EP0438338B1 - Process for making a product from pre-alloyed powders and the product obtained from the said process - Google Patents

Description

The present invention relates to a process for obtaining a product from pre-alloyed powders, and more particularly to a process for obtaining in which the powders are subjected to a densification treatment.

The present invention also relates to the products obtained using said process.

It is known that powder metallurgy, in development for several years, allows in particular the manufacture of parts, from metals, impossible or difficult to implement otherwise, for example in super-refractory alloys. As a result, powder metallurgy has, in particular, important applications in the field of aeronautical manufacturing (parts of turbines).

However, if the materials produced with the current production methods exhibit excellent mechanical properties at medium temperatures, due to the fineness of the grains, their behavior at high temperatures (above 650 ° C.) is not still satisfactory, in particular with regard to their creep resistance and their resistance to crack propagation.

An object of the invention is to provide a process for obtaining products from pre-alloyed powders made of super-refractory alloy which overcomes these drawbacks.

Surprisingly, while until now in powder metallurgy, the materials produced have been characterized by a very fine metallurgical grain, the invention proposes a process for obtaining comprising treatments intended to increase the size of the grains, the size powder particles being nevertheless always initially limited by sieving, and the scale of segregations being reduced to dimensions which do not exceed the size of said particles.

We know, in classical metallurgy, that it is possible enlarging the grain size of an alloy by subjecting it to a heat treatment. However, in the context of powder metallurgy, such heat treatments prove in practice to be ineffective, for example the alloy grains obtained from powders with a diameter of less than 106 micrometers being in particular very difficult to magnify beyond 7 in ASTM standard.

In seeking to analyze the causes of these limitations, the Applicant has observed that during the densification steps, for example by hot isostatic compaction or by spinning, the elements segregated on the surface of the powder particles (mainly carbon and oxygen ) precipitated there, thus forming stable networks which cannot be absorbed by further processing. One consequence of this phenomenon is to promote interparticle ruptures and to make it impossible to magnify the grain. As a result, the grain is limited to the size of the initial powder particles.

In one of its main aspects, the invention proposes a process for obtaining one of which is a treatment making it possible to attenuate the "decoration" of the particles by precipitation of the segregated elements. More particularly, the solution adopted by the applicant consists in subjecting, before compaction, the powders of the alloy to heat treatments under low pressure (or without pressure) by which the segregated elements precipitate internally, in phases stable at temperature. compaction and no longer during compaction on the surface of the particles.

To improve the moldability and resistance of sintered parts, it has already been proposed, in document JP-A-55.161002, to subject the powder, before densification, to a heat treatment making it possible to precipitate certain elements present in the alloy such as phosphorus or tin. However, the previous method consists in heating the powder above the melting point of these elements to bring them onto the surface of the powder particles whereas the object of the invention is, on the contrary, to precipitate the segregated elements inside particles to avoid precipitation on the surface.

Furthermore, as part of its analysis, the Applicant has also observed that the grain enlargement treatments by temperature rise were limited by burning phenomena, that is to say the start of local melting, which prevented bring the powder alloy to the desired temperatures.

To overcome this other limitation, the invention proposes, in combination with the aforementioned non-pressure or low-pressure treatments, to subject the powders or alloys to homogenization treatments, intended to structurally standardize the materials, in order to ascend as much as possible the temperature where these burning phenomena appear.

More particularly, in the case of alloys with structural hardening, the homogenization treatments according to the invention are heat treatments above the Solvus temperature of the alloy.

The subject of the invention is therefore a method of improving the behavior at high temperatures of a product obtained by a hot densification treatment of a pre-alloyed powder, said powder being subjected, in a manner known per se, to a heat treatment carried out, before the densification treatment, under low pressure (or without pressure), so as to allow the precipitation of elements segregated in stable phases.

According to the invention, the pre-alloyed powder consists of an alloy with structural hardening and the pretreatment under low pressure is carried out at a temperature allowing the precipitation of segregations in stable phases inside the powder particles and the densification treatment at high temperature and under pressure comprises at least one step of enlarging the metallurgical grains up to a size exceeding the limits of the particles.

Advantageously, the temperature at which the pretreatment is carried out under low pressure (or without pressure) is between the Solvus temperature of the alloy reduced by approximately 100 ° C. and its melting temperature.

Advantageously still, the powder is subjected to a heat treatment of homogenization at a temperature higher than the Solvus temperature of the alloy.

Preferably, the densification treatment comprises a step of compaction at low pressure.

More preferably, the densification treatment includes a consolidation step, in particular by hot isostatic compaction or by spinning. Processing densification can include both a low pressure compaction step and a later consolidation step.

In a preferred embodiment, the heat treatment at a temperature above the Solvus temperature of the alloy is carried out after the densification treatment.

In another preferred embodiment, the heat treatment at a temperature higher than the Solvus temperature of the alloy is carried out during the pre-treatment without pressure or under low pressure and / or during the densification treatment. The consolidation step can in particular comprise an isostatic compaction at a temperature higher than the Solvus temperature of the alloy.

The subject of the invention is also a product of structural hardening alloy obtained from such a process, and more particularly of product of nickel-based super-refractory alloy.

Finally, yet another object of the invention is the use of products obtained using said process, in critical parts operating at high temperatures, for example above 650 ° C., and more particularly the use of such products for the manufacture of critical parts in the field of aeronautical construction.

The description which follows is purely illustrative and not limiting. It should be read in conjunction with the accompanying drawings.

• La Figure 1 représente une micrographie illustrant, pour un premier alliage, la croissance des grains métallurgiques au-delà des limites des particules de poudre initiales, grâce à la mise en oeuvre d'un procédé conforme à l'invention;
• La Figure 2 est un graphe illustrant la croissance des grains métallurgiques d'un deuxième alliage, grâce à la mise en oeuvre d'un procédé conforme à l'invention;
• Les Figures 3 et 4 sont des micrographies illustrant respectivement, pour le même deuxième alliage, les différences de tailles de grains métallurgiques de matériaux obtenus, d'une part, avec un procédé d'obtention conforme à l'état de l'art, et, d'autre part, avec un procédé d'obtention conforme à l'invention.

In these drawings:

• FIG. 1 represents a micrograph illustrating, for a first alloy, the growth of metallurgical grains beyond the limits of the initial powder particles, thanks to the implementation of a process in accordance with the invention;
• Figure 2 is a graph illustrating the growth of metallurgical grains of a second alloy, thanks to the implementation of a method according to the invention;
• FIGS. 3 and 4 are micrographs respectively illustrating, for the same second alloy, the differences in size of metallurgical grains of materials obtained, on the one hand, with a process for obtaining conforming to the state of the art, and , on the other hand, with a process for obtaining according to the invention.

The process according to the invention was used to obtain a first alloy known under the trade name ASTROLOY® (registered trademark), the weight composition of which is as follows: 0.040% zirconium: 0.023% boron ; 0.020% carbon; 3.5% titanium; 4% aluminum; 5% molybdenum; 15% chromium; 17% cobalt; nickel balance. This alloy has a Solvius temperature of 1140 ° C.

A classic process for obtaining a product. based on pre-alloyed powders in the aforementioned weight proportions, consists of a densification treatment at high temperature and under pressure, for example a heat treatment under 100 MPa for six hours. Table A below gives, in ASTM standard, the grain sizes obtained as a function of the temperature of this heat treatment, this from powder particles having an average diameter less than 75 micrometers. TABLE A ASTROLOY Treatment (s) Grain sizes in ASTM standard State of the art processes 1120 ° C 6h 100 MPa 10 1160 ° C 6h 100 MPa 7 1200 ° C 6h 100 MPa 6

An example of a process for obtaining according to the invention consists, for its part, of a preliminary heat treatment of the powders under low pressure (less than 1 atm) or without pressure for 24 hours, then in a densification heat treatment, a stage is a stage of hot isostatic compaction, under 100 MPa for six hours, at 1160 ° C., this stage of hot isostatic compactin being able to be followed by a treatment, for four hours at a temperature of 1200 ° C.

The grain sizes obtained from initial powders having an average diameter of less than 75 μm are given in table B below: TABLE B ASTROLOY Treatment (s) Grain sizes in ASTM standard Processes 1120 ° C 24 h + 1160 ° C 6 h 100 MPa 5 conform 1160 ° C 24 h + 1160 ° C 6 h 100 MPa 3 to the invention 1160 ° C 24 h + 1160 ° C 6 h 100 MPa + 1200 ° C 4 h 1 to 3

It is clear from the comparison of Tables A and B, that the implementation of a heat pretreatment under low pressure or without pressure allows a significant increase in the grain sizes of the alloys obtained.

It is also noted, from Table B, that the grain sizes are much larger when the pretreatment is carried out at 1160 ° C., that is to say above the Solvus temperature of the alloy, than when performed at 1120 ° C.

In addition, it can also be seen that the results on the grain sizes are still very much improved when the obtaining process comprises a final treatment step at 1200 ° C., the metallurgical grain sizes produced being able to go up to a value of 1 in ASTM standard.

If we refer, moreover, to FIG. 1, which represents a micrograph of an ASTROLOY alloy obtained from that of the methods according to the invention cited in table B, which comprises a final processing step at 1200 ° C, it can be seen that the metallurgical grains, the limits of which appear in solid lines, have developed beyond the initial limits of the powder particles, which can be seen in dotted lines on the micrograph, due to the persistence low decorations despite the implementation process.

The process according to the invention was also applied to obtain a second alloy known under the trade name N18, the weight composition of which is as follows: 0.030% zirconium; 0.015% boron; 0.015% carbon; 0.25% hafnium; 4.35% titanium; 4.35% aluminum; 6.5% molybdenum; 11.5% chromium; 15.7% cobalt; nickel balance. This alloy has a Solvus temperature of 1195 ° C.

Table C gives the grain sizes obtained for this second alloy, from conventional production methods, that is to say by hot isostatic compaction at 100 MPa, for six hours, for different compaction temperatures, the initial powders with an average diameter of less than 75 ”m. TABLE C N 18 Treatment (s) Grain sizes in ASTM standard State of the art processes 1120 ° C 6h 100 MPa 10 1160 ° C 6h 100 MPa 9 1200 ° C 6h 100 MPa 7

Table D gives, for its part, values of grain sizes of alloys produced for initial powders of similar dimensions, from production methods in accordance with the invention. These production methods each include thermal pretreatments under low pressure (less than 1 atm) or without pressure, followed by a densification treatment. Such densification treatment may include a consolidation step by conventional hot isostatic compaction, whether or not followed by a subsequent consolidation step. The densification treatment can also consist of an isostatic compaction step under low pressure, followed by a posterior compaction step. TABLE D N 18 Treatment (s) Grain sizes in ASTM standard Processes in accordance with the invention 1160 ° C 24 h + 1200 ° C 6 h 100 MPa 5 1160 ° C 24 h + 1160 ° C 6 h 100 MPa + 1200 ° C 4 h 5 1170 ° C 24 h + 1170 ° C 4 h 10 MPa + 1200 ° C 6 h 100 PMa -1 to 2 1200 ° C 24h + 1160 ° C 6 h 100 MPa + 1200 ° C 6 h 100 MPa 4

It appears from the comparison of Tables C and D, that the heat pretreatment of the powders under low pressure makes it possible to produce alloys having large grain sizes, in particular having a value in ASTM standard less than or equal to 5.

It should also be noted that the results obtained on these grain sizes are all the better, as the pre-treatment or heat treatment temperatures are higher than the Solvus temperature of the alloy (1195 ° C.).

In addition, it will also be noted that the results obtained with an intermediate isostatic compaction step at low pressure (10 MPa) are very clearly superior to the others, the values of the grain sizes reached being able to go up to -2 in ASTM standard.

If we now refer to Figures 3 and 4, which respectively represent micrographs of an alloy N18 treated according to, on the one hand, the state of the art at 1160 ° C, for 6 hours, under 100 MPa, and, according to, on the other hand, a process in accordance with the invention consisting of a pretreatment at 1170 ° C for 24 hours, followed by isostatic compaction at low pressure at 1170 ° C, for 4 hours at 10 MPa, followed by further compaction after 1200 ° C, for 6 hours at 100 PMa, we see that l 'We clearly see in Figure 4, the limits of the metallurgical grains formed in the alloy by the process of obtaining according to the invention, while these same limits are difficult to distinguish on the micrograph shown in Figure 3, c 'is to say on the alloy obtained by a process according to the state of the art. The grains of the alloys obtained with the process according to the invention are very much higher than those obtained with the process according to the state of the art.

If we now refer to Figure 2, which is a graph giving the grain size, in ASTM standard, of the alloys obtained using a process according to the invention, as a function of the pre-treatment temperature to which said alloys are subjected for 24 hours, this pretreatment having been followed by a step of hot isostatic compaction at 1120 ° C, for six hours, under 100 MPa, extended by a step of compaction after 1200 ° C for six hours, at 100 MPa, there is a significant increase in the value of the grain sizes when the temperature of the initial pretreatment is above the Solvus temperature of the alloy (1195 ° C.).

Claims (12)

1. A method to improve the behaviour at elevated temperatures of a product obtained by a hot compaction treatment of a prealloyed powder said powder being subjected in a per se known manner to a heat treatment performed before the compaction treatment under low pressure (or without pressure) to allow precipitation of segregated elements according to stable phases,
      characterized in that the prealloyed powder is made of an alloy with structural hardening, in that the low pressure pretreatment is carried out at a temperature allowing precipitation of segregations in stable phases inside the powder particules, and in that the compaction treatment at elevated temperature and under pressure comprises at least a step of enlarging the sizes of metallurgical grains until a size going outer the limits of the particules.
2. A method according to claim 1, characterized in that the temperature at which is performed the pretreatment under low pressure (or without pressure) is comprised between the solvus temperature of the alloy reduced of about 100°C and its melting temperature.
3. A method according to any one of claims 1 or 2, characterized in that the powder is subjected to an homogenization heat treatment at a temperature above the solvus temperature of the alloy.
4. A method according to any one of claims 1 to 3, characterized in that the compaction treatment includes a stage of compaction at low pressure.
5. A method according to any one of claims 1 to 4, characterized in that the compaction treatment includes a stage of consolidation, in particular by hot isostatic pressing.
6. A method according to claims 4 and 5 taken in combination, characterized in that the compaction treatment includes a stage of low pressure compaction and a subsequent consolidation stage.
7. A method according to claim 3 taken in combination with any one of claims 4 to 6, characterized in that the heat treatment at a temperature above the solvus temperature of the alloy is performed after the compaction treatment.
8. A method according to claim 3 taken in combination with any one of claims 4 to 6, characterized in that the heat treatment at a temperature above the solvus temperature of the alloy is performed during the pretreatment without pressure or under low pressure and/or during the compaction treatment.
9. A method according to claim 8 taken in combination with one of claims 5 or 6, characterized in that the consolidation stage includes isostatic pressing at a temperature above the solvus temperature of the alloy.
10. A method according to one of the preceding claims, characterized in that the prealloyed powder is a nickel based super-refractory alloy.
11. Product comprising a structural hardening alloy obtained from the method according to one of claims 1 to 10.
12. Use of the alloy according to claim 11 for making critical pieces liable to work at temperatures above 650°C.

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