ITRM20000699A1 - PROCEDURE FOR SURFACE TREATMENT OF TITANIUM, PRODUCTS AND MANUFACTURES MADE OR COATED IN TITANIUM AND TREATED ACCORDING TO SUCH PROC - Google Patents

Abstract

Process for the surface treatment of Titanium comprising a heating step for items made of or coated with Titanium, for a present time interval at a temperature higher than the phase transition temperature of Titanium and in the presence of an atmosphere comprising one or more inert gases and a percent of Carbon gas. The invention further relates to items (3) made of or coated with Titanium treated according to such process.

Description

DESCRIPTION of the industrial invention entitled: "Procedure for the surface treatment of Titanium, products and artifacts made or coated in Titanium and treated according to this procedure"

DESCRIPTION

The present invention refers to a process for superficially treating a product coated or made of titanium, in order to impart improved mechanical properties to the surface itself.

Among the mechanical properties of titanium surfaces, non-sticking stands out, which makes this material suitable for applications in which products made or coated in titanium are immersed or are in any case put into contact with liquid or pasty fluids, in a condition in which it is required. a minimum friction between fluid and surfaces or a dirtiness as low as possible.

Possible applications are therefore available in many industrial sectors, such as the food and oil sectors, in the manufacture of paper, glues and similar pasty aggregates, for the manufacture of vessels, agitators, valves, augers, pipes and the like.

Also think of domestic applications, such as in coated non-stick pans and in all applications that involve the use of plastic substances, such as PTFE, subject to wear and deterioration.

The excellent non-stick characteristics have not been fully exploited so far because the surfaces in question, if subjected to continuous stress over time, are subject to wear and tear, phenomena that force costly replacement of components and which can lead to pollution.

The need is therefore felt to have a process for treating these products and manufactured articles on the surface, in such a way as to reinforce the surface without negatively affecting the appreciated non-stick properties.

The technical problem underlying the present invention consists in providing a process such as to satisfy said need. This problem is solved by carburizing the titanium surface. In fact, the process provided according to the present invention comprises a heating phase for products made or coated in titanium, for a predetermined period of time at a temperature not lower than 50 ° C to the phase transition temperature of the titanium and in the presence of a atmosphere comprising one or more inert gases and a percentage of carbon gas.

According to a preferred version of the process, this temperature is higher than 800 °, advantageously comprised between 850 ° C and 1050 ° C.

The pressure at which this step is carried out is reduced, preferably lower than 5.0 mbar and preferably lower than 1.0 mbar, for example equal to about 0.2 mbar, i.e. in vacuum conditions beyond the controlled atmosphere of process .

The length of the heating period essentially depends on the temperature and carbon content of the atmosphere: for carbon gases in a percentage greater than 25% and for temperatures equal to or greater than 850 ° C, treatment times are no less than 30 minutes. , preferably at least 1 hour and can extend up to 6 hours and beyond.

The preferred percentage for the volumetric concentration of the carbon gas is higher than 30%, advantageously equal to about 40%.

The possible inert gases that can be used are Nitrogen and Argon, while among the carbon gases the compounds of carbon and hydrogen are obviously preferred, having a molecular weight lower than 60. Three particularly effective gases are methane (CH4), propane ( C3H8) and acetylene (C2H2), but the list obviously does not exclude other gases of the same families.

The artifacts and products made of titanium or coated with titanium show an enrichment of carbon in the surface layers, an increase that leads to a marked improvement in the mechanical performance of the surface.

However, it is understood that this treatment does not involve excessive embrittlement of the surface itself.

In the case of the procedure described above, the effective penetration of the carbon atoms extends to a depth of 120 μm from the surface, with an appreciable content of carbon, oxides and titan nitrides.

It is understood that the invention refers to all those manufactured articles comprising a non-stick surface based on Titanium, treated according to the process described above. Only for the breadth of the possibilities of use, a list of possible products is not provided, but only some examples including bodies and shutters of valves, agitators, mixers, coatings for containers, components of machines operating on liquid or pasty fluids, augers , ducts and their coatings, parts of machines capable of sliding in contact with each other, slides, guides, wheels and others.

In the comparative examples that will be described below, titanium samples will be subjected to a heating step as defined above and each specimen will be subjected to measurements to determine the effects of the treatment on the mechanical-physical performance.

An example of application of the process itself will then be described, relating to a valve obturator.

Comparative Examples

Some carburetion treatments were carried out using a furnace for high vacuum treatments whose main characteristics are shown below:

* Cylindrical internal chamber dimensions: diameter 0 = 250 mm; height h = 400 mm.

* Maximum working temperature T = 2000 ° C (Tungsten resistors)

* Inert gases: Nitrogen and Argon

In total, approx. twenty-five treatments, the most significant of which are described in Tables 1.1 and 1.2. Specimens with dimensions equal to 200 x 100 x 3 mm were used, previously cleaned on the surface with acetone. Ten treatments were carried out in an atmosphere composed of Argon and Methane at 40%, the rest were carried out in an atmosphere of Argon and Acetylene at 40%. The working pressure for the furnace is fixed at 0.2 mbar, the treatment temperature has been changed (from 800 ° C to 1050 ° C), and the duration of the treatment (from 1 to 6 hours).

Tab. 1.1 - Carburation treatments carried out with methane.

Tab. 1.2 - Carburation treatments carried out with acetylene

Techniques such as optical microscopy, spectroscopic analysis, hardness measurements, surface erosion and spectroscopy procedures, X-rays, XPS, measurement of hydrogen content, wettability and tests were used to evaluate the effect of carburizing treatment on titanium sheets. wear resistance.

Optical microscope observations and spectroscopic analysis

As a first approximation, the surface carburization of the samples was evaluated by means of spectroscopic analyzes, through these techniques it is not possible to perform a quantitative analysis of the carbon, however it is possible to carry out a qualitative evaluation. Fig.l (CSM - 4.1) shows the spectra of Ti as is (a) and of sample A (b) representative of all the carbureted samples in question.

From the comparison between the heights of the peaks (note the first peak on the left of the spectrum, almost non-existent for the titanium sample as it is and higher in sample A), it appears that carbon is present in an equivalent quantity on the surfaces of the plates which have underwent the carburetion treatment and is greater than the carbon present on the surface of the Ti as it is. The treatments carried out have therefore increased, regardless of time and temperature, the concentration of surface carbon.

The sections of these plates were observed under an optical microscope following a metallographic attack conducted with Kroll's reagent (HF + HN03 + distilled water) which highlighted the structure of the materials and the thickness of the carburation layer.

From micrographs taken from the samples it was possible to observe that all the samples treated at 800 and 850 ° C, therefore below the phase transition temperature of titanium, show a polygonal structure, of an equiaxed type similar to that of Ti CP; the samples treated at 950 and 1050 ° C show a different structure with gods (more elongated grains, which indicate a partial transformation from the a-type structure to the P-type (acicular structure α + β).

In some micrographs the carburation layer was evident, of variable thickness according to the temperature and the duration of the treatment. Sample D shows a layer with a thickness of approx. 120 μm. In reality, subsequent EDS analyzes showed the presence of a carburized layer of approx. 15 μm, a nitride-containing phase of approx. 100 μm and then, for another 5 μm, up to 120 μm an additional carburation band.

All samples treated at 950 ° C show an evident cemented layer with a thickness of approx. 20-40 μm.

For the plates treated at temperatures of 800 and 850 ° C it is not possible to highlight, from the micrographs, the presence of any surface carburization.

Hardness measurements

The hardness measurements, conducted in section, allow to identify a hardness profile from edge to edge in order to evaluate the possible carburetion and its thickness.

The first measurement was made at approx. 10 - 25 μm from the surface, the subsequent ones every 100 μm up to 1 mm thick, then at half thickness (1.5 mm) and in the same way starting from the opposite surface. The most significant measures are shown in table 1.3.

In the case of samples in which the thickness of the carburized layer is of the order of a few μm, these measurements are not significant for the purpose of identifying the cemented band as they are carried out in the matrix and not in the carburized layer. For example, samples A, H, M and N, treated at 800 ° C for 1 or 2 hours, do not reveal an increase in hardness compared to titanium as it is. Tab. 1.3 - Results of micro-hardness measurements in section

These measurements show that sample D, treated at 1050 ° C for 6 hours, underwent an evident hardening process reaching a hardness under the skin of about 900 HV, far greater than that of Ti CP as it is, which generally shows a hardness around 150-200 HV. This sample, 100 μm from the surface, still shows a high hardness, equal to 593 HV.

Samples G, I, L, 0, P and Q, carburized at 850 and 950 ° C, also show a surface hardening of 400-500 HV, which however rapidly decreases to 200 HV at approx. 100 μm from the surface.

Other samples also show a slight hardening, much lower than that suffered by sample D.

Surface analysis

An analysis of the type that provides for surface erosion and spectroscopic measurements was performed on the carburized samples in order to evaluate the thickness of the carbide layer and the possible presence of unwanted elements such as O and N, in the various conditions heat treatment.

The analysis was also carried out on a sample of titanium as it is (TQ). used as a reference.

The results showed the following:

* In the TQ sample there is only a slight carbon surface contamination and a passivation oxide film;

* The graphs show that all samples have a carburized layer;

* The thickness of the carburized layer for the various samples is shown in table 1.4.

Tab. 1.4 - Thickness of the carburized layer of the titanium samples

A significant example is represented by sample E, in which the presence of carbon, oxygen and nitrogen is detected up to a depth of approx. 20 μm. The percentage by weight of oxygen in the carburized layer is approx. 3% is always lower than that of carbon. There is also nitrogen in quantities of approx. 1%.

The subsequent qualitative analysis of the samples by X-rays confirms the presence of the elements mentioned above, in particular it reveals the presence of a phase of titanium carbide nitride in which the stoichiometric ratio between carbon and nitrogen is not less than 0.7: 0.3. The presence of TiO is also highlighted.

XPS analysis was also performed on some samples by which the chemical composition of the first surface layers (max. Up to 60 nm), the various phases present and their variation as a function of depth were evaluated. It has been observed that titanium is present on the surface mainly as TiO2, and to a lesser extent as TiC or TiN; going in depth, the quantity of Ti02 decreases to the detriment of that relating to TiC and / or TiN. On the surface, carbon is mainly present as graphite or hydrocarbon; in depth the component relative to the carbide grows.

By way of example, table 1.5 shows the chemical composition of the first atomic layers of sample D.

Tab.1.5- Chemical composition in atomic percentage of sample D on the surface and at various depths

Hydrogen content

Measurements of the hydrogen content were performed on some carburized samples. It is observed in table 1.6 that all the samples show a hydrogen content lower than the maximum limit defined by the ASTM standards, set at 150 ppm. In particular, samples M and N treated at 800 ° C for 1h and 2h respectively show the lowest hydrogen content.

Tab. 1.6 - Hydrogen content of the carburized titanium samples

Wettability measures

Wettability measurements with deionized water were performed on all samples using a cleaning procedure. For each lamina, 6 repetitions were performed, except for insignificant measurements; the values obtained are shown in table 1.7. Despite the careful cleaning of the samples, there is a high dispersion of the data; in some cases a standard deviation of up to 27 ° was calculated. Observing the measurements obtained, however, it can be noted that for samples carburized in an acetylene atmosphere (M, N, 0, P and Q) the standard deviation is generally more contained.

Tab. 1.7 - Wettability measurements on carbide titanium samples

Some contact angle measurements were performed using a method different from the one used up to now, which allowed to obtain a greater reproducibility of the data. The method provides for the measurement of the advancing and receding angles, which are more stable than the static angle measured with the sessile drop method. Table 1.8 shows the results of the contact angle measurements performed with water.

Tab. 1.8 - Measures of wettability

For these samples only the mean value was provided

It can be observed that for samples A, I, N and Q the carburetion treatment caused an increase in the contact angle with respect to TQ and therefore improved the anti-adhesion properties of the

material.

Wear resistance tests

Adhesive wear tests were performed to evaluate the effect on wear resistance of the different treatments carried out on the titanium sheets.

To carry out this test, the SRV-Optimol ("Fretting") tribometer was used, with a sphere-plane contact geometry. The foil used is made of titanium while the sphere is of 100Cr6. The test consists in the oscillation (10Hz) of the sphere on the plate, with an amplitude of 3 mm and applying a load of 20N. The test duration is 15 minutes. Wear resistance is evaluated by measuring the weight loss (ΔΡ) of the materials after testing. The test conditions were chosen in such a way as to be able to discriminate the effect of the different treatments performed on titanium and the results obtained are shown in Table 1.9.

Tab. 1.9 - Results of the wear test

All the samples reveal an improvement in the tribological properties compared to Ti TQ, showing a significantly lower weight loss.

Samples treated at 950 ° C, for times between 1 and 6 hours, in both methane and acetylene atmospheres show a high resistance to wear; in particular sample E, carburized in a methane atmosphere at 950 ° C for 1 hour, shows a weight loss of only 0.0003 g, while samples L, P and Q show a weight loss varying between 0.0012 and 0.0016 g.

Sample G, treated at 850 ° C for 6h, also showed an improvement in the tribological properties, with a weight decrease of only 0.0012 g.

With reference to Figure 2, a manually operated valve 1 is shown. It is understood that the present application example is quoted for purely illustrative purposes, without any limitative intent.

This valve comprises a valve body 2 inside which a conduit is formed. In it, the valve 1 comprises an interceptor member, in particular a shutter capable of rotating by 90 ° between a closed or intercepted position and an open position, thanks to a knob 4.

The intercepting member in question must be intended for continuous use in which it must resist a certain pressure and is subjected to mechanical fatigue and friction with the other surfaces of the valve body.

Furthermore, the surface of the shutter must have a wettability as low as possible, so that the presence of the valve, with the duct open, does not constitute an excessive pressure drop.

The shape of the interceptor is foil or plate. Conveniently, this plate is made of titanium and is subsequently treated according to the procedure described above, for example in the same way as sample D of the comparative examples.

In this way, this member will have a prolonged resistance, greater reliability and will introduce a reduced pressure drop thanks to the reduced friction between its surfaces and the liquid that engages the valve.

A person skilled in the art, in order to satisfy further and contingent needs, may make numerous further modifications and variants to the above-described process and to the products and manufactured articles treated with it, all of which however fall within the scope of protection of the present invention, as defined by the claims attached.

Claims (12)

CLAIMS 1. Process for the surface treatment of Titanium comprises a heating phase for products made or coated with titanium, for a predetermined period of time at a temperature not lower than 50 ° C to the phase transition temperature of the titanium and in the presence of a atmosphere comprising one or more inert gases and a percentage of carbon gas. 2. Process according to claim 1, wherein the heating temperature is higher than 800 °. 3. Process according to claim 2, wherein the heating temperature is between 850 ° C and 1050 ° C. 4. Process according to claim 1, wherein the pressure at which this heating step is carried out is lower than 5 mbar, and preferably lower than 1 mbar. Process according to claim 1, wherein the duration of the heating period is at least 30 minutes and preferably at least 1 hour. 6. Process according to claim 1, wherein the percentage for the concentration of the carbon-containing gas is greater than 30% by volume. 7. Process according to claim 1, in which the inert gases that can be used are Nitrogen and Argon. 8. Process according to claim 1, wherein said carbon gases are composed of carbon and hydrogen. 9. Process according to claim 8, wherein said carbon and hydrogen compounds have a molecular weight lower than 60. 10. Process according to claim 9, wherein said usable carbon gases are selected from the group comprising: methane (CH4), propane (C3H8) and acetylene (C2H2). 11. Product or article (3), made of Titanium or comprising at least one surface coated with Titanium, treated by a process according to one of the preceding claims. 12. Product or manufactured article (3) according to the preceding claim, for example bodies and shutters of valves, agitators, mixers, coatings for containers, components of machines operating on liquid or pasty fluids, augers, ducts and their coatings, parts of suitable machines to crawl in contact with each other, slides, guides, wheels.