EP1180552A1 - Method for surface treatment of mechanical pieces subjected to wear and corrosion - Google Patents

Abstract

A method for the surface treatment of mechanical components involves consecutively nitriding the component and then oxidising it. The nitriding is carried out by immersing the component in a sulphur free nitriding bath of molten salts at a temperature of between 500 and 700oC and the oxidation is carried out in an oxidising aqueous solution at a temperature of less than about 200oC. An Independent claim is included for components treated by this method and having a rugosity of less than 0.5 mu m and with a surface free from tables.

Description

The present invention relates to a method of surface treatment of parts mechanical subjected to both wear and corrosion. More specifically, the invention relates to a method of surface treatment of mechanical parts subject to both wear and corrosion to give said parts a high resistance to wear and corrosion as well as a roughness favorable to lubrication. Even more specifically, the invention relates to a treatment method surface of mechanical parts whose lubrication must be controlled so precise, and whose roughness must therefore be controlled within a narrow range.

It is well known that the thickness of the oil film on the surface of a part depends much of the roughness of its surface: a perfectly polished part may not be wet with oil, while, conversely, a very rough part will be covered of a film whose thickness will be less than the height of the microreliefs, this will result in high risk of seizure.

Among the parts which could advantageously be treated according to the present invention, for example, the rods of cylinders and the valves of engines thermal. Regarding a cylinder rod, the thickness of the oil film at its surface must be perfectly controlled; too weak, the rod-seal contact is no longer lubricated and there is wear; too high, the resulting lubricant leakage affects the cylinder performance. Regarding a thermal engine valve, the film oil performs both the lubrication and dynamic sealing functions in the valve stem / valve guide contact; too polished a piece will provide a film thin oil and lubrication will be random, while high roughness will result in high oil consumption and loss of engine performance.

Numerous solutions are available to the skilled person when he is in the presence a member which must resist both wear and corrosion. It is so common to use thick deposits of micro-cracked "hard chrome". However, these disadvantages. On the technical level, the presence of an interface between steel and chrome can cause dramatic flaking for the desired functions, moreover, in the case of parts that operate intermittently like some cylinders, there is a risk of elimination of the residual lubricant film by the weather and therefore corrosion. Economically, this process requires deposition followed by machining, which makes it an expensive solution. Finally, in terms of the environment, chromium plating is still very largely carried out using baths containing chromium VI which is a major pollutant.

Another commonly used solution is to nitride the parts and then to oxidize; these two operations are often followed by a step of impregnating the surface porosity with a product further improving corrosion resistance. These operations are carried out successively, either in a salt bath as teach it for example the French patents FR-A-2 672 059 or FR-A-2 679 258, either in a gaseous atmosphere as taught for example by the European patent 0217420.

This combined nitriding and oxidation operation generally gives very good resistance to wear and corrosion, however, it leads systematically to an increase in the roughness of the part, bringing it to a level incompatible with what is required by applications within the scope of the invention.

This increase in roughness has led those skilled in the art to complete these processes by one or more polishing phases more or less advanced so that one leads to sequences such as nitriding-oxidation-polishing, or even nitriding-oxidation-polishing-oxidation. Such methods make it possible to efficiently fill the lubrication function, but are difficult to apply industrially because they impose a combination of different technologies (thermochemical and mechanical) which makes it both very expensive and of limited use, it is indeed difficult to control by polishing the roughness on a piece of complex shape.

Surprisingly, the Applicant has demonstrated that it is possible to obtain both a high resistance to wear and corrosion and a roughness conducive to lubrication by conducting nitriding and oxidation operations in special baths.

The objects defined above are satisfied by the present invention which provides a method of surface treatment of mechanical parts, making it possible to give say parts high resistance to wear and corrosion as well as roughness suitable for lubrication in which nitriding is carried out consecutively said part followed by an oxidation of said part, characterized in that said nitriding is carried out by immersing said part in a nitriding bath molten salts free of sulfur species, at a temperature between about 500 ° C and about 700 ° C, and in that said oxidation is carried out in a aqueous oxidizing solution at a temperature below about 200 ° C.

To be in accordance with the invention, the process must respect both the association following nitriding and oxidation, the two operations being carried out in liquid phase under the conditions specified above.

It is not, however, the consecutive association of a particular nitriding and a particular oxidation process, but of a whole inseparable because, in the case of the process according to the invention, there is an interaction very strong in between.

(1) L'opération de nitruration préalable (première étape) doit être exécutée dans un bain fondu exempt d'espèces soufrées. La température du bain est comprise entre environ 500°C et 700°C, par exemple à une température comprise entre environ 590°C et 650°C.Avantageusement, le bain comporte des cyanates et des carbonates alcalins et a la composition suivante:

Li⁺ =0,2 à 10%

Na⁺ = 10 à 30 %

K⁺ = 10 à 30%

CO₃ ^2- = 25 à 45 %

CNO^- = 10 à 40%

CN^- < 0,5%
en poids

Par exemple, le bain de nitruration de sels fondus contient les ions suivants, en pourcentage en poids :

Li⁺ = 2,8 à 4,2

Na⁺ = 16,0 à 19,0

K⁺ = 20,0 à 23,0

CO₃ ^2- = 38,0 à 43,0

CNO^- = 12,0 à 17,0

avec une quantité d'ions CN^- au plus égale à 0,5 % en poids.Une agitation par de l'air comprimé sera avantageusement prévue.Avantageusement, la durée d'immersion des pièces est au moins d'environ 10 minutes; elle peut aller jusqu'à plusieurs heures selon les besoins. Habituellement, la durée d'immersion des pièces est comprise entre environ 30 et 60 minutes.

(2) L'opération d'oxydation (deuxième étape) qui fait suite à la nitruration doit être conduite à une température inférieure à environ 200°C. La température du bain d'oxydation est de préférence comprise entre environ 110°C et 160°C. Mieux encore, la température du bain d'oxydation est comprise entre environ 125°C et environ 135°C. La composition du bain est avantageusement la suivante :

• HO^-   =   10,0 à 22%
• NO₃ ^-   =   1,8 à 11,8%
• NO₂ ^-   =   0 à 5,3%
• S₂O₃ ^2-   =   0,1 à 1,9%
• Cl^-   =   0 à 1,0%
• Na⁺   =   1,0 à 38%
  en poids

Par exemple, la solution aqueuse oxydante contient les ions suivants, en pourcentage en poids : OH^- = 17 à 18,5 NO₃ ^- = 4,0 à 5,5 NO₂ ^- = 1,0 à 2,5 Cl^- = 0,25 à 0,35 Na⁺ = 25 à 29. Par exemple, la solution aqueuse oxydante contient en outre 0,6 à 1,0 % en poids d'ions thiosulfate S₂O₃ ^2-.La durée d'immersion des pièces dans le bain d'oxydation est avantageusement comprise entre environ 5 et 45 minutes.Il est remarquable de constater que, après avoir été nitrurées puis oxydées conformément à l'invention, les pièces traitées peuvent ensuite subir une opération d'imprégnation avec la même efficacité que dans l'art antérieur. Bien que la rugosité finale soit nettement plus faible, l'affinité de la couche pour les produits d'imprégnation est au moins aussi élevée. Ce fait surprenant n'est encore pas expliqué scientifiquement à ce jour.L'invention fournit également une pièce traitée par le procédé ci-dessus, dans laquelle ledit procédé a provoqué des modifications superficielles. La pièce selon l'invention est caractérisée en ce que sa rugosité a une valeur Ra inférieure à environ 0,5 µm et en ce que sa surface est exempte de "tables".L'invention sera à présent décrite plus en détail dans les exemples non limitatifs suivants.

The two stages of the process, namely the nitriding stage and the oxidation stage, must meet the following conditions:

(1) The prior nitriding operation (first step) must be carried out in a molten bath free from sulfur species. The temperature of the bath is between approximately 500 ° C and 700 ° C, for example at a temperature between approximately 590 ° C and 650 ° C. Advantageously, the bath comprises cyanates and alkali carbonates and has the following composition:

Li ⁺ = 0.2 to 10%

Na ⁺ = 10 to 30%

K ⁺ = 10 to 30%

CO ₃ ^2- = 25 to 45%

NOC ^- = 10 to 40%

CN ^- <0.5%
in weight

For example, the nitriding bath of molten salts contains the following ions, in percentage by weight:

Li ⁺ = 2.8 to 4.2

Na ⁺ = 16.0 to 19.0

K ⁺ = 20.0 to 23.0

CO ₃ ^2- = 38.0 to 43.0

NOC ^- = 12.0 to 17.0

with a quantity of CN ions ^- at most equal to 0.5% by weight. Agitation by compressed air will advantageously be provided. Advantageously, the duration of immersion of the parts is at least about 10 minutes; it can go up to several hours as needed. Usually, the immersion time of the parts is between about 30 and 60 minutes.

(2) The oxidation operation (second step) following nitriding must be carried out at a temperature below about 200 ° C. The temperature of the oxidation bath is preferably between about 110 ° C and 160 ° C. More preferably, the temperature of the oxidation bath is between about 125 ° C and about 135 ° C. The composition of the bath is advantageously as follows:

• HO ^- = 10.0 to 22%
• NO ₃ ^- = 1.8 to 11.8%
• NO ₂ ^- = 0 to 5.3%
• S ₂ O ₃ ^2- = 0.1 to 1.9%
• Cl ^- = 0 to 1.0%
• Na ⁺ = 1.0 to 38%
  in weight

For example, the aqueous oxidizing solution contains the following ions, in percentage by weight: OH ^- = 17 to 18.5 NO ₃ ^- = 4.0 to 5.5 NO ₂ ^- = 1.0 to 2.5 Cl ^- = 0.25 to 0.35 Na ⁺ = 25 to 29. For example, the aqueous oxidizing solution also contains 0.6 to 1.0% by weight of thiosulfate ions S ₂ O ₃ ^2-. The period of immersion of the parts in the oxidation bath is advantageously between approximately 5 and 45 minutes. It is remarkable to note that, after having been nitrided and then oxidized in accordance with the invention, the treated parts can then undergo an impregnation operation with the same efficiency as in the prior art. Although the final roughness is significantly lower, the affinity of the layer for the impregnation products is at least as high. This surprising fact has not yet been scientifically explained to date. The invention also provides a part treated by the above method, in which said method has caused surface changes. The part according to the invention is characterized in that its roughness has a Ra value of less than about 0.5 μm and in that its surface is free from "tables". The invention will now be described in more detail in the examples following nonlimiting.

Example 1

Parallelepipedic specimens of dimensions 30 x 18 x 8 mm as well as 35 mm diameter rings, both made of non-alloy steel with 0.35% carbon and initial roughness Rmax = 0.6 µm, were first treated in a salt bath nitriding containing by mass 19% of cyanate ions, 37% of carbonate ions and 3.5% lithium ions, the rest being sodium and potassium ions. The pieces were immersed for 40 minutes at a temperature of 630 ° C.

On leaving the bath, the pieces were cooled in a water tank and then washed before being immersed for 15 minutes in an oxidizing brine at 135 ° C constituted at the rate of 85 kg of the following salt mixture (Table I) in % by weight for 75 liters of water: HO ^- 18% NO ₂ ^- 2% NO ₃ ^- 5% S ₂ O ₃ ^2- 1% Cl 0.3% Na ⁺ 27%

The parts were then washed in water at 80 ° C. and then neutralized in an oil-based solution soluble at 40 ° C before being dried.

The test pieces were characterized on the one hand by roughness and on the other hand by friction tests.

The roughness measurements carried out on the parts thus treated are grouped in Table II and compared with those obtained with the conventional methods listed, N1, N2, 0x1 and 0x2 corresponding for N1 to nitriding according to FR 72 05 498, N2 to nitriding according to (TF1), 0x1 to an oxidation according to FR 93 09814 and 02 to an oxidation according to FR 76 07858. The morphological parameters of the roughness patterns used to qualify the surface states are: Ra (arithmetic mean of length) and R (average depth arithmetic). TREATMENT BEFORE TREATMENT AFTER TREATMENT R (µm) Ra (µm) R (µm) Ra (µm) N2Ox2 UNPOLISHED 0.25 58 2.3 62 POLISHED 0.25 58 0.9 54 N3 + 0x3 UNPOLISHED 0.25 58 2.5 66 POLISHED 0.25 58 0.9 56 N1 + 0x3: processing according to the invention 0.25 58 0.85 52

It will be noted that the method according to the invention makes it possible to obtain a roughness equivalent to that of conventional processes followed by polishing.

For friction tests, the ring is pressed against the large face of the plate with a regularly increasing load from the initial value of 5 daN and with a constant sliding speed of 0.55 m / s. The friction surface of the wafer is oiled before the test. The results are collated in Table III. Treatment Duration of test (min) Cumulative wear of the two parts (µm) Coefficient of friction N2 + 0x2 Without polishing 30 30 0.4 After polishing 60 12 0.25 N3 + 0x3 Without polishing 30 34 0.43 After polishing 50 20 0.3 N1 + 0x1: processing according to the invention 60 10 0.2

Example 2

High-alloy steel cylinders containing 0.45% carbon, 9% carbon chromium and 3% silicon, are treated in a nitriding bath of composition identical to that of Example 1.

The parts are immersed for 30 minutes in the bath maintained at a temperature of 590 ° C then soaked in cold water. Once washed, they are oxidized in the brine described in Example 1 for 10 minutes at 130 ° C. and then at again washed in hot water.

Usually, on this type of steel, the roughness obtained with the processes standard oxidation nitrocarburizing or oxidized sulfo nitrocarburizing are relatively high due to the poor quality of the surface layers obtained (very porous layers and poorly adherent powdery oxides). For information, the Rz is usually of the order of 10 μm and it is often necessary to carry out a polishing or even micro-blasting operation to bring roughness Rz to the vicinity 2 µm.

The test pieces treated according to the range described in this example have an Rz between 2 and 2.5 µm without the need for polishing or microbeading.

Note: Rz = average roughness depths according to standard NF ISO 4287 from 1997 corrected 1998.

Example 3

Tests have been carried out to show to what extent the process according to the invention is an inseparable whole. In this context, test tubes cylindrical non-alloy steel 0.35% by mass of carbon were treated in associating different nitriding processes with the usual oxidation processes, including including that cited in Examples 1 and 2.

The nitriding step was carried out either, according to FR 72 05498, at 570 ° C., in a salt bath consisting by weight of 37% of cyanate ions and 17% of carbonate ions, the rest being K + alkaline cations, Na + and Li +, with in addition 10 to 15 ppm of S ² - ions, ie under the same conditions as those of Example 1.

The oxidation step was carried out either according to FR 9309814, at 475 ° C in a bath salts based on 13.1% carbonate ions, 36.5% nitrate ions, 11.3% ions hydroxides and 0.1% dichromate ions, the remainder being alkaline cations K +, Na + and Li +, either under the conditions described in Examples 1 and 2.

The roughness results are collated in Table IV below, bearing in mind that the starting roughness for all the test pieces is 0.3 μm Ra. TREATMENT ROUGHNESS AFTER TREATMENT R (µm) Ra (µm) N2 570 ° C + Ox2 475 ° C 2.3 62 N2 570 ° C + Ox1 130 ° C 2.6 66 N1 630 ° C + Ox2 475 ° C 2.4 63 N1 570 ° C + Ox1 130 ° C 0.9 54 N1 630 ° C + Ox1 130 ° C according to invention 0.85 52 N1 570 ° C + Ox1 110 ° C 0.9 55 N1 590 ° C + Ox1 150 ° C according to invention 0.85 51

Claims (15)

Method of surface treatment of mechanical parts, making it possible to give said parts a high resistance to wear and corrosion as well as a roughness favorable to lubrication in which nitriding of said part is carried out consecutively followed by oxidation of said part, characterized in that said nitriding is carried out by immersing said part in a nitriding bath of molten salts free of sulfur species, at a temperature between about 500 ° C and about 700 ° C, and in that said oxidation is carried out in an aqueous oxidizing solution at a temperature below about 200 ° C. Process according to Claim 1, characterized in that the nitriding bath of molten salts contains the following ions, in percentage by weight: Li ⁺ = 0.2 to 10 Na ⁺ = 10 to 30 K ⁺ = 10 to 30 CO ₃ ² - = 25 to 45 NOC ^- = 10 to 40 with a quantity of CN- ions at most equal to 0.5% by weight. Process according to Claim 2, characterized in that the nitriding bath of molten salts contains the following ions, in percentage by weight: Li ⁺ = 2.8 to 4.2 Na ⁺ = 16.0 to 19.0 K ⁺ = 20.0 to 23.0 CO ₃ ^2- = 38.0 to 43.0 NOC ^- = 12.0 to 17.0 with a quantity of CN- ions at most equal to 0.5% by weight. Process according to any one of Claims 1 to 3, characterized in that the mechanical part is immersed in the nitriding bath for a period of time at least equal to approximately 10 minutes. Method according to claim 4, characterized in that the mechanical part is immersed in the nitriding bath for a period of between approximately 30 and 60 minutes. Process according to any one of Claims 1 to 5, characterized in that the nitriding bath is agitated by compressed air. Process according to any one of Claims 1 to 6, characterized in that the aqueous oxidizing solution contains the following ions, in percentage by weight: OH ^- = 10.0 to 22.0 NO ₃ ^- = 1.8 to 11.8 NO ₂ ^- = 0 to 5.3 Cl ^- = 0 to 1.0 Na ⁺ = 1.0 to 38. Process according to Claim 7, characterized in that the aqueous oxidizing solution contains the following ions, in percentage by weight: OH ^- = 17 to 18.5 NO ₃ ^- = 4.0 to 5.5 NO ₂ ^- = 1.0 to 2.5 Cl ^- = 0.25 to 0.35 Na ⁺ = 25 to 29. Process according to any one of Claims 1 to 8, characterized in that the aqueous oxidizing solution additionally contains from 0.1 to 1.9% by weight of thiosulfate ions S ₂ O ₃ ^2- Process according to Claim 9, characterized in that the oxidizing aqueous solution also contains 0.6 to 1.0% by weight of thiosulfate ions S ₂ O ₃ ^2- Process according to any one of Claims 1 to 10, characterized in that the nitriding is carried out at a temperature between approximately 590 ° C and approximately 650 ° C. Process according to any one of Claims 1 to 11, characterized in that the oxidation is carried out at a temperature between approximately 110 ° C and approximately 160 ° C. Process according to claim 12, characterized in that the oxidation is carried out at a temperature between approximately 125 ° C and approximately 135 ° C. Process according to any one of Claims 1 to 13, characterized in that the period of immersion of the part in the oxidation bath is between approximately 5 and 45 minutes. Part treated by a method according to any one of Claims 1 to 14, in which said method has caused surface modifications, characterized in that its roughness has a Ra value of less than approximately 0.5 µm and in that its surface is free of "tables".