Note: Descriptions are shown in the official language in which they were submitted.
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MACHINABLE-GRADE, FERROUS POWDER
BLEND CONTAINING BORON NITRIDE
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to ferrous powder blends.
In one aspect, the invention relates to machinable-
grade, ferrous powder blends containing boron nitride
while in another aspect, the invention relates to the
use of boron nitride containing relatively minor
amounts of boric oxide and having an average particle
size of between 0.1 and 25 microns.
2. Description of the Prior Art
The making and using of ferrous powders are well
-15 known, and is described in considerable detail in
Kirk-Othmer's Encyclopedia of Chemical Technoloav,
Third Edition, Volume 19, at pages 28-62. Ferrous
powders can be made by discharging molten iron metal
from a furnace into a tundish where, after passing
through refractorv nozzles, the molten iron is
f~ subjected to granulation by horizontal water jets.
The granulated iron is then dried and reduced to a
powder, which is subsequently annealed to remove
oxygen and carbon. A pure iron cake is recovered and
then crushed back to a powder.
Ferrous powders have many applications, such as
powder metallurgy (P/M) part fabrication, welding
electrode coatings, flame cutting and
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scarfing. For P/M applications, the iron powder is
often blended with selected additives such as
lubricants, binders and alloying agents. A ferrous
P/M part is formed by injecting iron or steel powder
into a dye cavity shaped to some specific
configuration, applying pressure to form a compact,
sintering the compact, and then finishing the sintered
compact to the desired specifications.
Shaped P/M sintered compacts often require
machining as one of the finishing steps to produce the
desired P/M product. Where the P/M product is a mass-
produced product (for which the P/M process is well-
suited), then the speed and efficiency at which these
P/M products can be produced will depend in part on
the speed and efficiency of the machining step. The
speed and efficiency of the machining step is in turn
a function of, among other things, how easily the P/M
sintered compact can be cut by the machining tool.
Generally, the more difficulty in cutting the P/M
sintered compact, the more energy required of the
cutting tool, the shorter the life of the cutting
tool, and the more time required to complete the
machining step.
One of the methods for increasing the speed and
efficiency of the machining step is to make a P/M
sintered compact with a low coefficient of friction at
the interface of the cutting tool and compact, and
with improved chip formation properties. This can be
accomplished by blending the ferrous powder with a
friction-reducing agent, such as manganese or
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molybdenum sulfide, but these known agents for ferrous powde~s
while operative, are subject to improvement. For example, while
all agents are admixed with the ferrous powder prior to
sintering, some either adversely affect the dimensional changes
that are undergone by the compact during sintering, or generally
reduce the strength properties of the sintered compact, or both.
Too much dimensional change (shrinkage) can require a die change
by the P/M part manufacturer, a costly step to be avoided if
possible, and significant reduced strength properties of the - -
sintered compact generally reduce its ultimate usefulness. As
importantly, the friction reduction imparted to the P/M sintered
compact by known friction-reducing agents is open to considerable
improvement, and remains a principal objective in the on-going
effort to improve the speed and efficiency of the P/M process
machining step.
SUMMARY OF THE INVENTION
According to this invention, a machinable-grade, ferrous
powder blend is prepared from: -
A. at least about 85 wt. % of a ferrous powder having an
average particle size between about 50 and 300 microns;
and
B. from about 0.01 to 1.0 wt. % boron nitride powder having
an average particle size between 0.1 and 25 microns.
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P/M sintered compacts prepared from this ferrous powder blend
demonstrate improved machinability. In addition, the boron
nitride friction-reducing agent has minimal effect on both the
strength of the P/M sintered compact and the dimensional changes
that the compact undergoes during sintering.
DETAILED DESCRIPTION OF THE INVENTION
Essentially any ferrous powder having a maximum particle
size less than about 300 microns can be used in the composition
of this invention. Typical iron powders are the Atomet5 iron
powders manufactured by Quebec Metal Powders Limited of Sorel,
Quebec, Canada. These powders have an iron content in excess of
99 wt. % with less than 0.2 wt. % oxygen and O.l wt. % carbon.
Atomet0 iron powders typically have an apparent density of at
least 2.50 g/cm3 and a flow rate of less than 30 seconds per
50 g. Steel powders, including stainless and alloyed steel
powders, can also be used as the ferrous powders for the blends
of this invention, and Atomet lOOl, 4201 and 4601 steel powders
are representative of the ~teel and alloyed steel powders. These
powders contain in excess of 97 wt. % iron and have an apparent
density of 2.9-3.0 g/cm3 and a flow of 24-28 seconds per 50 g.
Atomet0 steel powder lOO1 is 99 plus wt. ~ iron, while Atomet~
steel powders 4201 and 4601 each contain 0.6 wt. % molybdenum and
0.5 and 1.8 wt. % nickel, respectively. Virtually any grade of
stainless steel powder can be used. Preferably, the ferrous
powder has a maximum particle size less than about 212 microns.
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Boron nitride is a commercially available white powder
which can have either a cubic or graphite-like hexagonal plate
crystalline structure. While boron nitride of either structure
can be used in this invention, the boron nitride of the hexagonal
structure is preferred, where boron nitride mixtures of both
structures are used, mixtures containing at least about 50 wt. %
of the hexagonal boron nitride are preferred. The boron nitride
used in this invention is characterized by having an average
particle size between about 0.1 and about 25 microns. Boron
nitride itself is a relatively inert material which is immiscible
with iron and stainless steel at temperatures below 1400 C and is
substantially unreactive with carbon below 1700 C. However, the
hygroscopicity generally associated with boron nitride is due in
large part to the presence of boric oxide, a residue from the
boron nitride manufacturing process. Since the shelf life of the
ferrous powder composition is dependent in part upon the amount
of water that is absorbed between the time the composition is
formed and the time it is used to prepare a P/M sintered compact,
the amount of boric oxide present in the boron nitride used to
make the compositions of this invention is typically less than
about 15 wt. % (based on the total weight of the boron nitride
plus any contaminants), and preferably less than about 5 wt. %.
Although not known with certainty, the boron nitride
particles when mixed with the iron particles are believe to
concentrate within or about the pores or crevices of the iron
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particles. This positioning of the boron nitride on the ferrous
particles keeps the boron nitride from interfering with the iron
particles during the sintering process and accordingly, from
materially impacting the dimensional changes that occur to the
/M compact during the slntering process. The preferred average
particle size of the boron nitride used in this invention is
between about 0.2 and about 15 microns, and more preferably
between about 0.5 and about 10 microns.
The ferrous powder blends of this invention are prepared
by blending from about 0.01 to about 1.0 wt. % boron nitride
powder with at least 85 wt. %, preferably at least 90 wt. %, of a
ferrous powder. The blending is performed in such a manner that
the resulting mixture of ferrous powder and boron nitride is
substantially homogeneous. Essentially any form of mixing can be
employed with conventional, mechanical mixing most typical.
The ferrous powder composition of this invention can
contain other materials in addition to the ferrous and boron
nitride powders. Binding agents such as polyethylene glycol,
polypropylene glycol, kerosene, and the like can also be present,
as well as alloying powders such as graphite, copper and/or
nickel. These materials, their use and methods of inclusion in
ferrous powder blends are well known in the art.
P/M sintered compacts having improved machinability
characteristics are the hallmark or this invention. These
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compacts are more easily machined than compacts made from ferrous
powder compositions not containing boron nitride powder as here
described, and thus the machining step of the P/M process
exhibits greater speed and efficiency as measured by such indicia
as cutting tool life, cutting tool power requirements, and length
of time required for the machining step of a particular compact.
These advantageous features are accomplished without any
significant negative impact on the sintered properties of the
ferrous powder blend.
The following examples are illustrative embodiments of
this invention.
SPECIFIC EMBODIMENTS
Atomet~ 28 iron powder was used to study the effect of
boron nitride additions on the sintering properties of P/M
compacts and on the strength and machinability of P/M sintered
compacts. Atomet~ 28 iron powder is 99+ wt. % iron and contains
about 0.18 wt. % oxygen and 0.07 wt. % carbon. It has an
apparent density of about 2.85 g/cm3 and a flow rate of about 26
seconds per 50 g. The screen analysis (U.S. mesh) was:
Screen Size Wt. %
on 100 5
-100 +140 28
-140 +200 23
-200 +325 24
-325 20
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This screen analysis translates to a maximum particle size of
about 212 microns. The boron nitride used in the examples of
Tables I and II had a graphite-like, hexagonal plate structure,
an average particle size of between about 0.5 and about 1 micron,
and contained between about 0.2 and about 0.4 wt. % boric oxide.
The Atomet~ 28 iron powder was first blended with about
0.5 wt. % zinc stereate (a lubricant) and varying levels of
graphite ranging from 0 through 0.9 wt. %. Various amounts of
boron nitride powder was then added to aliquots of the blend and
then mechanically mixed to form a substantially homogeneous
mixture (within 5% of the addition level). Test pieces were
compacted at 6.7 g/cm3 and then sintered for 30 minutes at
1120 C in a rich endothermic atmosphere. Sintered properties
were measured on standard transverse rupture bars in accordance
with Metal Powder Industries Federation test methods. The
reported values in the Table are averages of three measurements.
Machinability was evaluated using the drilling thrust
force test. High speed steel drills were inserted in the
rotating head of an industrial lathe and fed into the specimens
mounted on a load cell. Thrust forces were measured on test bars
measuring 31.8 mm by 12.7 mm by 12.7 mm compacted and sintered
according to the above-described procedures. Two holes of 6.4 mm
diameter and 10 mm diameter deep were drilled in each specimen.
No coolant was used during the drilling operation and the
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penetration rate was fixed at 40 mm/min and the speed of the
drill at 800 rpm for all tests. The thrust forces were measured
by the load cell and recorded on a high speed plotter. The
thrust force was used as a machinability index of the sintered
parts and the lower the thrust force, the better the
machinability (longer cutting tool life, less cutting tool power
requirements, and less time required to machine the sintered
compact).
The results of these tests are reported in Table I. As
these results demonstrate, the addition of boron nitride as
described in the specification significantly reduces the thrust
force as measured against sintered compacts made from ferrous
powder blends without boron nitride. The improved machinability
is independent of the carbon (graphite level. In addition, the
presence of the boron nitride has only a nominal effect on the
dimensional change of the compacts during sintering, on the
hardness and strength properties of the sintered compacts.
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TABLE I
EFFECTS OF BORON NITRIDE ON THE PROPERTIES OF
AUTOMET~ 28 - COMPACTS AND SINTERED COMPACTS
.... :
Thrust ~orce
Boron Added
Nitride Carbon T.R.S.l Hardness3 lb % reduction
twt. %2 (wt, %) tsi X Dim.Ch.2 (Rb~
0.0 0 67.0 -.28 38 107
0.1 0 71.7 -.28 41 105 -2
0.3 0 71.3 -.22 45 64 -33
0.5 0 67.7 -.23 48 56 -42
.0 .1 70.0 -.26 40 109
0.2 .1 71.5 -.24 47 90 -17
0.5 .1 70.0 -.21 45 58 -37
0.0 .3 79.4 -.20 51 120
0.1 .3 80.4 -.20 54 114 -5
0.2 .3 78.5 -.17 51 97 -21
0.3 .3 79.2 -.16 51 77 -33
0.5 .3 75.3 -.17 53 69 -40
0.0 .4 81.9 -.17 56 132
0.2 .4 82.3 -.16 56 ~ 113 -14
0.5 .4 76.0 --14 54 79 -40
0.0 .6 94.2 -.13 66 124
0.1 .6 89.4 -.13 65 118 -5
0.2 .6 95.1 -.12 68 96 -19
0.3 .6 91.7 -.12 66 85 -29
0.5 .6 78.6 -.10 64 72 -42
0.0 .7 98.0 -.13 68 126
0.2 .7 95.8 -.08 69 104 -18
0.5 .7 81.5 -.10 63 75 -42
0.0 .9 115.3 -.05 77 136
0.1 .9 112.0 -.03 76 130 -4
0.2 .9 113.3 -.06 76 111 -17
0.3 .9 107.7 -.02 76 98 -27
0.5 .9 91.1 -.05 70 80 -39
12Tranverse Rupture Strength
3Percent Dimensional Change
Rockwell
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In another set of experiments, test pieces were made
from similar materials and by the same procedures as used to
prepare the test pieces of Table I, except that manganese sulfide
was substituted for boron nitride in some test pieces. The
manganese sulfide had an average particle size of about
5 microns, and 0.5 wt. ~ of it was added to the blend of Atomet~
28 .iron powder, graphite and zinc stearate. The test pieces were
analyzed in the same manner as those analyzed in Table I. The
results of these analyses and their comparison to the boron
nitride containing test pieces are reported in Table II. As
these results demonstrate, test pieces containing manganese
sulfide performed better in the thrust force test than test
pieces without either manganese sulfide or boron nitride, but did
not perform as well as those containing boron nitride. Moreover,
the manganese sulfide had a much greater negative impact on the
transverse rupture strength of the test pieces than did boron
nitride. Neither the manganese sulfide nor boron nitride had
much negative impact on the dimensional change characteristics of
the ferrous powder during sintering.
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TABLE II
EFFECTS OF BORON NITRIDE AND MANGANESE SULFIDE ON THE
THE PROPERTIES OF ATOMET3 28 COMPACTS AND SINTERED COMPACTS
A. TRANSVERSE RUPTURE STRENGTH, tsi
Added
Carbon
wt. %Controll 0.5 wt.% Mns 0.2 wt.% BN0.3 wt.% BN
0.3 79.4 68.5 78.5 79.2
0.6 94.2 74.4 95.1 91.7
0.9 115.3 91.4 113.3 107.7
B. DIMENSIONAL CHANGE,
Control2
0.3 -0.20 -0.10 -0.17 -0.16
0.6 -0.13 -0.12 -0.12 -0.12
0.9 -0.05 +0.06 -0.06 -0.02
C. THRUST FORCE, lb
Controll
0.3 131 98 107 83
0.6 135 114 105 93
0.9 157 147 124 108
lControl = Atomet~ 28 Sintered Compact without BN or MnS
2Control = Atomet~ 28 Compact without BN or MnS
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In yet another set of experiments, test pieces were made
from similar materials and by the same procedures to prepare the
test pieces of Tables I and II, except that the boron nitride had
an average particle size of between about 3.2 and 6 microns and
contained between about 1.5 and about 2.2 wt. ~ boric oxide. The
test pieces were analyzed in the same manner as those analyzed in
Tables I and II, and the results are reported in Table III. Once
again, the results demonstrate that test pieces containing the
boron nitride performed better in the thrust force test than the
control test piece and at levels of 0.1 wt. ~ or less, without
significant negative impact in either transverse rupture
strength, dimensional change or hardness. At levels above
0.1 wt. ~ boron nitride addition of these added carbon levels,
the nominal marginal gain in the machinability index is adversely
offset by the considerable marginal loss in transverse rupture
strength.
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TABLE III
EFFECTS OF BORON NITRIDE WITH AN AGERAGE PARTICLE SIZE
BETWEEN 3-6 MICRONS ON THE PROPERTIES OF ATOMET~ 28
COMPACTS AND SINTERED COMPACTS
Boron Added % Dim. Ch.
Nitride Carbon TRS from Hardness Thrust Force
(wt-%) (%)lb/in2Die Size (Rb) lbs % Reduction
0 0.6 90,000 -0.10 64 115
0.1 0.6 83,700 -0.12 59 83 -30
0.2 0.6 72,200 -0.11 60 63 -47
O 0.9116,000 -0.09 76 125
0.05 0.9100,000 0.08 74 87 -30
0.01 0.990,900 -0.01 70 76 -39
0.02 0.972,200 -0.09 62 64 -41
While this invention has been described with specific
reference to particular embodiments, these embodiments are for
the purpose of illustration only and are not intended as a
limitation upon the scope of the following claims.
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