EP0638661B1 - Process for improving the wear resistance and corrosion resistance of ferrous metal workpieces - Google Patents

Abstract

The process consists in immersing the articles which have previously undergone a thermochemical diffusion of either nitriding or sulphonitriding or carbonitriding type, in a bath of molten salts made up of alkali metal carbonates, nitrates, hydroxides and their oxygenated salts, the relative anionic weight quantities of carbonates, nitrates and hydroxides, expressed for sodium salts and corresponding to the active, that is to say liquid, phase of the bath being the following: 11 < CO<2-> < 23 19 < NO3<-> < 37 6 < OH<-> < 19 while the weight quantity of oxygenated salts of alkali metals, expressed as Cr2O7<2-> equivalent, is the following: 0.05 < oxygenated anions < 0.5. This process makes it possible to guarantee a high degree of reproducibility of the results, whatever the type of articles treated.

Description

The present invention relates to a method for improving the resistance to wear and corrosion of ferrous metal parts, making it possible to guarantee a high degree of reproducibility of the results whatever the type of parts treated.

The great variety of surface treatments is partly explained by the multiplicity of situations faced by the technician in industrial practice, as well as by the extreme interweaving of the phenomena which lead to the alteration of surfaces.

Furthermore, the requirements are constantly becoming more stringent and an increasing number of parts must withstand stresses involving several phenomena, for example in mechanics: friction, wear, corrosion, or even more shock and fatigue. In these cases, the traditional methods prove to be insufficient.

In addition to this improvement in performance, the technician, driven by industrial demand, is increasingly committed to consolidating the industrialization of the processes in terms of quality, reliability and reproducibility, in other words to seek "zero defects".

The advantage of thermochemical diffusion treatments, and in particular those designated by nitriding, is known for improving the wear resistance of ferrous metal parts.

It is also known that a substantial improvement in resistance to wet corrosion can be obtained by growth, over the nitrided layer, of an oxidized surface layer.

The literature on these oxidation treatments is very abundant. Mention may in particular be made of the following patents.

Patent FR 2 306 268 from 1976 describes an oxidizing salt bath composed of alkali hydroxides, optionally with an alkali nitrate in an amount of 2 to 20% by weight. This salt bath, at preferred operating temperatures of 200 to 300 ° C, is essentially intended to simultaneously achieve controlled cooling of ferrous metal parts nitrides, leaving a cyanate / cyanide nitriding bath, and the destruction by oxidation of the cyanides entrained by the parts.

Furthermore, according to patent FR 2 463 821 of 1980, this same bath composed of alkali hydroxide, containing from 2 to 20% by weight of alkali nitrate, gives the nitrided parts a significant increase in corrosion resistance, if they are immersed in the bath between 250 and 450 ° C for a sufficient time, between 15 and 50 minutes. The study of this patent and in particular of its examples, which relate to a bath comprising by weight 37.4% of sodium hydroxide, 52.6% of potassium hydroxide and 10% of sodium nitrate, reveals improvements in resistance to corrosion by salt spray, which result in exposure times, before the appearance of traces of corrosion, almost doubled.

Patent FR 2,525,637 of 1982 describes a process for treating ferrous metal parts in an oxidizing salt bath to improve their resistance to corrosion, parts containing sulfur. This document teaches to immerse the parts in an oxidizing bath, comprising alkali hydroxides, nitrates and / or alkali nitrites and possibly alkali carbonates, with in addition from 0.5 to 15% by weight of a strong oxidant, in the species of oxygenated alkali metal salts whose normal redox potential compared to the hydrogen reference electrode is less than or equal to - 1 volt. Bichromates, permanganates, peroxycarbonates, iodates and periodates are cited as oxygenated salts, the alkali metals being sodium and potassium. The process described in this patent is further characterized in that an oxygen-containing gas is injected into the salt bath and in that the weight content of the bath in insoluble particles is kept at less than 3%. This process provides even better performance. It not only improves the corrosion resistance of the parts this time by a factor of almost 4, but also does not alter their resistance to wear and fatigue and even improve their anti-seizure properties in dry friction.

However, it has been found that these performances cannot in fact be achieved with the degrees of reliability and reproducibility required by industrial requirements. In the laboratory, differences in performance are relatively invisible. However, they become much sharper when it comes to dealing with industrial series. They are particularly observed when it is necessary to condition, according to the so-called "bulk" technology, large quantities of small parts, or else parts whose surface conditions are imperfect: the presence of disturbed areas such as stamping burrs or punching, crimping or folding folds, welding heterogeneities are all sources of faults, and therefore of corrosion.

However, on parts such as rods of cylinders or shock absorbers, or even axes of windscreen wipers or car starters, random corrosion resistance is absolutely unacceptable. The solution has long been to carry out successive retouching of the baths on a case-by-case basis and according to the more or less aberrant behaviors observed. However, this solution is not satisfactory, in particular given the industrial requirements explained above. It was therefore necessary to seek new solutions. An in-depth study work undertaken by the Applicant has enabled the development of a process which makes it possible to control on an industrial scale the intangibles that were encountered previously.

EP-A-0 497 663 describes a process aimed at improving the corrosion resistance of ferrous metal parts which have undergone nitriding and oxidation in a bath corresponding to the teaching of FR-A-2 525 637, by depositing on the surface of the treated parts, a coating of a polymer, for example FEP (fluoroethylene-propylene), having a thickness of 3 to 20 μm.

Thus, the subject of the present invention is a process, associating thermochemical diffusion and passivation by oxidation, making it possible to substantially improve the wear and corrosion resistance of parts made of ferrous metals, while guaranteeing a high degree of reproducibility , therefore a minimum dispersion.

$${\text{19 < NO}}_{\text{3}}{}^{\text{-}}\text{< 37}$$

$${\text{6 < OH}}^{\text{-}}\text{< 19}$$

tandis que la quantité pondérale de sels oxygénés de métaux alcalins, exprimée en équivalent Cr₂O₇ ^2- est la suivante : $$\text{0,05 < anions oxygénés < 0,5}$$

To this end, it proposes a process for improving the corrosion and wear resistance of ferrous metal parts in which the parts which have undergone a thermochemical diffusion of the type either nitriding, sulfonitriding or carbonitriding are immersed, in a bath of molten salts composed of carbonates, nitrates, hydroxides, as well as oxygenated salts of alkali metals chosen from dichromates, chromates, permanganates, peroxycarbonates, iodates and periodates, characterized in that the anionic relative amounts by weight of carbonates, nitrates and hydroxides, expressed for sodium salts and corresponding to the active phase, that is to say the liquid in the bath, are the following: $${\text{11 <CO}}_{\text{3}}{}^{\text{2-}}\text{<23}$$

$${\text{19 <NO}}_{\text{3}}{}^{\text{-}}\text{<37}$$

$${\text{6 <OH}}^{\text{-}}\text{<19}$$

while the quantity by weight of oxygenated salts of alkali metals, expressed in Cr ₂ O ₇ ^2- equivalent is as follows: $$\text{0.05 <oxygen anions <0.5}$$

The invention covers any composition containing other alkali metal salts than sodium, taken alone or in mixtures, and the percentages of which are converted into sodium salts are those indicated above.

For the remainder of the description and to facilitate its understanding, all the concentrations will be expressed in% by weight corresponding to the sodium salts and designated by the expression "sodium unit", which will serve to "normalize" the different mixtures, regardless of the associated metal cation (for example Na ⁺ , K ⁺ , Li ⁺ ).

The temperature of the bath is between 350 and 550 ° C. and preferably between 450 and 530 ° C., and the duration of immersion of the parts in the bath is greater than 10 minutes.

It can be seen that the composition according to the invention is, from a qualitative point of view, of the same type as that cited in the patent FR 2 525 637 previously mentioned. In quantitative terms, however, it stands out in a very significant way. This can be explained as follows.

We were able to determine that the root cause of the dispersions observed was linked to the existence of a lack of compactness in the nitrided and oxidized layers.

It has also been possible to determine that the improvement in the wear resistance is due to the nitrided layer essentially while the improvement in the corrosion resistance depends on both the nitrided layer and the oxidized layer: both providing anodic protection. The effectiveness of this protection depends directly on the integrity of the barrier layer: it is sought to have an oxidized, continuous and waterproof surface layer. The nature of the layer forming in the oxidizing bath is known and consists essentially of iron oxide type Fe ₃ O ₄ , which is perfectly inert. It was therefore not a question of doing better in terms of the nature of the layer but of finding a solution to guarantee its sealing. What therefore mattered was therefore to find a mode of implementation of this barrier-forming layer which is such that the expected result is achieved in all cases, that is to say on all types of parts and on all the parts of the same load or of several successive loads, which the known baths do not allow, as will emerge from the examples described below.

The oxidizing bath composition which is the subject of the present application differs from known baths by the fact that it combines a percentage of powerful oxidizing salt which is clearly lower than that cited in patent FR 2 525 637, in combination with nitrates and also different hydroxides.

It can also be noted that the cationic species of the bath are only defined by the nature of the corresponding metals, namely the alkali metals. It is indeed of little importance that one or more cations are present and in the case where at least two cations are present simultaneously, the relationships between them have only a slight influence.

There is no need to insist again on the complexity of molten salt baths and the difficulty there is in grasping the mechanisms of action and predicting their behavior by a theoretical approach, based on logical reasoning. It was therefore by the experimental route that the formulation of the bath according to the invention and its operating conditions were developed. To this end, the following assessment criteria were used: the corrosion resistance of the treated parts, their resistance to wear, the fluidity of the bath, the color of the parts, as well as the dispersion of the results.

Thanks to judiciously chosen experiments, it has been found that the composition according to the invention makes it possible to reconcile all of the criteria listed above.

Even if the mechanism of action of the bath could not be completely elucidated, one can nevertheless add a certain number of details and put forward some probable explanations.

Whatever its embodiment, the thermochemical treatment prior to oxidation produces on the surface of the parts a layer mainly composed of nitrides and / or carbonitrides, with also free iron in a minority proportion essentially present at the level of the defects of the layer. previous. It is probably this free iron which is responsible for the poor resistance to corrosion of parts that are simply nitrided, or carbonitrided.

Nitrates are, however, oxidizing agents of medium activity and if they are capable of oxidizing the free iron present in the layer, they would not however be powerful enough to destabilize the nitrides or carbonitrides.

On the contrary, a strong oxidant such as a dichromate or a chromate, or a permanganate could oxidize not only the free iron but also part of the nitrides, thus leading to the development of a more tight layer.

Too much oxidizing agent on the other hand would lead to a weakening of the layer by the effect of the residual stresses of which it is the seat with the appearance of cracks, corrosion and scaling, detrimental to its tribological properties. The presence of carbonates would go in the direction of a moderation of these oxidation reactions.

As for the temperature of the bath, below a certain threshold, fixed by the carbonate content of the bath, it does not make it possible to achieve sufficient fluidity of the molten salts, which on the practical level leads to significant consumption. of salts by removal with the charges of parts, as well as a significant decantation at the bottom of the crucible. Too high a temperature leads to premature degeneration of the bath, with a concomitant decrease in its effectiveness.

Finally, it should be noted that the oxidation operation, which is carried out according to reactions in heterogeneous liquid / solid phase, acts primarily and preferentially on the outer part of the nitrided, sulfonitrided or carbonitrided layer. It is therefore understandable that the morphology and the degree of porosity of this layer can have a significant influence on the intensity levels and on the kinetics of the reactions.

• une partie compacte au contact du substrat d'épaisseur comprise entre 6 et 12 µm
• une partie externe finement poreuse, d'épaisseur comprise entre 3 et 6 µm, le diamètre moyen des pores étant compris entre 0,1 et 2 µm.

As such, a preferred arrangement of the implementation of the invention consists in immersing in the oxidizing bath parts having previously undergone thermochemical diffusion with an adjustment of the parameters suitable for producing a layer in two parts:

• a compact part in contact with the substrate of thickness between 6 and 12 µm
• a finely porous external part, of thickness between 3 and 6 μm, the average pore diameter being between 0.1 and 2 μm.

The characteristics and advantages of the invention will emerge more clearly from the description which follows, relating to particular modes of implementation and accompanied by examples which will in particular make it possible to specify the respective roles of the various constituents of the oxidizing bath.

EXAMPLE 1 Description of a preferred embodiment and of the properties of the parts treated.

Parts made of non-alloy steel with 0.38% carbon were used, which were first subjected to a sulfonitriding treatment according to the teachings of the patents FR 2 171 993 and FR 2 271 307, by immersion for 90 minutes in a salt bath containing by weight 37% of cyanate ions and 17% of carbonate ions, the rest being alkaline cations K ⁺ , Na ⁺ and Li ⁺ , with in addition 10 to 15 ppm of S ^2- ions. The temperature of the molten salts was 570 ° C.

On leaving the bath, the pieces were immersed for 20 minutes in another bath, kept at the temperature of 475 ° C. and having the following composition, expressed in "sodium units":

CO ₃ ^2- : 13.1% NO ₃ ^- : 36.5% OH ^- : 11.3% Cr ₂ O ₇ ^2- : 0.1% Na ⁺ equivalent: 39%

The parts were then washed in water at pH 13.5 and then dried. Finally, they were the subject of characterizations, on the one hand in corrosion tests, on the other hand in friction tests.

a) Corrosion tests: the test pieces were in this case square plates, 50 mm side, protected on the edges by a varnish. Intensity / potential curves were drawn in aerated acid medium, which led to the following results:
(see TABLE on next page)

NATURE OF TESTS CORROSION POTENTIAL (or pricking) in mV / DHW * untreated 130 to 150 simply nitrided 175 to 225 nitrided then oxidized according to the process of the invention 1,000 to 1,300 * saturated calomel electrode.

It will be noted that at the values of 1,000 to 1,300 Mv / ECS obtained with the nitrided and then oxidized test specimens, the expression "corrosion potential" is practically an abuse of language, because at this level it is no longer a potential for piquration which is measured but rather the oxidation potential of the aqueous solution: the protection provided by the nitride / oxidized layer is practically perfect.

b) Friction tests: the test pieces in this case were rings with a diameter of 35 mm and parallelepipedic plates with dimensions 30 x 18 x 8 mm. The friction test is carried out dry, pressing the ring against the large face of the plate, with a regularly increasing load from the initial value of 10 daN and with a sliding speed of 0.55 m / s. The results obtained are summarized in the following table:

Nature of test pieces Duration of test (min) Cumulative wear of the two parts (µm) Coefficient of friction Not processed 2 seizure seizure simply nitrided 30 50 0.40 Nitrided / Oxidized according to the invention 60 35 0.25

EXAMPLE 2 Comparison of the process of the invention with the process described in patent FR 2 525 637.

This comparison was established from two oxidizing baths with a capacity of 120 kg of salts, both operating at the temperature of 460 ° C. and having the following respective compositions:

Composition of the Bath CO ₃ ^2- NO ₃ ^- OH ^- Cr ₂ O ₇ ^2- Na ⁺ equivalent according to FR 2 525 637 6.5 24.7 20.7 4.6 43.5 according to the invention 13.1 36.5 11.3 0.1 39

In each of these baths, a dozen loads of parts were treated, in this case axes of non-alloy steel with a diameter of 10 mm and a length of 100 mm, having a thread at one of their ends. Each load had 100 axes, for a total load weight of 10 kg.

The other operating conditions, as regards the prior nitriding, the duration of immersion of the parts in the oxidizing bath, and the final washing / drying operations, were the same as in Example 1.

The results obtained were qualified according to two reproducibility criteria, taking into account one of the color of the parts, the other of their resistance to corrosion in standardized salt spray.

As for the color, it can go from dark black (which is the optimum sought, for reasons of presentation of the treated parts), to reddish brown (which one seeks to avoid).

On all the parts treated, the following results were obtained:

According to FR 2 525 637 According to the invention Black color 65% 95% Brown color 35% 5%

With regard to corrosion tests, their duration corresponds to the period of time between the introduction of the parts into the salt spray enclosure and the appearance of the first puncture, the latter taking place in most cases at the level of the threaded part of the samples. This zone is in fact very disturbed on the metallurgical level, which generates numerous imperfections in the nitrided layer, which are as many possible primers of pitting corrosion.

The salt spray tests were carried out on samples of five pieces from each charge and the following results were obtained, after the appearance of the first bite:

According to FR 2 525 637 According to the invention Intervals for varying the durations of exposure to salt spray (hours) until the first bite occurs from 10 to 480 from 144 to 504 Average 245 280 Standard deviation 220 105

EXAMPLE 3 Influence of the content of dichromate or other oxidizing salts in the oxidation bath.

The procedure was as in Example 1, but varying from 0 to 1% the content of the oxidizing bath in Cr ₂ O ₇ ^2- anions.

In the absence of dichromate and whatever the bath temperature between 350 and 550 ° C., there is a significant dispersion of the color of the pieces between brown and black. In addition, in corrosion tests by plotting intensity / potential curves, low corrosion (or pitting) potentials are noted, varying from 100 to 300 mV / DHW, which is characteristic of the presence of leaks in the the passive layer.

The introduction into the dichromate bath makes it possible to find a regular black color of the parts and there is concomitantly an increase in the corrosion potential above 1000 mV / DHW.

The effect begins with 0.05% Cr ₂ O ₇ ^2- anion in the bath. The optimal influence is obtained with 0.2% Cr ₂ O ₇ ^2- ; beyond 0.2%, no further improvement is observed up to 0.5%; more than 0.5% of Cr ₂ O ₇ ^2- leads to a weakening of the layer which tends to flake.

The same effects, with the same contents, are obtained by replacing the dichromate by permanganate, or by chromate.

EXAMPLE 4 Influence of the nature of the components of the oxidation bath.

Three tests were carried out, operating as in Example 2, with baths whose compositions were:

CO ₃ ^2- NO ₃ ^- OH ^- Cr ₂ O ₇ ^2- Na ⁺ equivalent bath N ° 1 15.6 21.7 18 0.16 40.44 bath N ° 2 19.4 21.8 15.1 0.09 43.61 bath N ° 3 5.7 21.9 25.2 0 47.2 the composition of baths No. 1 and No. 2 being in accordance with the invention while the composition of bath No. 3 is not.

As in Example 2, the results obtained were qualified, on the one hand by the color regularity of the treated parts, on the other hand by their resistance to corrosion with standardized salt spray:

% OF PIECES WITH REGULAR BLACK COLOR bath N ° 1 96% bath N ° 2 70% bath N ° 3 45%

With regard to corrosion tests, batches of 5 test specimens taken from charges treated in each of baths 1 to 3 led to average holding times before the appearance of the first puncture, which are summarized in the following table:

Bath Average holding time before appearance of the first bite (hour) Standard deviation # 1 270 95 # 2 250 120 # 3 120 95

EXAMPLE 5 Conducting an oxidation bath.

An experimental bath of the same composition was used as in Example 1, in which loads of steel parts were treated regularly for several days. The following observations were made:

a) as the charges are treated, the carbonate content of the bath increases. This is due to the fact that when leaving the preliminary nitriding bath, the parts carry with them salts of said bath, which are composed essentially of carbonates and alkaline cyanates. These in turn transform into carbonates by reaction with the oxidizing salts.

• b) à leur sortie du bain oxydant, les pièces entraînent là encore des sels. Cette perte, jointe à celle liée à l'élimination des carbonates, se traduit par une baisse du niveau du bain oxydant.
• c) pour réajuster le niveau, on rajoute dans le bain du sel neuf, c'est-à-dire qu'on le réalimente en éléments actifs nitrates et bichromates (ou sels oxygénés équivalents). On trouve là l'explication du fait que, même s'ils ne sont présents dans le bain qu'en très faible quantité, les sels oxygénés ne disparaissent pas au fur et à mesure du traitement des charges de pièces et que leur effet est durable.
• d) en dehors de ce qui précède, la composition chimique du bain n'évolue que très peu dans le temps de façon naturelle.

When the saturation threshold is exceeded, the carbonates decant at the bottom of the crucible: they should therefore be removed.

• b) on leaving the oxidizing bath, the parts again entail salts. This loss, combined with that linked to the elimination of carbonates, results in a drop in the level of the oxidizing bath.
• c) to readjust the level, new salt is added to the bath, that is to say it is replenished with active elements nitrates and dichromates (or equivalent oxygenated salts). Here we find the explanation of the fact that, even if they are only present in the bath in a very small quantity, the oxygenated salts do not disappear during the treatment of the loads of parts and that their effect is lasting .
• d) apart from the above, the chemical composition of the bath changes very little over time in a natural way.

EXAMPLE 6 : Other modes of prior thermochemical diffusion.

If the sulfonitriding of steel parts is replaced by nitriding or carbonitriding in a salt bath, the same effects are observed as those previously described.

If now the thermochemical diffusion is carried out by ionic or gas route it is the same, except that the conduct of the oxidizing bath is modified compared to what was described in Example 5: in this case in fact it there is no longer the entrainment of the nitriding salts; the carbonation of the oxidizing bath, as well as its drop in level, are slower. To keep the oxidizing power of the bath constant, we are therefore led to make periodic additions of oxygenated salt, by regularly checking the composition of the bath by analysis.

Claims (5)

1. Method for improving the resistance to corrosion and wear of ferrous metal parts where the parts, which have previously undergone a thermochemical diffusion of the nitriding, sulphonitriding or carbonitriding type, are immersed in a bath of molten salts composed of carbonates, nitrates, hydroxides and oxygenated alkali metal salts chosen from amongst bichromates, chromates, permanganates, peroxycarbonates, iodates and periodates, characterised in that the relative anionic quantities by weight of carbonates, nitrates and hydroxides, expressed in relation to sodium salts and corresponding to the active phase, that is to say the liquid in the bath, are as follows: $${\text{11 < CO}}_{\text{3}}{}^{\text{2-}}\text{< 23}$$

   

   $${\text{19 < NO}}_{\text{3}}{}^{\text{-}}\text{< 37}$$

   

   $${\text{6 < OH}}^{\text{-}}\text{< 19}$$

   

   whilst the quantity by weight of oxygenated alkali metal salts, espressed in equivalents of Cr₂O₇ ^2-, is as follows: $$\text{0.05 < oxygenated anions < 0.5}$$

   
2. Method according to Claim 1, characterised in that the temperature of the bath is between 350 and 550°C.
3. Method according to Claim 1 or 2, characterised in that the temperature of the bath is between 450 and 550°C.
4. Method according to any one of the preceding claims, characterised in that the period of immersion of the parts in the bath is greater than 10 minutes.
5. Method according to any one of the preceding claims, characterised in that the thermochemical diffusion is carried out in such as way as to produce, on the surface of the part, a layer which has, in contact with the substrate, a compact part with a thickness of between 6 and 12 micrometres and on the outside a finely porous part with a thickness of between 3 and 6 micrometres, the mean diameter of the bores being between 0.1 and 2 micrometres.