Nitriding process and nitriding furnace thereof

Abstract: A nitriding process and nitriding furnace therefor wherein the parts to be treated are maintained at floating potentional in a vacuum furnace (9), current being provided to a metal screen (5) cathode surrounding the parts to be treated, the necessary heat being provided by radiation from screen (5) whereby nitriding gas is injected into the furnace such that it has to flow through screen (5) where the plasma necessary for the nitriding reaction is generated by glow discharge the plasma flowing around and reacting with the parts to be treated before it is evacuated at the bottom of furnace (9) through vacuum/exhaust conduit (2).

Description: The present invention relates to a novel glow discharge plasma nitriding process and a nitriding furnace therefore wherein the metal articles to be treated are at floating potential and wherein the necessary heat is provided and plasma generated by glow discharge at a metal screen constituting the cathode. The nitride hardening of metal articles (work pieces, tools and other metal articles) to improve their wear characteristics is well-known in the art. Three nitride hardening or nitriding processes are known namely nitriding by immersing the metal articles into molten salt baths, nitriding in the gas phase and finally nitriding in cold plasma. Currently two cold plasma processes providing the active reagents necessary i.e. ions, electrons and other active energized neutral gaseous particles for the thermochemical reactions occurring on the surface of the articles to be treated are known. The most common of these processes is the ionic nitriding process whereby the articles to be treated are placed inside a furnace where they constitute the cathode and where the grounded walls of the furnace constitute the anode. An electrical generator provides the current (pulsed or D.C.) necessary for heating the furnace and for generating a plasma. To generate the plasma a gas, such as nitrogen, hydrogen, methane or others depending on the desired hardening is introduced into a vacuum chamber where a glow discharge generates the active reagents (ions, electrons and other active, energized neutral gaseous particles) directly on and around the surface of the metal articles to be treated. In accordance with the second known process the active reagents are generated by microwave discharge in a plasma generator provided adjacent to and outside of the nitriding furnace. The plasma thus generated is directed into a vacuum furnace comprising the heated articles to be treated. This process is known in the art as post-discharge nitriding. While both processes provide the desired nitride hardening and improve the wear characteristics of the treated articles they suffer from several drawbacks. In the ionic nitriding process the articles to be treated constitute the cathode and provide the heat necessary for the nitriding process. The uneven shape and geometry of the articles to be treated make it very difficult to control the heat distribution in the furnace. Moreover as the number or articles as well as their shape or geometry vary from one load to another it is difficult to calibrate the furnaces, the heating characteristics varying with the load. This results in an uneven temperature throughout the chamber. Where, however, the temperature in industrial furnaces cannot be properly controlled the nitride hardening quality of the treated articles suffers. The dual function of the load i.e. articles to be treated and cathode and the difficulties of direct temperature measurements on the cathode can lead to hot spots or overheating of the cathode. Such hollow cathode problems destroy the shape and/or geometry of precisely machined articles rendering the articles useless. To prevent undesirable side effects of this order only articles having sensibly identical sizes, shapes and geometry should be treated simultaneously in a same load. The economic efficiency of these known furnaces is thus very unsatisfactory. Moreover the articles to be treated have to be thoroughly cleaned of every organic surface impurities and have to be degreased before they can be used as cathodes in the nitriding furnace in order to prevent hot spots on the cathode. In smaller furnaces the danger of unipolars arcs can be minimized. with larger furnaces, however, total current increases and thus the danger of unipolar arcs. These arcs impair the usefulness of the treated articles as they destroy the articles or modify the surface and geometry characteristics thereof. In the furnaces of the art the positively charged ions travel to and hit the negatively charged cathode i.e. the articles to be treated. These impacts can be so violent that metal atoms are knocked out of the lattice. The articles are subject to a sandblasting-like effect. While this surface impairment is not dramatic and quite tolerable for most articles it is undesirable for highly polished surfaces. These have thus to be repolished after the nitriding process. It is obvious for any man skilled the art that the use of the articles as a cathode and plasma generator in the furnace makes it very difficult to treat small bore articles or to treat economically a large number of small caliper articles in one load. While the nitride hardening conditions are difficult to control in small furnaces of the known art, the difficulties are compounded in larger, industrial size furnaces. The inventors of the post-discharge processes tried to overcome some of the difficulties discussed above. The processes necessitate, however a separate plasma generating chamber. The plasma generated in these chambers has to be transferred into the nitriding furnace in which the heated articles are disposed. The even and homogeneous distribution of the reagents on and around the articles to be treated is difficult to control. The problems are obviously magnified in large, industrial scale furnaces where it is very difficult to guarantee that sufficient plasma reaches distant areas of the furnace. In these large furnaces problems arise also due to the limited useful life to the plasma particles. These particles may no more be active when they reach distant (as compared to the gas entry) areas of the vacuum furnace. Unevenably treated articles are obtained. Whatever the process it is thus very difficult to obtain satisfactory results in large scale industrial furnaces and processes of the art. WO-A-97/14172 discloses a method and apparatus for plasma processing whereby the plasma is generated by means of thermionic filaments, the processing being carried out within a chamber at a gas pressure of 13.3 µPa to about 133 mPa (0.01 to about 100 millitorr). Efforts have been made for some time to improve the control of the nitride hardening conditions of the known furnaces and processes. A satisfactory economical solution has not yet been disclosed. It is therefor an object of the invention to provide a glow discharge nitriding process allowing better control of the nitriding conditions whereby the articles to be treated are heated by radiation from a metal screen cathode surrounding the articles to be treated a gas mixture being injected into the furnace such that it flows through the screen where the necessary plasma is generated by glow discharge before this highly ionised gas compound reaches and reacts with the articles which can be either maintained at floating potential or at a load of less than 1 kW. It is a further object of the invention to provide a glow discharge nitriding furnace allowing an economical simultaneous nitriding of articles having different shapes and geometry. This is achieved by a furnace wherein the reaction heat is provided by and the plasma is generated by glow discharge at a metal screen cathode surrounding the articles to be treated, gas entries being provided between the furnace wall and the metal screen cathode. These and other objects and advantages of the invention will become more readily apparent from the following detailed description of preferred embodiments thereof, when taken in conjunction with the accompanying drawing, showing the nitride hardening furnace of the present invention. In accordance with the novel process of the invention the articles to be treated are placed into a glow discharge nitriding furnace. Electric current is provided to a metal screen surrounding the articles to be treated. Heat to the furnace and articles is provided by radiation from the screen which constitutes the cathode of the furnace. Gas is introduced into the furnace between the grounded furnace walls constituting the counter electrode and the metal screen cathode so that the gas flows through the screen. At the screen plasma is generated by glow discharge such that a mixture of ions, electrons and other active energized neutral gaseous particles come into contact with the articles to be treated. The gases are evacuated at the bottom of the furnace. Referring now to the drawing the furnace (9) in accordance with the invention is constituted by an upper part (1a) and a bottom part (1b) joined by gas seal (3). A grounded generator (4) provides the necessary pulsed or D.C. current to a metal screen cathode (5) surrounding a support (8) maintained at floating potential on which the articles to be treated rest. This screen (5), heated by current from generator (4) heats by radiation the interior of the furnace (9). As the characteristics of this screen are known and remain constant in the furnace it is possible to control the furnace temperature within a narrow range by controlling the current provided to this screen. After placing the parts to be treated onto support (8) the upper part (1a) of the furnace is lowered onto the grounded bottom part (1b). A vacuum pump (not shown) eliminates the gases present in the furnace through vacuum/exhaust conduit (2). After the establishment of a pressure inferior to 20 hPa (mbar) within the furnace generator (4) is switched on to provide a power density of 20 - 50 W/dm 2 to screen (5). when the screen has reached the necessary temperature corresponding to an internal homogenous and uniform temperature of 300 to 600°C a gas mixture constituted of nitrogen and neutral gases such as hydrogen and/or argon is injected into the furnace at different levels through gas injection conduits (6). The gas injection conduits (6) enter the reactor outside of screen (5) such that the gases have to flow through screen (5). The glow discharge at the screen (5) generates the plasma of highly ionized gas constituted of ions, electrons and other active, energized neutral gaseous particles necessary for nitriding the articles on support (8). As the gases are continuously evacuated through vacuum/exhaust conduit (2) the plasma generated at the screen flows downward and around the articles on support (8). The articles are continuously bathed in a gentle flow of the active reagents before the plasma is evacuated through conduit (2). The gas injection conduits are distributed over the entire surface of the furnace and the vacuum exhaust conduit or conduits are disposed such that a constant homogeneous plasma flow around the articles to be treated is obtained. The actual location of these conduits will depend on the size and form of the furnace. Preferably the vacuum/exhaust conduit (2) is provided at the center and near the bottom surface of support (8). By providing the entry of vacuum/exhaust conduit (2) at the center and near the bottom surface of support (8) a continuous flow of plasma to the articles is guaranteed and any contact of these articles with the injected untreated gases is prevented. For most applications a furnace temperature of between about 300 and 600 °C is adequate. For special alloys, however higher temperature up to about 800 °C could be used. In contradistinction to the furnaces of the prior art, it is not the articles to be treated that are used as heating elements and as plasma generators. Rather metal screen (5) constitutes the cathode and is used both to heat the interior of the reactor and the parts to be treated and to generate the plasma of ions, electrons and other neutral particles necessary for the nitriding reaction. As current is no more applied to the articles to be treated all problems associated with overheating or hot spots, be they due to impurities remaining on the articles or to shape or geometry, have been overcome. With the process of the invention it is possible to treat parts, work pieces or tools or other articles without resorting to time consuming cleaning or degreasing processes. For the treatment of special steels articles, for example stainless steel articles, or other parts made of special materials depolarization or a surface activation is often required. In these instances cleaning and degreasing of these articles before they are loaded into the furnace is recommended. For the depolarization of the articles current is applied, as in the prior art process, to support (8) such that the articles to be treated constitute, for a short period of time, the cathode. After having achieved the depolarization either by plasma generation on the articles and/or by the above disclosed sandblasting-like effect the current to support (8) is switched off to allow the nitride hardening process of the invention to proceed with support (8) and the articles thereon at floating potential. For some nitriding processes, depending on the steel alloys that have to be treated, the geometry of the articles and/or the density of the load i.e. articles very close together it is preferable to apply a weak current to the support (8) and thus to the articles. The articles are thus no more at floating potential but constitute a weak cathode within the furnace. The weak cathode character will guarantee a more even distribution of the plasma on and around the articles to be treated and will thus further improve the homogeneous nitriding achieved by the process of the invention. The current applied in accordance with this invention will be very weak when compared to the current applied in the prior art. Thus, whereas in the prior art loads of 60 to 100 KW depending on the load and the size of the furnace were applied to the support the load applied in the process of this invention will be less than 1 KW. It is obvious to a man skilled in the art that the current to be applied will depend on the load of articles to be treated. Whatever this load of articles, the load should preferably not exceed 1 KW. The application of a weak current to the support (8) will guarantee a uniform homogeneous nitriding result for articles with a complicated geometry and for very high density loads and even for the bulk treatment of small articles. Considering that no current or only a very weak current is applied to the articles to be treated during the nitriding process no unipolar arc problems impairing the surface, shape or geometry characteristics of the articles can arise. As the articles are not on only weakly negatively charged there is no violent impact of positively charged ions onto these articles. The bathing of the articles on support (8) by the plasma generated by glow discharge at screen (5) thus not only guarantees that the entire surface inclusive of any holes or recesses is equally and continuously in contact with freshly generated plasma and that the entire surface of the articles is uniformly treated but the gentle flow of the plasma on and around the articles does not lead to a sandblasting-like effect such that the surfaces of the articles are not at all impaired. For the novel process of the invention the amount and speed of injection of the gas mixture into the furnace are not critical. It is only necessary to ascertain that a sufficient amount of gas is injected to provide the ions and particles necessary for the nitriding reaction. Typically a mixture of nitrogen and neutral gases such as hydrogen and/or argon is used. It is however possible to add other active gases to this mixture such as. methane, propane, hydrogen sulfide, carbon fluoride etc. Indeed, it is self evident that the apparatus and process disclosed may not only be used for nitride hardening processes but also for nitride-carbide hardening, oxynitride carbide hardening, sulfo nitride hardening. The different types of hardening obtained depend only on the composition of the reactive gases injected into the furnace. For carrying out the process of the invention in the novel furnace the composition, size and other characteristics of metal screen (5) cathode are not critical. Due to the fact that the heating of the furnace is no more obtained from the radiation of varying quantities of articles of different shapes and geometry it is possible to precisely calibrate the furnaces of the invention. It is sufficient to vary the power density provided to the screen to control the furnace temperature within narrow limits and obtain a uniform temperature throughout the furnace. In the novel furnace and process of the invention the plasma generated at the screen (5) flows gently around the articles to be treated independently of the size and form of the furnace. The novel process and furnace allows the economical treatment of articles of different size, bore, shape or geometry in a single load even the treatment of articles in bulk in the furnace without any impairment of the nitride hardening or other surface, shape of geometry characteristics of the articles thus treated. As the furnace is no more heated by current applied to the articles to be treated hot spots or other overheating problems do no more occur. The provision of heat by radiation from screen (5) guarantees an uniform temperature profile throughout the furnace. As the radiation heat can be controlled by the amount of current provided to a screen having a known size and characteristics the temperature control becomes easy. By a judicious distribution of the gas injection conduits (6) to guarantee an ample and continuous supply of plasma to the articles to be treated, furnaces with two or more super imposed supports (8) can be built thus further improving the economics of the inventive process. It is evident to men skilled in that art that the furnace of the invention can further be provided with devices known in the art, such as measuring devices, look through glasses, forced cooling devices which do not form part of the present invention. It is also possible to sputter rare earth elements for example lanthanum onto the articles to be treated. The rare earth elements have a catalyzing effect and speed up the diffusion of the plasma into the metal lattice of the articles.

Claims: A glow discharge nitriding process wherein a plasma is generated in a furnace (9) constituted of an upper part (1a) and an electrically grounded bottom part (1b) and a gas seal 3 therebetween, said furnace (9) comprising metal articles be treated on a support maintained a temperature of about 300 to 800° C and at a pressure inferior to about 20 hPa (mbar), characterised by providing current to a metal screen cathode (5) surrounding the articles to be treated resting on a support (8) wherein the counter electrode is constituted by the walls of the furnace (9), heating the furnace (9) and the articles to be treated by radiation from this screen (5), the articles to be treated being maintained at floating potential, and by injecting a gas mixture into the furnace (9) such that the gas flows through the metal screen cathode (5) where the plasma necessary for the nitriding reaction is generated by glow discharge, the plasma thus generated flowing to the articles to be treated and the gases being evacuated through a conduit (2) provided beneath the articles to be treated. A glow discharge nitriding process wherein a plasma is generated in a furnace (9) comprising an upper part (1a) and an electrically grounded bottom part (1b) and a gas seal (3) therebetween, said furnace (9) comprising metal articles to be treated on a support maintained at a temperature of about 300 to 800° C at a pressure inferior to about 20 hPa (mbar), characterised by providing current to a metal screen cathode (5) surrounding the metal articles to be treated resting on a support (8) wherein the counter electrode is constituted by the walls of the furnace (9), heating the furnace (9) and the articles to be treated by radiation from this screen (5) , a weak current being applied to the support (8) and the articles such that the articles to be treated constitute a weak cathode and the load does not exceed 1 kW, injecting a gas mixture into the furnace (9) such that the gas flows through the metal screen cathode (5) where the plasma necessary for the nitriding reaction is generated by glow discharge, the plasma thus generated flowing to the articles to be treated and the gases being evacuated through a conduit (2) provided beneath the articles to be treated. Process according to claim 1 or 2, characterised in that the gas mixture is constituted of nitrogen or nitrogen and hydrogen and/or argon. Process according to any of claims 1, 2, or 3, characterised in that a power density of about 20 to 50 W/dm2 is applied to screen (5). A process according to any of claims 1, 2, 3 or 4, characterised in that the gas mixture comprises additionally methane, propane, hydrogen sulfide and/or carbon fluoride. A glow discharge nitriding furnace constituted of an upper part (1a) and an electrically grounded bottom part (1b) and a gas seal (3) therebetween, the walls of the furnace (9) forming a counter electrode, a support (8) for metal articles to be treated, such that during the operation of the furnace the articles can be nitrided at a floating potential, a gas exhaust/vacuum conduit (2) and comprising an electrically grounded current generator (4) for the nitride hardening of the articles, characterised in that it comprises a metal screen (5) surrounding support (8), whereby the generator (4) is connected to this metal screen (5) which constitutes the cathode of the furnace, the furnace further comprising gas injection conduits (6) disposed around the furnace and between the furnace wall and metal screen (5). Furnace according to claim 6, characterised by providing at least two super imposed supports (8) within metal screen (5).

Inventors: GEORGES JEAN, GEORGES, JEAN
Applicants: PLASMA METAL S A, PLASMA METAL S.A.
Application year: 1997
Main classification: C23C8
IPC codes: C23C8