US2235835A - Powder metallurgy - Google Patents

Description

arch 25, c. G. GoETzEL 2,235,835

POWDER METALLURGY Filed Oct. 18, 1958 2 Sheets-Sheet 2 Cia; 'aewef oeZzeZ ATTORNEYS Patented Mar. 25, 1941 POWDER TAILURGY Claus Guenter Goetzel, New York, N. Y., ignor to Hardy Metallurgical Company, New York, N. Y., a corporation of Delaware Application October 18, 1938, Serial No. 235,597

12 Claims.

This invention relates to powder metallurgy and particularly to the heat treatment of masses of metal powder particles to bond them together into a continuous metal structure. The inven- 5 tion contemplates an improved method and apparatus for determining or establishing and maintaining appropriate conditions for such heat treatment.

It has been customary heretofore to heat masses of metal powders, particularly coherent masses or briquettes that have been compacted by pressure with or without the aid of a binder, in order to establish metallic bonds between the particles. The briquettes of metal powder may be heated to the point of incipient fusion so that grain boundaries are destroyed, but such practice (especially if the surfaces of the briquettes are relatively unsupported or unconned) tends to bring about undesirable distortion. Adequate 20 bonding of metal powder particles may also be accomplished by maintaining the brlquette at an elevated temperature but below the fusion point until sufiicient diffusion of metal occurs between particles. This is known as diffusion '25 welding and is preferred when articles of accurate configuration are to be made because it involves less distortion of an unsupported briquette, i. e., one that is heat treated while unconned in a mold. Optimum conditions for diffusion welding vary depending upon the metallic constituents and the degree of consolidation of the briquette prior to heat treatment. Hence, it is frequently necessary to carry out a series of experiments with various time periods and temperatures in order to determine a desirable set of conditions for heat treating a given kind of briquette. Even then optimum conditions may not be determined unless a very extensive series of tests is made.

As a result of my investigations, I have developed a method and apparatus whereby optimum or desirable conditions for heat treatment of any given mass of metal powder may be determined quickly and accurately and without resort to the cut and try expediente of the prior art. The method is based upon my observation that a mass of 'metal powder undergoing heat treatment exerts a varying influence on an a1- ternating current flowing in a circuit with which the mass is inductively associated. Thus, if the metal powder mass, preferably a, briquette, is heated while disposed as the core of an induction coil energized by alternating current, I have found that the effectiveV inductance of the coil changes gradually until the mass reaches a temperature at which diiusion welding occurs rapidly and thereafter remains substantially constant until the mass is near to being fused. Changes in the impedance or effective inductance of the coil may be observed by noting variations in either the characteristics of the circuit or the character of the alternating current or both which the changes bring about, and my invention contemplates making such observations and employing them as an index and ultimately as a control of proper conditions of heat treatment. Thus, in accordance with my invention a mass of metal powder particles which are to be bond- Y ed together into a more or less continuous metal structure, is subjected to heating While disposed in inductive relationship with a conductor in a circuit energized by an alternating current, and. the changes in the impedance (eective inductance) of the circuit, as manifested by changes in a. characteristic of the current, is noted as the temperature of the mass is raised. Depending upon the circuit employed, the impedance (or eifective inductance) change may be manifested by a change in any one or more of a number of current characteristics, such, for example, as amperage, voltage, phase relationship between amperage and voltage waves, wave form, frequency or wave length. One or more of these characteristics is observed while the heating progresses so that an index to the eiective inductance of the conductor (coil) is obtained. When the mass attains a proper temperature for good diffusion welding, the eifective inductance of the conductor which changes as the temperature rises, becomes substantially constant and the 4 aforementioned variations substantially cease. When this occurs the temperature of the mass may be observed. Having determined this temperature for a specimen briquette, like briquettes may without further testing be raised to and maintained at this temperature with the assurance that diffusion welding will occur relatively promptly and thoroughly. The same procedure may be used as a basis for automatic heat treatment control by employing means for maintaining the temperature of the mass substantially constant or for interrupting the heating or both when the effective inductance of the coil reaches a constant value.

Ina preferred modification of my invention, the temperature of a metal powder briquette is raised while it is disposed in inductive relationship with al conductor (preferably a coil in which the mass is disposed as a core) energized by an alternating current, the frequency (or conversely, the wavel length) of which varies with the effective inductance of the conductor. 'I'he frequency or wave length of the current is determined as the temperature is raised. When the frequency (or wave length) attains a substantially constant value, the proper treatment temperature has been reached, and the mass should be maintained at or about this temperature until adequate diffusion has occurred.

The mass may be heated by any appropriate means such as a furnace heated by fuel or electrical resistance. However, as explained in greater detail hereinafter, I prefer to raise the temperature of the briquette by means of eddy or short circuit currents induced within it by means of the field set up by the alternating current flowing in the conductor. These eddy currents preferably are of high frequency, because (as disclosed by Charles Hardy in his copending application Serial No. 233,387, filed October 5, 1938), the presence of these eddy currents in the briquettel not only brings about a rapid rise in its temperature but also accelerates the rate at which diffusion welding occurs. Automatic means may be employed for controlling the current input into the apparatus when the frequency of the current of the high frequency induction coil becomes constant to the end that the heat treatment of the briquette proceeds at a proper temperature. These and other features of my invention will be more thoroughly understood in the light of the following detailed description taken in conjunction with the accompanying drawings, in which Fig. l is a diagrammatic representation of apparatus of my invention adapted to the determination of the proper treatment temperature of a powder metal mass in order to bring about diffusion Welding; and

Fig. 2 is a modified form of the apparatus of Fig. 1 adapted for automatic heat treatment control.

Referring now to Fig. 1, the apparatus comprises a spark gap type high frequency generator or converter I0 energized by a source II of alternating current qf commercial frequency, say 60 cycles per second, and commercial vpotential, say 220 volts. The source is connected to a step-up transformer l2 having a primary coil I3 and a secondary coal I5. The primary is connected with the power source through a switch I4 and a variable react-ance HA. The secondary coil l5 of the transformer is connected to an oscillatory circuit including a high frequency induction coil I6 of a furnace I1, a condenser I8 and a spark gap I9. The spark gap isin series with the coil i5 in a low frequency circuit IIA and is also in series with the coil I8 in a high frequency circuit I 6A. The condenser is connected between the spark gap and the coil Il. The capacitance of the condenser is chosen to provide the required wave length for the furnace and type of Vbriquette to be use d.

'Ihe high frequency induction coll should be of low ohmic resistance and may be made conveniently as a helix of copper tubing through which a cooling fluid is circulated from a source (not shown). Disposed within the induction coil and preferably concentric therewith is a" muiile 20 made of refractory non-conductive material such as quartz and having end portions extending substantially outside the ends of the coil. One end portion is provided with an integrally formed observation port 2| closed by a quartz window 22. Below the port is an inlet nipple 23 adapted to be connected to a source of non-oxidizing, inert, or reducing gas in order that a suitable atmosphere may be maintained in the mufile. The other end of the muiile is closed by a, removable but tight fitting plug 24 provided with an'outlet nipple 25 through which the gas may be withdrawn either'to evacuate the muiiie or assure a circulation of gas therethrough. The plug is also provided with a seal 26 through which projects a thermocouple 21 that extends to the heating zone within the muille, and i. connected to a pyrometer type voltmeter 21A.

A metal powder briquette 28 to be heat treated is disposed within the muille adjacent the hot junction of the thermocouple.

Frequency may, of course, be determined by observing wave length. A radio wave meter 29 is provided. It has a pick-up coil 30 placed within the field of the induction coil. The pickup coil is connected in series with a rectifier Il and an indicator 32, such as a hot wire ammeter. A variable condenser 33 for tuning purposes is shunted across the pick-up coil. Any suitable wave or frequency meter may be employed in place of the simple one illustrated.

It is important that the means provided for supplying high frequency current .to the induction coil be such that a change in the impedance or'eiective inductance of the coil brings about a substantial change in the wave length of the high frequency circuit. The oscillatory circuit illustrated in Fig. 1 is only one of a number which will permit change of the impedance to effect the wave length (and conversely the frequency), but it is particularly well adapted to the present purpose because it permits a current of suitable wave length (say of the order of 3000 meters, or less) to be employed, and thus shortens markedly the time for heating the briquette up to optimum temperature and also :ihortens the following interval during which the eddy currents complete the diffusion welding.

In the operation of the apparatus of Fig. 1, one or more relatively cold briquettes (formed by compressing a mass oi' metal powders in any of the heretofore customary manners into a coherent mass of desired configuration, such for example asa gear) is placed in the muille, and a suitable inert, reducing or non-oxidizing atmosphere, say one of dry hydrogen, is established within the muiiie, although with certain materials an atmosphere of air or a vacuum may be employed. The induction coil is then energized by the high frequency alternating current. As

. in. the briquette.`

'I'he temperature of the briquette rises rapidly. As the temperature rises the setting of the variable condenser of the radio wave meter is altered by the operator to keep the meter in tune with the current of the induction coil. It will be observed that the wave length gradually decreases, which is -to say that the frequency gradually increases as the brlquette is heated, until at a certain temperature dependent upon the metallic constituents and degree of consolidation of the briquette, a substantially constant wavelength is attained. When this condition is reached, the temperature of. the briquette is determined by means of the thermocouple. This temperature is the one at which heat treatment should be conducted; When it is attained, the variable reactance oh the input slde.of the transformer is increased to cut down the power input to the coil and consequently the heat sup- The power input is rethe heat losses from the furnace so that no further rise in temperature of the briquette occurs, and the briquette is held for a short time, usually only a few minutes, until diffusion weldlng is complete. With certain metals having a high diffusion rate, Welding is complete practically as soon as the wave length attains a constant value, and the current may be turned off the induction coil immediately by means of the switch. Otherwise the heating should be con;- tinued at the reduced current input for a time which will vary depending upon the type of briquette employed, and will in general be short for din'icultly fusible metals and relatively longer for low-melting point metals and alloys. When the only quantity sought is the optimum treatment temperature or the constant wave length and adequate welding is unnecessary, the current may be turned off as soon as the temperature of the wave length has been observed.

My invention is applicable to the heat treatment of briquettes of any metals or metal mixture. The change of effective inductance of the induction coil is apparently due to a change in the nature of its core, which is the briquette undergoing heat treatment. At first the briquette is composed of numerous particles or small aggregates of particles at least partially insulated from each other by films of occluded or adsorbed gas, or films of oxidation products or binder. When an appropriate temperature is reached, however, welding begins to occur very rapidly, especially when diffusion is accelerated by high frequency eddy currents, and so a more or less continuous metal structure is formed from the discrete particles. The character of the core is thus changed; eddy currentsA in it pursue longer paths; and this reacts upon the energizing current by affecting the eiective inductance of the coil.

The practice of the invention is well illustrated in the following tests, in which the induction coil comprised about 60 turns of inch O. D. water cooled copper tubing wound in a helix about 31/2 inches through. The coil was energized by a Lepel high frequency spark gap converter (Type C6 in Test I, Type C-2 in Tests II to V, inclusive) which took current of 60 cycles, 220 volts and converted it to current of slightly less than 3000 meters. The wave length of the currentgin the coil was determined with a standard wave meter which had no direct connection to the energizing current, but received waves on a pick-up coil placed about 18 inches from the induction coil. A muflle of the type described hereinbefore was placed within the coil, and the thermocouple was disposed in the muille for determining the temperature of the briquettes. l

'IEsr No. I.-Heat treatment of'copper powder briquettes From electrolytically deposited copper'powder of minus 150 mesh and with relatively 'clean surfaces, twelve briquettes 3,/8 inch square and 3 inches long were made by compression in a mold under a force of 50 tons per square inch. The briquettes thus formed were removed from the mold and placed in the mule separated by thin layers of alundum powder. An atmosphere of dry hydrogen was produced in the furnace and the current was turned on. The following table gives temperatures of the briquettes, wave lengths and frequency of the current employed to heat the briquettes; and thepower input in kilowatts. During this and the following tests, the voltage of the current supplied to the spark gap converter had a potential of approximately 222 volts.

L Tempera- P ture of Wave Frequency, ower Time, minutos the brilength, cycles IEEE quoctes, meters per second Watts 20' 2590 115, 80() 2. 1 593 2560 117, 200 2. 1 782 255() 117, 700 1. 8 815 2540 8, 200 1. 8 827 2540 118, 200 1. 8 827 2540 118, 200 1. 8 sgg 2540 118, m Off l Current supplied.

2 Current reduced.

i Current orf.

4 Briquettes cooled in inutile.

It will be observed that when the briquettes reached a temperature of approximately 815 C., the wave length of the energizing high frequency current attained a substantially constant value of 2540 meters and that additional heating did not substantially change the wave length. However, if the briquet-tes had .been heated to the point at which they became plastic and distortion occurred, reduction in wave length would have been encountered.

The bars which were made from the briquettes were subjected to physical tests and microscopic examination and were found to be mechanically sound and comparable in strength to bars of cast copper..

TEST No. II.-Heat treatment of copper-tin briquettes to produce bronze bars Copper powder (90 parts by weight) and tin powder parts by weight) in a dry condition were subjected to mixing for eight hours to assure a uniform mixture. The mesh size of the powder mixture was all minus 150 (Tyler scale). The mixture was pressed into briquettes inch square by 3 inches long in molds under a force of 50 tons per square inch and twelve of the briquettes were heat treated in the muiile. An atmosphere of dry hydrogen was maintained in the muflle and the briquettes were separated from each other by thin layers of alundum powder. The following table gives values for time. temperature. wave length, frequency and power input during the heattreatment.

Temperal ture o! Wave Frequency, Pmowr Time, minutes the bulenig, you ggg que" m 9' md watts m w 118, m 2. 1 m HQ 7m 2. 1 251) '119, 'l 2., l 2510 110, m 3. 0 060 m) In 1m 3. 0 7T] 2490 12), 2. 7 821 24m 121', 1m 2. 7 821 w 121, 1w 2. 7 827 24R) 121, 1w 2. 7 121, 1w 0H olf. I Briquettes cooled in munie.

It will be noted that in Test No. II, the conditions were substantially like those in the previous test except that the optimum temperature of heat treatment was approximately 821 C. instead of 815 C. for copper, and that the presence of tin was apparently effective in reducing the wave length.

The resulting bronze'bars were subjected to ,physical tests and microscopic examination. It

was found that the copper Aand tin had alloyed and formed a homogeneous mass. Segregations of impurities such as oxides, etc., originally present upon the surfaces of the powder particles had occurred. 'I'he segregations were substantially spherical in form and did not affect appreciably the strength of the bars, which was comparable to that of bars made from cast bronze. In short, the products of the test were in every respect satisfactory.

Tssr IIL-Heat treatment of iron powder briquettes The briquettes in this case were made from iron powder obtained by hydrogen reduction of iron oxide. 'Ihe powder had a mesh size of minus 150 and had clean metallic surfaces. The briquettes were formed under a force of 50 tons per square inch. Twelve of the briquettes in rela- Vtively unsupported condition, i. e., with unconiined surfaces. were placed in the muille separated by layers of alundum powder and subjected to the effect of the eddy currents produced as described hereinbefore. The following table gives data observed during the progress of the heat treatment. During heat treatment, an atmosphere of dry hydrogen was maintained with- The resulting iron bars were entirely satisfactory. They were reduced to 116 of their original thickness by' hot rolling before any cracks appeared. and no evidence of distortion was apparent.

Trs'r IV.-Heat treatment of'iron nickel powder mixture to produce binary alloy Nickel powder (4% by weight) produced from nickel carbonyl and 'iron powder (96% by weight) produced by hydrogen reduction of iron oxide were mixed in adry condition for eight hours. The mixture had a mesh size of minus 150 and was formed into twelve briquettes inch square by 3 inches long under a force of 50 tons per square inch. 'I'he twelve briquettes, separatedvfrom veach other by layers ofralundum powder, were disposed in the muiiie in an atmosphere of dry hydrogen and subjected to heating by eddy currents induced in them. The following table gives data observed during the progress of the heat treatment.

l Briquettes cooled in muil'le.

The resulting bars of iron nickel alloy were entirely satisfactory. No appreciable distortion occurred in heat treatment so that the bars were of accurate configuration. The micrographic structure and the physical properties, such as tensile and compressive strengths, of the bars were good.

It will be observed that the optimum treatment temperature was in .the neighborhood of 1l00 C., as indicated bythe thermocouple disposed within the maille, and that this temperature corresponded to a wave length of 2510-2520 meters. The power input of 4.5 kilowatts which was established after six minutes of treatment was sufflcient to maintain the briquettes approximately at optimum treatment temperature. In other words, enough heat was generated by this power to make up for the heat lost by conduction, convection and radiation from the briquettes.

Tasr No. V.-Heat treatment of powder metals to produce ternaryv stainless steel alloys .pressive force of 70 tons per squareinch. The

twelve briquettes wereiflrst heated in the muille in 'an atmosphere of dry hydrogen. s Approximately 15 minutes were consumed in raising the briquetteto optimum heat treatment temperature which was maintained for about an hour. Thereafter, the resulting bars were allowed to Temperatureb, C., 'Ilemiea- P as o serv ure F ower tra am n e. fia 122221?, m u pyr ca e i oeter thermometers secqnd watts through couple muifle port 20 2820 106, 400 5.0 150 2800 107, 200 5.0 310 2770 108, 300 5. 440 2760 108, 700 5. 0 590 2740 109, 600 5.0 750 2730 109, 900 5.0 880 2710 110, 800 5. 0 990 2690 lll, 600 5.0 1070 2670 112, 300 5. 0 1100 2660 112. 800 5. 0 1130 26,40 113, 700 5. 6 1170 2620 119, 600 5. 6 1225 2600 115. 400 5. 6 1260 2590 115, 900 6. 0 1310 2570 116, 800 6.0 1340 2550 117, 300 6. 0 1390 2530 118, 000 6. 0 1395 2520 119, 100 4. 5 1400 2520 119, 100 4. 5 1400 2520 119, 100 4. 5 1395 2520 119, 100 4. 5 1400 2530 118, 700 4. 1410 2520 119, 100 4. 5 1400 2530 118, 700 4. 5 1390 2530 `118. 700 4. 5 1390 2530 118, 700 Off l Power on.

l Power increased by reducing reactance.

I Power increased again by reducing reactance. 4 Power decreased by increasing reactance.

5 Power oil'.

Bars cooled in muie.

The bars produced in these tests were excellent. 'I'heir corrosion resisting properties were high. Their mechanical properties were comparable in every respect to those of stainless steel produced by processes involving fusion. There was no distortion.

As the foregoing results show, optimum treatment temperatures corresponded to a wave length of 2520-2530 meters. Optimum treatment temperature was in the neighborhood of 1400o C. The temperatures observed with the optical pyrometer and the thermocouple did not check exactly, but it is'believed that the optical pyrometer readings are less exact in the high temperature range at which the work was done.

When my invention is employed for control purposes it is not necessary to observe the temperature at which frequency and wave length become constant, as will be apparent from the following description of lthe apparatus of Fig. 2, which is a modication of that of Fig, 1, like parts of the apparatus being designated by the same numerals as in Fig. l. Hence, difficulties in obtaining exact readings of high temperatures may be eliminated.

The apparatus of Fig. 2 differs from that of Fig. 1, in that it is adapted to automatic heat treatment control, and is provided with a relay the particles of the briquette.

4U in addition to the indicator of the wave meter. This relay is adapted to operate when the current flow of the meter circuit approaches a maximum, which will be when the meter circuit is tuned to resonance with the current in the high frequency induction coil. The relay is connectable in the wave meter circuit in place of the indicator by a double pole double throw switch 4I. Three leads 42, 43, are provided'on the outlet side of the relay. 'I'he center lead 43 is connected to an adjustable tap 65 on the coil of the variable reactance MA. .The outer lead 44 is.

connected to the lower end of said coil and the inner lead 42 is connected-to the power source through a time switch 46 which is also connected to the center lead.

In the operation of the apparatus of Fig. 2, the appropriate wave length value for the heat treatment of a given specimen powder mass is determined prelirninarily either with the apparatusof Fig. 1, or by heating the specimen in the apparatus of Fig. 2 with the indicator 32 connected in the wave meter circuit through switch 4I while adjusting the variable condenser 33 so that this circuit is tuned and a maximum current value is obtained. 'Ihe relay is adjusted to operate under the influence of a current approaching this value, and the switch @il is thrown to connect the relay to the meter circuit; the tap on the coil of the variable reactance is adjusted so that when the relay operates the power input to the furnace will be just enough to balance heat losses and hold a constant heat; and the time switch is set so that it will operate after an interval sucient to permit adequate bonds to be formed between With the apparatus thus set, another briquette or a plurality thereof is introduced into the muiile; current is applied by closing the switch i4; and thereafter heat treatment control is automatic. Thus, when the current in the high frequency coil attains a constant wave length, an increase in current in the meter circuit causes the relay to operate. The relay, which at rest makes a contact between inner lead 42 and outer lead da, operates to brealk this contact and establish one between lead 42 and middle lead 43. When this occurs, the time switch is energized and begins to operate and at the same time the variable reactance is increased so that the power input to the high frequency induction coil is decreased to the proper degree. The temperature of the briquette is thus held constant at the proper point for a predetermined interval of time until the time switch opens the circuit, deenergizes the high frequency induction coil and allows thebriquette to cool. The settings of the variable condenser in the wave meter circuit, the tap on the variable reactance coil and the time switch necessarily are predetermined for any type of powder metal briquette, but when once determined permit completely automatic control of heat treatment.

As indicated hereinbefore, determination of proper heat treatment conditions and automatic control thereof are preferably based upon observations of constant frequency of a current flowing in a circuit having a conductor in inductive relationship with a briquette to be heated. Howmeter an appropriate electrical measuring device for the particular electrical value to he determined. My invention contemplates determination and control of proper heat treatment conditions by such means.

From the foregoing test data it will be seen that the desirable or optimum treatment temperature indicated by the method of my invention (i. e., the temperature attained by a powder metal mass when changes in effective inductance of an inductively associated conductor substantially cease to occur) approaches but is lower than the melting point of the metal mass or product which results from the bonding together of the metal particles of the briquette by diifusion welding. This temperature relation is shown by a comparison of the indicated proper treatment temperatures and the melting points of the products of the foregoing tests:

Desirable; Meielttnl treatmeu po n o Test No' temperature. Pmdwt product,

QC. C.

815 Cowen...V 1080 R21 Bronze 1000 i093 en .,A 1530 1110 Nickel steel $1530 1400 I Stainhss steel.. 1450+ It is not necessary substantially to exceed the indicated treatment temperature in order to obtain rapid bonding together of the particles, and distortion of the briquettes tends to.increase as this indicated temperature is exceeded. Consequently, efforts should be made to hold the mass at or about the indicated temperature and inno case should the melting point be approached too closely.

I claim:

l. A process for determining a desirable temperature for heat treatment of a coherent mass of metal powder particles to bring about bonding therebetween which comprises disposing the mass in inductive relationship with a conductor energized by a source of alternating current, the frequency of which varies with the eifective inductance of the conductor, heating the mass thus clisposed to an elevated temperature, determining the frequency of said current during the heating. and observing the temperature of the mass at which said frequency becomes substantially constant.

2. In the heat treatment of a mass of metal powder particles to bring about bonding therebetween, the improvement which comprises disposing the mass in inductive relationship with a coil energized by an alternating current the frequency of which varies with the effective inductance of the coil, heating the mass thus disposed, determining the frequencyduring the heating, raising the temperature of the mass until the frequency becomes substantially constant and thereafter maintaining the mass at an elevated temperature but not substantially in excess of the temperature at which the frequency became substantially constant.

3. In the heat treatment of a. mass of metal -powders to 'bring about bonding therebetween, the improvement which comprises disposing the mass in inductive relationship with a coil energized by an alternating current the frequency of which varies with the effective inductance of the coil, heating the mass thus disposed, determining the frequency during the heating, and heating the mass until the frequency becomes substantially constant, and maintaining the mass for a considerable period of time. at a temperature in the neighborhood of that attained in the mass when the frequency became substantially constant.

4. In the heat treatment of a mass 'of metal powder particles to bring about bonding therebetween, the improvement which comprises disposing the mass in inductive relationship with a coil energized by an alternating current the frequency of which varies with the effective inductance of the coiL'heating the mass thusdisposed by eddy currents induced within it lby said alternating current, determining the frequency during the heating, continuing to raise thejtemperature of the mass by said heating until the frequency becomes substantially constant and maintaining the mass for a considerable period of time at an elevated temperature but not substantially in excess of the temperature at which the frequency became substantially constant.

5. In the heat treatment of a mass of metal powder particles to bring about metallic bonding therebetwee the improvement which comprises disposing sai mass in inductive relationship with a coll energized by an alternating current of a high frequency that varies with the effective inductance of the coil, heating the mass thus disposed by eddy currents induced within it by said. alternating current, determining `the frequency during the heating, raising the temperature of the mass by said heating until the frequency becomes substantially constant, and then reducing the input of power into said coil while maintaining the mass in inductive relationship therewith, the reduction of input of power into the coil being such that the mass is maintained at an elevated temperature for a considerable period of time but not substantially in excess of the temperature at which the frequency became substantially constant.

6. In the heat treatment of a mass of metal powder particles to bring about metallic bonding therebetween, the improvement which comprises disposing said mass in inductive relationship with a coil energized by an alternating current of a high frequency that varies with the eiiective inductance of the coil, heating the mass thus disposed by eddy currents induced within it by `said alternating current, determining the frequency during the heating, and raising the temperature of the mass by said heating until the frequency becomes substantially constant, and then reducing the input of power into said coil to a point such that the temperature of the mass remains in the neighborhood of that which it attained when the frequency became substantially constant..

7. In the heat treatment of a mass of metal powder particles to establish metallic bonds therebetween, the improvement which comprises disposing a specimen mass of said powders in inductive relationship with a coil energized by alternating current the frequency of which varies with the effective inductance of the coil, heating the mass thus disposed to an elevated temperature, determining the frequency during the heating, observing the temperature of the specimen mass when said frequency becomes substantially constant and thereafter subjecting other masses corresponding. in composition to the specimen t0 heat treatment at a temperature approximating the temperature observed when the frequency became substantially constant.

8. A process for determining a desirable temperature of heat treatment for a coherent mass of metal powder particles to bring about bonding therebetween which comprises heating the mass while it is disposed in inductive relationship with a conductor energized by an alternating current in a circuit, determining changes in the impedance of said circuit during said heating and observing the temperature of the mass when the impedance attains a substantially constant character.

9. A process for determining a desirable temperature for heat treatment of a coherent mass of metal powder particles to bring about bonding therebetween which comprises heating the mass while it is disposed in inductive relationship with a conductor energized by an alternating current in a. circuit, observing a change of impedance of the circuit as manifested by a change in a current characteristic during the heating, and observing the temperature of' the mass when the impedance becomes substantially constant.

10. A process for determining a desirable temperaturefor heat treatment of a coherent mass of metal powder particles to bring about bonding therebetween which comprises heating the mass while it is disposed in inductive relationship with a conductor energized by an alternating current in a circuit, observing changes in impedance in the circuit as manifested by changes in a characteristic of the current during the heating, and

maintaining the temperature of the mass sub- I stantially constant when changes in the characteristic of said current substantially cease to occur.

11. A process for determining a desirable temperature for heat treatment ofa coherent mass of metal powder particles to bring about the bonding therebetween which comprises disposing the mass in inductive relationship with a conductor in a circuit energized by an alternating current and thereby inducing in said mass eddy currents of suiicient intensity to heat the mass to a relatively high temperature, determining changes in impedance in said circuit and observing the temperature in said mass when changes in impedance substantially cease to occur.

1 2. In the heat treatment of masses of metal powder particles to bring about bonding therebetween, the improvement which comprises disposing the mass in inductive relationship with a coil energized by an alternating current in a circuit, heating the mass by means of internal eddy currents induced by said alternating current, determining changes in impedance of the circuit during the heating, observing the temperature at which the impedance becomes substantially constant, and thereafter heating substantially similar masses of metal powder to the temperature at which said frequency became substantially constant and maintaining them in the neighborhood of said temperature for a considerable period of time.

CLAUS GUENTER GOETZEL.

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