EP0018263A1 - Process for chromizing steel articles, and chromized steel articles - Google Patents

Abstract

The invention relates to a method for chromizing metallic pieces such as steel pieces and to chromized metallic pieces obtained thereby, said metallic pieces, particularly steel, comprising a chromized surface layer with a hardness at least equal to 1200 Vickers degrees and a thickness at least equal to 12 microns as well as an adjacent sub-layer with a hardness less than that of the core of said piece. The chromized surface layer has a depth preferably at least equal to 20 microns, more preferably 30 microns, and the hardness of the sublayer presents a maximum variation of 25 Vickers degrees with respect to that of the core of said piece.

Description

The present invention relates to a method for chromizing metal parts such as steel parts, of the type in which chromium is diffused in order to obtain a surface layer of chromium and in which the chromium supply is obtained by gas phase decomposition of chromium halides, the decomposition of the halide and the diffusion of chromium in the steel being carried out simultaneously by heating the parts to temperatures not exceeding 1300 ° C., in the halide atmosphere, the actual chromizing operation being preceded by a surface nitrogen addition operation.

The actual chromization is known, for example, from the works of Doctor GALMICHE and from the ONERA patents n ° 1.012.401 and their two additions 60.539 and 60.686.

The application of these known treatment methods to the surface of mild steel leads to the production of surface layers with a high chromium content which have the advantage of being stainless but do not exhibit high hardnesses. This is "soft" chromization which finds its applications in the field of oxidisability.

Thanks to this type of chromization process, it is easy to obtain increasingly deep or thick layers by simply increasing the duration of the treatment cycle and therefore the diffusion time of the chromium.

The application of these same known treatments to the surface of steels containing carbon con also results in the production of surface layers with a high chromium content. But this time, in the surface layer, chromium also combines with carbon to give chromium carbides which give the surface layer great hardness in addition to its stainlessness. This is hard chromium plating which finds its applications in the field of steels which must have a resistance to wear possibly combined with the oxidisability of said steels.

• a) l'affinité chrome-carbone étant très grande, il se forme dès les premières heures du cycle de traitement, une couche de carbure de chrome stable qui forme rapidement une sorte d'écran gênant fortement ou empêchant l'absorption du chrome par l'acier et par conséquent l'augmentation du temps de traitement n'accroît pratiquement plus la profondeur de diffusion du chrome, c'est-à-dire l'épaisseur de la couche chromisée.
• b) la vitesse de diffusion du carbone dans l'acier étant plus grande que celle du chrome, on constate au contraire que l'augmentation du temps de traitement provoque un "pompage" du carbone de l'acier de base par le chrome superficiel, aboutissant ainsi à la formation d'une couche d'acier décarburée immédiatement sous la couche chromisée. La présence de cette sous-couche décarburée est souvent une gêne pour la tenue mécanique des couches superficielles fortement sollicitées, car la résistance mécanique de l'acier décarburé est très amoindrie.
• c) la profondeur de diffusion pourrait être accrue par l'augmentation de la température de chromisation, si l'on n'était alors pas limité par le problème du grossissement du grain de l'acier sous jacent, qui a pour inconvénient de fragiliser la sous-couche ce qui peut être néfaste dans certaines applications dans lesquelles les pièces chromisées sont soumises à de fortes sollicitations mécaniques.

But in this type of hard chromization if one seeks to obtain as previously increasingly deeper layers by simply increasing the duration of the treatment cycle one meets the following pitfalls:

• a) since the chromium-carbon affinity is very high, a stable layer of chromium carbide is formed from the first hours of the treatment cycle, which quickly forms a sort of screen which greatly hinders or prevents the absorption of chromium by steel and therefore increasing the processing time practically no longer increases the depth of diffusion of chromium, that is to say the thickness of the chromized layer.
• b) the rate of diffusion of the carbon in the steel being greater than that of the chromium, it is noted on the contrary that the increase in the treatment time causes a "pumping" of the carbon of the base steel by the surface chromium, thus resulting in the formation of a layer of decarburized steel immediately below the chromized layer. The presence of this decarburized sub-layer is often a hindrance for the mechanical strength of the highly stressed surface layers, since the mechanical resistance of the decarburized steel is very much reduced.
• c) the diffusion depth could be increased by increasing the chromization temperature, if one were not then limited by the problem of the magnification of the grain of the underlying steel, which has the disadvantage of weakening the undercoat which can be harmful in certain applications in which the chromized parts are subjected to strong mechanical stresses.

In the current state of the art and in the field of hard chromization, it is therefore very difficult to exceed depths of about fifteen microns, for the surface layer actually hardened beyond a hardness of the order 1200, preferably 1400 Vickers points. This minimum hardness limit also depends on the effective chemical composition of the steel to be treated.

It has already been proposed to provide a surface nitrogen supply before the chromizing operation (see for example French patent 1,410,647 or the corresponding US patent 3,282,746). The conditions of this preliminary treatment (temperature above the critical temperature and duration less than two hours - presence of carbon in the treatment gases) do not make it possible to obtain a hard chromized surface layer with a thickness greater than 20 microns. In addition, according to the known method, the nitrides present in the surface layer remain isolated in the form of an intermediate strip.

The object of the present invention is to propose a hard chromization process of the aforementioned type and making it possible to produce steel parts having a hard chromized surface layer comprising carbo-nitrides, a layer having a thickness greater than at least 20 microns, preferably 30 microns, and a fine grain undercoat with little or no decar burée.

This object is achieved by the fact that prior to the actual chromizing operation, the introduction of nitrogen into the surface layer of the metal part is made so that the nitrogenous surface layer has a content greater than 0.8 % nitrogen over a depth or thickness at least equal to 0.5 mm.

Thanks to the invention, there are then obtained after the actual chromization operation hard chromized layers comprising carbo-nitrides instead of separate carbides and / or separate nitrides, these layers being of a thickness greater than at least 20 microns and having a high resistance to wear.

• - d'une part, sur l'affinité chrome-carbone et par conséquent sur la vitesse de formation du carbure de chrome conduisant même, pour une teneur suffisante en azote, à la formation de carbonitrures de chrome au lieu de carbures de chrome et,
• - d'autre part, sur les vitesses respectives de diffusion du carbone et du chrome dans l'acier et,
• - par ailleurs, sur la cinétique de grossissement du grain de l'acier.

The Applicant's experience has led to highlight the beneficial influence of nitrogen

• - on the one hand, on the chromium-carbon affinity and therefore on the rate of formation of chromium carbide leading even, for a sufficient nitrogen content, to the formation of chromium carbonitrides instead of chromium carbides and,
• - on the other hand, on the respective diffusion rates of carbon and chromium in the steel and,
• - moreover, on the magnification kinetics of the steel grain.

As a result, the presence of nitrogen in the surface layers of the steel parts makes it possible to achieve a different balance of the chromium and carbon elements with respect to the base steel during the chromization reaction, this different balance thus allowing to obtain much deeper chromized surface layers without producing significantly decarburized undercoats and without enlarging the grain in undercoat.

The introduction of nitrogen into the surface layers of the pieces of steel to be chromized leads to a sort of activation of said layers.

The surface introduction of nitrogen can be carried out by heating the workpiece to a temperature between 400 and 800 ^° C. and in any case at a temperature below the lower critical point of the treated steel, and for the periods between 12 and 150 hours in a nitrogen generating medium which, by way of example may consist of molten nitrous salts, ammonia or nitrogen gases, ionized or not.

This technique is quite analogous to conventional nitriding techniques but, however, the optimum temperatures and heating times must be chosen according to the steel grades considered and especially according to the expected later effect during the final chromization phase.

Indeed, surface introductions that are too poor in nitrogen would not have sufficient action either on the depth of the chromized layer, or on the formation of chromium carbonitrides, and too superficial nitrogen introductions could effectively act on the depth. of the chromized layer proper but would not have the expected action on the decarburization and the grain size of the undercoat.

It is therefore important to prepare nitrogenous surface layers previously containing more than 0.8% nitrogen over a depth at least equal to 0.5 mm. Preferably, nitrogenous surface layers are prepared having a nitrogen content of between 1 and 2% over a depth of 0.5 to 1.0 mm. On a steel of type 32 CDV 13 such satisfactory nitrogenous layers can be obtained by treatment cycles in an ammonia atmosphere of 24 hours at 700 ° C or 90 hours at 560 ° C.

• - soit par une opération conduite dans une installation séparée et préalablement à l'opération de chromisation proprement dite, avec un refroidissement de la pièce à traiter entre l'introduction superficielle d'azote et ladite opération de chromisation,
• - soit dans l'installation de chromisation proprement dite, lors d'une phase initiale destinée seulement à l'introduction d'azote et suivie du cycle de chromisation proprement dit.

This introduction of nitrogen can be carried out:

• either by an operation carried out in a separate installation and prior to the actual chromization operation, with cooling of the part to be treated between the surface introduction of nitrogen and said chromization operation,
• - or in the actual chromization installation, during an initial phase intended only for the introduction of nitrogen and followed by the actual chromization cycle.

During the final chromization phase, the chromium halide atmosphere conforms to the already known technique, the temperature and holding time parameters can be conformed to the already known technique, but they can also be modified according to the prior introduction of nitrogen; compared to what they would be on the same non "activated" steel according to the present invention. In particular, the chromization temperatures can be reduced in order to limit the magnification of the grains of the undercoat.

• 1) Dans la sous-couche proche, et de façon plus précise sous une profondeur de l'ordre du millimètre, la présence effective d'azote a pour effet direct l'obtention d'un grain beaucoup plus fin pour une température égale de chromisation.
• 2) Dans la sous-couche profonde et de façon plus précise dans tout le coeur des pièces d'acier il n'y a pas présence d'azote et le grossissement des grains pourrait s'effectuer comme dans une opération de chromisation connue. Cependant, dans la mesure où l'invention permet de pratiquer la chromisation à température plus basse, cela a pour effet indirect de provoquer un grossissement de grain plus faible.

It is indeed necessary to distinguish two different mechanisms both beneficial to the fineness or smallness of the grains:

• 1) In the near sub-layer, and more precisely under a depth of the order of a millimeter, the effective presence of nitrogen has the direct effect of obtaining a much finer grain for an equal chromization temperature. .
• 2) In the deep sub-layer and more precisely throughout the core of the steel parts there is no presence of nitrogen and the magnification of the grains could be carried out as in a known chromization operation. However, insofar as the invention makes it possible to practice chromization at a lower temperature, this has the indirect effect of causing a lower magnification of the grain.

After chromization, the steel parts can preferably undergo a regenerative heat treatment in order to reduce the grain sizes and improve the resilience of the base steel.

By way of example, the comparison drawing shows the comparison of the chromized layers obtained on the same steel grade 32 CDV 13 (AFNOR) by known chromization techniques without or with prior activation by nitrogen and in the three cases followed by a heat treatment of grain regeneration by heating under vacuum, quenching and tempering.

In the appended drawing, FIG. 1 is a micro-hardness diagram showing for the steel grade 32 CDV 13 (AFNOR) three examples of chromized steel parts and, FIGS. 2 to 4 show micro-photos of the surface layer and of the adjacent underlay of these three examples of chromized steel parts, the enlargement factor being in all three cases of 200.

In FIG. 1, the hardness of different levels of the steel parts is indicated on the ordinate, hardness measured in Vickers points and on the abscissa the thickness or depth of the layers in microns from the surface of the part of steel considered.

In this FIG. 1, the curve a in solid lines represents the hardness of a piece of steel in Vickers degrees as a function of the depth of the level considered. This piece of steel, the transverse structure of which is illustrated in FIG. 2, has not been previously activated by nitrogen, but only one chromium-plating operation for 12 hours at 940 ° C.

It can be seen that the chromized surface layer 1 has a depth or thickness of 12 microns and has a Vickers hardness greater than 1,500 points and reaching more than 1,800 points. As soon as one leaves the chromized surface layer 1 towards the core of the steel part, one enters a sub-layer 2 whose Vickers hardness decreases rapidly to a minimum value of 260 and then slowly rises to Vickers hardness at the heart of the hardness steel part which is of the order of 350 and is reached at a depth of 77 microns. The thickness of this sublayer or decarburization zone 2 is of the order of 65 microns which is more than 5 times the thickness of the chromized surface layer 1 and the minimum hardness of the sublayer 2 is only about 3/4 of the Vickers hardness at the heart of the steel part. The decrease in the hardness of the underlay compared to that of the core of the steel part clearly shows the extent of the decarburized zone and the harmful influence of the inevitable carbon pumping during the known chromization operation. It can also be seen from FIG. 2 that the size of the grain in the sub-layer is significant (Afnor index 4).

Curve b of FIG. 1 shows the hardness of a piece of steel of the same aforementioned grade without activation with nitrogen, subjected to a chromization operation also for 12 hours, but at a higher temperature, namely 980 ° C. This increase in temperature brings the depth of the chromized surface layer 1 to 16 microns, but on the other hand the corresponding structure illustrated in FIG. 3 shows an even larger and unfavorable grain size (Afnor index 3) than that of the structure. shown in Figure 2 and the diagram of microhardness shows at the bottom of curve b between a depth of 16 microns and 85 microns (sublayer 2) an even deeper and more intense decarburization zone (minimum Vickers hardness 240 points) than that of the example in the curve a and of FIG. 2. Here the Vickers hardness of the undercoat 2 is of the order of 2/3 of the Vickers hardness at the heart of the steel part and the undercoat 2 with its decarburized zone a a thickness at least four times greater than that of the chromized surface layer 1.

Curve c of FIG. 1 corresponds to a piece of steel having undergone prior activation by the surface introduction of nitrogen in accordance with the present invention (treatment cycle of 90 hours at 560 ° C.). This curve c in FIG. 1 shows the Vickers hardness of a piece of steel of the same grade as the previous examples, but is obtained after the same chromization cycle as the example of curve a, that is 12 hours at 940 ° C. As can be seen in Figures 1 and 4, the chromized layer 1 has a depth of 50 microns. The structure; illustrated by FIG. 4 also shows the fineness of the grain obtained in the sub-layer 2 over a depth of 1 mm and the diagram of microduretures (FIG. 1) shows for curve c a much less thick decarburization zone (20 to 30 microns ) and less intense (minimum Vickers hardness around 330 points) than in the two previous examples.

It is noted that, in the case of the example according to the invention, the thickness of the decarburized zone is very small, is less than that of the chromized layer 1 and is only approximately equal to half that of said chromized layer 1. As for the hardness of the decarburized zone of the underlayer 2, it is at least equal to about 95% of the hardness of the core of the steel piece. In other words, the loss of hardness in the underlayment compared to the base steel at The core of the steel part is inside 25 Vickers points and affects a weakly decarburized area with a thickness of less than 25 microns. Here we have a very good grain fineness in the sublayer 2, grain fineness which is at least equal to or greater than the Afnor6 index (index 7) over a thickness at least equal to 0.5 mm (1 mm) .

As previously mentioned, it is important that carbon-containing steel intended for hard chromium plating has a surface layer containing at least 0.8% nitrogen over a depth greater than 0.5 mm. It has been found that to the extent that the nitrogen content of the surface layer before chromization exceeds 0.8% over a depth of 0.5 mm, there is found after said chromization in the sublayer a higher residual nitrogen content at 0.4% over a depth greater than 0.5 mm, this nitrogen content preventing significant decarburization and a significant enlargement of the grain of the undercoat.

In addition, when the nitrogen content of the surface layer of the steel is, before chromization, greater than 0.8% over a depth greater than 0.5 mm, after said chromization is found in the chromized surface layer the presence of chromium carbonitrides with a nitrogen content greater than 2%.

We also find in all cases an undercoat which over a thickness at least equal to 1 mm has a fine grain of Afnor index equal to or greater than 7.

The invention relates not only to the new hard chromization process, but also to the pieces of steel treated with nitrogen with a view to chromization as well as to the pieces of steel chromized according to the present process and having a chromized surface layer. hard with a depth of at least 20 microns and a Vickers hardness of at least 1200 points.

Claims (12)

1. A method for chromizing metal parts such as steel parts, of the type in which chromium is made to diffuse on the surface of metal parts in order to obtain a surface layer of chromium and in which the addition of chromium is obtained by the gas phase decomposition of chromium halides, the decomposition of the halide and the diffusion of chromium in the steel being carried out simultaneously by heating said parts to temperatures not exceeding 1300 ° C. in the atmosphere of halide, this chromisation operation proper being preceded by a surface nitrogen supply operation, characterized in that the introduction of nitrogen into the surface layer of the metal part is made so that the surface layer nitrogen has a content greater than 0.8% nitrogen over a depth or thickness at least equal to 0.5 mm. 2. Method according to claim 1, characterized in that the surface introduction of nitrogen is carried out by heating the metal part to be treated at a temperature between 400 ° C and 800 ° C and in any case at a lower temperature at the lower critical point of the treated metal part, in a nitrogen generating medium. 3. Method according to one of claims 1 and2, characterized in that the duration of the nitrogen introduction cycle is between 12 and 150 hours. 4. Method according to one of claims 1 to 3, characterized in that the surface introduction of nitrogen into the metal part is immediately followed by the actual chromization operation. 5. Method according to one of claims 1 to 3, characterized in that after the surface nitrogen introduction cycle is allowed to cool the nitrogenous metal piece before subjecting it to the actual chromization operation. 6. Metal part, in particular steel, comprising a chromized surface layer with a hardness at least equal to 1200 Vickers points and a depth at least equal to 12 microns as well as an adjacent undercoat of lower hardness to that of the core of said part, characterized in that the chromized surface layer has carbonitrides and has a depth at least equal to 20 microns, preferably 30 microns, and that the hardness of the underlay has a maximum deviation of 25 Vickers points relative to that of the heart of said piece. 7. Metal part according to claim 6, characterized in that the presence of chromium carbonitrides in the chromized surface layer corresponds to a minimum nitrogen content at least equal to 2%. 8. Metal part according to claim 6, and having a sub-layer having a decarburized zone, characterized in that the thickness of the decarburized zone is less than the depth of the chromized surface layer. 9. Metal part according to claim 8, characterized in that the decarburized zone has a depth or thickness less than 30 microns. 10. Metal part according to one of claims 6 to 9, characterized in that the sub-layer has fine grain of Afnor index equal to or greater than 6 over a thickness or depth at least equal to 0.5 mm. 11. Metal part according to one of claims 6 to 10, characterized in that the residual nitrogen content.in the sublayer is greater than 0.4%. 12. Metal part according to claim 11, characterized in that the sub-layer has residual nitrogen over a thickness or depth greater than 0.5 mm.

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