to 
failure obtained by clean cutting-off the fractured surface and polishing 
the cut area. The worst field of view is obtained by viewing and analyzing 
a plurality of views of the cut polished surface. In the present 
invention, 50 fields of view are analyzed to obtain a worst field of view. 
Residual uninfiltrated porosity and maximum pore size data is obtained by 
measurement under magnification. The volume percent porosity is obtained 
from the area measurement following a procedure outlined in pages 446-449 
of the National Bureau of Standards Publication 431, dated January 1976 
(incorporated herein by reference thereto). Preferably, the worst field of 
view has a porosity less than about 5 percent and a maximum pore size of 
residual uninfiltrated porosity of less than about 75 micrometers. 
Also, for purposes of the present application, the term "powder metal iron 
or steel" includes as starting materials plain carbon steels, e.g., SAE 
1025, tool steels, e.g., M2, stainless steels, e.g., 316L(AISI), and low 
alloy steels such as 4600. Typical alloying elements may be nickel, 
molybdenum, chromium, silicon and boron. Tool steels may contain such 
elements as vanadium and tungsten. The carbon content may be augmented as 
described below. 
Also, in a preferred embodiment the infiltrant is copper, containing 
typically an alloying constituent such as iron, tin, zinc, silver, 
lithium, silicon, manganese, chromium, zirconium, and combinations thereof 
in amounts generally less than 5% by weight and preferably from 0.1% to 
3%. The amount of any iron should be less than 2.0 to 3.0% by weight. 
It is also desirable for best results to control the microstructure of the 
infiltrated ferrous powder metal body in such a way that the diffusion of 
copper into the iron matrix is kept within a certain range. Control of the 
cleanliness of the ferrous metal matrix provides additional improvement. 
As above indicated, the ferrous powder metal body infiltrated with copper 
or a copper alloy is desirably characterized as having after infiltration 
an overall density of about 7.5 g/cm.sup.3 and a diffusion depth of copper 
into the ferrous powder metal matrix of less than about 4 micrometers as 
determined by chemical etching, or less than about 8 micrometers as 
determined by electron dispersive X-ray analysis (EDXA). 
Preferably, the ferrous powder metal body has a diffusion depth of copper 
or copper alloy of less than about 3 micrometers as determined by chemical 
etching or less than about 5 micrometers as determined by EDXA. 
An important aspect in the present invention conducive to staying within 
the diffusion depth parameters stated above is employing as the powdered 
metal an iron powder having a carbon content in the range of about 0.3 to 
about 1.4%, based on the weight of the copper-free skeleton. The percent 
carbon is the amount (weight percent) of carbon added to the iron powder 
for preparing a so-called green part or body. During presintering and 
infiltrating a portion of this carbon is lost due to the formation carbon 
monoxide and/or carbon dioxide (carbon oxides), the oxygen content of the 
iron powder being the source of oxygen. Carbon may also be lost through 
the formation of hydrocarbons with any hydrogen in the presintering and 
infiltrating atmosphere. Typical losses due to these causes amount to from 
0.1% to 0.2%, or an average of 0.15% based upon the copper-free iron or 
steel skeleton. 
In the Examples herein, the percent carbon, for the purpose of convenience, 
is generally expressed in terms of percent carbon (or graphite) added. 
Accordingly, in terms of combined carbon, i.e., the carbon concentration 
of the finished part based on the amount of the steel matrix only the 
critical range is between about 0.15 percent and about 1.25%. A preferred 
range is about 0.25% to about 1.05%. It is understood that the percent 
carbon added can be in the form of carbon (as graphite) blended in with 
the iron powder or carbon alloyed with the iron. 
Although we desire not to be bound by a particular theory, it is believed 
that these percents carbon inhibit the diffusion of the copper into the 
ferrous metal particles during infiltration, and in so inhibiting 
diffusion, more than offset the negative aspect of the addition of carbon 
on impact strength. Optimum results are obtained with an added carbon 
content of about 0.9%. 
As an alternative to carbon, or in combination with carbon, one can employ 
about 0.1% to about 0.2% of an additive such as boron, which will inhibit 
diffusion in the same manner as carbon. 
The "diffusion depth" of copper is determined by measuring the copper 
concentration at various depths, for instance one, two or three 
micrometers, and plotting the copper concentration data against depth data 
on semi-logarithmic paper. 
The copper concentration is plotted along the linear scale starting from 
zero, and the depth is plotted along the logarithmic scale starting from 
zero. The connection of the experimental points forms approximately a 
straight line and the "diffusion depth" is the point of intersection of 
the straight line with the logarithmic scale at zero percent copper 
concentration. 
In those cases where the iron or steel contains prior to infiltration, a 
more or less uniformly distributed base amount of copper, (a base 
concentration, for instance about several percent copper, can be 
tolerated) the same procedure for determining diffusion depth is employed, 
except that only the experimental points close to the surface of the steel 
particle are used to form said straight line. A new base line is then 
drawn parallel to the logarithmic scale at the level of the base amount of 
copper, and then the point of intersection obtained by extrapolation of 
the straight line with the new base line establishes the "diffusion 
depth". 
The following specific examples are illustrative of the present invention 
and will enable those skilled in the art to prepare yet other examples 
utilizing the teachings of these examples.

EXAMPLE I 
Izod impact specimens made of A1000SP iron powder, 0.9%, Lonza 25 graphite 
and 0.75% zinc stearate as a lubricant were pressed to 7.04 g/cm.sup.3 
density. An IP204LD infiltrant slug weighing 14% of the green specimen was 
placed on one end of the specimen. The sample specimens were sintered and 
infiltrated in a vacuum furnace with the following temperature profile: 
Heat to 1400.degree. F. and hold for 10 minutes. 
Heat to 1900.degree. F. and hold for 60 minutes. 
Heat to 2050.degree. F. and hold to 7 minutes. 
The process of the present invention is desirably carried out in an 
inactive furnace atmosphere, including vacuum. The pressure may range from 
near vacuum to atmospheric or above. Useful furnace atmospheres include, 
vacuum, helium, argon, dissociated ammonia, "synthetic nitrogen", 
nitrogen, hydrogen, carbon monoxide, or mixtures of two or more thereof, 
etc. "Synthetic nitrogen" is formed by thermal decomposition of methanol 
in the presence of variable amounts of nitrogen. Dissociated ammonia is 
formed by the thermal decomposition of ammonia and yields a gas containing 
nitrogen, hydrogen and possibly traces of ammonia. The dew point of the 
atmosphere used should be less than about 35.degree. F. although dew 
points as high as 60.degree. F. may be used. When nitrogen is present in 
the atmosphere used, there is some tendency to react with the iron to form 
nitrides. However, this has been found to be very slight and thus 
insignificant to the results obtained. Also, when carbon monoxide and 
carbon dioxide are present in the gas mixture, the quantities of each 
should be thermodynamically balanced and take into account the carbon 
content of the powdered metal. A preponderance of carbon dioxide can cause 
oxidation of deleterious amounts of iron, whereas an excess of carbon 
monoxide can cause reduction to carbon and effect perhaps harmful change 
in the carbon content. If, however, these gases are thermodynamically 
balanced in the system the deleterious tendencies are cancelled out and 
the gas remains relatively inert. 
The specimens were then austenitized at 1650.degree. F. for 15 minutes and 
tempered at 1300.degree. F. for 60 minutes. The specimens were then 
shortened to the proper length for Charpy unnotched impact strength 
testing. The results for two bars were 107 and 124 ft-lbs. respectively. 
These results fall between the single and double run processes. 
A1000SP has the following analysis: 
Carbon 0.01% 
Sulfur 0.15% 
H.sub.2 loss 0.17% 
Oxygen 0.12% 
Nitrogen 0.0014% 
Phosphorus 0.006% 
Silicon 0.01% 
Manganese 0.16% 
Copper 0.04% 
Nickel 0.04% 
Chromium 0.04% 
A typical analysis of the infiltrant material IP-204LD is as follows: 
Iron 2-3% 
Manganese 0.5-1.5% 
Lubricant 0.5-1.0% 
Other 0.5-1.0% 
Copper Balance. 
Instead of zinc stearate, any thermally decomposable lubricant for aiding 
the compacting operation may be used. Another example is Acrawax C 
(Chemical Abstracts Reg. No. 110-30-5) or 
N,N'-1,2-ethanediylbisoctadecanamide. 
EXAMPLE II 
Samples were prepared as in Example I above and the sintering and 
infiltration were performed sequentially in a single furnace run. In order 
to obtain better control of temperature and time, the experiments were 
performed in a vacuum furnace using distinct temperature/time profiles. In 
this set of experiments, the vacuum furnace was operated under a helium 
pressure of about 300 micrometers of Hg. The heat up rates from room 
temperature to 1900.degree. F. and from 1900.degree. F. to 2050.degree. F. 
were 100.degree. F. per minute and 50.degree. F. per minute, respectively. 
The holding times at 1900.degree. F. and 2050.degree. F. as well as the 
impact properties (after austenitizing, water quenching and tempering at 
1200.degree. F. for 1 hour) are shown in the following Table I. 
TABLE I 
______________________________________ 
Pre- Charpy 
Presintering 
sintering Infil. 
Infilt. 
Impact 
Impact Temperature 
Time Temp. Time Strength 
Bar No. 
.degree.F. Minutes. .degree.F. 
Min. ft. lbs. 
______________________________________ 
II-1 1900 0 2050 7 98 
II-2 1900 0 2050 7 150 
II-3 1900 0 2050 7 98 
II-4 1900 30 2050 7 160 
II-5 1900 30 2050 7 162 
II-6 1900 30 2050 7 228 
II-7 1900 30 2050 10 142 
II-8 1900 30 2050 10 142 
II-9 1900 30 2050 10 142 
II-10 1900 60 2050 7 187 
II-11 1900 60 2050 7 120 
II-12 1900 60 2050 7 206 
______________________________________ 
The infiltrated densities were between 7.78 g/cm.sup.3 and 7.90 g/cm.sup.3. 
The specimens showed no sign of erosion when viewed under a low 
magnification stereo-microscope. The tensile strengths of these specimens 
are about 100 to 110 ksi. It is clear from these properties that it is now 
possible to obtain excellent, never before possible combinations of impact 
an tensile strength with single run processing. Even with zero 
presintering time at the lower temperature, unnotched Charpy impact 
strengths were between about 100 to 150 ft. lbs. At 30 minutes 
presintering the average impact strength to about 162 ft. lbs. and 
individual values were as high as 228 ft. lbs. The benficial effect of 
presintering at a temperature below the melting point of the infiltrant is 
attributed to the formation of a stronger matrix skeleton. 
EXAMPLE III 
Samples were prepared and processed as in Example I except that the vacuum 
furnace was pressurized with nitrogen to a pressure of about 750 mm Hg, 
that is nearly atmospheric pressure, for both the presintering and 
infiltration steps. Also, presintering time was 30 minutes and 
infiltration time was 14 minutes instead of the 7 and 10 min of Example I. 
The impact strengths are shown in the following Table II. 
TABLE II 
______________________________________ 
Unnotched Charpy 
Impact Bar No. 
Impact Strength Ft. Lbs. 
______________________________________ 
III-1 121 
III-2 72 
III-3 107 
______________________________________ 
The infiltrated densities of these specimens were between 7.80 and 7.85 
g/cm.sup.3. Although the impact strengths of this example were lower than 
those of Example I for the same length of presintering, this example 
demonstrates that attractive strength properties are obtainable also when 
sintering and infiltration during a single furnace run are performed in an 
atmosphere such as nitrogen. 
EXAMPLE IV 
Samples were prepared and processed as in Example I except that the single 
Furnace run operation was carried out in a laboratory tube furnace using 
an atmosphere of nitrogen and hydrogen, obtained from cylinders in a 
volume ratio of 1 to 3, that is corresponding in analysis to that of 
dissociated ammonia. The impact strengths are shown in the following Table 
III: 
TABLE III 
______________________________________ 
Pre- Charpy 
Presintering 
sintering Infil. 
Infilt. 
Impact 
Impact Temperature 
Time Temp. Time Strength 
Bar No. 
.degree.F. Minutes. .degree.F. 
Min. ft. lbs. 
______________________________________ 
IV-1 1900 0 2050 10 111 
IV-2 1900 0 2050 10 121 
IV-3 1900 0 2050 10 146 
IV-4 1900 30 2050 10 238 
IV-5 1900 30 2050 10 119 
IV-6 1900 30 2050 10 93 
.sup. IV-7.sup.(1) 
1900 30 2050 10 123 
.sup. IV-8.sup.(1) 
1900 30 2050 10 175 
.sup. IV-9.sup.(1) 
1900 30 2050 10 149 
IV-10 1900 60 2050 10 104 
IV-11 1900 60 2050 10 188 
IV-12 1900 60 2050 10 133 
______________________________________ 
.sup.(1) The slugto-metal matrix ratio for these three specimens had been 
increased to 0.21. 
The densities of the infiltrated specimens varied between 7.75 and 7.89 
g/cm.sup.3. The overall results are similar to those shown in Example I 
demonstrating that excellent properties for single run furnace processing 
are also possible when the furnace atmosphere is a reducing one. 
EXAMPLE V 
Samples were prepared and processed as in Example I except that the single 
furnace run operation was carried out in a laboratory tube furnace using 
an atmosphere of nitrogen and hydrogen, obtained from cylinders in a 
volume ratio of 1 to 3, that is corresponding to that of dissociated 
ammonia. The impact strengths are shown in the following Table IV: 
TABLE IV 
______________________________________ 
Impact Presintering Unnotched Charpy Impact 
Bar No. Temperature .degree.F. 
Strength Ft. Lbs. 
______________________________________ 
V-1 1875 178 
V-2 1875 181 
V-3 1875 188 
V-4 1900 180 
V-5 1900 125 
V-6 1900 184 
V-7 1925 238 
V-8 1925 137 
V-9 1925 185 
V-10 1950 216 
V-11 1950 183 
V-12 1950 171 
______________________________________ 
The infiltrated densities of these specimens varied between 7.88 and 7.90 
g/cm.sup.3. This series of experiments demonstrates that it is possible to 
obtain excellent impact strength using single furnace run processing over 
a relatively narrow range of presintering temperatures above about 
1875.degree. F.