EP0018263B1 - 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 steel parts, of the type according to which chromium is diffused in order to obtain a surface layer of chromium and according to which the chromium contribution is obtained by decomposition in the gas phase d 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 operation of actual chromization being preceded by a surface nitrogen addition operation.

The actual chromization is known, for example, from the work of Doctor GAL-MICHE and from the French patents ONERA n ° 1 012 401 and their two additions 60 539 and 60686.

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 also leads to 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, the following pitfalls are encountered:

• 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, we note on the contrary that the increase in the treatment time causes a "pumping of the carbon from the base steel by the surface chromium, resulting thus 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, because 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 around fifteen Jlm, for the surface layer actually hardened beyond a hardness of the order 1,200, preferably 1,400 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 wm. 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 μm, preferably 30 µm, and a fine grain undercoat with little or no decarburization.

This object is achieved by the fact that prior to the actual chromizing operation, the introduction of nitrogen into the surface layer of the steel part is made so that the nitrogen surface layer has a content greater than 0 , 8% nitrogen over a depth or thickness at least equal to 0.5 mm.

The surface introduction of nitrogen is carried out by heating the part to be treated at a temperature between 400 and 800 ° C and in any case at a temperature below the lower critical point of the treated steel, and during the times 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.

Thanks to the invention, one then obtains after the actual chromizing operation of the hard chromized layers comprising carbonitrides instead of separate carbides and / or separate nitrides, these layers being of a thickness greater than at least 20 μm 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 as an 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.

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 steel not "activated in accordance with 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 at 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 nitrogen present and the grain enlargement 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 may 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 microphotos of the surface layer and of the adjacent undercoat of these three examples of chrome-plated steel parts, the enlargement factor in all three cases being 200.

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

In this figure 1, the curve has a solid line 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 subjected to prior activation by nitrogen, but only a chromium-plating operation of 12 hours at 940 ° C.

It can be seen that the chromized surface layer 1 has a depth or thickness of 12 μm 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 steel part, hardness which is of the order of 350 and is reached at a depth of 77 µm. The thickness of this sublayer or decarburization zone 2 is of the order of 65 μm 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 is also noted 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 μm, 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 FIG. 2 and the microdurethane diagram shows at the bottom of curve b between a depth of 16 μm and 85 μm (sublayer 2) an even deeper and more intense decarburization zone (minimum Vickers hardness 240 points) than that of the example of curve a and of FIG. 2. Here the Vickers hardness of the underlay 2 is of the order of 2/3 of the Vickers hardness at the heart of the steel piece and the layer 2 with its decarburized zone has 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 wm. The structure illustrated in 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 the curve c a much less thick decarburization zone (20 to 30 Jlm) 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 underlay with respect to the base steel at the heart of the steel part is less than 25 Vickers points and affects a weakly decarburized zone with a thickness of less than 25 μm. We have here a very good grain fineness in the sub-layer 2, grain fineness which is at least equal to or greater than the Afnor 6 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 residual nitrogen content greater than 0.4% over a depth greater than 0.5 mm, this nitrogen content preventing significant decarburization and 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 chromizing process, but also to the pieces of steel chromized according to the present process and having a hard chromized surface layer with a depth at least equal to 20 μm and having a Vickers hardness at least equal. at 1,200 points.

Claims (8)

1. Method for chromizing steel pieces, of the type wherein chromium is diffused on the surface of steel pieces with a view to obtaining a surface layer of chromium and wherein the supply of chromium is obtained by the decomposition in gaseous phase of chromium halides, the decomposition of the halide and the diffusion of the chromium into the steel being achieved simultaneously by heating the said pieces at temperatures nor exceeding 1 300 °C in the halide atmosphere, this chromizing operation proper being preceded by a superficial deposit of nitrogen, characterized in that the introduction of the nitrogen in the surface layer of the steel piece is so conducted that the nitrogenous surface layer has a nitrogen content greater than 0,8 % over a depth or thickness at least equal to 0,5 mm, that this superficial introduction of nitrogen is effected by heating the steel piece to be treated to a temperature varying between 400 °C and 800 °C and in any case to a temperature lower than the lowest critical point of the steel piece to be treated, in a nitrogen-producing medium and that the duration of the nitrogen introducing cycle is between 12 and 150 hours.

2. Method according to claim 1, characterized in that the superficial introduction of nitrogen in the steel piece is immediately followed by the chromizing operation proper.

3. Method according to claim 1, characterized in that after the cycle of superficial introduction of nitrogen, the nitrogen-containing steel piece is left to cool before being subjected to the chromizing operation proper.

4. Steel piece, comprising a chromized surface layer with a hardness at least equal to 1 200 Vickers degrees, a depth at least equal to 20 µm and presenting carbonitrides, as well as a sub-layer presenting a decarburized zone adjacent the said chromized layer and with a hardness lower than that of the core of the said steel piece, characterized in that the thickness of said decarburized zone is less than that of the chromized superficial layer and that the reduction of hardness in that decarburized zone of the sub-layer is less than 25 Vickers degrees with respect to the hardness of the steel in the core of the piece.

5. Steel piece according to claim 4, characterized in that the decarburized zone has a depth or thickness less than 30 um.

6. Steel piece according to one of claims 4 and 5, characterized in that the sub-layer presents a fine grain of Afnor index equal to or greater than 6 over a thickness or depth at least equal to 0,5 mm.

7. Steel piece according to one of claims 4 to 6, characterized in thàt-the residual nitrogen content in the sub-layer is greater than 0,4 %.

8. Steel piece according to claim 7, characterized in that the sub-layer presents residual nitrogen over a thickness or depth greater than 0,5 mm.

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