JP7044307B2 - Vacuum curing method of metal work and manufacturing method of metal work using this - Google Patents

Description

The present invention relates to a vacuum carburizing method, a vacuum nitriding method, a vacuum curing method such as a vacuum carburizing nitriding method, and a method for manufacturing a metal work (processed product) using the vacuum hardening method.

Here, the metal work broadly includes steel products, titanium or its alloys, or aluminum or its alloys that can be carburized or nitrided. Further, the metal work includes both a case where the metal work is entirely made of a carburized or nitridable material and a case where at least a part thereof is made of a carburizing or nitridable material. Even if the entire metal work can be carburized or nitrided, a carburized layer or a nitriding layer may be formed only on a part of the metal work by applying a hardening preventing means to a part that does not need to be hardened.

As a means for hardening the surface layer of a steel metal work, for example, a vacuum carburizing method is widely used. The vacuum carburizing method introduces a hydrocarbon gas into a processing furnace in a high-temperature vacuum atmosphere and incorporates carbon separated from the hydrocarbon gas into the surface layer of the metal work, compared to gas carburizing performed under atmospheric pressure. There are advantages such as the advantage that the amount of hydrocarbon gas used can be suppressed, the advantage that the carburizing time can be shortened, and the high safety because there is no diffusion of hydrocarbon gas.

In this vacuum carburizing method, the carburizing effect such as carburizing amount and carburizing depth depends on various factors such as the amount and type of carburizing gas introduced, the degree of vacuum, and the temperature inside the furnace (surface temperature of the metal work). In particular, the amount of carbide gas introduced (to be exact, the amount introduced per unit time and unit volume) is an important factor.

The hydrocarbon gas comes into contact with a metal work or the like and separates into carbon and hydrogen in the processing furnace, but the degree of carburization depends on the amount of carbon infiltrated (contacted) per unit surface surface of the metal work. Therefore, conventionally, hydrocarbon gas is excessively introduced so as to maintain a sufficient carbon content. That is, it can be said that conventionally, the hydrocarbon gas is pushed into the processing furnace to ensure the carburizing.

When the hydrocarbon gas is decomposed, the concentration of hydrogen in the processing furnace increases, so that the degree of separation of the hydrocarbon gas can be grasped from the hydrogen concentration. Patent Document 1 pays attention to this point, detects, for example, a hydrogen concentration from the thermal conductivity of the atmospheric gas in the processing furnace, and based on this hydrogen concentration, the composition of the atmospheric gas in the processing furnace (separation of hydrocarbon gas). It is disclosed that the degree) is analyzed and the amount of hydrocarbon gas introduced is controlled based on the analysis result.

In other words, the hydrocarbon gas comes into contact with the surface of the steel member and decomposes into carbon and hydrogen, but the degree of decomposition of the hydrocarbon gas is the surface surface of the metal work, the degree of oxidation, and the sooting of the steel furnace wall. It depends on various conditions such as the degree, type and amount of hydrocarbon gas introduced, and degree of vacuum. Therefore, in Patent Document 1, the hydrogen concentration is set to a set value (hydrogen concentration in a state where normal decarburization is performed). By feedback-controlling the amount of hydrocarbon gas introduced, a sufficient amount of carbon is ensured even if the conditions inside the furnace change.

By the way, the inside of the processing furnace is not a complete vacuum, and a small amount of air remains, but the partial pressure of oxygen constituting the air changes according to the amount of hydrogen separated from the hydrocarbon gas. Therefore, Patent Document 1 also discloses that the oxygen concentration in the processing furnace is detected and the amount of hydrocarbon gas introduced is controlled so that the oxygen concentration becomes constant.

As the hydrocarbon gas, acetylene, propane, methane, etc. are used. While acetylene decomposes into carbon and hydrogen in one step, a hydrocarbon gas having a large molecular weight such as propane is used. Some are directly decomposed into carbon and hydrogen in the first stage, and the rest are decomposed into hydrocarbon gas having a small molecular weight, and then decomposed into carbon and hydrogen in the second stage. In any case, since the entire hydrocarbon gas does not come into contact with the metal work, there is a considerable proportion of the gas discharged to the outside of the processing furnace without being decomposed into carbon and hydrogen.

Carburizing is usually combined with quenching (and tempering), and hardening by quenching imparts compressive residual stress to the metal work, improving the mechanical properties of the metal work. And one of the problems with metal workpieces carburized using hydrocarbon gas is hydrogen embrittlement. It can be said that the cause and mechanism of hydrogen embrittlement are almost unclear, but many countermeasures have been proposed.

Most of the hydrogen embrittlement countermeasure approaches try to eliminate the brittleness by removing hydrogen, assuming that the cause of the brittleness is the presence of hydrogen. As an example, Patent Document 2 describes that a high effect (improvement of mechanical properties) was obtained by removing hydrogen.

On the other hand, there is an idea that the main cause of hydrogen embrittlement is not the hydrogen itself but the formation of voids (vacuums) between the crystal lattices of the steel material due to the infiltration of hydrogen. A method of performing vacuum carburizing in a state where the above is not generated is disclosed.

In addition, there is Non-Patent Document 1 as an academic research paper on the cause and mechanism of hydrogen embrittlement. According to Non-Patent Document 1, hydrogen promotes the formation of pores in a metal structure, and even if hydrogen is removed, the pores are not removed, so that the crack growth resistance is reduced and the fracture process is promoted. "Hydrogen-enhanced plastic-induced vacancy theory" has been proposed.

As an example of controlling the amount of hydrocarbon gas introduced in the vacuum carburizing method, Patent Document 4 measures the carbon potential value (CP value) by analyzing the amounts of hydrogen and methane in the furnace. It is disclosed that the amount of hydrocarbon gas introduced is controlled so that the C / P value becomes a desired value. Further, in Patent Document 5, a parameter called "theoretical hydrogen partial pressure ratio" is used to compare the time change of the theoretical hydrogen partial pressure ratio with the time change of the actual hydrogen partial pressure ratio, and the actual change of the hydrogen partial pressure ratio is obtained. It is disclosed to control so as to approach the change of the theoretical hydrogen partial pressure ratio.

Japanese Patent No. 3531736 Japanese Unexamined Patent Publication No. 2002-339045 Japanese Unexamined Patent Publication No. 2015-78416 Japanese Unexamined Patent Publication No. 53-15232 Japanese Unexamined Patent Publication No. 2012-7240

"Pressure Technology", Vol. 46, No. 4, pp. 190-199, published by Japan High Pressure Technology Association, written by Michihiko Nagumo, "Hydrogen Embrittlement as a Fracture Phenomenon-Hydrogen Embrittlement Mechanism-"

As described above, in the vacuum carburizing method, the amount of carbon in the processing furnace is an important factor that influences the degree of carburizing, and it can be said that the control of Patent Document 1 is also to secure the required amount of carbon. That is, the introduction amount of the hydrocarbon gas is controlled in order to secure the required carbon amount, and the hydrogen concentration and the oxygen concentration are used as a means for maintaining the carbon amount. This point can be said to be the same in Patent Documents 4 and 5.

In the past, securing the amount of carburizing was the highest priority, and a larger amount of hydrocarbon gas than the theoretical value required for carburizing was pushed into the processing furnace, and as a result, means for removing hydrogen that had penetrated into the steel. Therefore, it can be said that the post-treatment was performed as in Patent Document 2. On the other hand, Patent Document 3 seeks the main cause of hydrogen embrittlement in voids (pores) formed in steel, and it can be said that the idea of Patent Document 3 is theoretically supported by Non-Patent Document 1.

After all, the infiltration of hydrogen into steel during carburizing should be suppressed as much as possible, and it is thought that by suppressing the infiltration of hydrogen itself, brittleness (particularly delayed fracture) caused by hydrogen can be eliminated or significantly suppressed. .. In the vacuum nitriding method, ammonia gas is generally used as the nitrogen-containing gas, but since hydrogen is generated by the decomposition of ammonia, hydrogen brittleness appears as a serious problem in the vacuum nitriding method as well as in the vacuum carburizing method.

Since Patent Document 3 carburizes without using a gas containing no hydrogen, the purpose of preventing hydrogen embrittlement can be surely achieved. Therefore, it can be said that Patent Document 3 can make a great contribution to related industries, but research is still required to improve the carburizing property of non-hydrogen-containing gas. Further, although the vacuum carburizing equipment (or the vacuum carburizing nitriding equipment) is widely used in practice, if the same effect as that of Patent Document 3 can be obtained by using the existing equipment or the same hydrocarbon gas as the conventional one. It can be said that the profits received by the industry are extremely large.

The invention of the present application was made on the basis of such recognition and knowledge, and the point of suppressing the infiltration of hydrogen is in line with Patent Document 3, from the viewpoint of effectively utilizing existing equipment and raw materials. Therefore, it is intended to easily provide a metal work with measures against hydrogen embrittlement.

As described above, the hydrocarbon gas comes into contact with the surface of the metal work and decomposes into carbon and hydrogen. Therefore, even if a hydrocarbon gas is used as the carburizing gas, the problem of hydrogen embrittlement can be significantly improved if the decomposed hydrogen does not infiltrate into the metal work or the infiltration amount is small. From this point of view, the inventors of the present application repeated thoughts, research and experiments, and came to complete the invention of the present application. The term "metal work" as used in the present invention refers to a work placed in a processing furnace as a curing treatment target, and includes both a single member and a plurality of members.

The present invention includes a method for vacuum curing a metal work and a method for manufacturing the metal work. Of these, the vacuum curing method for metal workpieces is
"The preparatory process for arranging the metal work in the processing furnace and
It is provided with a curing treatment step of introducing a curing gas into a processing furnace in which the metal work is arranged under a high temperature vacuum atmosphere to form a cured layer on the surface of the metal work.
As the curing gas, a gas in which carbon and hydrogen are bonded or a gas in which nitrogen and hydrogen are bonded is used, and the curing gas introduced into the processing furnace in the curing treatment step is used in the metal work. It is divided into a portion that is contacted and decomposed and a portion that is discharged without contacting the metal work, and carbon or nitrogen separated by contacting the metal work enters the surface of the metal work and the cured layer is formed. A carburized layer or a nitriding layer is formed. "
In the configuration
" In the curing treatment step, hydrogen separated from the curing gas is produced by controlling the introduction amount of the curing gas per unit time so that the amount of the curing gas in contact with the metal work does not become excessive. Preventing or suppressing invasion of the metal work "
(The "high temperature" of the present invention means the temperature at which the curing gas decomposes, and it differs depending on the type of gas and the type of metal work, etc. Generally, it is in the range of about 700 to 1100 ° C. This point is obvious to those skilled in the art).

Then, in the first invention, in the above basic configuration,
" As a control of the amount of curing gas introduced per unit time in the curing treatment step, in order to evenly adhere carbon or nitrogen to the entire surface of the untreated metal work in a state where hydrogen is prevented from entering. Based on the reference gas amount preset as the amount to be introduced into the processing furnace ,
i) Introduce an amount smaller than the standard gas amount,
ii) Introduce the same amount as the standard gas amount,
iii) Introduce an amount larger than the reference gas amount within the preset allowable range.
One of the three controls is continuously performed , or a plurality of controls are combined . "
It is configured as.

The first invention can be developed in various ways. As an example, in the second invention,
" In the curing treatment step, an amount of curing gas smaller than the reference gas amount is continuously introduced."
It is configured as.

Further, as another development example of the first invention, in the third invention,
" In at least a part of the curing treatment step, the amount of the curing gas introduced is such that the amount of the curing gas is equal to or more than the reference gas amount within an allowable range in which hydrogen separated from the curing gas can be suppressed from entering the metal work. Be controlled "
It is configured as.

The basic feature of the present invention is to prevent or suppress the invasion of hydrogen separated from the curing gas into the metal work, and for this purpose, the amount of the curing gas introduced is determined based on the reference gas amount. Although controlled, the reference gas amount varies depending on the total surface area of the metal work placed in the processing furnace. Therefore, it is necessary to set the reference gas amount each time the metal work is put into the processing furnace and processed. The fourth invention responds to such a request.

That is, the fourth invention is described in any one of the first to third inventions.
"The curing treatment step includes an initial step that also serves as detection of the reference gas amount, and a continuous step in which the introduced gas amount is controlled based on the reference gas amount.
Using a sample whose surface area is known, the sample is directly or indirectly detected by the decomposition rate of the curing gas when the curing gas is introduced in an amount set as the detection introduction amount per unit time. Set it as a value,
In the initial step, the curing gas is introduced by the amount introduced for detection to directly or indirectly detect the decomposition ratio of the curing gas, and the actual value regarding the decomposition ratio is compared with the sample value. By doing so, the surface area of the metal work is set, and the reference gas amount is set based on the set surface area. "
It is configured as.

As can be understood as the opposite interpretation of the fourth invention, the curing treatment step can adopt a configuration that does not include an initial step of detecting the reference gas amount. That is, it is possible to control the amount of curing gas introduced based on the reference gas amount throughout the entire curing treatment process. For example, if the steps related to the same metal work are continuous a plurality of times and the reference gas amount has already been obtained by the preceding work, the initial step can be omitted.

The manufacturing method can be specified as the fifth invention. This fifth invention is
" A cured layer is formed on the surface by the vacuum curing method according to any one of the first to fourth inventions, and then quenching is performed. "
It is configured as.

In the present invention, the type of metal work to be carburized is not limited, and anything that can be put into a processing furnace is targeted. The following can be illustrated, albeit very little. That is, rod-shaped screws (or rod-shaped fasteners) such as drill screws, tapping screws and wood screws, gears, pulleys, chain sprocket, components of power transmission devices such as chains, various rotating shafts and arms, and various casings (housings). ), Balls, rollers, inner races, outer races, retainers, rods and pistons of hydraulic cylinders and pneumatic cylinders, drive shafts of solenoid valves, etc. that make up the bearings. The rotating shafts include a wide variety of shafts such as engine cam shafts and crank shafts, input shafts and output shafts of stepless transmissions, rotating shafts of water pumps for internal combustion engines, and spindles of various motors.

By the way, Patent Document 1 describes that the hydrogen concentration in the processing furnace is controlled to be the hydrogen concentration under "normal carburizing conditions". This can be said to mean that if the normal carburizing conditions are maintained, the desired carburizing will be performed even if the conditions are different. The problem here is that, firstly, hydrogen embrittlement occurred under "normal carburizing conditions", and secondly, a large amount of attention was paid only to securing the required carbon amount in any state. It was the introduction of hydrocarbon gas. This point will be described with reference to FIG. 1 (A).

FIG. 1A shows a state in which acetylene (C ₂ H ₂ ) is introduced into a processing furnace 1 having a gas inlet 2 and a gas discharge port 3 and acetylene is brought into contact with the surface of the metal work W to carburize. It is displayed schematically. In this state, a larger amount of acetylene than the theoretical value is introduced so that sufficient carbon can be separated and a desired amount of carbon can be obtained (the amount of acetylene becomes rich inside the processing furnace 1). However, it is not known how rich it is.)

Since hydrogen is the gas having the smallest mass among the substances, the hydrogen separated on the surface of the metal work W basically diffuses into the inside of the processing furnace 1 and is discharged from the gas discharge port 3. However, if the amount of acetylene introduced is set to a high value for the purpose of maintaining the amount of carbon, the partial pressure of hydrogen becomes high and the emission of gas from the processing furnace 1 tends to be delayed. Insufficient diffusion (emission) creates an environment in which hydrogen stays on the surface of the metal work W, or the rate of hydrogen scattering is significantly reduced, creating an environment in which hydrogen is easily captured on the surface of the metal work W. Hydrogen is taken in between the crystal lattices of the steel constituting the metal work W (because the hydrogen molecule is the smallest, it is easily taken in between the atoms constituting the crystal lattice of the steel).

Most of the hydrogen taken into the surface layer of the metal work W is released into the atmosphere with the passage of time (or by tempering), and remains in the structure of the metal work W. Although the amount is small, the voids (pores) generated by the infiltration of hydrogen remain in the structure as they are, and the voids are not closed and the structure is not repaired. It is presumed that the thinning force is reduced, and as a result, plastic deformation gradually progresses due to repeated loading and continuous loading, eventually leading to fracture.

On the other hand, in the present invention, as schematically shown in FIG. 1 (B), the amount of acetylene introduced is controlled so that the amount of hydrogen generated is within the permissible limit. Therefore, hydrogen is discharged from the gas discharge port 3 with no or almost no retention near the surface of the metal work W. To be precise, the amount of acetylene introduced is controlled so that even if hydrogen is generated, it is not infiltrated into the metal work W and is quickly eliminated. This makes it possible to prevent or remarkably suppress the infiltration of hydrogen into the steel. This point will be further described.

The amount of hydrogen generated per unit time is roughly determined by the type (composition) of the hydrocarbon gas, the amount of the hydrocarbon gas introduced per unit time, the degree of vacuum of the processing furnace 1, and the total surface area of the metal work W. If the type of hydrocarbon gas and the degree of vacuum are constant, the amount of separation between carbon and hydrogen (that is, the amount of carbon and hydrogen generated per unit time) and the introduction of hydrocarbon gas per unit time are fixed. It fluctuates according to the amount (directly proportional).

Further, not all of the hydrocarbon gas introduced into the processing furnace 1 comes into contact with the metal work W, and some hydrocarbon gas is discharged from the processing furnace without coming into contact with the metal work W, and the hydrocarbon gas is emitted. Even if carbon is separated by contact with the metal work W, some carbon is discharged to the outside of the processing furnace without being subjected to hydrocarbon engraving. That is, the hydrocarbon gas used for carburizing includes a loss discharged without being decomposed and a loss discharged without contributing to carburizing even if decomposed.

Further, in the vacuum carburizing method, as shown in (C), first, the carbon atoms separated from the hydrocarbon gas adhere evenly to the surface of the metal work W, and then the carbon atoms infiltrate into the inside over time (. The mechanism of diffusion) is adopted, and the minimum (necessary and sufficient) amount of hydrocarbon gas introduced (accurately) to realize a state in which carbon atoms adhere evenly to the surface of the metal work W. (Time x introduction amount) can be defined as the reference gas amount (the introduction amount in anticipation of the above loss).

Then, as a means for detecting the amount of the reference gas, for example, the concentration of hydrogen generated when the amount of the reference gas can be defined as the reference hydrogen concentration. Since it is discharged to the outside, it can be said that hydrogen does not infiltrate into the metal work W.

However, in the past, since there was no tool such as the standard hydrogen concentration and the standard gas amount was not grasped, the amount of hydrocarbon gas that greatly exceeded the standard gas amount (many times) was introduced into the processing furnace. As a result, hydrogen was also excessively generated at a concentration exceeding the reference hydrogen concentration, and as a result, a large amount of hydrogen stayed near the surface of the metal work W and stayed. It can be said that a large amount of hydrogen had infiltrated into the structure of the metal work W.

On the other hand, in the present invention, by grasping the reference gas amount by using a detection means such as the reference hydrogen concentration, the introduction amount of the hydrocarbon gas can be controlled so that the hydrogen generation amount is within the allowable range. As a result, hydrogen generated by decomposing the hydrocarbon gas can be quickly discharged to the outside of the processing furnace without staying near the surface of the metal work. Therefore, the generation of brittleness due to the infiltration of hydrogen can be prevented or remarkably suppressed. In addition, variations in the curing process can be suppressed.

In the second invention of the present application, the amount of the curing gas introduced into the processing furnace is set to be lower than the reference gas amount, so that hydrogen is prevented from entering the metal work due to the vacuum curing process. , It is possible to prevent hydrogen embrittlement from occurring in the metal work.

By the way, the mechanical properties required for the metal work W are various, and the degree to which hydrogen infiltration is allowed also greatly differs depending on the type and grade of the metal work W. Therefore, it may not be necessary to set the introduction amount of the hydrocarbon gas lower than the reference gas amount. In the third invention, paying attention to this point, a curing gas having a reference gas amount or more is introduced into the processing furnace. Specifically, it is estimated that the amount of hydrogen invaded can be significantly reduced as long as it is up to about 1.5 times the standard gas amount.

The above description has been about a carburizing method using a hydrocarbon gas, but the above description is generally valid for vacuum nitriding using a hydrogen nitride gas such as ammonia. Further, there is a vacuum carburizing nitriding method in which a vacuum carburizing method and a vacuum nitriding method are combined, and the same applies to this case. Regarding nitriding, first, the separated nitrogen is combined with the iron on the surface of the metal work to form a layer of iron nitride on the surface of the metal work, and then the nitrogen is separated from the iron nitride and penetrates into the inside. It is presumed to go. Since hydrogen is generated in proportion to the decomposition rate of gas such as ammonia even in vacuum nitriding, the amount of gas introduced can be controlled by using the hydrogen concentration.

As the carburizing of the metal work progresses, the ratio of the hardening gas introduced into the processing furnace to the carburizing per unit time changes. Therefore, as in the embodiment, it is possible to correct the amount of the curing gas introduced based on an index such as oxygen partial pressure.

The control of the amount of the curing gas introduced based on the reference gas amount is specifically performed by using a numerical value that changes according to the amount of the curing gas introduced. For example, it is possible to detect the hydrogen concentration (hydrogen partial pressure) in the furnace and control the actual hydrogen concentration (hydrogen partial pressure) to be equal to or higher than the reference hydrogen concentration (partial pressure).

How much hardening gas is required for carburizing or nitrification of the metal work W largely depends on the total surface area of the metal work. If the total surface area is small, the amount of curing gas may be small, and if the total surface area is large, the required amount of curing gas is also large. In addition, the contact property of the curing gas with respect to the metal work changes depending on the volume of the furnace.

Therefore, the problem is how to detect the total surface area of the metal work. If the metal work is a simple flat plate, it can be calculated, but in the case of a metal work W with a complicated shape such as a screw or gear, it is troublesome to calculate the total surface area. Often there are. When metal works with different shapes and dimensions are mixed, it becomes more troublesome to calculate the total surface area. Further, even if it is a simple flat plate, when carburizing a large number of pieces at once, the number must be calculated one by one, so it is very troublesome to calculate the total surface area.

On the other hand, in the configuration of claim 4, in the initial step, a numerical value representing the decomposition rate (for example, hydrogen concentration) is detected as a control index, and the actual value is compared with the sample value to determine the total surface area of the metal work. Since it can be grasped, the reference gas amount of the metal work suitable for each treatment can also be grasped.

Therefore, regardless of the shape and number of metal workpieces, an appropriate amount of curing gas can be introduced and the curing treatment can be performed in a state where the invasion of hydrogen is prevented or suppressed. What is important here is that the reference gas amount can be detected while performing the curing treatment, and therefore, it is excellent in efficiency.

The total surface area of the metal work does not necessarily have to be detected as an actual value. For example, when the hydrogen concentration is used as an index to know the degree of decomposition of the curing gas, the numerical value of the hydrogen concentration (average hydrogen concentration) that appears in the initial process is grouped into several groups, and the reference gas amount corresponding to each group is used. It is also possible to set. In this case, there are some variations, but it is realistic.

(A) is a schematic diagram of a conventional example, and (B) and (C) are schematic views of the present invention. It is a conceptual diagram of a vacuum carburizing facility. In the figure which shows embodiment, (A) is a figure which shows the flow of a process, (B) is a figure which shows the amount of hydrocarbon gas introduced, (C) is a figure which shows the change of hydrogen concentration, (D) is oxygen partial pressure. It is a figure which shows the change of. (A) is a diagram showing a simple relationship between the hydrogen concentration and the amount of hydrocarbon gas introduced, and (B) is a diagram showing a state in which the relationship between the hydrogen concentration and the amount of hydrocarbon gas introduced is corrected by the oxygen partial pressure. be. It is a figure which shows the experimental result of a reference example, (A) is a graph explaining a material, (B) is a graph which shows the relationship between a stress range and the number of fracture repetitions, (C) is a side view of a test piece.

(1). Outline of equipment Next, the invention of the present application will be described with reference to the drawings. First, the outline of the vacuum carburizing equipment shown in FIG. 2 will be described. This equipment is the same as the conventional one, and as the main elements, the processing furnace (carburizing chamber) 1 in which the gas introduction port 2 and the gas discharge port 3 are opened inside and the processing furnace 1 are adjacent to each other through the door 4. It has a cooling chamber 5 and a quenching chamber 6 arranged below the cooling chamber 5. In the quenching chamber 6, a quenching tank 7 in which a cooling liquid (water system or oil) is stored is arranged. Although not shown, the processing furnace 1 and the cooling chamber 5 are provided with doors for taking in and out the metal work W.

Although only one gas introduction port 2 and one gas discharge port 3 are displayed, it is possible to provide a plurality of each. In general, four or more gas introduction ports 2 are provided, and about two gas discharge ports 3 are often provided.

A cylinder 8 is connected to the gas introduction port 2 via a gas introduction pipe 9, and a first valve 10 is arranged in the middle of the gas introduction pipe 9. The cylinder 8 is filled with a hydrocarbon gas as a carbon source. In this embodiment, acetylene is used as the hydrocarbon gas. The gas discharge port 3 is connected to the vacuum pump 12 via the first discharge pipe 11, and a second valve 13 is interposed in the middle of the first discharge pipe 11. The first discharge pipe 11 is also connected to the cooling chamber 5 via the second discharge pipe 14, and the third valve 15 is arranged in the middle of the second discharge pipe 14.

A large number of heat transfer heaters 16 are arranged inside the processing furnace 1, and each heater 16 is connected to a power source 17. Further, the carburizing equipment is provided with a hydrogen concentration sensor 18 as a means for directly detecting the hydrogen concentration.

As disclosed in Patent Document 1, the hydrogen concentration sensor 18 detects the thermal conductivity of the gas in the processing furnace 1, and is provided in the bypass pipe 19 provided in the first discharge pipe 11. The thermal conductivity is finally converted into an electric signal and used for control. If conditions such as performance allow, the hydrogen concentration sensor 18 can be exposed inside the processing furnace 1. An oxygen sensor 20 faces the processing furnace 1. The oxygen sensor 20 is an example of a partial pressure detecting means for a trace amount of non-hydrogen gas.

The vacuum carburizing equipment has a control device (control means) 21. The control device 21 may be a set with the vacuum carburizing equipment, or may be replaced with a personal computer. In any case, it has a memory, a monitor, an input means, and the like. The valves 10, 13, 15, the heater power supply 17, the hydrogen concentration sensor 18, and the oxygen sensor 20 are electrically connected to the control device 21. Although not shown, the processing equipment is provided with a digital temperature sensor and a vacuum gauge, which are also connected to the control device 21.

(2). Basic mode of treatment pattern Next, an example of the vacuum carburizing treatment pattern will be described with reference to FIGS. 3 and 4. The basic pattern of the vacuum carburizing treatment is the same as the conventional one, which is shown in FIG. 3 (A).

That is, the vacuum carburizing treatment includes a heating step of putting the heated metal work W into the processing furnace 1 to raise the temperature inside the furnace, a soaking step of maintaining the set temperature and heating the metal work W evenly, and the treatment furnace 1. A carburizing step in which acetylene is introduced into the metal work W to infiltrate the decomposed carbon into the metal work W in a vacuum atmosphere, a diffusion step in which the introduction of acetylene is stopped and carbon is diffused in the surface layer of the metal work W, and the temperature inside the furnace is adjusted to some extent. It is provided with a temperature lowering step of gradually lowering the temperature and a soaking step of maintaining a certain temperature to stabilize the temperature of the metal work W. Further, it has either a gas cooling step of moving to the cooling chamber 5 and cooling with nitrogen gas, or a quenching step of charging the quenching liquid into a quenching liquid for quenching.

The carburizing temperature is the same as before. It depends on the material of the metal work W, but is, for example, about 750 to 1100 ° C. The degree of vacuum of the processing furnace 1 is not significantly different from that of the conventional one, and the opening degree of the second valve 13 is automatically controlled so as to be maintained at, for example, about 4 to 10 Torr.

In this example, the carburizing step consists of an initial step of first introducing acetylene and a continuing step of introducing acetylene at a certain time interval thereafter, and the continuing step consists of introducing acetylene into a plurality of carburizing blocks I. It is a pulse control method that is divided into ~ IV. In this example, the continuation process is composed of four carburizing blocks I to IV, but the number and intervals of the carburizing blocks can be set arbitrarily.

In the initial step, a certain amount of acetylene per unit time is continuously introduced into the processing furnace 1 for a predetermined time. On the other hand, in each of the blocks I to IV constituting the continuation step, the introduction amount Q of acetylene is basically controlled with the hydrogen concentration ρ as a variable, and is also corrected by the oxygen partial pressure P. Further, in the present embodiment, as shown in FIG. 4, ON / OFF of the introduction of acetylene is repeated at short time intervals in each of the carburizing blocks I to IV constituting the continuation process. Therefore, each of the carburized blocks I to IV is also pulse-controlled to turn on / off the introduction of acetylene at short time intervals.

When shifting from the initial process to the continuation process, and when the introduction of acetylene is cut in the continuation process, the inside of the processing furnace is in a gas-free state in which acetylene does not exist, and even in this gas-free state, The processing furnace 1 is maintained in a predetermined vacuum atmosphere (the degree of vacuum is higher than when acetylene is introduced).

(3). Setting (calculation) of total surface area
As shown in FIG. 3C, in the initial step, the hydrogen concentration ρ rapidly rises from zero due to the introduction of a predetermined amount of acetylene, and the rate of increase sharply decreases. Therefore, the range in which the rate of increase is significantly reduced is set as the stabilized state, and the average value in the stabilized state is set as the reference hydrogen concentration ρ0 in the initial step. For example, assuming that the initial step is n minutes in total, the stable time T0 is (n-1) minutes between 1 and n minutes after the introduction of acetylene, and the average value of the hydrogen concentration ρ at T0 is the reference hydrogen concentration ρ0. Set as. The average value may be plotted for each unit time and their weighted average may be obtained.

The surface area (total surface area) of the metal work W is calculated from the reference hydrogen concentration ρ0. As a preparation for this, multiple types of samples whose areas are known in advance are produced, and each sample is exposed to a hydrocarbon gas to determine the hydrogen concentration. By detecting this, the relationship between the absolute value of the surface area of the metal work W and the hydrogen concentration is created as a map (correspondence table).

Conceptually speaking, a large number of samples such as 1 m ² sample, 2 m ² sample, 3 m ² sample, 4 m ² sample, 5 m ² sample, etc. are manufactured, and each of them is used as an initial step. Record the hydrogen concentration (ρ1, ρ2, ρ3 ...) Generated when carburized under the same conditions, and create a relational expression (map) between the surface area and the hydrogen concentration.

Then, the average value ρ0 of the hydrogen concentration detected from the actual metal work W is compared with the reference hydrogen concentration ρ1, ρ2, ρ3 ... Of each sample, and the actual average value ρ0 is the closest reference hydrogen concentration. Select ρ1, ρ2, ρ3 ..., set the total surface area of the sample to which this reference hydrogen concentration belongs to the total surface area of the metal work W (pseudo setting), and use the set total surface area as a reference. The saturated hydrogen concentration set in the case of the total surface area is set as the saturated hydrogen concentration of the actual metal work W.

For example, if the metal work W is set (simulated) as the total surface area of 3 m ² , the target hydrogen concentration ρ3'that allows for a safety factor is set from the standard hydrogen concentration ρ3 in the case of the total surface area of 3 m ² , and this target hydrogen concentration is set. Let ρ3'be the upper limit of the hydrogen concentration ρ. The safety factor depends on the permissible amount of hydrogen intrusion. Basically, when preventing the intrusion of hydrogen (when aiming for zero hydrogen intrusion), it can be said that the safety factor may be, for example, about 0.8 to 0.95.

On the other hand, if some intrusion is allowed, it should be 1.0 or more. When the actual hydrogen concentration becomes twice the standard hydrogen concentration ρ1, ρ2, ρ3 ..., the amount of hydrogen generated becomes excessive (same as before), and the invasion of hydrogen into the metal work can be suppressed. It is presumed that it is hard to say. Therefore, in reality, the safety factor is usually smaller than 2.0, and is preferably estimated to be about 1.5 or less. It is also possible to set the target hydrogen concentration ρ3'to the reference hydrogen concentration ρ1, ρ2, ρ3 ... (The safety factor is 1.0).

Further, the lower limit value of the hydrogen concentration ρ is also set, and for convenience, the lower limit value in the case of the total surface area of 3 m ² is displayed as ρ3 ″. The lower limit value is the reference hydrogen concentration ρ1, ρ2, ρ3 ... It may be higher, may be the same as the reference hydrogen concentration ρ1, ρ2, ρ3 ..., Or may be lower than the reference hydrogen concentration ρ1, ρ2, ρ3 ... In this case, the target hydrogen concentration ρ3', which is the control upper limit value, can be 0.9 times the reference hydrogen concentration ρ1, ρ2, ρ3 ..., And the lower limit value can be 0.85 times the target hydrogen concentration (upper limit value). ) And the lower limit value can be made different depending on the material and shape of the metal work W. The reference hydrogen concentration can also be called a saturated hydrogen concentration.

When a predetermined amount of acetylene is introduced in a predetermined time in the initial step, the portion of acetylene in contact with the metal work is decomposed to generate hydrogen, but the amount of hydrogen generated is directly proportional to the surface area of the metal work W. Further, since the inside of the processing furnace 1 has a vacuum atmosphere, acetylene comes into contact with the surface of the metal work W evenly if a certain amount is maintained as an initial process. Therefore, in the initial step, the separated carbon is evenly adhered to the surface of the metal work W.

Therefore, in the initial process, regardless of the size of the surface area of the metal work W, the minimum amount of acetylene (reference gas amount) necessary for evenly adhering carbon to the entire surface of the metal work W and the reference gas amount are used. The corresponding hydrogen concentration can be detected. To be precise, the hydrogen concentration can be used to detect the reference gas amount of the metal work. Carburizing is also performed in the initial process.

(4). Control of acetylene introduction amount As shown in Fig. 4 (A), the actual hydrogen concentration ρ changes from moment to moment, but ρ is maintained between ρ3'and ρ3 ″. , The amount of acetylene introduced Q is controlled. That is, when the actual hydrogen concentration ρ approaches the saturated hydrogen concentration (upper limit value ρ3'), the amount of acetylene introduced is reduced, and the actual hydrogen concentration ρ approaches the lower limit value ρ3 ″. By increasing the amount of acetylene introduced, the actual hydrogen concentration ρ is maintained within a predetermined range (level).

As a result, it is possible to prevent excessive generation of hydrogen in the processing furnace 1 and prevent or significantly suppress the infiltration of hydrogen into the metal work W. That is, carburizing can be performed using acetylene (hydrocarbon gas) in a state where the harm of hydrogen is eliminated or suppressed. The upper limit value Q3 ′ and the lower limit value Q3 ″ corresponding to the saturated hydrogen concentration are set for the acetylene introduction amount Q.

In the above embodiment, even if the total surface area is different, those in a certain range are treated uniformly in group management (zone management), but since there is a proportional relationship between the total surface area of the metal work W and the hydrogen concentration. For example, based on the hydrogen concentration meter results of 5 samples of 1 m ² , 2 m ² , 3 m ² , 4 m ² , 5 m ² , 1.5 m ² reference hydrogen concentration, 2.5 m ² reference hydrogen concentration, 3. It is possible to create a reference hydrogen concentration of 5 m ² and a reference hydrogen concentration of 4.5 m ² , and by preparing many zones in this way, errors can be reduced and fine control can be performed.

By the way, as described above, in the continuation step of the present embodiment, each of the carburized blocks I to IV is pulse-controlled to open and close the gas introduction port at short time intervals, and therefore, gas introduction for a short time. A large number of gas introduction units are performed intermittently. Therefore, looking at the change in hydrogen concentration in each gas introduction unit, the hydrogen concentration rises from zero and stabilizes, and the concentration after stabilization is the above-mentioned upper limit value (saturated hydrogen concentration) and lower limit value. The introduction amount Q of acetylene is controlled so as to be within the interval.

In FIG. 4, the hydrogen concentration and the amount of gas introduced fluctuate for each unit of the pulse, but this is a convenient display for easy understanding, and in reality, each gas is displayed. Even in the introduction unit, the hydrogen concentration ρ and the acetylene introduction amount Q change in high and low. However, when the time of each gas introduction unit is short, it is possible to control the amount of acetylene introduced in the next gas introduction unit based on the hydrogen concentration in the previous gas introduction unit.

As an embodiment of the present invention, the introduction amount Q of acetylene can be made larger than the reference gas amount within an allowable range. The pattern can vary. First, there is an embodiment in which the introduced amount Q is always larger than the reference gas amount (when the hydrogen concentration is controlled, the actual hydrogen concentration always exceeds the reference hydrogen concentration). Secondly, there is an embodiment in which the introduced amount Q becomes the same as the reference gas amount or exceeds the reference gas amount.

Thirdly, there is an embodiment in which the introduced amount Q varies between an amount smaller than the reference gas amount and an amount larger than the reference gas amount. The third aspect is a case where the reference gas amount is targeted and a certain fluctuation range is provided above and below the reference gas amount, but in this case, it is presumed that hydrogen hardly invades the metal work. Fourth, although the introduction amount is basically smaller than the reference gas amount, there is an embodiment in which the introduction amount is temporarily increased to the reference gas amount or more.

(5). Correction control using oxygen partial pressure In FIG. 3 (D), the change in oxygen partial pressure in the processing furnace 1 is displayed. Looking at the initial process, the partial pressure P of oxygen drops sharply from the partial pressure of oxygen before the introduction of the carburized gas and shifts to a stabilized region, and in the stable region, it is slightly increased according to the carburizing of the metal work W. Shows an upward trend. Looking at each carburizing blocks I to IV, in the state before each carburizing blocks I to IV, acetylene is not introduced in the processing furnace 1, so that the value decreases from the upper limit corresponding to the degree of vacuum and is stable. It shifts to the region, and in each stable region, it tends to increase slightly as the carburizing of the metal work W progresses.

In the continuous step of carburizing, carburizing also progresses with the passage of time, so that the average value of the oxygen partial pressure P in the stable region tends to increase toward the rear carburizing block. In addition, since each carburizing block I to IV is pulse-controlled in which a large number of gas introduction units are intermittently repeated, the oxygen partial pressure P is stabilized after abruptly decreasing for each gas introduction unit. The average value of the oxygen partial pressure P tends to increase with time even when viewed in each gas introduction unit. Therefore, the display in FIG. 3D corresponds to the display of the transition of the average value of the oxygen partial pressure P of each gas introduction unit.

As the carburization of the metal work W progresses, the decomposition performance of acetylene on the surface of the metal work W decreases, and the amount of carbon and hydrogen generated per unit time and unit amount of acetylene tends to decrease. Along with this, the oxygen partial pressure P tends to increase. Therefore, the oxygen partial pressure P is an index for grasping the progress of carburizing.

Then, due to various factors such as the progress of carburizing, even if the hydrogen concentration ρ is within the preset range, if the carbon required for carburizing is insufficient (the amount of acetylene is insufficient) or the carbon is excessive. (In excess of acetylene). That is, it may be difficult to set the amount of acetylene so that the amount of carbon can be separated without excess or deficiency only by controlling the hydrogen concentration ρ.

Therefore, as shown in FIG. 4B, the upper limit value P3'and the lower limit value P3'are set for the oxygen partial pressure P, and when the oxygen partial pressure P exceeds the upper limit value P3', the hydrogen concentration ρ is set. The amount of acetylene introduced is increased even within the range, and when the oxygen partial pressure P is below the lower limit value P3 ″, the hydrogen concentration ρ is controlled to decrease the amount of acetylene introduced even within the set range. Therefore, the amount of acetylene introduced after being corrected by the oxygen partial pressure P has changed as shown by the dotted line in FIG. 4 (B). As a result, an appropriate amount of carbon is supplied to the metal work W while preventing hydrogen from entering the metal work W.

Note that P3'and P3 "indicate the case where the surface area of the metal work W is 3 m ² , which is consistent with FIG. 4 (A). The hydrogen concentration and the amount of gas introduced in (B) are the same. The graph of (A) is used as it is, and the actual amount of acetylene introduced is displayed in a bar graph on the graph of the amount of gas introduced. In FIG. 4 (B), the control unit time is used as t for feedback. The timing of the control of the acetylene introduction amount Q based on the oxygen partial pressure P is delayed by the time of t.

In this way, by increasing the amount of acetylene introduced in response to an increase in the oxygen partial pressure P, while preventing the infiltration of hydrogen, the required amount of carbon in contact with the metal work W is secured to accurately perform carburizing. It can be carried out. Although the amount of acetylene introduced is controlled so as not to exceed the upper limit value Q3', it is possible to set Q3' in FIG. 4 (B) to a value higher than Q3' in FIG. 4 (A). ..

In the case of the vacuum carburizing and nitriding method, as shown in FIG. 3A, ammonia is introduced into the processing furnace 1 in the heat equalizing step after carburizing and nitriding is performed. In this case as well, the nitriding process is performed by combining the initial process and the continuous process as in the process in the carburizing process. The control procedure is not essentially different from that of carburizing. Since ammonia has a higher proportion of hydrogen in the molecule than acetylene, it can be said that the amount of ammonia introduced is preferably suppressed as compared with acetylene.

When only immersion is performed, acetylene may be replaced with ammonia in the above description. As a matter of course, the reference hydrogen concentration and the target hydrogen concentration are different between carburizing and nitrification even if the surface area of the metal work W is the same.

(6). Reference experiment example Next, a reference experiment example and a comparative example will be described. In this example, a round bar made of boron steel (JIS 10B21) was used as the test piece. As shown in FIG. 5C, the sample has a reference diameter D of 7.15 mm, a total length of 100 mm, a male threaded portion S serving as a gripping portion in the testing machine of M8, and a length of 15 mm each, at an intermediate position. A notch with an opening angle of 60 degrees was formed as a fracture prediction portion. The outer diameter of the notch was about 4.6 mm. Seven examples and seven comparative examples were prepared.

In the reference example, pulse carburizing was performed for about 14 minutes under the conditions applied to ordinary boron steel, and the mixture was quenched at 890 ° C. Oil was used as the coolant. The comparative example is carburized by the gas carburizing method performed under atmospheric pressure, and the carburized gas (propane) is excessively supplied as in the conventional case. Therefore, the amount of diffusible hydrogen in the comparative example was 2.1 ppm immediately after carburizing. On the other hand, in the examples, the acetylene gas was set to a lean state and carburized so that the target hydrogen concentration was about 90% of the reference hydrogen concentration, and the diffusible hydrogen concentration was 0.05 ppm. The diffusible hydrogen amount A immediately after carburizing is a measured value from room temperature to 300 ° C., and the diffusible hydrogen amount B after tempering is a measured value after tempering at 360 ° C.

Most of the hydrogen that has penetrated into the metal work W is discharged in the tempering process. The same applies to the comparative example, and the amount of diffusible hydrogen after tempering was measured and found to be 0.04 ppm. On the other hand, the example was 0.03 ppm. Therefore, the apparent amount of residual hydrogen does not differ much between the examples and the comparative examples.

Then, a fatigue test was conducted between the reference example and the comparative example. It was set in a fatigue tester, and a load was repeatedly applied in the axial direction until it broke, and the relationship between the stress range and the repeating cycle (SN curve) was obtained. The control waveform was a sine wave, the stress ratio was 0.1, the frequency was 30 Hz, and the maximum number of repetitions was 5.0 × ¹⁰⁶ . The graph shown in FIG. 5B shows the results, and it can be seen that the implemented product has high durability in the stress range of 300 to 400 MPa, which is a particular problem for steel products.

Further, for example, even if the amount of acetylene gas introduced is the same as or slightly larger than the reference gas amount, it is presumed that the invasion of hydrogen into the metal work is significantly suppressed. From this, it can be easily inferred that the product of the present invention has high durability.

The invention of the present application can be embodied in various ways. For example, a mass spectrometer can be used instead of the hydrogen concentration sensor as a means for setting the reference gas amount. That is, it is possible to measure the concentration of hydrogen gas generated by decomposition with a mass spectrometer and set the reference gas amount based on the numerical value. The detection unit of the mass spectrometer can be arranged inside the processing furnace or can be arranged in the gas discharge path. The mass spectrometer can also be used as a means for detecting the partial pressure of oxygen.

It is also possible to calculate the total surface area of the metal work and determine the reference gas amount based on that surface area. When a specific member is hardened in a large amount, it can be said that the efficiency can be improved by such a method. In addition, when processing the same metal work that has been processed in the past, it is possible to set the reference gas amount using the past data, and in this case as well, the procedure for setting the reference gas amount is performed. It can be omitted.

The invention of the present application can actually be applied to a curing treatment of a metal work. Therefore, it can be used industrially.

W Metal work 1 Processing furnace 2 Gas introduction port 3 Gas discharge port 5 Cooling room 6 Quenching room 8 Hydrocarbon gas cylinder (acetylene cylinder)
10, 13, 15 Valve 12 Vacuum pump (vacuum source)
18 Hydrogen concentration sensor 20 Oxygen sensor 21 Control device

Claims (5)

The preparatory process for arranging the metal work in the processing furnace and
It is provided with a curing treatment step of introducing a curing gas into a processing furnace in which the metal work is arranged in a high temperature vacuum atmosphere to form a cured layer on the surface of the metal work.
As the curing gas, a gas in which carbon and hydrogen are bonded or a gas in which nitrogen and hydrogen are bonded is used, and the curing gas introduced into the processing furnace in the curing treatment step is used in the metal work. It is divided into a portion that is contacted and decomposed and a portion that is discharged without contacting the metal work, and carbon or nitrogen separated by contacting the metal work enters the surface of the metal work and the cured layer is formed. As a carburized layer or a nitriding layer is formed,
It is a composition called
Further, in the curing treatment step, hydrogen separated from the curing gas is separated by controlling the introduction amount of the curing gas per unit time so that the amount of the curing gas in contact with the metal work does not become excessive. Is a method of preventing or suppressing the invasion of the metal work.
In order to control the amount of the curing gas introduced per unit time in the curing treatment step , carbon or nitrogen is evenly adhered to the entire surface of the untreated metal work in a state where hydrogen is prevented from entering. Based on the standard gas amount preset as the amount to be introduced into the processing furnace ,
i) Introduce an amount smaller than the standard gas amount,
ii) Introduce the same amount as the standard gas amount,
iii) Introduce an amount larger than the reference gas amount within the preset allowable range.
One of the three controls is continuously performed , or a plurality of controls are combined .
Vacuum curing method for metal workpieces. In the curing treatment step, an amount of curing gas smaller than the reference gas amount is continuously introduced.
The vacuum curing method for a metal work according to claim 1. In at least a part of the curing treatment step, the introduction amount of the curing gas is controlled so as to be equal to or more than the reference gas amount within an allowable range in which hydrogen separated from the curing gas can be suppressed from entering the metal work. Be done,
The vacuum curing method for a metal work according to claim 1. The curing treatment step includes an initial step that also serves as detection of the reference gas amount, and a continuous step in which the introduced gas amount is controlled based on the reference gas amount.
Using a sample whose surface area is known, the sample is directly or indirectly detected by the decomposition rate of the curing gas when the curing gas is introduced in an amount set as the detection introduction amount per unit time. Set it as a value,
In the initial step, the curing gas is introduced by the amount introduced for detection to directly or indirectly detect the decomposition ratio of the curing gas, and the actual value regarding the decomposition ratio is compared with the sample value. By doing so, the surface area of the metal work is set, and the reference gas amount is set based on the set surface area.
The vacuum curing method for a metal work according to any one of claims 1 to 3. A cured layer is formed on the surface by the vacuum curing method according to any one of claims 1 to 4, and then quenching is performed.
Manufacturing method of metal work.

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