US2927231A

US2927231A - Glow discharge apparatus

Info

Publication number: US2927231A
Application number: US624122A
Authority: US
Inventors: Bucek Hans
Original assignee: Elektrophysikalische Anstalt Bernhard Berghaus
Current assignee: Elektrophysikalische Anstalt Bernhard Berghaus
Priority date: 1955-11-22
Filing date: 1956-11-23
Publication date: 1960-03-01
Anticipated expiration: 1977-03-01
Also published as: DE1060217B; GB852070A; FR1210232A; CH349284A

Description

March 1, 1960 H. BUCEK 2,927,231

GLOW DISCHARGE APPARATUS Filed Nov. 23, 1955 2 Sheets-Sheet 1 Elsi.

MarCh 1, 196 H. BUCEK 2,927,231

GLOW DISCHARGE APPARATUS Filed Nov. 23, 1956 2 Sheets-Sheet 2 7 INVENTOR.

BY W

A Tran 4 Y United States Patent GLOW DISCHARGE APPARATUS Hans Bucek, Zurich, Switzerland, assignor to Elektrophysikalische Anstalt Bernhard Berghaus, Vaduz, Liechtenstein Application November 23, 1956, Serial No. 624,122

Claims priority, application Switzerland November 22, 1955 4 Claims. (Cl. 31326) This invention relates to a method of surface treatment of bodies by means of bombarding the surfaces involved in the process with ions of a foreign material in an electric glow discharge, a pre-determined temperature being maintained at said surfaces.

In similar processes it is sought in most cases to bring the surfaces up to the specified temperature with as little energy as possible and to maintain them at that temperature for the duration of the process, as the cost of the treatment is in general closely related to the consumption of energy, particularly in processes of long duration. Accordingly, it is usual in such processes to take all reasonable precautions with a view to reducing heat losses due to dissipation, radiation and convection. in the first place, the treatment vessels are sufficiently insulated and the heat losses of the bodies treated are minimized. In a great number of processes on surfaces of specified temperature, it would be very desirable to intensify the actions and at the same time shorten the duration of the process. However, this cannot be achieved by increasing [the transformation ofenergy, as it is not permissible to exceed the specified temperature of the surfaces involved in the process. Accordingly, science in this field isseeking to find other ways to increase the effectiveness of such processes. 1 V

For instance, the above-mentioned group of processes on the surface of bodies in partial vacuum in an electric glow discharge, also includes the surface treatment of metallic blanks (nitriding, carburization, purifying, roughening, oxidation, reduction, etc.). Further, metallurgical and chemical processes (melting, dehydration, etc.) have been successfully performed by means of such glow discharges. Broadly speaking, these thermal processes working on the glow discharge system, present the advantage of being admirably economical, apart from-incidental advantages in technical respects and in the quality of the resulting products. Heat is supplied to the surfaces involved in the process by means of an electric glow discharge produced in the immediate neighborhood of the surfaces.

As these processes imply a predetermined temperature on the surfaces involved, the transformation of energy in the glow discharge must obviously be adjusted to the heat requirements of the bodies and materials used, these heat requirements in their turn depending chiefly on the losses caused by dissipation and radiation and possibly also on the energy consumption of the chemical reactions. For instance, in nitriding steel bodies on the glow discharge system, an energy transformation of app. 1.5 watts per sq. cm. of surface treated is required for the purpose of maintaining the optimum temperature of the steel body of SOD-550 C.

It has already been suggested that the duration of the process or the quality of the products might be successfully influenced with a higher energy transformation of the glow discharge. But since the pre-determined temperature of the surfaces involved in the process may not be exceeded, it was suggested by way of one of several 2,927,231 Patented Mar. 1, 1960 possibilities to operate on the system of impulses. If the process is performed with periodicallyalternating high and low energy transformation, it is possible to increase the energy transformation in the high performance intervals at the expense of the intermissions, Without the mean value of the energyand heat-supply exceeding the value determined by the temperature specified. Processes operating with glow discharge impulses have actually produced the advantages expected, but it is obvious that only a limited increase in effectiveness can be achieved with the temperature-governed mean value of the energy transformation.

Also, it was suggested to achieve independence from the temperature-governed mean energy transformation in similar thermal surface-treating processes, by cooling the blanks. The process according to the present invention belongs to the last-named class and is characterized by the fact that the distribution of the foreign substances diffused into the surface of the body is influenced by means for fast heat dissipation from the surfaces involved in the process and concurrently by increasing the discharge energy on these surfaces at the rate of the energy increase required for maintaining the temperature specified.

Below; the invention is set forth in several practical examples of performing the process, with the aid of 'Figs. 1 to 4, of which Fig. 1 shows a longitudinal section through a practical embodiment of a receptacle for the glow discharge treatment of hollow shafts pursuant to the invention,

Fig.2 shows a diagram Wlith several examples of the trend of the depth of hardness in nitrided steel parts,

Figs. 3 and 4 each show a longitudinal section through other practical embodiments of devices for glow discharge treatment of blanks according to this invention.

For the purpose of putting this process into practice, it is necessary to abandon the principle commonly applied in most thermal processes conducted on an industrial scale,.according to which the heat losses must be minimized with "a view to making the process an economical proposition. It is well-known that in the common thermal gas nitriding processes on steel bodies, for instance, great importance is attached to good heat insulation of the nitriding chambers, especially when these chambers are heated electrically. Similarly, in the common glow discharge processes, for instance in the surface treatment of blanks in a glow discharge vessel in partial vacuum, it is usual in order to reduce heat losses to arrange one or several bright sheet cylinders acting as a radiation reflector in a line around the blank, and to prevent any appreciable dissipation of heat through the suspension and securing arrangements.

In contrast to this general principle in thermal processes, the dissipation of heat from the surfaces involved in the processclaimed under this invention is rendered as effective as possible, even to the extent of providing a system-to cool the surfaces, if possible. Obviously in order to obtain a specified temperature of the surfaces, the supply 'of energy to them must be increased correspondingly. This increase in heat supply on the one hand and the more efiective heat dissipation on the other, produce an increased'energy transformation on every surface element of the surfaces involved in the process. This is the very'efrect that is intended, although the sole effect of the energy increase appears to be the heating of the refrigerant and the deterioration of the economical aspect.

However, surprising it may seem, it was found-at least in the cases so far investigated-that the larger energy transformation resulted in 'a corresponding reduction of the time required for the process, the overall energy requirements in this process remaining approximately the same. The'time saved in this manner repre- I sents a remarkable technical progress, apart from the surface treatment of a hollow-shaft 1in a discha'rg'e ves-.

sel 2 of metal. The blank 1 to be treated on its outside, for instance by metalizing or. nitriding, is electrically insulated from the metal vessel 2 and represents the one electrode as being concentrically encompassed by a counter-electrode 3. Both electrodes are connected to current lead-ins passing through but insulated from the walls of the vessel, as indicated at 4. Between blank 1 and counter-electrode 3 is applied the tension of the current source 5, for instance 500 volts, by which an electric glow discharge is produced between these parts with different potentials at a gas pressure of 1 to 20 mm. Hg in the vessel. The energy transformation' of this discharge is largely concentrated on to the glow edge and the cathode drop space formed in the immediate neighbourhood of the cathode. Where direct current is used, the negative pole of the tension source 5 must naturally contact the blank 1. However, alternating current of the usual frequency (50 cycles) may also be used.

The hollow shaft 1 is'cooled in this case on the inside by a refrigerant, such as oil, flowing ofi through the pipe 7 made of insulating material, such as china. The refrigerant stream can be regulated or shut off by means of the valve 8 at the supply pipe 6 made of insulating material. Also, the discharge vessel 2 provided with double-walls is expediently cooled by a refrigerant flowing in through the sleeve 9 and flowing out through the sleeve 10. The interior of the vessel is maintained at the desired underpressure by means of thevacuum pump '11 through the suction pipe 12. The gas" provided for the process can be-conducted into the interior of the vessel through the valve 13 and the pipe 14. The surface temperature of the blank 1 to be treated is 'monitored by means of a radiation pyrometer 17 through an aperture 15 in the counter-electrode 3 and a window 16 inthe wall of the vessel. i

It is expedient in starting up the apparatus, to shut off the refrigerant stream through the hollow shaft 1 by means of the valve 8 and to perform a so-called starting operation, as described in the'copending application of Bernhard Berghaus and Hans Bucek, Serial No. 473,895, filed December 8, 1954, and entitled Starting and Carrying Out of Processes Using Electric .Glow Discharges.

When the end state of the discharge and the specified temperature of the hollow shaft surface, as indicated on the monitor control 17, are reached at the'end of the .starting operation, the valve 8 is slowly opened for .cooling the inside of the hollow shaft 1, and no't.before. .The subsequent Eslow drop of the temperatures; on: the

hollow shaft surface is supervised on the indicator 17 .and the energy transformation inthe glow discharge is stepped up to a corresponding degree, either by increasing the gas pressure in the vessel 2 or by increasing the difference of tension between the hollow shaft 1 and the counter-electrode 3. The refrigerant stream is boosted until the desired energydensity in watt per sq. cm. of surface of the hollow shaft 1 is attained, while at the same time the energy transformation is steadily increased so as to maintainthe surface temperature specified- If so desired, an automatic pilotmay be provided for controlling the rate of heat dissipation and the increase of energy, this pilot being governed by the radiation pyrometer 17.

A nitriding process on steel bodies with the composition 0.3 C, 0.4 Mn, 2.5 Cr and 0.6 Mo, which was conducted in an analogous manner, resulting in a larger depth of penetration and in improvement in hardness by 5 Rockwell C over an uncooled treatment of the same duration. This nitriding process was performed in 48 hours at a treatment temperature of 520 C. in a nitrogenous atmosphere of 10 mm. Hg and with an increase of the energy density to 1.3 times the value by the fast rate of heat dissipation. An examination of the trend of depth of the surface, hardness showed that the curve of hardness depth can be appreciably influenced by this increased flow of energy on the surface of the blank. Fig. 2 shows the hardness depth curve A obtained without cooling and energy increase and, by way of com parison, a. hardness depth curve B obtained with the same steel in the treatment described above. By properly selecting gas pressure and energy transformation, the trend of the hardnessdepth curve B can be influenced and made to correspond to curve C for instance. These experimental'findings have proven, that it is possible to influence the improved surface portion produced here by intro-diffused nitrogen, by increasing the energy transformation on the body surfaces treated, despite unchanged maintenance of the surface temperature prevailing thereon. On the one hand, the duration of the. process for reaching'a specific penetration'depth of the intro-diffused substance is shortened, which represents a definite progress over the diffusion methods hitherto in common use, because it has been generally assumed that the diffusion speed cannot be influenced at a particular surface temperature. 0n the other hand, the quality of the surface portion produced by introdilfusion differs from that of the nitriding layers-obtained without energy increase, in particular in respect of surface hardness and also of the hardness depth trend and the mechanical properties, such as chipping resistance, ductility, porosity and change in volume.

Whereas prior to my invention it has not been'possible in similar nitriding processes with a steel of given alloy components, to influence the hardness depth trend except by a'suitable pretreatment, the nitriding temperature and the duration of treatment, my improved process makes it possible to influence in the manner desired the structure and the properties of, the surface portion to be modified, in applying the optimum temperature, by

selecting the energy transformation on the surfaces inproperties. 7

Finally it might be mentioned that according to experimental results, the process described permits of producing a far, greater regularity of the structure along the improved steel surface of the surfaces involved in the process, than before. For instance, the variations in hardness values in surfaces nitrided in this manner, are at least50 percent lower than usual.

Another, practical example of an apparatus for performing the process in treating the surface of the interior wall of pipes by means of a glow discharge, is outlined in Fig. 3. Here, a detachable cooling device is mounted on to the outside jacket of thepipe 20 to be treated, this device consisting for instance of the cooling cylinder 21, the

end walls

22 and 23 of which are provided each with a bore fitting the pipe 20, against which these end walls are sealed by means of the

elastic packings

24 and 25, respectively. In this manner, a circular space 26 is formed around the pipe 20, through which a liquid or eeevasl gaseous cooling agent can how by way of inlet 27 and outlet 28. The two mouths of the pipe 20 protrude from the cooling cylinder 21 into dome-like extensions formed by the

caps

29 and 30 secured in a gas-tight manner to the

end walls

22 and 23 respectively. The two domelike extensions are interconnected by the bore of the pipe 20 and jointly form the glow discharge space. A hollow counter-electrode 31 extends through the pipe 20 and is passed through the

caps

29 and 30 with

insulators

32 and 33, respectively. This hollow electrode 31 is connected to one of the poles of the tension source 34, while the other pole stands in connection with the cooling cylinder 21, and, through the lead 35, with the pipe 20. Through

pipes

36 and 37, the treating gas is conducted into the gas discharge chamber and pumped out of it, respectively.

In operation, a glow discharge is generated between the inside wall of the pipe 20 and the hollow electrode 31, and when the pipe 20 operates at least intermittently as the cathode, this discharge heats up the inside wall. Owing to the cooling of the pipe 20 by the coolant in the chamber 26, heat is dissipated fast from the inside wall of the pipe involved in the process, so that the energy transformation of the glow discharge and the energy impingement on to the inside wall can be increased. The hollow electrode 31 can also be cooled from the inside by a stream of liquid or gas. This is expedient when alternating current is used, for instance, for the purpose of avoiding an excessive heating up of the hollow electrode acting intermittently as the cathode. However, also where direct current is used in conjunction with a hollow electrode acting as the anode, it might be expedient to cool the electrode in order to dissipate the heat originating from the inside wall of the pipe.

The means to be provided pursuant to the process of this invention for the fast dissipation of heat from the surfaces involved in the process, do not necessarily require a coolant stream as in the practical examples indicated in Figs. 1 and 3. Especially in the case of small blanks, cooling might be satisfactorily effected merely by intense heat dissipation or heat radiation. A practical example of this case is outlined in Fig. 4, in which all minor details are omitted. This example pertains to the surface treatment of the inside wall of a metal sleeve 40. For fast heat dissipation, here the sleeve 40 is inserted into a massive metal block 41 of good heat conductivity, for instance of copper, this block presenting an accurately fitting bore 42. The mouth of the sleeve 40 faces a discharge chamber 43 formed by a cowl 45 secured to the block 41 with the insulating ring 44 in an electrically insulated and gastight fashion. An electrode bar 47 is fitted inside the cowl with insulator 46 in a gastight fashion, this bar extending into the sleeve 40 and being connected outside the cowl 45 to one of the poles of a tension source 48, this pole also contacting the metal block 41. The other pole of the tension source 48 is connected to the metal cowl 45 insulated from the block 41 and from the electrode bar 47.

If an underpressure gas atmosphere of a few mm. Hg is produced in a suitable manner in the chamber 43 and a direct current source 48 of 400 to 1000 volts is used, a glow discharge is produced when the cowl 45 operates as the anode, a glow edge appearing close to the inside wall of the sleeve 40 and the outside of the bar electrode 47, although these two parts have the same negative potential. By selecting a suitable diameter for the bar electrode 47 and a corresponding gas pressure in the chamber 43, these cathodic glow edges and consequently the associated cathode drop spaces can be made to touch, which by experience results in a considerable increase of the energy density in the inside space of the sleeve (so-called hollow cathode effect). The heat produced in this manner on the inside wall of the sleeve 40 is dissipated fast by the large mass of the metal block 41. Normally, the intense radiation and air cooling achieved by virtue of the large outside surface of the block 41 is sutficient to prevent the temperature of the inside wall of 6 the sleeve 40 from increasing beyond the value specified. If not, an additional liquid or gas system can be provided for cooling the block 41.

The described heating up of the sleeve 40 by means of the hollow cathode effect can be effectively influenced with this type of apparatus by regulating the gas pressure in the chamber 43, thus permitting safe control. The efiect desired can be obtained both with a direct current source and an alternating current source.

This hollow cathode eifect can also be applied for the purpose of producing a higher energy transformation in the predetermined ranges of the surfaces involved in the process.

In the process of the present invention, the increased energy density of the glow discharge results in a more intensive ionic bombardment on the surface acting as the cathode, it being possible that both the mean kinetic energy and the number of gas ions impinging per unit of time increase. Apart from the fact that the metal surface as a result is heated more intensely, which, incidentally, is exactly compensated by the fast heat dissipation to the coolant, it is probable that a deeper penetration into the outer layers of the lattice structure of the body and a larger quantity of atomic gas at the limit layer play a part.

However, it must be explicitly stated that several preliminary investigations on the effect of the increased passage of heat in radiation heating of metallic objects in respect of the surface treatment, have shown an excellent effect of the larger flow of energy through the surfaces involved in the process on the acceleration and quality of the process. Also the impingement with strong heat impulses plays a part.

The process of the invention is not necessarily restricted to the surface teratment of metallic and other bodies, but all processes taking place on the surface of solid and liquid material masses or layers can be embodied in the process claimed under this invention.

I claim:

1. Apparatus for the diffusion treatment of a metallic body in a gaseous atmosphere, comprising a discharge chamber having connections for evacuating the same and having at least one insulated current lead-in, gas supply and gas suction lines connected to said chamber, means for suspending a metallic body within the chamber and for electrically insulating the same from the walls of the chamber, means for electrically connecting the body with a current lead-in which is at least periodically connected to the negative pole of a source of current, a metallic member providing a counter-electrode for producing with the said body a glow discharge therebetween, the other pole of the source of current being connected to said metallic member, said metallic body being hollow, and electrically insulated means for conducting a cooling agent to and from the holow interior of said body and in direct contact therewith to promote the withdrawal of heat.

2. Apparatus for the diffusion treatment of a cylindri cal metallic body in a gaseous atmosphere, comprising a metallic discharge chamber adapted to surround said body and having gas supply and gas suction conduits connected thereto, at least one electrically insulated cur rent lead-in connected with said body in the interior of the chamber, said lead-in being adapted to be connected externally of the chamber and at least temporarily with the negative pole of a source of current, the other pole of the source of current being connected with the wall of the chamber, a cooling jacket about the chamber and providing an annular cooling space, conduits for charging a cooling agent into and discharging the same from said jacket, and means for directing a current of cooling agent into direct contact with said metallic body to promote the withdrawal of heat.

3. Apparatus for the diffusion treatment of a cylindrical metallic body in a gaseous atmosphere, comprising a metallic discharge chamber adapted to surround said body and having gas supply and gas suction conduits connected thereto, at least one electrically insulated current lead-in connected with said body in the interior of the chamber, said lead-in being adapted to be connected externally of the chamber and at least temporarily with the negative pole of a source of current, the other pole of the source of current being connected with the wall of the chamber, a cooling jacket about the chamber and providing an annular cooling space, conduits for charging a cooling agent into and discharging the same from said jacket, and electrically insulated supply and discharge conduits for cooling agent connected with the hollow interior of the cylindrical body for directing a current'of cooling agent into direct contact with said metallic body to promote the withdrawal of heat.

4. Apparatus for the diffusion treatment of the interior surfaces of hollow metallic bodies of relatively small dimensions in a gaseous atmosphere comprising a discharge chamber composed of a massive metallic block provided with a bore forming a socket and a cowl overlying said block, means for evacuating the chamber, gas a supply and gas suction conduits connected with the chamber, at least one current lead-in electrically insulated from the metallic socket, said lead-in being connected exter- References Cited in the file of this patent UNITED STATES PATENTS 2,371,27 Berghaus et-al Mar. 13, 1945 2, 77,071 I'Carne Apr. 27, 1954 2,755,159 Bernieret a1, July 17, 1956 V FOREIGN PATENTS 427,623 Great Britain jA i. 23, 1935 I 529,544 Great Britain Nov. 22, 1940 OTHER REFERENCES Archiv fiir das Eisenhuettenwesen, Bennek and Ruediger, vol. 18, July-August 1944, No; 12, pages 61-67.