US2916409A

US2916409A - Process and apparatus for the treatment of ferrous articles

Info

Publication number: US2916409A
Application number: US624169A
Authority: US
Inventors: Bucek Hans
Original assignee: Elektrophysikalische Anstalt Bernhard Berghaus
Current assignee: Elektrophysikalische Anstalt Bernhard Berghaus
Priority date: 1950-11-09
Filing date: 1956-11-23
Publication date: 1959-12-08
Anticipated expiration: 1976-12-08
Also published as: US2946708A; GB810802A; FR69955E; GB828699A; FR1126104A; LU34829A1; CH342980A; DE1141850B; BE553492A

Description

mac. 8, 1959 H. BUCEK PROCESS AND APPARATUS FOR THE TREATMENT OF FERROUS ARTICLES Filed NOV. 23, 1956 FIG.3

s O O O *3 3 3 o INVENTOR HA NS BUCEK ATTORNEY d S P r 2,916,409

1 Patented Dec. 8, 1959 sion of approximately 1.5 Watts per square centimeter of 2,916,409 surface to be treated is necessary to maintain the most PROCESS 'AND APPARATUS FOR THE TREAT- I MENT F FERROUS ARTICLES Hans Buc ek, Zurich, Switzerland, assignor to Elektrophysikalische Anstalt, Bernhard Berghaus, Vaduz, Liechtenstein Application November 23, 1956, Serial No. 624,169

Claims priority, application Switzerland November 9, 1950 14 Claims. (Cl. 148-10) This invention relates to the production of improved surface layers on tubes of iron, steel and steel alloys, and particularly over the interior surface of gun barrels.

The present application is a continuation-in-part of my copending applications, Serial No. 255,751, filed November 5, 1951, now abandoned, and Serial No. 534,376, filed September 14, 1955.

It is the general object of the invention to provide an improved process and apparatus for the treatment of ferrous articles for improving their surface properties and characteristics with the aid of an electric glow discharge.

More specifically, it is an object of the invention to produce surface hardening of articles of iron, steel and 'steel alloys in a more effective manner than heretofore and with shorter periods of treatment.

Other objects and advantages of the invention will become apparent from the detailed description thereof hereinafter.

The invention can be applied to the treatment of articles of various kinds, but is of particular value for the treatment of the barrels of various types of weapons, such as cannon, machine guns and other automatic weapons, and the like, and will accordingly be described in detail in connection with such articles.

Because of the great stresses arising during the firing of modern weapons, the barrels are rapidly worn and their inner surfaces destroyed. In particular, owing to the rapid succession of firing, gun barrels are subject to intensive abrasion, so that they become useless within a short period of time and must be replaced.

In accordance with the present invention, it is possible to increase considerably the durability of such tubes against mechanical stresses, particularly of the inner walls, by means of a treatment of such tubes whereby their inner surfaces, or parts thereof, are improved substances, by bombarding the inner walls of the tubes with ions of such foreign substance or substances in an electric glow discharge.

Ions of nitrogen, hydrogen, carbon, silicon, boron, and

'various metals, among others, can be used in this pressure and temperature.

As electric glow discharges generally require a predetermined temperature at the surfaces to be improved 'by such processes, the energy conversion in the glow dis charge must, of course, be in accord with the heat requirement of the tubes or other workpieces, which in turn is conditioned by the losses due to conduction and radiation, as well as by any energy consumption of the chemical reactions involved. For example, in the elec- "trical glow nitriding of steel bodies an energy converthrough the action of what may be termed foreign V favorable temperature of the steel body of about 500- 550" C. a

It has already been suggested that the time of treatment or the quality of the surface layer could be influenced favorably by a higher energy conversion of the glow discharge than has heretofore been applied. However, as the predetermined temperature of the surfaces to be improved may not be exceeded, an impulselike method of operation has been proposed as the only possibility of employing higher temperatures. If the process is conducted with periodically'alternating high and low energy conversion, then an energy increase is possible in the high output intervals at the expense of the intervening intervals, in which the energy conversion is reduced, without 'causing' the time-average value of the energy and heat supply to exceed the value set by the prescribed temperature. The method of operating with glow discharge impulses has actually brought the expected advantages, but because the average value' of the energy conversion is set by the temperature limitation, only a limited increase of effectiveness could be achieved.

According to the present invention not only the preferred impulse type of operation of anelectric glow discharge for the surface treatment of ferrous articles, but also the constant voltage type of process, is improved in such manner that higher energy densities per unit area of the surface being treated, or a higher total value of energy conversion, is made possible without exceeding any more or less critical temperature limits. This is accomplished by simultaneously employing measures during the bombardment with the ions of the foreign material which lead to an increased heat removal at least from the surfaces to be improved, while the energy supply by the ions to these surfaces is increased by the amount necessary to maintain the predetermined temperature. In this Way a considerable and predetermined increase of the energy conversion at these surfaces is made possible.

In the known nitriding processes the greatest importance is placed on good heat insulation of the nitriding chamber, as for instance in the known thermalgas nitriding processes of steel articles, particularly when an electrically heated chamber is involved. Similarly, it has been the practice in the heretofore known glow discharge processes, for instance, during the surface treatment of workpieces in a glow discharge chamber at subpressures, to array one or more bright tin cylinders as radiation reflectors, one behind the other, around the workpiece to reduce the loss of heat, and likewise to prevent any considerable heat conduction through the hanging and holding means as much as possible.

Contrary to this general principle of operation for thermal and glow discharge processes, a large amount of heat, and generally as much heat as possible is conducted away from the tube or other article in the practice of the present invention. This is accomplished, for example, by constructing the gas discharge chamber with double walls and deliberately cooling the same by means of a flowing coolant. To achieve the predetermined temperature of the tube in such case, the energy supply to the same must necessarily be correspondingly increased. This increased heat supply on the one hand, and the improved heat conduction on the other hand, effect an increased energy conversion at each surface element of the areas subjected to the process. This very effect is desired, even though the increase of energy appears merely to heat the cooling agent and an impairment of the economy of the process is accordingly to be expected.

It has, however, developed in a most surprising manner, that the increased energy conversion makes possible a decrease of the treatmenttime heretofore required in a given case, so that the total energy consumption remains approximately the same. An important technical advance has thereby been achieved because of the saving of time alone, as well as an increased economy in the use of the apparatus for the carrying out of the treatment by reason'of a higher output. Aside from the saving of time, however, I have found that the higher thermal intensity makes it possible to effect the production of surface layers of an improved quality in all the various types of glow discharge treatments I have tested.

The results of tests have established that by an increase of the energy conversion at the treated surfaces of the tubes, the improved surface zone created by inter-diffused nitrogen can definitely be influenced, despite the fact that the surfaceternperature is maintained unchanged. n the one hand the duration of the treatment for achieving a certain depth of penetration of the diffused material is shortened, which constitutes an important ad vantage over the diffusion methods heretofore suggested especially as it was considered impossible previously to influence the diffusion speed while keeping the surface temperature constant. On the other hand, the quality of the surfacezone obtained by the inter-diffusion in accordance with my improved process is. diiferent from the nitrided layers obtained without energy increase, and is superior with respect both to surface hardness as well as the hardness depth pattern, and the mechanical characteristics, such as Splinterresistance, ductility,'porosity' and volume change.

Whereas it was possible heretofore in nitriding processes of this type and in the case of'a steel with given alloy components to influence the hardness depth pattern only by way of-a suitable pre-treatment, according to the present process the structure and the characteristics of the obtained surface zone can be controlled in the desired manner, employing the'optimum temperature, by the selection of the degree of energy conversion at the sure faces to be improved. Of course, in the case of a tube made of an as yet unknown steel alloy, and as is usual in other metallurgical processes, the characteristics of the surface zone resulting from different degrees of energy conversion at the surface and at different surface temperatures and in dependence on the duration of the treatment must be determined experimentally by means of several test pieces, in order to establish the optimum relationship of the several factors involved for improving tubes of such steel in a corresponding manner. However, the results obtained will be unequivocally reproducible, and a single series of experiments will result in a formula for the production of tube surfaces with pre-determined characteristics.

It should also be mentioned that according to experimental results, by the process of my invention a much higher degree of uniformity of structure can be achieved along the improved steel surface than has been possible heretofore. For example, fluctuations in the hardness values of surfaces nitrided by my process are at least 50% less than in the best nitrided surfaces heretofore obtained.

When gas pressures of 2 mm. of Hg and hi her are used, the electric glow discharge is preferably conducted with energy supplied periodically for short intervals. In this case the short-period energy impulses should be at least 10% higher than the permanent tension. The pause intervals should be made equal to or larger than the periods of increased energy supply. The glow discharge in the case of a nitriding process is powered with about 0.2 to 5 Watts per cm. of surface area of the workpiece. Other gases, preferably hydrogen, can be added to the treatment gas. Normal ammonia gas or a mixture of nitrogen and hydrogen at a ratio of 1:10 to 1 :25 can be used for nitriding.

The treatmentgases are purified before they are introduced into the treatment chamber, so that even smallquantities of undesired gases are removed, depending on the type of treatment involved, for instance oxygen, or when working, for example, with nitrogen, also any small quantities of hydrocarb'onsthat may be present. This purification of the treatment gases can be and preferably ness of about V mm. or more. In the preferred man-,

her of carrying out the present invention, the tubes are I connected as the cathode of the glow. discharge. The current conductors to the electrodes should be insulated and shielded by means of a gap against the gas discharge in known manner.

As already mentioned, his advantages in many cases, particularly when working with higher pressures, to supply the glow discharge energy in impulses, i.e., to use electric impulses which with longer duration would lead to considerably higher temperatures of the gun barrels. The high energy, however, is supplied only for short intervals, and pauses or intervals at lower energy levels, are interposed, which are about as long as but preferably considerably longer than, the intervals with high energy. For example, a base or maintenance voltage. of 400 volts is applied and this is superimposed at intervals of 3 to 5 seconds byvoltage impulses of several tenths of a second duratiorn'the amplitude of which is a multiple of, for instance ten times, that of the base voltage. The gas pressure can then be increased to several millimeters of mercury; A particular advantage'of this periodic mode of treatment lies in the fact that the temperatures in the core of .the inner wall of the tube can be maintained so low that this part of the material is not influenced in any way. A further advantage is that the high energy glow discharge also affects with sufficient intensity also the central parts of the interior which are farthest from the tube ends.

In the present process operating with a glow discharge as heat source for the metal surfaces to be thermally treated, in contrast to a thermal treatment by heat radiation, a further advantageous effect in addition to the increased energy conversion at the treated surface can be realized. The increased energy density of the glow discharge results in a more intensive ionic bombardment on the surface which acts as the cathode, and indeed both the average kinetic energy as well as the number of impinging gas ions per unit of time can be increased. In addition to the thus resulting increased supply of heat to the metal surface, which is completely compensated by the rapid heat flow-off to the cooling agent, a deeper penetration into the outermost layers of the grid or lattice structure of the body and a quantitatively greater supply of atomic gas at the boundary layer appear also to be of influence.

With proper choice of the ionic energy and ionic density for the steel alloy to be treated at any time, an improved surface zone can be produced which, compared with an identical steel tube after treatment with the usual gas nitriding process with catalytic gas dissociation, has at least the same hardness but greater ductility, resistance to wear and to splintering. This is due principally to the fact that in the case of glow-discharge treatment, in contrast to the usual gas nitriding, the formation of a brittle surface layer is avoided. Consequently, the consumed mass of foreign material-in the case of nitriding, the quantity of nitriding nitrogen which must continually be charged into the gaseous atmosphere, is at a maxiby about 30% greater than the theoretical consumption through enrichment of the steel surface. Also in this respect the present invention is distinguished from the known gas-nitriding processes in which the requirement of foreign material exceeds the theoretical consumption by about 100 to 500%. In the use of the present invention in connection with the treatment of firearm barrels and particularly artillery barrels, they are mechanically completely finished prior to the treatment and brought to the prescribed end tolerance's. The glow-nitriding produces neither any substantial increase in volume of the nitrided surface nor a con traction of the barrel. After the inner wall parts of the barrel which are contacted by the projectile have been subjected to an ionic bombardment in the above-described manner, no after t'reatment of a mechanical nature is necessary. 7 j t t The prevention of any substantial increase in thevoltune of the nitrided surface, that is, any substantial change the dimension of the treated tube or other article, is accomplished by the aid of the additional gas above mentioned, such as hydrogen, or a rare gas. This added gas efiects compensation, by cathode disintegration, of the increase of volume which occurs in the normal nitriding, of the surface of a ferrous article with nitrogen or ammonia. The cathode disintegration effected by the added hydrogen or rare gas is quantitatively different from the cathode disintegration occurring during the glow discharge treatment in the nitrogen atmosphere. Nor- 'rrially hydrogen will cause decrease in the cathode disintegration, whereas the rare gases, e.g., argon, krypton, neon an'd xenon cause an increase in cathode disintegration. By the proper selection of the added gas and of its -quantity, a substantially complete compensation of thenormalvolume'increase caused by nitriding canv be effected. 'These added gases can be employed in such man'ner thateither their proportion in the gas atmosphere or their'partial pressures remain constant throughout the treatment. This can easily be achieved by regulating the gas-supply and, if necessary, the gas removal. The additional gas can, however, also be fed intermittently, e.g., by periodically interposing breaks, which may last from fractions of a minute up to several minutes, or it can be fed during part of the treatment only. In my copending application, Serial No. 255,751, filed November 9, 1951, there is described in greater detail the use of added gases 'for compensating for volume changes caused by treatments in an electric glow discharge, and it will be understood that the procedures described in said application may be combined with the novel modifications described herein.

, In order to facilitate entry of the glow discharge into the inside of the tubes, an axially tensioned wire or a cylinder of smaller. outside diameter than the internal tube diameter can be provided, to which also an electrical tension is applied. For carrying out the process 'of the present invention, a total electric output of at least 3 to 10 kilowatts is preferably used. The articles to be improved may be and preferably are preliminarily subjected to an ionic bombardment of reducing gases, such -as hydrogen and the like, before the actual treatment. 'In this case it is advantageous to keep the articles at an increased temperature by means of a glow current. This pre-treatment improves the technological characteristics "of thematerial, for instance, with respect to the notch strength, etc., by removing small quantities of sulfur or phosphorus, and it prepares the surface of the articles inan'excellent manner for receiving the actual improving element.

' 1 In one mode of carrying out the process of the invention, a gun barrel is hung vertically in a gas discharge chamber and is connected to the negative pole of a direct current source, for instance, a current rectifier, which is adapted-to supply a current of several amperes at a tension of several hundred volts, i.e.,--sufiicient to maintain the article at the necessary temperature. The glow discharge is effected in agaseous atmosphere-at a pressure of, for instance, several millimeters of mercury. The positive pole of the tension source is connected to an anode arranged inside the gas discharge chamber or to the metallic wall of such chamber itself. The gas discharge is thus effected between the gun barrel as cathode and the anode. If desired, the glow discharge can also be powered with alternating current, as in that case the gun barrel acts as the cathode for part of the time. A conducting wire or cylinder may be disposed through the tube, and should have a positive voltage with respect to the tube for at least part of the time.

The invention will be further described by reference to the accompanying drawings forming part of this specification, wherein:

Fig. l is a diagram illustrating the hardness-depth curves of gun barrels treated in accordance with the invention and according to known processes;

Fig. 2 is a view in vertical section of an apparatus for carrying out the process of the invention; while Fig. 3 is a diagram showing the relation between muzzle velocity and number of firings in barrels of automatic weapons.

A satisfactory embodiment of an apparatus for carrying out our improved process for the surface treatment of the inner walls of tubes by means of a glow discharge is shown schematically in Fig. 2. In this construction, a removable cooling device is secured to the outer surface of the tube 20 to be treated, this cooling device consisting, for example, of cooling cylinder 21, whose end walls 22 and 23 each have a bore suitable for receiving tube 20 and which are sealed against the same by the elastic seals 2.4- and 25, respectively. In this manner an annular chamber 26 is created around tube 20, through which chamber a liquid or gaseous cooling medium can flow from inlet 27 to outlet 28. The two muzzles of tube 20 project from the cooling cylinder 21 into dome-shaped enlargements formed by the closure caps 29 and 30 and are fastened to the front walls 22 and 23 in gas-tight manner. The dome-shaped enlargements are connected with each other through the bore of tube 20 and jointly form the glow discharge chamber. A hollow counter-electrode 31 extends axially through tube 20 and is led through the closure caps 29 and 30 by way of

insulators

32 and 33. The hollow electrode 31 is connected to one pole of a direct or alternating tension source 34 whose other pole is connected to tube 20 by way of cooling cylinder 21 and connecting wire 35. When direct current is used, it is preferably connected to the positive poleas anode. The treating gas is led into and pumped out of the gas discharge chamber by way of conduits 37 and 36, respectively.

During operation, a glow discharge is created between the inner wall of tube 20 and the hollow electrode 31, which heats the inner wall of such tube, the latter operating at least intermittently as the cathode. Because of the cooling of tube 20 by the cooling'medium in chamber 26, heat is led off rapidly from the inner wall of the tube, so that the energy conversion of the glow discharge and the energy applied to the inner walls can be increased over the amount normally required to maintain the predetermined temperature in the workpiece. The hollow electrode 31 can also be cooled from its inside by means of a stream of liquid or gas. This is advisable, for example, if alternating current is used, in order to avoid excessive heating of the hollow electrode which in this case acts intermittently as cathode. But also when direct current is used with the hollow electrode as the anode, cooling of the latter can be advantageous, in order to lead off the heat radiated from the inner wall of the tube For nitriding, a nitrogen-containing gas is employed which was purified prior to being charged into the glow discharge chamber over ferrosilicon at elevated temperatures. The gas may consist, for example, of a mixture of nitrogen and hydrogen, or ammonia gas in pure form or enriched with nitrogen ondiluted with hydrogen or another suitable gas, and any :of these gases may contain also a rare gas (argon, neon, xenon or krypton). Then an electric tension is applied of such magnitude, and the gas discharge pressure is so adjusted that the tube assumes a temperature suitable for nitride-hardening, which as a rule is at approximately 500 to 550 C. The time of treatment, depending on the desired depth of nitriding, is ordinarily between about 12 to 30 hours, but can be higher, e.g., 48 hours, or lower, depending on the particular requirements.

A nitriding process conducted in this manner-on a steel tube containing 0.3% carbon, 0.4% manganese, 2.5% chromium, and 0.6% molybdenum at a temperature of 520 C., and a treatment time of 48 hours in a nitrogen-containing atmosphere at 10 mm. Hg, as well as an increase of the energy density by the rapid heat removal to 1.3 times its usual value, resulted in an increased penetration depth and thereby an increase of the hardness measurement value by Rockwell C as contrasted with a treatment of the same duration but without cooling and under otherwise identical conditions. Examination of the depth pattern of the surface hardness showed that the hardness-depth curve was markedly influenced by this increased energy flow at the surface of the inner wall of the tube.

Fig. 1 shows a hardness-depth curve obtained without deliberate cooling and without energy increase, and, for purposes of comparison, a hardness-depth curve 11 obtained with the same steel according to the treatment described herein. By a suitable choice of gas pressure and energy conversion or density, the course of hardnessdepth curve 11 can be influenced, at will, and can, for instance, be made similar to curve 12.

In place of the apparatus shown in Fig. 2, there can be employed also metallic glow discharge chambers which are long enough to receive a tube and have a sutficient diameter to receive several tubes arranged parallel to each other and without disturbing each other.

The firearm barrells treated in accordance with the invention display definitely improved properties, both compared to the untreated as well as to gas-nitrided tubes (by radiant heat) of the same grade of steel. The improvement in the case of gun barrels resides particular ly in the fact that the rifting of the inner wall, despite the hardening effected by the glow treatment, remains free, on subsequent firing, from splinters which are visible without optical magnification. Instead, there follows gradually a smoothening of the walls of the rifling grooves, somewhat as in the case of non-nitrided firearm barrels.

Examination of a gas-nitrided (by radiant heat) and a glow-nitrided firearm barrel according to the invention in respect of the surface condition after an equal number of firings (within the range of utility of the thermally nitride barrel) was made on photographs taken with a tube camera, and at three different distances from the cartridge case of the Weapon. The thermally nitrided surface was seen to be badly splintered, indicating a high de gree of brittleness. On the other hand, the glow-nitrided surface showed no visible splintering, and hence, despite the hardness, the treated surface was tough and ductile. It was also clearly evident that in the case of glow-nitrided barrels, the grooves were well maintained, Whereas in the case of the gas-nitrided barrels, the grooves were greatly deformed. The uniform smoothening of the glow-nitrided groove walls was likewise clearly visible. The glownitridcd surface produced in accordance with the present invention is therefore at the same time harder, more splinter-proof and more durable than the nitrided surfaces of an identical barrel produced by the usual heating in a nitrogen-containing atmosphere.

This difference in properties was established also by comparing test pieces of steel, identical in constitution, after thermal nitriding and after glow-nitridinglon the basis of hardness testing irnpressions. In the case of known thermally nitrided surfaces, impressions of only $5 inch diameter showed fractures and splintering about the impression circle, whereas impressions of the 3/ inch and even inch diameter in the case of the Surfaces treated according to the present invention had clean circular outlines without any evidence of fracture or splintering. In fact, the impressions of the V inch diameter, which were deeper than the "71 inch impressions, and corresponded to 60 Rockwell-C, had a raised ridge of uniform outline about their circumferences, showing. that the hardened surface was capable of flow under pressure Without rupture. t

On firing tests with radiant heat-nitrided and glow nitrided gun barrels of identical construction and identical quality of steel, it has been found also that the life of the glow-nitrided barrel is considerably longer than that subjected to radiant heat nitriding. The diagram shown in Fig. 3 illustrates by way of example the muzzle velocity V and the barrel temperature T upon the basis of measurements on a radiant heat-nitrided barrel (full line) and on a glow-nitrided barrel (dotted line), in dependence on a number of firings. Theradiant heatnitrided barrel was tested in six groups, each with firings and. each with a cooling pause, and after 6 firings during which it attained a maximum temperature. of about 280 C., showed a drop in the muzzle velocity V of 10%, so that the barrel became unfit for further use. j

The glow-nitrided barrel was tested in a considerably more rigorous manner in groups in which each barrel was subjected to 3X firings before a coolinginterval was interposed. Despite the fact that this tube had to withstand temperatures up to 430 C., the muzzle velocity V after 6 firings remained practically unchanged. Tests have in fact shown that the glow-nitrided barrels of the present invention make possible a firing number as high as 250% and more as compared with radiant heat nitrided barrels made under otherwise identical conditions, before the V falls more than 10%.. Investigations conducted at different firing frequencies prescribed according to tactical requirements showed in all cases, and with reference to the necessary number of firings before there occurred a drop in the muzzle velocity of 10%, an improvement of at least 50% in the case of the glow-nitrided barrels.

Even in the case of continuous firing with automatic weapons, that is, of rapid fire weapons with high cadence, there has been established an increase of at least 10% in the number of firings before the first appearance of transverse splinters int the case of barrels treated in accordance with the invention. With a firingspeed de' termined by tactical requirements, corresponding to the useful life span of thefirearm barrel in question, the number of firings, up to the first appearance of transverse splintering, could be increased by at least 50% by the glow-nitriding of the barrels, in comparison with radiant heat-nitrided barrels.

Reference has been made hereinabove to theuse of metals for improving the surface of ferrous articles by deposition of such metals on such surface in an electric glow discharge conducted in accordance with the invention, and diffusion of'the metal into the surface layer of the article. While the metal can be vaporized in the glow discharge chamber in an electrically heated crucible, I prefer to effect such vaporization either by means of the cylinder 31 or a wire used in place thereof which is provided with a film of the metal to be deposited, or by coating the surface to be treated with a layer of the metal. The wire or cylinder 31 disposed along the axis of the tube 20 operates in the above described process to insure uniform dispersion. If such coated wire or cylinder is under a negative tension, a sputtering of the on its surface.

same results and the evaporated material is deposited on the inner wall of the tube, and on longer duration, it is diffused into the tube.

The necessary high temperatures at the surface of the inner wall of the tube in this case can be attained by means of sufiicient periodic impulses of high energy and of short duration. By suitable choice of the gas pressure in relation to the distance of the central wire or cylinder from the inner surface of the tube, a considerable increase of the energy of discharge in the interior of the tube can be achieved.

An example of the diffusion of aluminum is contained in my c'opending application Serial No.-255,751, and the process therein described can be conducted according to the principles of the present invention. Other metals which can be deposited by my present process are chromium, nickel, tungsten, manganese, molybdenum, and in general all of the metals commonly added to iron and steel to produce ferrous alloys. Further examples of metal deposition and diffusion in which the novel features of the present invention can be incorporated are the following:

A shaft 10 of carbon steel (carbon content 0.3%) is treated in such manner that a film of tungsten is provided To this end, a counter-electrode consists of a thin cylinder of tungsten surrounding the shaft; it may, however, consist also of a cylinder of another metal, such as iron, which is coated on its inner surface with tungsten. (In the case of treatment of the inner wall of a tube, there will be employed a wire or cylinder disposed along the axis of the tube.) The discharge gap is subjected to an alternating current of 480 to 550 volts at 50 or 60 cycles per second. The chamber is filled with a gas atmosphere at 8 to 10 mm. Hg, and such degree of pressure is maintained throughout the treatment.

During the first portion of the process, lasting from about 18 to 20 hours, a glow discharge is maintained about the workpiece in an atmosphere of pure hydrogen, the workpiece being at a temperature of about 550 C., the voltage being regulated to a corresponding value within the above-mentioned limits.

The temperature at the counter-electrode can be somewhat higher in order to obtain as high an evaporation of tungsten as possible. At the end of the stated period of treatment, the workpiece has been provided with a surface zone containing tungsten through the deposition of the tungsten and its difiusion into the metallic surface of the workpiece, such tungsten zone adhering strongly to the workpiece and being capable of withstanding high mechanical stresses. However, the treatment results in an increase of the shaft diameter by about 10 to 20 microns, which may be undesirable for the surfaceimprovement of precision parts.

Accordingly, after the termination of the first stage of treatment, the hydrogen atmosphere is pumped out of the chamber and replaced by an atmosphere of argon, at a pressure of 0.8 to 1.5 mm. Hg, which pressure is maintained throughout the second stage of treatment. In such second stage, the workpiece is subjected to a glow discharge for about 8 to 10 hours at a somewhat lower temperature than in the first stage, for example, at 450 C. In this second stage, the glow discharge effects, as has been found in actual operation, a preponderating evaporation of metallic particles from the surface of the workpiece. Consequently, there occurs a reduction in the diameter of the workpiece which is not attended by any disadvantageous eifect on the improved surface zone. The evapo- 1 0 into the chamber, the latter opened andfthe workpiece is removed. I V

If desired, the first and second stages can be carried out also in relatively large number of first stages of shorter duration and in alternation. For example, the first treatment can last for about 4 hours in a hydrogen atmosphere, and thereupon the workpiece is subjected to the second stage in an argon atmosphere for about 1 /2 to 2 hours, this cycle being repeated five times, so thatthe total time of treatment is about the same as indicated hereinabove.

The just-described procedure gives satisfactory results, but better results are obtained, especially in the first stage of the process, if an intensive cooling of the workpiece is provided coupled with an increase in the energy conversion at the surface of the treated-article. The voltage can in such case be increased by about 100 volts or more, and can be still more greatly increased periodically if the glow discharge is pulse-wise, as described hereinabove.

For producing a chromium-containing zone, a shaft of the same type is used as in the precedingexample. There is employed a counter-electrode of sheet iron with a layer of chromium on its inner surface produced, for example, electrolytically. There is provided in the gas discharge chamber an atmosuphere composed of 80'volume percent of hydrogen and 20 volume percent of argon at a pressure of 6 to 8 mm. of Hg, and these proportions and this pressure are maintained throughout the treatment. The current supply is obtained from two direct current sources, of which one is'at about 600 volts and the other at about 450 volts. The two current sources are alternatingly connected to the workpiece and the counter-electrode, so that the workpiece is at a 600 volt operating'tension for 20 seconds as a cathode with a current density corresponding to a specific output of about 8 watts/cmF, and subsequently at an operating voltage of 450 volts for 100 seconds, as anode. Through this latter rather long interval, the counter-electrode is operated at a specific capacity of 3 watts/ems". This periodically repeated op erating voltage cycle produces at the workpiece a temperature of about 580 C., while the temperature of the counter-electrode is considerably higher, in order to increase the evaporation.

With this method of operation, there results a chromium deposition upon thesurface'of the workpiece at high ionic energy, which facilitates diffusion into the mass of metal of the workpiece. At the same time, however, a suificient quantity of metallicparticles from the surface of the workpiece are expelled, so that increase in diameter is kept within permissible and predetermined limits. After 14 to 16 hours of treatment, the current is interrupted and after cooling of the workpiece, at the subpressure, to below 200 C., the chamber is opened and the workpiece removed.

When the procedure just described is supplemented by a vigorous cooling, as by means of a discharge chamber provided with cooling jacket, the voltages and energy conversion at the workpiece can be considerably increased,

ration of metal from the surface of the workpiece. can

continue until there is complete compensation of the previously obtained diameter increase. I

Upon completion of the second stage of the process, the current is interrupted, but the workpiece is allowed to remain in. the argon atmosphere until the temperature-has dropped toabout 150 to 200 C. Air is then introduced with further improvement of the treated surface and with reduction of the total time of treatment.

What has been stated hereinabove with respect to the details of the process is to beconsidered as by way of example only, as such details can be changed of supplemented, depending on the given conditions, and must be adjusted in each 'case to the size of the tubes, or other workpieces, the inner diameter of a tubular workpiece and the other variables. Such an adjustment is'generally possible by change of the applied electric tensiorn-gas pressure, the composition of gas, the degree of cooling, the choice of the corresponding periodic tension impulses; and in the case of tubular workpieces, also by the'use of tension-carrying'electrodes in the interior of'the tube.

Iclaim: I, 1; Process for the treatment of ferrous articlesfor improving at least part of the surface thereof with another substance, which comprises subjecting the said part of the surface to be improved to an ionic bombardment with said other substance in an electric glow discharge in a gaseous atmosphere and thereby depositing said substance on' said surface at a predetermined elevated temperature, withdrawing heat from said surfaceat .a predetermined rate by cooling the same and simultaneously increasing the rate at which energy is conducted by the ionic bombardment by a corresponding amount so as to maintain the predeterminedtemperature, whereby'an increased energy conversion is caused to occur at said surface, without rise of the surface temperature abovethe predetermined temperature.

2. Process for the treatment of tubes of ferrous alloys forimproving at least part of the interior surface thereof with another substance, which comprises subjecting the said part of the surface to be improved to an ionic born.-

bardment with said other substance in an electri glow discharge in a gaseous atmosphere and thereby depositing said substance on said surface at a predetermined elevated temperature, withdrawing heat from said interior surface at a predetermined rate by cooling the same and simultaneously increasing the rate at which energy is conducted by the ionic bombardment by a corresponding amount so as to maintain the predetermined temperature, whereby an increased energy conversion is caused to occur at said surface without substantial ,rise of the surface temperature above the predetermined temperature.

3.. Process according to claim 2, wherein thecooling is effected by withdrawingheat from the exterior surface at the tube andwherein the energy supplied to the interior surface amounts to 0.2 to watts/cmF.

4. Process according to claim 2 wherein the intensity of the ionic bombardment is varied in periodic and cyclic sequence according to a pre-selected time program, and wherein the gas pressure in the interior of the tube is maintained at a value of over 2 mm. Hg.

5. Process according to claim 2, wherein the ionic bombardment has a pre-selected mean valueof intensity and includes periodically alternating intervals during which the intensity exceeds the mean intensity by at least and intervals during which the intensity is below the mean intensity, and including thesteps of producing the higher intensity intervals by superimposingelectrical impulses on the glow discharge, and maintaining the gas pressure in the interior of the tube at a value of over 2 mm. Hg.

6. Process according to claim 1 for enriching at least part of the surface ofthe treated article with diifused nitrogen, wherein the article is subjected to a glow discharge in an atmosphere containing free nitrogen and hydrogen in the proportion of 1:10 to 1:25.

7. Process according to claim 2, wherein the tube is preliminarily reduced at a first temperature in a reducing atmosphereand is then treated at a second predetermined temperature in a gaseous atmosphere containing the substance tokbe applied thereto.

8. Process according to claim 2, wherein the cooling withdraws a quantity of heat corresponding to an increase of the supplied energy of about 30% compared with operation without cooling, whereby variations in the properties of the treated surface parts are reduced.

9. Process according to claim 2, wherein a gaseous element is employed which combines with the wall of the tube, and wherein the quantity of gaseous element charged into the glow discharge atmosphere exceeds the theoretical. amount required for. such combination by at most 30%. i l

10. Process according to claim 2, for the treatment of gun barrels, whereinthe barrel is first machined to its end tolerances and wherein at least those parts of the inner wall of the barrelwhich the projectile contacts are subjected to the defined ionic bombardment, said treated barrel being utilized without after-treatment of said parts.

terminal ofadirect electric currentsource capableof maintaining in said chamber an electrical glow discharge ofsuch kind as to effect a cathodic disintegrationof the surface jofisaid object, evacuating said chamber to a pressure of less than .001 mm. of mercury, introducing nitrogen into the chamber for eifecting said treatment, introducing also hydrogen into said chamber,gmaintaining the molecular ratio of hydrogen to nitrogen at approximately l.8:l,. charging direct current through said object at such a voltage that it is maintained at'a temperature ofabout. 520 C., and continuously pumping gas out ofthe chamber while charging hydrogen and nitrogen thereinto in the above-mentioned ratio, whereby the quantity of materialremovedfrom themetallic, object by cathodic disintegration is approximately equal to the increase of volume of the metallic object caused by said the inner conductor to the other pole thereof, a chamber adapted to enclose the tube for the whole length of the latter and to form a cooling space therewith, means for conducting, a cooling agent to and withdrawing it from thecooling space, and means for charging a gas to and withdrawing the same from the interior of the tube.

'13. Apparatus for the treatment of the inner wall of a tube by means of an ionic bombardment in an electric glow discharge-comprising a tubular chamber open at both its ends, detachable metallic covers sealing the ends of the chamber, means for supporting ,a tube to be treated within the chamberin such manner that the tube forms with the chamber an annular cooling space which is sealed from the interiorofthe tube, an inner conductor passing through the covers and insulated therefrom, said conductor extending along the axis of the tube to be treated, terminals for connecting the tube with an at least intermittently negative pole of a source of current and for connecting the inner conductor to the other pole thereof, a chamber adapted to enclose the tube for the whole length of the latter and to form a cooling space therewith, means for conductinga cooling agent to and withdrawing it fromthelcooling space, and means for charging a gas to and withdrawing the same from the interior of the tube. i

' 14. Apparatus according to claim 13, wherein the inner conductor is hollow, and means for charging a cooling agent into the conductor and for withdrawing the same therefrom. r 1

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Physical Review, pp. 769775, vol. 15, April 15, 1939,

Claims

1. PROCESS FOR THE TREATMENT OF FERROUS ARTICLES FOR IMPROVING AT LEAST PART OF THE SURFACE THEREOF WITH ANOTHER SUBSTANCE, WHICH COMPRISES SUBJECTING THE SAID PART OF THE SURFACE TO BE IMPROVED TO AN IONIC BOMBARDMENT WITH SAID OTHER SUBSTANCE IN AN ELECTRIC GLOW DISCHARGE IN A GASEOUS ATMOSPHERE AND THEREBY DEPOSITING SAID SUBSTANCE ON SAID SURFACE AT A PREDETERMINED ELEVATED TEMPERATURE, WITHDRAWING HEAT FROM SAID SURFACE AT A PREDETERMINED RATE BY COOLING THE SAME AND SIMULTANEOUSLY INCREASING THE RATE AT WHICH ENERGY IS CONDUCTED BY THE IONIC BOMBARDMENT BY A CORRESPONDING AMOUNT SO AS TO MAINTAIN THE PREDETERMINED TEMPERATURE, WHEREBY AN INCREASED ENERGY CONVERSION IS CAUSED TO OCCUR AT SAID SURFACE, WITHOUT RISE OF THE SURFACE TEMPERATURE ABOVE THE PREDETERMINED TEMPERATURE.