BACKGROUND OF THE INVENTION Nitride hardening is well known for improving the wear and fatigue resis- tance of metal parts.

Salt baths nitriding is conventionally used for treating small articles in bulk such as e. g. screws, nails, needles, bolts and the like. Liquid salt baths give a relatively uniform heating and prevent oxidation. A liquid salt bath typically consists of 60 to 70 percent sodium salts, mainly sodium cyanide, and 30 to 40 percent potassium salts, mainly potassium cyanide.

Although salt bath nitriding has been performed for many decades in the industry, it has the major disadvantage that it uses chemical species which are hard to control and harmful to the workers and the environment. Furthermore, spent salt baths must be disposed of as hazardous waste and are therefore generally stored in underground dumps.

It is to be reminded that for larger parts, such as e. g. camshafts, gears, sleeves, pinion shafts, extrusion dies, fly wheels etc. , other nitriding processes are generally used, namely gas nitriding and plasma nitriding.

Gas nitriding typically uses nitrogeneous gas (generally ammonia) to sup- ply the nitrogen to the parts to be nitridided. The nitriding conditions are however more difficult to control than in plasma nitriding.

Plasma (ionic) nitriding is an environmentally friendly technique, which can be applied to a variety of materials. It uses non-toxic precisely controlled gas mixtures, and allows a consistent treatment of parts.

In a basic plasma nitriding process, the parts to be treated are placed in- side a nitriding furnace where they constitute the cathode and where the grounded walls of the furnace constitute the anode. An electrical generator provides the current necessary for heating the load and for generating a plasma. To generate the plasma a gas-containing nitrogen and e. g. hydro- gen, methane or others depending on the desired hardening-is introduced into the furnace in a controlled manner to obtain a reduced gas pressure.

Current is supplied to the cathode, thereby initiating a glow discharge that generates the active reagents (ions, electrons and other active, energized neutral gaseous particles) directly on and around the surface of the metal parts to be treated. In the furnace, the parts are placed on a supporting table in a precise and organised manner to distribute the load in the furnace and provide plasma flow paths between parts.

A major disadvantage of this process is that the parts to be treated consti- tute the cathode and provide the heat necessary for the nitriding process. This mainly causes problems of overheating of the parts and of arcing, potentially damaging the shape and/or geometry of precisely machined parts and render- ing them useless.

An improved plasma nitriding process is described in US 5,989, 363, which allows a better control of the nitriding conditions. According to this process, the articles to be treated are at floating potential and are heated by radiation from a metal screen cathode surrounding the parts to be treated. A gas mixture is injected into the furnace such that it flows through the metal screen cathode, where the necessary plasma is generated by glow discharge, the plasma further reaching and reacting with the metal parts methodically placed on a supporting plate.

The process of US 5,989, 363 overcomes the defects of the above- described basic plasma nitriding process. The metal screen cathode allows a precise control of the temperature. Moreover, as current is not applied to the articles, all problems associated with overheating or hot spots, due to impurities

or shape geometry of the articles, are eliminated. However this process requires that the articles be methodically placed on the supporting table, which is not feasible with articles in bulk such as e. g. needles.

JP-A-55065356 describes a method for nitriding articles in a plasma fur- nace comprising a basket for receiving a load of articles in bulk. The basket is formed as a cylindrical cage that is mounted on a horizontal drive shaft for rotating the basket about its axis and mixing the articles received therein. A nitriding plasma is created by glow discharge on the articles in the basket, which are cathodically polarised when in contact with the basket, the latter being connected to a cathode lead via the drive shaft. The structure of the furnace of JP-A-55065356 is rather complex and thus inappropriate for indus- trial use. Furthermore, the basket forms a sort of Faraday cage surrounding the articles in bulk, which affects the efficiency of the plasma generation. Another problem is that the plasma is formed on the articles in movement in the furnace, which may be damaged due e. g. to arcing.

Hence, in spite of the advantages of gas nitriding and plasma nitriding, these technologies are considered inappropriate for hardening small articles in bulk at industrial level, and only salt bath nitriding is used in practice.

There is thus a need for an alternative nitriding process for small articles in bulk.

SUMMARY OF THE INVENTION According to the present invention, a process for nitriding articles in bulk comprises the following steps: loading articles in bulk in a mixing basket; treating the articles in bulk received in the mixing basket by means of a ni- triding plasma in a nitriding furnace under controlled temperature and pressure, the nitriding plasma being generated outside the mixing basket; and operating the mixing basket to mix the articles in bulk during generation of

the nitriding plasma.

Contrary to the conventional practice, the process according to the pre- sent invention proposes to carry out the nitriding of articles in bulk by means of a nitriding plasma. It will be appreciated that in the present process the articles are received in a mixing basket and that the plasma is formed outside the basket, whereby active particles (species) from the nitriding plasma come into contact with the articles in the mixing basket to react therewith. In other words, the plasma is not directly formed on the articles, which means in particular that the articles do not form the cathode for a glow discharge plasma generation. It will be appreciated that this allows mixing the articles in the plasma nitriding furnace, without any risk of severe arcing on the moving articles. Furthermore, the use of a basket in the furnace allows treating a load of articles without having to place them individually and in an organised manner in the nitriding furnace. The mixing in the basket leads to a homogenous and reproducible treatment of the articles, which will thus exhibit uniform nitride layers over their whole surface. Depending on the selected nitriding conditions, the present process allows forming a'or E layer at the surface of the articles.

The mixing basket is inclined by a predetermined angle, so that its opera- tion (normally by rotation) produces a tumbling movement of the articles in the mixing basket and results in a gentle mixing of the articles. The rotating speed may be between 1 and 5 rpm, depending on the dimensions of the articles. The mixing basket is advantageously inclined by an angle between 5 and 45°.

It is further to be noted that the mixing basket is open upwards, which permits a prompt and easy access to the articles contained therein, without the need to remove any wall part of the basket. This further means that the basket is permanently open, which also facilitates the plasma circulation through the basket.

The present process is particularly adapted for treating small articles in bulk such as e. g. screws, nails, needles, bolts and the like. It can be used to treat a wide range of materials including carbon and low-alloy steels, tool

steels, stainless steels, cast irons, sintered steels, and even titanium.

It is to be noted that contrary to salt bath nitriding which is extremely toxic, the process of the invention is environmentally friendly, since plasma nitriding generally employs non-toxic precisely controlled gas mixtures. A further advantage of the present process is that with adaptations, it can be imple- mented in some existing furnaces, in particular in a furnace such as described in US 5,989, 363.

The nitriding plasma is preferably generated from a gas mixture compris- ing nitrogen. Typically, a gas mixture comprising nitrogen and a neutral gas such as hydrogen and/or argon can be used in most processes.

The nitriding gas may also comprise additional active gases to simultane- ously allow the introduction of other elements into the articles. Such active gases may be methane, propane, hydrogen sulfide and/or carbon fluoride, in case nitride-carbide hardening, oxy-nitride carbide hardening or sulfo-nitride hardening is to be carried out.

In order to provide an enhanced contact between the articles and the ac- tive particles from the nitriding plasma, a gas flow is preferably maintained in the furnace in such a way as to carry the active particles of the nitriding plasma through the articles in the mixing basket.

The controlled temperature and pressure in the furnace depend on the materials to be treated, as well as on the nitriding gas and the desired nitrided diffusion case. A temperature between 300 and 600°C is adequate for most applications; but it may be increased up to 800°C for some special alloys. The pressure in the furnace is preferably controlled so as to generally be inferior to 10 millibars.

It will be understood that a load of articles in bulk in the mixing basket may include articles of different sizes and shapes. In such a case, the articles have different properties and there is generally not one single optimal pressure.

To take these differences into account, it is advantageous to vary the pressure in the furnace during the process. The pressure may be varied continuously in

a range from 0,01 to 0,8 millibar and more preferably in a range from 0,001 to 8 millibars.

In the present process, the articles need not be biased and are generally maintained at floating potential. For some nitriding processes, depending on the articles'materials and sizes as well as on the load, it is however preferable to apply a cathodic bias to the articles in bulk in the mixing basket, so as to improve the plasma distribution around the articles and control the bombard- ment of particles on the articles.

The nitriding plasma is generated by glow discharge on a metal screen cathode surrounding the mixing basket. This metal screen is used both to heat the interior of the furnace and the articles to be treated, and to generate the plasma of ions, electrons and other neutral particles necessary for the nitriding reaction. As a result, a homogenous heating and plasma generation are achieved about the mixing basket. The current applied to the metal screen cathode may be in the range of 20 to 60 W/dm2.

It remains to be noted that although the present process is particularly well suited for the nitriding of metallic articles, it may also advantageously be implemented for the nitriding of plastic articles in bulk. In such a case, the treatment temperature in the furnace is controlled to be generally lower than for metallic articles, and will normally be of at least 150°C. The composition of the nitriding gas may also be adapted to the type of polymer. Plastic articles to be nitrided according to the present process may consist of thermoplastic or thermosetting polymers.

According to a further aspect of the present invention, a nitriding furnace for nitriding articles in bulk comprises: a processing chamber; a mixing basket in the processing chamber for receiving a load of articles in bulk to be treated; plasma generating means for producing a nitriding plasma outside the

mixing basket in order to treat the articles in bulk in the processing chamber; and means for operating the mixing basket in such a way as to mix the articles in bulk.

The mixing basket is inclined by a predetermined angle and is open up- wards. The inclination of the basket permits to obtain a tumbling movement of the articles. By properly selecting the inclination angle it is thus possible to obtain a gentle mixing.

A merit of the present invention is thus to have found a furnace for carry- ing out the plasma nitriding of articles in bulk that is adapted for industrial use.

In particular, the mixing basket and its operating means allow to easily load the articles in bulk into the furnace and ensure a reproducible and uniform nitriding of the articles. To reduce the duration of furnace stops due to loading and unloading, a set of interchangeable mixing baskets may be used.

The mixing basket is preferably designed as a container having a bottom and side walls defining a loading space, and with an open top for load- ing/unloading the articles to be treated (i. e. without any lid or wall that would form a basket completely surrounding the articles contained therein).

In a preferred embodiment, the mixing basket is rotatably mounted and the means for operating the mixing basket comprises driving means coupled to the mixing basket for rotating the latter. This coupling can be achieved by means of a toothed ring affixed to the mixing basket and a cooperating pinion driven by the driving means.

The furnace also preferably comprises means for varying the inclination angle of the mixing basket between 5 and 45°. Such an inclination enhances the mixing, and causes the articles to tumble in the mixing basket. For the same purpose, the mixing basket is advantageously provided with inner mixing ribs.

The mixing basket is preferably made of metallic meshed material so that

the active particles from the nitriding plasma can flow therethrough. To encour- age the flow of these active particles through the mixing basket, a gas outlet is preferably provided underneath the mixing basket.

Depending on the size of the processing chamber and of the mixing bas- ket, the furnace may comprise two or more mixing baskets, which can be directly or indirectly coupled to the driving means.

The plasma generating means comprises a metal screen cathode sur- rounding the mixing basket. Such a screen is used for both heating purposes and plasma generation. As the characteristics of the 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 supplied to this screen. In this embodiment, the plasma generation means shall preferably comprise a nitrid- ing gas injection conduit disposed around the metal screen cathode, to provide a homogeneous distribution of gas.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with ref- erence to the accompanying drawings, in which: Fig. 1 : is a schematic sectional view through a preferred embodiment of a nitriding furnace in accordance with the invention, the furnace being equipped with one mixing basket; and Fig. 2: is a schematic sectional view through the furnace of Fig. 1 with three mixing baskets; and Fig. 3: is a graph illustrating the pressure (P) in the furnace in function of the time (t) in a preferred operating mode of the furnace.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT According to the present process, articles to be nitrided are loaded in bulk in a mixing basket and treated with a nitriding plasma in a nitriding furnace, while being mixed. It is to be noted that the plasma is formed outside the mixing

basket, i. e. not directly on the articles in bulk. Fig. 1 shows a preferred nitriding furnace 10, in which the present process may be implemented.

The furnace 10 has an external wall comprising an upper part 12 and a lower part 14 joined by a gas seal 15. A processing chamber 16 is delimited by an inner wall 18. The furnace 10 comprises plasma generating means that are designed to generate a nitriding plasma by glow discharge on a metal screen cathode 20. The metal screen cathode 20 is preferably a metal grid arranged to form a cylindrical cathode. The furnace walls (outer 12,14 and inner 18 walls) are earthed and constitute the anode. A generator 22 provides the necessary pulsed or D. C. current to the metal screen cathode 20. Nitriding gas is intro- duced by an intake line 23 entering the furnace through its upper part 12. If desired, the intake line 23 may advantageously be connected to a helical gas injection conduit (not shown) passing in between the screen 20 and the inner wall 18. The injection of nitriding gas by the helical conduit should be designed in such a way that the nitriding gas flows through the screen 20 and towards the center of the processing chamber 16, thereby obtaining an homogeneous distribution of the nitriding gas into the processing chamber 16.

At the metal screen cathode 20, plasma is generated by glow discharge, whereby a mixture of ions, electrons and other active energised particles flow towards the center of the processing chamber 16 to be in contact with the articles to be treated. Gases are evacuated at the bottom of the furnace through a vacuum/exhaust conduit 26.

It will be appreciated that the present furnace comprises a mixing basket 28, which is designed to receive articles in bulk to be treated. The mixing basket 28 is preferably supported on a stand 30, which rests on a central workable 32. As can be seen, the mixing basket 28 is surrounded by the metal screen cathode 20, whereby an homogeneous nitriding plasma is generated outside the mixing basket 28 and in the vicinity thereof.

Mixing of the articles in the mixing basket 28 is preferably carried out by rotating the basket 28 on its stand 30 about the basket's axis, e. g. in the

direction of arrow 31. As can be seen, the stand 30 is designed to support the mixing basket 28 at a certain inclination angle-preferably adjustable-with regard to the workable 32, so as to enhance the mixing.

The use of such an inclined (or tilted) basket permits to produces a gentle tumbling of. the articles in the basket. In other words, the articles gently roll one over another as the basket is rotated. In addition, the use of a stand 30 that is adapted to vary the inclination angle of the mixing basket is advantageous since it permits to adapt the mixing (tumbling) in function of the size and shape of the articles.

To rotate the mixing basket 28, the latter is coupled to driving means. The driving means comprises an electric motor 34 coupled to a pinion 36 located in the processing chamber 16 at the height of the mixing basket 28. The motor 34 is located outside the furnace 10 and connected to the pinion 36 by means of a rotating shaft 42, which enters the furnace via an opening provided with a vacuum tight seal (not shown). The mixing basket 28 comprises at its bottom a toothed ring 38 cooperating with the pinion 36 of the driving means. It will be understood that ring 38 and pinion 36 should be designed to cooperate with each other at the selected inclination angle, and to withstand the heat in the furnace 10.

The mixing basket 28 advantageously consists of metallic meshed mate- rial, so as to allow the flow of gas therethrough. It is preferably designed as a cylinder closed at one end, on which it rests on the stand 30; the load- ing/unloading of the basket 28 being made through the opposite open end, as indicated by arrow 40. Most preferably, the mixing basket comprises a rigid structure defining the general cylindrical shape (or other) that is featured with metallic meshed material to form porous bottom and side walls. The metallic meshed material is advantageously removably mounted in the rigid structure so as to allow the selection of the meshed material for the bottom and side walls in function of the dimensions of the articles to be treated.

The mixing basket 28 preferably further comprises inner mixing ribs (not

shown), e. g. protruding on its side walls, to enhance the mixing. If required, the bottom of the basket 28 may include reinforcing spokes (not shown).

It is to be noted that in the present furnace 10, the metal screen 20 consti- tutes the cathode and is used both to (1) heat the interior of the furnace 10 and the articles to be treated and (2) to generate the plasma of ions, electrons and other neutral particles involved in the nitriding reaction. As the plasma generat- ing current is not applied to the articles to be treated, all problems associated with overheating, hot spots and especially arcing are eliminated. Furthermore, since the characteristics of the screen 20 are known and remain constant in the furnace 10, it is possible to control the furnace temperature within a narrow range by controlling the current supplied to this screen 20. The provision of heat by radiation from the screen 20 ensures a uniform temperature profile throughout the furnace 10.

The metal screen cathode 20 may be made from materials varying from low alloyed steels to inconel steel, depending on the applications.

As has been understood, the articles in the mixing basket 28 do not form the cathode of the glow discharge, and are generally maintained at floating potential. If desired, a weak cathodic bias (e. g. about 600 to 700 V) can be applied to the articles in bulk in the mixing basket. This allows to control the bombardment of active particles on the articles, improving their distribution about the articles and thus improving the nitriding conditions. As only a weak current is applied to the articles in this case, no arcing problems can arise. For example, articles consisting of noble steels having a good affinity for nitrogen (especially stainless steels) may generally be processed at floating potential and more basic steels having a lower affinity for nitrogen (e. g. carbon steels) shall be processed under cathodic bias.

According to the present process, nitriding of articles in bulk is carried out as follows. After loading the articles in bulk in the mixing basket 28, the upper part 12 is lowered onto the lower part 14. A vacuum pump (not shown) is activated to evacuate the gases present in the furnace 10 through conduit 26.

After the pressure has dropped below 10 mbar within the furnace, the electric generator is switched on to provide a current of 20 to 60 W/dm2 to the metal screen cathode. As heating begins, the driving means are also preferably activated to rotate the basket 28 so as to homogeneously heat up the articles.

When the screen 20 has reached the necessary temperature correspond- ing to an internal homogeneous and uniform temperature of 300 to 800°C, a nitriding gas mixture, e. g. consisting of nitrogen and argon and/or hydrogen, is injected into the furnace 10 via intake conduit 23. The introduction of gas is controlled to provide the desired pressure in the processing chamber 16, and preferably to maintain a constant flow of the gas mixture from the periphery of the furnace 10 to its center. Hence, the gas mixture flows through the screen 20 toward the center of the furnace 10 and the glow discharge at the metal screen 20 generates the plasma of highly ionised gas constituted of ions, electrons and other active, energised neutral gaseous particles involved in the nitriding reaction. In the center of the furnace 10, the active particles from the plasma reach the articles in bulk in the mixing basket 28 and react with the articles, while the mixing basket 28 is advantageously continuously rotated so as to mix the articles.

It is to be noted that the motion of gases through the mixing basket 28 is encouraged by the fact that the gas/vacuum exit 26 is located underneath the mixing basket 28.

Example : About 2500 steel needles (used in the manufacture of silent chains) having a diameter of 2 mm and a length of about 8 mm were loaded in a mixing basket in a furnace as shown in Fig. 1, the mixing basket being inclined by 25°. The mixing basket, having a diameter of 400 mm, was filled to about 35%.

The needles were treated for 8 hours in the furnace at the following oper- ating conditions: - Temperature : 525°C ; - Pressure : 2,5 millibars ;

- mixing basket rotating speed: 3 rpm; - nitriding gas mixture: 30% of N2 and 70% of H2 (so-called y'mixture).

The needles exhibited a uniform diffusion layer with a y'compound layer, the diffusion layer having a depth of 0,15 to 0,20 mm and the compound layer of about 7 Am.

It will be understood that, depending on the size of the basket 28 and of the furnace 10, the furnace 10 may comprise more than one mixing basket, which is economically interesting.

In Fig. 2, the furnace of Fig. 1 is represented with three mixing baskets 28, 28'and 28" ; for simplicity, same reference numbers are used as in Fig. 1. As can be seen, the three mixing baskets 28,28'and 28"are super imposed in the mixing chamber 16, the three baskets being each rotatably mounted on a common supporting structure (not shown) resting on the central workable 32.

Each mixing basket 28,28'and 28"comprises a respective toothed ring 38,38' and 38"cooperating with a respective pinion 36,36'and 36"mounted on a common shaft 42 and driven by the motor 34.

Since both the metal screen cathode 20, and optionally the helical gas in- jection conduit, extend over the whole height of the processing chamber 16, plasma is homogeneously formed about the three mixing baskets 28,28'and 28".

In an alternative, not shown, embodiment, the mixing baskets 28,28'and 28"are disposed side-by-side on the central workable 32. A first mixing basket 28 has its toothed ring meshing with the pinion 36 which is driven by the motor 34. The second mixing basket is arranged on the workable 32 in such a way that its toothed ring 38'meshes with the toothed rings 38 and 38"of the first and third mixing baskets 28 and 28"so that the three baskets 28,28'and 28" are rotationally driven by the single pinion 36.

It remains to be noted that, in some cases, it is advantageous to vary the

pressure in the furnace during the nitriding process, so that the plasma genera- tion does not occur at a constant pressure. This is typically of interest with a load of articles of different sizes and shapes, in which case the articles have different properties and there is generally not one single optimal pressure.

For example, the pressure may be varied in a periodical manner from a minimal pressure Pmin (e. g. 0.01 mbar) up to a maximal pressure Pmax (e. g.

8 mbar) and back to Pmin, as shown in Fig. 3. The duration of the rising and decreasing phases may be varied in function of the desired treatment duration at a given pressure range Dp. As can be seen, when the pressure in the furnace is controlled in the manner illustrated in Fig. 3 and supposing that Dp is an optimal pressure range for treating some of the articles in the furnace, then during each period the articles will be treated at the optimal pressure range Dp during a period of time: T = t1 + t2.

In practice, the pressure in the furnace may be increased by closing the exhaust conduit 26 (typically equipped with adjustable valve means) and maintaining the inlet flow of nitriding gas. When reaching Pmax the pressure may then be decreased to Pmin at a desired rate by adjusting the opening cross-section of the exhaust conduit 26.

LIST OF REFERENCE SIGNS 10 furnace 12 upper part 14 lower part 15 gas seal 16 processing chamber 18 inner wall 20 metal screen cathode 22 generator 23 intake line 26 vacuum/exhaust conduit 28,28', 28"mixing basket 30 stand 31 arrow 32 workable 34 electric motor 36,36', 36"pinion 38 toothed ring 40 arrow 42 rotating shaft