CA2285720A1 - Nitriding process - Google Patents

Abstract

A nitriding process wherein a plasma is generated in a furnace comprising the parts to be treated on a support at a pressure inferior to about 20 mbar whereby the temperature is maintained below 560°C and 3 to 15% of cyclic aliphatic hydrocarbon gases are added to the nitriding gases.

Description

Nitriding process The present invention relates to an novel plasma nitriding process wherein a cyclic or aliphatic hydrocarbon is added to the nitriding gases.
The nitride hardening of metal parts (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 energised neutral gaseous particles for the thermo-chemical reactions occurring on the surface of the parts to be treated are known.
The most common of these processes is the ionic nitriding process whereby the parts 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 generating a plasma.
To generate the plasma a gas, such as nitrogen, hydrogen 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, energised neutral gaseous particles) directly on and around the surface of the metal parts to be treated.
In accordance with the second known process the active reagents are obtained from a plasma source (by microwave or glow discharge) provided adjacent to and outside of the nitriding furnace. The plasma thus generated is directed into a vacuum furnace comprising the heated parts 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 parts they suffer from several drawbacks.
In the ionic nitriding process the parts to be treated constitute the cathode and provide the heat necessary for the nitriding process. The uneven shape and geometry of the parts to be treated make it very difficult to control the heat distribution in the furnace. Moreover as the number or parts 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 load applied to the parts to be treated and the difficulties of direct temperature measurements on the cathode can lead to hot spots or overheating of the parts.
This overheating can destroy the shape and/or geometry of precisely machined articles rendering the parts useless.
To prevent undesirable side effects of this order only articles having sensibly identical sizes, shapes and geometry could 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 and arcing on the parts to be treated.

In smaller furnaces the danger of unipolar arcs can be minimised. With larger furnaces, however, total current increases and thus the danger of unipolar arcs. These arcs impair the usefulness of the treated parts as they destroy the parts 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 parts to be treated. These impacts can be so violent that metal atoms are knocked out of the lattice. The parts are subject to a sandblasting-like effect. While this surface impairment is not dramatic and quite tolerable for most parts it is undesirable for highly polished surfaces. These have thus to be re-polished after the nitriding process.
It is obvious for any man skilled in the art that the use of the parts as a cathode and for plasma generation in the furnace makes it very difficult to treat small bore articles or to treat economically a large number of small calibre 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 parts are disposed. The even and homogeneous distribution of the reagents on an around the parts 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 parts are obtained.
Whatever the process it is thus very difficult to obtain satisfactory results in large scale industrial furnaces and processes of the art.
EP application 0872569 describes a nitriding process and a nitriding furnace therefore wherein the parts to be treated are maintained at a floating or biased potential on a support in a vacuum furnace where current is provided to a metal screen cathode surrounding the parts to be treated.
The necessary heat is provided by radiation from the screen whereby nitriding gas is injected into the furnace such that it has to flow through the screen, where the plasma necessary for the nitriding reaction is generated by glow discharge, the active reagents flow around and react with the parts to be treated before they are evacuated at the bottom of the furnace. This improved nitriding furnace and process allowed the simultaneous treatment of loads constituted of articles having different complex shapes and geometry.
While the process and furnace described in EP
application 0872569 thus constituted a major improvement of the known nitriding processes, this process still suffered from drawbacks albeit less than the prior art processes.
Indeed, it was difficult to obtain a satisfactory nitriding of articles having a complex shape or geometry respectively to obtain any nitriding at the points of contact of the different articles in bulk loads respectively on the surfaces resting on a support within the nitriding furnace.

The articles treated had thus blind or black spots where no nitriding occurred.
During nitriding of steel articles nitrogen atoms 5 and/or carbon atoms where carbonitriding or carbonising is involved penetrate into the articles to be treated forming a superficial layer enriched with nitrogen; where the nitrogen is present in the form of a solid solution or more generally in the form of nitrides. At temperatures between 300 and 600°C the following is usually obtained:
At the outer surface (to a depth of about 1 - 30 Nm) a combination layer (often called white layer) composed of y' Fe4N or of E Fe~Z_;~N , or a mixture of both.
Below this layer, to a depth which might reach up to about 0.8 mm, a diffusion layer composed of nitrides of alloy elements of the steel, (e. g. chromium, manganese etc.) precipitated in the steel's lattice.
With lesser quality steels a y' and/or a diffusion layer cannot be obtained to a satisfactory degree (lack of alloy elements to obtain nitride precipitates). For this reason, a treatment of nitrocarburising at higher temperatures (560°C - 650°C) is carried out in order to obtain a thick, up to 30 um, E compound layer.
It is known that the formation of the E Fe~z_3~N layer and especially its thickness, and to a lesser extend the formation of the y' Fe4N layer, on the treated articles can be improved by the addition of a hydrocarbon gas to the nitrogen-hydrogen mixture and by carrying out the reaction at temperatures above 560°C typically between 560°C and 620°C.
It is therefore an object of the invention to provide a nitriding process allowing better control of the nitriding conditions such that articles having complex shapes or geometry can be treated in one load in a nitriding furnace where a Y' or E and/or diffusion layer is generated over the entire surface of each article even at the points of contact of the articles in bulk loads whereby an aliphatic or cyclic hydrocarbon gas is added to the nitriding gases and where the nitriding process is carried out at a temperature below 560°C and typically between 300°C and 560°C.
It is a further object of the invention to provide a nitriding process whereby the pressure in the nitriding furnace is continuously and randomly varied to allow the active reagents to reach and flow over the entire surfaces of the articles to be treated.
It is also an object of the invention to provide a nitriding process whereby oxygen is added to the nitriding gases.
It is a further object of the invention to disclose an improved nitriding process to be used in a furnace disclosed in EP patent application 0872569.
The use of hydrocarbons in the nitriding processes of the art was known. However, only in order to improve the formation and thickness of the compound E layer, on the articles constituted of steel of an inferior quality.
It has been discovered that quite surprisingly and only at certain conditions the addition of aliphatic or cyclic hydrocarbon gases to the nitriding gases can considerably improve the overall nitriding appearance and quality of complexly shaped articles. Considering that it was known that the addition of hydrocarbons to the nitriding gases at temperatures above 560°C favoured the formation and the thickness of the compound E or Fe~2_»N

layer and at a lesser extend a similar reaction was expected at the lower temperatures. It was thus quite unexpected and quite surprising to note that below 560°C
the addition of hydrocarbons benefits equally the formation of the E Fe~2_3~N layer but also the Y' Fe,N layer and/or of the diffusion layer over the entire surface of the treated articles.
while the exact process occurring within the furnace is not known, it is surmised that the addition of hydrocarbons somehow increases the flowability of the active reagents and thus allows to these reagents to reach not only the easily accessible surface areas but literally every nook and cranny of the articles to be treated, and thus even those parts of the surfaces where the articles touch or where the articles are supported within the furnace.
The desired results are achieved at a temperature between 300°C and 560°C whereby between 3 and 15~ of cyclic or aliphatic hydrocarbon gases are added to the nitriding gases. All nitriding gases can be used, however, typically a mixture of nitrogen and hydrogen is used.
While the results thus achieved were much improved with respect to the prior art processes, the results were not completely satisfactory. A more even treatment of the complexly shaped articles can be obtained by adding the hydrocarbons as disclosed above but by simultaneously and continuously, over the entire process duration, varying the pressure within the furnace randomly between about 0.5 and 10 mbar. Preferably the pressure is varied by increasing or decreasing the gas flow to the furnace. These pressure variations seem to contribute to the distribution of the active reagents evenly over the entire surface.
In accordance with a preferred embodiment of the present invention the furnace (9) is constituted by an upper part (la) and a bottom part (1b) joined by gas seal (3). A 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 (la) of the furnace is lowered onto the grounded bottom part (lb). 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 mbar within the furnace generator (4) is switched on to provide a current sufficient to heat screen (5) to a temperature which is sufficient to heat up the load to process temperature. When the screen has reached the necessary temperature allowing an internal homogenous and uniform temperature of 300° C to 560° C a gas mixture constituted of nitrogen and neutral gases such as hydrogen and/or argon as well as a cyclic or aliphatic hydrocarbons 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 ionised gas constituted of ions, electrons and other active, energised neutral gaseous particles necessary for nitriding the parts 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 parts on support (8).
The parts are continuously bathed in a gentle flow of the active reagents before the gaseous medium is evacuated through conduit (2).
The gas mixture constituted of hydrogen and/or argon, nitrogen and cyclic or aliphatic hydrocarbons is fed into the furnace at different points and rates such that the pressure within the furnace varies continuously and randomly between 0.5 and 10 mbar.
This preferred embodiment allows the treatment of articles of very complex shapes either alone or in combination with other articles having different shapes or allows the nitriding of bulk loads where even at the point of contact of the different articles or at the point of contact of the article with the support within the furnace the desired compound and diffusion layers are generated.
Analysis of thus treated articles showed that a compound y' and E and/or a diffusion layer had been formed over the entire interior and exterior surfaces of the articles which is proof that the active particles flowed around and entered all interstices of the complexly shaped articles thus treated.
In the nitriding process as described the temperature control is of paramount importance as it must not exceed 560°C as otherwise the formation of the E Fe,z_3~N layer is favoured to the detriment of the formation of the y' Fe,N
and especially the diffusion layer. The hydrocarbons to be added to the nitriding gas mixture can be any hydrocarbons.
However, with hydrocarbons with higher carbon numbers undesirable side effects such as a carbon or graphite deposition within the nitriding furnace have been observed.
It is thus preferable to use low carbon number easily available hydrocarbons such as methane, ethane, butane or their derivates.

A carbon deposition on the parts to be treated or in the furnace can also occur where the volume of hydrocarbons added to the gas mixture is too high. The exact amount of cyclic or aliphatic hydrocarbons to be added to the 5 nitriding gases is quite difficult to determine as it depends on the shape (surface area) of the parts to be treated and the carbon absorption ability of the different steel types.

10 As it is possible to treat simultaneously different parts constituted of different steel types determining, for each load, the exact amount of hydrocarbons necessary to achieve the object of the invention would at best be very difficult and at worst be impossible.
These problems can be overcome by adding to the gas mixture, depending on the amount of hydrocarbons used, a small volume i.e. 1 to 4~ of oxygen where the oxygen content must, however, be less than 30~ of the total volume of hydrocarbons. The oxygen will combine with the excess carbon which will thus be evacuated as CO or CO2 and no blackening of the parts or the furnace will occur.
Furthermore the oxygen contributes to improve the reactivity of the gas mixture and thus contributes to a better and even faster diffusion of the nitride layer.
The oxygen could be added in the form of OZ gas or any gaseous molecule containing oxygen.
while a temperature maintained between 300°C and 560°C
and the addition of hydrocarbons in a amount of about 3 to 15°s with respect to the nitriding gases allows a considerable increase of the efficiency of the nitriding process, the pulsing or variation of the pressure between 3 and 15 mbar and the addition of 1 to 4~ oxygen allows a perfect nitriding of the entire surfaces even the most difficultly accessible surfaces of the articles especially if performed in a process as disclosed in the preferred embodiment.
It is evident to men skilled in the art that the process of the invention can be used with any known devices or processes of the art. Thus it is possible to sputter rare earth elements, for example lanthanum, onto the parts to be treated. The rare earth elements have a catalysing effect and speed up the diffusion of the active reagents into the metal lattice of the parts.

Claims (7)

1- A nitriding process wherein a plasma is generated in a furnace comprising the parts to be treated on a support at a pressure inferior to about 20 mbar, characterised by maintaining the temperature below 560C and by adding 3 to 15% of cyclic or aliphatic hydrocarbons gases to the nitriding gases.

2 - Process according to claim 1 characterised in that hydrocarbons with a low number of carbons are used.

3- Process according to any of claims 1 or 2, characterised in that the hydrocarbon used is methane, ethane, butane and their derivates.

4- Process according to any of claims 1 or 3 characterised in that the pressure within the furnace is varied continuously but randomly between about 0.5 and 10 mbar by varying the amount of gas mixture fed to the furnace.

5- Process according to claims 1 to 4, characterised in that, oxygen is added in an amount of 1 to 4% of the total gas mixture where the amount of oxygen must however be less than 30% of the total hydrocarbon volume of the gas mixture.

6- Process according to claim 5, characterised in that the oxygen is added in the form of O2 gas, water vapour or any other gaseous molecule containing oxygen

7- Use of a process according to any of claims 1 through 6 in a furnace as disclosed in EP application 0872569.