CA1090622A - Method for improving the sinterability of iron powder derived from comminuted scrap metal - Google Patents
Method for improving the sinterability of iron powder derived from comminuted scrap metalInfo
- Publication number
- CA1090622A CA1090622A CA280,180A CA280180A CA1090622A CA 1090622 A CA1090622 A CA 1090622A CA 280180 A CA280180 A CA 280180A CA 1090622 A CA1090622 A CA 1090622A
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- CA
- Canada
- Prior art keywords
- turnings
- particles
- iron
- copper
- metal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F8/00—Manufacture of articles from scrap or waste metal particles
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
- B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
- B22F1/17—Metallic particles coated with metal
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W30/00—Technologies for solid waste management
- Y02W30/50—Reuse, recycling or recovery technologies
Abstract
ABSTRACT or THE DISCLOSURE
A method is disclosed for embrittling scrap metal by dissolving the carbon in ferrous scrap metal having a critical predetermined carbon content, quenching the metal to produce substantially all martensite and impacting the treated metal while simultaneously coating with an anti-oxidizing agent
A method is disclosed for embrittling scrap metal by dissolving the carbon in ferrous scrap metal having a critical predetermined carbon content, quenching the metal to produce substantially all martensite and impacting the treated metal while simultaneously coating with an anti-oxidizing agent
Description
The present invention relates to recovery of scrap metal.
There are strong economic as well as ecological incentives favoring development of a process for the direct -conversion of machine scrap or swarf into a powder which can be utilized by the ferrous powder metallurgy industry. -At Ford Motor Company alonff an estimated 105,000 tons of low alloy steel machine turnings are generated by the various manufacturing plants and sold as scrap on the open market, destined to be part of a furnace charge in some ferrous melting operation. Its utility, even as a furnace charge material, is limited by high bulk-to-weight ratios and residual machine oil content. The alloying elements present ~ in most of this kind of scrap represent a valuable resource ''t~ if they could be economically recovered. In melting opera-;; tions much of this alloy content is oxidized and lost to ~; Qlag. The ecological advantages of direct conversion of swarf to powder with no intermediate melting operation stem from use of a cleaner, less polluting process and the theoret-ical 100% recovery of valuable alloying elements. Economically, ~ .
1 direct conversion is also very attractive. Based oA current
There are strong economic as well as ecological incentives favoring development of a process for the direct -conversion of machine scrap or swarf into a powder which can be utilized by the ferrous powder metallurgy industry. -At Ford Motor Company alonff an estimated 105,000 tons of low alloy steel machine turnings are generated by the various manufacturing plants and sold as scrap on the open market, destined to be part of a furnace charge in some ferrous melting operation. Its utility, even as a furnace charge material, is limited by high bulk-to-weight ratios and residual machine oil content. The alloying elements present ~ in most of this kind of scrap represent a valuable resource ''t~ if they could be economically recovered. In melting opera-;; tions much of this alloy content is oxidized and lost to ~; Qlag. The ecological advantages of direct conversion of swarf to powder with no intermediate melting operation stem from use of a cleaner, less polluting process and the theoret-ical 100% recovery of valuable alloying elements. Economically, ~ .
1 direct conversion is also very attractive. Based oA current
2 prices for iron powders or prealloyed iron powders and the
3 price of swarf on the open market, a significant differential
4 is available for conversion costs and profit. In spite of theQe incentives, technological problems exist which have 6 thwarted previous attempts to employ powder made from swarf 7 in standard powder metallurgy sintering operations.
B8 ~
9 It is well known that ordinary carbon or low alloy steel compositions are incompatible with the sintering 11 processes currently employed in this country. At temperatures 12 f 1120-11480C (2050-21000F) under furnace atmospheres of 13 endothermic generator gas, elements such as silicon, manganese, 14 chromium and vanadium can oxidize at the surfaces of powder particles and create a barrier to the establishment of sinter 16 bonds between particles. As a result, makers of powder 17 metallurgy ~teels for sintered parts are forced to employ a 18 limited number of elements not easily oxidized under these 19 conditions, such as nickel, copper, and molybdenum. Some pre-alloyed powder metallurgy steels designed for sintering and 21 hot forming are currently available with decreased manganese 22 concentrations and only trace amounts of chromium or silicon.
23 Hardenability of the latter prealloyed powders is enhanced by 24 use of additional copper, nickel or molybdenum.
2~ Chemical composition does not present the only deterrent 26 to a direct conversion Process from swarf to powder. Most of the .. _ . . . _ . .. . . _ . . . . . . .. . .... . .. . . . _ _ _ _ . _ .. _ ..
27 energy required in mechanically comminutinq brittle materials 28 to powder is for comminution by brittle fracture. Energy input, 29 therefore, is related solely to the new surface area created.
Steel scrap, being quite malleable, undergoes considerable 1 plastic deformation during most mechanical comminution 2 processes. Energy requirements for comminution of steel, as 3 a result, are considerably greater than for an eauivalent 4 brittle material, with large accompanying heat losses. The S work hardening that accompanies excessive plastic deformation 6 would also make the product powder difficult to ld, 7 necessitating expensive annealing to restore compressibility 8 properties. The ductility of steel swarf tend to make powders 9 produced by conventional mechanical comminution coarse and flake-shaped. The unusual shape and particle size distribution 11 detracts also from compressibility as well as green strength, 12 flow properties and the ability to reproduce fine detail from 13 the compacting die.
14 One approach to overcoming the energy problem is to employ cryogenics, cryogenic powder making is a relatively new 16 mode of providing a powdered raw material which can be put to 17 use in powder metallurgy techniques and other applications.
18 Cryogenic powder holds great promise because it can provide 19 powdered material at a significantly lower cost and it may result in more useable physical properties, if not enhanced 21 physical properties, for a sintered powdered part.
22 Essentially cryogenic powder maki~g comprises 23 subjecting scrap metal, or other solid starting metal material, 24 to a temperature below the ductile-brittle transition temperature of said metal, such as -(30-40)~F for ferrous based material.
26 The metal becomes so brittle at such depressed temperatures that 27 agitation within a conventional ball mill will reduce the scrap 28 or starting metal material to a powder form over a predetermined 29 period of time and stress from the ball milling elements. At the same time, any oil or other organic materials coating the scrap 1 metal, particularly scrap metal in the form of machine turnings, 2 will also freeze and be removed during the impaction by the 3 ball milling elements; such frozen debris can be screened and 4 separated.
To insure that the scrap metal is in the embrittled 6 condition at the point of impaction, it is necessary to direct 7 a supply of liquid nitrogen against the scrap metal immediately 8 prior to introducing the scrap metal into the mill itself. The 9 comminuted particles resulting from a predetermined amount of ball milling under such embrittled conditions, produces 11 metal~particle shapes which are irreqular, not sPhe,rica,l, _ ~ , ., . _ _ . ., .. . . . . . ... , ., . _ . . , . , _ 12 The layer-like or fla~e configur,atio,,n res,ults ,,~
13 fxom the two facts: (a) the scrap metal was machine turnings 14 which have a thin ribbon-like configuration with a large surface-to-volume ratio, and (b) comminution takes place by 16 fracture insuring irregularity of small broken sections of the 17 ribbon resembling flakes. Expensive annealing and further ball-18 milling is required by the prior art.
19 When such cryogenically produced powder is subjected to conventional powder metallurgy techniques, with a compacted 21 quantity of such powder being heated to a sintering temperature, 22 oxidation of ingredients such as manganese and silicon will 23 typically take place prior to diffusion and completion of the 24 sintering step. Such oxidation results because these el~ements require more sintering atmosphere control than is normally 26 possible in current, more stringent operations. Moreover, such 27 powder, when mixed with alloy powders or higher carbon powders 28 demand an uneconomical and inefficient sintering temperature with 29 attendant loss of hardenability.
In accordance with one aspect of the present invention, there is provided a method of making an intermediate powder, comprising: (a) selecting metallic turnings contain-ing oxidizable ingredients during sintering at conventional .-:
temperatures and atmospheres, t~le turnings having a surface-to-volume ratio of at least 60:1, (b) heating the turnings to a temperature at which all carbon in the turnings dissolves, the heating being carried out under an atmosphere and for a period of time effective to provide a solidification structure having at least 8Q% martensite at a selected cooling rate, (c) cooling the turnings at a selected rate, (d) impacting the heat treated turnings at am~ient temperature conditions with a fragmenting force, the impacting being -carried out by the use of elements laden with a metal protective against oxidizing of the ingredients and having a hardness less than that of the resulting particles to promote transfer to the particles upon impact between the elements and particles, the protective metal being completely soluble in the metal of the particles and being relativ.ely easy to abrade, and (e~ continuing to carry out the impaction of step (d2 to provide both coating of the particles with a thin envelope of the protective metal and cold ~orking of substantially each particle having a size greater than 124 -microns to thereby promote at least one cold work strain site therein.
The present invention also includes a method of making a powder compact, comprising: (al selecting metallic machine turnings comprised substantially of ferrous based . material, the turnings having a surface-to-volume ratio of at least 60:1 and a hypoeutectoid composition, ~b~ heating the turnings to.a temperature at which all carbon in the turnings dissolves, the heating being carried out under an atmosphere and for a period of time effective to provide a martensitic structure therein of at least 80% at a selected cooling rate, (c) cooling the turnings at a selected rate, (d) milling the heat treated turnings to provide chips of about -30 mesh, (e~ repeatedly impacting a charge of the milled chips at ambient temperature conditions with a frequenting ~orce, the impacting being carried out with a plurality of copper or iron laden elements having a transverse dimension at least 50 times the largest dimension of any chip, (f) continuing the impacting to simultaneously coat substantially each of the resulting particles with a thin copper or iron shell and to stress substantially each of the particles above 124 microns to effect cold working therein and to deliberately establish at least one non-natural -defect site in each of the particles above 124 microns, and (g) compacting a predetermined quantity of the copper or iron coated particles into a desired shape.
The invention is described further, by way of illus-tration, with reference to the accompanying drawings, in which:
Figure 1 is a schematic flow diagram of a method mode of this invention; and Figures 2a and 2b are photographs, different magnifications, of a powder specimen processed according to Figure 1.
Referring to Figure 1, a hypoeutectoid steel or hypoeutectoid alloy steel which is normally ductile can be made brittle ~y heat treatment according to this invention.
This method comprises (1~ selecting ferrous scrap pieces with a carbon content in the range of 0.0-0.6~C; C2~ heating the ~ 7 ~
selected ferrous based scrap pieces 25 to a temperature where all carbon is put into solution, such as in furnace 30;
(3~ subjecting the pieces 25 to a quench, such as in tank 29, severe enough to form pieces 26 having the martensite phase, (A) continuously impacting the heated treated pieces ` to form a powder 27, such as in a ball mill 28; and (S) preferably heat treating the powder for decarburization and softening so that it may be more readily adaptable to all present powder metallurgy techniques. If the powder is coated with a material through which carbon is readily diffused ~such as purer iron~ the decarburizing treatment may be carried out after the coating is applied. If a copper coating impervious to carbon is employed, decarburization should be provided before the coating is applied. The - brittleness of martensite can be attributed to the distor-tional effect of carbon atoms in the martensite crystal lattice. The greater the supersaturation of carbon the higher the distortion and thus the more highly stressed the material becomes. Hardness data on as quenched martensite indicate that at a~out 0.6% by weight of carbon, the distortional effect of carbon atoms begins to abate, but continues to a lesser extent as carbon is increased.
The employment of such an em~rittli`ng heat treatment to allow comminution of ductile machine chips into powder is unique. The process as descri~ed above would be practical if the chips could be quenched rapidly and to a low enough temperature to form 100% martensite~ Unfortunately, with the low carbon concentrations normally employed in machining steels, this is not usually practical. Thus the heat treatment sequence requires formation of martensite and may involve additional or alternative carburization (2b2 during ~ 8 lO90~;Z2 the carbon solution treatment. This enables a less severe quenching schedule to be employed and still form martensite in appreciable proportions. The most desirable quench would be one that produces a completely martensitic microstructure with carbon concentrations in excess of 0.6%, the most brittle structure possible since little or no plastic deformation can be introduced before fracture occurs.
However, sufficient embrittlement for the purposes of comminution can be obtained with lesser fractions of martensite with the tradeoff coming in energy expended for comminution. The more ductile components in the micro-structure, the greater the proportion of the energy input is consumed by plastic deformation rather than comminution. -Optimization of the process is directly related to the ~-specific composition of the steel chips.
The effectiveness of the embrittlement process was tested by degreasing and heating a batch of machining chips for two hours at 1000C in a carburizing atmosphere. The original composition of the chips was typically SAE 8617 as shown in Table I appearing at the end of this disclosure;
also listed is the composition for the same material that was carburized. Because 8617 steel i8 highly alloyed, quenching was not necessary to form martensite with 1~
carbon present, only a rapid cool. The cooling was carried out in a furnace under atmosphere to avoid decarburization upon cooling. The same procedure would have to be followed if a quench was employed, i.e. maintaining the chips under atmosphere until they are immersed completely in the quench medium. The heat treated chips were then subjected to one pass through a hammer mill and reduced to a size distribution as follows:
.~
Mesh size Fraction +30 50%
-30/+60 20%
-60/+100 12%
-100/+140 7%
-140/+200 7%
-200/+325 3%
-325 1%
Similar chips, in the untreated condition, would not pass through the hammer mill without destroying the internal screens due to their ductility and toughness in the as-received condition.
Subsequent grinding to reduce average particle size was performed in a ball mill, using only the -30 mesh particles. Using a 4" diameter x 4" length alumina ball mill charged with 158 grams of chips, 2730 grams of 3/8n-1/2" diameter iron balls and 350 cc. of reagent grade benzene, a grinding period of about l60 hours produced 225 grams of coated powder more than 98~ of the product having a screen analysis of -325 mesh. In previous experimental work, with untreated chips of the same alloy, complete elimination of the +60 fraction was never achieved with comparable milling times Csee Table IIl. Although the grinding experiment performed on the embrittled powder resulted in a product that was too fine for commercial application it did establish two facts:
Cl~ The embrittling heat treatment makes hammer milling of the machining chips possible and markedly improves grinding efficiency in the ball mill grinding;
(2) Simultaneous grinding and iron coating is possible.
Final chemical analysis of the ball mill product revealed about a 10 weight % pic~up of iron, as coating.
Subsequent experiments have been established that shorter grinding times will produce coarser products more in a~
~oso~22 line with commercial requirements and have indicated that a continuous ball milling operation where the -100 mesh product is swept away continuously by overflow of the milling liquid is the most practical way of achieving the most desirable product. The particle product is illustrated in Figures 2a and 2b for different particles (at 500X and lOOX, respectively~.
The light outer rim is the anti-oxidation envelope and appears best in Figure 2a.
TABLE I: CHEMICAL ANALYSES OF 8617 CHIPS ~:~
C Mn P S Si Cu Ni Mb Cr As Rec'd0.20 0.76 0.008 0.0260.33 0.20 0.44 0.20 0.65 Heat Treated 1.14 " " " " " " " n TABLE II: SCREEN ANALYSIS OF LOW CARBON 8617 POWDER
Untreated Chips Millinq Tine +60-60/+100-100/+200 -200/+325-325 Rec'd ~b Milling) 59.816.8 12.4 4.9 4.9 Milled 8 Hrs. 44.2 18.0 19.3 8.2 10.3 Milled 72 Hrs. 31.5 ll,Q 25.5 22.5 9.5
B8 ~
9 It is well known that ordinary carbon or low alloy steel compositions are incompatible with the sintering 11 processes currently employed in this country. At temperatures 12 f 1120-11480C (2050-21000F) under furnace atmospheres of 13 endothermic generator gas, elements such as silicon, manganese, 14 chromium and vanadium can oxidize at the surfaces of powder particles and create a barrier to the establishment of sinter 16 bonds between particles. As a result, makers of powder 17 metallurgy ~teels for sintered parts are forced to employ a 18 limited number of elements not easily oxidized under these 19 conditions, such as nickel, copper, and molybdenum. Some pre-alloyed powder metallurgy steels designed for sintering and 21 hot forming are currently available with decreased manganese 22 concentrations and only trace amounts of chromium or silicon.
23 Hardenability of the latter prealloyed powders is enhanced by 24 use of additional copper, nickel or molybdenum.
2~ Chemical composition does not present the only deterrent 26 to a direct conversion Process from swarf to powder. Most of the .. _ . . . _ . .. . . _ . . . . . . .. . .... . .. . . . _ _ _ _ . _ .. _ ..
27 energy required in mechanically comminutinq brittle materials 28 to powder is for comminution by brittle fracture. Energy input, 29 therefore, is related solely to the new surface area created.
Steel scrap, being quite malleable, undergoes considerable 1 plastic deformation during most mechanical comminution 2 processes. Energy requirements for comminution of steel, as 3 a result, are considerably greater than for an eauivalent 4 brittle material, with large accompanying heat losses. The S work hardening that accompanies excessive plastic deformation 6 would also make the product powder difficult to ld, 7 necessitating expensive annealing to restore compressibility 8 properties. The ductility of steel swarf tend to make powders 9 produced by conventional mechanical comminution coarse and flake-shaped. The unusual shape and particle size distribution 11 detracts also from compressibility as well as green strength, 12 flow properties and the ability to reproduce fine detail from 13 the compacting die.
14 One approach to overcoming the energy problem is to employ cryogenics, cryogenic powder making is a relatively new 16 mode of providing a powdered raw material which can be put to 17 use in powder metallurgy techniques and other applications.
18 Cryogenic powder holds great promise because it can provide 19 powdered material at a significantly lower cost and it may result in more useable physical properties, if not enhanced 21 physical properties, for a sintered powdered part.
22 Essentially cryogenic powder maki~g comprises 23 subjecting scrap metal, or other solid starting metal material, 24 to a temperature below the ductile-brittle transition temperature of said metal, such as -(30-40)~F for ferrous based material.
26 The metal becomes so brittle at such depressed temperatures that 27 agitation within a conventional ball mill will reduce the scrap 28 or starting metal material to a powder form over a predetermined 29 period of time and stress from the ball milling elements. At the same time, any oil or other organic materials coating the scrap 1 metal, particularly scrap metal in the form of machine turnings, 2 will also freeze and be removed during the impaction by the 3 ball milling elements; such frozen debris can be screened and 4 separated.
To insure that the scrap metal is in the embrittled 6 condition at the point of impaction, it is necessary to direct 7 a supply of liquid nitrogen against the scrap metal immediately 8 prior to introducing the scrap metal into the mill itself. The 9 comminuted particles resulting from a predetermined amount of ball milling under such embrittled conditions, produces 11 metal~particle shapes which are irreqular, not sPhe,rica,l, _ ~ , ., . _ _ . ., .. . . . . . ... , ., . _ . . , . , _ 12 The layer-like or fla~e configur,atio,,n res,ults ,,~
13 fxom the two facts: (a) the scrap metal was machine turnings 14 which have a thin ribbon-like configuration with a large surface-to-volume ratio, and (b) comminution takes place by 16 fracture insuring irregularity of small broken sections of the 17 ribbon resembling flakes. Expensive annealing and further ball-18 milling is required by the prior art.
19 When such cryogenically produced powder is subjected to conventional powder metallurgy techniques, with a compacted 21 quantity of such powder being heated to a sintering temperature, 22 oxidation of ingredients such as manganese and silicon will 23 typically take place prior to diffusion and completion of the 24 sintering step. Such oxidation results because these el~ements require more sintering atmosphere control than is normally 26 possible in current, more stringent operations. Moreover, such 27 powder, when mixed with alloy powders or higher carbon powders 28 demand an uneconomical and inefficient sintering temperature with 29 attendant loss of hardenability.
In accordance with one aspect of the present invention, there is provided a method of making an intermediate powder, comprising: (a) selecting metallic turnings contain-ing oxidizable ingredients during sintering at conventional .-:
temperatures and atmospheres, t~le turnings having a surface-to-volume ratio of at least 60:1, (b) heating the turnings to a temperature at which all carbon in the turnings dissolves, the heating being carried out under an atmosphere and for a period of time effective to provide a solidification structure having at least 8Q% martensite at a selected cooling rate, (c) cooling the turnings at a selected rate, (d) impacting the heat treated turnings at am~ient temperature conditions with a fragmenting force, the impacting being -carried out by the use of elements laden with a metal protective against oxidizing of the ingredients and having a hardness less than that of the resulting particles to promote transfer to the particles upon impact between the elements and particles, the protective metal being completely soluble in the metal of the particles and being relativ.ely easy to abrade, and (e~ continuing to carry out the impaction of step (d2 to provide both coating of the particles with a thin envelope of the protective metal and cold ~orking of substantially each particle having a size greater than 124 -microns to thereby promote at least one cold work strain site therein.
The present invention also includes a method of making a powder compact, comprising: (al selecting metallic machine turnings comprised substantially of ferrous based . material, the turnings having a surface-to-volume ratio of at least 60:1 and a hypoeutectoid composition, ~b~ heating the turnings to.a temperature at which all carbon in the turnings dissolves, the heating being carried out under an atmosphere and for a period of time effective to provide a martensitic structure therein of at least 80% at a selected cooling rate, (c) cooling the turnings at a selected rate, (d) milling the heat treated turnings to provide chips of about -30 mesh, (e~ repeatedly impacting a charge of the milled chips at ambient temperature conditions with a frequenting ~orce, the impacting being carried out with a plurality of copper or iron laden elements having a transverse dimension at least 50 times the largest dimension of any chip, (f) continuing the impacting to simultaneously coat substantially each of the resulting particles with a thin copper or iron shell and to stress substantially each of the particles above 124 microns to effect cold working therein and to deliberately establish at least one non-natural -defect site in each of the particles above 124 microns, and (g) compacting a predetermined quantity of the copper or iron coated particles into a desired shape.
The invention is described further, by way of illus-tration, with reference to the accompanying drawings, in which:
Figure 1 is a schematic flow diagram of a method mode of this invention; and Figures 2a and 2b are photographs, different magnifications, of a powder specimen processed according to Figure 1.
Referring to Figure 1, a hypoeutectoid steel or hypoeutectoid alloy steel which is normally ductile can be made brittle ~y heat treatment according to this invention.
This method comprises (1~ selecting ferrous scrap pieces with a carbon content in the range of 0.0-0.6~C; C2~ heating the ~ 7 ~
selected ferrous based scrap pieces 25 to a temperature where all carbon is put into solution, such as in furnace 30;
(3~ subjecting the pieces 25 to a quench, such as in tank 29, severe enough to form pieces 26 having the martensite phase, (A) continuously impacting the heated treated pieces ` to form a powder 27, such as in a ball mill 28; and (S) preferably heat treating the powder for decarburization and softening so that it may be more readily adaptable to all present powder metallurgy techniques. If the powder is coated with a material through which carbon is readily diffused ~such as purer iron~ the decarburizing treatment may be carried out after the coating is applied. If a copper coating impervious to carbon is employed, decarburization should be provided before the coating is applied. The - brittleness of martensite can be attributed to the distor-tional effect of carbon atoms in the martensite crystal lattice. The greater the supersaturation of carbon the higher the distortion and thus the more highly stressed the material becomes. Hardness data on as quenched martensite indicate that at a~out 0.6% by weight of carbon, the distortional effect of carbon atoms begins to abate, but continues to a lesser extent as carbon is increased.
The employment of such an em~rittli`ng heat treatment to allow comminution of ductile machine chips into powder is unique. The process as descri~ed above would be practical if the chips could be quenched rapidly and to a low enough temperature to form 100% martensite~ Unfortunately, with the low carbon concentrations normally employed in machining steels, this is not usually practical. Thus the heat treatment sequence requires formation of martensite and may involve additional or alternative carburization (2b2 during ~ 8 lO90~;Z2 the carbon solution treatment. This enables a less severe quenching schedule to be employed and still form martensite in appreciable proportions. The most desirable quench would be one that produces a completely martensitic microstructure with carbon concentrations in excess of 0.6%, the most brittle structure possible since little or no plastic deformation can be introduced before fracture occurs.
However, sufficient embrittlement for the purposes of comminution can be obtained with lesser fractions of martensite with the tradeoff coming in energy expended for comminution. The more ductile components in the micro-structure, the greater the proportion of the energy input is consumed by plastic deformation rather than comminution. -Optimization of the process is directly related to the ~-specific composition of the steel chips.
The effectiveness of the embrittlement process was tested by degreasing and heating a batch of machining chips for two hours at 1000C in a carburizing atmosphere. The original composition of the chips was typically SAE 8617 as shown in Table I appearing at the end of this disclosure;
also listed is the composition for the same material that was carburized. Because 8617 steel i8 highly alloyed, quenching was not necessary to form martensite with 1~
carbon present, only a rapid cool. The cooling was carried out in a furnace under atmosphere to avoid decarburization upon cooling. The same procedure would have to be followed if a quench was employed, i.e. maintaining the chips under atmosphere until they are immersed completely in the quench medium. The heat treated chips were then subjected to one pass through a hammer mill and reduced to a size distribution as follows:
.~
Mesh size Fraction +30 50%
-30/+60 20%
-60/+100 12%
-100/+140 7%
-140/+200 7%
-200/+325 3%
-325 1%
Similar chips, in the untreated condition, would not pass through the hammer mill without destroying the internal screens due to their ductility and toughness in the as-received condition.
Subsequent grinding to reduce average particle size was performed in a ball mill, using only the -30 mesh particles. Using a 4" diameter x 4" length alumina ball mill charged with 158 grams of chips, 2730 grams of 3/8n-1/2" diameter iron balls and 350 cc. of reagent grade benzene, a grinding period of about l60 hours produced 225 grams of coated powder more than 98~ of the product having a screen analysis of -325 mesh. In previous experimental work, with untreated chips of the same alloy, complete elimination of the +60 fraction was never achieved with comparable milling times Csee Table IIl. Although the grinding experiment performed on the embrittled powder resulted in a product that was too fine for commercial application it did establish two facts:
Cl~ The embrittling heat treatment makes hammer milling of the machining chips possible and markedly improves grinding efficiency in the ball mill grinding;
(2) Simultaneous grinding and iron coating is possible.
Final chemical analysis of the ball mill product revealed about a 10 weight % pic~up of iron, as coating.
Subsequent experiments have been established that shorter grinding times will produce coarser products more in a~
~oso~22 line with commercial requirements and have indicated that a continuous ball milling operation where the -100 mesh product is swept away continuously by overflow of the milling liquid is the most practical way of achieving the most desirable product. The particle product is illustrated in Figures 2a and 2b for different particles (at 500X and lOOX, respectively~.
The light outer rim is the anti-oxidation envelope and appears best in Figure 2a.
TABLE I: CHEMICAL ANALYSES OF 8617 CHIPS ~:~
C Mn P S Si Cu Ni Mb Cr As Rec'd0.20 0.76 0.008 0.0260.33 0.20 0.44 0.20 0.65 Heat Treated 1.14 " " " " " " " n TABLE II: SCREEN ANALYSIS OF LOW CARBON 8617 POWDER
Untreated Chips Millinq Tine +60-60/+100-100/+200 -200/+325-325 Rec'd ~b Milling) 59.816.8 12.4 4.9 4.9 Milled 8 Hrs. 44.2 18.0 19.3 8.2 10.3 Milled 72 Hrs. 31.5 ll,Q 25.5 22.5 9.5
Claims (6)
1. A method of making an intermediate powder, comprising:
(a) selecting metallic turnings containing oxidizable ingredients during sintering at conventional temperatures and atmospheres, said turnings having a surface-to-volume ratio of at least 60:1.
(b) heating said turnings to a temperature at which all carbon in said turnings dissolves, said heating being carried out under an atmosphere and for a period of time effective to provide a solidification structure having at least 80% martensite at a selected cooling rate, (c) cooling said turnings at a selected rate, (d) impacting said heat treated turnings at ambient temperature conditions with a fragmenting force, said impacting being carried out by the use of elements laden with a metal protective against oxidizing of said ingredients and having a hardness less than that of the resulting particles to promote transfer to said particles upon impact between said elements and particles, said protective metal being completely soluble in said metal of said particles and being relatively easy to abrade, and (e) continuing to carry out the impaction of step (d) to provide both coating of said particles with a thin envelope of said protective metal and cold working of substantially each particle having a size greater than 124 microns to thereby promote at least one cold work strain site therein.
(a) selecting metallic turnings containing oxidizable ingredients during sintering at conventional temperatures and atmospheres, said turnings having a surface-to-volume ratio of at least 60:1.
(b) heating said turnings to a temperature at which all carbon in said turnings dissolves, said heating being carried out under an atmosphere and for a period of time effective to provide a solidification structure having at least 80% martensite at a selected cooling rate, (c) cooling said turnings at a selected rate, (d) impacting said heat treated turnings at ambient temperature conditions with a fragmenting force, said impacting being carried out by the use of elements laden with a metal protective against oxidizing of said ingredients and having a hardness less than that of the resulting particles to promote transfer to said particles upon impact between said elements and particles, said protective metal being completely soluble in said metal of said particles and being relatively easy to abrade, and (e) continuing to carry out the impaction of step (d) to provide both coating of said particles with a thin envelope of said protective metal and cold working of substantially each particle having a size greater than 124 microns to thereby promote at least one cold work strain site therein.
2. The method of claim 1, in which the turnings are comprised of hypoeutectoid iron based metal having a carbon content less than 0.6%, and heating is carried out in a carburizing atmosphere to increase the carbon content of said turnings to at least 0.6%.
3. The method of claim 1, in which said impaction is carried out by a ball mill having ball elements comprised of solid copper or iron.
4. The method of claim 1, in which the carbon content of said untreated as well as treated turnings is between 0.09 and 0.6% the cooling rate of step (b) being sufficient-ly rapid to quench said turnings and produce substantially a complete martensitic solidification structure.
5. A method of making a powder compact, comprising:
(a) selecting metallic machine turnings comprised substantially of ferrous based material, said turnings having a surface-to-volume ratio of at least 60:1 and a hypoeutectoid composition, (b) heating said turnings to a temperature at which all carbon in said turnings dissolves, said heating being carried out under an atmosphere and for a period of time effective to provide a martensitic structure therein of at least 80% at a selected cooling rate, (c) cooling said turnings at a selected rate, (d) milling said heat treated turnings to provide chips of about -30 mesh, (e) repeatedly impacting a charge of said milled chips at ambient temperature conditions with a fragmenting force, said impacting being carried out with a plurality of copper or iron laden elements having a transverse dimension at least 50 times the largest dimension of any chip, (f) continuing said impacting to simultaneously coat substantially each of the resulting particles with a thin copper or iron shell and to stress substantially each of said particles above 124 microns to effect cold working therein and to deliberately establish at least one non-natural defect site in each of said particles above 124 microns, and (g) compacting a predetermined quantity of said copper or iron coated particles into a desired shape.
(a) selecting metallic machine turnings comprised substantially of ferrous based material, said turnings having a surface-to-volume ratio of at least 60:1 and a hypoeutectoid composition, (b) heating said turnings to a temperature at which all carbon in said turnings dissolves, said heating being carried out under an atmosphere and for a period of time effective to provide a martensitic structure therein of at least 80% at a selected cooling rate, (c) cooling said turnings at a selected rate, (d) milling said heat treated turnings to provide chips of about -30 mesh, (e) repeatedly impacting a charge of said milled chips at ambient temperature conditions with a fragmenting force, said impacting being carried out with a plurality of copper or iron laden elements having a transverse dimension at least 50 times the largest dimension of any chip, (f) continuing said impacting to simultaneously coat substantially each of the resulting particles with a thin copper or iron shell and to stress substantially each of said particles above 124 microns to effect cold working therein and to deliberately establish at least one non-natural defect site in each of said particles above 124 microns, and (g) compacting a predetermined quantity of said copper or iron coated particles into a desired shape.
6. The method of claim 5, in which said copper or iron laden elements consist of solid copper or iron balls having a diameter substantially about 0.5 inches, said copper or iron balls operating within a revolving housing of a ball mill, said housing being rotated so as to impact said copper or iron balls with said comminuted particles at a predetermined rate and stress frequency so as to produce said copper or iron coated particles and defect sites therein.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/735,468 US4129443A (en) | 1975-06-06 | 1976-10-26 | Method for improving the sinterability of iron powder derived from comminuted scrap metal |
| US735,468 | 1976-10-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1090622A true CA1090622A (en) | 1980-12-02 |
Family
ID=24955939
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA280,180A Expired CA1090622A (en) | 1976-10-26 | 1977-06-09 | Method for improving the sinterability of iron powder derived from comminuted scrap metal |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS5362711A (en) |
| CA (1) | CA1090622A (en) |
| DE (1) | DE2736274C2 (en) |
| GB (2) | GB1580378A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| KR100245398B1 (en) * | 1991-02-01 | 2000-03-02 | 시드니엠.카우프만 | Method of recycling scrap metal |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1494887A (en) * | 1975-06-06 | 1977-12-14 | Ford Motor Co | Method of making sintered shapes |
-
1977
- 1977-05-19 GB GB21083/77A patent/GB1580378A/en not_active Expired
- 1977-06-09 CA CA280,180A patent/CA1090622A/en not_active Expired
- 1977-06-14 GB GB2476677A patent/GB1527449A/en not_active Expired
- 1977-08-11 DE DE2736274A patent/DE2736274C2/en not_active Expired
- 1977-10-26 JP JP12769677A patent/JPS5362711A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| DE2736274A1 (en) | 1978-04-27 |
| GB1580378A (en) | 1980-12-03 |
| JPS5543484B2 (en) | 1980-11-06 |
| JPS5362711A (en) | 1978-06-05 |
| DE2736274C2 (en) | 1985-03-07 |
| GB1527449A (en) | 1978-10-04 |
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