FR3133394A1 - METHOD FOR TREATING AN IRON ALLOY PART TO IMPROVE ITS CORROSION RESISTANCE - Google Patents

Abstract

METHOD FOR TREATING AN IRON ALLOY PART TO IMPROVE ITS CORROSION RESISTANCE The present invention relates to a process 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 on 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. Figure for abstract: Fig. 1

Description

METHOD FOR TREATING AN IRON ALLOY PART TO IMPROVE ITS CORROSION RESISTANCE Technical field of the invention

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.

Technology background

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

Conventionally, the parts can receive one or more treatments to improve some of their performances, including friction properties, wear resistance, fatigue resistance, or even corrosion resistance.

Among these different treatments, nitriding is an interesting choice. It consists of immersing a ferrous metal part in a medium 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 towards the heart of the part leads to significant surface hardening.

Nitriding encompasses nitrocarburizing, which is a variation of nitriding, in which carbon enters the part in addition to nitrogen. The ARCOR process, developed by the Applicant and implemented by him for many years, and described in the remainder of this text, is a preferred example of a nitrocarburization process. In the remainder of this text, we group nitriding and nitrocarburizing under the single term “nitriding”.

Nitriding can be done from a gas phase, a plasma phase, or from a liquid phase.

Liquid phase nitriding, also called nitriding in molten salt baths, has the advantage of allowing significant hardening to a thickness of several tens of microns in a time of just a few hours, making it a first choice. order. 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 cations of alkali metals, such as lithium, sodium , or even potassium for example.

Nitriding hardens the surface of the part and thus improves its resistance to wear. It can also improve sliding and seizure properties.

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

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

The performance of parts having undergone phosphating, whatever their application, essentially depends on two characteristics: crystallinity and the quality of adhesion to a substrate of the phosphate layer.

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

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 in fact conditions its reactivity towards the active agents in the phosphating bath, as well as the growth of the layer of phosphate complexes.

It is implied that the parts can be washed, degreased and dried before the nitriding and/or phosphating operations. These routine operations do not modify the chemistry or structure of the parts to be treated. In some cases, however, other preprocessing operations are implemented.

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.

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

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

In the particular case of cast iron, gray cast iron is cast iron in which carbon crystallizes in the form of graphite inclusions, which can take the form of long and narrow strips of graphite, 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 was 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 processing.

Lamellar cast iron, in which the inclusions are in the form of strips, 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.

Gray cast iron being a cheap material, it is used for the manufacture of brake discs for motor 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, and are therefore eliminated.

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

Furthermore, if the vehicle is stored for a long period before its sale, the presence of unsightly corrosion on the brake discs harms the appearance of the vehicle and may prevent the purchase.

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 during the first braking the phosphate layer is removed.

Brief description of the invention

Faced with this situation, the Applicant sought a solution to simplify the processes for treating mechanical parts by nitriding then phosphating, particularly in view of the constraint of surface preparation before the actual phosphating operation, which increases costs. of production.

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.

To this end, the invention proposes a process for treating an iron alloy part to improve its resistance to corrosion and its mechanical strength, comprising:
- a nitriding or nitrocarburizing operation in a molten salt bath, forming a combination layer on the part, then
- 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.

The Applicant realized that carrying out a nitriding operation in the presence of chlorides (that is to say chloride ions Cl ^- and by extension to chemical species containing chloride ions such as chlorine salts) then a phosphating operation with zinc and iron (zinc/iron type, Zn/Fe), or with manganese and iron (manganese/iron type, Mn/Fe), 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.

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 gas phase, but also eliminates the need for prior degraphitization. For applications requiring a low coefficient of friction, Zinc can be replaced at least partially by Manganese.

Conversely, in the case where we wish to maintain a relatively high coefficient of friction, for example for brake discs, then it is appropriate to avoid the presence of Manganese ions in the phosphating bath.

The phosphating bath therefore comprises, in addition to the usual PO ₄ ^3- phosphate ions, zinc Zn ²⁺ ions and/or manganese Mn ²⁺ ions, as well as iron Fe ²⁺ ions.

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

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, resistance to wear and resistance to corrosion.

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

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;
• chlorides include alkali metal chlorides;
• alkali metal chlorides are lithium, sodium and/or potassium chlorides;
• the molten salt bath comprises cyanate ions CNO ^- , carbonate ions (CO ₃ ) ^2- , and alkaline ions preferably chosen from lithium ions Li ⁺ , and/or potassium ions K ⁺ , and/or ions sodium Na ⁺ ;
• the molten nitriding salt bath includes:

• from 25% to 60% alkali metal chlorides,
• from 10% to 40% alkali metal carbonates,
• from 20% to 50% alkali metal cyanates,
• 3% or less 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 treated part and the expected performance:

• 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 (gram per liter), preferably between 5 g/L and 20 g/L, relative to the total volume of the phosphating bath;

• 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, for example ratio to the total volume of the phosphating bath;
• 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 can 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 ions. manganese;
• 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 L and 5 g/L;
• the bath possibly comprising nitric acid or nitrates;
• the rest of the bath being essentially made up of water.

• the phosphating layer has a thickness of between 3 µm and 40 µm, preferably between 5 µm and 30 µm, and more preferably between 5 µm and 20 µm;
• the combination layer measures between 5 µm and 40 µm thick, and preferably between 15 µm and 25 µm;
• the part is made of gray cast iron.

The process of the invention, in the case where the phosphating bath does not contain manganese ions, applies particularly to a brake disc. The invention therefore also relates to a process for treating a gray cast iron part to improve its resistance to corrosion, the process being as described previously.

Another object of the invention is an iron alloy part capable of being obtained by the treatment method described above, comprising:
- a nitriding layer on the surface of the iron alloy, comprising a combination layer containing nitrides,
- 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.

The invention also relates to a gray cast iron part capable of being obtained by the treatment method described above, comprising:
- a nitriding layer on the surface of the gray cast iron, comprising a combination layer containing nitrides,
- 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.

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

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, to the extent that it has not undergone prior degraphitization. These graphite inclusions are located at least at one surface of the part on which the combination layer is formed.

The part is preferably a brake disc. In this case, the phosphating is 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 figures

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:

There is a micrograph of a part according to one embodiment of the invention, said part being a lamellar cast iron brake disc.

There is a photograph of the piece from the after nitriding-phosphating treatment and before a salt spray test.

There is a photograph of a part not in accordance with the invention, said part being a lamellar cast iron brake disc.

There is a micrograph of the part of the after nitriding-phosphating treatment and before a salt spray test.

Detailed description of embodiments of the invention

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

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

The treated parts (P) were subjected to a salt spray, and their resistance to corrosion is evaluated based on the duration of appearance of a threshold of pitting and/or rust dripping.

Then, 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 friction coefficient were therefore carried out.

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

With reference to the micrograph illustrated on the , the ARCOR nitrocarburization 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 µm, and is formed at the surface of the substrate (S) which makes up the part (P).

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

Nitriding was carried out in a molten salt bath.

Phosphating provides, on the surface of the part (P), above the combination layer (1), a layer of metal phosphate (2). The metal phosphate layer (2) typically has a thickness of around 20 µm.

On the micrograph of the , an additional layer (a) is visible: it is a layer of aluminum not forming part of the part (P), but deposited in order to make the section necessary for the micrograph.

We can also observe the inclusions (G) of graphite, of lamellar shape, which emerge on the surface of the part (P).

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

The enclosure is then opened every 24 hours to check the appearance of corrosion spots, which indicate the beginning 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.

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

No. Pretreatment Media
nitriding Salt of
nitriding Post treatment Duration before corrosion threshold Result C4 Without Liquid ARCOR V Oxidation 1 24h N.G. C5 Without Liquid ARCOR L Without 24h N.G. C8 Without Liquid ARCOR L Oxidation 2 24h N.G. C9 Without Liquid ARCOR L Oxidation 1 48h N.G. C10 = P Without Liquid ARCOR L Ph (ZnFe) >96h OK C13 Without Liquid ARCOR L Ph (ZnCa) 24h N.G. C14 Without Liquid ARCOR L Ph (Zn) 24h N.G. C16 Without Liquid ARCOR V Ph (ZnFe) 24h N.G. C17 Without Liquid ARCOR V Ph (ZnCa) 24h N.G.

“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 specification requested. It corresponds to what is described in patent FR2972459. The “ARCOR L” nitriding bath contains chloride ions.

“ARCOR V” nitriding 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 specification requested. It corresponds to the nitriding part which is described in patent FR2812888. The “ARCOR V” nitriding bath does not contain chloride ions.

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

“Oxidation 2” post-treatment is an oxidation process in aqueous solution with treatment temperatures that can vary between 120°C and 140°C. Salts are essentially made up of nitrates, nitrites and carbonates associated with alkaline Na+ cations.

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 ²⁺ ions (Me = Zn). Iron is provided in the form of iron powder.

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 ²⁺ and Ca ²⁺ ions (Me = Zn, Ca).

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 include only Zn ²⁺ ions.

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

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

With reference to the photograph of the , a part (C14) not corresponding to the invention presents, after 48 hours of testing, more than 50 corrosion points (pc). The treatment referenced C14 is therefore not satisfactory.

In reference to the , a micrograph of a part (C14) was taken after treatment, and before salt spray testing. It can be seen that the border 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), to the extent that 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 iron from the combination layer makes the surface of the substrate (S) too rough which could increase the porosity of the phosphate layer.

This phenomenon is not observed when the phosphating bath includes Zn ²⁺ ions and Fe ²⁺ ions (test C10).

Of all the different combinations of nitriding and phosphating/oxidation tested on parts (P) which have not undergone degraphitization, only the combination of “ARCOR L” nitriding and “Ph” phosphating ((Zn and/ or Mn) + Fe)” allows sufficient corrosion resistance to be obtained (test C10).

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

We note 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.

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.

On this subject, the friction performances of such a part (P) are close to those of the prior art. The process according to the invention is therefore particularly suitable for the treatment of brake discs.

The process according to the invention is, however, less expensive than the solutions of the prior art, because it makes it possible to dispense with degraphitization. The nitriding step in particular is inexpensive, because the nitriding salts used (chlorides) are economical and their dosage is not critical.

In other words, the process 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, resistance wear and corrosion resistance.

In addition, nitriding and phosphating are both 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 required to complete the complete treatment is greatly reduced compared to equivalent gas nitriding.

Other ferrous alloys can be considered to make the part (P), such as steel or white cast iron. However, gray cast iron has the advantage of being the least expensive material.

Claims (13)

Process for treating a part (P) made of iron alloy to improve its resistance to corrosion 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. Process according to claim 1, in which the molten salt bath does not contain sulfur. A method according to claim 1 or claim 2, wherein the chlorides comprise alkali metal chlorides. A method according to claim 3, wherein the alkali metal chlorides are lithium, sodium and/or potassium chlorides. Process according to any one of the preceding claims, in which 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 cyanide ions,
the percentages being expressed in weight relative to the total weight of the bath. 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;
- 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;
- 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 ;
- 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. Method according to any one of the preceding claims, in which the phosphating layer (2) has a thickness of between 3 µm and 40 µm, preferably between 5 µm and 30 µm, and more preferably between 5 µm and 20 µm. Method according to any one of the preceding claims, in which the combination layer (1) has a thickness of between 5 µm and 40 µm, preferably between 15 µm and 25 µm. Method according to any one of the preceding claims, in which the part (P) is made of gray cast iron. Part (P) in gray cast iron capable of being obtained by the treatment method according to any one of the preceding claims, 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. Part (P) according to claim 10, in which the phosphating layer (2) has a thickness of between 3 µm and 40 µm, preferably between 5 µm and 30 µm, and more preferably between 5 µm and 20 µm. 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. Part (P) according to any one of claims 10 to 12, said part being a brake disc.