CA1327462C - Machinable-grade, ferrous powder blend containing boron nitride - Google Patents

Abstract

ABSTRACT OF THE INVENTION

The machinability characteristics of P/M ferrous sintered compacts are improved when the compact is prepared from a ferrous powder having a maximum particle size less than about 300 microns, and from 0.01 to 1.0 wt. % of a boron nitride powder having an average particle size between about 0.1 and 25 microns.

Description

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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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- . ~ , ~: -~`` 1327462 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.

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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--` 1327462 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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', ',` ' ', ' :, , ~ , , : " ' -, -~` 1327462 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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1327~62 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 . . .
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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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-` 1327~62 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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13274~2 , TABLE I

EFFECTS OF BORON NITRIDE ON THE PROPERTIES OF
AUTOMET~ 28 - COMPACTS AND SINTERED COMPACTS
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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 -, ! ~

, , , 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.

, TABLE II

EFFECTS OF BORON NITRIDE AND MANGANESE SULFIDE ON THE

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

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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Claims (11)

1. A machinable-grade, powder blend comprising:

a. at least 85 wt. % of a ferrous powder having a maximum particle size less than about 300 microns; and b. from 0.01 to 1.0 wt. % boron nitride powder having an average particle size between about 0.1 and 25 microns.

2. The powder blend of Claim 1 where the maximum particle size of the ferrous powder is less than about 212 microns.

3. The powder blend of Claim 1 where the ferrous powder comprises at least 90 wt. % of the blend.

4. The powder blend of Claim 1 where the boron nitride comprises between about 0.02 and .5 wt. % of the blend.

5. The blend of Claim 3 where the boron nitride comprises between about 0.05 and .3 wt. % of the blend.

6. The blend of Claim 5 where the boron nitride powder contains less than about 15 wt. % boric oxide.

7. The blend of Claim 5 where the boron nitride powder contains less than about 5 wt. % boric oxide.

8. The blend of Claim 7 where the boron nitride powder has an average particle size between about 0.2 and 15 microns.

9. The blend of Claim 7 where the boron nitride powder has an average particle size between about 0.5 and 10 microns.

10. The blend of Claim 1 where the crystalline structure of the boron nitride is predominately graphite-like, hexagonal plates.

11. A ferrous shape made from compacting the powder blend of Claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.