EP0010484B1 - Improvement in the chromising of steel in the gaseous phase - Google Patents

Abstract

The object of the present invention is an improvement in chromizing methods, constituted by a method of chromizing steels to a depth e greater than 30 microns, usable for steels with a carbon content of at least 0.2%, especially for steels for construction work and steels for tools, characterized by the combination of three successive treatments, the first of these three treatments consisting of an ionic nitriding of a surface layer between 100 and 350 microns thick, this ionic nitriding being realized in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature of between 450 DEG C. and 650 DEG C., for between 5 and 40 hours, so as to obtain between 1.5% and 2.5% nitrogen in the nitrided layer, the second of these treatments consisting of a chromizing by gaseous method forming chromium carbides, lasting between 5 and 30 hours, and realized at temperatures of between 850 DEG C. and 1,100 DEG C., the third of these three treatments being a thermal treatment comprising a quenching in oil of the chromized piece followed by a tempering at a temperature of between 600 DEG C. and 650 DEG C., lasting between 30 minutes and 10 hours, depending on the size of the piece treated.

Description

The present invention relates to an improvement in chromium-plating by gas of steels with more than 0.2% carbon, more particularly structural steels and tool steels; this improvement makes it possible to significantly increase the thickness of the chromized layer, as well as its toughness.

The chromization of steels by gas is well known. The formation of chromium-based diffusion alloys on the surface of steels has already been described in numerous invention patents. The transport of chromium to the surface of the material to be treated is carried out by means of halides, which are the only chromium compounds in the vapor state at diffusion temperatures. The passage of chromium in solid solution in the metal is made by exchange between the chromium halide and the iron according to a reaction which, in the case of chlorides, can be written:

To obtain sufficient diffusion, the reaction must be carried out at high temperature and in the austenitic range, that is to say above 850 ° C. for the usual steels. In steels whose carbon content is greater than 0.2%, the surface reaction of carbon and chromium causes on the one hand the formation of a film of chromium carbides, on the other hand a diffusion of carbon towards the area. The surface film consists of two types of carbides, M ₂₃ C ₆ , richer in chromium towards the surface and M ₇ C ₃ , poorer in chromium, towards the metal substrate.

In what precedes and in what follows, M indicates a metal such as der (Fe), chromium (Cr), nickel (Ni), etc ...

The surface film has a thickness between 12 and 18 micrometers, and a hardness level between 1200 and 1800 in the Vickers hardness scale. Chromium thus diffuses into steel over a depth generally close to 15 micrometers. In known methods, this depth of chromization never exceeds 20 micrometers.

• 1 ) la formation du deuxième type de carbures M₂₃C₆.
• 2) l'obtention de couches superficielles de carbures relativement minces.

The affinity of chromium for carbon is such that it forms very quickly, during the rise in processing temperature, a film of carbides of type M, C ₃ on the surface of the parts. This film hinders the penetration of chromium inside the steel by diffusion; the result is:

• 1) the formation of the second type of carbides M ₂₃ C ₆ .
• 2) obtaining relatively thin surface layers of carbides.

These thin, two-phase layers have the drawback of being relatively fragile, due to the state of the stresses which are found in the curving phases after heat treatment. The carbide M, C ₃ , of columnar structure, is in particular in a state of extension stresses, which causes the formation of cracks which are often at the origin of the spallings observed.

On the other hand, successive treatments of gas nitriding (at temperatures above 830 ° C) and chromization by gas diffusion, as described for example in French patent no. 1,410,647, made it possible to increase the hardness of the chromized layers, but the thicknesses of the coatings produced, constituted by nitrides and by chromium carbides, therefore two-phase, were as low as those of the previously described layers, obtained by non-chromized preceded by gas nitriding.

• 1 ) La diffusion du carbone vers la surface est ralentie; en effet, le coefficient de diffusion du carbone en volume dans la phase austénitique du fer est de l'ordre de 10-⁸ cm²/sec à 900°C, alors qu'il est voisin de 2.10^-6 cm²/sec à la même température dans la phase ferritique. Il en résulte donc une réduction de la vitesse de formation des carbures en surface et une accentuation de la diffusion du chrome en profondeur.
• 2) Le carbure M₂₃C₈, de structure cubique à faces centrées a une maille a de 10,6 A°, pratiquement trois fois plus grande que celle de l'austénite (a=3,6 A°), Ce carbure précipite donc beaucoup plus facilement dans la structure austénitique que le carbure M₇C₃ de structure hexagonale.

The main object of the present invention is to find a means for obtaining on the surface only one type of carbides, and this in a layer of greater thickness. A rapid passage of the part to be treated in the austenitic phase could constitute a solution for two reasons:

• 1) The diffusion of carbon towards the surface is slowed down; Indeed, the volume of carbon diffusion coefficient in the austenitic phase of the iron is of the order of 10- ⁸ cm ² / sec at 900 ° C, while it is around 2.10 ^-6 cm ² / sec to the same temperature in the ferritic phase. This therefore results in a reduction in the rate of formation of carbides at the surface and an accentuation of the diffusion of chromium at depth.
• 2) The carbide M ₂₃ C ₈ , of cubic structure with centered faces has a mesh a of 10.6 A °, practically three times larger than that of austenite (a = 3.6 A °), This carbide precipitates therefore much more easily in the austenitic structure than the carbide M ₇ C ₃ of hexagonal structure.

Therefore, a first solution for increasing the thickness and the toughness of the carbide layers can be constituted by an initial treatment phase consisting of a rapid rise in temperature, in particular in the range 600-900 ° C where usually precipitate carbides M _I C ₃ in the ferritic structure. However, such a solution would present risks with regard to the initiation of cracking and spalling during heating, in particular for parts of relatively alloyed steels having geometries which are ill-suited to the high stresses of thermal origin which would result from such a cycle of heater.

The present invention, while applying the principle of chromizing the part in the austenitic phase, avoids the risks mentioned above by first producing a nitrided layer, but without a combination layer, that is to say in such a way that it does not in any way formation of a surface layer of iron and chromium nitrides. Indeed, a surface layer of iron nitrides would remain relatively stable, even at high temperatures, and would constitute a real barrier opposing the diffusion of chromium, this barrier being reinforced by the formation of chromium nitrides linked to an additional contribution. nitrogen.

Thus, the absence of a combination layer on the surface of the parts to be subsequently treated by chromization is a necessary condition for good diffusion of the chromium in depth, and this condition is achieved in the present invention.

To this end, the subject of the present invention is a process for chromizing steels over a depth e greater than 40 micrometers, usable for steels having a carbon content at least equal to 0.2%, in particular for structural steels and for tool steels, consisting of three successive treatments, namely: a nitriding treatment, a gas chromizing treatment and a thermal treatment, and characterized in that the nitriding treatment consists of ionic nitriding of a surface layer with a thickness between 100 and 350 micrometers, produced in an atmosphere consisting of a mixture of nitrogen and hydrogen, at a temperature between 450 ° C and 650 ° C, for a period between 5 and 40 hours , so as to obtain between 1.5% and 2.5% of nitrogen in the nitrided layer, in that the chromization by gas-forming form of chromium carbides, lasting between 5 and 30 hours, is carried out at temperatures between 850 ° C and 1,100 ° C, and in that the heat treatment comprises an oil quenching of the chromized part followed by tempering at a temperature between 600 ° C and 650 ° C, a duration of between 30 minutes and 10 hours depending on the size of the part treated.

According to a particular characteristic of the present invention, the ionic nitriding forming the first of the three treatments and carried out under a nitrogen and hydrogen atmosphere is carried out under a partial nitrogen pressure at most equal to 150 Pascals, and under a total gas pressure. between 200 and 1000 Pascals.

According to another particular characteristic of the present invention, the chromization forming the second of the three treatments, carried out by the known technique of cements under a reducing atmosphere based on hydrogen, uses a pulverulent mixture based on ferro-chromium and on chloride of ammonium, the latter representing by weight only 0.4% to 1% of the pulverulent mixture, the ferro-chromium powder preferably having a chromium content of between 50% and 75% and a particle size of between 0.5 millimeter and 4 millimeters, without aluminous or magnesium binder.

In all of the above and in all that follows, the term "ionic nitriding" should be understood to mean a thermochemical treatment of a metal surface by ion bombardment with rarefied gas, performing surface nitriding of the metal part placed in a cathode under an atmosphere. nitrogen and hydrogen at a temperature between 450 ° C and 650 ° C.

The main interest of ionic nitriding is that it makes possible the exploitation of all the possibilities offered by the balance diagrams between the elements constituting the treated steel and I-nitrogen. Indeed, the thermochemical treatments of metal surfaces by ion bombardments, and more particularly nitriding, are based on the properties of the electric discharge in rarefied gases, in this case mixtures of nitrogen and hydrogen with possibly hydrocarbons. The reactive gas atmosphere can be chosen independently of the need for its pyrolitic cracking since its activation is obtained by ionization. It is therefore possible to adjust the partial nitrogen pressure so that the phase or phases provided for in the binary iron-nitrogen balance diagram are superficially formed. This is thus for the lowest nitrogen pressures, only a diffusion layer is formed, a solid solution of nitrogen in the iron a at temperatures generally between 450 ° and 570 ° C. In this same temperature range, an increase in the partial pressure of nitrogen leads first of all to the formation of a combination layer of nitrides y '(Fe ₄ N), then nitrides y' and e (Fe ₂ , ₃ N).

It is possible to obtain in the das steels with more than 0.2% carbon, and more especially construction steels and tool steels, nitrogen diffusion layers of thicknesses between 100 and 350 micrometers , this ionic nitriding being carried out in an atmosphere constituted by a mixture of nitrogen and hydrogen, at a temperature of between 450 and 570 ° C., for a period of between 5 and 40 hours so as to obtain, for example, at depths from 50 to 200 micrometers from the surface of the nitrogen contents in solid solution of between 1.5 and 2.5%.

As will be understood, one of the main advantages of the invention consists, thanks to ionic nitriding, in obtaining a nitrided layer without combination layer, that is to say without iron and chromium nitrides, in a way reliable and repetitive by adjusting the partial pressure of nitrogen as a function of the treatment temperature and the chemical composition of the steel. So, without the risk of cracking, the steel on the surface can quickly pass into the austenitic phase at moderate temperature, because of a nitrogen content of the order of 1.5% to 2.5%.

After which gas chromization can be carried out at greater depth, for example up to 50 micrometers and even more, and with the formation on the surface of a single type of carbonitrides, in Cr ₂ (C, N), this which results in a significant increase in the toughness of the coating.

In order to clearly understand the invention, an embodiment of the improvement according to the invention will be described below, by way of non-limiting example, in which a chromium-molybdenum-vanadium steel of the type is treated 35 CDV 12, therefore at 0.35% carbon, in order to obtain a chromization depth of 50 micrometers.

The ionic nitriding which constitutes the first of the three successive treatments according to the invention is carried out here in a metal enclosure provided with heat shields and cooled by circulation of water, which enclosure constitutes the anode connected to the ground. The electrical parameters are chosen in such a way that the current increases with the direct voltage produced by the generator and that the sample to be nitrided which constitutes the cathode is covered by the corona corresponding to the abnormal discharge regime. Near the cathode surface, the gaseous ions are formed and accelerated towards the sample and cause it to heat up, which is continued until the temperature chosen to carry out the thermochemical treatment. The temperature regulation is obtained using a thermocouple protected by an alumina sheath and placed in the sample under conditions which prevent arcing.

The pressure at which the thermochemical treatment is carried out is generally between 250 and 800 Pascals; a primary pump is sufficient to create the initial vacuum, then to allow the renewal of the nitriding gas near the sample. The nitriding gas mixture is composed of nitrogen and hydrogen. The partial pressures of nitrogen P _N for which oh obtains a solid solution of nitrogen in the ferrite network are between 10 and 50 Pascals. The temperature is set on average at 520 ° C and does not deviate from the range between 510 and 530 ° C. Leaving aside the temperature rise and the low pressure setting of the atmosphere, the duration of ion nitriding at good pressure and at the right temperature is 25 hours. By this first treatment, the average nitrogen content of the steel between 50 and 200 micrometers deep reaches 2.196, and the nitrided layer does not contain iron nitrides, nor chromium nitrides.

The metal piece of CDV 12 steel 35 thus nitrided is then extracted from the ion nitriding furnace and introduced into a case-hardening box which will carry out the second treatment according to the invention, which is gas chromization.

The cementing agent used is a powder consisting for 99.5% of ferro-chromium with 60/70% of chromium and for 0.5% of ammonium chloride, without alumina or magnesia. This powder has a particle size between 0.5 and 4 mm, with an average size close to 2.7 mm. This powder is placed at the bottom of the case-hardening box, which has the shape of a vertical cylinder, and it is covered by a partition on which the piece of steel to be chromized is placed. At the upper part of the case-hardening box there is a ferro-chromium reserve in a basket used for the direct regeneration of the active vapor of chromium chloride CrCI ₂ . Introduced hydrogen creates a reducing atmosphere.

The enclosure is brought to an average temperature of 950 ° C, not deviating from the range 920 ° C-980 ° C, for a period of 20 hours.

• Au chauffage, le chlorure d'ammonium se dissocie. L'ion chlore ainsi libéré agit sur le chrome du ferro-chrome pour former du chlorure de chrome CrCI₂ à l'état de vapeur, qui produit la chromisation superficielle selon la réaction (1) mentionnée ci-dessus.

The following phenomena occur in the cementation box:

• When heated, the ammonium chloride dissociates. The chlorine ion thus released acts on the chromium of ferro-chromium to form chromium chloride CrCI ₂ in the vapor state, which produces the surface chromization according to the reaction (1) mentioned above.

The ferrous chloride vapors from reaction (1) react on the chromium reserve placed at the top of the body, which regenerates gaseous chromium chloride CrCI ₂ which participates in the chromization according to (1).

After 20 hours of holding at 920 ° C-980 ° C, the chromized part undergoes the third treatment according to the invention, that is to say that it is extracted from the case hardening box, it is immediately soaked in oil , then it is introduced into a tempering oven maintained at a temperature of the order of 625 ° C., for 2 hours.

• - que la couche superficielle contenant les carbonitrures de chrome a une épaisseur voisine de 50 micromètres,
• -que les carbonitrures de chrome de cette couche superficielle sont à peu près exclusivement du type Cr₂(C, N),
• - que la dureté de cette couche est comprise entre 1800 et 2000 dans l'échelle de Vickers,
• -qu'elle se fissue sous une charge de 1 kilogramme-force.

After income, we observe:

• - that the surface layer containing the chromium carbonitrides has a thickness close to 50 micrometers,
• that the chromium carbonitrides of this surface layer are almost exclusively of the Cr ₂ (C, N) type,
• - that the hardness of this layer is between 1800 and 2000 in the Vickers scale,
• -that it is released under a load of 1 kilogram-force.

The chromized coating thus obtained in the present example according to the invention is to be compared with that of a chromization of known type, not preceded by ionic nitriding.

• la couche superficielle contenant les carbures de chrome a une épaisseur voisine de 15 micromètres,
• - on y observe deux phases de carbures de chrome, l'une en M₂₃C₆, surtout en surface, l'autre en M,C₃, vers le substrat métallique,
• - la dureté de la couche superficielle est comprise entre 1200 et 1800 dans l'échelle de Vickers, avec des hétérogénéités liées à des porosités superficielles,
• - la charge à partir de laquelle apparaissent les fissures aux angles des empreintes Vickers est de 300 grammes-force.

In this case of known type:

• the surface layer containing the chromium carbides has a thickness close to 15 micrometers,
• - two phases of carbides of chromium, one in M ₂₃ C ₆ , especially at the surface, the other in M, C ₃ , towards the metallic substrate,
• the hardness of the surface layer is between 1200 and 1800 in the Vickers scale, with heterogeneities linked to surface porosities,
• - the load from which the cracks appear at the corners of the Vickers footprints is 300 grams-force.

It is understood that it is possible, without departing from the scope of the invention, to imagine variants and refinements of details, as well as to envisage the use of equivalent means.

Claims (3)

1. Process for the chromising of steels to a depth e of more than 40 micrometres, which can be used for steels having a carbon content equal to at least 0.2%, consisting of three successive treatments, namely: a nitrogen hardening treatment, a chromising treatment by a gaseous method and a heat treatment, and characterised in that the nitrogen hardening treatment consists of ionic nitrogen hardening of a surface layer with a thickness of between 100 and 350 micrometres, carried cut in an atmosphere consisting of a mixture of nitrogen and hydrogen, at a temperature between 450°C and 650°C, for a period of between 5 and 40 hours, so as to obtain between 1.5% and 2.5% of nitrogen in the layer subjected to nitrogen hardening, in that the chromising by a gaseous method, forming chromium carbides and lasting between 5 and 30 hours, is carried out at temperatures between 850°C and 1,100°C, and in that the heat treatment comprises oil- quenching of the article subjected to chromising, followed by tempering at a temperature between 600°C and 650°C, lasting between 30 minutes and 10 hours, depending on the size of the article treated.

2. Process for the chromising of steels, according to Claim 1, characterised in that the ionic nitrogen hardening, forming the first of the three treatments and carried out under an atmosphere of nitrogen and hydrogen, is carried out under a partial nitrogen pressure equal to at most 150 Pascals and under a total gas pressure of between 200 and 1,000 Pascals.

3. Process for the chromising of steels, according to either one of Claims 1 and 2, in which the chromising, forming the second of the three treatments, is carried out by the technique known for cementation agents, under a hydrogen-based reducing atmosphere, and uses as the cementation agent a pulverulent mixture based on ferro-chromium and ammonium chloride, characterised in that the pulverulent mixture contains between 0.4% and 1% of ammonium chloride and in that the ferro-chromium powder has a chromium content of between 50% and 75% and a particle size of between 0.5 millimetre and 4 millimetres, with no alumina binder and no magnesia binder.