EP0076745A1 - Process for treating the surfaces of steels against wear, and coating obtained - Google Patents

Abstract

The present invention relates to a method of treatment against surface wear of parts made of steels with a carbon content of at least 0.15% and the coating obtained. The process is characterized in that a supply of sulfur is carried out successively on the surface to be treated, for example by deposition of metallic sulfide and chromization in the vapor phase. The coating obtained is characterized by the presence of a layer of chromium sulfide on the surface and by the partial recrystallization of the layer of underlying chromium carbide M7C3. The invention can be applied to any mechanical system in motion subjected to friction. Its advantage is to increase the resistance to wear and the lifetime of the treated system while limiting the wear of the untreated antagonist in friction.

Description

Tectonic scope of the present invention is that of surface treatment of the surface layers of steel parts in order to im- _li orer their mechanical properties. More particularly, the technical sector of the present invention relates to treatments which aim to increase the resistance to wear of structural steel and tool steel parts subjected to friction.

The main treatments of this type currently practiced on tool steels are nitriding, oxidation, chromium plating, hard chromium plating and boriding. Direct sulfurization of a substrate has never been used for treatments of this type because this element leads to the embrittlement of steels. Sulfur is used in combined form the molybdate _d ene or iron for antifriction cemne solid lubricant application and not in wear resistance.

By the known chromizing treatments, coatings are obtained which have good wear resistance. These coatings generally consist of layers based on chromium carbides M ₂₃ C ₆ and M ₇ C ₃ , M denoting a metal which apart from chromium (Cr) can be iron (Fe), molybdenum (Mo) vanadium (V) etc ... The layers obtained have a limited thickness; for example, the layers obtained by chromizing treatment on steels having a carbon content of at least 0.15% have a total thickness of carbides of the order of 10 to 15 μm.

However, the lifetime of a system, thus treated, and subjected to friction, depends on the kinetics of wear of this layer of carbides. For a given stressing rode, this lifetime is linked, on the one hand, to the total thickness of the hard surface layer, on the other hand, to the crystallization morphology of the carbides M ₇ C ₃ . In these known chromizing treatments, M ₇ C ₃ has a columnar structure which is harmful to good wear resistance. Furthermore, in the case of dry friction contact with antagonists of lesser hardness, these undergo very significant wear.

The aim of the present invention is to produce a surface coating of steel parts which improves the wear resistance of the part subjected to friction and makes it possible to increase the service life of systems solicited in friction while by limiting the wear of the antagonists in contact by friction with said parts.

The subject of the present invention is a method of treatment against surface wear of steel parts with a carbon content of at least 0.15%, characterized in that a supply of sulfur and an vaporization control.

By prior supply of sulfur on the base steel, the exchange conditions between the chromium deposited during the subsequent chromization and the elements of the substrate are modified and the sulfur is thus allowed to combine with the chromium. An original surface layer composed of chromium sulfide is obtained. The surface thus treated has satisfactory wear resistance and also offers the advantage of greatly reducing the wear of the antagonist in uncoated steel. During friction, the sulfur reacts by contact with the antagonist and forms on the latter transfer layers rich in sulfur species favoring its least wear. This treatment also makes it possible to increase the total thickness of the hard layer on the surface. The carbon content greater than or equal to 0.15% is necessary for the constitution of sublayers of carbides which participate in the resistance to wear.

The sulfur is preferably added by depositing a metal sulphide on the steel surface, for example by depositing a varnish based on molybdenum disulphide or by depositing an iron-molybdenum alloy sulphide of formulation (Fe - Mo) ₃ S ₄ .

Direct sulfurization of the substrate is indeed to be avoided since sulfur leads to the embrittlement of steels. On the other hand, by exchange reaction with metallic elements, it is possible to transport chromium from a chromium halide in the vapor phase.

Advantageously, an iron-mlyndene deposition is carried out prior to the deposition of iron-molybdenum sulfide.

If the deposit of iron-molybdenum sulphide is applied directly to the part, its adhesion over time is quite weak. On the other hand, a prior sulfur-free deposit significantly increases adhesion.

Chromization is carried out at 950 ° C for 15 hours and without the flow of hydrogen.

Indeed, hydrogen can desulfurize the deposit by the formation of H ₂ S and thus prevent the formation of chromium sulfide on the substrate surface.

The usual heat treatments can be practiced on steels treated by this process. However, water quenching is not recommended, as it can introduce cracks in the coating made according to the invention.

The subject of the invention is also a surface coating against wear of steel parts with a carbon content greater than or equal to 0.15%, characterized in that it consists of a surface layer of chromium sulfide and a layer underlying the surface layer consisting of chromium carbides M ₂₃ C ₆ and M ₇ C ₃ . In the following description, all the percentages are percentages by mass.

The coating obtained by the process according to the invention is characterized in that it consists of a surface layer of chromium-molybdenum sulphide, the molybdenum being substituted in the hexagonal network of chromium sulphide at a mass content less than 18% and a layer underlying the surface layer consisting of chromium carbides M ₂₃ C ₆ and M ₇ ^C ₃ .

The carbide layer is made up of two sublayers consisting successively of chromium carbide M ₂₃ C ₆ and chromium carbide M ₇ C ₃ .

The chromium carbide M ₇ C ₃ sublayer is recrystallized over part of its thickness from the substrate.

There is indeed a change in the crystallization mode of M ₇ C ₃ carbides. Molybdenum codiffuses with chromium in M ₇ C ₃ carbides, thus making it possible to reduce the quantity of chromium in this phase and to reach the critical content of 60% of chromium for which the recrystallization of M ₇ C ₃ occurs. This increases the toughness of the carbide layer and thereby its wear resistance.

Other advantages and characteristics of the invention will appear in the description of an embodiment of the invention which is not limiting, which follows.

The steel used for this description is the low alloy steel 35CD4, widely used in industrial manufacturing. Any other steel with a carbon content of at least 0.15% could be used.

The sequenced processing according to the invention comprises two parts, the order of which must be respected. The first part of this treatment consists of a treatment making it possible to deposit a sulphide of the iron-molybdenum sulphide or molybdenum sulphide type. The treatment mode only matters with regard to the uniformity in thickness and distribution of the deposit. It is thus possible to make deposits of the varnish type based on molybdenum disulphide or to carry out deposits by the "sprettering-magnetron" technique of (Fe-Mo) ₃ S ₄ . The latter technique allows good control of the thickness and distribution of the initial deposit.

A deposit of at least 5 µm is necessary to obtain good results. A thickness of 8 to 15 µm is best suited to subsequent chromizing conditions.

When the parts are treated without prior iron-molybdenum deposition, deposit faience is observed before chranization. Treatments with prior deposition of iron-molybdenum were therefore carried out. They did not show any lack of adhesion.

After depositing the sulphide layer, the chromizing treatment is carried out according to average parameters such as:

If one wishes to avoid the surface formation of a sulfur-free chromium layer, it is necessary in this case to suppress the flow of hydrogen usually provided in these types of treatment.

It should be noted that the type of cement and the static or dynamic mode of transport do not condition the nature of the coating here; they only modulate the thicknesses of relative layers.

Two main layers of equal thicknesses, respectively based on sulfide and chromium carbides, make up the coating obtained after chromization.

The external sulphide layer, formed from the (Cr, Mo) S phase which crystallizes in the hexagonal system, is quite heterogeneous as regards its distribution of molybdenum and sulfur. The iron is almost zero in content and Mo at a rate of at most 18%.

The underlying carbide layer is divided into two carbide sublayers M ₂₃ C ₆ and M ₇ C ₃ . The latter carbide, rich in molybdenum, is partially recrystallized.

During the exchanges, the iron was therefore pumped from the initial deposit of iron-molybdenum-sulfur, to give rise to deposit of chromium. This last element, diffusing through the whole of the initial deposit, has migrated to the interface of the substrate to form there with the carbon of the steel, the sequence of carbides obtained in conventional chromization, with however some different characteristics. The M ₂₃ C ₆ carbide undercoat is much larger here; and the carbide M ₇ C ₃ is partially recrystallized.

On the surface, the general morphology of the layers is in the form of crystals with a spherular front. These crystals are distributed in domains separated by fairly shallow "channels".

In section, the coating. is divided into two main layers A and B with respective thicknesses of 9 µm.

Layer A has a two-phase appearance with basaltic domains. These areas are generally set back from the outside surface. They correspond to channels.

Layer B is made up of two sublayers B ₁ and B ₂ Reversed after basic attack, these sublayers correspond to carbides; the sublayer 13 ₂ has the typical morphology of the carbide M ₇ C ₃ with recrystallization.

Layer A consists essentially of the elements chromium, sulfur, molybdenum. The two-phase aspect observed in metallography seems to be due only to relative variations in sulfur and molybdenum. The iron is almost zero in content, and molybdenum up to 18%.

Based on the data in the chromium-sulfur binary diagram, it appears from the level of chromium concentration in this layer that the phase of which it is composed is chromium sulfide CrS. This phase crystallizes in the hexagonal system (a = 3.45 A, c = 5.76A c / a = 1.67).

Layer B is made up of two layers formed of carbides where no sulfur is detected. The chromium concentration levels and the shape of the carbon distribution profiles indicate that the sublayers B ₁ and B ₂ correspond respectively to the carbides M ₂₃ C ₆ and M ₇ C ₃ . It should be noted that the molybdenum content in these carbides is high. But molybdenum can largely replace this type of phase.

Contrary to the results recorded during the previous analyzes, we notice here that the molybdenum instead of being pumped in the substrate to migrate towards the carbides in formation, in fact diffused until in this one starting from the deposit initial.

In section, the coating is divided into two main sub-layers with respective thickness of 9 µm, for a total thickness of 18 µm.

The surface layer, the two-phase appearance of which is only due to relative variations in molybdenum, consists of the (Cr, Mo) S phase. The hardness of this phase is 770 + 50 Hv _0.02 .

The second layer, revealed by metallographic attack in basic medium, consists of chromium carbides enriched in molybdenum M ₂₃ C ₆ and M ₇ C ₃ . The carbide M ₇ C ₃ is in this case partially recrystallized, which ensures an increase in toughness to this layer of carbide. The hardness of carbide M ₂₃ C ₆ is 1400 + 200 Hv _0.02 , that of carbide M ₇ C ₃ is 2700 + 500 Hv _v0.02 for the recrystallized domain.

It should be noted that by direct chromization with identical treatment parameters the layer had a total thickness of carbide of 13 μm.

• - disque plans en acier 35CD4,
• - pions cylindriques à bout plat de diamètre 1,5mm en acier 35CD4,traités thermiquement pour une dureté de 310Hv_0.5,
• - effort normal 1N, soit une contrainte statique normale de 0,56 MPa,
• - vitesse circonférentielle : 500 tr.mn^-1
• - vitesse linéaire de contact : 41 m. mn^-1
• - température de laboratoire : 20°C,
• - frottement sec,
• - distance de sollicitation : 50 km.

A tribological characterization of this layer was carried out. In the standard wear test on a tribometer in disc-disc configuration, the test parameters were as follows:

• - 35CD4 steel flat disc,
• - cylindrical pins with flat end of 1.5mm diameter in 35CD4 steel, heat treated for a hardness of 310Hv _0.5 ,
• - normal force 1N, i.e. a normal static stress of 0.56 MPa,
• - circumferential speed: 500 rpm ^-1
• - linear contact speed: 41 m. mn ^-1
• - laboratory temperature: 20 ° C,
• - dry friction,
• - operating distance: 50 km.

• a) la couche composée de sulfure de chrome est moins usée que la couche obtenue par chromisation directe;
• b) le pion antagoniste, qui n'a pas de traitement spécifique anti-usure, est ici très peu endommagé. Cette usure particulièrement faible est assurée par un film transfert à base de soufre,oxygène et carbone mis en évidence par spectrométrie Auger.
  

Identical tests being carried out on 35CD4 chromized discs directly with the same processing parameters, the results of table n ° show that:

• a) the layer composed of chromium sulfide is less used than the layer obtained by direct chromization;
• b) the antagonist counter, which has no specific anti-wear treatment, is very little damaged here. This particularly low wear is ensured by a transfer film based on sulfur, oxygen and carbon highlighted by Auger spectrometry.
  

The advantage of the invention resides in the fact that it is possible to increase the overall performance of wear resistance of systems subjected to dry friction or poorly lubricated. Furthermore, this gain in quality can be acquired from various implementation techniques without fundamentally modifying the characteristics obtained. A change in technology for carrying out the conventional chromization treatment requires only a fine-tuning of the parameters, which is of obvious interest from an economic point of view.

The economic benefit also lies in improving the lifespans and reliability of the parts.

The invention can be applied to any moving mechanical system, made of steel, in particular weapon mechanisms or engine transmission mechanisms, etc.

Claims (13)

₁ - Treatment process, against surface wear of steel parts with carbon content of at least 0.15%, characterized in that successively carries out on this surface a supply of sulfur and chromization in the vapor phase . 2 - A method according to claim 1, characterized in that the addition of sulfur is done by depositing on the surface of the steel a metal sulfide. 3 - Process according to claim 2, characterized in that a varnish deposition based on molybdenum disulfide is carried out. 4 - Process according to claim 2, characterized in that one carries out a deposition of sulphide of iron-malybdenum alloy of formulation (Fe-Mo) ₃ S ₄ . 5 - Process according to claim 3 or 4, characterized in that one carries out, prior to the deposition of iron-molybdenum sulfide, a deposition of iron-molybdenum. 6 - Process according to claim 5, characterized in that firstly a layer of iron-molybdenum at least 1 µm thick is deposited and then a layer of iron-wlybdenum sulfide of at least 4 µm . 7 - Method according to one of claims 1 to 6, characterized in that the chromization is carried out at 950 ° C for 15 h and without hydrogen flow. 8 - Method according to one of the preceding claims, characterized in that a subsequent quenching heat treatment is carried out, with the exception of water quenching. 9 - Surface coating against wear of steel parts with carbon content greater than or equal to 0.15%, characterized in that it consists of a surface layer of chromium sulphide and a layer underlying the surface layer consisting of chromium carbides M ₂₃ C ₆ and M ₇ C ₃ . 10 - Surface coating of steel parts obtained by the method according to claims 1 to 8, characterized in that it consists of a surface layer of chromium-molybdenum sulfide, the molybdenum being substituted in the hexagonal network of the sulfide of chromium at a content of up to 18% by mass, a layer underlying the surface layer consisting of chromium carbides M ₂₃ C ₆ and M ₇ C ₃ . 11 - Surface coating according to claim 9 or 10, characterized in that the underlying layer of carbides consists of two sublayers constituted successively by chromium carbide M ₂₃ C ₆ and chromium carbide M ₇ C ₃ . 12 - Surface coating according to claim 11, characterized in that the underlay of chromium carbide M ₇ C ₃ is recrystallized over part of its thickness from the substrate. ₁₃ - surface covering according to one of claims 10 to 12, charac- t _ERI in that it is divided into two main layers of metal sulfide and metal carbides with respective thicknesses of 9 .mu.m at least, a total thickness of at least 18 µm.