A method for surface hardening a ferrous-alloy article and the resulting product

Description

Background of the Invention

Field of the Invention

[0001] This invention relates to a method for surface hardening a ferrous alloy article which forms a carbonitride layer, particularly a layer of carbonitride of at least one element selected from V, Nb, Ta, on the surface of the article and to the ferrous alloy article obtained thereby.

Description of the Prior Art:

[0002] The inventors of this invention have previously developed a method for surface hardening of a ferrous alloy article containing at least 0.2% by weight of carbon. A layer of carbide of an element selected from V, Nb and Ta is formed on the article surface by treatment thereof with, for example, a molten salt, powder or gas containing this element. This method described in DE-A-2 829 976 is widely used to prolong the life of, for example, a tool or mold.

[0003] The carbide layer is formed by thermal diffusion and combination of vanadium, niobium or tantalum, and carbon in the ferrous allow article. The amount of carbon in the article immediately below the carbide layer is reduced as it is consumed for the formation of the carbide layer. As a result, when the article is hardened, its portion immediately below the carbide layer is likely to obtain a lower degree of hardness than the inner portion of the article. If this article is used for an application in which it is subjected to a high stress, it is likely that the portion immediately below the carbide layer may be deformed to cause the cracking or peeling of the carbide layer. If the article is a bar having a diameter of no more thaa_1 mm or has a small thickness like the cutting edge of a sharp knife, the prior method for surface treatment reduces the amount of carbon throughout the article and thereby makes it difficult to harden the article satisfactorily.

[0004] A layer of a carbide containing nitrogen or a carbonitride has recently been found to be higher in toughness than a carbide layer and has come to replace the carbide layer for the purpose of coating the surface of an article. This carbonitride coating treatment is presently carried out by a method employing a gas, such as titanium tetrachloride. This method not only requires substantial caution for protecting the health of workers, but also is complicated and necessitates expensive equipment.

Summary of the Invention

[0005] An object of this invention is to improve a previously-developed method for a carbide layer and provide a method which effectively forms (on the surface of a ferrous-alloy article) a surface layer composed of the carbonitride of at least one element selected from V, Nb, Ta and an inner layer composed of a ferrous alloy containing a solid solution of nitrogen.

[0006] Another object of this invention is to provide a method which easily and quickly forms a carbonitride layer.

[0007] A further object of this invention is to provide a ferrous alloy article which can be satisfactorily hardened.

[0008] These objects are attained by the features according to claim 1.

[0009] This invention is applicable to a ferrous alloy article containing at least 0.2% by weight of carbon. The presence of at least 0.2% by weight of carbon in the ferrous alloy is essential to form a carbonitride layer of good quality. In the event a ferrous alloy article contains less than 0.2% by weight of carbon, it is carburized before, during or after nitriding so that its surface portion may contain at least 0.2% by weight of carbon prior to its treatment for the formation of a nitrocarbide thereon.

[0010] The ferrous alloy article produced by the method of this invention has (on its surface) a surface layer composed of the carbonitride of an element selected from V, Nb, Ta and an inner layer composed of a ferrous alloy containing a solid solution of nitrogen immediately below the surface layer of the article, and can be hardened and quenched satisfactorily.

[0011] These and other objects, features and advantages of this invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the invention is shown by way of illustrative example.

Brief Description of the Drawings

[0012] 

Figures 1 and 4 are graphs showing the hardness of the base material in the ferrous alloy article treated in accordance with this invention in Examples 1 and 3, respectively.

Figures 2, 3 and 6 to 11 are graphs showing the results of X-ray microanalysis of the surface portion of the ferrous alloy article treated in accordance with this invention in Examples 1, 3 and 5 to 10; and

Figure 5 is a microphotograph of 400 magnifications showing the cross section of the surface layer formed in accordance with this invention in Example 4.

Detailed Description

[0013] The surface hardening treatment of this invention includes nitriding. It is possible to employ any method, such as gas nitriding, gas soft nitriding, salt bath soft nitriding or glow discharge nitriding. Each of these methods is conventional and well known. It is preferable to form a nitrided layer having a high nitrogen content and a large depth. In the event carburization treatment follows nitriding, it is particularly important to form a nitrided layer having a sufficiently large depth and a sufficiently high nitrogen content, since diffusion of nitrogen into the inner or center portion of an article during carburization leads to a reduction in the amount of nitrogen in its surface portion. The carburization must be effected in a short time. Such carburization is well known and is readily effected by any skilled artisan.

[0014] The formation of a carbonitride layer can be effected by various processes, including molten salt dipping, molten salt electrolysis, powder packing, slurry process and fluidized furnace process. These processes and required compositions are, per se, known, but not in the context of the present invention. This treatment combines an element selected from V, Nb and Ta in a treating agent and the carbon and nitrogen in the ferrous alloy article. This treatment is carried out at a temperature of at least about 600°C and lower than the melting point of the material to be treated. An appropriate temperature range is from 800°C to 1200°C. The period of time required for the treatment is in the range of from 30 minutes to 24 hours. When an electrolysis process is employed, it is appropriate to apply a cathode current density of from 0.01 to 3 A/cm².

[0015] The treatment provides a surface layer having an increased thickness when the temperature and period of time are increased. An increase in the thickness of the layer, however, tends to develop fine pores in the layer and to lower its pitting and wear resistance. Although the critical thickness beyond which those defects are intolerable depends on the element (V, Nb, Ta) employed, nitriding conditions, the kind of ferrous alloy article and treatment conditions for the carbonitride layer, there is usually no appreciable problem when the thickness does not exceed 10 ₁₁m.

[0016] The treatment of this invention forms on the surface of a ferrous alloy article a surface layer composed of said carbonitride; the surface layer is outstanding in wear resistance and toughness. As the formation of the layer according to this invention is effected by the combination of not only carbon, but also nitrogen, in the article with the element selected from V, Nb and Ta, it obtains a predetermined thickness within a shorter period of time than a carbide layer does. The treatment of this invention also forms a ferrous alloy layer containing a solid solution of nitrogen immediately below the carbonitride layer. This makes it possible to obtain a sufficiently high degree of hardness in the area immediately below the surface layer. Therefore, it is possible to harden the article satisfactorily, even when the article has only a small thickness.

Example 1

[0017] An untreated sample and a nitrided sample, each having a diameter of 20 mm, were formed from alloy tool steel for hot forming use, AISI H13 (equivalent to Japanese Industrial Standard (JIS) SKD61). The nitrided sample was prepared by two hours of immersion in a salt bath containing cyanide and having a temperature of 560°C. These samples were subjected to molten salt dipping, i.e. immersed in a bath of molten borax containing 20% by weight of vanadium oxide (V₂0₅) and 10% by weight of boron carbide (B₄C) and having a temperature of 1000°C for from 30 minutes to 16 hours, and oil quenched. After the molten borax adhering to the samples had been removed by washing, each sample was cut in cross section and the cut section was polished; the thickness of the exposed surface layer was then measured. A layer thickness of 8 or 6 pm was obtained on the nitrided samples by immersing the same for eight or four hours, respectively, in the molten salt bath for forming the carbide layer but twice as much time, i.e. 16 or 8 hours, was required to obtain the same layer thickness on the untreated samples. The cross-sectional hardness distribution of the article was examined, and the results are shown in Figure 1. Curve B indicates that the unnitrided sample suffered from a reduction in hardness immediately below the surface layer (of vanadium carbide), but, as is obvious from curve A, the nitrided sample did not substantially show any such hardness reduction.

[0018] The X-ray microanalysis of the cross section of the nitrided sample, which had been treated with molten borax for an hour, revealed the presence of C and N, as well as V, in the layer as shown in Figure 2. The lattice constant obtained by X-ray diffraction confirmed that the layer was one of vanadiuim carbonitride expressed as V (C, N). The layer was found to contain 10% by weight of nitrogen. A ferrous alloy layer containing a solid solution of nitrogen was found to have been formed immediately below the carbonitride layer. It coincided with the area in which no substantial hardness reduction took place as shown by curve A in Figure 1.

Example 2

[0019] A nitrided sample and an unnitrided sample were prepared from carbon steel for machine structural use, AISI H13 (equivalent to JIS S45C). The nitrided sample was prepared by three hours of glow discharge nitriding (ion nitriding) at 555°C. The samples were each 7 mm in diameter. They were immersed in a bath of molten borax containing 20% by weight of Fe-V powder and having a temperature of 900°C for from 30 minutes to 16 hours. The cross-sectional examination of the samples for layer thickness indicated that a surface layer grew at a faster rate (about 1.5 times faster) on the nitrided sample than on the unnitrided sample. A surface layer of V (C, N) and an inner layer composed of a ferrous alloy containing nitrogen and located immediately below the surface layer were formed in the nitrided sample.

Example 3

[0020] The procedures of Example 1 were repeated for the preparation of unnitrided and nitrided samples made of alloy tool steel for hot forming use, AISI H13 (equivalent to JIS SKD61). They were immersed in a bath of molten borax containing 20% by weight of iron-niobium-tantalum (Fe-Nb-Ta) powder for 30 minutes to four hours. The formation of 4 and 5 µm thick layers required one and two hours, of immersion for the carbide layer formation, respectively, on the nitrided sample, and two and four hours, respectively, on the unnitrided sample.

[0021] The nitrided sample which had been subjected to 30 minutes of thermal diffusion treatment in the molten borax bath was found by X-ray microanalysis to contain N, as well as C, in its surface layer, as shown in Figure 3. As Ta and Nb also showed a similar change in concentration, the results of the X-ray diffraction could be interpreted to conclude that there had been formed a surface layer of the carbonitride expressed as Nb, Ta (C, N) and an inner layer composed of a ferrous alloy containing a solid solution of nitrogen. As is obvious from Figure 4, the unnitrided sample (curve B) showed a substantial reduction in hardness immediately below the surface layer, but the nitrided sample (curve A) did not show any such phenomenon.

Example 4

[0022] A nitrided sample made of alloy tool steel for hot forming use, AISI H13 (equivalent to JIS SKD61) was prepared as described in Example 1, and subjected to molten salt electrolysis. Namely, it was immersed in a bath of molten borax containing 10% by weight of niobium oxide (Nb_z0₅) and having a temperature of 1000°C. The sample was employed as a cathode and subjected to two hours of electrolysis at a cathode current density of 0.05 A/cm² in a graphite vessel employed as an anode. A layer having a smooth surface (as shown by the microphotograph of Figure 5 was formed, and it was found to be composed of a carbonitride Nb (C, N). An inner layer (composed of a ferrous alloy containing nitrogen) was found to exist immediately below the surface carbonitride layer, as was also the case in the following examples.

Example 5

[0023] A sample made of carbon steel for machine structural use, AISI 1045 (equivalent to JIS S45C) was subjected to salt bath nitriding at 570°C for 90 minutes and then immersed for the carbide layer formation for 30 minutes in a nitrogen-protected bath of barium chloride (BaCl₂) containing 10% by weight of Fe-V powder and having a temperature of 1000°C. A layer having a thickness of 3 pm was thereby formed. The X-ray microanalysis of the layer indicated that it was a layer of the carbonitride expressed as V (C, N), as shown in Figure 6. Moreover, the microscopic examination of the sample revealed the presence of a structure containing nitrogen in the area of the article adjoining the carbonitride layer.

Example 6

[0024] A sample made of alloy tool steel for cold forming use, AISI D2 (equivalent to JIS SKD11) in the shape of a round bar was subjected to gas soft nitriding at 570°C for 150 minutes, and was subjected to powder packing, i.e. buried in a powder mixture consisting of Fe-V powder and 10% by weight of potassium borofluoride (KBF₄) powder, having a particle size of -100 mesh and placed in a stainless steel vessel. The vessel was heated at 600°C for 16 hours in an atmospheric furnace. After the vessel had been taken out of the furnace and air cooled, the sample was taken out of the powder. The sample was found by X-ray microanalysis to have a layer composed of V, N and C as shown in Figure 7, i.e., a layer of the carbonitride expressed as V (C, N).

Example 7

[0025] A round bar made of carbon steel for machine structural use, AISI 1045 (equivalent to JIS S45C) was subjected to gas nitriding at 500°C for 60 hours, and the procedures of Example 6 were repeated for treating the sample, except that a heating temperature of 650°C was employed. A layer composed of V, N and C was found on this sample, too, as shown in Figure 8, and this V (C, N) layer was formed on the outermost surface of the sample.

Example 8

[0026] A round bar made of carbon tool steel, AISI W 1-9 (equivalent to JIS SK4) was gas nitrided as described in Example 7, and treated at 1000°C for five hours with a powder mixture consisting mainly of Fe-V and containing 5% by weight of ammonium chloride (NH₄CI). A layer composed of V, N and C was formed as shown in Figure 9. The X-ray diffraction of this surface layer revealed diffraction lines indicating VC and VN, and thereby confirmed that the surface layer was composed of the carbonitride expressed as V (C, N).

Example 9

[0027] A bar made of carbon tool steel, AISI W 1-9 (equivalent to JIS SK4) was nitrided as described in Example 6, and treated at 1000°C for five hours with a powder mixture consisting of Fe-Nb-Ta and 10% by weight of KBF₄ and having a particle size of -100 mesh. A surface layer composed of Nb, Ta, C and N (as shown in Figure 10) was formed. The layer was found to be of the carbonitride Nb, Ta (C, N).

Example 10

[0028] A slurry was formed from a powder mixture consisting of 40% by weight of alumina (AI₂0₃), 55% by weight of Fe-V and 5% by weight of ammonium chloride (NH₄CI) by using a solvent prepared by dissolving ethyl cellulose in ethyl alcohol. The slurry was applied in a thickness of 3 to 5 mm onto a sample made of carbon tool steel, AISI W 1-9 (equivalent ot JIS SK4) which had been gas nitrided as described in Example 7. The sample was placed in a stainless steel vessel, and heated at 1000°C for five hours in an argon gas atmosphere. The surface layer thereby formed was found by X-ray microanalysis to be composed of V (C, N).

Example 11

[0029] A powder mixture consisting of 60% by weight of alumina (AI₂0₃), 38.8% by weight of Fe-V and 1.2% by weight of NH₄CI was placed in a fluidizing furnace, and fluidized by the argon gas introduced into the furnace through the bottom thereof. A bar made of alloy tool steel for hot forming use, AISI H13 (equivalent to JIS SKD61), which had been salt-bath nitrided as described in Example 1, was placed in the furnace, taken out after eight hours, and air hardened. A layer of the carbonitride V (C, N) was formed on the bar, as shown in Figure 11.

Example 12

[0030] A standard thread-cutting tap made of high speed tool steel, AISI M2 (equivalent to JIS SKH9) and having a pitch diameter of 8 mm was dipped in a bath of molten borax containing 30% by weight of V₂0₅ and 15% by weight of B₄C, and treated at 1025°C for an hour, whereby a vanadium carbide layer was formed on the surface of the tap. The tap was then heated at 1190°C for 30 minutes in a vacuum furnace and thereafter gas hardened.

[0031] The tap was subjected to salt bath soft nitriding at 560°C for 20 minutes, and treated in a bath of molten borax as hereinabove described, whereby a vanadium carbonitride layer was formed on the surface of the tap. The tap was then hardened.

[0032] The tap treated in accordance with this invention and a commercially-available nitrided tap were tested for thread cutting in carbon steel material, AISI 1045 (equivalent to JIS S45C), and the life of each such tap was examined. About 1500 holes could be cut by the commercially-available tap, about 2500 holes by the tap on which the carbide layer had been formed, and about 3000 holes by the tap on which the carbonitride layer had been formed. This example also established that a ferrous alloy layer containing nitrogen could be formed immediately below the surface layer in accordance with this invention and enabled the satisfactory hardening and quenching of a ferrous alloy article.

[0033] As is obvious from the foregoing description, this invention makes it possible to form a surface layer composed of the carbonitride of at least one element selected from V, Nb, Ta and an inner layer of a ferrous alloy containing nitrogen in a ferrous alloy article by nitriding it and introducing said element to combine it by thermal diffusion with the nitrogen and carbon in the ferrous alloy. The carbonitride layer defines a surface having good properties, and the inner layer enables the material immediately below the surface layer to be hardened and quenched satisfactorily. The treatment of this invention can be carried out very quickly. This invention is particularly useful if applied to cutting tools, as it can greatly prolong their life.

Claims

1. A method for the surface hardening treatment of a ferrous alloy article, comprising the steps of: nitriding the surface of a ferrous alloy article containing at least 0.2% by weight of carbon, and subjecting the surface of said article to thermal diffusion treatment with 'a treating agent containing at least one element selected from V, Nb, Ta to provide the surface of said article with a surface layer composed of a carbonitride and an inner ferrouos alloy layer containing a solid solution of nitrogen and located immediately below said surface layer.

2. A method as set forth in claim 1, wherein said nitriding is selected from gas nitriding, gas soft nitriding, salt bath soft nitriding and glow discharge nitriding.

3. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out by a process selected from molten salt dipping, molten salt electrolysis, powder packing, slurry treatment and powder fluidization.

4. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out in a molten salt bath containing at least one element selected from V, Nb, Ta, and a borate.

5. A method as set forth in claim 4, wherein said article is dipped in said molten salt bath.

6. A method as set forth in claim 4, wherein said article is employed as a cathode, and subjected to electrolysis in said molten salt bath at a cathode current density of from 0.01 to 3 A/cm².

7. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out by burying said article in powder of said treating agent.

8. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out by coating said article with a slurry of said treating agent.

9. A method as set forth in claim 1 or 2, wherein said thermal diffusion treatment is carried out by fluidizing powder of said treating agent and placing said article in said fluidized powder.

10. A method as set forth in any of claims 1 to 9, wherein said treating agent comprises of at least one compound selected from vanadium oxide, niobium oxide and tantalum oxide.

11. A method as set forth in any of claims 1 to 9, wherein said treating agent comprises at least one alloy selected from the group consisting of ferrovanadium, ferroniobium and ferrotantalum.

12. A method as set forth in any of claims 1 to 11, wherein said thermal diffusion treatment is carried out at a temperature of at least 600°C, but lower than the melting point of said article.

13.-A method as set forth in claim 12, wherein said thermal diffusion treatment is carried out at a temperature of from 800°C to 1200°C.

14. A method as set forth in claim 12, wherein said thermal diffusion treatment is continued for from 30 minutes to 24 hours.

15. A method as set forth in claim 5, wherein said nitriding is carried out in a salt bath.

16. A method as set forth in claim 7, wherein said nitriding is gas soft nitriding.

17. A ferrous alloy article having a surface layer composed of a carbonitride of at least one element selected from V, Nb, Ta and an inner ferrous alloy layer containing a solid solution of nitrogen and located immediately below said surface layer.

Ansprüche

1. Verfahren zum Oberflächenhärten eines Gegenstandes aus einer Eisenlegierung mit den folgenden Schritten: die Oberfläche eines Gegenstandes aus einer Eisenlegierung mit wenigstens 0,2 Gew.-% Kohlenstoff wird nitriert und einer thermischen Diffusionsbehandlung unterworfen mit einem Behandlungsmittel, das wenigstens eines der Elemente Vanadium, Niob, Tantal enthält, um die Oberfläche des Gegenstandes zu versehen mit einer Oberflächenschicht aus Carbonitrid und einer inneren Eisenlegierungsschicht, die eine feste Lösung aus Stickstoff enthält und unmittelbar unterhalb der Oberflächenschicht liegt.

2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Nitrierung als Gasnitrierung, als Gasweichnitrierung, als Salzbadweichnitrierung oder als Glimmentladungsnitrierung durchgeführt wird.

3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung durchgeführt wird als Eintauchbehandlung in eine Salzschmelze, als Elektrolysebehandlung in einer Salzschmelze, durch Einbetten in Pulver, durch eine Behandlung in einer Aufschlämmung oder durch ein Wirbelschichtverfahren.

4. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung durchgeführt wird in einer Salzschmelze, die wenigstens eines der Elemente Vanadium, Niob, Tantal und ein Borat enthält.

5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß der Gegenstand in die Salzschmelze eingetaucht wird.

6. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß der Gegenstand als Kathode verwendet wird und in der Salzschmelze einer Elektrolysebehandlung unterworfen wird, wobei die Kathodenstromdichte im Bereich zwischen 0,01 bis 3 Alcm² liegt.

7. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung durchgeführt wird durch Einbetten des Gegenstandes in das Pulver des Behandlungsmittels.

8. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung durchgeführt wird durch Beschichten des Gegenstandes mit einer Aufschlämmung des Behandlungsmittels.

9. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung durchgeführt wird, indem pulverförmiges Behandlungsmittel verwirbelt und der Gegenstand in das Wirbelbett gebracht wird.

10. Verfahren nach einem der Ansprüche 1 bis 9, dadurch gekennzeichnet, daß das Behandlungsmittel wenigstens eine der Verbindungen Vanadiumoxid, Nioboxid bzw. Tantaloxid enthält.

11. Verfahren nach einem der Ansprüche 1 bis 9, dadurch gekennzeichnet, daß das Behandlungsmittel wenigstens eine Legierung enthält, die aus der Gruppe Ferrovanadium, Ferroniob und Ferrotantal ausgewählt ist.

12. Verfahren nach einem der Ansprüche 1 bis 11, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung bei einer Temperatur von wenigstens 600°C aber unterhalb des Schmelzpunktes des Gegenstandes durchgeführt wird.

13. Verfahren nach Anspruch 12, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung bei einer Temperatur zwischen 800°C und 1200° ausgeführt wird.

14. Verfahren nach Anspruch 12, dadurch gekennzeichnet, daß die thermische Diffusionsbehandlung während einer Zeitdauer von 30 Minuten bis 24 Stunden durchgeführt wird.

15. Verfahren nach Anspruch 5, dadurch gekennzeichnet, daß die Nitrierbehandlung in einem Salzbad durchgeführt wird.

16. Verfahren nach Anspruch 7, dadurch gekennzeichnet daß die Nitrierbehandlung als Gasweichnitrierung durchgeführt wird.

17. Gegenstand aus einer Eisenlegierung mit einer Oberflächenschicht aus einem Carbonitrid wenigstens eines der Elemente Vanadium, Niob, Tantal und einer inneren Eisenlegierungsschicht, die eine feste Lösung aus Stickstoff enthält und unmittelbar unterhalb der Oberflächenschicht liegt.

Revendications

1. Procédé de traitement de durcissement superficiel d'un objet en alliage ferreux, comprenant les stades de: nitruration de la surface d'un objet en alliage ferreux contenant au moins 0,2% en poids de carbone, et de soumission de la surface de l'objet à un traitement de diffusion thermique avec un agent de traitement'contenant au moins un élément choisi parmi V, Nb, Ta pour munir la surface de l'objet d'une couche superficielle composée d'un carbonitrure et d'une couche intérieure d'alliage ferreux, contenant une solution solide d'azote et disposée immédiatement en-dessous de la couche superficielle.

2. Procédé suivant la revendication 1, dans lequel la nitruration est choisie parmi la nitruration par un gaz, la nitruration douce par un gaz, la nitruration douce au bain de sel et la nitruration par décharge luminescente.

3. Procédé suivant la revendication 1 ou 2, dans lequel le traitement de diffusion thermique est effectué par un procédé choisi parmi l'immersion dans un sel fondu, l'électrolyse en sel fondu, l'enrobage dans une poudre, le traitement en suspension et la fluidisation d'une poudre.

4. Procédé suivant la revendication 1 ou 2, dans lequel le traitement de diffusion thermique est effectué dans un bain de sel fondu contenant au moins un élément choisi parmi V, Nb, Ta et un borate.

5. Procédé suivant la revendication 4, dans lequel l'objet est immergé dans le bain de sel fondu.

6. Procédé suivant la revendication 4, dans lequel l'objet est monté en cathode et est soumis une densité du courant cathodique de 0,01 à 3 A/_cm2. cm².

7. Procédé suivant la revendication 1 ou 2, dans lequel le traitement de diffusion thermique est effectué en enterrant l'objet dans de la poudre de l'agent de traitement.

8. Procédé suivant la revendication 1 ou 2, dans lequel le traitement de diffusion thermique est effectué en revêtant l'objet d'une suspension de l'agent de traitement.

9. Procédé suivant la revendication 1 ou 2, dans lequel le traitement de diffusion thermique est effectué en fluidisant de la poudre de l'agent de traitement et en plaçant l'objet dans la poudry fluidisée.

10. Procédé suivant l'une des revendications 1 à 9, dans lequel l'agent de traitement comprend au moins un composé choisi parmi l'oxyde de vanadium, l'oxyde de niobium et l'oxyde de tantale.

11. Procédé suivant l'une des revendications 1 à 9, dans lequel l'agent de traitement comprend au moins un alliage choisi parmi le ferrovanadium, le ferroniobium et le ferrotantale.

12. Procédé suivant l'une des revendications 1 à 11, dans lequel le traitement de diffusion thermique est effectué à une température d'au moins 600°C, mais inférieure au point de fusion de l'objet.

13. Procédé suivant la revendication 12, dans lequel le traitement de diffusion thermique est effectué à une température de 800°C à 1200°C.

14. Procédé suivant la revendication 12, dans lequel le traitement de diffusion thermique est pour-suivi pendant 30 minutes à 24 heures.

15. Procédé suivant la revendication 5, dans lequel la nitruration est effectuée dans un bain de sel.

16. Procédé suivant la revendication 7, dans lequel la nitruration est une nitruration douce par un gaz.

17. Objet en alliage ferreux, ayant une couche superficielle composée d'un carbonitrure d'au moins un élément choisi parmi V, Nb, Ta, et une couche intérieure d'alliage ferreux, contenant une solution solide d'azote et disposée immédiatement en-dessous de la couche superficielle.

Drawing