JP2007169789A - Method for producing semi-manufactured product or component having high density - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semi-manufactured product or a component having high density particularly in the central part while having a fine structure where crystal grains are fined. <P>SOLUTION: This invention refers to a method for producing a component or a semi-manufactured product having an average relative density of >98.5% and a relative core density of >98.3%, and composed of a substance selected from a group comprising molybdenum, molybdenum alloys, tungsten and tungsten alloys. The method includes: a sintering stage where relative density D is allowed to satisfy 90%<D<98.5% and the ratio of closed pores to the whole porosity is made higher than 0.8; and a stage where hot isostatic pressing is performed at a temperature satisfying (0.40 to 0.65)×a solidus temperature under 50 to 300 MPa. The component manufactured in this way enables the remarkable increase of pot life upon use, e.g., as an electrode. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention provides a semi-finished product comprising a group of materials having an average relative density greater than 98.5% and a relative core density greater than 98.3% and comprising molybdenum, molybdenum alloys, tungsten and tungsten alloys, or The present invention relates to a method for manufacturing a component.

  The refractory metals molybdenum, tungsten and their alloys are usually produced by powder metallurgy. Using ore concentrate as a starting material, chemically processed to form an intermediate and then reduced to obtain a metal powder. In this case, the reducing agent is hydrogen. The alloy elements can be mixed before, during or after the reduction.

Typical molybdenum alloys are TZM (Ti—Zr—C alloyed Mo), Mo—La ₂ O ₃ , Mo—Y ₂ O ₃ and Mo—Si—B. The tungsten-based, AKS-W (potassium (K) _{W-doped), W-ThO 2, W} -La 2 O 3, W-Ce 2 O 3, W-Y 2 O 3 and AKS-W-ThO ₂ Is mentioned. AKS-W and AKS-W-ThO ₂ are used in particular in the lighting technology, ie in particular for filaments and electrodes. AKS-W is made by adding K, which takes the form of small bubbles, stabilizes the growth of crystal grains, and as a result can maintain a stable microstructure for a long time at high operating temperatures. This is extremely important for the working life characteristics of the electrodes of particularly powerful lamps such as metal halide lamps and short arc lamps whose surface temperature reaches 2600 ° C.

  The powder can be compressed and densified by a die processing press method or a cold isotropic processing press method. A semi-finished product having a large size is preferably produced by a cold isostatic pressing method. In the case of a wire rod or a small roll sheet bar, either a die processing press method or a cold isostatic pressing method can be used. When using molybdenum powder having a typical particle size of 2 to 5 μm by the Fisher method and tungsten powder having a typical particle size of 1.5 to 4.5 μm by the Fisher method, 0.11 to 0.17 ( Molybdenum) and fractional bulk densities in the range of 0.13 to 0.22 (tungsten) are obtained. When the pressing pressure is 200 to 500 MPa, a fractional green density of 0.6 to 0.68 is achieved in both molybdenum and tungsten.

  The green product is sintered in the next process step. In this case, the sintering process is carried out so that the sintered body has as low a porosity as possible and has a finely crystallized microstructure. Molybdenum and tungsten are usually sintered in hydrogen with a dew point of less than 0 ° C. The normal sintering temperature is 1800-2200 ° C. for molybdenum and 2100-2700 ° C. for tungsten. Normal sintering time is 1 to 24 hours. Since this sintering process is determined by boundary diffusion of crystal grains, sintering at a lower temperature is possible when the grain size is small. However, depending on the particle size, the pore size in the semi-finished product to be sintered is also determined. For example, when the particle size of the molybdenum powder used is reduced from 10 μm to 2.6 μm as measured by the Fisher method, the pore size is reduced to 1/3.

  However, the disadvantage of finely crystallized powder is that the proportion of adsorbed gas, especially oxygen, is high. This is because during the sintering process, oxygen reacts with hydrogen in the sintering gas to produce water vapor. Since the gas permeability of the unsintered compact is low and it is further lowered during the sintering process, water vapor cannot be sufficiently removed particularly from the center of the sintered body. This occurs inevitably when a fine crystallized powder having a particle size of less than 4.5 μm as measured by the Fisher method is used.

When the water vapor content in the sintered body is high, it triggers the CVT (Chemical Vapor Transport) reaction. Due to mass transport through the gas phase, this CVT reaction reduces the specific surface area, and as a result, the driving force for sintering, particularly within the sintered body, decreases. This process is powerful in the case of molybdenum and tungsten alloys, and the additive during sintering releases oxygen-containing species and promotes the generation of water vapor. Such examples are observed with AKS-W, Mo-La ₂ O ₃ or W-La ₂ O ₃ . Therefore, particularly in the case of these alloys, the size of the sintered body is limited by the gas phase reaction. When the size of the sintered body is large or when using extremely fine crystallized powder, the sintered density obtained at the center of the sintered body is small, or coarser powder is used. It will be lower than the case.

  After the sintering process, molybdenum, tungsten and their alloys are usually subjected to a processing heat treatment. By this heat treatment, desired molding, porosity reduction / removal, and desired mechanical and microstructural properties can be set. By increasing the degree of molding, the density can be increased to the theoretical density, and the size of the crystal grains can be reduced. Thus, the size of the crystal grains is highly dependent on the selected forming temperature and intermediate annealing temperature.

  As already mentioned, when using fine grained powders or using alloys containing chemical species that release oxygen or water during the sintering process, the size of the sintered body is limited. Arise. If an attempt is made to produce a larger dimension product from this sintered body, the degree of formability possible there may not be sufficient to close the pores, especially in the center of the sintered body.

  This applies, for example, to AKS tungsten used as a lamp electrode material. In particular, for short arc lamps, an anode having a maximum diameter of 55 mm is used. It is the dimensional stability that determines the pot life of such electrodes. Electrode deformation begins with heat-induced stress. This thermally induced stress can lift, for example, the flat area of the electrode. Then, the arc concentrates on this raised part, and local overheating occurs. This can cause melting of the electrode in that region.

  Furthermore, evaporation of the electrode material becomes active with local overheating. The evaporated electrode material accumulates on the lamp sphere, resulting in a significant reduction in luminous flux.

  As a result of research so far, it has been clarified that the above-mentioned lifting occurs with the creep phenomenon. If the material contains pores, these creep phenomena become significant. This is because the pores act as void and sinking sources. Further, since the pores suppress heat dissipation, a local temperature rise becomes remarkable.

  In addition to this, the fine grained electrode material exhibits a longer pot life. This is thought to be due to the fact that in a coarse grained material, the damage is concentrated at several grain boundaries, and as a result the arc is concentrated, so that a self-enhancing effect occurs there.

  It is therefore an object of the present invention to provide a semi-finished product or component having a high density, especially in the central part, while having a finely grained microstructure.

  This problem is solved by a method having the features of claim 1.

Using the method of the present invention to produce a semi-finished product or component of molybdenum, tungsten and their alloys having an average relative density greater than 98.5% and a relative core density greater than 98.3% it can. Here, the average relative density means the average density with respect to the weight of the unit volume, and the core density means the density of the central part of the semi-finished product or the component among those skilled in the art. Since the core volume with respect to the entire volume is not specified, in the following description, the core volume for obtaining the core density is defined as follows. That is,
10% of the product of the direction perpendicular to the deformation and the size of the deformation direction closest to the center of the total surface area.

In the deformed state, the semi-finished product or component preferably has a number of crystal grains greater than 100 grains / mm ^{2 in} a direction perpendicular to the direction of deformation.

  In the method according to the present invention, commercially available molybdenum and tungsten powders having a particle size range of 0.5 to 10 μm as measured by the Fisher method are used.

  Alloying elements may be added to these powders before, during or after the reduction process. These powders are compressed and densified under a pressure of 100 to 500 MPa by a normal compression densification process, for example, a pressing method or a cold isotropic pressing method.

Sintering occurs at a temperature of (0.55-0.92) × solidus temperature. At this time, the sintering temperature is selected, a sintered density corresponding to 90 to 98.5% of the theoretical density is obtained, and the ratio of the closed pores to the total pores is made larger than 0.8. If the relative density is higher than 98.5%, it is not possible to produce a target product, ie a component or semi-finished product with a number of grains greater than 100 grains / mm ² .

When the ratio of the closed pores to the whole pores is larger than 0.8, the desired property can be surely obtained by the hot isostatic press which is the next step. If the value is lower than 0.8, a molding step of 2% <ψ <60% is required after the sintering process. ψ is defined as follows.
((Initial cross-sectional surface area−cross-sectional surface area after molding process) / initial cross-sectional surface area) × 100.
Thereby, the pore of a peripheral part is closed reliably.

  The hot isostatic pressing method is carried out at a temperature of (0.40 to 0.65) × solidus temperature under a pressure of 50 to 300 MPa without using a mold. If the temperature is below 0.4 × solidus temperature, the goal of an average relative density greater than 98.5% and a relative core density greater than 98.3% in the component or semi-finished product cannot be achieved. If the temperature is higher than 0.65 × solidus temperature, it is not desirable because the crystal grains become coarse due to normal or abnormal crystal grain growth. If the pressure is lower than 50 MPa, the desired density cannot be obtained in the same manner. If the pressure exceeds 300 MPa, the method according to the invention is no longer profitable.

  In the subsequent process, a compacted part is molded by hot isostatic pressing. The forming degree ψ at this time is 15 to 90%. If the forming degree ψ is less than 15%, a relative core density greater than 98.3% cannot be obtained. If the forming degree exceeds 90%, this method cannot be profitable. This is because high-density products can be produced without using the hot isostatic pressing method according to the present invention.

  The method according to the invention has been found to be particularly useful for the production of electrodes having a diameter of 15 to 55 mm for use in discharge lamps. When the diameter is less than 15 mm, this type of electrode can be produced economically by conventional production methods. The upper limit of 55 mm is a number derived from the limit power of this type of lamp.

  The raw material for the electrode is preferably subjected to forming by a radial forging method or a rolling method. As a result of the test, it was found that the electrode manufactured by the method of the present invention has a pot life which is 20% longer on average than the electrode manufactured by the conventional manufacturing method.

  The present invention is described in more detail using the following examples.

Example In order to produce an AKS-W electrode, an AKS-W powder having a particle size of 4.1 μm as measured by the Fisher method was used. The powder was compressed and densified by a cold isostatic pressing method at a press pressure of 200 MPa to form a green compact. Next, sintering was performed at 2250 ° C. in hydrogen. The finished sintered rod showed an average relative density of 92.0% as measured using the buoyancy method. The percentage of closed pores was greater than 95%. The measurement was performed by the mercury porosimeter method. In the next step, the sintered body was compressed and densified by hot isostatic pressing at a temperature of 1750 ° C. and a pressure of 195 MPa over 3 hours. The relative average density after hot isostatic pressing operation was 97.9%. The rod was then formed on a radial forging machine. The forming degree ψ was 67%. After the molding process, the rod exhibited an average relative density of 99.66% and a relative core density of 99.63%. The grain size was measured after the green state and after annealing at 1800 ° C. for 4 hours. In an unmolded state, it was about 10,000 crystal grains / mm ² in both the central portion and the peripheral portion of the rod. In the annealed state, a very fine crystallized microstructure was formed, and the average number of crystal grains was about 800 at the center of the rod and 850 grains / mm ² at the periphery.

  Chemical analysis of the finished rod gave results of potassium: 15 μg / g, silicon: 6 μg / g, carbon: less than 5 μg / g, oxygen: 7 μg / g.

  Using the material prepared according to the present invention, a 2.5 kW short arc lamp anode for movie projection was produced and confirmed to have an average pot life of 2060 hours. For comparison, a material using the same manufacturing process was used except that compression densification by hot isostatic pressing operation after the sintering process was not performed, and the average pot life was 1710 hours. Met.

Claims (9)

A method of producing a component or semi-finished product having a mean relative density greater than 98.55% and a relative core density greater than 98.3% from a group of materials including molybdenum, molybdenum alloys, tungsten and tungsten alloys. A method comprising the following steps:
A step of preparing a powder having a particle size of 0.5 to 10 μm as measured by the Fisher method;
Pressurizing the powder under a pressure of I00 to 500 MPa,
Sintering at a temperature of (0.55 to 0.92) × solidus temperature and setting the relative density D to 90% <D <98.5%,
A process of hot isostatic pressing at a temperature of (0.40 to 0.65) × solidus temperature and a pressure of 50 to 300 MPa without using a mold, and a forming degree ψ Is formed so that 15% <ψ <90%. The method of claim 1, wherein the component or semi-finished product has an average grain number greater than 100 grains / mm ² in a deformed state.   The method according to claim 1 or 2, characterized in that, before the hot isostatic pressing, the sintered body is further formed by 2% <ψ <60%.   4. The method according to claim 1, wherein in the sintered body, a ratio of closed pores to whole pores is larger than 0.8. 5.   The method according to one of claims 1 to 4, characterized in that the component or semi-finished product consists of potassium-doped tungsten (AKS-W) and has a potassium content of 5 to 70 μg / g.   The method according to claim 1, wherein the forming is performed by a radial forging process or a rolling process, and a rod is manufactured by this method.   The method according to claim 6, wherein the rod has a diameter of 15 to 55 mm.   8. A method according to claim 6 or 7, characterized in that the rod is used to produce a lamp electrode.   9. The method of claim 8, wherein the lamp electrode is used for a short arc lamp.