METHOD FOR TREATMENTING AN IRON ALLOY PART TO IMPROVE ITS CORROSION RESISTANCE

Abstract

METHOD FOR TREATMENT OF AN IRON ALLOY PART TO IMPROVE ITS CORROSION RESISTANCE The present invention relates to a method for treating an iron alloy part (P) 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), followed by - a phosphating operation of the part (P) forming a phosphating layer (2) on the surface of the part, characterized in that the molten salt bath contains chlorides, and the phosphating operation is carried out in a phosphating bath containing zinc and / or manganese ions, and iron ions. Figure for the abstract: Fig. 1

Description

Title of the invention: METHOD FOR TREATMENT OF AN IRON ALLOY PART TO IMPROVE ITS CORROSION RESISTANCE Technical field of the invention

[0001] The present invention relates to a method for 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. Technological background

[0002] In automotive, aeronautical or industrial applications, mechanical parts are generally subjected to significant stresses in service.

[0003] In a conventional manner, parts can receive one or more treatments to improve some of their performance characteristics, including friction properties, wear resistance, fatigue resistance, or corrosion resistance.

[0004] Among these different treatments, nitriding is an interesting choice. It consists of immersing a ferrous metal part in a medium capable of releasing 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.

[0005] Nitriding encompasses nitrocarburizing, which is a variant of nitriding in which carbon enters the workpiece in addition to nitrogen. The ARCOR process, developed by the Applicant and implemented by them for many years, and described hereafter, is a preferred example of a nitrocarburizing process. In the remainder of this text, nitriding and nitrocarburizing are referred to collectively as "nitriding."

[0006] Nitriding can be carried out from a gaseous phase, a plasma phase, or from a liquid phase.

[0007] Liquid-phase nitriding, also called molten salt bath nitriding, has the advantage of allowing significant hardening to a thickness of several tens of microns in just a few hours, making it a first-rate choice. Molten salt baths are typically at temperatures of around 600°C and, in practice, contain cyanates and carbonates, as well as cations, which are usually alkali metal cations, such as lithium, sodium, or potassium, for example.

[0008] Nitriding hardens the surface of the part and thus improves its wear resistance. It can also improve the sliding and seizing up.

[0009] It is often combined with another treatment, for example if good corrosion resistance is required.

[0010] Phosphating is, for example, such a treatment, which improves the performance of parts in terms of corrosion resistance.

[0011] The performance of parts that have undergone phosphating, whatever their application, depends essentially on two characteristics: the crystallinity and the quality of adhesion of the phosphated layer to a substrate.

[0012] In general, the aim is therefore to obtain during the phosphatation step a fine and regular crystallization, leading to the densest possible phosphate layer.

[0013] These two characteristics are directly dependent on the quality of the preparation of the metallic surface of the substrate before the actual phosphating operation. The physicochemical state of this surface determines its reactivity towards the active agents of the phosphating bath, as well as the growth of the phosphate complex layer.

[0014] It is understood that the parts may be washed, degreased, and dried before nitriding and / or phosphating. These routine operations do not alter the chemistry or structure of the parts to be treated. In some cases, however, other pretreatment operations are carried out.

[0015] It is known to those skilled in the art that the crystallinity of the phosphate layer can be improved by a pretreatment consisting of depositing finely dispersed titanium compounds onto the surface to be phosphated. The applicant has also demonstrated in patent EP 0560641 that nitriding in the presence of sulfur species facilitates phosphatation and thus ensures fine crystallinity.

[0016] However, the sulfur content must be low (on the order of tens of ppm) and the control and maintenance of the correct sulfur content increases the complexity and cost of such nitriding.

[0017] The cost can then become prohibitive for the processing of inexpensive materials, for example very low alloy steels or cast irons.

[0018] In the specific 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 reach the surface of the part, can disrupt surface treatments and thus constitute points of entry for corrosion. Therefore, in order to improve corrosion resistance, it has been proposed to carry out a degraphitization treatment before nitriding (described in document WO2016143712). However, such a pretreatment further increases the time and overall cost of the cast iron treatment. grey.

[0019] Lamellar cast iron, in which the inclusions are in the form of lamellae, has the advantage of being less expensive than other cast irons. Spheroidal cast iron requires the addition of a crystallizing element (rare earth, lithium, magnesium), which increases its cost.

[0020] In general, grey cast iron is a fairly inexpensive material, but rather difficult to protect against corrosion.

[0021] Since gray cast iron is an inexpensive material, it is used for manufacturing brake discs for motor vehicles. Often, brake discs are untreated and exhibit very low corrosion resistance. During braking, corrosion on the disc surface is abraded by the brake pads and thus eliminated.

[0022] However, with the emergence of electric vehicles and the recovery of electrical energy during braking, mechanical brakes are used less and less and any corrosion attacks present on the surface of the discs are no longer eliminated: corrosion, unsightly and detrimental to the quality of braking, spreads on the brake discs.

[0023] Moreover, in the event of long-term storage of the vehicle before its sale, the presence of unsightly corrosion on the brake discs detracts from the appearance of the vehicle and may prevent the purchase.

[0024] It is therefore necessary to offer inexpensive yet corrosion-resistant mechanical parts, particularly brake discs. In the case of brake discs, phosphating alone is insufficient because the phosphate layer is removed during the first braking application. Brief description of the invention

[0025] Faced with this situation, the Applicant sought a solution to simplify the processes of 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.

[0026] The process should preferably be adapted to the treatment of grey cast iron despite the intrinsic constraints of this material. However, it should be applicable to any cast iron or low-alloy steel, or even in principle to all iron alloys.

[0027] To this end, the invention proposes a method for treating an iron alloy part to improve its corrosion resistance and mechanical strength, comprising: - a nitriding or nitrocarburizing operation in a molten salt bath, forming a compound layer on the part, then - a phosphating operation on the part, forming a phosphating layer on the surface of the part,

[0028] mainly characterized in that the molten salt bath contains chlorides, and the phosphatation operation is carried out in a phosphatation bath which contains zinc ions and / or manganese ions, and iron ions.

[0029] The Applicant has observed 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 phosphatizing operation with zinc and iron (of the zinc / iron, Zn / Fe type), or with manganese and iron (of the manganese / iron, Mn / Fe type), or with zinc and manganese and iron (of the zinc / manganese / iron, Zn / Mn / Fe type), makes it possible to obtain a part exhibiting good corrosion resistance properties and good mechanical strength.

[0030] Furthermore, in the specific case of a gray cast iron part, the process of the invention not only leads to the aforementioned advantageous mechanical properties, in particular, mechanical strength relatively similar to the current standard for gray cast irons, namely nitriding followed by gas oxidation, but also eliminates the need for prior degraphitization. For applications requiring a low coefficient of friction, zinc can be at least partially replaced by manganese.

[0031] Conversely, in the case where it is desired to maintain a relatively high coefficient of friction, for example for brake discs, then it is necessary to avoid the presence of Manganese ions in the phosphating bath.

[0032] The phosphatation bath therefore includes, in addition to the usual phosphate ions PO43, zinc ions Zn2+ and / or manganese ions Mn2+, as well as iron ions Fe2+.

[0033] As previously stated, the process of treating a part according to the invention does not include a degraphitization operation of the part prior to phosphating, i.e. before phosphating or even before nitriding.

[0034] 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.

[0035] The process according to the invention is therefore advantageously, but not exclusively, applicable to iron alloy parts which usually require degraphitization, in particular grey cast iron parts.

[0036] In other respects, the treatment process according to the invention has the following different characteristics taken alone or in technically possible combinations: - the molten salt bath does not contain sulfur; - chlorides include alkali metal chlorides; - Alkali metal chlorides are lithium, sodium and / or potassium chlorides; - the molten salt bath includes cyanate ions CNO, carbonate ions (CO3)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 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,

[0037] the percentages being expressed in weight relative to the total weight of the bath; - The zinc, manganese and iron ion content in the phosphating bath is adjusted as follows, depending on the alloy of the part being treated and the expected performance: • 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 (grams per liter), 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 of between 1 g / L and 40 g / L, preferably between 5 g / L and 20 g / L. In addition, and optionally, 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 zinc and manganese ions; • 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; • the bath possibly containing nitric acid or nitrates; • the rest of the bath being essentially made up of water. - the phosphatizing layer has a thickness 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 grey cast iron.

[0038] The process of the invention, in the case where the phosphating bath does not contain of 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 corrosion resistance, the method being as described above.

[0039] Another object of the invention is an iron alloy part that can be obtained by the processing method described above, comprising: - a nitriding layer on the surface of the iron alloy, comprising a compound layer containing nitrides, - a phosphating layer arranged on and in contact with the nitriding layer, comprising metallic phosphates,

[0040] mainly characterized in that the metallic phosphates comprise zinc and / or manganese, and iron.

[0041] The invention also relates to a grey cast iron part that can be obtained by the treatment process described above, comprising: - a nitriding layer on the surface of the grey cast iron, comprising a compound layer containing nitrides, - a phosphating layer arranged on and in contact with the nitriding layer, comprising metallic phosphates,

[0042] mainly characterized in that the metallic phosphates comprise zinc and / or manganese, and iron.

[0043] Preferably, the phosphating layer of the part has a thickness between 3 pm and 40 pm, preferably between 5 pm and 30 pm, and even more preferably between 5 pm and 20 pm.

[0044] Preferably, the combination layer has a thickness of between 5 pm and 40 pm, preferably between 15 pm and 25 pm.

[0045] The part has graphite inclusions on one surface of the part on which the compound layer is formed, insofar as it has not undergone prior degraphitization. These graphite inclusions are located at least on one surface of the part on which the compound layer is formed.

[0046] 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. Description of the figures

[0047] Other advantages and features of the invention will become apparent from the following description, given by way of illustrative and non-limiting example, with reference to the following attached figures:

[0048] [Fig-1] [Fig.1] is a micrograph of a part according to an embodiment of the invention, said part being a lamellar cast iron brake disc.

[0049] [Fig.2] [Fig.2] is a photograph of the part of [Fig.1] after treatment of nitriding-phosphating and before a salt spray test.

[0050] [Fig.3] Fig.3 is a photograph of a part not conforming to the invention, said part being a lamellar cast iron brake disc.

[0051] [Fig.4] [Fig.4] is a micrograph of the part in [Fig.3] after treatment of nitriding-phosphating and before a salt spray test.

[0052] Detailed description of embodiments of the invention

[0053] The Applicant's approach was to carry out several series of tests implementing different treatments of a part (P) made of grey cast iron.

[0054] The first problem addressed is to find a process giving the part (P) sufficient corrosion resistance, despite the fact that the nitriding bath does not contain sulfur and that, moreover, the said part is made of non-degraphitized grey cast iron and therefore particularly difficult to protect from corrosion.

[0055] The treated parts (P) were subjected to a salt spray, and their corrosion resistance is evaluated from the duration of the appearance of a threshold of pitting and / or rust runs.

[0056] Next, the second problem was to validate the proper functioning of the part (P) thus treated, according to a particular use of brake disc: comparative measurements of coefficient of friction were therefore carried out.

[0057] In particular, the applicant studied the effects of the following two operations.

[0058] With reference to the micrograph illustrated in [Fig. 1], the nitrocar- treatment The ARCOR buration (trademark registered by the Applicant) provides, from the surface towards the core of the part (P), a juxtaposed combination layer (1) and diffusion zone (1'). The combination layer (1) typically has a thickness between 5 and 30 µm, and forms at the surface of the substrate (S) that makes up the part (P).

[0059] 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.

[0060] The nitriding was carried out in a bath of molten salts.

[0061] Phosphating provides, on the surface of the part (P), above the layer of combination (1), a layer of metallic phosphate (2). The metallic phosphate layer (2) typically has a thickness of about 20 pm.

[0062] On the micrograph of [Fig.1], an additional layer (a) is visible: this is an aluminum layer not part of the part (P), but deposited in order to make the section necessary for the micrograph.

[0063] In addition, lamellar graphite inclusions (G) can be observed, which open onto the surface of the part (P).

[0064] A first series of corrosion resistance tests was conducted. The parts (P) manufactured from grey cast iron, then treated, are placed in an enclosure and subjected to a salt spray test.

[0065] The enclosure is then opened every 24 hours to check for the appearance of corrosion spots, which indicate the beginning of localized corrosion. These spots are counted, and if their number exceeds a predefined threshold, for example 50, then the part (P) is considered corroded and the test is stopped.

[0066] The longer the time before the threshold is reached, the more corrosion-resistant the part. The minimum corrosion resistance is set by the Applicant at a duration greater than or equal to 96 hours.

[0067] Table 1 below illustrates the corrosion tests most relevant to understanding the invention:

[0068] [Tables 1] No. Pretreatment Nitriding Media Nitriding Salt Posttreatment Time to Corrosion Threshold Result C4 Without Liquid ARCOR V Oxidation 1 24h NG C5 Without Liquid ARCOR L Without 24h NG C8 Without Liquid ARCOR L Oxidation 2 24h NG C9 Without Liquid ARCOR L Oxidation 1 48h NG C10 = P Without Liquid ARCOR L Ph (ZnFe) >96h OK C13 Without Liquid ARCOR L Ph (ZnCa) 24h NG C14 Without Liquid ARCOR L Ph (Zn) 24h NG C16 Without Liquid ARCOR V Ph (ZnFe) 24h NG C17 Without Liquid ARCOR V Ph (ZnCa) 24h NG

[0069] An “ARCOR L” nitriding process 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.

[0070] An “ARCOR V” nitriding process is a ferritic nitrocarburizing process with treatment temperatures that can vary between 500°C and 700°C depending on the steel being treated and the required specification. It corresponds to the nitriding portion described in patent FR2812888. The “ARCOR V” nitriding bath does not contain chloride ions.

[0071] A post-treatment "Oxidation 1" is an oxidation process in a bath of molten salts at a temperature of 450°C.

[0072] A post-treatment "Oxidation 2" 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 alkali Na+ cations.

[0073] A "Ph (ZnFe)" post-treatment is a phosphatation process with treatment temperatures that can vary between 60°C and 70°C. The salts used are of the (H2PO4)2Me type and contain Zn2+ ions (Me = Zn). The iron is supplied in the form of iron powder.

[0074] A "Ph (ZnCa)" post-treatment is a phosphatation process with treatment temperatures that can vary between 80°C and 90°C. The salts used are of the type (H2PO4)2Me and comprise Zn2+ and Ca2+ ions (Me = Zn, Ca).

[0075] A "Ph (Zn)" post-treatment is a phosphatation process with treatment temperatures that can vary between 60°C and 70°C. The salts used are of the (H2PO4)2Me type and consist only of Zn2+ ions.

[0076] Based on the corrosion tests carried out, it is possible to draw the following conclusions.

[0077] With reference to the photograph in [Fig.2], it can be seen that a part (P) according to the invention shows no corrosion points (photo taken after 48 hours of testing).

[0078] With reference to the photograph in [Fig. 3], a part (C14) not conforming to the invention exhibits, after 48 hours of testing, more than 50 corrosion points (pc). The treatment referenced C14 is therefore unsatisfactory.

[0079] With reference to [Fig. 4], a micrograph of a part (C14) was taken after treatment and before salt spray testing. It can be seen that the boundary between the phosphating layer (2) and the compound layer (1) is less distinct than on the parts (P) according to the invention. It can be assumed that the deposition of the phosphating layer (2) attacked the compound layer (1), since phosphating with zinc alone removes Fe ions from the substrate. Unlike iron alloy substrates phosphated with zinc but not nitrided, we observe that phosphating with zinc alone on nitrided parts does not produce a good corrosion protection. One hypothesis for this observation is that the dissolution of iron from the combination layer makes the surface of the substrate (S) too rough, which could increase the porosity of the phosphate layer.

[0080] This phenomenon is not observed when the phosphating bath includes Zn2+ ions and Fe2+ ions (CIO test).

[0081] Of all the different combinations of nitriding and phosphating / oxidizing tested on parts (P) not having undergone degraphitization, only the combination of an “ARCOR L” nitriding and a “Ph ((Zn and / or Mn) + Fe)” phosphating allows sufficient corrosion resistance to be obtained (CIO test).

[0082] Further tests were then carried out to verify that the characteristics of the treated part (P) are compatible with use as a brake disc.

[0083] It is observed 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.

[0084] However, a part (P) which has not undergone degraphitization, and which has undergone nitriding “ARCOR L” and phosphatizing “Ph (ZnFe)” is entirely compatible with such use.

[0085] In this respect, the friction performance of such a part (P) is close to that of the prior art. The process according to the invention is therefore particularly suitable for the treatment of brake discs.

[0086] The process according to the invention is, however, less expensive than prior art solutions, 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.

[0087] In other words, the process according to the invention requires only nitriding and phosphatizing 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.

[0088] Furthermore, since both nitriding and phosphating are carried out by bath, there is no need to change the part setup to move from one step to the other, and industrialization is facilitated. In particular, the time required to complete the entire treatment is significantly reduced compared to equivalent gas nitriding.

[0089] Other ferrous alloys could be considered for making the part (P), such as steel or white cast iron. However, gray cast iron has the advantage of being the least expensive material.

Claims

Claims

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

2. The method of claim 1, wherein the molten salt bath does not contain sulfur.

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

4. A method 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: - from 25% to 60% of alkali metal chlorides, - from 10% to 40% of alkali metal carbonates, - from 20% to 50% of alkali metal cyanates, - 3% or less of cyanide ions, the percentages being expressed by weight relative to the total weight of the bath.

6. Method according to any one of the preceding claims, in which: - 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, preferably between 5 g / L and 20 g / L, relative to the total volume of the phosphating bath; - 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.

7. A method according to any preceding claim, 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. 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. Method according to any one of the preceding claims, in which the part (P) is made of gray cast iron.

10. A part (P) made of gray cast iron obtainable by the treatment method according to any one of the preceding claims in combination with claim 9, comprising: - a nitriding layer in contact with the gray cast iron, comprising a combination layer (1) containing nitrides, - a phosphating layer (2) arranged on and in contact with the combination layer (1), comprising metal phosphates, characterized in that the metal phosphates comprise zinc and / or manganese, and iron, and said part (P) has graphite inclusions (G) at a surface of the part (P) on which the combination layer (1) is formed.

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 any one of claims 10 or 11, said part being a brake disc.

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