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By Harry K. Ihrig

The hot-working of iron and steel is an art dating back to antiquity, but until about 25 years ago, relatively few alloying elements were used, and these were present only in small percentages. With the exception of sulphur, the amounts of other elements in steels did not markedly affect their hot-workability. The references in the literature on the effect of other elements on the hot-workability of steel are usually reports of failure of these steels in forging or rolling.1-8 With the increased use of alloying elements, and their use in larger amounts, it was observed that many of the new steels were red short in certain critical ranges or at all hot-working temperatures. The stainless steels are particularly difficult to hot-work. The determining of the correct conditions for piercing, rolling and forging of these alloys by experimentation in the mill often results in large losses of these expensive metals. It also is found that the hot-workability limits of many types vary from heat to heat, and hence poor results are obtained on changing to a new heat after a previous one has rolled satisfactorily. To determine the effect on hot-workability of iron to which various elements have been added, it was necessary to devise a method that could be used on small quantities of metal. In such a method, the minimum number of variables should be present, and the procedure should be simple, quick and correlatable with large-scale hot-working operations. Sauveur,9 in the first Henry Marion Howe lecture in 1924, described the effect of carbon on the ductility of steels in torsion near their critical points. He found that carbon increases the plasticity of gamma iron. In the 1930 Campbell Memorial lecture,10 he gives the effect of carbon, chromium, and chromium and nickel on the ductility of steels under 2000°F. in torsion. Ellis11 has studied the malleability of steels at various elevated temperatures by means of a single-blow drop-hammer test. In this test the percentage decrease in height of standard cylinders is taken as a measure of their malleability. He has shown that in the low-alloy steels studied, carbon has a greater effect than nickel, chromium and vanadium. In 1938 the first tests were run on a new quantitative hot-workability test in the laboratories of the Globe Steel Tubes CO. Thum12 mentioned it in 1940. An illustration of the apparatus was published in January 1942.'' The effect of a few alloying elements on the hot-workability of metals was shown in a description of the test in 1944.14 Description of Test Method The method adopted after a trial of several others is a rapid twisting of bars to failure in a furnace. The standard-size bar was chosen as 9/16 in. It is prepared from larger pieces by forging and cold-

Jan 1, 1947

By Harry K. Ihrig

The hot-working of iron and steel is an art dating back to antiquity, but until about 25 years ago, relatively few alloying elements were used, and these were present only in small percentages. With the exception of sulphur, the amounts of other elements in steels did not markedly affect their hot-workability. The references in the literature on the effect of other elements on the hot-workability of steel are usually reports of failure of these steels in forging or rolling.1-8 With the increased use of alloying elements, and their use in larger amounts, it was observed that many of the new steels were red short in certain critical ranges or at all hot-working temperatures. The stainless steels are particularly difficult to hot-work. The determining of the correct conditions for piercing, rolling and forging of these alloys by experimentation in the mill often results in large losses of these expensive metals. It also is found that the hot-workability limits of many types vary from heat to heat, and hence poor results are obtained on changing to a new heat after a previous one has rolled satisfactorily. To determine the effect on hot-workability of iron to which various elements have been added, it was necessary to devise a method that could be used on small quantities of metal. In such a method, the minimum number of variables should be present, and the procedure should be simple, quick and correlatable with large-scale hot-working operations. Sauveur,9 in the first Henry Marion Howe lecture in 1924, described the effect of carbon on the ductility of steels in torsion near their critical points. He found that carbon increases the plasticity of gamma iron. In the 1930 Campbell Memorial lecture,10 he gives the effect of carbon, chromium, and chromium and nickel on the ductility of steels under 2000°F. in torsion. Ellis11 has studied the malleability of steels at various elevated temperatures by means of a single-blow drop-hammer test. In this test the percentage decrease in height of standard cylinders is taken as a measure of their malleability. He has shown that in the low-alloy steels studied, carbon has a greater effect than nickel, chromium and vanadium. In 1938 the first tests were run on a new quantitative hot-workability test in the laboratories of the Globe Steel Tubes CO. Thum12 mentioned it in 1940. An illustration of the apparatus was published in January 1942.'' The effect of a few alloying elements on the hot-workability of metals was shown in a description of the test in 1944.14 Description of Test Method The method adopted after a trial of several others is a rapid twisting of bars to failure in a furnace. The standard-size bar was chosen as 9/16 in. It is prepared from larger pieces by forging and cold-

Jan 1, 1947

By Harry K. Ihrig

Jan 1, 1947

By Harry K. Ihrig

Jan 1, 1947

By George Stern

The purpose of this investigation has been to study one method for making ordinary carbon steel by the powder metallurgy technique. This method con- sists of pressing and subsequently sintering an intimate mixture of iron powder and gaphite in a suitable atmosphere to Produce a steel. Two types of iron Powder have been studied, electrolytic and reduced. The sinkring of mixtures of loose Car-bonyf iron and carbon black for several hours at I000 to 1100 C. (1830' to 2010°F.), followed by forging of the resultant ingots to bars, has been studied by Offermann.' TrailZ has reported on the properties of hydrogen-reduced and electrolytic iron powders, both containing 0.5 per cent carbon. Methods for making steel bodies by powder metallurgy have also been sug- gested by Goetzel13 who recommends the addition of steel powder of well-defined carbon content to pure iron powder, and Volterra,~ who suggests compaction and subsequent sintering of a mixture of steel powder, iron oxide and iron powder. Additional methods for making steel bodies by powder metallurgy have been reported in the literature by Kohering Allen,6 Balke,7 Wulff,8 Boegehold,9 and others.,0,11,12 It is felt that the method of pressing iron powder and graphite, as described herein, lends itself best to production be-Cause of the lubricating action of the

Jan 1, 1946

By George Stern

The purpose of this investigation has been to study one method for making ordinary carbon steel by the powder metallurgy technique. This method con- sists of pressing and subsequently sintering an intimate mixture of iron powder and gaphite in a suitable atmosphere to Produce a steel. Two types of iron Powder have been studied, electrolytic and reduced. The sinkring of mixtures of loose Car-bonyf iron and carbon black for several hours at I000 to 1100 C. (1830' to 2010°F.), followed by forging of the resultant ingots to bars, has been studied by Offermann.' TrailZ has reported on the properties of hydrogen-reduced and electrolytic iron powders, both containing 0.5 per cent carbon. Methods for making steel bodies by powder metallurgy have also been sug- gested by Goetzel13 who recommends the addition of steel powder of well-defined carbon content to pure iron powder, and Volterra,~ who suggests compaction and subsequent sintering of a mixture of steel powder, iron oxide and iron powder. Additional methods for making steel bodies by powder metallurgy have been reported in the literature by Kohering Allen,6 Balke,7 Wulff,8 Boegehold,9 and others.,0,11,12 It is felt that the method of pressing iron powder and graphite, as described herein, lends itself best to production be-Cause of the lubricating action of the

Jan 1, 1946

By Otto H. Henry, J. J. Cordiano

Though powder metallurgy is one of the oldest of metallurgical processes, it is in its infancy as a branch of the modern field of metallurgy. As early as 3000 B.C., the ancients produced implements and weapons of iron by reducing iron ore in a charcoal fire and forging the resultant sponge while still hot into the desired shape. Melting and casting soon superseded the powder metallurgy practice, and it was not until the turn of the twentieth century that powder metallurgy was applied to the commercial manufacture of tungsten lamp filaments. From this beginning, the use of the powder metallurgy process spread" to the manufacture of porous oil-impregnated bearings, cemented carbide tools, contact points, diamond grinding wheels, clutch facings, starter brushes, resin-bonded radio cores, magnets, metallic filters, and other parts. More recently, and particularly during the present war, there has been a definite trend toward the manufacture of machine parts by this process in competition with the more conventional fabricating methods. Notwithstanding these many developments, scientific progress has been slow. While many articles on powder metallurgy have appeared in the press and various publications, there have been too few contributions to our technical societies, with the result that there is a dearth of the type of information necessary for the full development of this science. It is of primary importance that the results of research and development be freely disseminated if powder metallurgy is to realize its fullest potentialities. Purpose of Investigation Excluding the refractory metal field, all commercial powder metallurgical products made by a cold compressing and sintering operation contain voids. These voids can be substantially eliminated by subsequent severe working and heat-treatment, but ' such practice has not proved economical in most applications. By applying heat during the compressing operation, plastic deformation of the powder particles is substantially increased, and the resultant compact or briquette is practically free of voids. However, hot-pressing has many attendant problems, which at present preclude its use in the manufacture of precision parts. The more important of these are to devise: 1. A die steel capable of withstanding high pressures at elevated temperatures. 2. A high-temperature lubricant capable of preventing excessive die wear and welding of the compact to the die walls. 3. A method of retaining a neutral or reducing atmosphere in the compacting zone of the die assembly. 4. A means of ejecting the hot-pressed specimen into a neutral atmosphere until reasonably cold.

Jan 1, 1946

By Otto H. Henry, J. J. Cordiano

Though powder metallurgy is one of the oldest of metallurgical processes, it is in its infancy as a branch of the modern field of metallurgy. As early as 3000 B.C., the ancients produced implements and weapons of iron by reducing iron ore in a charcoal fire and forging the resultant sponge while still hot into the desired shape. Melting and casting soon superseded the powder metallurgy practice, and it was not until the turn of the twentieth century that powder metallurgy was applied to the commercial manufacture of tungsten lamp filaments. From this beginning, the use of the powder metallurgy process spread" to the manufacture of porous oil-impregnated bearings, cemented carbide tools, contact points, diamond grinding wheels, clutch facings, starter brushes, resin-bonded radio cores, magnets, metallic filters, and other parts. More recently, and particularly during the present war, there has been a definite trend toward the manufacture of machine parts by this process in competition with the more conventional fabricating methods. Notwithstanding these many developments, scientific progress has been slow. While many articles on powder metallurgy have appeared in the press and various publications, there have been too few contributions to our technical societies, with the result that there is a dearth of the type of information necessary for the full development of this science. It is of primary importance that the results of research and development be freely disseminated if powder metallurgy is to realize its fullest potentialities. Purpose of Investigation Excluding the refractory metal field, all commercial powder metallurgical products made by a cold compressing and sintering operation contain voids. These voids can be substantially eliminated by subsequent severe working and heat-treatment, but ' such practice has not proved economical in most applications. By applying heat during the compressing operation, plastic deformation of the powder particles is substantially increased, and the resultant compact or briquette is practically free of voids. However, hot-pressing has many attendant problems, which at present preclude its use in the manufacture of precision parts. The more important of these are to devise: 1. A die steel capable of withstanding high pressures at elevated temperatures. 2. A high-temperature lubricant capable of preventing excessive die wear and welding of the compact to the die walls. 3. A method of retaining a neutral or reducing atmosphere in the compacting zone of the die assembly. 4. A means of ejecting the hot-pressed specimen into a neutral atmosphere until reasonably cold.

Jan 1, 1946

By Laurence Delisle, Arron Finger

The alloys formed by the addition of nickel to iron by conventional metallurgical procedures show physical properties that differ widely from those of the individual metals. The effect of alloying on the mechanical properties is most pronounced in alloys containing between 15 and 25 per cent nickel. The tensile strength of an alloy containing, for example, 20 per cent nickel and 80 per cent iron (with 0.06 per cent carbon), in an annealed condition, is above 150,000 lb. per sq. in.$ owing to a martensitic type of transformation, as compared with about 50,000 lb. per sq. in. for pure iron in the same condition. The object of this work was to determine whether the same beneficial alloying effect coulcl be obtained in alloys of iron and nickel prepared by powder metallurgy technique. Procedure Bars and pellets for tension and compression tests, respectively, were made. Two groups of specimens were prepared: (I) by cold-pressing the powders and sintering; (2) by cold-pressing the powders, sintering, repressing and resintering. Raw Materials Nickel powder supplied by Metals Disintegrating Co., marked MD-151, was used. A typical analysis of this powder was as follows: sulphur, 0.040 per cent; copper, 0.18; iron, 0.23; manganese, 0.002; magnesium, o.oo15; lead, 0.035; tin> 0.025; silicon, 0.08; chromium, 0.008. Electrolytic iron powder supplied by Plastic Metals Inc. was used. A repre-sentative chemical analysis for impurities other than gases, supplied by the vendor, gave the following results: carbon, 0.005 per cent; manganese, 0.002; silicon, 0.003; phosphorus, 0.001; sulphur, 0.004; nickel, 0.008. The fractions from these powders passing a 325-mesh sieve were reduced in hydrogen at 600°C. for one hour, to clean the sur-faces of the particles. The reduced pow-ders, slightly caked, were ground in a mortar and screened again through a 325-mesh sieve. Mixing The proper proportions of the powders were mixed overnight on rolls, in glass jars fitted with iron-wire baffles to break aggregates of particles that might form. Pressing All specimens were compacted on hy-draulic presses. The pellets were com-pacted in a cylindrical die, 34-in. diameter. The ratio of thickness to the diameter of the pellets was maintained as nearly as possible at 0.9, as specified for standard short compression pieces. The tensile bars were made by compacting 30 grams of powder. Their shape is shown in Fig. I. The compacting pressures for the pellets and the tensile bars were:

Jan 1, 1946

By Arron Finger, Laurence Delisle

The alloys formed by the addition of nickel to iron by conventional metallurgical procedures show physical properties that differ widely from those of the individual metals. The effect of alloying on the mechanical properties is most pronounced in alloys containing between 15 and 25 per cent nickel. The tensile strength of an alloy containing, for example, 20 per cent nickel and 80 per cent iron (with 0.06 per cent carbon), in an annealed condition, is above 150,000 lb. per sq. in.$ owing to a martensitic type of transformation, as compared with about 50,000 lb. per sq. in. for pure iron in the same condition. The object of this work was to determine whether the same beneficial alloying effect coulcl be obtained in alloys of iron and nickel prepared by powder metallurgy technique. Procedure Bars and pellets for tension and compression tests, respectively, were made. Two groups of specimens were prepared: (I) by cold-pressing the powders and sintering; (2) by cold-pressing the powders, sintering, repressing and resintering. Raw Materials Nickel powder supplied by Metals Disintegrating Co., marked MD-151, was used. A typical analysis of this powder was as follows: sulphur, 0.040 per cent; copper, 0.18; iron, 0.23; manganese, 0.002; magnesium, o.oo15; lead, 0.035; tin> 0.025; silicon, 0.08; chromium, 0.008. Electrolytic iron powder supplied by Plastic Metals Inc. was used. A repre-sentative chemical analysis for impurities other than gases, supplied by the vendor, gave the following results: carbon, 0.005 per cent; manganese, 0.002; silicon, 0.003; phosphorus, 0.001; sulphur, 0.004; nickel, 0.008. The fractions from these powders passing a 325-mesh sieve were reduced in hydrogen at 600°C. for one hour, to clean the sur-faces of the particles. The reduced pow-ders, slightly caked, were ground in a mortar and screened again through a 325-mesh sieve. Mixing The proper proportions of the powders were mixed overnight on rolls, in glass jars fitted with iron-wire baffles to break aggregates of particles that might form. Pressing All specimens were compacted on hy-draulic presses. The pellets were com-pacted in a cylindrical die, 34-in. diameter. The ratio of thickness to the diameter of the pellets was maintained as nearly as possible at 0.9, as specified for standard short compression pieces. The tensile bars were made by compacting 30 grams of powder. Their shape is shown in Fig. I. The compacting pressures for the pellets and the tensile bars were:

Jan 1, 1946

By E. I. Larsen, E. F. Swazy, F. R. Hensel

High strength, high-conductivity copper-base alloys have found considerable use in the resistance welding and electrical industry in the form of castings, forgings, or wrought products. There are a number of parts utilized by these industries which would lend themselves to manufacture by powder metallurgical methods provided the required physical and electrical properties are obtainable with such methods. The present paper describes the results of a study of certain copper-base powder metallurgical products susceptible to precipitation hardening. Copper Powders The general types of commercially available copper powders are flake, spherical and electrolytic. Micrographs of these three types are shown in Figs. I, 2 and 3. The difference in particle shape is pronounced and obvious. The type of powder used is determined, or at least indicated, by the final properties desired in the finished article. The type of powder employed during the present investigation was electrolytic powder shown in Fig. 3, and was chosen because of its particular adaptability to the work described in this Paper. The screen and chemical analyses of this powder are shown in Table I. In the preparation of compacts of high electrical conductivity, the purity of the copper powder is of importance in addition to other factors, such as density and the state of internal stresses. These variables were studied briefly by pressing a number of compacts to various pressed densities, sintering them for one hour in hydrogen at 975OC and repressing. The size of the finished compacts was 1/4, by % by 8 in. The time interval from the application to the termination of pressure was approximately 20 sec. Some of the compacts were annealed before testing. The results are listed in Figs. 4 to 8, inclusive. In Fig. 4 the curves of pressed density vs. compacting pressure, and sintered density versus compacting pressure show the typical direct relationship of pressure and density. Compacting pressures exceeding about 30 tons per sq. in. result fiequently in expansion during sintering for the type of powder used. Fig. 5 indicates that variations in the initial pressed density within the range of about 5 to 6 grams per c, c., affect only slightly the pressure required to obtain a

Jan 1, 1946

By E. I. Larsen, E. F. Swazy, F. R. Hensel

High strength, high-conductivity copper-base alloys have found considerable use in the resistance welding and electrical industry in the form of castings, forgings, or wrought products. There are a number of parts utilized by these industries which would lend themselves to manufacture by powder metallurgical methods provided the required physical and electrical properties are obtainable with such methods. The present paper describes the results of a study of certain copper-base powder metallurgical products susceptible to precipitation hardening. Copper Powders The general types of commercially available copper powders are flake, spherical and electrolytic. Micrographs of these three types are shown in Figs. I, 2 and 3. The difference in particle shape is pronounced and obvious. The type of powder used is determined, or at least indicated, by the final properties desired in the finished article. The type of powder employed during the present investigation was electrolytic powder shown in Fig. 3, and was chosen because of its particular adaptability to the work described in this Paper. The screen and chemical analyses of this powder are shown in Table I. In the preparation of compacts of high electrical conductivity, the purity of the copper powder is of importance in addition to other factors, such as density and the state of internal stresses. These variables were studied briefly by pressing a number of compacts to various pressed densities, sintering them for one hour in hydrogen at 975OC and repressing. The size of the finished compacts was 1/4, by % by 8 in. The time interval from the application to the termination of pressure was approximately 20 sec. Some of the compacts were annealed before testing. The results are listed in Figs. 4 to 8, inclusive. In Fig. 4 the curves of pressed density vs. compacting pressure, and sintered density versus compacting pressure show the typical direct relationship of pressure and density. Compacting pressures exceeding about 30 tons per sq. in. result fiequently in expansion during sintering for the type of powder used. Fig. 5 indicates that variations in the initial pressed density within the range of about 5 to 6 grams per c, c., affect only slightly the pressure required to obtain a

Jan 1, 1946

By Claus G. Goetzel

Pressing of powdered metal parts is best done in the direction of the shortest extension of the piece, to avoid too great a loss of pressing force through internal iriction. As long as curved surfaces, re-cessej, or offsets occur only parallel to this shortest axis of pressure, no special difficulties arise: the cross sections, perpendicular to the pressure, do not vary, nor does the shape of the parts change during compression except for a gradually reduced thickness (Fig. IA). The problem becomes more dificult, however, when the curved surfaces or recesses are perpendicular to the shortest axis of the part (Fig. IB) Two processes for molding uniformly dense parts with complicated shapes from powdered metals are described in detail in this paper. Both can be employed successfully by those trained in the art. The first refers to curved parts; the second is especially adapted to parts having one or more recesses or steps. Both methods, as set forth here, are applied under the condition that presents the more difficult problem. For clarity, before description of the two practical methods, the variable factors in molding powdered metal parts will be reviewed, then an idealized procedure will be considered briefly. Variable Factors in Molding Powdered Metal Parts Compression Ratio Compression ratio is defined as the proportion of the final relative density of the compact to the apparent density of the powder. For iron powder of an apparent density of 3.0, for example, a compression ratio of 2 1/2:1 would be required to produce a compressed compact of 7.5 density; whereas for another iron powder having an apparent density of only 1.5, a compression ratio of 5:1 would be required to produce the same density. The lower the powder's apparent density, the greater must be the compression ratio for pressing compacts of high density.

Jan 1, 1946

By Claus G. Goetzel

Pressing of powdered metal parts is best done in the direction of the shortest extension of the piece, to avoid too great a loss of pressing force through internal iriction. As long as curved surfaces, re-cessej, or offsets occur only parallel to this shortest axis of pressure, no special difficulties arise: the cross sections, perpendicular to the pressure, do not vary, nor does the shape of the parts change during compression except for a gradually reduced thickness (Fig. IA). The problem becomes more dificult, however, when the curved surfaces or recesses are perpendicular to the shortest axis of the part (Fig. IB) Two processes for molding uniformly dense parts with complicated shapes from powdered metals are described in detail in this paper. Both can be employed successfully by those trained in the art. The first refers to curved parts; the second is especially adapted to parts having one or more recesses or steps. Both methods, as set forth here, are applied under the condition that presents the more difficult problem. For clarity, before description of the two practical methods, the variable factors in molding powdered metal parts will be reviewed, then an idealized procedure will be considered briefly. Variable Factors in Molding Powdered Metal Parts Compression Ratio Compression ratio is defined as the proportion of the final relative density of the compact to the apparent density of the powder. For iron powder of an apparent density of 3.0, for example, a compression ratio of 2 1/2:1 would be required to produce a compressed compact of 7.5 density; whereas for another iron powder having an apparent density of only 1.5, a compression ratio of 5:1 would be required to produce the same density. The lower the powder's apparent density, the greater must be the compression ratio for pressing compacts of high density.

Jan 1, 1946

By F. N. Rhines

Sintering may be defined as the process by which powders bond themselves into coherent bodies, usually, although not necessarily, under the influence of pressure and elevated temperature. For the sake of clarity and simplicity, the present survey will exclude from consideration the sintering of mixed or alloyed powders, with attendant complications, but at the risk of overlooking information of general applicability. Sintering in the presence of a liquid phase is thus automatically eliminated. The sintering of alloys involves much additional subject matter and deserves separate consideration. Since the literature of the subject is extensive and widely scattered, it will not be feasible to consider the contributions individually. Instead, the subject matter will be classified according to experimental observations and theory, and will be presented in summary form. It should be noted that the appended list of references does not constitute a complete bibliography; it includes only those works quoted below. More extensive bibliographies are included in several of the references; see particularly references 7, 9, 28, 34, 38, 50, 52, 54, a "Bibliography on Powder Metallurgy" published by the Library of Congress and a pamphlet published by the Metals Disintegrating Co. entitled "The Field of Powder Metallurgy and Bibliography." 11. Summary of Experimental Observations The following classified list is composed of general statements of established fact derived from the experimental results reported in the published literature. A few contested observations are included, because of their potential importance; these are labeled as such. Any satisfactory and complete theory must be capable of explaining all of the uncontested facts. Obviously, some important facts may be missing from this list and these may hold the key to the correct explanation of the sintering process, but the possible lack of such material should not deter attempts to understand the subject on the basis of what is available here. A, The Initial Bond at Room Temperature a. To a limited extent uncompressed powders and massive metals adhere at room temperature. Clean tin powder forms a weak "cake" on standing at room temperature, the strength increasing with time. Clean gold surfaces adhere on light con-tact, as do those of many other substances, including nickel.27 b. Upon compression the strength of the bond- is increased progres-ivelv. There is a limit of pressure

Jan 1, 1946

By F. N. Rhines

Sintering may be defined as the process by which powders bond themselves into coherent bodies, usually, although not necessarily, under the influence of pressure and elevated temperature. For the sake of clarity and simplicity, the present survey will exclude from consideration the sintering of mixed or alloyed powders, with attendant complications, but at the risk of overlooking information of general applicability. Sintering in the presence of a liquid phase is thus automatically eliminated. The sintering of alloys involves much additional subject matter and deserves separate consideration. Since the literature of the subject is extensive and widely scattered, it will not be feasible to consider the contributions individually. Instead, the subject matter will be classified according to experimental observations and theory, and will be presented in summary form. It should be noted that the appended list of references does not constitute a complete bibliography; it includes only those works quoted below. More extensive bibliographies are included in several of the references; see particularly references 7, 9, 28, 34, 38, 50, 52, 54, a "Bibliography on Powder Metallurgy" published by the Library of Congress and a pamphlet published by the Metals Disintegrating Co. entitled "The Field of Powder Metallurgy and Bibliography." 11. Summary of Experimental Observations The following classified list is composed of general statements of established fact derived from the experimental results reported in the published literature. A few contested observations are included, because of their potential importance; these are labeled as such. Any satisfactory and complete theory must be capable of explaining all of the uncontested facts. Obviously, some important facts may be missing from this list and these may hold the key to the correct explanation of the sintering process, but the possible lack of such material should not deter attempts to understand the subject on the basis of what is available here. A, The Initial Bond at Room Temperature a. To a limited extent uncompressed powders and massive metals adhere at room temperature. Clean tin powder forms a weak "cake" on standing at room temperature, the strength increasing with time. Clean gold surfaces adhere on light con-tact, as do those of many other substances, including nickel.27 b. Upon compression the strength of the bond- is increased progres-ivelv. There is a limit of pressure

Jan 1, 1946

By E. I. Larsen, E. F. Swazy, F. R. Hensel

Experience has indicated that hard bronzes are not suitable for bearing applications where high bearing loads and speeds are involved. It is the general practice to utilize softer materials for these purposes, such as copper, lead, silver, babbitt, lead-base alloys, and more recently aluminum-base alloys, preferably supported by a steel backing. It was the objective of the present investigation to study a new material produced by powder metallurgical methods susceptible to precipitation-hardening. Test MAteriaIs Preliminary tests were made with compacts containing copper, nickel and silicon in quantities sufficient to form 3 to 5 per cent nickel silicide. It mas found that this type of alloy was susceptible to the formation of a soft surface shell when sintered in tank hydrogen. The depth of the shell depended On the length of sintering time and sintering temperature, and varied in thickness from a few thousandths to as much as One eighth inch. This soft shell resulted from the preferential oxidation of silicon, thereby reducing the efficiency of age-hardening by the nickel silicide compound. The body of the sintered compacts below the soft shell could be precipitation-hardened to a hardness of 70 to 85 Rockwell B, by quenching from goo°C. and age-hardening at 450°C. A series of tests was made with various addition agents, in order to decrease the rate of preferential oxidation, and it was found that phosphorus was most beneficial in this respect. After considerable experimentation, the following nominal com-position was accepted as standard: 2.4 per cent nickel; 0.8 silicon; 0.3 phosphorus; balance copper. The physical propertties and certain bearing properties of this alloy are described in the present paper Preparation of Compacts Several methods of incorporating the hardening ingredient (nickel silicide) were studied. Compacts formed from copper, nickel and silicon powders were found to be inferior to those fabricated from prealloyed nickel silicide master alloys, because of the rapid oxidation of pure silicon during sintering. The use of a prealloyed nickel silicide hardener containing 25 per cent silicon and 75 per cent nickel could be further improved by preparing a master alloy, incorporating nickel, silicon, phosphorus and copper, having the following nominal composition~ 25 per cent nickel; 7 silicon; 3 phosphorus 64 copper. This alloy is brittle in the cast condition and can be readily powdered. Sintering Procedure The blended powders were pressed at '5 to 40 tons per sq' in., the pressure depending on the size and shape of the compacts. Sintering was carried out in hydrogen or cracked ammonia atmospheres at

Jan 1, 1946

By E. I. Larsen, E. F. Swazy, F. R. Hensel

Experience has indicated that hard bronzes are not suitable for bearing applications where high bearing loads and speeds are involved. It is the general practice to utilize softer materials for these purposes, such as copper, lead, silver, babbitt, lead-base alloys, and more recently aluminum-base alloys, preferably supported by a steel backing. It was the objective of the present investigation to study a new material produced by powder metallurgical methods susceptible to precipitation-hardening. Test MAteriaIs Preliminary tests were made with compacts containing copper, nickel and silicon in quantities sufficient to form 3 to 5 per cent nickel silicide. It mas found that this type of alloy was susceptible to the formation of a soft surface shell when sintered in tank hydrogen. The depth of the shell depended On the length of sintering time and sintering temperature, and varied in thickness from a few thousandths to as much as One eighth inch. This soft shell resulted from the preferential oxidation of silicon, thereby reducing the efficiency of age-hardening by the nickel silicide compound. The body of the sintered compacts below the soft shell could be precipitation-hardened to a hardness of 70 to 85 Rockwell B, by quenching from goo°C. and age-hardening at 450°C. A series of tests was made with various addition agents, in order to decrease the rate of preferential oxidation, and it was found that phosphorus was most beneficial in this respect. After considerable experimentation, the following nominal com-position was accepted as standard: 2.4 per cent nickel; 0.8 silicon; 0.3 phosphorus; balance copper. The physical propertties and certain bearing properties of this alloy are described in the present paper Preparation of Compacts Several methods of incorporating the hardening ingredient (nickel silicide) were studied. Compacts formed from copper, nickel and silicon powders were found to be inferior to those fabricated from prealloyed nickel silicide master alloys, because of the rapid oxidation of pure silicon during sintering. The use of a prealloyed nickel silicide hardener containing 25 per cent silicon and 75 per cent nickel could be further improved by preparing a master alloy, incorporating nickel, silicon, phosphorus and copper, having the following nominal composition~ 25 per cent nickel; 7 silicon; 3 phosphorus 64 copper. This alloy is brittle in the cast condition and can be readily powdered. Sintering Procedure The blended powders were pressed at '5 to 40 tons per sq' in., the pressure depending on the size and shape of the compacts. Sintering was carried out in hydrogen or cracked ammonia atmospheres at

Jan 1, 1946

By John Wulff, Richard P. Seelig

The importance of the pressing operation in the forming of articles by powder metallurgy depends to a great extent on the type of product to be made. While in some few cases the pressing is merely a means of compacting loose powder particles into a solid mass preliminary to a sintering treatment, in most applications it constitutes a molding operation with the purpose of shaping the raw material in powder form into a certain predetermined, accurately defined part not altered in shape by subsequent sintering. In still other cases, no pressing at all but sintering in molds gives useful articles. A limited number of products are made by the simultaneous application of pressure and heat. The factors that control the product of the pressing operation are raw material, lubricant, type, size and shape of die and compact, magnitude and time of application of pressure, as well as temperature of pressing. All such factors need to be controlled; yet, depending on the function of the pressing operation, greater emphasis may be placed on processing factors other than the pressing operation. In any complete discussion of pressing, subsequent sintering cannot be disregarded. For the sake of brevity, the present paper has been limited to the mechanics of pressing and only occasionally refers to sintering. For the same reason, hot-pressing has been omitted as beyond the scope of this paper In any survey of the literature on the pressing of metal powders, the 1938 paper of Balshinl on the theory of the process of pressing deserves mention. The first attempts at an interpretation of pressing phenomena were made by Balshin' and RakovskL2 The experimental results of both are of great interest. Since the experimental results are not described, the work was repeated, using our own interpretation of the possible techniques, and is reported in the following pages. The basic theory of Balshin,' based on a hydro-dynamic analogy, however, leads to hut another way of plotting density and pres- , sure against height of a pressed compact. Since it does not give any new insight into the phenomena, nor account for the influence of side-wall friction, a discussion of the theory has been omitted from this paper. Density Gradients in Compacts The process of pressing metal powders in a die (disregarding for this consideration the powder filling and ejection operations) may be divided into three phases: (I) packing, (2) elastic and plastic deformation, (3) cold-working with or without fragmentation. It must be emphasized that these three steps do not follow each other in sequence; on the contrary, in practice

Jan 1, 1946

By John Wulff, Richard P. Seelig

The importance of the pressing operation in the forming of articles by powder metallurgy depends to a great extent on the type of product to be made. While in some few cases the pressing is merely a means of compacting loose powder particles into a solid mass preliminary to a sintering treatment, in most applications it constitutes a molding operation with the purpose of shaping the raw material in powder form into a certain predetermined, accurately defined part not altered in shape by subsequent sintering. In still other cases, no pressing at all but sintering in molds gives useful articles. A limited number of products are made by the simultaneous application of pressure and heat. The factors that control the product of the pressing operation are raw material, lubricant, type, size and shape of die and compact, magnitude and time of application of pressure, as well as temperature of pressing. All such factors need to be controlled; yet, depending on the function of the pressing operation, greater emphasis may be placed on processing factors other than the pressing operation. In any complete discussion of pressing, subsequent sintering cannot be disregarded. For the sake of brevity, the present paper has been limited to the mechanics of pressing and only occasionally refers to sintering. For the same reason, hot-pressing has been omitted as beyond the scope of this paper In any survey of the literature on the pressing of metal powders, the 1938 paper of Balshinl on the theory of the process of pressing deserves mention. The first attempts at an interpretation of pressing phenomena were made by Balshin' and RakovskL2 The experimental results of both are of great interest. Since the experimental results are not described, the work was repeated, using our own interpretation of the possible techniques, and is reported in the following pages. The basic theory of Balshin,' based on a hydro-dynamic analogy, however, leads to hut another way of plotting density and pres- , sure against height of a pressed compact. Since it does not give any new insight into the phenomena, nor account for the influence of side-wall friction, a discussion of the theory has been omitted from this paper. Density Gradients in Compacts The process of pressing metal powders in a die (disregarding for this consideration the powder filling and ejection operations) may be divided into three phases: (I) packing, (2) elastic and plastic deformation, (3) cold-working with or without fragmentation. It must be emphasized that these three steps do not follow each other in sequence; on the contrary, in practice

Jan 1, 1946