Sintered molybdenum part

Abstract

A powder-metallurgical sintered molybdenum part which is present as a solid body has the following composition: a molybdenum content of 99.93% by weight, a boron content B of 3 ppmw and a carbon content C of 3 ppmw, with a total content BaC of carbon and boron being in a range of 15 ppmwBaC50 ppmw, an oxygen content O in a range of 3 ppmwO20 ppmw, a maximum tungsten content of 330 ppmw and a maximum proportion of other impurities of 300 ppmw. A powder-metallurgical process for producing such a sintered molybdenum part is also provided.

Claims

1. A powder-metallurgical sintered molybdenum part being present as a solid body, the sintered molybdenum part comprising the following composition: a. a molybdenum content of 99.93% by weight; b. a boron content B in a range of 5B45 ppmw and a carbon content C in a range of 5C30 ppmw, with a total content BaC of carbon and boron being in a range of 15 ppmwBaC50 ppmw; c. an oxygen content O in a range of 3 ppmwO20 ppmw, d. a maximum tungsten content of 330 ppmw; e. a maximum proportion of other impurities of 300 ppmw; and f. a maximum proportion of additives of Zr, Hf, Ti, V and Al of 50 ppmw.

2. The sintered molybdenum part according to claim 1, wherein said oxygen content O is in a range of 5O15 ppmw.

3. The sintered molybdenum part according to claim 1, which further comprises: a maximum proportion of contamination by silicon, rhenium and potassium of 20 ppmw in total.

4. The sintered molybdenum part according to claim 1, which further comprises a total content of molybdenum and tungsten of 99.97% by weight.

5. The sintered molybdenum part according to claim 1, wherein said carbon and said boron are present in dissolved form in a total amount of at least 70% by weight based on said total content of carbon and boron.

6. The sintered molybdenum part according to claim 1, wherein said boron and said carbon are finely dispersed and are present in an increased concentration in a region of large angle grain boundaries.

7. The sintered molybdenum part according to claim 1, wherein the sintered molybdenum part has sections and has a preferential orientation of at least one of large angle grain boundaries or large angle grain boundary sections perpendicular to a main forming direction.

8. The sintered molybdenum part according to claim 1, wherein the sintered molybdenum part has a partially or fully recrystallized structure, at least in sections.

9. The sintered molybdenum part according to claim 1, which further comprises a weld connection for joining the sintered molybdenum part to a further sintered molybdenum part having a composition identical the sintered molybdenum part, said weld connection including a weld zone having a molybdenum content of 99.93% by weight.

10. The sintered molybdenum part according to claim 1, wherein the following applies at least at a grain boundary section of a large angle grain boundary and an adjoining grain: a total proportion of carbon and boron in a region of said grain boundary section is at least one and one half times a total proportion of carbon and boron in a region of a grain interior of said adjoining grain, measured in atom percent by three-dimensional atom probe tomography; a three-dimensional, cylindrical region having a cylinder axis running perpendicular to said grain boundary section and a thickness running along said cylinder axis of 5 nm which, relative to a cylinder axis direction, laid centrally around said grain boundary section is selected for said region of said grain boundary section; and a three-dimensional, cylindrical region having identical dimensions and an identical orientation and having a center 10 nm away from said grain boundary section in said cylinder axis direction is employed for said region of said grain interior.

11. The sintered molybdenum part according to claim 10, wherein said total proportion of said carbon and said boron in said region of said grain boundary section is at least three times said total proportion of said carbon and said boron in said region of said grain interior of said adjoining grain.

12. A process for producing a sintered molybdenum part, the process comprising the following steps: producing the sintered molybdenum part having a molybdenum content of 99.93% by weight, a boron content B in a range of 5B45 ppmw and a carbon content C in a range of 5C30 ppmw, a total content BaC of carbon and boron in a range of 15 ppmwBaC50 ppmw, an oxygen content O in a range 3 ppmwO20 ppmw, a maximum tungsten content of 330 ppmw, a maximum proportion of other impurities of 300 ppmw and a maximum proportion of additives of Zr, Hf, Ti, V and Al of 50 ppmw, by: a. pressing a powder mixture composed of molybdenum powder and boron-containing and carbon-containing powders to give a green body; and b. sintering the green body in an atmosphere protecting against oxidation for a residence time of at least 45 minutes at temperatures in a range of 1600 C.-2200 C.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1: graph of a 3-point bending test on specimens of various sintered molybdenum parts;

(2) FIG. 2: corresponding graph as in FIG. 1 within inclusion of further specimens of sintered molybdenum parts;

(3) FIG. 3: graph of the elongation at break of various sintered molybdenum parts in a tensile test;

(4) FIG. 4: graph of the breaking strength of various sintered molybdenum parts in a tensile test;

(5) FIG. 5: three-dimensional reconstruction of a sample point of a sintered molybdenum part 15B15C according to the invention determined by atom probe tomography, showing the elements carbon (C), boron (B), oxygen (O) and nitrogen (N); and

(6) FIG. 6: graph of the linear or one-dimensional concentration profile of the elements C, B, O and N corresponding to the three-dimensional reconstruction shown in FIG. 5 along the cylinder axis drawn in in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

(7) In FIG. 1 the 3-point bending test for two sintered molybdenum parts 30B15C and 15B15C according to the invention and for a conventional sintered molybdenum part Mo pure is prepared. In FIG. 2, further sintered molybdenum parts 30B, B70, B150, C70, C150 are additionally included. The sintered molybdenum parts had the following compositions (insofar as of importance for the present invention):

(8) TABLE-US-00001 30B15C 15B15C 30B B70 B150 C70 C150 Mo pure B content [ppmw] 30 15 30 70 150 <5 <5 <5 C content [ppmw] 15 15 9 8 9 70 150 6 O content [ppmw] 9 9 8 5 6 7 <5 14 W content [ppmw] 330 330 330 330 330 330 330 330 Other impurities [ppmw] 300 300 300 300 300 300 300 300

(9) The bending angles shown in FIGS. 1 and 2 for the various sintered molybdenum parts were determined by means of a 3-point bending test. For this purpose, cuboidal test specimens having dimensions of 6*6*30 mm from the various sintered molybdenum parts were used in each case. The 3-point bending test was carried out in accordance with DIN EN ISO 7438 using a correspondingly configured test apparatus. The respective maximum bending angle attained, which was attained for the various test specimens at the test temperatures indicated in each case, before fracture of the test specimen occurred is plotted in FIGS. 1 and 2. This bending angle is firstly characteristic for the ductility, i.e. the higher the achievable bending angle, the higher the ductility of the respective sintered molybdenum part. Furthermore, the transition from ductile to brittle behaviour can be shown by means of the temperature dependence of the maximum achievable bending angle.

(10) As the comparison of the sintered molybdenum parts 30B15C and 15B15C according to the invention with the conventional sintered molybdenum part Mo pure in FIG. 1 shows, the test specimens configured according to the invention attain significantly greater bending angles at the same test temperatures. At a test temperature of 60 C., in particular, the test specimen 30B15C attains a bending angle of 99, the test specimen 15B15C attains a bending angle of 94 and the test specimen Mo pure attains a bending angle of only about 2.5. At a test temperature of 20 C., the test specimen 30B15C attains a bending angle of 82, the test specimen 5B15C attains a bending angle of 40 and the test specimen Mo pure attains a bending angle of only about 2.5. As the temperature dependence of the bending angle for the individual test specimen shows, the transition from ductile to brittle behaviour can be shifted to significantly lower temperatures in the case of sintered molybdenum parts according to the invention, in particular from 110 C. in the case of Mo pure to 10 C. in the case of 30B15C and to 0 C. in the case of 15B15C. The transition from brittle to ductile behaviour is assigned to the temperature at which a bending angle of 20 is attained for the first time. Furthermore, comparison of the test specimens 30B15C and 15B15C shows that a somewhat higher addition of boron leads, especially in the temperature range from about 20 C. to 50 C., to a further increase in the ductility, while the ductility in the other temperature ranges is comparable. For many applications, a B content of 15 ppmw and a C content of 15 ppmw will be sufficient, particularly when a very low proportion of additional elements is sought.

(11) As the comparison with the further test specimens B70, B150, C70, C150 in FIG. 2 shows, a significantly higher B or C content also leads to only a limited increase in the ductility (when the low limit values for oxygen, W content and other impurities as defined above are adhered to), with this increase being restricted essentially to the temperature range from about 20 C. to 50 C. Furthermore, the transition from ductile to brittle behaviour is shifted only slightly to lower temperatures, when the test specimen 30615C is taken as comparative measure representative of the present invention. With a view to the objective of the invention of providing very pure molybdenum, this graph shows that a significantly improved ductility is achieved by means of the composition ranges according to the invention without additives (elements/compounds) having to be added to any appreciable extent. The test specimen 30B, for which the transition from ductile to brittle behaviour lies at higher temperature than in the case of the test specimens 301315C and 15615C makes it clear that the effect of boron alone is limited and a minimum content of both carbon and boron (of, for example, in each case at least 10 ppmw, in particular in each case at least 12 ppmw) in combination has a particularly advantageous effect.

(12) FIGS. 3 and 4 show the results of tensile tests which were carried out in accordance with DIN EN ISO 6892-1 method B on correspondingly dimensioned test bars of the sintered molybdenum parts Mo pure, 30615C, 15615C, 150B, 70B, 30B, 150C, 70C. The elongation at break (in % of the change in length L relative to the initial length L) of the various test bars is shown in FIG. 3, while the breaking strength Rm (in MPa; megapascal) of the various test bars is shown in FIG. 4. Here too, it can again be seen that the sintered molybdenum parts of the invention 30615C, 15615C and 30B lead to a significant increase in both materials parameters compared to Mo pure. Furthermore, it can be seen from the test bars 70C, 150C, 70B, 150B that greater additions of boron and/or carbon (while adhering to the low limit values for oxygen, W content and other impurities as are defined above) lead to a further increase only to a small extent. Thus, the tensile tests also confirm that excellent materials properties can be achieved within the composition ranges defined according to the invention, without additives (elements/compounds) being required to an appreciable extent.

(13) FIG. 5 depicts a three-dimensional reconstruction of a sample point of a sintered molybdenum part 15B15C according to the invention determined by atom probe tomography. In this depiction, the position of the C atoms in the sample point is shown in red, that of the B atoms is shown in violet, that of the O atoms is shown in blue and that of the N atoms is shown in green. Furthermore, the Mo atoms are indicated as small dots in order to make the shape of the sample point visible. Even in a shades-of-grey depiction (in as the patent text), the positions of the various atoms are readily discernible by the different shades of grey. The three-dimensional reconstruction is also described qualitatively in the following and also supplemented quantitatively by the one-dimensional concentration profile of FIG. 6. In particular, it can be seen in FIG. 5 that the C and B atoms are distributed uniformly in the Mo base material in the upper part of the sample point, which corresponds to the region of the grain interior. In the lower part of the sample point, an area in which the B and C atoms are greatly concentrated runs perpendicular to the longitudinal extension of the sample point. As explained above in respect of atom probe tomography, this makes the profile of a grain boundary section 2 located in the sample point visible, since the B and C atoms are greatly concentrated in this.

(14) As described above in respect of atom probe tomography and shown in graph form in FIG. 5 by the three-dimensional cylinder 4, a measurement cylinder 4 is drawn by the measurement software in the three-dimensional reconstruction in such a way that its cylinder axis 6 runs perpendicular to the plane spanned by the grain boundary section 2 in order to determine the segregation of B and C quantitatively in the region of the grain boundary section relative to the region of the grain interior. In the present case, a measurement cylinder 4 having a length of 20 nm (along the cylinder axis) and a diameter of 10 nm was selected. In the depiction in FIG. 5, the grain boundary section 2 is located centrally (based on the cylinder axis 6) within the measurement cylinder 4.

(15) The linear concentration profile of the elements C, B, O and N along the cylinder axis 6 of the measurement cylinder 4 was subsequently determined in the manner explained above in respect of atomic probe tomography. FIG. 6 shows the resulting linear concentration profile in graph form. The grain boundary section can be seen from the great increase in the concentration of the elements B and C (cf. in particular, the values in the range 9 nm-3 nm along the axis Distance). As can be seen from FIG. 6, the oxygen content is increased only slightly in the region of the grain boundary and the N content is substantially constant at a low level, which is advantageous with regard to the grain boundary strength.

(16) In the following, the further procedure in order to express the proportion of B and C in the region of the grain boundary section 2 as a ratio to the proportion thereof in the region of the grain interior will be described more specifically with the aid of FIG. 6. As has been described above in detail in respect of this evaluation, five adjoining discs (each having a thickness of 1 nm) of the measurement cylinder 4, in which the sum of the measured concentrations B and C is a maximum, are selected as the three-dimensional cylindrical region representative of the grain boundary section. These are in the present case the measured values at the distances 9, 10, 11, 12 and 13 nm. As the cylindrical region of the grain interior to be examined, the five adjoining discs whose central disc is at a distance of 10 nm from the central disc of the cylindrical region of the grain boundary section are selected. These would be, in the depiction of FIG. 6, the measured values at the distances 3, 2, 1, 0, 1 (the latter value in the present case not encompassed by the measurement cylinder). The proportions of B, C and of B and C in total were subsequently determined for these two regions (of the grain boundary section and also of the grain interior) and expressed as a ratio to one another, as is described in detail above. As can be seen from the depiction in graph form in FIG. 6, the proportion of carbon and boron is in each case individually and also in total at least three times as high in the region of the grain boundary section as in the region of the grain interior of the adjoining grain. Furthermore, it can be seen from FIG. 6 (and also from FIG. 5) that B and C are (particularly in the grain interior) finely and uniformly distributed and also greatly concentrated in the region of the large angle grain boundaries.

Production Example

(17) Molybdenum powder produced by reduction by means of hydrogen was used for the powder-metallurgical production of a sintered molybdenum part according to the invention. The grain size determined by the Fisher method (FSSS in accordance with ASTM B330) was 4.7 m. The molybdenum powder contained 10 ppmw of carbon, 470 ppmw of oxygen, 135 ppmw of tungsten and 7 ppmw of iron as impurities. Including the amount of B and C present after reduction in the molybdenum powder (in the present case: C content of 10 ppmw; B not detectable), such amounts of C- and B-containing powder (39 ppmw of C and 31 ppmw of B) were added that a total proportion of 49 ppmw of carbon and 31 ppmw of boron was set in the molybdenum powder. The powder mixture was homogenized by mixing for 10 minutes in a ploughshare mixer. Subsequently, this powder mixture was introduced into appropriate tubes and cold isostatically pressed at a pressing pressure of 200 MPa at room temperature for a time of 5 minutes. The pressed bodies produced in this way (round rods each weighing 480 kg) were sintered in indirectly heated sintering plants (i.e. heat transfer to the material being sintered by thermal radiation and convection) at a temperature of 2050 C. for a time of 4 hours in a hydrogen atmosphere and subsequently cooled. The sintered rods obtained in this way had a boron content of 22 ppmw, a carbon content of 12 ppmw and an oxygen content of 7 ppmw. The tungsten content and the proportion of other metallic impurities remained unchanged.

(18) The sintered molybdenum rods according to the invention were deformed on a radial forging machine at a temperature of 1200 C., with a diameter reduction from 240 to 165 mm being carried out. Ultrasonic examination of the rod having a density of 100% did not display any cracks even in the interior and metallographic polished sections confirmed this finding.

(19) Welding Test:

(20) Sintered molybdenum parts according to the invention in sheet form were welded to one another by means of a laser welding process. The following welding parameters were set: Laser type: Trumpf TruDisk 4001 Wavelength: 1030 nm Laser power: 2.750 W (watt) Focus diameter: 100 m (micron) Welding speed: 3600 mm/min (millimetres per minute) Focus position: 0 mm Protective gas: 100% argon

(21) Studies on the microstructure showed that a uniform, relatively fine-grain microstructure had been formed even in the region of the welding zone. The welded sintered molybdenum parts had a comparatively high ductility even in the region of the weld connection, which was confirmed in a bending test in which bending angles of >70 were attained.

(22) EBSD Analysis to Determine the Drain Boundaries:

(23) The EBSD analysis which can be carried out using a scanning electron microscope is explained below. For this purpose, a cross section through the sintered molybdenum part to be examined was produced in the sample preparation. The preparation of a corresponding polished section is carried out, in particular, by embedding, grinding, polishing and etching of the cross section obtained, with the surface subsequently also being ion-polished (to remove the deformation structure on the surface arising from the grinding operation). The measurement arrangement is such that the electron beam impinges at an angle of 20 on the prepared polished section. In the scanning electron microscope (in the present case: Carl Zeiss Ultra 55 plus), the distance between the electron source (in the present case: field emission cathode) and the specimen is 16.2 mm and the distance between the specimen and the EBSD camera (in the present case: DigiView IV) is 16 mm. The information given in parenthesis relate in each case to the instrument types used by the applicant, but it is in principle also possible to use other instrument types which permit the functions described in a corresponding way. The acceleration voltage is 20 kV, a magnification of 500 is set and the spacing of the individual pixels on the specimen, which are scanned in succession, is 0.5 m.

(24) In the EBSD analysis, large angle grain boundaries (e.g. running around a grain) and large angle grain boundary sections (e.g. having an open beginning and end) which have a grain boundary angle which is greater than or equal to the minimum rotation angle of 15 can be made visible within the area examined on the specimen. Large angle grain boundaries or large angle grain boundary sections within the specimen area examined are always determined and shown between two scanned points by the scanning electron microscope when an orientation difference between the crystal lattice of 15 is found between the two scanned points. For the present purposes, the orientation difference is in each case the smallest angle which is required to make the respective crystal lattices present at the scanned points to be compared coincide. This procedure is carried out at each scanned point in respect of all scanned points surrounding it. In this way, a grain boundary pattern of large angle grain boundaries and/or large angle grain boundary sections is obtained within the specimen area examined.