KR100273924B1 - The wear and corrosion resistance promoting method for ferro-metal - 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

How to improve wear resistance and corrosion resistance of ferrous metal parts

The present invention relates to a method for improving the wear resistance and corrosion resistance of ferrous metal parts which guarantees highly reproducible results regardless of the type of part treated.

The great variety of surface treatments has been partly explained by the fact that the various conditions faced by technicians and the phenomena that cause surface degradation are extremely interdependent.

Moreover, the requirements are becoming more stringent, and a large number of parts must withstand some of the phenomena, such as friction, wear, corrosion, even shock and fatigue in the mechanical industry. In this case, the conventional method is insufficient to cope with the above situation.

At the request of the industry, apart from these improvements, technicians have become increasingly interested in research on "integrity" in other words in terms of quality, reliability and reproducibility.

It has been recognized that thermochemical diffusion treatment has the advantage of improving the wear resistance of ferrous metal parts. In particular, it is well-known as a nitriding process.

In addition, a significant improvement in wet corrosion resistance can be achieved by growing an oxide surface layer on top of the nitride layer.

There is much literature on such oxidation treatments. This document includes the following patents.

In 1976, French patent FR-A-2 306 268 describes an oxidizing salt bath comprising an alkali hydroxide having an alkali nitrate of 2 to 20% by weight. And such salt baths, which are preferably used at temperatures between 200 ° C. and 300 ° C., indicate that cyanide / cyanide is removed as oxidation by controlled cooling of the nitrided ferrous metal parts leaving the nitriding bath and the accompanying cyanides. Intended as a substitute.

The French patent FR-A-2 463 821 of 1980 has a part nitrided in the same alkali hydroxide hydroxide containing 2 to 20% by weight alkali nitrate with a temperature of 250 ° C to 450 ° C for a sufficiently long period of 15 to 50 minutes. If soaked, the corrosion resistance of these parts is markedly increased. This French patent and especially its examples on baths comprising 37.4w% sodium hydroxide, 52.6w% potassium hydroxide and 10w% sodium nitrate have shown in practice that the exposure time prior to the appearance of doubling corrosive traces. Improved salt spray corrosion resistance reflected accordingly.

French patent FR-A-2 525 637 in 1982 describes a method of treating these parts in an oxidized salt bath to improve the corrosion resistance of the sulfur-containing ferrous metal parts. The specification teaches the immersion of parts in an oxidizing bath consisting of alkali hydroxide, alkali nitrate and / or nitrite alkali and optionally alkali carbonate and additionally 0.5w% to 15w% of a powerful oxidant, in which case oxygen oxides of alkali metals Normal oxygen reduction potential is less than -1 volts relative to the hydrogen reference electrode. The oxygen flame is composed of dichromate, permanganate, carbonate peroxide, iodide and periodate, and sodium and potassium which are alkali metals. The method described in this French patent is further characterized in that the gas containing oxygen is blown into the salt bath and the insoluble particles in the bath are maintained at 3 w% or less. Thus, this method can achieve better performance. This method can improve the corrosion resistance of the parts by almost four times, and also improve their anti-adhesion resistance to dry friction without deterioration in wear resistance or fatigue resistance.

However, it became clear that this performance could not be achieved at the level of reliability and reproducibility required by the industry. In the laboratory it is relatively difficult to detect changes in performance. However, the amount of performance change is more prominent when dealing with industrial size quantities. These are particularly evident when large quantities of small parts have to be treated “in large quantities” or when the surface condition of the parts to be processed is incomplete: pressing and stamping burrs, crimping and bending wrinkles and weld defects. The presence of such imperfections has the potential for corrosion starting sites.

Uneven corrosion resistance is not entirely acceptable for parts such as automotive jack pistons or damper piston rods and wipers and starting motor spindles. The long-term solution is to adapt the bath from time to time as needed, depending on the observed changes in behavior. Such a solution is unsatisfactory to meet industrial requirements, as stated earlier. This gives the need to find new solutions. The inventor has carried out in-depth research to reach an improvement method capable of controlling the impurity encountered on an industrial scale.

It is therefore an object of the present invention to provide a method of combining a passivation by oxidation which can ensure a high reproducibility and therefore a minimum dispersion, and which can significantly improve the wear resistance and corrosion resistance of the ferrometallic parts. .

To this end, the present invention uses the nitriding, nitriding or nitrocarburizing thermochemical diffusion technology to immerse the pre-treated iron-based metal parts in a molten salt bath containing carbonate, nitrate, hydroxide and oxygenated alkali metal salts to A method for improving abrasion resistance is provided wherein the relative anion weight ratio of carbonate, nitrate and hydroxide corresponding to the active phase in terms of sodium salt equivalent and liquid in the bath is

11 〈CO ₃ ^2- 〈23

19 <NO ₃ ^- <37

6 <OH ^- <19 and

The weight ratio of the oxygenated alkali metal salt converted into the equivalent of Cr ₂ O ₇ ^2- is 0.05 <oxygenated anion <0.5.

The present invention includes all compositions containing, alone or in combination, other alkali metal salts than sodium in percentage as equivalent to sodium salt equivalents as specified above.

In the remainder of the specification, for ease of understanding, all concentrations of equivalent sodium salts referred to as "sodium units" provide a standard for various mixtures regardless of the metal cations involved (e.g., Na ⁺ , K ⁺ , Li ⁺ ). Converted in weight percent.

The temperature of the bath is 350 ° C. to 550 ° C. and 450 ° C. to 530 ° C. is preferred, and the part is immersed in the bath for at least 10 minutes.

From a qualitative point of view, the composition of the present invention is of the same kind as specified in the French patent FR-A-2 525 637 described above. But in quantitative terms, this is clearly distinguished from the latter.

This can be explained as follows.

It has been known that the main cause of dispersion observed previously is due to the presence of defects in densification in the nitrided and oxidized layer.

The improvement in wear resistance is known to be due in large part to the nitrided layer while the improvement in corrosion resistance depends on both the nitrided and oxidized layers. These two layers provide anode protection. This protective effect depends directly on the integrity of the boundary layer. The requirement is for a continuous, impermeable oxide surface layer. The layer formed in the oxidation bath is known to essentially constitute a fully inert Fe ₃ O ₄ iron oxide. Therefore, there is no possibility to improve the properties of this layer, and the requirement is to guarantee the impermeability of this layer. Therefore, it is important to find a way to obtain this boundary layer in all cases, ie at the same or several consecutive charges, so that the intended result is obtained for all types of parts and all parts. This was not possible with the prior art desires as apparently obtained in the examples described below.

The composition of the oxidation bath of the present invention is conventionally known by the fact that it is formulated with another concentration of nitrates and hydroxides and consists in a substantially lower percentage of strongly oxidizing salts than taught in French patent FR-A-2 525 637 It is distinguished from swear words.

Note that the cationic group of the bath is defined only by the nature of each metal, namely alkali metal. It is not important whether only one or several cations are present and their relative proportions do not affect the results even if at least two cations are present at the same time.

It is no longer necessary to understand the complexity of molten salt baths and the mechanisms that work in the baths and to emphasize the difficulty of predicting their behavior by theory based on logical reasons. Therefore, the formalization of the bath of the present invention and its operating conditions are determined by experiment. The following evaluation criteria were adopted for the purpose of corrosion resistance of treated parts, their wear resistance, bath fluidity, color of parts and dispersion of the results.

Carefully chosen experiments have shown that the composition of the present invention can meet all of the above criteria.

Although the mechanisms acting in bathing cannot be explained in detail in all respects, some evidence and likely explanations can nevertheless be given.

Regardless of how this works, the first thermochemical treatment produces a layer consisting primarily of nitrides and / or carburized nitrides with a smaller percentage of free iron on the surface of the part, mainly where the underlying layer is defective. Perhaps these free irons are simply responsible for the poor corrosion resistance of nitrided or carburized parts.

But nitrates are mildly active oxidants, and although they can oxidize the free iron present in the layers, they are not strong enough to destabilize the nitrides or carburites.

On the other hand, powerful oxidants such as dichromate, chromate or permanganate can oxidize some nitrides as well as free iron, making the layer more impermeable.

On the other hand, excessive oxidants can make this layer vulnerable due to all residual cracks, internal stresses that cause the onset and scaling of corrosion, which are detrimental to the frictional properties of the part. The presence of carbonates induces this oxidation reaction in a slightly gentle direction.

Sufficient fluidity of the molten salt, with regard to the temperature of the bath below a certain threshold (threshold) set by the amount of carbonate in the bath, as well as a significant settling at the bottom of the crucible, resulting in the high consumption of salt through the removal of these together with the parts. It is impossible to achieve. Excessive temperature causes premature deterioration of the bath with a decrease accompanying its efficiency.

Finally, the oxidation reaction occurring in the heterogeneous liquid / solid phase first acts on the outside of the nitrified and nitrified or carburized layer. Therefore, the shape and porosity of these layers have a negligible effect on the reaction rate and strength level.

In this regard, in a preferred embodiment of the present invention, the parts previously immersed in the

A dense part having a thickness of 6 µm to 12 µm in contact with the substrate, and

Thermochemical diffusion treatment with a controlled variable to form two sublayers consisting of finely pore outer portions having a thickness of 3 to 6 μm and an average pore diameter of 0.1 to 2 μm.

The features and advantages of the present invention will become apparent from the following descriptions relating to the accompanying embodiments, specifying the function of each of the various components of the oxidation bath, among other embodiments of the invention.

Example 1 Preferred Embodiments of the Present Invention and Characteristics of Processed Parts

The unalloyed 0.38% carbon steel component was first immersed in a salt bath for 90 minutes and subjected to sulphur nitriding under French patent FR-A-2 171 993 and French patent FR-A-2 271 307. At this time, the salt bath used is 37w% cyanate ion, 17w% carbonate ion, the residue is composed of alkali cations such as K ⁺ , Na ⁺ , Li ⁺ with S ² ions in the range not to deviate from 10ppm to 15ppm. At this time, the temperature of the molten salt was 570 degreeC.

The parts removed from the bath were then immersed in another bath for 20 minutes in the bath having the following composition in terms of "sodium units" and held at 475 ° C.

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 with water at pH 13.5 and dried.

Finally, the corrosion and wear test characteristics of these parts were investigated.

a) Corrosion test: The test specimen is a 50mm ² square plate with protective varnish on the edge. Intensity / potential curves were plotted against the degassed acid vehicle which showed the following results.

* Saturated mercury chloride electrode

The reduced "corrosion potential" for values of 1000 Mv / ECS to 1300 Mv / ECS obtained with oxidized test specimens after nitriding is somewhat approximated at this level because it is the oxidation potential of the aqueous solution rather than the measured fitting potential.

The protective film provided by the nitride / oxide layer is actually perfect.

b) Abrasion test: Test specimens are 35 mm diameter rings and 30 mm × 18 mm × 8 mm equilibrium hexahedral plates.

The abrasion test was carried out in dry condition by pressing the ring against the larger side of the plate with increasing load from the initial value of 10 decaN at a sliding speed of 0.55 m / s.

The results obtained are summarized in the table below.

Example 2

Comparison of the method of the present invention with the method described in French patent FR-A-2 525 637

Comparisons were made based on two oxidation baths, each with the following composition and a salt capacity of 120 kg with a temperature of 460 ° C.

About 10 parts were processed in each bath.

The part was an unalloyed steel spindle with a diameter of 10 mm and a length of 100 mm with threads at one end. Each charge consists of 100 spindles with a total weight of 10 kg.

Other operating conditions (initial nitriding, duration of immersion of parts in the oxidizing bath, final flush / dry operation) are the same as in Example 1.

The results obtained were graded according to two criteria for reproducibility.

One was based on the color of the parts and the other on their standardized salt spray corrosion resistance.

The color ranged from dark black (optimum for the appearance of the treated parts) to reddish brown (avoided).

The results obtained for all treated parts are as follows.

For corrosion tests, the duration of the test corresponds to the time that the component is inserted into the salt spray zone and, in most cases, until the first fitting appears in the threaded portion of the specimen. This area is very unusual from a metallurgical point of view and causes numerous defects in the nitride layer, providing many possible places for corrosion fittings.

The salt spray test was carried out on five parts from each charge and the following results were obtained after the first fitting appearance.

Example 3

Effect of Dichromate or Other Oxide Salts in Oxidation Baths

The procedure is the same as in Example 1 except that the Cr ₂ O ₇ ^2- anion concentration in the oxidation bath is changed from 0 to 1%.

Without dichromate and when all bath temperatures were between 350 ° C. and 550 ° C., there was marked dispersion of the part collar in brown to black.

In addition, low corrosion (i.e. fitting) potential was measured between 100mV / ECS and 300mV / ECS in the corrosion test by indicating the intensity / potential curve, thereby evaluating the characterization of the presence of impermeable defects in the passivation layer. .

The introduction of dichromate into the bath makes it possible to achieve normal blackening of the parts and the accompanying increase in corrosion potential above 1000 mV / ECS.

This effect started with 0.05% Cr ₂ O ₇ ^2- anion in the bath and the optimum was at 0.2% Cr ₂ O ₇ ^2- ;

There was no better improvement from 0.2% or more to 0.5%.

At 0.05% Cr ₂ O ₇ ^2- this layer is fragile and attempts to form a scale.

The same effect was obtained by replacing dichromate with permanganate or chromate at the same concentration.

Example 4

Effect of Properties of Oxidation Bath Components

Three tests were carried out using the procedure of Example 2 and a bath having the following composition.

Bath numbers 1 and 2 were according to the invention, whereas the composition of bath number 3 did not follow the invention.

The uniformity of the treated part collars and their standardized salt spray corrosion resistance were investigated as a result obtained as in Example 2.

According to the corrosion test, a set of five test specimens taken from the amount of charge treated in each of the bath numbers 1 to 3 represents the average time before the first fitting appearance as summarized in the following table.

Example 5

Oxidation bath control

The experimental bath was prepared in the same composition as in Example 1, and the loading amount of the steel parts was treated for several days to obtain the following observation.

a) The amount of carbonate in the bath increased as more charge was processed.

This is because the components leaving the nitriding bath prior to such treatment are usually accompanied by a salt consisting of alkali cyanate and alkali carbonate.

Cyanate reacts with oxidized salts and turns into carbonates.

When the saturation threshold (threshold) is exceeded, the carbonate precipitates at the bottom of the crucible and must be removed.

b) The parts also carry salts when leaving the oxidizing bath. These losses, combined with the losses associated with the removal of carbonates, lower the level of the oxidation bath.

c) new salts are added to the bath to fill to the same level.

That is, the bath is filled with active nitrates and dichromates (or equivalent oxygenates).

Although they are only present in the bath in very small quantities, this explains why oxygen flames do not disappear as the charge of the part is processed and also why their effects last long.

d) Apart from what is stated above, the chemical composition of the bath does not change inherently with time.

Example 6 Another method of preliminary thermochemical diffusion.

The same effect is achieved as stated above if the nitrification of steel parts is replaced by nitriding or nitrification in the salt bath.

The same applies if the thermochemical diffusion is via an ionic or gaseous path, except that the control of the oxidation bath has changed compared to that specified for Example 5.

In other words, when nitride salts are no longer accompanied, the slowdown in the level of carbon chloride and oxidation bath is further slowed down. Therefore, it is required to add oxygen flame to the bath periodically to keep the bath's oxidizing power constant.

Claims (6)

Improving corrosion resistance and wear resistance in such parts comprising immersing pretreated iron-based metal parts in a molten salt bath containing carbonates, nitrates, hydroxides and oxygenated alkali metal salts using nitriding, nitriding or nitrification thermochemical diffusion techniques In this method, the relative anion weight ratio of carbonate, nitrate and hydroxide corresponding to the active phase converted into sodium salt equivalent and liquid in the bath is 11 〈CO ₃ ^2- 〈23 19 <NO ₃ ^- <37 6 <OH ^- <19 and How the ratio by weight of oxygenated alkali metal salt in terms of equivalent weight of Cr ₂ O ₇ ^2-, characterized in that 0.05 <oxygenated anions <0.5. The method of claim 1 wherein the temperature of the bath is from 350 ° C. to 550 ° C. 7. The method of claim 1 wherein the temperature of the bath is from 450 ° C. to 550 ° C. 7. The method of claim 1 wherein the part is immersed in the bath for at least 10 minutes. The method of claim 1 wherein the oxygenated alkali metal salt is selected from dichromate, chromate, permanganate, percarbonate, iodide and periodate. 2. The thermochemical diffusion of claim 1, wherein the thermal chemical diffusion is between 6 micrometers and 12 micrometers in thickness, the surface layer having a dense portion in contact with the substrate over the part and having a thickness of 3 micrometers to 6 micrometers on average. And a microporous portion having a pore diameter of 0.1 micrometers to 2 micrometers on the outside.