Method for treating a part made of iron alloy for improving the Anti-corrosion properties thereof

Abstract

The present invention relates to a method for treating a part (P) made of iron alloy for improving the anti-corrosion and mechanical strength properties thereof, the method comprising: - a step of salt bath nitriding or salt bath nitrocarburising to form a combination layer (1) on the part (P); and subsequently - a step of phosphating the part to form a phosphating layer (2) on the surface of the part, which method is characterised in that the bath of molten salts contains chlorides and the phosphating step is carried out in a phosphating bath containing zinc ions and / or manganese ions, and iron ions.

Description

[0001] DESCRIPTION

[0002] Title: METHOD FOR TREATING AN ALLOY PART

[0003] IRON TO IMPROVE ITS RESISTANCE TO CORROSION

[0004] Technical field of the invention

[0005] The present invention relates to a method of treating an iron alloy part by nitriding and phosphating to improve its corrosion resistance, as well as to an iron alloy part treated by nitriding and phosphating.

[0006] Technological background

[0007] In automotive, aeronautical or industrial applications, mechanical parts are generally subjected to significant stresses in service.

[0008] Traditionally, parts can receive one or more treatments to improve some of their performance, including friction properties, wear resistance, fatigue resistance, or even corrosion resistance.

[0009] Among these different treatments, nitriding is an interesting choice. It consists of immersing a ferrous metal part in an environment likely to release nitrogen, and the formation of iron nitrides on the surface of the part as well as the diffusion of nitrogen from the surface to the core of the part leads to significant surface hardening.

[0010] Nitration includes nitrocarburizing, which is a variant of nitriding, in which carbon enters the part in addition to nitrogen. The ARCOR process, developed by the Applicant and implemented by it for many years, and described in the remainder of this text, is a preferred example of a nitrocarburizing process. In the remainder of this text, nitriding and nitrocarburizing are grouped together under the single term "nitriding".

[0011] Nitriding can be done from a gas phase, a plasma phase, or from a liquid phase. Liquid phase nitriding, also called molten salt bath nitriding, has the advantage of allowing significant hardening over a thickness of several tens of microns in times of just a few hours, which makes it a first-rate choice. Molten salt baths are typically at temperatures of around 600°C, and in practice contain cyanates and carbonates, and also contain cations which are usually alkali metal cations, such as lithium, sodium, or potassium for example.

[0012] Nitration hardens the surface of the part, improving its wear resistance. It can also improve sliding and gripping properties.

[0013] It is often combined with another treatment, for example if good corrosion resistance is required.

[0014] Phosphating is, for example, such a treatment, making it possible to improve the performance of parts in terms of corrosion resistance.

[0015] The performance of phosphated parts, whatever their application, depends essentially on two characteristics: crystallinity and the quality of adhesion of the phosphate layer to a substrate.

[0016] Generally speaking, we therefore seek to obtain during the phosphating stage a fine and regular crystallization, leading to the densest possible phosphate layer.

[0017] These two characteristics are directly dependent on the quality of the preparation of the metal surface of the substrate, before the actual phosphating operation. The physicochemical state of this surface conditions its reactivity towards the active agents of the phosphating bath, as well as the growth of the layer of phosphate complexes. It is implicit that the parts can be washed, degreased and dried before the nitriding and / or phosphating operations. These routine operations do not modify either the chemistry or the structure of the parts to be treated. In some cases, however, other pretreatment operations are implemented.

[0018] It is thus known to those skilled in the art that the crystallinity of the phosphate layer can be improved by a pretreatment which consists of depositing finely dispersed titanium compounds on the surface to be phosphated. The applicant has also demonstrated in patent EP 0560641 that nitriding in the presence of sulfur species makes it possible to facilitate phosphating and thus guarantee fine crystallinity.

[0019] However, the sulfur species content must be low (in the order of tens of ppm) and controlling and maintaining the correct sulfur species content increases the complexity and cost of such nitriding.

[0020] The cost can then become prohibitive for the processing of inexpensive materials, for example very low alloy steels or cast irons.

[0021] In the particular case of cast iron, gray cast iron is a cast iron in which carbon crystallizes in the form of graphite inclusions, which can take the form of long, narrow graphite flakes, nodules (spheroids), or vermicules. These inclusions, when they emerge on the surface of the part, can disrupt surface treatments and thus constitute initiation points for corrosion. Thus, in order to improve corrosion resistance, it has been proposed to carry out a degraphitization treatment before nitriding (described in document WO2016143712). However, such pre-treatment further increases the duration and overall cost of gray cast iron treatment.

[0022] Lamellar cast iron, in which the inclusions are in the form of lamellae, has the advantage of being cheaper than other cast irons. Spheroidal cast iron requires the addition of a crystallizing element (rare earth, lithium, magnesium), which increases its cost. Generally speaking, gray cast iron is a relatively inexpensive material, but rather difficult to protect against corrosion.

[0023] Since cast iron is a cheap material, it is used to manufacture brake discs for automotive vehicles. Brake discs are often untreated and have very low corrosion resistance. During braking, corrosion attacks on the surface of the discs are abraded by the pads, thus eliminating them.

[0024] However, with the emergence of electric vehicles and electrical energy recovery during braking, mechanical brakes are used less and less and possible corrosion attacks on the surface of the discs are no longer eliminated: corrosion, unsightly and harmful to the quality of braking, spreads to the brake discs.

[0025] Furthermore, if the vehicle is stored for a long time before being sold, the presence of unsightly corrosion on the brake discs will affect the appearance of the vehicle and may prevent it from being purchased.

[0026] It is therefore appropriate to offer mechanical parts, particularly brake discs, that are inexpensive but resistant to corrosion. In the case of brake discs, phosphating alone is not sufficient because the phosphate layer is removed during the first braking.

[0027] Brief description of the invention

[0028] Faced with this situation, the Applicant sought a solution to simplify the processes for treating mechanical parts by nitriding and then phosphating, particularly in view of the constraint of preparing the surface before the actual phosphating operation, which increases production costs. The process should preferably be adapted to the treatment of gray cast iron despite the intrinsic constraints of this material. However, it must be able to be applied to any cast iron or low-alloy steel, or in principle to all iron alloys.

[0029] To this end, the invention proposes a method for treating an iron alloy part to improve its corrosion resistance and its mechanical strength, comprising:

[0030] - a nitriding or nitrocarburizing operation in a molten salt bath, forming a combination layer on the part, then

[0031] - a phosphating operation of the part forming a phosphating layer on the surface of the part, mainly characterized in that the molten salt bath contains chlorides, and the phosphating operation is carried out in a phosphating bath which contains zinc ions and / or manganese ions, and iron ions.

[0032] The Applicant has noticed that carrying out a nitriding operation in the presence of chlorides (i.e. chloride ions Cl' and by extension to chemical species containing chloride ions such as chlorine salts) followed by a phosphating operation with zinc and iron (zinc / iron, Zn / Fe type), or with manganese and iron (manganese / iron, Mn / Fe type), or with zinc and manganese and iron (zinc / manganese / iron, Zn / Mn / Fe type), makes it possible to obtain a part with good corrosion resistance properties and good mechanical strength.

[0033] Furthermore, in the particular case of a gray cast iron part, the process of the invention not only leads to the aforementioned advantageous mechanical properties, in particular, a mechanical strength relatively similar to the current standard for gray cast iron, namely nitriding then oxidation in the gas phase, but also makes it possible to dispense with prior degraphitization. For applications requiring a low coefficient of friction, Zinc can be replaced at least partially by Manganese. Conversely, in the case where it is desired to maintain a relatively high coefficient of friction, for example for brake discs, then it is advisable to avoid the presence of Manganese ions in the phosphating bath.

[0034] The phosphating bath therefore includes, in addition to phosphate ions PCL 3 ' usual, zinc ions Zn 2+ and / or manganese ions Mn 2+ , as well as iron ions Fe 2+ .

[0035] As indicated previously, the method for treating a part according to the invention does not include an operation of degraphitizing the part, prior to phosphating, that is to say before phosphating or even before nitriding.

[0036] The absence of degraphitization makes it possible to reduce production time and costs, without however degrading the quality of the part obtained, in particular its good properties of hardness, wear resistance and corrosion resistance.

[0037] The method according to the invention therefore applies advantageously, but not exclusively, to iron alloy parts which usually require degraphitization, in particular gray cast iron parts.

[0038] According to other aspects, the treatment method according to the invention has the following different characteristics taken alone or according to their technically possible combinations: the molten salt bath does not contain sulfur; the chlorides comprise alkali metal chlorides; the alkali metal chlorides are lithium, sodium and / or potassium chlorides; the molten salt bath comprises cyanate ions CNO', carbonate ions (CCL) 2 ', and alkali ions preferably chosen from lithium ions Li + , and / or potassium ions K + , and / or sodium ions Na + ; the nitriding molten salt bath includes:

[0039] • from 25% to 60% of alkali metal chlorides,

[0040] • from 10% to 40% of alkali metal carbonates, • from 20% to 50% of alkali metal cyanates,

[0041] • 3% or less of cyanide ions, the percentages being expressed by weight relative to the total weight of the bath; the content of zinc, manganese and iron ions in the phosphating bath is adjusted as follows, depending on the alloy of the part treated and the expected performance:

[0042] • when the phosphating bath contains zinc ions but not manganese ions, said zinc ions are present at a mass concentration of between 1 g / L and 40 g / L (grams per liter), preferably between 5 g / L and 20 g / L, relative to the total volume of the phosphating bath;

[0043] • when the phosphating bath contains manganese ions but not zinc ions, said manganese ions are present at a mass concentration of between 1 g / L and 40 g / L, preferably between 5 g / L and 20 g / L, relative to the total volume of the phosphating bath;

[0044] • when the phosphating bath contains zinc ions and manganese ions, the total of these ions is present at a mass concentration of between 1 g / L and 40 g / L, preferably between 5 g / L and 20 g / L. In addition and optionally, the manganese ions may represent between 10% and 90% by weight, preferably between 25% and 75% by weight, and advantageously between 40% and 60% by weight of the total weight of the zinc ions and the manganese ions;

[0045] • the iron ions being in all cases present in the phosphating bath at a mass concentration of between 1 g / L and 8 g / L, preferably between 1 g / L and 6 g / L, and more preferably between 1 g / L and 5 g / L;

[0046] • the bath possibly including nitric acid or nitrates;

[0047] • the remainder of the bath being essentially made up of water. the phosphating layer has a thickness of between 3 pm and 40 pm, preferably between 5 pm and 30 pm, and even more preferably between 5 pm and 20 pm; the combination layer measures between 5 pm and 40 pm in thickness, and preferably between 15 pm and 25 pm; the part is made of gray cast iron.

[0048] The method of the invention, in the case where the phosphating bath does not contain manganese ions, is particularly applicable to a brake disc. The invention therefore also relates to a method for treating a gray cast iron part to improve its resistance to corrosion, the method being as described above.

[0049] Another object of the invention is an iron alloy part capable of being obtained by the treatment method described above, comprising:

[0050] - a nitriding layer on the surface of the iron alloy, comprising a combination layer containing nitrides,

[0051] - a phosphating layer arranged on and in contact with the nitriding layer, comprising metal phosphates, mainly characterized in that the metal phosphates comprise zinc and / or manganese, and iron.

[0052] The invention also relates to a gray cast iron part capable of being obtained by the treatment method described above, comprising:

[0053] - a nitriding layer on the surface of the gray cast iron, comprising a combination layer containing nitrides,

[0054] - a phosphating layer arranged on and in contact with the nitriding layer, comprising metal phosphates, mainly characterized in that the metal phosphates comprise zinc and / or manganese, and iron.

[0055] Preferably, the phosphating layer of the part has a thickness of between 3 pm and 40 pm, preferably between 5 pm and 30 pm, and even more preferably between 5 pm and 20 pm.

[0056] Preferably, the combination layer has a thickness of between 5 μm and 40 μm, preferably between 15 μm and 25 μm. The part has graphite inclusions at a surface of the part on which the combination layer is formed, provided that it has not undergone prior degraphitization. These graphite inclusions are located at least at a surface of the part on which the combination layer is formed.

[0057] The part is preferably a brake disc. In this case, the phosphating is a zinc-iron phosphating. The invention therefore also relates to a brake disc having the characteristics of the gray cast iron part described above.

[0058] Description of figures

[0059] Other advantages and characteristics of the invention will appear on reading the following description given by way of illustrative and non-limiting example, with reference to the following appended figures:

[0060] [Fig. 1] Figure 1 is a micrograph of a part according to one embodiment of the invention, said part being a lamellar cast iron brake disc.

[0061] [Fig. 2] Figure 2 is a photograph of the part in Figure 1 after nitration-phosphating treatment and before a salt spray test.

[0062] [Fig. 3] Figure 3 is a photograph of a part not in accordance with the invention, said part being a lamellar cast iron brake disc.

[0063] [Fig. 4] Figure 4 is a micrograph of the part in Figure 3 after nitration-phosphating treatment and before a salt spray test.

[0064] Detailed description of embodiments of the invention

[0065] The Applicant's approach was to carry out several series of tests implementing different treatments of a part (P) in gray cast iron.

[0066] The first problem addressed is to find a process that gives the part (P) sufficient corrosion resistance, despite the fact that the nitriding bath does not contain sulfur and that, moreover, the part is made of non-degraphitized gray cast iron and is therefore particularly difficult to protect from corrosion.

[0067] The treated parts (P) were subjected to a salt spray, and their resistance to corrosion is evaluated from the duration of appearance of a threshold of pitting and / or rust drips.

[0068] Then, the second problem was to validate the correct functioning of the part (P) thus treated, according to a particular use of brake disc: comparative measurements of the coefficient of friction were therefore carried out.

[0069] In particular, the applicant studied the effects of the following two operations.

[0070] With reference to the micrograph illustrated in Figure 1, the ARCOR nitrocarburizing treatment (trademark registered by the Applicant) provides, from the surface towards the core of the part (P), a combination layer (1) and a diffusion zone (1') juxtaposed. The combination layer (1) typically has a thickness of between 5 and 30 pm, and is formed at the surface of the substrate (S) which makes up the part (P).

[0071] The diffusion zone (1') typically has a thickness of a few tens or hundreds of microns, for example 300 pm, and is measured from the surface of the substrate (S) of the part (P), towards the core of said part.

[0072] The nitrification was carried out in a molten salt bath.

[0073] Phosphating brings, to the surface of the part (P), above the combination layer (1), a layer of metal phosphate (2). The layer of metal phosphate (2) typically has a thickness of about 20 pm.

[0074] In the micrograph of Figure 1, an additional layer (a) is visible: this is a layer of aluminum that is not part of the part (P), but was deposited in order to make the cut necessary for the micrograph. We can also observe the graphite inclusions (G), in lamellar form, which open onto the surface of the part (P).

[0075] A first series of corrosion resistance tests was conducted. The parts (P) made from gray cast iron, then treated, are placed in an enclosure and subjected to a salt spray test.

[0076] The enclosure is then opened every 24 hours to check for the appearance of corrosion points, which indicate the start of localized corrosion. These points are counted, and if their number is greater than a predefined threshold, for example 50, then the part (P) is considered corroded and the test stops. The longer the time before the threshold is reached, the more resistant the part is to corrosion. The minimum corrosion resistance is set by the Applicant at a duration greater than or equal to 96 hours.

[0077] Table 1 below illustrates the corrosion tests most relevant to understanding the invention: [Table 1] An “ARCOR L” nitriding is a ferritic nitrocarburizing process with treatment temperatures that can vary between 530°C and 650°C depending on the steel to be treated and the required specification. It corresponds to what is described in patent FR2972459. The “ARCOR L” nitriding bath contains chloride ions.

[0078] An “ARCOR V” nitriding bath is a ferritic nitrocarburizing process with treatment temperatures that can vary between 500°C and 700°C depending on the steel to be treated and the required specification. It corresponds to the nitriding part which is described in patent FR2812888. The “ARCOR V” nitriding bath does not contain chloride ions.

[0079] An “Oxidation 1” post-treatment is an oxidation process in a molten salt bath at a temperature of 450°C.

[0080] An “Oxidation 2” post-treatment is an oxidation process in aqueous solution with treatment temperatures that can vary between 120°C and 140°C. The salts consist mainly of nitrates, nitrites and carbonates associated with Na+ alkali cations.

[0081] A “Ph (ZnFe)” post-treatment is a phosphating process with treatment temperatures that can vary between 60°C and 70°C. The salts used are of the (H2PO4)2Me type and include Zn ions 2+ (Me = Zn). Iron is supplied in the form of iron powder.

[0082] A “Ph (ZnCa)” post-treatment is a phosphating process with treatment temperatures that can vary between 80°C and 90°C. The salts used are of the (H2PO4)2Me type and include Zn ions 2+ and that 2+ (Me = Zn, Ca).

[0083] A “Ph (Zn)” post-treatment is a phosphating process with treatment temperatures that can vary between 60°C and 70°C. The salts used are of the (H2PO4)2Me type and only contain Zn ions. 2+ . Based on the corrosion tests carried out, the following conclusions can be drawn.

[0084] With reference to the photograph in figure 2, we see that a part (P) according to the invention does not present any corrosion points (photo taken after 48 hours of testing).

[0085] With reference to the photograph in Figure 3, a part (Cl 4) not corresponding to the invention shows, after 48 hours of testing, more than 50 corrosion points (pc). The treatment referenced C14 is therefore not satisfactory.

[0086] With reference to Figure 4, a micrograph of a part (Cl 4) was taken after treatment, and before salt spray testing. It can be seen that the boundary between the phosphating layer (2) and the combination layer (1) is less clear than on the parts (P) according to the invention. It can be assumed that the deposition of the phosphating layer (2) attacked the combination layer (1), since phosphating with Zn alone takes Fe ions from the substrate. Unlike iron alloy substrates phosphated with zinc but not having undergone nitriding, we observe that phosphating with zinc alone on nitrided parts does not provide good corrosion protection. One hypothesis for this observation is that the dissolution of the iron from the combination layer makes the surface of the substrate (S) too rough, which could increase the porosity of the phosphate layer.

[0087] This phenomenon is not observed when the phosphating bath includes Zn ions 2+ and Fe ions 2+ (CIO test).

[0088] Of all the different combinations of nitriding and phosphating / oxidation tested on parts (P) that had not undergone degraphitization, only the combination of “ARCOR L” nitriding and “Ph ((Zn and / or Mn) + Fe)” phosphating provides sufficient corrosion resistance (CIO test). Other tests were then carried out to verify that the characteristics of the treated part (P) are compatible with use as a brake disc.

[0089] It is noted that the presence of Manganese ions in the phosphating bath gives the part (P) a lower coefficient of friction, which is not compatible with use as a brake disc.

[0090] However, a part (P) which has not undergone degraphitization, and which has undergone “ARCOR L” nitriding and “Ph (ZnFe)” phosphating is entirely compatible with such use.

[0091] In this regard, the friction performance of such a part (P) is close to that of the prior art. The method according to the invention is therefore particularly suitable for the treatment of brake discs.

[0092] The process according to the invention is, however, less expensive than the solutions of the prior art, because it eliminates the need for degraphitization. The nitriding step in particular is inexpensive, because the nitriding salts used (chlorides) are economical and their dosage is not critical.

[0093] In other words, the method according to the invention only requires nitriding and phosphating to modify the chemistry and macroscopic structure of the part (P), and thus give it good mechanical properties, in particular mechanical strength, wear resistance, and corrosion resistance.

[0094] In addition, since both nitriding and phosphating are carried out by bath, there is no need to change the assembly of the parts to move from one stage to another and industrialization is facilitated. In particular, the time taken to complete the treatment is significantly reduced compared to equivalent gas nitriding. Other ferrous alloys can be considered to produce the part (P), such as steel or white cast iron. However, gray cast iron has the advantage of being the least expensive material.

Claims

DEMANDS 1. A process for treating a part (P) made of iron alloy to improve its corrosion resistance and mechanical strength, comprising: - a nitriding or nitrocarburizing operation in a molten salt bath, forming a compound layer (1) on the part (P), then - a phosphatizing operation of the part (P) forming a phosphatizing layer (2) on the surface of the part, characterized in that the molten salt bath contains chlorides, and the phosphatizing operation is carried out in a phosphatizing bath containing zinc ions and / or manganese ions, and iron ions.

2. A process according to claim 1, wherein the molten salt bath does not contain sulfur.

3. A process according to claim 1 or claim 2, wherein the chlorides comprise alkali metal chlorides.

4. A process according to claim 3, wherein the alkali metal chlorides are lithium, sodium and / or potassium chlorides.

5. A method according to any one of the preceding claims, wherein the molten salt bath comprises: - 25% to 60% alkali metal chlorides, - 10% to 40% alkali metal carbonates, - 20% to 50% alkali metal cyanates, - 3% or less of cyanide ions, the percentages being expressed by weight relative to the total weight of the bath.

6. A method according to any one of the preceding claims, wherein: - when the phosphatizing bath contains zinc ions but not manganese ions, said zinc ions are present at a mass concentration between 1 g / L and 40 g / L, preferably between 5 g / L and 20 g / L, relative to the total volume of the phosphatizing bath; - when the phosphatizing bath contains manganese ions but not zinc ions, said manganese ions are present at a mass concentration between 1 g / L and 40 g / L, preferably between 5 g / L and 20 g / L, relative to the total volume of the phosphatizing bath; - when the phosphatizing bath contains zinc ions and manganese ions, the total of these ions is present at a mass concentration between 1 g / L and 40 g / L, preferably between 5 g / L and 20 g / L; - iron ions being in all cases present in the phosphatation bath at a mass concentration between 1 g / L and 8 g / L, preferably between 1 g / L and 6 g / L, and more preferably between 1 g / L and 5 g / L.

7. A method according to any one of the preceding claims, wherein the phosphating layer (2) has a thickness of between 3 pm and 40 pm, preferably between 5 pm and 30 pm, and more preferably between 5 pm and 20 pm.

8. A method according to any one of the preceding claims, wherein the combination layer (1) has a thickness of between 5 pm and 40 pm, preferably between 15 pm and 25 pm.

9. A method according to any one of the preceding claims, wherein the part (P) is made of grey cast iron.

10. Part (P) made of grey cast iron obtainable by the processing method according to any one of the preceding claims, comprising: - a nitriding layer in contact with the grey cast iron, comprising a combination layer (1) containing nitrides, - a phosphatizing layer (2) arranged on and in contact with the combination layer (1), comprising metallic phosphates, characterized in that the metallic phosphates comprise zinc and / or manganese, and iron.

11. Part (P) according to claim 10, in which the phosphating layer (2) has a thickness of between 3 pm and 40 pm, preferably between 5 pm and 30 pm, and more preferably between 5 pm and 20 pm.

12. Part (P) according to claim 10 or claim 11, said part (P) having graphite inclusions (G) at a surface of the part (P) on which the combination layer (1) is formed.

13. Part (P) according to any one of claims 10 to 12, said part being a brake disc.

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