Glass-melting component

Abstract

A process for producing a glass melting component composed of refractory metal. A surface zone of the glass melting component is densified at least in sections by application of local compressive stress. As a result the surface zone has its porosity reduced compared to a volume section which is located underneath the surface zone and which has residual porosity.

Claims

1. A glass melting component composed of refractory metal, the component comprising: a surface zone and a volume section underneath said surface zone; said surface zone being, at least in sections, densified and having a reduced porosity relative to a residual porosity of said volume section underneath said surface zone; said surface zone and said volume section being composed of the refractory metal; and said surface zone having a coarser grain structure than said volume section of the glass melting component; wherein an average grain size in said surface zone lies in a range from 40 μm to 1000 μm.

2. The glass melting component according to claim 1, being a glass melting component produced by a powder-metallurgical process.

3. The glass melting component according to claim 1, wherein the porosity of said surface zone is not more than 1%.

4. The glass melting component according to claim 1, wherein the porosity of said surface zone is at least 1.5 percentage points lower than the porosity of said underlying volume section.

5. The glass melting component according to claim 1, wherein said surface zone has a depth in the range from 50 μm to 1000 μm.

6. The glass melting component according to claim 1, wherein a surface of said surface zone has a roughness Ra of less than 0.30 μm.

7. The glass melting component according to claim 1, wherein the glass melting component is a crucible.

8. The glass melting component according to claim 1, wherein the glass melting component is a glass melting electrode.

9. The glass melting component according to claim 1, comprising an interior wall and an outer wall each having a respective said densified surface zone.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1a-b working examples of glass melting components according to the invention, schematically in cross section

(2) FIG. 2a-b metallographic sections of samples which have been densified in a surface zone (unetched to determine the local porosity)

(3) FIG. 3a-d metallographic sections of samples which have been densified in a surface zone and heat treated (etched to determine the grain size)

(4) FIG. 4a-b a schematic depiction of the process

DESCRIPTION OF THE INVENTION

(5) FIGS. 1a and 1b schematically show working examples of glass melting components 1 according to the invention in cross section. In FIG. 1a, the glass melting component 1 is configured as a crucible. The crucible can be provided, for example, for growing sapphire single crystals. In this case, the crucible is supplied with a charge of aluminum oxide which is melted in the crucible. The material of the crucible is typically tungsten or molybdenum. A surface zone 2 has been densified according to the invention on an interior wall of the crucible by application of a mechanical compressive stress, as a result of which the surface zone 2 has a lower porosity than underlying volume sections 3 of the glass melting component 1. The densification of the surface zone 2 is preferably carried out by smooth rolling. In smooth rolling, a rolling body is pressed against and conveyed over the surface. In the case of rotationally symmetric components, this can occur, for example, on a lathe.

(6) The relative density of pressed/sintered crucibles (corresponding to the underlying volume sections 3) is, for example, 96%. This corresponds to a porosity of 4%.

(7) The porosity of the surface zone 2 after densification is, for example, 0.02%. Due to the densified surface zone 2, the crucible is significantly more resistant to corrosive attack by the aluminum oxide melt than a conventional crucible. The virtually pore-free microstructure of the surface zone 2 offers less surface area to a melt for corrosive attack than does a microstructure having residual porosity.

(8) The crucible which has been densified at the surface according to the invention is preferably subjected to a heat treatment above the recrystallization temperature of the densified surface zone 2. As a result of the method of production, underlying volume sections 3 of the pressed/sintered crucible do not have any plastic deformation, for which reason no or only minor recrystallization occurs in underlying volume sections 3. The recrystallization in the surface zone 2 results in strong grain growth in the surface zone 2. The temperature required for setting the desired coarse-grain microstructure can be determined by a person skilled in the art by means of experiments. Here, for example, heat treatments at various temperatures can be carried out. The temperature at which an advantageous coarse-grain microstructure is obtained can be determined by metallographic evaluation of the samples.

(9) The resulting microstructure in the surface zone 2 has significantly coarser grains than the microstructure in underlying volume sections 3. The coarse-grain microstructure of the surface zone 2 brings about a further-increased resistance of the glass melting component 1 to corrosive attack by a melt. Since grain boundaries always represent weak points for corrosive attack, a coarse-grain microstructure is more corrosion-resistant than a microstructure having fine grains.

(10) The heat treatment preferably takes place in situ during use of the crucible. The typical use temperatures for crucibles for growing sapphire single crystals are ≥2000° C. Thus, the desired coarse-grain microstructure is formed in the first use of the crucible without a separate heat treatment having to be carried out.

(11) In FIG. 1b, the glass melting component 1 is configured as glass melting electrode. Glass melting electrodes serve to heat glass melts by direct passage of electric current through the glass melt.

(12) Glass melt electrodes are typically made of forged and/or rolled molybdenum rods. Owing to the low degrees of deformation of rod material, glass melting electrodes have, for example, a residual porosity of 3%, corresponding to a relative density of 97%.

(13) The glass melting electrode according to the present working example has a densified surface zone 2 having a porosity of, for example, 0.02%, while the porosity is 3% in underlying volume sections 3 of the glass melting electrode.

(14) As indicated for the working example of the crucible, a heat treatment to recrystallize the surface zone 2 is preferably also carried out here.

(15) The glass melting electrode according to the working example is significantly more resistant to corrosive attack by glass melts than a conventional glass melting electrode.

(16) FIGS. 2a-b show optical micrographs of unetched metallographic polished sections through the surface zone 2 and underlying volume sections 3 of pressed/sintered molybdenum round plates in which a surface zone 2 has been densified by smooth rolling with a Ø6 mm ball at a rolling pressure of 150 bar or 250 bar.

(17) The surface zone 2 is in each case located at the left-hand margin of the picture. It can be seen even from the pictures that the porosity in the surface zone 2 has been significantly reduced compared to the underlying volume sections 3. The following table shows the results of the porosity determination on the samples of FIGS. 2a-b.

(18) TABLE-US-00004 TABLE 4 Results of the porosity determination on the samples of FIGS. 2a-b Rolling pressure FIG. Position Porosity [%] 150 bar FIG. 2a Surface zone 0.4 500 μm from 4.6 the surface 250 bar FIG. 2b Surface zone 0.2 500 μm from 5.1 the surface

(19) The quantitative evaluation gave a porosity of the surface zone 2 of about 0.4% for a rolling pressure of 150 bar and of about 0.2% for a rolling pressure of 250 bar. The porosity in underlying volume sections 3, on the other hand, is about 4-5%.

(20) In summary, it can be seen from FIGS. 2a-b and the quantitative microstructural analysis that application of compressive stress, in this example by means of smooth rolling, to the surface zone 2 of a pressed/sintered sample leads to virtually complete elimination of pores. The depth of the densified surface zone 2 is about 200 μm in the case of the samples subjected to a rolling pressure of 150 bar, and is about 300 μm in the case of the samples subjected to a rolling pressure of 250 bar.

(21) The determination of the porosity was carried out by means of quantitative microstructural analysis via proportion by area of pores. For this purpose, a black-and-white picture is produced from the optical micrograph and the proportion by area of the pores is determined on representative sections of the picture using an image analysis program.

(22) FIGS. 3a-d show optical micrographs of polished sections through the densified surface zone 2 and parts of underlying volume sections 3 of samples of pressed/sintered molybdenum which have been densified in the surface zone 2. The polished sections were etched in order to make the grain boundaries visible.

(23) The parameters in the densification of the surface zone 2 were 150 bar (FIGS. 3a and 3b) or 250 bar (FIGS. 3c and 3d) rolling pressure using a Ø6 mm rolling ball as rolling body.

(24) FIGS. 3a and 3c show the initial state (“IS”) after densification of the surface zone 2 by application of mechanical compressive stress before a heat treatment. In the initial state before a heat treatment, a fine-grain sintered microstructure is present both in the surface zone 2 and in underlying volume sections 3.

(25) FIGS. 3b and 3d show the microstructure after recrystallization by means of a heat treatment at 2100° C. and a hold time of two hours. The coarsening of the grain structure in the surface zone 2 can clearly be seen. In undeformed underlying volume sections 3, a fine-grain microstructure remains.

(26) For better discernibility, individual grains are outlined. Detail A in FIG. 3a shows, by way of example, a grain in the surface zone 2 in the initial state. The grain size is about 40 μm. Detail B in FIG. 3b shows a grain in the surface zone 2 of a sample which has been heat treated at 2100° C. The grain size is about 250 μm. The grain sizes in undeformed underlying volume sections 3 remain unchanged. It can be seen therefrom that the recrystallization bringing about grain growth takes place only in the surface zone 2 into which driving force for the recrystallization in the form of mechanical work of deformation has been introduced by the mechanical densification.

(27) The results of a quantitative microstructural analysis in respect of the grain sizes are summarized in the following table.

(28) TABLE-US-00005 TABLE 5 Results of the quantitative microstructural analysis of the samples of FIG. 3 Rolling GS GS NG pressure State Position [μm] ASTM [1/mm.sup.2] 150 bar IS Surface zone 33 6.5 900 (FIG. 3a) 500 μm from 30 6.8 1056 the surface 2100° C./2 h Surface zone 64 4.6 240 (FIG. 3b) 500 μm from 38 6.1 676 the surface 250 bar IS Surface zone 35 6.3 784 (FIG. 3c) 500 μm from 35 6.4 812 the surface 2100° C./2 h Surface zone 64 4.6 240 (FIG. 3d) 500 μm from 41 5.9 576 the surface

(29) Table 5 shows the results of the quantitative microstructural analysis of the samples depicted in FIGS. 3a-d. The abbreviations have the following meanings: IS—initial state after smooth rolling, GS—average grain size, NG—number of grains. The average grain size in μm, the grain size determined by the ASTM method (“ASTM number”) and the number of grains were determined in accordance with ASTM E112-13. The evaluation was carried out by the line intercept method on optical micrographs of etched polished sections. The measurement accuracy in the determination of the grain size in μm is about 5%.

(30) It can be seen that the grain size in the surface zone 2 after the heat treatment is almost doubled compared to the undeformed microstructure in underlying volume sections 3.

(31) The heat treatment above the recrystallization temperature of the deformed surface zone 2 thus leads to significant grain growth. Since the number of grain boundaries in a coarse-grain microstructure is reduced, and diffusion of impurities in the high-temperature range takes place predominantly along the grain boundaries, a coarse-grained surface zone 2 results in reduced grain boundary diffusion. Corrosive attack by diffusion of melt along grain boundaries is made more difficult. In addition, the coarse-grain microstructure of the surface zone 2 improves the creep resistance of a glass melting component 1 produced in this way.

(32) Finally, the resulting very smooth surface of the surface zone 2 reduces possible bubble formation caused by oxides in a melt. A smooth surface offers fewer nuclei for the formation of gas bubbles.

(33) This leads to significantly improved properties of glass melting components 1 produced in this way.

(34) The densification of the surface zone 2 by application of local compressive stress is inexpensive and can in principle be employed for all refractory metal materials.

(35) FIG. 4a shows a schematic depiction of the process of the invention. In the example shown, the glass melting component 1 is a crucible. A surface zone 2 of the glass melting component 1 having residual porosity in underlying volume sections 3 is densified by application of local compressive stress by means of a rolling body 4 and the surface zone 2 becomes virtually pore-free. The process is effected here by pressing-on of a spherical rolling body 4.

(36) The glass melting component 1 is then preferably subjected to a heat treatment above the recrystallization temperature of the refractory metal forming the glass melting component 1, as shown schematically in FIG. 4b. This heat treatment leads to coarse grain formation in the previously deformed surface zone 2.

(37) The apparently paradoxical route of subjecting the glass melting component 1 which has previously been plastically deformed in a surface zone 2 to a heat treatment at a temperature above the recrystallization temperature of the refractory metal is thus chosen. A heat treatment above the recrystallization temperature leads in all cases to any increases in hardness due to cold hardening and also any residual compressive stresses present being degraded. In the prior art, however, it is precisely this, namely an increase in hardness and induction of residual compressive stresses, which is intended as a result of application of local compressive stress. For this reason, a heat treatment above the recrystallization temperature is not carried out in the prior art on components which have previously been treated for the purpose of increasing the hardness and/or introducing residual compressive stresses.

(38) However, as indicated above, the coarse-grain microstructure resulting from this heat treatment leads to improved resistance of the glass melting component 1 to corrosive attack by glass melts.