Patent Application
20200199713
The present invention relates to a method for producing a platinum group metal or a platinum group-based alloy, and more particularly, to production of a molten in got in a method for producing a platinum group metal or a platinum group-based alloy.
A platinum group metal or a platinum group-based alloy is designed using heat resistance, oxidation resistance, and chemical resistance of the platinum group metal, and is widely used as a high temperature member or a corrosion-resistant product. The platinum group metal as used herein collectively refers to Pt, Pd, Rh, Ir, Ru, and Os.
Processes for production of a platinum group metal or a platinum group-based alloy generally include a compounding step, a melting step, a deformation processing step, and the like for an alloy raw material. In the melting step, a melting method for producing a molten ingot can be classified into several types. A platinum group metal as a main component has a very high melting point (1,500° C. or higher), and hence an induction heat melting furnace or an energy beam melting furnace having a melting ability of 2,000° C. or higher is used.
Energy beam melting includes nonconsumable arc melting, vacuum plasma melting, atmospheric pressure plasma arc melting, electron beam melting, and the like, and those melting processes are common in being performed through radiation of an energy beam to a raw material in a water-cooled copper crucible. The molten raw material is in the form of a plate, a wire, powder, or the like including an ingot and scrap, and is appropriately compounded by a predetermined amount for use.
The energy beam melting is broadly divided into two types according to methods employed in the water-cooled copper crucible. One method is the use of a boat-shaped water-cooled copper crucible. The boat-shaped water-cooled copper crucible is a water-cooled copper crucible having a cavity (hollow) in the shape of a circle, a rectangle, or the like, and a whole quantity of a raw material placed in the cavity is molten to obtain a molten ingot (Patent Literature 1).
Another method is the use of a water-cooled copper crucible having a through cavity. In this method, while a raw material rod as the raw material is held horizontally, one end of the raw material rod is exposed to an energy beam to be molten, and the molten metal is continuously dropped, to thereby form a molten pool in the cavity receiving the dropped molten metal. A bottom portion of the molten pool is continuously pulled down to obtain a rod-like molten ingot (Patent Literature 2). The raw material rod is generally produced by melting the raw material in advance.
When the molten raw material is partially or entirely of powder, and the raw material in the form of powder is molten as it is, the raw material is thrown up or scattered due to a flow of the energy beam. In some cases, in order to prevent the powder from being thrown up, the powder is subjected to compression molding in advance with a molding method such as press molding or CIP mold (Patent Literature 3).
In the powder subjected to the compression molding, particles are brought into contact with each other and entangled to form a united appearance. Thus, even when an energy beam is radiated to the particles, the particles are prevented from being blown off and thrown up. A molded body generally has a relative density of from about 30% to about 60% and includes voids to a considerable extent, and an atmospheric gas or a residual gas exists in the voids.
Further, the molded body merely has a united appearance. Thus, the molded body is easily broken down due to an impact caused by being dropped or other factors, and also causes powder that exfoliates from a surface of the molded body during conveyance, thereby degrading a material yield. The material yield as used herein refers to a ratio of a mass of the molten ingot to a mass of the molten raw material.
Incidentally, when an energy beam is radiated to the molded body, the molded body is heated with conducted heat, radiated heat, and Joule heat, and a temperature of the molded body abruptly rises mainly at an irradiated portion thereof. At this time, the gas existing in the voids abruptly expands, and hence the particles merely united in appearance are flicked to the outside of the water-cooled copper crucible. At the same time, the partly molten metal is also flicked out. As a result, the mass of the molten ingot is reduced accordingly. That is, the material yield is degraded, thereby causing a large economic loss in production of a very expensive platinum group metal.
Further, when the raw material is a powder mixture, the material yield is degraded, and the composition may also be changed. When the molded body is broken to cause a piece thereof to be dropped, cause powder on the surface of the molded body to exfoliate, or cause the molten metal to scatter while being molten, a component contained in that portion is not included in the molten ingot, and hence accurate alloy composition cannot be obtained. Further, in the energy beam melting using the boat-shaped water-cooled copper crucible, an energy beam for melting is radiated from above. However, the raw material is generally turned upside down and molten also from an opposite side and this operation is repeated to obtain a molten ingot having uniform composition. At this time, a dropped piece and exfoliated powder may be left unmolten at a corner of the boat-shaped cavity in the water-cooled copper hearth. Such a case may also prevent the alloy composition from being accurate.
The present invention has been made in view of problems of the related art described above, and an object of the present invention is to provide a method for producing a platinum group metal or a platinum group-based alloy having a high material yield.
According to the present invention, provided is a method for producing a platinum group metal or a platinum group-based alloy, including: a preparing step of weighing a raw material that is partially or entirely of powder and, when the alloy is to be produced, mixing the weighed raw material to obtain a powder mixture; a molding step of molding and solidifying the prepared raw material to obtain molded bodies; a sintering step of sintering the molded bodies in a furnace to obtain a sintered body as a joined body; a melting step of melting the sintered body to produce a molten ingot; and a deformation processing step (plastic working step) of processing the molten ingot, in which, in the sintering step, the molded bodies are sintered in the furnace in a stacked state to produce the sintered body having a relative density of 70% or more.
The preparing step is a step of weighing the raw material in accordance with a desired amount of the molten ingot. When the alloy is to be produced, each raw material is weighed so as to obtain a predetermined alloy composition. The raw material may have any shape. However, the raw material is at least partially or entirely of powder.
The molding step is a step of molding and solidifying the raw material, which is partially or entirely of powder in the whole quantity of the raw material, to obtain the molded bodies, and a well-known dry molding method such as uniaxial pressing, tableting, cold isostatic pressing (CIP), or rubber pressing is suitable therefor. The shape can be appropriately selected from among the shape of a disc/cylinder, the shape of a sharp-edged plane figure including a polygon/prism, a briquette having no regular shape, and the like. The number of the molded body can be determined depending on a shape and dimensions of the water-cooled copper crucible, and may be one or more.
The sintering step is a step of sintering to substantially unify the molded bodies. A well-known sintering furnace (firing furnace) such as a gas furnace or an electric furnace can be used therefor, and both a batch type and a continuous type are suitable. A sintering temperature can be appropriately selected depending on the kind of the raw material, but a range that is 1,200° C. or higher and does not exceed a melting point of the raw material is more suitable for a platinum group metal or a platinum group-based alloy having a melting point that is above 1,500° C. An atmosphere, an inert gas, or a vacuum can be applied as a sintering atmosphere, and are appropriately selected depending on the kind of the raw material. In the sintered body, individual particles are unified, thereby increasing the strength, and increasing the density through sintering shrinkage. It is preferred that the sintered body has a relative density of 70% or more.
With such a sintered body, particles are unified to increase the strength, thereby preventing a piece of the molded body from dropping and the powder from being exfoliated. Further, occurrence of scattering during melting in the melting step can be suppressed, and hence a change in the alloy composition can also be suppressed.
As described above, when the molded body is used as a raw material rod in a melting step in a pull-down method, the molded body may be broken during melting due to insufficient strength. Further, the molded body may collapse with a small force. Thus, there is a difficulty in grasping the molded body with a raw material rod feeding mechanism and using the molded body as it is. According to the present invention, particles are unified through sintering, and a high strength is obtained. Thus, the molded body can be used as a raw material rod without the fear of breakage and collapse.
In the sintering step, a plurality of molded bodies can be sintered in a stacked state into the sintered body as a joined body. The shape of the molded body can be appropriately selected from among the shape of a disc/cylinder, the shape of a sharp-edged plane figure including a polygon/prism, and the like. Specifically, in the sintering step, when the molded bodies are sintered in a stacked state, particles in the individual molded bodies and also particles in contact with each other at an interface between the stacked molded bodies are sintered and unified. In such a manner, a rod-like sintered body (joined body) can be obtained. There is an advantage in that, through appropriate selection of dimensions and the number of the molded bodies to be stacked, changes can be made as necessary from a very small and short raw material rod to a long raw material rod. In particular, the rod-like sintered body is suitable for use as a raw material rod in a melting step of the pull-down system.
With regard to a related-art raw material rod, before a melting step, a molten ingot is produced in advance in an energy beam melting furnace using a boat-shaped water-cooled copper crucible, and the molten ingot is used as a long raw material rod. The molten ingot produced in such a manner has no regular shape. Specifically, the shape of the boat-shaped water-cooled copper crucible is transferred to a bottom portion of the molten ingot and thus, the bottom portion has a regular shape, but a side surface and an upper surface of the molten ingot are in the shape of the solidified molten metal as it is. When a latent heat at constant volume in the melting is high as in the case of a platinum group metal or a platinum group-based alloy, the molten metal is liable to be solidified immediately after the molten metal separates from the energy beam (heat source). Thus, occurrence of a burr on a side surface and waviness on the upper surface of the molten ingot are conspicuous, and the raw material rod has an indefinite sectional area. The latent heat at constant volume (kJ/cm3) as used herein is latent heat necessary for melting a substance per unit volume, and is defined by heat of fusion (kJ/mol), molar mass (g/mol), and density (g/cm3).
When pull-down melting is performed with such a raw material rod, there is a difficulty in dripping the molten metal at a constant speed. Thus, at a portion having a small sectional area, the molten metal to be dripped becomes insufficient, and hence a defect such as a pore is liable to occur in the molten ingot. At a portion having a large sectional area, the molten metal to be dripped becomes excessive, and hence a trouble that the dripped molten metal overflows the cavity in the water-cooled copper crucible and is solidified is liable to occur.
According to the present invention, molded bodies having fixed dimensions can be sintered in the sintering step, and the molded bodies can be used as a raw material rod having fixed dimensions, and hence such problems do not arise. Further, manufacture of a raw material rod requires a dedicated melting facility (melting furnace, crusible, and the like). However, according to the present invention, such a facility is unnecessary, and a general electric furnace or the like can be used to produce the raw material rod (sintered body) very conveniently.
The molded body used for the raw material rod may have an appropriate shape. However, when the molded body is shaped to be substantially rectangular parallelepiped by uniaxial pressing, the molding is particularly easy. Further, such a shape is very convenient in stacking the molded bodies in the sintering step (claim 2).
Incidentally, a pressure in the furnace in the energy beam melting differs depending on the melting method and the molten raw material (high vacuum to atmospheric pressure). In particular, an electron beam melting furnace requires a high vacuum region of 0.1 Pa or less. When the vacuum is high as described above, a pressure difference with a gas component remaining in voids in the sintered body is large, and hence subtle scattering may occur. Therefore, the suitable pressure in the furnace in the melting is 1 Pa or higher.
The melting step is a step of producing the molten ingot using the sintered body as a raw material. Not only the energy beam melting described above but also related-art melting furnaces or melting methods widely used for producing a platinum group precious metal and a platinum group-based alloy are applicable. For example, a sufficient induced current cannot be obtained through induction heat melting of a powder raw material due to a small contact area between particles. Thus, the induction heat melting is regarded as inappropriate. However, according to the present invention, the particles are substantially unified through the sintering, and a sufficient induced current can be obtained. Thus, induction heat melting is also applicable.
Further, in the melting step, an energy beam melting furnace with the water-cooled copper crucible having a through cavity is used. One end of the rod-like sintered body (joined body) as a raw material rod is exposed to an energy beam to be molten, and the molten metal is continuously dripped, to thereby form a molten pool in the cavity receiving the dripped molten metal. A bottom portion of the molten pool is continuously pulled down to obtain the rod-like molten ingot. Specifically, it is suitable to use the rod-like sintered body (joined body) as a raw material rod in the melting step of the pull-down system.
Further, according to the present invention, provided is a method for producing a platinum group metal or a platinum group-based alloy, including: a preparing step of weighing a raw material that is partially or entirely of powder and, when the alloy is to be produced, mixing the weighed raw material to obtain a powder mixture; a molding step of molding and solidifying the prepared raw materials to obtain molded bodies, a sintering step of sintering the molded bodies to obtain a sintered bodies, a melting step of melting the sintered body using energy beam melting that uses a boat-shaped water-cooled copper crucible having a cavity formed therein to produce a molten ingot, and a deformation processing step (plastic working step) of processing the molten ingot. In the sintering step, a shape and dimensions of each sintered body are determined so as to conform to the cavity, and, in the melting step, the sintered bodies conforming to the cavity are densely arranged in the cavity of the boat-shaped water-cooled copper crucible to produce the molten ingot.
In the boat-shaped water-cooled copper crucible in the energy beam melting, a circular or rectangular cavity (hollow) is generally formed in an upper surface of copper in which a water-cooling circuit is provided. The molten raw material is placed in the cavity, and an energy beam is radiated from above to perform heating and melting. Through designing the shape and the dimensions of the sintered body so as to conform to the cavity, more molten ingots can be obtained. Specifically, when a molded body in the shape of a cylinder or a disc is sintered and arranged in a circular cavity, and a molded body in the shape of a cube, a rectangular parallelepiped, or a hexagonal column is sintered and arranged in a rectangular cavity, two-dimensional dense arrangement of the molded bodies can be made, and hence stacking of the molded bodies is also easy.
The deformation processing step is a step of processing the molten ingot into a desired shape such as a plate or a wire, and a well-known method is applicable thereto. Deformation processing of the molten ingot produced according to the present invention can be performed as in the case of a related-art molten ingot obtained through steps without the sintering step.
For example, when processing into a plate is performed, forging and rolling are performed. When processing into a wire is performed, forging, groove rolling, and wire drawing are performed. In any of the cases, depending on the extent of work hardening, heat treatment is given as appropriate midway during the processing to perform softening. After the processing into a plate or a wire, depending on the intended use, processing such as cutting, bending, or welding can also be performed. Further, with regard to each processing, both cold working and hot working, in which a material is heated when processed, are applicable.
As described above, according to the producing method of the present invention, as compared to a related-art producing method, scattering of a raw material during melting can be effectively suppressed, and the material yield of an expensive platinum group metal or an expensive platinum group-based alloy can be improved.
Further, as compared to the molded body, the sintered body has a higher strength and is not broken easily, and hence powder can be prevented from being exfoliated during conveyance. Such feature is advantageous in that part of the raw material is prevented from dropping or exfoliating, and hence change in composition does not occur. Further, there is also an advantage in that, when the sintered body is used as a raw material rod, the sintered body can be supported and grasped in an apparatus without difficulty.
Further, there is also an advantage in that, as compared to the molded body, the sintered body has a higher density, that is, a smaller volume per the same mass, and hence more raw material can be mounted on the water-cooled copper crucible, thereby contributing to improvement in productivity.
A method for producing an electrode chip of a spark plug for an internal combustion engine is taken as an example and is described more in detail.
For an electrode chip of a spark plug, an iridium-base alloy or a platinum-base alloy is preferred for use. In this example, a whole quantity of a raw material is of powder, and Ir powder and Pt powder are used.
(Raw Material Preparing Step)
Predetermined amounts of the respective powders are weighed so as to obtain predetermined composition, and a V-mixer is used to mix the powders to obtain a uniform powder mixture.
(Molding Step)
The powder mixture is charged in a hopper of an automatic press forming machine (uniaxial pressing). A rectangular cavity having short sides of 20 mm and long sides of 50 mm is formed in a molding die, and four corners thereof have an R of 2 mm. The molded body is substantially in the shape of a rectangular parallelepiped having dimensions of 20 mm×20 mm×50 mm with corners thereof having an R of 2 mm (
When the two exemplary molded bodies are molten as they are as in the related art, there can be visually recognized a state in which part of the heated powder and molten metal scatter in the melting furnace to cause sparkling. Further, the strength is to such an extent that a touch with a hand may cause the powder to attach to a finger, and that a corner of the molded body is broken when the molded body is dropped from a height of about 5 cm.
(Sintering Step)
Five molded bodies are vertically stacked with surfaces of 20 mm×20 mm thereof being upper and lower surfaces, and these are counted as one unit (
(Melting Step)
The raw material rod is horizontally grasped by a raw material rod feeding mechanism of an atmospheric pressure plasma arc melting furnace (pull-down system) and is continuously molten and dripped in an argon atmosphere of from 0.9 atm (atmospheric pressure) to 1.2 atm, and a bottom portion of a water-cooled copper crucible is pulled down. With this, a cylindrical ingot having a diameter of φ 35 mm is obtained. The state of scattering is not observed during melting, and an effect of the sintering step can be confirmed. Further, although the raw material rod is in a cantilever state at this time, the raw material rod is not broken, and powder does not exfoliate during the melting step.
(Deformation Processing Step)
The molten ingot is formed into a square rod by hot forging, and then, into a wire having a substantially rectangular section by hot groove rolling. The molten ingot is further formed into a round wire having a predetermined outer diameter by hot drawing using a die.
(Cutting Step)
The round wire is cut into lengths suitable for a wire saw. A plurality of wires are arranged so as to be in parallel with one another, fixed with a resin, and cut by the wire saw, to thereby obtain electrode chips for a spark plug each having a predetermined length.
Further description is given using Examples.
In Table 1, there are shown results. Evaluation was made in accordance with the following criteria.
Reduction in mass represents reduction in mass of the molten ingot from the raw material powders at the time of the compounding, and is expressed in percentage. The reduction in mass which is more than 3% was denoted by x, and 3% or less was denoted by ∘.
With regard to powder exfoliation, when a sintered body or a molded body before being molten was picked up with fingers, and attachment of powder to the fingers was observed, the powder exfoliation was denoted by x. When attachment was not observed at all, the powder exfoliation was denoted by ∘.
With regard to a molten state, visual observation was made during the melting. When a sparkling-like scattering phenomenon was continually observed, the molten state was denoted by x. When the phenomenon was observed once in a while, the molten sate was denoted by A. When the phenomenon was hardly observed, the molten state was denoted by ∘.
With regard to comprehensive judgment, these results were taken into consideration. When the effect of the present invention was not recognized, the comprehensive judgement was denoted by x. When the effect was recognized, the comprehensive judgement was denoted by ∘. When the effect was more considerable, the comprehensive judgement was denoted by ∘∘.
According to the description above (Description of Embodiments), molded bodies were prepared with an automatic press forming machine (uniaxial pressing). A molding pressure was 200 MPa. Then, the molded bodies were sintered at different temperatures for three hours. Table 1 shows a relative density of each of the molded body and the sintered bodies prepared at different sintering temperatures. The sintered bodies having a relative density of 70% or more were obtained at sintering temperature of 1200° C. or higher.
In Example 1, five molded bodies each having dimensions of 20 mm×20 mm×50 mm were prepared. The five molded bodies were vertically stacked with surfaces of 20 mm×20 mm thereof being upper and lower surfaces. The molded bodies in a stacked state were arranged in a setter formed of carbon, and the molded bodies, together with the setter, were inserted into an atmospheric furnace including a carbon heater. Sintering was performed to obtain a sintered body at 1,500° C. for 3 hours under argon airflow. With regard to the density of the molded body calculated from the dimensions and the mass, the relative density was 52%. The density of the sintered body was 74%. Using the sintered body as the raw material rod, a molten ingot having a diameter of about φ35 mm×a length of L 150 mm was manufactured.
When visual observation was conducted during the melting (under a pressure of 1.1×105 Pa), the scattering phenomenon was not at all observed. The reduction in mass of the molten ingot from the raw material preparing step was 0.6% or less. Further, after the sintering until the melting was completed, the raw material rod was not broken or exfoliated.
Almost no scattered material was left in the furnace after the melting, and attachment of the scattered material to the water-cooled copper crucible was not recognized.
In Example 2, the same procedure as in Example 1 was repeated except that the sintering temperature was changed to 1300° C. In Example 3, the same procedure as in Example 1 was repeated except that the sintering temperature was changed to 1200° C. The density of the sintered bodies obtained in Examples 2 and 3 were 74% and 71%, respectively. Using the sintered body as the raw material rod, a molten ingot having a diameter of about φ35 mm×a length of L 150 mm was manufactured. In Examples 2 and 3 as in the case of Example 1, when visual observation was conducted during the melting (under a pressure of 1.1×105 Pa), the scattering phenomenon was not at all observed. The reduction in mass of the molten ingot from the raw material preparing step was 0.6% or less. Further, after the sintering until the melting was completed, the raw material rod was not broken or exfoliated.
Almost no scattered material was left in the furnace after the melting, and attachment of the scattered material to the water-cooled copper crucible was not recognized.
In Reference Example, molded bodies were prepared as in the case of Example 1. Each molded body was substantially in the shape of a rectangular parallelepiped having dimensions of 20 mm×20 mm×50 mm with corners having an R of 2 mm. Such molded bodies were individually sintered without being stacked to obtain sintered bodies of about 16 mm×16 mm×44 mm. The density of each sintered body calculated from the dimensions and the mass was 74%. The sintered bodies were placed on a boat-shaped water-cooled copper crucible and were molten by vacuum plasma melting to manufacture a molten ingot of about 15 mm×30 mm×100 mm. The pressure during the melting was adjusted to be 5×10−1 Pa (Ar).
In visual observation during the melting, occasional scattering was observed. A small amount of the scattered material was left in the furnace after the melting, with part thereof being attached to the water-cooled copper crucible.
The reduction in mass of the molten ingot was 2.5%. Further, with regard to the shape of the molten ingot, the bottom portion was approximately smooth along the shape of the boat-shaped water-cooled copper crucible. However, there was a burr on a side surface, and the upper surface was solidified in a wavy state.
In Comparative Example 1, after the raw material powder was mixed using a V-mixer, the CIP method was used to manufacture a cylindrical molded body having a diameter of φ 30 mm. The molding pressure was 300 MPa. With regard to the density of the molded body calculated from the dimensions and the mass, the relative density was 48%. The molded body was cut into lengths of about 30 mm, placed on a boat-shaped water-cooled copper crucible, and molten by arc melting to manufacture a molten ingot of about t 15 mm×w 30 mm×L 100 mm. The pressure during the melting was adjusted to 8×104 Pa (Ar).
The strength of the molded body was not low to such an extent that a touch by a hand causes the molded body to be broken. However, when the molded body was taken out of a CIP die, powder attached to a finger, and powder attached to an inner wall of the CIP die could be confirmed.
In visual observation during the melting, continuous scattering from a molten portion until the molded body was entirely molten away was confirmed. The scattered material was left in the furnace after the melting, and attachment thereof to the water-cooled copper crucible was conspicuous. Further, the scattered material and powder that exfoliated from the molded body were left in a corner of the bottom portion of the boat-shaped water-cooled copper crucible. As described above, part of the compounded raw material powders was left unmolten and the reduction in mass of the molten ingot from the raw material preparing step was 3.2%.
With regard to the shape of the molten ingot, there were a burr and waviness as in the case of Example 2.
In Comparative Example 2, a molded body manufactured as in the case of Comparative Example 1 was placed on a boat-shaped water-cooled copper crucible, and a molten ingot of about t 15 mm×w 30 mm×L 100 mm was manufactured by vacuum plasma melting. The pressure during the melting was adjusted to 5×10−1 Pa (Ar).
In visual observation during the melting, continuous scattering from a molten portion until the molded body was entirely molten away was confirmed. More scattered material was left in the furnace after the melting, and attachment thereof to the water-cooled copper crucible was more conspicuous. Further, the scattered material and powder that exfoliated from the molded body were left in a corner of the bottom portion of the boat-shaped water-cooled copper crucible. As described above, part of the compounded raw material powders was left unmolten, and the reduction in mass of the molten ingot from the raw material preparing step was 4.5%.
From the results described above, it was confirmed that, in a method of directly melting a molded body without performing the sintering step, the reduction in mass was larger, and the material yield was lower, whereas, in melting the sintered body having a relative density of 70% or more according to the present invention, the reduction in mass was smaller.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-182792 | Sep 2014 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15307149 | Oct 2016 | US |
| Child | 16808727 | US |
