EP0340264B1 - Process for manufacturing semi-finished products from sintered refractory metal alloys - Google Patents

Abstract

In a process for manufacturing semi-finished products from sintered refractory metal alloys with a stacked microstructure, the sinter feed formed at least at 85 % is subjected prior to subcritical annealing to an intermediate annealing for at least 20 minutes at a temperature not less than 700°C and not greater than that at which no further recrystallization occurs. Following this intermediate annealing, the hot feed is deformed by a further 3 % to 30 %. The process makes it possible to manufacture semi-finished products with a good stacked microstructure and of substantially greater dimensions or of the same dimensions and a substantially better stacked microstructure than can be obtained with known manufacturing processes.

Description

The invention relates to a process for the production of semi-finished products from sintered refractory metal alloys with a stacked structure, in which the sintered material is brought to a degree of deformation of at least 85% by mechanical forming in several forming steps and then subjected to a recrystallization annealing.

In order to improve the heat resistance and creep resistance properties at high temperatures in the case of refractory metals, various types of alloying of refractory metals have been developed in the past.

According to a known method, which is limited to powder metallurgy, a refractory base metal is doped with certain elements and subjected to high mechanical deformations with a degree of deformation of at least 85% in the course of production. After the forming process has been completed, the material is subjected to a recrystallization annealing. This results in a very specific structure of the refractory metal alloy, the so-called stack structure, which is characterized by elongated structure grains, the ratio of length to width of which is at least 2: 1.

Known refractory metal alloys of this type are e.g. Tungsten and molybdenum alloys that are doped with small amounts of aluminum, silicon and potassium, or with silicon and potassium.

wobei A_a die Querschnittsfläche des gesinterten Ausgangsmaterials und A_e die Querschnittsfläche des fertigen Endproduktes ist. Um die Umformung zu erleichtern und Risse im Material zu vermeiden ist es wichtig, daß die notwendige Umformtemperatur während des gesamten Umformprozesses aufrecht erhalten wird, so daß zwischen einzelnen Umformschritten in der Regel neu angewärmt werden muß. Nach abgeschlossenem Umformprozeß wird das Material einer Rekristallisationsglühung unterzogen. Die Rekristallisationstemperatur ist abhängig von der Art der Legierung und vom speziellen Umformgrad. Je höher der Umformgrad ist, desto höher ist für diesen Legierungstyp auch die für die Rekristallisation erforderliche Temperatur.In the production of these alloys, the sintered starting material is heated to temperatures of approximately 1350 ° C. to approximately 1450 ° C. and by mechanical shaping, for example rolling or round hammers and Drawing, formed in several forming steps to a final degree of forming of at least 85%. The degree of deformation is a measure of the plastic deformation and thus material compression achieved and is calculated in percent $$\frac{{\text{A}}_{\text{a}}{\text{- A}}_{\text{e}}}{{\text{A}}_{\text{a}}}\text{x 100}$$

where A _{a is} the cross-sectional area of the sintered starting material and A _{e is} the cross-sectional area of the finished end product. In order to facilitate the forming and to avoid cracks in the material, it is important that the necessary forming temperature is maintained during the entire forming process, so that it is usually necessary to reheat between individual forming steps. After the forming process has been completed, the material is subjected to a recrystallization annealing. The recrystallization temperature depends on the type of alloy and the specific degree of deformation. The higher the degree of deformation, the higher the temperature required for recrystallization for this type of alloy.

A disadvantage of this process for the production of refractory metal alloys with a stacked structure is that only semi-finished products of relatively small dimensions, e.g. for sheets, sheet thicknesses of a maximum of about 2 mm and for wires with a maximum diameter of about 1.7 mm. In the case of semi-finished products that exceed these dimensions, the formation of a satisfactory stack structure is generally no longer achievable.

EU A1 119 438 describes special molybdenum alloys with a stacked structure in which the molybdenum is doped with about 0.005 to 0.75% by weight of at least one of the elements aluminum, silicon and potassium. This prior publication also mentions that by additionally doping this alloy with 0.3 to 3% by weight of at least one compound selected from the group of Oxides, carbides, borides and nitrides of the elements La, Ce, Dy, Y, Th, Ti, Zr, Nb, Ta, Hf, V, Cr, Mo, W and Mg the high temperature properties of the alloy can be further improved.
In the production of these special molybdenum alloys, the sintered starting material is formed with a degree of deformation of at least 85%, preferably of 95% and more. As a particularly advantageous measure, a first recrystallization annealing is recommended there after reaching a degree of deformation between 45% and 85%. This is followed by further shaping to the predetermined degree of deformation and a final recrystallization annealing. When further shaping to the desired degree of deformation, no special regulations regarding the successive degrees of deformation are mentioned. This special manufacturing process results in a certain improvement in the creep resistance and the thermal behavior of these alloys compared to alloys that are produced without this intermediate recrystallization annealing. However, even using this manufacturing process, it is not possible to manufacture a semi-finished product from molybdenum alloys with a stacked structure which have larger dimensions in the sheet thickness or in the wire diameter than mentioned at the beginning.

The object of the present invention is to provide a method for producing semifinished products from sintered refractory metal alloys with a stacked structure, according to which the semifinished product can also be produced with comparatively large dimensions, or according to which, when producing semifinished products, a dimension that is comparatively the same mentioned prior art significantly improved stack structure is achieved.

According to the invention, this is achieved in that the sintered material, which has been formed to at least 85%, is subjected to intermediate annealing prior to recrystallization annealing for at least 20 minutes at a minimum temperature of 700 ° C. and a maximum temperature at which just no recrystallization occurs, and subsequently to the intermediate annealing deformed by a further 3% to 30% when heated.

By combining the special intermediate annealing of the starting material, which has been formed to at least 85%, with a subsequent forming within a very special forming area, it is achieved in a completely surprising way that semifinished products made of sintered refractory metal alloys compared to semifinished products which are produced by known processes, with training a good stacked structure can be produced with significantly larger dimensions or has a much better stacked structure with the same dimensions than according to the described prior art.

Thus, with the methods according to the invention, sheet thicknesses of up to approx. 10 mm and bar diameters of up to approx. 50 mm can be achieved in the production of sheet metal, while at the same time forming a satisfactory stack structure.

The intermediate annealing according to the invention and the subsequent shaping can be repeated one or more times, it being possible to carry out repetitions both before and after or after a first recrystallization annealing. Only the first intermediate annealing and the subsequent forming must be carried out before a first recrystallization annealing. It is also important that intermediate annealing and forming are only carried out in combination with one another as long as the material has not yet undergone a first recrystallization annealing.

Additional recrystallization annealing following a repetition cycle Intermediate annealing and forming can bring about an additional improvement in the stack structure compared to only once for the purpose of recrystallization annealing.

In the case of a repetition cycle, the further transformations from 3% to 30% each relate to the cross section of the material during the previous annealing.

The method according to the invention is particularly suitable for refractory metal alloys made of molybdenum, tungsten, chromium and alloys of these metals with one another which are used to achieve the stack structure with aluminum, potassium and silicon or also with compounds and / or mixed phases from the group of oxides, nitrides, carbides , Borides, silicates or aluminates with a melting point above 1500 ° C.

The production process according to the invention is explained in more detail below by examples.

example 1

bei 1800°C und 10 N/mm² Belastung.
Eine weitere Verbesserung des Stapelgefüges wurde dadurch erreicht, daß das 5 mm starke Blech vor der abschließenden Rekristallisationsglühung bei 1100°C während 5 Stunden nochmals zwischengeglüht und anschließend in einem Schritt auf 4,5 mm Stärke fertiggewalzt wurde. Die Kriechgeschwindigkeit dieses Bleches betrug 2,5 . 10⁻⁵ $$\frac{\text{m/m}}{\text{h}}$$

bei 1800°C und 10 N/mm² Belastung.
Ebenso ist es möglich, das 5 mm Blech nach der Rekristallisationsglühung in einem Schritt auf 4,5 mm fertigzuwalzen.
In diesem Fall kann sowohl die nochmalige Zwischenglühung bei 1100°C als auch eine nochmalige abschließende Rekristallisationsglühung entfallen.Potassium silicate solutions were sprayed into molybdenum oxide, which was then reduced to molybdenum metal powder in a first stage at about 650 ° C in H₂ countercurrent to MoO₂ and in a second stage at about 1100 ° C. The amount sprayed in was so dimensioned that the metal powder contained 0.175% by weight of silicon and 0.152% by weight of potassium.
The molybdenum powder with an average grain size of approx. 5 µm was then pressed on a die press with 3 MN to form sheets with the dimensions 550 mm x 200 mm x 70 mm.
The plates were then sintered under H₂ protective gas with a heating time of 3 hours and a holding time of 5 hours at 1800 ° C.
The sintered plates, starting at a forming temperature of approx. 1400 ° C, were rolled out in steps of approximately 10% to form sheet of 5.6 mm thickness. After annealing under H₂ protective gas at 1100 ° C for 5 hours, the sheet was finish-rolled at 800 ° C in one step to 5 mm thickness.
After the final recrystallization annealing at 1900 ° C for 15 minutes, the sheet metal structure showed a stacked structure. The creep speed of this sheet was 6. 10⁻⁵ $$\frac{\text{m / m}}{\text{H}}$$

at 1800 ° C and 10 N / mm² load.
A further improvement of the stack structure was achieved in that the 5 mm thick sheet was annealed again for 5 hours at 1100 ° C. before the final recrystallization annealing and then finished rolled to 4.5 mm thickness in one step. The creep speed of this sheet was 2.5. 10⁻⁵ $$\frac{\text{m / m}}{\text{H}}$$

at 1800 ° C and 10 N / mm² load.
It is also possible to finish-roll the 5 mm sheet to 4.5 mm in one step after the recrystallization annealing.
In this case, the repeated intermediate annealing at 1100 ° C as well as another final recrystallization annealing can be omitted.

Example 2

bei 1800°C und 10 N/mm² Belastung.98.8% by weight of molybdenum powder with an average particle size of approximately 5 µm was mixed with 1.2% by weight of La (OH) ₃ powder with an average particle size of 0.4 µm in a compulsory mixer and on a die press pressed with 3 MN into sheets with the dimensions 170 mm x 400 mm x 54 mm.
Then the plates were sintered under H₂ protective gas with a heating time of 3 hours and a holding time of 4 hours at 2000 ° C.
The sintered plates were rolled at a forming temperature of approx. 1400 ° C, with gradations of around 10% each, to sheet 2.2 mm thick.
After annealing under H₂ protective gas at 1100 ° C for 5 hours, the sheet was finish-rolled to 700 mm in one step to 2 mm. After a final recrystallization annealing at 2300 ° C for 15 minutes, the sheet metal structure showed a stack structure, the structure grains having an average length to width ratio of 5: 1 exhibited. The sheet creep speed was 1.5. 10⁻⁴ $$\frac{\text{m / m}}{\text{H}}$$

at 1800 ° C and 10 N / mm² load.

Example 3

95.3% by weight of molybdenum powder with an average grain size of approx. 5 µm were mixed with 4.7% by weight of La (OH) ₃ powder with an average grain size of 0.4 µm under the same conditions as in Example 2 mm sheet metal processed.
The final recrystallization annealing was carried out at 2300 ° C for 15 minutes. In the stack structure formed afterwards, the structure grains had an average length to width ratio of more than 10: 1.

Example 4

Blue tungsten oxide powder was mixed with potassium silicate and aluminum chloride solutions and under H₂ protective gas at a temperature of about 1000 ° C to doped metal powder with an average grain size of 5 microns with 0.16 wt.% Potassium, 0.19 % By weight silicon and 0.027% by weight aluminum reduced.
The powder was washed with hydrofluoric acid and cold isostatically pressed with 3 MN to square bars with a cross section of 2 cm x 2 cm. Then the rods were sintered under H₂ protective gas after a heating time of 5 hours at 2600 ° C for 5 hours. The sintered bars were hammered into bars with a diameter of 7 mm, starting with forming temperatures of 1600 ° C., in increments of approximately 10% each, and then drawn into wires with a diameter of 5.15 mm. After annealing under H₂ protective gas at 1250 ° C for 3 hours, the wires were pulled to a diameter of 5 mm in one step.
The stacked structure was formed during a recrystallizing annealing at 2300 ° C. for 15 minutes.

Example 5

A potassium silicate solution was added to molybdenum oxide powder in such a way that a mixture of molybdenum with 0.20% by weight potassium and 0.315% by weight silicon was present after the reduction. This doped molybdenum powder was mixed with the same amount of chromium powder and pressed on a die press with 3 MN to form plates with the dimensions 400 mm × 170 mm × 40 mm.
The plates were then sintered under H₂ protective gas with a heating time of 3 hours and a holding time of 7 hours at 1700 ° C. The sintered plates were rolled at a forming temperature of approx. 1200 ° C, with gradations of approximately 10% in each case to sheet metal of 3.3 mm thickness.
After annealing in vacuo at 880 ° C for 5 hours, the sheet was finish-rolled at 700 ° C to 3 mm.
With a final recrystallization annealing at 1700 ° C for 15 minutes, the stack structure was formed.

Example 6

bei 1800°C und 10 N/mm² Belastung.
Die restlichen gesinterten Platten wurden erfindungsgemäß bei einer Umformtemperatur von ca. 1400°C beginnend, mit denselben Abstufungen von jeweils etwa 10 % Umformgrad, zu Blech von 2,2 m Stärke gewalzt.
Nach einer Glühung unter H₂-Schutzgas bei 1100°C während 5 Stunden wurde das Blech bei ca. 700°C in einem Schritt auf 2 mm fertiggewalzt. Bei einer abschließenden Rekristallisationsglühung bei 1900°C während 15 Minuten wies das Blech eine gute Stapelgefügestruktur auf. Die Kriechgeschwindigkeit des Bleches betrug 3,1 . 10⁻⁴ $$\frac{\text{m/m}}{\text{h}}$$

bei 1800°C und 10 N/mm² Belastung (gegenüber 1,6 . 10⁻² $$\frac{\text{m/m}}{\text{h}}\text{)}$$

.In this example, the manufacture of semi-finished products of the same dimensions is compared once according to the prior art and once according to the method according to the invention.
It can be seen that the creeping speed of the semifinished product produced according to the invention is comparatively significantly lower and, consequently, the stack structure is formed, while the semifinished product which was manufactured according to the prior art has no stack structure.
A potassium silicate solution was added to molybdenum oxide powder in such a way that 0.175% by weight of silicon and 0.152% by weight of potassium were contained in the reduced molybdenum metal powder. The doped metal powder with an average grain size of approx. 5 µm was pressed on a die press with 3 MN to form plates with the dimensions 400 mm x 170 mm x 47 mm.
The plates were then sintered under H₂ protective gas with a heating time of 3 hours and a holding time of 5 hours at 1700 ° C. Some of these plates were rolled according to a state-of-the-art manufacturing process, starting at a forming temperature of approx. 1400 ° C., with gradations of approximately 10% in each case, into sheet metal of 2 mm thickness.
With a final recrystallization annealing at 1900 ° C for 15 minutes, no stack structure was formed. The structure remained essentially fine-grained and was not elongated. The sheet creep speed was 1.6. 10⁻² $$\frac{\text{m / m}}{\text{H}}$$

at 1800 ° C and 10 N / mm² load.
The remaining sintered plates were rolled according to the invention at a forming temperature of approximately 1400 ° C., with the same gradations of approximately 10% degree of deformation, into sheet metal of 2.2 m thickness.
After annealing under H₂ protective gas at 1100 ° C for 5 hours, the sheet was finish-rolled at about 700 ° C in one step to 2 mm. During a final recrystallization annealing at 1900 ° C for 15 minutes, the sheet had a good stack structure. The sheet creep speed was 3.1. 10⁻⁴ $$\frac{\text{m / m}}{\text{H}}$$

at 1800 ° C and 10 N / mm² load (compared to 1.6. 10⁻² $$\frac{\text{m / m}}{\text{H}}\text{)}$$

.

Claims (4)

1. A process for manufacturing semifinished products from sintered refractory metal alloys with elongated strain structure, in which a strain of at least 85 % is produced in the sintered material by mechanical deformation in several forming steps and the sintered material is then subjected to a recrystallization annealing,
   characterized in that
   the sintered material with a strain of at least 85 % is subjected before the recrystallization annealing for at least 20 minutes, at a minimum temperature of 700 °C and a maximum temperature of that at which recrystallization does not yet quite occur, to an intermediate annealing and following the intermediate annealing is deformed in a heated-up state by a further 3 % to 30 %.
2. Process for manufacturing semifinished products from sintered molybdenum alloys with elongated strain structure according to Claim 1, characterized in that the intermediate annealing takes place for at least 20 minutes at a temperature between 950 °C and 1300 °C.
3. Process for manufacturing semifinished products from sintered tungsten alloys with elongated strain structure according to Claim 1, characterized in that the intermediate annealing takes place for at least 20 minutes at a temperature between 1250 °C and 1700 °C.
4. Process for manufacturing semifinished products according to one of Claims 1 to 3, characterized in that the deformation after the intermediate annealing takes place with a strain of 10 % relative to the sintered material deformed by at least 85 %.