US20170043400A1

US20170043400A1 - Sintered body production method, degreased body production method, and heating furnace

Info

Publication number: US20170043400A1
Application number: US15/230,779
Authority: US
Inventors: Hidefumi Nakamura
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 2015-08-11
Filing date: 2016-08-08
Publication date: 2017-02-16
Also published as: JP6631080B2; JP2017036483A; CN106424721A

Abstract

A sintered body production method includes a first step in which temperature elevation is started based on a first temperature elevation program in a firing furnace in a state where a molded body containing a metal powder and an organic binder is placed in the firing furnace, a second step in which a vacuum degree or a dew point in the firing furnace is measured in the course of elevating the temperature, and a third step in which temperature elevation is performed by applying a second temperature elevation program in place of the first temperature elevation program when the vacuum degree or the dew point measured in the second step does not satisfy a predetermined condition, and temperature elevation is performed by applying the first temperature elevation program when the vacuum degree or the dew point measured in the second step satisfies the predetermined condition.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2015-158957 filed on Aug. 11, 2015. The entire disclosure of Japanese Patent Application No. 2015-158957 is hereby incorporated herein by reference.

BACKGROUND

1. Technical Field
The present invention relates to a sintered body production method, a degreased body production method, and a heating furnace.
2. Related Art
A powder metallurgy method for producing a sintered body by firing a molded body containing a metal powder enables the production of a metal product having a shape close to a desired shape, and therefore is widely used in many industrial fields.
There have been known several methods for producing a molded body, and one of these methods is a metal injection molding (MIM) method in which a metal powder and an organic binder are mixed and kneaded, followed by injection molding using the resulting kneaded material. A molded body produced by this MIM method is subjected to degreasing processing (debinder processing), thereby removing the organic binder, and then, fired. In firing, metal powder particles are bound to one another and sintered, whereby a desired metal product (sintered body) is obtained.
For example, JP-A-2-138443 (PTL 1) discloses a production method for a sintered magnetic material in which a metal powder and an organic binder are kneaded, followed by injection molding processing and degreasing processing, and thereafter, further two-step sintering processing under different conditions is performed. It is disclosed that as an example of the two-step sintering processing, sintering at a relatively low temperature and sintering at a relatively high temperature are performed.
However, in the sintering processing, a large amount of a gas is unintentionally generated from a material to be processed, etc., and the processing conditions may sometimes change in the course of the processing. If the processing conditions change in this manner, this change may affect the progress of the sintering phenomenon of the material to be processed, and favorable sintering may not be performed.

SUMMARY

An advantage of some aspects of the invention is to provide a sintered body production method capable of producing a high-quality sintered body even if the processing environment changes in the course of the firing step, a degreased body production method capable of producing a degreased body which has been sufficiently degreased even if the processing environment changes in the course of the degreasing step, and a heating furnace capable of performing sufficient processing for a material to be processed even if the processing environment changes in the course of the firing step or the degreasing step.
The advantage can be achieved by the following configurations.
A sintered body production method according to an aspect of the invention is a method for producing a sintered body by firing a molded body containing a metal powder and an organic binder, and includes a first step in which temperature elevation is started based on a first temperature elevation program in a firing furnace in a state where the molded body is placed in the firing furnace, a second step in which a vacuum degree or a dew point in the firing furnace is measured in the course of elevating the temperature, and a third step in which temperature elevation is performed by applying a second temperature elevation program in place of the first temperature elevation program when the vacuum degree or the dew point measured in the second step does not satisfy a predetermined condition, and temperature elevation is performed by applying the first temperature elevation program when the vacuum degree or the dew point measured in the second step satisfies the predetermined condition.
According to this configuration, a high-quality sintered body can be produced even if the processing environment changes in the course of the firing step.
In the sintered body production method according to the aspect of the invention, it is preferred that the second temperature elevation program is a program in which a temperature elevation rate is increased among the factors constituting the first temperature elevation program.
According to this configuration, the decrease in the reaction rate of the reduction reaction of a metal oxide generated when the vacuum degree or the dew point does not satisfy the predetermined condition can be compensated by the acceleration of the reduction reaction by the increase in the temperature elevation rate. As a result, the occurrence of sintering failure can be suppressed by preventing a state where the reaction rate of the reduction reaction is decreased from continuing for a long time.
In the sintered body production method according to the aspect of the invention, it is preferred that the second step is performed again after the third step.
According to this configuration, it is possible to verify whether an effect of applying the second temperature elevation program is exhibited. That is, in the third step after the second step which is performed again, a determination is made again as to whether the vacuum degree or the dew point satisfies the predetermined condition, and therefore, a suitable temperature elevation program can be selected according to the state of the vacuum degree or the dew point. As a result, the variation in the quality of the sintered body can be kept to the minimum, or the effect of the second temperature elevation program can be exhibited to the maximum.
In the sintered body production method according to the aspect of the invention, it is preferred that in the second step, the time integral of the vacuum degree or the time integral of the dew point in the firing furnace is calculated, and the predetermined condition in the third step is a condition associated with the time integral of the vacuum degree or a condition associated with the time integral of the dew point.
According to this configuration, as compared with the case where the condition associated with the measurement value of the vacuum degree or the dew point is used as the predetermined condition as it is, a determination as to whether the vacuum degree or the dew point satisfies the predetermined condition in the third step can be made based on a relatively strict predetermined condition. Due to this, the change in the vacuum degree or the dew point can be dealt with in good time, and thus, a state where the reaction rate of the reduction reaction is decreased can be prevented from continuing for a long time.
A degreased body production method according to an aspect of the invention is a method for producing a degreased body by degreasing a molded body containing a metal powder and an organic binder, and includes a first step in which temperature elevation is started based on a first temperature elevation program in a degreasing furnace in a state where the molded body is placed in the degreasing furnace, a second step in which the concentration of a decomposed gas of the organic binder in the degreasing furnace is measured in the course of elevating the temperature, and a third step in which temperature elevation is performed by applying a second temperature elevation program when the concentration of the decomposed gas measured is within a predetermined range, and temperature elevation is performed by applying the first temperature elevation program when the concentration of the decomposed gas measured is outside the predetermined range.
According to this configuration, a degreased body which has been sufficiently degreased can be produced even if the processing environment changes in the course of the degreasing step.
A heating furnace according to an aspect of the invention includes a furnace main body, a heating unit which heats the inside of the furnace main body, an output adjustment unit which adjusts the output of the heating unit, a measurement unit which measures a vacuum degree or a dew point in the furnace main body, and a control unit which has a function of controlling the operation of the output adjustment unit based on a temperature elevation program and a function of rewriting the temperature elevation program based on the measurement result of the measurement unit.
According to this configuration, in the case where this heating furnace is used as a firing furnace, a high-quality sintered body can be produced even if a factor of the processing environment changes in the course of the firing step.
A heating furnace according to an aspect of the invention includes a furnace main body, a heating unit which heats the inside of the furnace main body, an output adjustment unit which adjusts the output of the heating unit, a measurement unit which measures the concentration of a decomposed gas of an organic binder in the furnace main body, and a control unit which has a function of controlling the operation of the output adjustment unit based on a temperature elevation program and a function of rewriting the temperature elevation program based on the measurement result of the measurement unit.
According to this configuration, in the case where this heating furnace is used as a degreasing furnace, a degreased body which has been sufficiently degreased can be produced even if a factor of the processing environment changes in the course of the degreasing step.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.

FIG. 1 is a cross-sectional view showing a firing furnace to which a first embodiment of a heating furnace according to the invention is applied.

FIG. 2 is a cross-sectional view taken along the line A-A in FIG. 1.

FIG. 3 is a cross-sectional view showing a degreasing furnace to which a second embodiment of a heating furnace according to the invention is applied.

FIG. 4 is a process chart for explaining an embodiment of a sintered body production method according to the invention.

FIG. 5 is an exemplary temperature profile showing a first temperature elevation program, an exemplary temperature profile showing a second temperature elevation program, and an exemplary graph showing a change in the measurement value of a vacuum degree.

FIG. 6 is a process chart for explaining an embodiment of a decreased body production method according to the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a sintered body production method, a degreased body production method, and a heating furnace according to the invention will be described in detail with reference to preferred embodiments shown in the accompanying drawings.
Heating Furnace

First Embodiment

First, a firing furnace to which a first embodiment of a heating furnace according to the invention is applied will be described. The firing furnace is a heating furnace with which a molded body containing a metal powder and an organic binder is sintered by heating.
FIG. 1 is a cross-sectional view showing the firing furnace to which the first embodiment of the heating furnace according to the invention is applied, and FIG. 2 is a cross-sectional view taken along the line A-A in FIG. 1. Incidentally, in the following description, for the sake of convenience of explanation, the upper side in FIGS. 1 and 2 is referred to as “upper” and the lower side in FIGS. 1 and 2 is referred to as “lower”.
A firing furnace 1 shown in FIG. 1 includes a furnace main body 2 in the shape of a cylinder having a long axis in the horizontal direction, a partition wall 4 in the shape of a cylinder provided in the furnace main body 2, a heater 5 provided along the inner wall of the partition wall 4, a gas introduction system 6 which introduces a gas into the furnace main body 2, and a gas exhaust system 7 which exhausts a gas. Hereinafter, the respective sections of the firing furnace 1 will be described in detail.
The furnace main body 2 shown in FIG. 1 is in the shape of a cylinder having an axis line in the horizontal direction, and one end (the right end in FIG. 1) in the extension direction of the axis line is closed, and the other end (the left end in FIG. 1) is openable. Then, a material to be processed can be inserted into or extracted from the inside from the other end side of the furnace main body 2.
The furnace main body 2 is composed of, for example, an iron-based alloy such as heat-resistant steel. The inner wall surface of the furnace main body 2 may be in the shape of a polygon such as a tetragon, a hexagon, or an octagon in the cross section perpendicular to the axis line of the furnace main body 2, but is preferably in the shape of a circle such as an ellipse or an elongated circle as well as a true circle as shown in FIG. 2. According to this, the temperature distribution inside the furnace main body 2 can be more easily made uniform.
The partition wall 4 is provided inside the furnace main body 2 and is in the shape of a cylinder having an axis line in parallel with the axis line of the furnace main body 2. The shape of the inner wall surface of the partition wall 4 is a tetragon in the cross section perpendicular to the axis line of the partition wall 4.
Such a partition wall 4 is composed of, for example, a metal material such as stainless steel or heat-resistant steel or a carbon material such as carbon.
Inside the partition wall 4, a stage 3 in the shape of a flat plate is provided. The stage 3 is fixed to the inner wall surface of the furnace main body 2 through a leg section 31 passing through the partition wall 4. This stage 3 is configured such that a material to be processed W to be subjected to firing processing in the firing furnace 1 can be placed thereon.
To the furnace main body 2, the gas introduction system 6 for introducing a firing gas is connected. The gas introduction system 6 shown in FIG. 1 includes a pipe 61 through which a gas can flow and a gas source (not shown). One end of the pipe 61 is connected to the furnace main body 2, and the other end of the pipe 61 is connected to the gas source. According to this, a firing gas can be introduced into the furnace main body 2 through the pipe 61.
Further, to the furnace main body 2, the gas exhaust system 7 for exhausting a gas is connected. The gas exhaust system 7 shown in FIG. 1 includes a pipe 71 through which a gas can flow and an exhaust pump 72. One end of the pipe 71 is connected to the furnace main body 2, and the other end of the pipe 71 is connected to the exhaust pump 72. According to this, a gas inside the furnace main body 2 can be exhausted by the exhaust pump 72 through the pipe 71.
Inside the partition wall 4, a heater 5 (heating unit) is provided. By supplying electric power to the heater 5, the atmosphere inside the furnace main body 2, the partition wall 4, the stage 3, and the like are heated, and the material to be processed W is heated accompanying this.
To the heater 5, an output adjustment section 52 is connected through a wire 51. The output adjustment section 52 changes the output of the heater 5 so as to change the temperature of the material to be processed W.
In the furnace main body 2 shown in FIG. 2, a total of four heaters 5 are provided so as to surround the material to be processed W, that is, one heater 5 is provided on each of the upper side, lower side, and both lateral sides of the material to be processed W. Further, in FIG. 1, three sets of these four heaters 5 are arranged along the axis line of the furnace main body 2. Therefore, in the furnace main body 2 shown in FIGS. 1 and 2, a total of twelve heaters 5 are provided. To each of these heaters 5, the output adjustment section 52 is individually connected. In FIGS. 1 and 2, the illustration of part of the heaters 5 and the output adjustment sections 52 is omitted. The above-mentioned number and arrangement of the heaters 5 are examples and the invention is not limited thereto.
Inside the partition wall 4, a thermocouple 8 (temperature measurement unit) is provided. To the thermocouple 8, a temperature adjustment section 82 (temperature adjustment unit) is connected through a wire 81. Further, the temperature adjustment section 82 and the output adjustment section 52 are connected to each other through a wire 83. Each of the symbols X, Y, and Z shown in FIG. 1 indicates the connection destination of the wire 83. That is, the wires 83 are laid so that the members indicated by the symbol X, the members indicated by the symbol Y, or the members indicated by the symbol Z shown in FIG. 1 are electrically connected to each other, respectively.
By the thermocouple 8, the wire 81, the temperature adjustment section 82, and the wire 83, the temperature inside the partition wall 4 is measured, and an appropriate output value is indicated to the output adjustment section 52 based on the measurement. The output adjustment section 52 supplies electric power to the heater 5 according to this output value.
The firing furnace 1 according to this embodiment further includes an integrated control section 85 (control unit) and a wire 86 which electrically connects the integrated control section 85 to each temperature adjustment section 82. The integrated control section 85 can simultaneously control the operations of a plurality of temperature adjustment sections 82. According to this, even if a temperature difference occurs inside the firing main body 2, the entire inside of the firing main body 2 can be controlled to have a desired temperature.
The firing furnace 1 according to this embodiment further includes a vacuum meter 91 (measurement unit) which measures a vacuum degree inside the furnace main body 2 and a dew point meter 92 (measurement unit) which measures a dew point inside the furnace main body 2. This vacuum meter 91 is electrically connected to the integrated control section 85 through a wire 911. Further, the dew point meter 92 is electrically connected to the integrated control section 85 through a wire 921.
The integrated control section 85 has a function of controlling the operations of the temperature adjustment section 82 and the output adjustment section 52 based on a first temperature elevation program which has been set in advance and a function of rewriting the first temperature elevation program into a second temperature elevation program based on the measurement result of the vacuum meter 91 or the dew point meter 92, and controlling the operations of the temperature adjustment section 82 and the output adjustment section 52 based on the second temperature elevation program.

Second Embodiment

Next, a degreasing furnace to which a second embodiment of the heating furnace according to the invention is applied will be described. The degreasing furnace is a heating furnace with which a molded body containing a metal powder and an organic binder is heated to thermally decompose and remove at least part of the organic binder, whereby a degreased body is obtained.
FIG. 3 is a cross-sectional view showing the degreasing furnace to which the second embodiment of the heating furnace according to the invention is applied.
Hereinafter, the second embodiment will be described, however, in the following description, different points from the above-mentioned first embodiment will be mainly described, and the description of the same matter will be omitted. Further, in the drawing, the same reference numerals are given to the same components as those of the above-mentioned embodiment.
A degreasing furnace 10 shown in FIG. 3 has the same configuration as the firing furnace 1 according to the first embodiment except that a gas concentration sensor 93 (measurement unit) which measures the concentration of a thermally decomposed gas of the organic binder is included in place of the vacuum meter 91 which measures a vacuum degree inside the furnace main body 2 and the dew point meter 92 which measures a dew point inside the furnace main body 2.
The gas concentration sensor 93 is electrically connected to an integrated control section 85 through a wire 931.
The integrated control section 85 has a function of controlling the operations of a temperature adjustment section 82 and an output adjustment section 52 based on a first temperature elevation program which has been set in advance and a function of rewriting the first temperature elevation program into a second temperature elevation program based on the measurement result of the gas concentration sensor 93, and controlling the operations of the temperature adjustment section 82 and the output adjustment section 52 based on the second temperature elevation program.
The gas concentration sensor 93 may be any as long as it is a sensor capable of measuring the concentration of a thermally decomposed gas of the organic binder. Examples of the type of the thermally decomposed gas of the organic binder include hydrocarbon-based gases such as methane, ethylene, ethane, propylene, propane, butane, pentane, hexane, and formaldehyde, and derivatives thereof.
As a detection method of a sensor which detects the concentration of, for example, methane among these, a thermal catalytic method, an infrared absorption spectrophotometric method, a thermal conductivity detection method, and the like are known. In the invention, any sensor employing any of these detection methods can be used.

Sintered Body Production Method

Next, an embodiment of a sintered body production method according to the invention will be described. In the following description, a method using the above-mentioned firing furnace 1 will be described.
FIG. 4 is a process chart for explaining an embodiment of the sintered body production method according to the invention.
The sintered body production method shown in FIG. 4 includes [1] a first step in which a molded body (material to be processed) is placed in the firing furnace 1 and temperature elevation is started based on a first temperature elevation program, [2] a second step in which a vacuum degree or a dew point in the firing furnace 1 is measured in the course of elevating the temperature, [3] a third step in which temperature elevation is performed by applying a second temperature elevation program when the vacuum degree or the dew point measured does not satisfy a predetermined condition, and temperature elevation is performed by applying the first temperature elevation program when the vacuum degree or the dew point measured satisfies the predetermined condition, and [4] a fourth step in which in the case where the first temperature elevation program is selected in the third step, and when a termination condition for the first temperature elevation program is satisfied at that time point, the first temperature elevation program is terminated, and when the termination condition for the first temperature elevation program is not satisfied at that time point, the operation is returned to the second step. Hereinafter, the respective steps will be sequentially described.
[1] First, a molded body as a material to be processed W is placed in the firing furnace 1. This molded body contains a metal powder and an organic binder. Specifically, this molded body is produced using a mixture of a metal powder and an organic binder by any of a variety of molding methods such as a powder compaction molding (compression molding) method, a metal injection molding (MIM) method, and an extrusion molding method. The molded body obtained in this manner is in a state where the organic binder is uniformly distributed in gaps between the particles of the metal powder.
The molded body to be placed in the firing furnace 1 may be subjected to degreasing processing as needed. That is, as the material to be processed, a degreased body may be placed in the firing furnace 1. Examples of this degreasing processing include a method of heating the molded body and a method of exposing the molded body to a gas which decomposes the organic binder.
In this degreasing processing, it is also possible to use the above-mentioned heating furnace according to the invention. Such degreasing processing will be described in detail later.
Subsequently, in a state where the molded body is placed in the firing furnace 1, the gas in the furnace main body 2 of the firing furnace 1 is exhausted. By doing this, the pressure inside the furnace main body 2 is reduced, so that the inside of the furnace main body 2 is brought into a vacuum state.
The vacuum degree in the furnace main body 2 is not particularly limited, but is preferably 10⁻¹⁵Pa or more and 10³Pa or less, more preferably 10⁻⁴Pa or more and 10²Pa or less. By setting the vacuum degree in the furnace main body 2 within the above range, the concentrations of oxygen, nitrogen, water, and the like in the furnace main body 2 are sufficiently decreased, and therefore, the molded body can be fired while suppressing oxidation, nitridation, and the like of the metal powder in the molded body.
Further, by setting the vacuum degree in the furnace main body 2 within the above range, a reaction between carbon contained in the molded body and oxygen contained in the metal powder can be induced. This corresponds to a reduction reaction of a metal oxide by carbon. Further, in the case where a gas is generated from the molded body in the course of firing, this gas can be exhausted promptly to the outside of the firing furnace 1. According to this, it is possible to suppress the decrease in the reaction rate of the reduction reaction or the stop of the reduction reaction due to the decrease in the vacuum degree by the generated gas.
The furnace main body 2 may be brought into a vacuum state, but may be brought into a reducing atmosphere instead.
Examples of the reducing atmosphere include an atmosphere of a reducing gas (firing gas) such as hydrogen or carbon monoxide, and an atmosphere containing these reducing gases. These atmospheres enable the firing of the molded body while suppressing oxidation, nitridation, and the like of the metal powder in the molded body.
Further, by bringing the inside of the furnace main body 2 into a reducing atmosphere, a reaction between the reducing gas and oxygen contained in the metal powder can be induced. This corresponds to a reduction reaction of a metal oxide by the reducing gas such as hydrogen.
The concentration of the reducing gas in the furnace main body 2 is not particularly limited, but is preferably 10 vol % or more, more preferably 50 vol % or more. By setting the concentration of the reducing gas within the above range, the reaction rate of the reduction reaction of the metal oxide by the reducing gas is sufficiently accelerated, and thus, the occurrence of sintering failure can be suppressed.
The dew point in the furnace main body 2 is not particularly limited, but is preferably −30° C. or lower, more preferably −40° C. or lower. By setting the dew point in the furnace main body 2 within the above range, the concentration of water vapor in the furnace main body 2 is sufficiently decreased, and therefore, it is possible to suppress the oxidation of the metal powder in the molded body, and also it is possible to suppress the decrease in the reaction rate of the reduction reaction of a metal oxide or the stop of the reduction reaction due to the reducing gas.
Subsequently, while maintaining the inside of the firing furnace 1 in a vacuum state or a reducing atmosphere, the first temperature elevation program which has been set in advance in the integrated control section 85 is started. By doing this, the temperature adjustment section 82 and the output adjustment section 52 are operated based on the first temperature elevation program, and temperature elevation in the firing furnace 1 is started (first step).
Here, the first temperature elevation program specifies the relationship between the elapsed time and the set temperature. By specifying the elapsed time in fine steps in advance, the temperature elevation rate for each elapsed time can be specified in more detail.
The integrated control section 85 causes the temperature adjustment section 82 and the output adjustment section 52 to operate based on such a relationship between the elapsed time and the set temperature. That is, the relationship between a certain set temperature and the output of the heater 5 necessary for realizing the set temperature varies depending on the structure or the like of the firing furnace 1, and therefore, such a relationship has been found in advance. By allowing the integrated control section 85 to hold such a relationship in advance, the necessary output of the heater 5 can be indicated based on the set temperature specified by the first temperature elevation program.
FIG. 5 is an exemplary temperature profile showing the first temperature elevation program. In FIG. 5, the horizontal axis represents the elapsed time, and the vertical axis represents the set temperature, and the temperature profile showing the first temperature elevation program is represented by a broken polygonal line.
The first temperature elevation program shown in FIG. 5 includes Step 1 (S1) of elevating the temperature from room temperature to 700° C., Step 2 (S2) of maintaining the temperature at 700° C., Step 3 (S3) of elevating the temperature from 700° C. to 1,000° C., Step 4 (S4) of maintaining the temperature at 1,000° C., Step 5 (S5) of elevating the temperature from 1,000° C. to 1,250° C., Step 6 (S6) of maintaining the temperature at 1,250° C., and Step 7 (S7) of lowering the temperature from 1,250° C. to room temperature.
[2] Subsequently, the vacuum degree or the dew point in the firing furnace 1 is measured in the course of elevating the temperature (second step). The measurement value is sent to the integrated control section 85.
For example, when the inside of the furnace main body 2 is in a vacuum state, the vacuum degree is measured. When the inside of the furnace main body 2 is in a reducing atmosphere, the dew point is measured.
The vacuum degree or the dew point may be monitored at all times, and the measurement value may be sent to the integrated control section 85 at any time.
[3] Subsequently, the integrated control section 85 makes a determination as to whether the vacuum degree or the dew point measured satisfies a predetermined condition. Since the vacuum degree or the dew point has an influence on the reaction rate of the reduction reaction of a metal oxide, the predetermined condition is appropriately set in consideration of the same. That is, the upper limit of the vacuum degree or the dew point is set so that the reaction rate is not decreased to such an extent that sintering failure occurs, and in the case where the measurement value or the value calculated from the measurement value falls within the range lower than the upper limit, the vacuum degree or the dew point is determined to “satisfy the predetermined condition”.
In the case where the vacuum degree or the dew point measured is determined not to satisfy the predetermined condition, that is, the measurement value of the vacuum degree exceeds the upper limit or the measurement value of the dew point exceeds the upper limit, the integrated control section 85 rewrites the first temperature elevation program into the second temperature elevation program. According to this, thereafter, the integrated control section 85 causes the temperature adjustment section 82 and the output adjustment section 52 to operate based on the second temperature elevation program, and temperature elevation in the firing furnace 1 is performed (third step).
Here, the second temperature elevation program specifies the relationship between the elapsed time and the set temperature in the same manner as the first temperature elevation program, however, this relationship is different from that of the first temperature elevation program. Specifically, in the second temperature elevation program, the temperature elevation rate may be specified to be lower than that in the first temperature elevation program, but is preferably specified to be higher than that in the first temperature elevation program. According to this, the decreased amount of the reaction rate of the reduction reaction can be compensated by the acceleration of the reduction reaction by the increase in the temperature elevation rate. That is, the decrease in the reaction rate of the reduction reaction is indirectly found from the measurement value of the vacuum degree or the dew point, and is reflected in (fed back to) the temperature elevation program, whereby the decrease in the reduction reaction can be promptly compensated. According to this, the occurrence of sintering failure can be suppressed by preventing a state where the reaction rate of the reduction reaction is decreased from continuing for a long time.
FIG. 5 shows an exemplary temperature profile showing the second temperature elevation program and an exemplary graph showing a change in the measurement value of the vacuum degree. The temperature profile showing the second temperature elevation program is represented by a solid polygonal line, and the change in the measurement value of the vacuum degree is represented by an alternate long and short dash line.
As shown in FIG. 5, in the temperature profile showing the second temperature elevation program, when the vacuum degree is increased, the temperature elevation rate is increased in response to this as compared with that of the temperature profile showing the first temperature elevation program. In the temperature profile showing the second temperature elevation program shown in FIG. 5, the temperature elevation rate is increased in the course of Step 1 (S1). That is, a portion in which this temperature change occurs is defined as Step 8 (S8), the temperature elevation rate in Step 8 is higher than the temperature elevation rate in Step 1.
The temperature elevation rate in Step 8, that is, the temperature elevation rate, which is higher than that in the first temperature elevation program, in the second temperature elevation program is preferably 101% or more and 200% or less, more preferably 110% or more and 180% or less of the temperature elevation rate in Step 1. According to this, while suppressing excessive sintering, the lack of the reduction reaction can be compensated in a short time.
In the example shown in FIG. 5, in Step 8, the temperature is elevated beyond the set temperature in Step 2, however, the second temperature elevation program is not limited to such a setting. For example, the temperature elevation may be suppressed up to the set temperature in Step 2 in the same manner as the first temperature elevation program.
Further, in FIG. 5, an example in which the temperature elevation rate is increased in Step 8 is illustrated, however, the setting in Step 8 is not limited thereto, and may be, for example, a setting in which the temperature is elevated beyond the temperature in Step 2 without changing the temperature elevation rate.
Further, the setting in Step 8 shown in FIG. 5 is a setting in which after the temperature is elevated, the temperature is lowered, however, the set temperature in Step 2 may be changed to high without lowering the temperature.
In the case where such rewriting into the second temperature elevation program is not performed although the vacuum degree or the dew point is increased, for example, in the case where a step in which the progress of the first temperature elevation program is temporarily halted is performed in place of rewriting into the second temperature elevation program, a state in which the reaction rate of the reduction reaction is decreased continues for a long time. In this case, a metal oxide which cannot be reduced in the end is easy to remain, and therefore, sintering failure may occur.
On the other hand, by performing rewriting into the second temperature elevation program, the occurrence of sintering failure can be suppressed in the end. As a result, a high-quality sintered body can be efficiently produced.
On the other hand, in the case where the vacuum degree or the dew point measured is determined to satisfy the predetermined condition, that is, the measurement value of the vacuum degree is lower than the upper limit or the measurement value of the dew point is lower than the upper limit, the integrated control section 85 continuously performs temperature elevation in the firing furnace 1 based on the first temperature elevation program (third step).
Incidentally, after rewriting into the second temperature elevation program is performed in this step, the operation is returned to the second step again as shown in FIG. 4 in this embodiment. That is, in this embodiment, the second step is repeated a plurality of times. By doing this, it is possible to verify whether rewriting into the second temperature elevation program exhibits an effect.
In the case where rewriting into the second temperature elevation program exhibits an effect, in this step (third step), the vacuum degree or the dew point is determined to satisfy the predetermined condition. In this case, the integrated control section 85 writes back the second temperature elevation program into the first temperature elevation program. According to this, thereafter, the integrated control section 85 performs temperature elevation in the firing furnace 1 based on the original first temperature elevation program again. By doing this, when the vacuum degree or the dew point which has been increased is decreased, the operation can be promptly returned to temperature elevation based on the first temperature elevation program. According to this, the sintering process can be allowed to proceed substantially as designed, and therefore, the variation in the quality of the sintered body can be kept to the minimum.
In the case where, although rewriting into the second temperature elevation program is performed, the effect of rewriting is not yet sufficiently exhibited, the vacuum degree or the dew point is determined not to satisfy the predetermined condition. In this case, the second temperature elevation program is continuously applied, and the second step and the third step are repeated until the effect is exhibited and the predetermined condition is satisfied. According to this, the decrease in the reaction rate of the reduction reaction can be kept to the minimum by promptly decreasing the vacuum degree or the dew point which has been increased. In other words, temperature elevation based on the second temperature elevation program is continued until the effect of rewriting into the second temperature elevation program is exhibited, and therefore, the effect of the second temperature elevation program is exhibited to the maximum.
In the case where the second step and the third step are repeated a plurality of times in this manner, the content of the second temperature elevation program may be updated every time the steps are repeated. For example, by updating the second temperature elevation program in which the temperature elevation rate is further increased, the decrease in the vacuum degree or the dew point can be further accelerated.
[4] In the case where the first temperature elevation program is continuously used in the third step, thereafter, a determination is made as to whether the termination condition for the first temperature elevation program is satisfied as shown in FIG. 4. Then, in the case where the termination condition is satisfied, the first temperature elevation program is terminated, and the temperature elevation in the firing furnace 1 is terminated. By doing this, the sintered body is cooled to room temperature by natural cooling or forced cooling.
On the other hand, in the case where the termination condition is not satisfied, the operation is returned to the second step again as shown in FIG. 4 in this embodiment. By doing this, temperature elevation based on the first temperature elevation program is continued (fourth step).
The termination condition for the first temperature elevation program may be, for example, an elapsed time from the start of the operation, or may be an elapsed time from a specific step, or may be the measurement value of the vacuum degree or the dew point in the firing furnace 1 or the value calculated from the measurement value. Such a termination condition can be a condition capable of sufficiently sintering the molded body.
The measurement value of the vacuum degree or the dew point in the second step described above may be subjected to calculation processing as needed after it is sent to the integrated control section 85. That is, the integrated control section 85 (in the third step) may perform calculation processing of the measurement value of the vacuum degree or the dew point and determine whether the obtained calculated value satisfies the predetermined condition.
Examples of this calculation processing include time differential processing and time integration processing. Among these, by performing time integration processing, the changes in the vacuum degree or the dew point over time can be integrated. Then, the integrated control section 85 determines whether the time integral of the vacuum degree or the dew point over time satisfies the predetermined condition. As a result, it becomes easy to find the change in the vacuum degree or the dew point at an earlier stage than in the case where the measurement value itself is monitored. That is, in the case where the measurement value itself is monitored and a determination is made in the third step, there is no choice but to set a somewhat mild condition for the determination in consideration of the fluctuation in the measurement value. On the other hand, by using the time integral value, the sensitivity to the fluctuation in the measurement value is reduced. Therefore, the condition for the determination can be relatively strictly set, and the change in the vacuum degree or the dew point can be dealt with in good time. As a result, the excessive deterioration of the vacuum degree or the dew point can be suppressed, and thus, a state where the reaction rate of the reduction reaction is decreased can be prevented from continuing for a long time.
A factor of the processing environment such as a vacuum degree or a dew point is a physical amount which is increased accompanying the progress of the reduction reaction, the thermal decomposition of the organic binder, the detachment of a component attached to the inner wall surface of the firing furnace 1, or the like. As described above, the reduction reaction of a metal oxide is likely to be affected by the increase in the vacuum degree or the dew point directly in a short time. Due to this, by making the determination in the third step based on such a physical amount, a state where the reaction rate of the reduction reaction is decreased can be prevented from continuing for a long time. In addition, the vacuum degree or the dew point has a small time lag when it is measured by a measurement unit, that is, it is a physical amount whose change can be found in a short time, and therefore is useful as a physical amount to be used for making the determination in the third step.

Degreased Body Production Method

Next, an embodiment of a degreased body production method according to the invention will be described. In the following description, a method using the above-mentioned degreasing furnace 10 will be described.
FIG. 6 is a process chart for explaining the embodiment of the degreased body production method according to the invention.
Hereinafter, the embodiment of the degreased body production method will be described, however, in the following description, different points from the above-mentioned embodiment of the sintered body production method will be mainly described, and the description of the same matter will be omitted.
The degreased body production method shown in FIG. 6 is the same as the sintered body production method shown in FIG. 4 except that the degreasing furnace 10 is used in place of the firing furnace 1, the concentration of a thermally decomposed gas of the organic binder is measured in the second step, and a determination is made as to whether the concentration of the thermally decomposed gas measured satisfies a predetermined condition in the third step.
That is, the decreased body production method shown in FIG. 6 includes [1] a first step in which a molded body (material to be processed) is placed in the degreasing furnace 10 and temperature elevation is started based on a first temperature elevation program, [2] a second step in which the concentration of a thermally decomposed gas of an organic binder in the degreasing furnace 10 is measured in the course of elevating the temperature, [3] a third step in which temperature elevation is performed by applying a second temperature elevation program when the concentration of the thermally decomposed gas measured does not satisfy a predetermined condition, and temperature elevation is performed by applying the first temperature elevation program when the concentration of the thermally decomposed gas measured satisfies the predetermined condition, and [4] a fourth step in which in the case where the first temperature elevation program is selected in the third step, and when a termination condition for the first temperature elevation program is satisfied at that time point, the first temperature elevation program is terminated, and when the termination condition for the first temperature elevation program is not satisfied at that time point, the operation is returned to the second step.
In the second step, the concentration of the thermally decomposed gas of the organic binder in the degreasing furnace 10 is measured in the course of elevating the temperature. The measurement value is sent to the integrated control section 85.
In the third step, the integrated control section 85 makes a determination as to whether the concentration of the thermally decomposed gas measured satisfies a predetermined condition. Since a factor of the processing environment such as the concentration of the thermally decomposed gas has an influence on the reaction rate of the thermal decomposition reaction of the organic binder, the predetermined condition is appropriately set in consideration of the same. That is, if the concentration of the thermally decomposed gas is increased, the thermal decomposition rate may be decreased to cause degreasing failure (result in insufficient degreasing), and therefore, the upper limit of the concentration of the thermally decomposed gas is set so that the thermal decomposition rate is not decreased to such an extent that degreasing failure occurs, and in the case where the measurement value or the value calculated from the measurement value falls within the range lower than the upper limit, the concentration of the thermally decomposed gas is determined to “satisfy the predetermined condition”.
Other than this, the second step to the fourth step of the embodiment of the degreased body production method are the same as those of the embodiment of the sintered body production method except that the vacuum degree or the dew point in the embodiment of the sintered body production method is replaced by the concentration of the thermally decomposed gas.
Hereinabove, the sintered body production method, the degreased body production method, and the heating furnace according to the invention have been described with reference to the embodiments shown in the drawings, however, the invention is not limited thereto.
For example, in the heating furnace according to the invention, the partition wall 4 may be omitted from the above-mentioned embodiment, or the position of the heater 5 may be between the partition wall 4 and the furnace main body 2. Further, the above-mentioned embodiment is a so-called batch-type furnace, however, the heating furnace according to the invention may be a continuous furnace.
Further, in the sintered body production method according to the invention and the degreased body production method according to the invention, an arbitrary step may be added to each of the above-mentioned embodiments.

Claims

What is claimed is:

1. A sintered body production method, which is a method for producing a sintered body by firing a molded body containing a metal powder and an organic binder, comprising:

a first step in which temperature elevation is started based on a first temperature elevation program in a firing furnace in a state where the molded body is placed in the firing furnace;

a second step in which a vacuum degree or a dew point in the firing furnace is measured in the course of elevating the temperature; and

a third step in which temperature elevation is performed by applying a second temperature elevation program in place of the first temperature elevation program when the vacuum degree or the dew point measured in the second step does not satisfy a predetermined condition, and temperature elevation is performed by applying the first temperature elevation program when the vacuum degree or the dew point measured in the second step satisfies the predetermined condition.

2. The sintered body production method according to claim 1, wherein the second temperature elevation program is a program in which a temperature elevation rate is increased among the factors constituting the first temperature elevation program.

3. The sintered body production method according to claim 2, wherein the second step is performed again after the third step.

4. The sintered body production method according to claim 1, wherein

in the second step, the time integral of the vacuum degree or the time integral of the dew point in the firing furnace is calculated, and

the predetermined condition in the third step is a condition associated with the time integral of the vacuum degree or a condition associated with the time integral of the dew point.

5. A degreased body production method, which is a method for producing a degreased body by degreasing a molded body containing a metal powder and an organic binder, comprising:

a first step in which temperature elevation is started based on a first temperature elevation program in a degreasing furnace in a state where the molded body is placed in the degreasing furnace;

a second step in which the concentration of a decomposed gas of the organic binder in the degreasing furnace is measured in the course of elevating the temperature; and

a third step in which temperature elevation is performed by applying a second temperature elevation program when the concentration of the decomposed gas measured is within a predetermined range, and temperature elevation is performed by applying the first temperature elevation program when the concentration of the decomposed gas measured is outside the predetermined range.

6. A heating furnace, comprising:

a furnace main body;

a heating unit which heats the inside of the furnace main body;

an output adjustment unit which adjusts the output of the heating unit;

a measurement unit which measures a vacuum degree or a dew point in the furnace main body; and

a control unit which has a function of controlling the operation of the output adjustment unit based on a temperature elevation program and a function of rewriting the temperature elevation program based on the measurement result of the measurement unit.

7. A heating furnace, comprising:

a furnace main body;

a heating unit which heats the inside of the furnace main body;

an output adjustment unit which adjusts the output of the heating unit;

a measurement unit which measures the concentration of a decomposed gas of an organic binder in the furnace main body; and