WO1999055921A1

WO1999055921A1 - Method for carburizing or carbonitriding metal parts

Info

Publication number: WO1999055921A1
Application number: PCT/FR1999/000707
Authority: WO
Inventors: Didier Domergue; Meryem Buffin; Thierry Sindzingre
Original assignee: L'air Liquide, Societe Anonyme Pour L'etude Et L'exploitation Des Procedes Georges Claude
Priority date: 1998-04-27
Filing date: 1999-03-26
Publication date: 1999-11-04
Also published as: CA2324943A1; CN1298456A; FR2777910B1; AU2939499A; FR2777910A1; EP1076726A1; JP2002513083A; BR9910018A

Abstract

The invention concerns a method for carburizing or carbonitriding metal or metal alloy parts, which consists in contacting the parts, for at least a carbon-enriching phase, with an enriching atmosphere comprising hydrogen and carbon monoxide, the atmosphere CO/H2 ratio being different by 1/2.

Description

CEMENTATION OR CARBONITRU PROCESS OF PARTS

METALLIC

The present invention relates to the field of atmospheres used in heat treatment furnaces. It is more particularly interested in the atmospheres used for the carburizing and carbonitriding processes of metal parts, in particular steel parts.

Such atmospheres frequently arise from an endothermic generator in which a mixture of air and hydrocarbon is sent to a catalytic reactor (most commonly based on nickel) at a temperature above 1000 ° C., making it possible to produce the partial oxidation of the hydrocarbon.

Such carburizing atmospheres can also be obtained by the in-situ reaction (inside the heat treatment furnace) of an air / hydrocarbon mixture, or also by the processes commonly called in the industry "nitrogen-methanol processes "in which a mixture of nitrogen and liquid methanol, in desired proportions (for example 40-60, or 30-70, or even 20-80), is sent inside the heat treatment oven, giving rise cracking methanol in situ, and the corresponding production of hydrogen and carbon monoxide. In all these cases (endothermic generator, nitrogen / methanol atmospheres, etc.) the CO / H ₂ ratio in the oven is close to 1/2.

By way of illustration, a typical composition of carburizing atmosphere inside the furnace (such as obtained for example from an endothermic generator operating on methane or else of a nitrogen-methanol process implementing a ratio 40 % - 60%) is as follows: 20% CO, 40% H ₂ , 0.1% CO ₂ , 0.3% H ₂ 0, 1, 3% CH ₄ , the rest of the atmosphere being made up of nitrogen.

We know that one of the ways in which the person skilled in the art of heat treatments characterizes a heat treatment atmosphere, and in particular its activity in terms of carburizing, is the "carbon potential" of the atmosphere, a parameter which characterizes the capacity of the atmosphere to supply carbon to a part to be cemented, and which (depending on the evaluation systems adopted) can in particular be expressed as a function of the CO and CO ₂ concentration of the atmosphere as well as the temperature at which treatment is carried out. By way of illustration, reference is made to the following works dealing with the role and calculation of the carbon potential involved in the carburizing of metal parts: the work by G. KRAUSS, entitled "Principles of heat treament of steel" published by "The American Society for Metals", as well as to the book "Metals Handbook 9th Edition, vol. 4, Heat treating", published in 1981 by "The American Society for Metals", or to the summary article by Daniel W. McCurdy published in May 1996 in the journal "Materials Australia".

The carbon potential of a gas mixture then represents the carbon content, expressed as a percentage by mass, of the austenite which is in equilibrium with this atmosphere.

If many models for calculating the carbon potential, based on the composition of the atmosphere, are reported in the literature, the fact remains that one of the most precise and rigorous (because absolute) methods of determining this carbon potential is the so-called method "Flashy".

It is based on the notion of thermodynamic balance between the carburizing atmosphere and the carbon contained in a treated part. It therefore consists in bringing together and in equilibrium in a furnace under given temperature and atmosphere, a shim of a low carbon steel (for example of the XC10 type with 0.1% carbon) of small dimensions (for example a shim of 80 mm long, 35 mm wide and 0.05 mm thick, these small dimensions guarantee the possibility of achieving a balance). The carbon potential reached under these fixed atmosphere and temperature conditions is then rigorously evaluated by direct analysis of the carbon content of the foil, for example by total chemical determination of the carbon after combustion of the foil in an oxygen stream ( CO ₂ dosage).

Thus, by way of illustration, an atmosphere whose carbon potential is equal to 0.7 is in equilibrium with an austenite containing 0.7% of carbon, this atmosphere then decarburizing up to 0.7% of carbon an austenite which contains more carbon, and cementing up to 0.7% carbon an austenite which contains less.

As can be seen from the references in the literature cited above, from 1950 to 1987, the state of the art in controlling the carbon potential of heat treatment atmospheres rested essentially on the use of one of the following three reactions (where C _a represents the carbon adsorbed on the surface of the part):

H ₂ + CO → H ₂ O + C _a (1)

2CO → C _a + CO ₂ (2) CO → C _a + 1/2 O ₂ (3)

We can therefore see that, throughout the ages, several formulas for evaluating the carbon potential have been and are used in the literature, thus reaction (1) clearly shows that if the CO and H ₂ contents of the atmosphere are known , the measurement of the H ₂ O concentration makes it possible to calculate the carbon potential of the atmosphere.

The use of reaction (2) shows that if the CO content of the atmosphere is known, the measurement of the CO ₂ concentration of the atmosphere here again makes it possible to evaluate the carbon potential of the atmosphere. This evaluation method was very widely adopted in the 60s for its great stability compared to a control which would be based on the measurement of the dew point.

Finally, we see that reaction (3) shows that for a CO content of the known atmosphere, a measurement of the oxygen content makes it possible to evaluate the carbon potential. The appearance of zirconia oxygen sensors on the market in the 1970s made this method of assessing carbon potential according to reaction (3) quickly become a world standard.

One of the methods commonly used to increase the carbon potential of an atmosphere is to add to the carburizing atmosphere a small amount of a gas rich in hydrocarbons, generally methane or propane, this additional gas reacting with water. , CO ₂ , or oxygen, thus making it possible to increase the CO and H ₂ contents according to the following reactions:

CH ₄ + CO ₂ → 2CO + 2H ₂ CH ₄ + H ₂ 0 → CO + 3H ₂

As we can see, the control but above all the regulation of the carbon potential across the eras was based on the control and regulation of one or more of the species among CO ₂ , CO, H ₂ , O ₂ , or even H ₂ O.

In addition, it has always been recommended to limit the injection of hydrocarbon as well as the level of carbon potential set up, in order to avoid deposits of soot (see for example the synthesis work of Dominique Ghiglione et al, “Practice of Thermal Treatments”, published in May 1997 by Engineering Techniques and the Technical Association for Thermal Treatment-ATTT).

The cementation processes reported in the literature typically use two types of phase, that is to say two types of contacting of the part to be cemented with a controlled atmosphere: a) a first phase called "enrichment" »In carbon during which the part is brought into contact, at a temperature generally between 780 ° C. and 980 ° C. (depending on whether it is carburizing or carbonitriding), with an atmosphere which comprises hydrogen and carbon monoxide, the carbon potential of which is generally in a range from 0.9 to 1.3 (for conventional steels), to obtain a given carbon profile in the surface part of the part. b) a so-called “diffusion” phase, during which the part is brought into contact with an atmosphere whose carbon potential is lower than the carbon potential set up during the enrichment phase (usually 0.7 to 0.9 for conventional steels), in order to obtain a carbon flux transferred from the gas phase to the part to be treated zero or almost zero, diffusion phase allowing a diffusion of the carbon previously introduced inside the part, and thus a carbon concentration profile inside the part (and in particular on the surface) required and chosen on metallurgical criteria.

Depending on the parts treated and the applications targeted, there is a very wide variety of cementation processes in industry, the duration of which varies very widely, typically from processes lasting only one hour, to processes lasting nearly 24 hours.

It is therefore understandable that it would be extremely advantageous, for economic reasons of productivity, to be able to have accelerated carburizing processes making it possible to significantly reduce the carburizing time.

The present invention aims in particular to propose an accelerated case hardening process.

To do this, the invention relates to a method of carburizing or carbonitriding metal parts (therefore based on metal or metal alloy, in particular ferrous), according to which the parts are brought into contact, during at least one carbon enrichment phase, with a carbon enrichment atmosphere which comprises hydrogen and carbon monoxide, a process of the kind where the carbon potential of said enrichment atmosphere can be varied by controlled additions of a gaseous species such as a hydrocarbon, or a mixture of hydrocarbons, or even a gaseous mixture comprising oxygen or capable of releasing oxygen, characterized by the implementation of the following measures a) said controlled addition is carried out in order to reach the level of carbon potential of the desired atmosphere making it possible to reach or even exceed the carbon saturation of the austenite of said metal or metal alloy; b) the residual content of the atmosphere in at least one hydrocarbon is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual content of the atmosphere in said at least one hydrocarbon at a preset value.

As will be understood from reading the above, the level of carbon potential set up and regulated makes it possible to obtain and maintain on the surface of the part a carbon content at least equal to the saturation value in carbon of the austenite of the metal or metal alloy considered.

By way of illustration, reference will conventionally be made to phase diagrams well known to those skilled in the art of heat treatments for iron or its alloys, with for example for pure iron, saturation of the austenite, for a temperature between about 895 ° C and 927 ° C, obtained for a carbon content of the order of 1, 2 to 1, 3%, beyond, the carbon precipitating in austenite in the form of carbides.

The process of carburizing or carbonitriding metal parts according to the invention may moreover adopt one or more of the following characteristics: - said contacting of the parts with an enrichment atmosphere is carried out at a temperature between 780 ° C and

980 ° C;

- the hydrocarbon whose residual content in the enrichment atmosphere is regulated is methane CH; - the hydrocarbon, the residual content of which is regulated in the atmosphere, is one of the decomposition by-products of a CxHy hydrocarbon where x>1;

- the carbon potential of the atmosphere is regulated to a value greater than or equal to 0.7%;

- the carbon potential of the atmosphere is regulated to a value greater than or equal to 1.3%;

- the carbon potential of the atmosphere is regulated to a value less than or equal to 4%; - The residual content of the hydrocarbon enrichment atmosphere is regulated to a value between 0.1% and 5% by volume;

the residual CO ₂ content of the atmosphere is less than or equal to 2% and preferably less than or equal to 1.5% by volume;

- one proceeds after the or each carbon enrichment phase of the parts, to a diffusion phase, by bringing the parts into contact with a diffusion atmosphere which comprises hydrogen and carbon monoxide, the carbon potential of said diffusion atmosphere being less than said regulated value of the carbon potential of the enrichment atmosphere; - we carry out several enrichment / diffusion cycles of carbon metal parts;

- the said value of the carbon potential of the diffusion atmosphere is obtained which is lower than the regulated value of the carbon potential of the enrichment atmosphere by adding carbon dioxide CO ₂ ; - the residual content of the diffusion atmosphere is measured in

CO ₂ and the carbon potential of the diffusion atmosphere is regulated at said desired level, by regulating the residual CO ₂ content of the atmosphere at a predefined value.

The invention also relates to a process for carburizing or carbonitriding parts based on metal or metal alloy, according to which the parts are brought into contact, during at least one carbon enrichment phase, at a given enrichment temperature, with an enrichment atmosphere comprising hydrogen and carbon monoxide, of the kind where the carbon potential of said enrichment atmosphere can be varied by controlled additions of a gaseous species such as a hydrocarbon or a mixture of hydrocarbons, or a gas mixture containing oxygen or capable of releasing oxygen, characterized by the implementation of the following measures: a) the said controlled addition is carried out in order to reach the level of carbon potential of the atmosphere sought for reaching or even exceed the carbon saturation of the austenite of said metal or metal alloy; b) the sensitivity of the carbon potential of the enrichment atmosphere to the vicinity of the carbon potential reached during step a) is evaluated, as a function of additions to the atmosphere of hydrocarbon or gaseous mixture comprising oxygen or capable of releasing oxygen; c) according to the result of the evaluation of step b), the carbon potential of the atmosphere is regulated at said desired level by adopting one or the other of the following paths:

- the residual content of the atmosphere in at least one hydrocarbon is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual content of the atmosphere in said at least one hydrocarbon to a value predefined;

- the residual content of the atmosphere in water vapor is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual content of the atmosphere in water vapor to a predefined value ;

- The residual oxygen content of the atmosphere is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual oxygen content of the atmosphere at a predefined value;

- The residual CO ₂ content of the atmosphere is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual CO ₂ content of the atmosphere at a predefined value.

The process of carburizing or carbonitriding metal parts according to the invention may moreover adopt one or more of the following characteristics:

- the contact with the enrichment atmosphere takes place at a temperature between 780 ° C and 980 ° C;

- Said hydrocarbon whose residual content is regulated is methane; - Said hydrocarbon whose residual content is regulated is one of the decomposition by-products of a CxHy hydrocarbon where x>1;

- the carbon potential of the atmosphere is regulated at a value in the range from 0.7% to 4%; - The residual content of said at least one hydrocarbon is regulated in the range from 0.1 to 5% by volume;

the residual CO _{2 content} of said enrichment atmosphere is less than or equal to 2% by volume;

- the residual CO _{2 content} of said enrichment atmosphere is less than 1.5% by volume;

- we proceed, after the or each carbon enrichment phase, to a diffusion phase, during which the parts are brought into contact with a diffusion atmosphere comprising hydrogen and carbon monoxide, the carbon potential of which is lower than said regulated value of the carbon potential of the enrichment atmosphere;

- the carbon potential of the diffusion atmosphere is less than 1%;

- the said value of the carbon potential of the diffusion atmosphere is obtained which is lower than the regulated value of the carbon potential of the enrichment atmosphere by adding carbon dioxide CO ₂ ;

- The residual CO _{2 content} of the diffusion atmosphere is measured and the carbon potential of the diffusion atmosphere is regulated by regulating the residual CO ₂ content of the atmosphere at a predefined value. Other characteristics and advantages of the present invention will emerge from the following description of embodiments given by way of illustration but in no way limiting, made in relation to the appended drawings in which:

- Figure 1 is a representation of curves giving on the ordinate the carbon content for foils of XC10 cemented steel, as a function of the CO ₂ content of the enrichment atmosphere, a curve being represented for each value of the ratio CO / H ₂ from the enrichment atmosphere;

- Figure 2 shows, for pellets (bulk samples) of XC10 case-hardened steel, four curves providing the carbon profile of the parts (carbon content of the parts on the ordinate as a function of the depth in micron in the room), the four curves having been respectively obtained for a carbon potential of the atmosphere of 0.5%, 0.78%, 1, 25% and 3.81%;

- Figure 3 shows, for the case of CO / H ₂ atmosphere at 70% CO, the carbon content measured in the foil as a function of the residual CH _{4 content} measured in the atmosphere.

FIG. 1 therefore represents the results obtained in terms of carburizing on low carbon steel foils (0.1% carbon by weight, rectangular pieces of 80 mm x 35 mm x 0.05 mm), treated in all cases during a duration of 15 min, this time of 15 min having demonstrated to be sufficient for the foil to reach thermodynamic equilibrium with the carburizing atmosphere.

Each foil after treatment was analyzed by complete combustion in oxygen and analysis of the combustion CO ₂ , making it possible to obtain the total carbon content of the foil, and therefore to deduce therefrom the value of the carbon potential of the atmosphere.

All tests were carried out in a brand pot oven

AICHELIN, whose volume is close to 0.15 m ³ , the foils being introduced into the oven as soon as the treatment temperature of 925 ° C is reached and as soon as the desired composition of the atmosphere in the oven is sufficiently stabilized. As already mentioned, the holding time of the foil in the oven is 15 min.

The work carried out by the Applicant as reported in Figure 1 therefore shows the carbon content measured in the foil as a function of the residual CO _{2 content} in the atmosphere, this for different enrichment atmospheres characterized by their ratio

CO / H ₂ (the rightmost curve on the graph being obtained for the ratio

90/10, then follow in order the curves obtained for the reports

70/30, 60/40, 50/50, 30/70, 20/80, the last curve shown furthest to the left on the graph being obtained for a nitrogen / methanol atmosphere in a 40/60 ratio).

A certain number of remarks are immediately imperative when reading these curves:

- firstly the possibility of obtaining, for certain atmosphere conditions, carbon contents in the foil (therefore atmospheric carbon potentials) extremely high, in practice widely 10

greater than 1, even reaching 3 to 4%, including for CO / H ₂ ratios other than 1/2;

- for all these curves, the observation of a first domain (domain obtained by adding CH ₄ to the atmosphere allowing the consumption of CO ₂ to form CO and hydrogen) for which, below a certain value of the residual CO _{2 content} (depending on the CO / H ₂ ratio considered), the residual CO _{2 content} in the atmosphere varies extremely little or even more at all while the carbon potential increases very strongly; - it is therefore conceivable that in this first extremely interesting field, since using very high carbon potentials, the value of the carbon potential of the atmosphere is difficult to adjust as a function of the CO ₂ content of the atmosphere (as is conventionally done in the literature) since this CO ₂ content varies very little; - similarly, given the presence of a significant hydrocarbon content in the atmosphere for this first domain, it will also be difficult to adjust the carbon potential using the traditional method of the oxygen sensor (problem pollution of the probe);

- for each curve, a significant variation in slope occurs in the vicinity of a carbon potential located between 1, 2 and 1, 3%, that is to say in the vicinity of the carbon potential of saturation of the austenite of l 'steel considered (at the treatment temperature practiced ie 925 ° C), it can then be extremely advantageous according to the invention, from this range, to regulate the carbon potential of the atmosphere to a given value, by a measurement the hydrocarbon content of the atmosphere and a regulation of this hydrocarbon content at a predefined level taking into account the targeted carbon potential;

- in a second part of the graph, on the other hand, that is to say for high CO ₂ contents (in practice beyond 1.5 to 2% by volume), second part obtained by adding CO ₂ in the atmosphere (causing the residual hydrocarbon content of the atmosphere to fall to very low levels), gives rise to a fall in the carbon potential to values below 0.5%, then below 0.25 %;

- it is therefore easy to understand the advantage according to the invention of regulating a high carbon potential (greater than or equal to 1, 2 or 1, 3, or even greater than 2 or 3%), during a carbon enrichment phase, on content 11

residual hydrocarbon from the enrichment atmosphere (the corresponding residual CO ₂ content being in this extremely insensitive range), whereas it will advantageously be used during a phase of diffusion of carbon inside the room, by bringing the part into contact with an atmosphere of lower carbon potential (for example less than 1, typically located between 0.7 and 0.9%), regulating the CO ₂ content of the atmosphere to a desired level given the targeted “diffusion” carbon potential of less than 1, the corresponding CH content of the atmosphere then varies extremely little (very low sensitivity); - we also see on reading this figure 1 the existence of what can be called a third domain, intermediate between the two previously mentioned domains, at least for certain CO / H ₂ ratios (typically greater than or equal to 30/70). This intermediate range is characterized by a “softer” variation of the carbon potential as a function of the CO ₂ content of the atmosphere, in a range of carbon potential situated here between approximately 1.2 and approximately 2.5%. We then conceive for the CO / H ₂ ratios considered and the intermediate domains considered, the advantage of being able to set up high carbon potentials (since they are much higher than the saturation of austenite) while retaining here the possibility of regulating or these carbon potentials by regulating the CO ₂ content of the carburizing atmosphere.

We can see very clearly in this figure that the limits of each domain will depend on the curve considered, that is to say the CO / H ₂ ratio used, but we can also see that the beam represented was for a temperature given treatment temperature (925 ° C) and that each treatment temperature will then give its own beam and therefore its own domains for each curve.

FIG. 3, as already mentioned, illustrates one of the curves representing the carbon content measured in the foil as a function this time not of the CO ₂ content measured in the atmosphere, but as a function of the residual CH ₄ content in the atmosphere, the example shown here concerning the case of the CO / H ₂ atmosphere at 70% CO already mentioned in the context of FIG. 1.

We then find in this figure the considerations already highlighted above: 12

- in a range of carbon potential less than or equal to 1.2 to 1.3%, we find the range of very small variation in the residual CH _{4 content} in the atmosphere, corresponding, as we have seen above, to a range of significant variation in the residual CO _{2 content} ; - from the level of carbon potential located in a range from approximately 1.2 to 1.3%, there is a large variation in the residual CH _{4 content} in the atmosphere (evolving in this case typically between 0 , 1% and 3%), resulting in a strong growth of the carbon potential until reaching a value greater than 3%, corresponding, as previously described in the context of FIG. 1, to a range of low value and very small variation in the residual CO _{2 content} in the atmosphere.

FIG. 2 then illustrates for its part the advantage of working with high carbon potential (3.81%) on the carbon content introduced into a piece of XC10 steel (cylindrical solid samples of 30 mm in diameter and 5 mm in diameter). thickness), as a function of the depth expressed in microns, by providing the comparative results obtained for four different carbon potentials (respectively 0.5%, 0.78%, 1, 25% and 3.81%).

The samples were obtained under the following experimental conditions: one hour of treatment in the same AICHELIN oven at 925 ° C, the initial atmosphere introduced into the oven being a binary CO / H ₂ atmosphere at 50% C0 and 50% d 'hydrogen.

The analysis of the samples, once treated, was carried out by Glow Discharge Spectrometry (SDL). The carbon potential of the atmosphere (0.5, 0.78, etc.) was determined in each of the 4 cases by introducing into the oven, in addition to the solid pellet to be cemented, a foil, and by analyzing this foil. after treatment (characteristics and procedure already described in the context of Figure 1) °. It is understood that in 1 hour of carburizing, the balance of the foil is largely reached since we saw it previously it was already reached in 15 minutes.

The carburizing atmospheres in each case are then the following: 13

a) case of carbon potential equal to 0.5%

CO = 49.5%, H ₂ = 48.6%, CO ₂ = 1.82%, CH ₄ = 0.12% and H ₂ O = 19.6 ° C. b) case of carbon potential equal to 0.78% CO = 49.3%, H ₂ = 49.7%, CO ₂ = 1.04%, CH ₄ = 0.18% and

H ₂ O = 11.6 ° C. c) case of carbon potential equal to 1.25%

CO = 50%, H ₂ = 49.4%, CO ₂ = 0.55%, CH ₄ = 0.29% and H ₂ O = 2.9 ° C. d) case of carbon potential equal to 3.81%

CO = 48%, H ₂ = 49.1%, CO ₂ = 0.25%, CH ₄ = 2.66% and H ₂ O = - 8.4 ° C.

We then clearly see that the carbon profile obtained for a carbon potential of 3.81% largely dominates the other carbon profiles obtained for carbon potential values which are not located on the substantially vertical part of the CO / H ₂ curve = 50/50 (Figure 1) representing the evolution of the carbon potential as a function of the CO ₂ content.

The amount of carbon introduced into the sample is consequently much greater in this case, for the same treatment time of one hour.

In view of the curves shown in Figure 2, we can see the great practical interest in being able to regulate the carbon potential of an enrichment atmosphere at high values (ie higher than that corresponding to the saturation of austenite) . Indeed, such conditions make it possible to accelerate the carburizing kinetics (a given carbon profile can here, considering FIG. 2, be obtained more quickly by working initially with a carburizing atmosphere with higher carbon potential than those traditionally used in the literature. ). It is indeed well known that at a given temperature, the cementation depth is expressed as a function of the square root of time (see the work by G. KRAUSS cited above).

To summarize the comments made below concerning FIGS. 1 to 3, we can understand the advantage of one aspect of the invention, according to which the parts are brought into contact, during at least one phase of carbon enrichment with an enrichment atmosphere, and the following measurements are carried out: 14

a) a controlled addition of a species such as a hydrocarbon is carried out, in order to reach or even exceed the carbon saturation of the austenite of the metal or metal alloy considered; b) the sensitivity of the carbon potential of the enrichment atmosphere to the vicinity of the carbon potential reached during step a) is evaluated, as a function of additions of CO ₂ or else of hydrocarbon in the atmosphere (as in as previously seen, depending on the CO / H ₂ ratio considered, an intermediate domain of “soft” variation in the carbon potential may or may not be observed as a function of the CO ₂ content of the atmosphere); c) according to the result of the evaluation of step b), the carbon potential of the atmosphere is regulated at said desired level by measuring the residual content of the atmosphere in at least one species from among the hydrocarbons, CO ₂ , H ₂ O, or O ₂ , and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual content of the atmosphere in said species.

Claims

15 CLAIMS

1. A method of carburizing or carbonitriding parts based on metal or metal alloy, according to which the parts are brought into contact, during at least one carbon enrichment phase, with an enrichment atmosphere comprising hydrogen and carbon monoxide, characterized in that the CO / H ₂ ratio of the atmosphere is other than 1/2.

2. A carburizing or carbonitriding method according to claim 1, characterized in that the carbon potential of the atmosphere is regulated to a desired level by adopting one or the other of the following routes:

- the residual content of the atmosphere in water vapor is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual content of the atmosphere in water vapor to a predefined value ; - The residual oxygen content of the atmosphere is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual oxygen content of the atmosphere at a predefined value;

3. A method of carburizing or carbonitriding parts based on metal or metal alloy, according to which the parts are brought into contact, during at least one carbon enrichment phase, at a given enrichment temperature, with an atmosphere d enrichment comprising hydrogen and carbon monoxide, of the type in which the carbon potential of said enrichment atmosphere can be varied by controlled additions of a gaseous species such as a hydrocarbon or a mixture of hydrocarbons, or a gas mixture comprising oxygen, characterized by the implementation of the following measures: 16

a) said controlled addition is carried out in order to reach the level of carbon potential of the desired atmosphere making it possible to reach or even exceed the carbon saturation of the austenite of said metal or metal alloy; b) the sensitivity of the carbon potential of the enrichment atmosphere to the vicinity of the carbon potential reached during step a) is evaluated, as a function of additions to the atmosphere of hydrocarbon or of mixture comprising oxygen or capable of releasing oxygen; c) according to the result of the evaluation of step b), the carbon potential of the atmosphere is regulated at said desired level by adopting one or the other of the following paths:

4. Method according to claim 3, characterized in that said contacting with the enrichment atmosphere is carried out at a temperature between 780 ° C and 980 ° C.

5. Method according to claim 3 or 4, characterized in that said hydrocarbon whose residual content is regulated is methane.

6. Method according to claim 3 or 4, characterized in that said hydrocarbon, the residual content of which is regulated, is one of the decomposition by-products of a hydrocarbon CxHy where x> 1. 17

7. Method according to one of claims 3 to 6, characterized in that one regulates the carbon potential of the atmosphere to a value in the range from 0.7% to 4%.

8. Method according to one of claims 3 to 7, characterized in that one regulates the residual content of said at least one hydrocarbon in the range from 0.1 to 5% by volume.

9. Method according to one of claims 3 to 8, characterized in that the residual CO _{2 content} of said enrichment atmosphere is less than or equal to 2% by volume.

10. Method according to claim 9, characterized in that the residual CO _{2 content} of said enrichment atmosphere is less than 1.5% by volume.

11. Method according to one of claims 3 to 10, characterized in that one proceeds, after the or each carbon enrichment phase, to a diffusion phase, during which the parts are brought into contact with an atmosphere diffusion comprising hydrogen and carbon monoxide, whose carbon potential is lower than said regulated value of the carbon potential of the enrichment atmosphere.

12. Method according to claim 11, characterized in that the carbon potential of the diffusion atmosphere is less than 1%.

13. Method according to claim 11 or 12, characterized in that one obtains the said value of the carbon potential of the diffusion atmosphere lower than the regulated value of the carbon potential of the enrichment atmosphere by addition of carbon dioxide CO ₂ .

14. Method according to one of claims 11 to 13, characterized in that the residual CO _{2 content} of the diffusion atmosphere is measured and the carbon potential of the diffusion atmosphere is regulated by regulating the residual CO ₂ content of the atmosphere at a predefined value.

15. A method of carburizing or carbonitriding parts based on metal or metal alloy, according to which the parts are brought into contact, during at least one carbon enrichment phase, with an enrichment atmosphere comprising hydrogen and carbon monoxide, of the kind where the carbon potential of said enrichment atmosphere can be varied by controlled additions of a gaseous species such as a hydrocarbon or a mixture of hydrocarbons, or a mixture 18

gaseous containing oxygen or capable of releasing oxygen, characterized by the implementation of the following measures: a) said controlled addition is carried out in order to reach the level of carbon potential of the desired atmosphere making it possible to reach or even to exceed the carbon saturation of the austenite of said metal or metal alloy; b) the residual content of the atmosphere in at least one hydrocarbon is measured and the carbon potential of the atmosphere is regulated at said desired level, by regulating the residual content of the atmosphere in said at least one hydrocarbon at a preset value.

16. The method of claim 15, characterized in that said contacting with the enrichment atmosphere is carried out at a temperature between 780 ° C and 980 ° C.

17. The method of claim 15 or 16, characterized in that said hydrocarbon whose residual content is regulated is methane.

18. The method of claim 15 or 16, characterized in that said hydrocarbon, the residual content of which is regulated, is one of the decomposition by-products of a CxHy hydrocarbon where x> 1.

19. Method according to one of claims 15 to 18, characterized in that the carbon potential of the atmosphere is regulated to a value lying in the range from 0.7% to 4%.

20. Method according to one of claims 15 to 19, characterized in that one regulates the residual content of said at least one hydrocarbon in the range from 0.1 to 5% by volume.

21. Method according to one of claims 15 to 20, characterized in that the residual CO _{2 content} of said enrichment atmosphere is less than or equal to 2% by volume.

22. The method of claim 21, characterized in that the residual CO _{2 content} of said enrichment atmosphere is less than 1.5% by volume.

23. Method according to one of claims 15 to 22, characterized in that one proceeds, after the or each carbon enrichment phase, to a diffusion phase, during which the parts are brought into contact with an atmosphere diffusion comprising hydrogen and carbon monoxide, whose carbon potential is lower than said regulated value of the carbon potential of the enrichment atmosphere. 19

24. The method of claim 23, characterized in that the carbon potential of the diffusion atmosphere is less than 1%.

25. The method of claim 23 or 24, characterized in that one obtains the said value of the carbon potential of the diffusion atmosphere lower than the regulated value of the carbon potential of the enrichment atmosphere by adding carbon dioxide CO ₂ .

26. Method according to one of claims 23 to 25, characterized in that the residual CO _{2 content} of the diffusion atmosphere is measured and the carbon potential of the diffusion atmosphere is regulated by regulating the residual CO ₂ content of the atmosphere at a predefined value.