EP0789086A2 - Process for preparing a metallic material having high hardness and its use - Google Patents

Abstract

Manufacturing a hard, wear-resistant alloy comprises isostatically hot-pressing an alloy powder containing mainly at least iron, nickel and/or chromium, with pressure, temperature and treatment duration chosen to obtain a material with fine grains, free of fissures and hollows. Also claimed are the use of the above method to manufacture a solid piece of the alloy; and for applying a coating on a metal substrate by hot-pressing.

Description

The invention relates to a method of manufacturing a metallic material of high hardness, resistant to wear and to applications of this method, in particular to the coating and to the production of parts undergoing friction in service, such as lids and tap seats. More particularly, the invention is used to obtain coatings or parts of wear-resistant alloy which are used in the construction of nuclear power plants and which must contain as little cobalt as possible.

In nuclear power plants, parts that are highly stressed in friction are used, such as lids or valve seats, parts for pumps for circulating fluid in the nuclear power station, pawls of the control cluster mechanisms. or support pins located in the equipment inside the nuclear reactor vessel.

These parts can be made of a material such as stainless steel covered with a wear-resistant coating or even be made in solid form of a wear-resistant material.

It is known to produce wear-resistant coatings on friction parts, by melting and depositing a coating metal by a process such as the TIG (Tungsten Inert Gas) welding process, the PTA (Plasma d 'Arc Transferred') or using a blowtorch.

It is known to use cobalt-based alloys, such as Stellites, to produce wear-resistant coatings by one of the deposition processes mentioned above.

When cobalt-based alloy coatings are made on friction parts used in nuclear power plants, these coated parts are likely to release cobalt under the effect of wear and corrosion, this cobalt can be entrained by a fluid such as the reactor coolant coming into contact with the friction part. The cobalt entrained by the reactor cooling water passes through the core of the reactor in which it is activated. The cobalt then becomes one of the most important radioactive elements present in the nuclear power station, so that this cobalt is at the origin of a significant part of the dose rate which one is confronted with during repair or maintenance operations. , during the nuclear reactor shutdowns.

It is therefore very important, in order to reduce the doses received by the maintenance personnel of nuclear power plants, to reduce or even eliminate the cobalt-based alloys used in nuclear power plants.

It has therefore been proposed to use alloys based on nickel or iron and containing chromium to replace the wear-resistant cobalt alloys.

FR-A-2,405,306 has proposed, for example, an alloy based on nickel and comprising cobalt only in the form of a residual element, which can have, under certain conditions, a hardness comparable to that of a cobalt alloy such as Haynes Stellite n ° 6.

Such a nickel alloy can be used in the form of powder, welding rods or coated or filled wires for the production of coatings, in particular by the TIG or PTA processes.

It has turned out, however, that the coatings obtained by these processes using the nickel-based alloy of FR-A-2,405,306 did not exhibit wear resistance characteristics quite comparable to those of cobalt alloys.

The same is true of the massive parts which can be obtained by melting the alloy in the induction furnace and molding.

In general, the known nickel alloys containing in particular chromium as an alloying element and whose compositions are adjusted to obtain high hardness, when they are used in the form of coatings obtained by a process such as the PTA process, do not exhibit wear resistance characteristics that are entirely comparable to those of cobalt alloys such as Stellites.

Iron-based alloys containing in particular chromium as an alloying element and whose compositions are adjusted to obtain high hardness are difficult to use and generally have insufficient corrosion resistance, when they are used as a base material or as a coating for producing parts used in an environment such as the primary circuit of a pressurized water nuclear reactor.

It may also be desirable, when it is possible to use cobalt alloys as an anti-wear material, to increase the hardness and the wear resistance of these alloys.

The object of the invention is therefore to propose a method of manufacturing a metallic material of great hardness, resistant to wear, which has characteristics comparable to those of cobalt alloys, as regards resistance to wear and corrosion, in particular in a nuclear medium, even if it contains cobalt only in the state of a residual element and which may even have improved wear resistance characteristics compared to alloys of cobalt.

For this purpose, a powder of a metal alloy is submitted mainly containing at least one of the iron and nickel elements as well as chromium, at high temperature isostatic compression, the pressure, temperature and duration of the isostatic compression treatment being defined to obtain a homogeneous and isotropic material with fine grains, substantially free from cracks and defects volume.

The method according to the invention can be used in particular for producing wear-resistant coatings on parts such as stainless steel parts.

In order to clearly understand the invention, we will now describe, by way of nonlimiting examples, with reference to the appended figures, several embodiments of the method according to the invention, used in particular for producing resistant coatings wear on friction parts used in nuclear power plants.

FIG. 1 is a diagram giving the rate of wear of materials of different compositions obtained by the process according to the invention and of a material obtained by a coating process according to the prior art.

FIG. 2A is a micrograph obtained with an electron microscope of a coating material according to the prior art.

FIG. 2B is a micrograph obtained with an electron microscope of a hard material produced by the method of the invention.

To carry out the implementation of the method according to the invention and according to a preferred embodiment, use is made of a powder of an alloy containing mainly iron or nickel as well as chromium.

The composition of the alloy of iron and / or nickel and chromium can be variable according to the characteristics of hardness and in particular of hardness to hot sought for the product made from the powder of the alloy.

In all cases, the powder is produced by spraying with a jet of inert gas, for example an alloy produced in the liquid state in an oven.

The properties of the metal powder are obtained by adjusting the parameters relating to spraying and by sieving.

The method involves subjecting the metal powder to isostatic compression at high temperature.

Such hot isostatic compression is carried out inside an oven where the powder is subjected to a high temperature and to a very high pressure in contact with an inert gas such as argon. Isostatic compression of the powder is carried out inside a deformable metal mold which is introduced into the isostatic compression enclosure inside the oven.

In the case of the manufacture of a solid part by the method of the invention, the mold is completely filled with metallic powder. In the case of the production of a coating on a part, the powder is contained in the space between the part to be coated and the mold. In this case, it is known to carry out, beforehand, a surface treatment of the part to be coated in order to favor the attachment of the deposit to the part.

In general, the powder is subjected to a pressure of the order of 1000 bars to 1500 bars and at a temperature between 0.8 and 1 times the solidus temperature, for a period ranging from 1 hour to a few hours, for example a duration of 1 to 5 hours.

The pressure, the temperature and the duration of the isostatic compression are determined so that the material has good structural homogeneity, that it is made up of fine grains and that it is substantially free from cracks and volume defects. In addition, the material obtained by isostatic compression has perfectly isotropic properties.

We will now give below, without implied limitation, several exemplary embodiments of materials by the process according to the invention, these materials based on iron and / or nickel having different compositions.

Example 1 :

A metal powder is produced from an alloy belonging to a first family of nickel-based alloys containing chromium, boron and silicon and having a relatively low carbon content.

This family of nickel-based alloys will itself be subdivided into a first subfamily designated by the letter A and a second subfamily designated by the letter B.

The implementation of the method according to the invention using an alloy of subfamily A will be described below in the form of example la and the implementation of the method according to the invention using an alloy of the sub -family B will be described in the form of example 1b.

Example 1a :

• carbone 0,2 à 0,6 %,
• silicium 1,25 à 3,50 %,
• bore 2 à 3 %,
• chrome 7 à 14 %,
• fer 1,25 à 3,25 %,

   le solde de l'alliage étant constitué par du nickel à l'exception d'impuretés inévitables, parmi lesquelles le cobalt présente une teneur inférieure ou égale à 1 % en poids.In the manner described above, a powder of a nickel alloy belonging to the subfamily A is produced. The alloys of the subfamily A are defined by the contents by weight of alloying elements given below :

• carbon 0.2 to 0.6%,
• 1.25 to 3.50% silicon,
• boron 2 to 3%,
• chromium 7 to 14%,
• iron 1.25 to 3.25%,

the balance of the alloy consisting of nickel with the exception of unavoidable impurities, among which the cobalt has a content less than or equal to 1% by weight.

The alloys of the first subfamily A correspond in particular to alloys known under the trade names "Colmonoy 4" "Deloro 40" and "TY 15.40" or designated by the acronym RNiCr-A according to the AWS.5.13 standard.

The hot isostatic compression of the alloy powder is carried out at a temperature between 900 and 980 ° C, for several hours and at a pressure of about 1000 bars to 1500 bars.

Preferably, the isostatic compression is carried out at a temperature of 920 ° C.

The hardness of the material obtained by hot isostatic compression is greater than 40 HRC (Rockwell hardness), which can be compared favorably to the hardness of materials obtained from alloys of the same type by a coating process according to the art. prior.

In fact, when a coating is carried out using one of the alloys mentioned above, by a method according to the prior art, for example by the PTA method, the Rockwell hardness of the coating obtained is between 38 and 45.

Example 1b :

• carbone de 0,3 à 0,8 %,
• silicium de 3 à 5 %,
• bore de 2 à 4 %,
• chrome de 10 à 16 %,
• fer de 2 à 5 %,

   le solde de l'alliage étant constitué par du nickel et des impuretés inévitables parmi lesquelles le cobalt présente une teneur pondérale inférieure ou égale à 1 %.The alloys belonging to the second subfamily B are defined by the following weight contents in alloying elements:

• carbon from 0.3 to 0.8%,
• 3 to 5% silicon,
• boron from 2 to 4%,
• chromium from 10 to 16%,
• iron from 2 to 5%,

the balance of the alloy consisting of nickel and unavoidable impurities, among which the cobalt has a weight content less than or equal to 1%.

The hot isostatic compression is carried out at a temperature between 900 and 980 ° C, preferably at 920 ° C, at a pressure of about 1000 bars to 1500 bars, for several hours.

The hardness of the alloy obtained is at least 50 HRC, which can be compared favorably to the hardnesses of coatings obtained by a process according to the prior art, for example the PTA process applied to alloys of the second subfamily. B, the hardnesses obtained in this case being between 46 and 54 HRC.

Typical alloys of the family B consist of the alloys known under the trade names "Colmonoy 5", "Deloro 50" and "TY 12.50" or designated by the acronym RNiCr-B according to the AWS.5.13 standard.

Example 2

A nickel-based alloy belonging to a second family of alloys having a high chromium content and a high carbon is produced.

Such nickel-based alloys belonging to the second family are for example described in FR-A-2,405,306 and known under the trade name "PY 150".

• carbone 1,4 à 2,5 %,
• silicium 0 à 2 %,
• chrome 25 à 33 %,
• molybdène 6 à 15 %,

   le solde de l'alliage étant constitué par du nickel à l'exception des impuretés inévitables parmi lesquelles le cobalt doit présenter une teneur pondérale inférieure ou égale à 1 %.The alloys of the second family can be defined by the following weight contents of alloy elements:

• carbon 1.4 to 2.5%,
• 0 to 2% silicon,
• chromium 25 to 33%,
• molybdenum 6 to 15%,

the balance of the alloy consisting of nickel, with the exception of unavoidable impurities, including the cobalt must have a weight content less than or equal to 1%.

• carbone 1,65 %,
• silicium 1,1 %,
• chrome 29 %,
• molybdène 7,8 %,
• fer inférieur à 1 %,

   le solde de l'alliage étant constitué par du nickel et des impuretés inévitables parmi lesquelles le cobalt est dans une teneur pondérale inférieure ou égale à 1 %.Typically, an alloy of the second family can have the following composition:

• carbon 1.65%,
• 1.1% silicon,
• chromium 29%,
• 7.8% molybdenum,
• iron less than 1%,

the balance of the alloy consisting of nickel and unavoidable impurities among which the cobalt is in a weight content less than or equal to 1%.

Isostatic hot compression of the powder is carried out at a temperature above 1000 ° C, preferably at 1100 ° C for several hours and at a pressure of about 1000 bar.

The hardness of the alloy obtained is at least 36 HRC, which compares favorably with the hardness of products obtained by processes according to the prior art from alloys of the second family, for example coatings obtained by PTA or solid molded parts whose hardness is between 30 and 35 HRC.

Example 3

A third family of high hardness alloys consists of iron-based alloys with a high chromium content and whose carbon content is relatively low.

• chrome 22 à 30 %,
• nickel 7 à 25 %,
• carbone 0,2 à 0,5 %,
• manganèse environ 2 %,
• molybdène 6 à 12 %,
• silicium environ 2 %,
• tungstène de 1 à à 4 ,
• vanadium environ 1 %,

   le solde de l'alliage étant constitué par du fer et des impuretés inévitables parmi lesquelles le cobalt présente une teneur pondérale inférieure ou égale à 1 %.The alloys of this third family can be defined by the weight contents of alloy elements given below:

• chromium 22 to 30%,
• nickel 7 to 25%,
• carbon 0.2 to 0.5%,
• manganese about 2%,
• molybdenum 6 to 12%,
• about 2% silicon,
• tungsten from 1 to 4,
• vanadium about 1%,

the balance of the alloy consisting of iron and unavoidable impurities, among which the cobalt has a weight content less than or equal to 1%.

A typical alloy of the third family is the alloy known by the trade name "Cenium Z 20".

The hot isostatic compression is carried out at a temperature above 1000 ° C., preferably at 1100 ° C., for several hours and at a pressure of approximately 1000 bars. The hardness of the alloy obtained is greater than 45 HRC.

Example 4

A fourth family of high hardness alloys containing no cobalt consists of iron-based alloys with a high chromium content and a high carbon content.

• chrome 22 à 30 %,
• nickel 0 à 10 %,
• carbone 1 à 3 %,
• manganèse 0,3 à 15 %,
• vanadium environ 4 %,

   le solde de l'alliage étant constitué par du fer à l'exception d'impuretés résiduelles inévitables parmi lesquelles le cobalt présente une teneur pondérale inférieure ou égale à 1 %.The alloys of this fourth family can be defined by the weight contents of the following alloying elements:

• chromium 22 to 30%,
• nickel 0 to 10%,
• carbon 1 to 3%,
• manganese 0.3 to 15%,
• vanadium about 4%,

the balance of the alloy consisting of iron with the exception of unavoidable residual impurities among which the cobalt has a weight content less than or equal to 1%.

An example of such alloys belonging to the fourth family is the alloy known by the trade name "Norem".

The hot isostatic compression is carried out at a temperature above 1000 ° C., preferably at 1100 ° C., for several hours, at a pressure of approximately 1000 bars. The hardness of the alloy obtained is at least 45 HRC.

During the implementation of isostatic compression, the pressure and duration of treatment parameters are linked together. It is thus possible, by increasing the pressure, to limit the compression time or vice versa, to limit the pressure by increasing the compression time.

The holding temperature of the furnace during the implementation of isostatic compression must be within a range of between 0.8 and 1 times the temperature of the solidus of the alloy, i.e. the starting melting point. of the alloy when its temperature is increased, from the solid state. In this way, the temperature, during hot isostatic compression, is lower than the burning temperature of the alloy.

The conditions for implementing isostatic compression are defined in particular so as to preserve as much as possible the fine-grained structure of the powder in the alloy after isostatic compression. In other words, the isostatic compression process must not cause a magnification of the powder grains initially introduced into the mold.

In the case of iron-based alloys, after the isostatic compression, the alloy is preferably cooled, for example at a speed of 300 to 400 ° C / h while controlling the cooling. One can consider a heat treatment after isostatic compression in order to refine the properties of the alloy.

Comparative wear tests of hard materials produced by the process according to the invention and of materials according to the prior art

Piondisk type wear tests were carried out on materials produced by the process according to the invention and on materials produced by a process according to the prior art.

The wear tests are carried out by rubbing a hard coated disc with a pin of Stellite 12 cobalt alloy.

The tests are carried out at 300 ° C, with a contact pressure between the pin and the disc of 100 MPa and at a friction speed of 10 mm / s. A wear rate is determined from weight loss measurements of the coated samples.

FIG. 1 is a diagram giving the wear rate of coatings obtained by the hot isostatic compression process according to the invention (CIC process) with different alloys and, for comparison, the wear rate of a coated Stellite 6 cobalt alloy obtained by the PTA process.

In FIG. 1, the points relating to the coating obtained by the process according to the invention are marked by black squares whereas the comparative point relating to a coating of Stellite 6 obtained by PTA is represented by a white square.

On the abscissa axis, we have given the standard names of the alloys used to make the coatings.

Comparatively, a coating of Stellite 6 was produced on the one hand by the PTA process and on the other hand by the CIC process according to the invention.

The wear rate of the coating obtained by the PTA process according to the prior art is 1 while the wear rate of the coating obtained by the CIC process according to the invention is 0.6.

The wear rate, in the case of the cobalt alloy Stellite 6 was therefore practically halved using the CIC method according to the invention.

In addition, the results relating to coatings obtained by the process according to the invention and employing alloy powders of the first family of nickel alloys belonging to the first subfamily A and to the second are shown. subfamily B, respectively.

The alloy of the first subfamily A is designated by the standardized designation R Ni Cr -A and the alloy powder of the second subfamily B by the standardized designation R Ni Cr -B.

The standardized designations R Ni Cr -A and R Ni Cr -B come from AWS 5.13 of the American Welding Society relating to the definition of welding materials and hard coatings.

The coating obtained from the material of the first subfamily A of nickel alloys has a wear rate close to 0.4 which is still lower than the wear rate of the Stellite 6 coating obtained by the CIC process which is 0.6.

The coating of material of subfamily B has a wear rate of 0.3 lower than the wear rate of the coating of material of first subfamily A.

Referring to FIG. 2A, it can be seen that in the case of a coating obtained by PTA from a material of the first subfamily (R Ni Cr -A), the hardening phases 1 appearing in black on the micrographs of FIG. 2A are distributed according to formations between which the material has zones 2 free of hardening precipitate having a large extent.

This results in limited hardness and wear resistance of the coating material.

In FIG. 2B, the microstructure of a coating of material of the first subfamily (R Ni Cr -A) obtained by the CIC process has been shown.

The magnification obtained under the microscope is six times greater in the case of FIG. 2B than in the case of FIG. 2A. It is clear from the higher magnification micrograph of FIG. 2B that the hardening phases 3 constitute very fine precipitates which are distributed homogeneously throughout the coating material.

This results in greater hardness and a wear resistance very greatly improved in the case of the coating obtained by the process according to the invention using isostatic compression.

In addition, the material obtained by the method of the invention is free from internal defects such as cracks or volume defects such that they can appear with conventional methods (TIG, PTA, torch). In fact, hot isostatic compression makes it possible to avoid the appearance of internal defects such as those which appear during the molding or the welding of a metallic material.

It has also been shown that when molded or welded parts are subjected to a hot isostatic compression operation, the small internal defects due to molding or welding tend to close. Hot isostatic compression therefore has a curative effect in the case of materials obtained by molding or welding.

In addition, one of the advantages of the hot isostatic compression method is that it makes it possible to shape alloys which are difficult to produce by conventional methods such as forging and rolling.

Isostatic hot compression also provides a material whose structure is perfectly homogeneous and whose properties are isotropic.

Due to the homogeneous structure of the materials, they have better resistance to generalized corrosion and to localized corrosion due to the homogeneous distribution of chromium.

The products obtained by the process according to the invention, despite their high hardness, are not very brittle and exhibit significant ductility (1% elongation at 20 ° C and 1.5% at 350 ° C). This results in improved resistance to thermal shock of the products obtained by the process according to the invention.

The invention is not limited to the embodiments which have been described.

Thus, hot isostatic compression can be carried out under conditions of pressure, temperature and duration different from those which have been indicated.

The characteristics of the alloy powder used, for example the particle size of this powder and the shape of the grains, can be variable and adapted to the intended use.

The process according to the invention can be carried out using powders of iron or nickel alloys having compositions different from those which have been indicated above.

Claims (20)

Process for the manufacture of a metallic material of great hardness, resistant to wear, characterized in that a powder of a metallic alloy is submitted mainly containing at least one of the elements iron and nickel as well as chromium, at isostatic compression at high temperature, the pressure, the temperature and the duration of the isostatic compression treatment being defined to obtain a homogeneous and isotropic material with fine grains, substantially free of cracks and volume defects. Process according to Claim 1, characterized in that the metallic powder consists of an alloy based on nickel and / or iron containing cobalt only as a residual element. Process according to Claim 2, characterized in that the metal alloy is based on nickel and contains boron, silicon as well as a low carbon content. Process according to Claim 3, characterized in that the metal alloy contains by weight, from 0.2 to 0.6% of carbon, from 1.25 to 3.50% of silicon, from 2 to 3% of boron , from 7 to 14% of chromium and from 1.25 to 3.25% of iron, the balance of the alloy consisting of nickel and unavoidable residual impurities including cobalt in a weight content less than or equal to 1 %. Process according to Claim 3, characterized in that the metal alloy contains by weight, from 0.3 to 0.8% of carbon, from 3 to 5% of silicon, from 2 to 4% of boron, from 10 to 16% chromium and 2 to 5% iron, the balance of the alloy consisting of nickel, with the exception of unavoidable residual impurities, among which cobalt is present in a weight content less than or equal to 1% . Process according to either of Claims 4 and 5, characterized in that the hot isostatic compression is carried out at a temperature between 900 and 980 ° C and preferably at 920 ° C, for several hours, at a pressure d '' around 1000 bars to 1500 bars. Process according to Claim 2, characterized in that the metal alloy contains by weight from 1.4 to 2.5% of carbon, from 0 to 2% of silicon, from 25 to 33% of chromium and from 6 to 15 % molybdenum, the balance of the alloy consisting of nickel, with the exception of unavoidable impurities among which the cobalt has a weight content less than or equal to 1%. Process according to Claim 7, characterized in that the metal alloy contains approximately 1.65% of carbon, 1.1% of silicon, 29% of chromium, 7.8% of molybdenum and less than 1% of iron, the balance of the alloy consisting of nickel with the exception of unavoidable impurities among which the cobalt has a weight content less than or equal to 1%. Process according to either of Claims 7 and 8, characterized in that the hot isostatic compression is carried out at a temperature above 1000 ° C, preferably at 1100 ° C for several hours at a pressure of about 1000 bar . Process according to Claim 2, characterized in that the metallic alloy contains by weight 22 to 30% of chromium, 7 to 25% of nickel, 0.2 to 0.5% of carbon, approximately 2% manganese, 6 to 12% molybdenum, about 2% silicon, 1 to 4% tungsten and about 1% vanadium, the balance of the alloy consisting of iron and impurities inevitable among which cobalt has a weight content less than or equal to 1%. Process according to Claim 10, characterized in that the hot isostatic compression is carried out at a temperature above 1000 ° C, preferably at 1100 ° C, for several hours at a pressure of about 1000 bar. Process according to Claim 2, characterized in that the metal alloy contains by weight 22 to 30% of chromium, from 0 to 10% of nickel, from 1 to 3% of carbon, from 0.3 to 15% of manganese and approximately 4% of vanadium, the balance of the alloy consisting of iron with the exception of unavoidable impurities among which the cobalt has a weight content less than or equal to 1%. Process according to Claim 12, characterized in that the isostatic compression is carried out at a temperature above 1000 ° C, preferably at 1100 ° C, for several hours at a pressure of approximately 1000 bar. Process according to any one of Claims 1 to 13, characterized in that the hot isostatic compression is carried out at a temperature situated in a range between 0.8 and 1 times the temperature of the solidus of the alloy. Process according to any one of Claims 1 to 14, characterized in that the hot isostatic compression is carried out in an argon atmosphere. Process according to either of Claims 11 and 13, characterized in that the metallic material is cooled, after hot isostatic compression, at a speed of between 300 and 400 ° C / h. Use of a method according to any one of claims 1 to 16 for the production of a wear-resistant coating on a metal part. Use of a method according to any one of claims 1 to 16 for the production of a solid piece of metal alloy. Use according to any one of claims 17 and 18 characterized in that the coated metal part or the metal alloy part is a friction part used in a nuclear power station, such as a cover or a valve seat, a part for a pump, a pawl of a control cluster mechanism or a key of the internal equipment of the nuclear power plant reactor. Use according to claim 19, characterized in that the metal alloy contains cobalt only as a residual element.

Images