Vacuum carburizing method and vacuum carburizing furnace

Abstract

This vacuum carburizing method involves: repeatedly executing a carburizing gas supply pattern including a first supplying step for supplying a carburizing gas into a heating chamber charged with a workpiece, a first stopping step for stopping the supplying of the carburizing gas in the first supplying step, a second supplying step for resuming the supplying of the carburizing gas after the first stopping step, and a second stopping step for stopping the supplying of the carburizing gas in the second supplying step to carburize the workpiece; and discharging gas in the heating chamber in either the first supplying step or the second supplying step and in the first stopping step and the second stopping step.

Description

Reduced-pressure carburizing method and reduced-pressure carburizing furnace

[0001] The present invention relates to a reduced-pressure carburizing method and a reduced-pressure carburizing furnace.

[0002] Conventionally, carburizing, which hardens the workpiece surface by dissolving carbon in the workpiece surface (part surface), has been used as a heat treatment to improve the durability of steel automotive parts, machine parts, etc. Vacuum carburizing, which performs carburizing in a vacuum atmosphere, is also known as a carburizing treatment that can reduce the amount of carbon dioxide emitted from the carburizing furnace in which the carburizing treatment is performed.

[0003] As a conventional technique relating to vacuum carburization, Patent Document 1 describes a method of increasing the amount of vacuum exhaust so as to maintain the degree of vacuum as the supply amount of acetylene-based gas increases.

[0004] Patent Document 2 describes a vacuum carburizing apparatus that introduces carburizing gas into a carburizing chamber in pulses. In this vacuum carburizing apparatus, the intake and exhaust sides of a vacuum pump are connected in a bypass manner by a return pipe line equipped with an on-off valve, and control means is provided that opens the on-off valve when the introduction of carburizing gas begins and closes the on-off valve when the introduction of carburizing gas ends.

[0005] Japanese Patent Application Publication No. 2007-224357 Japanese Patent No. 4807660

[0006] When multiple workpieces are subjected to vacuum carburizing treatment simultaneously, the surface carbon concentration of each workpiece is likely to vary depending on the position of each workpiece in the heating chamber to which the carburizing gas is supplied. In other words, when multiple workpieces are subjected to vacuum carburizing treatment, there is a problem that the surface carbon concentration of each workpiece (hereinafter referred to as carburizing variation) is likely to increase, even for workpieces treated in the same lot. However, the methods described in Patent Documents 1 and 2 leave room for improvement in terms of reducing carburizing variation, and the development of new carburizing treatment technologies is desired.

[0007] The present invention has been made in view of the above circumstances, and has as its object to reduce variations in carburization of multiple workpieces processed in the same lot when carburizing multiple workpieces simultaneously.

[0008] In response to the above-mentioned problems, the inventors discovered a reduced-pressure carburizing method that combines on / off supply control of carburizing gas, which repeatedly starts and stops the supply of gas to a heating chamber containing a workpiece, with on / off exhaust control, which repeatedly starts and stops exhausting the air in the heating chamber. Furthermore, they discovered that in this reduced-pressure carburizing method, exhausting the gas at a specific timing relative to the carburizing gas supply pattern makes it possible to homogenize the diffusion of the carburizing gas within the heating chamber and reduce variations in carburizing, leading to the completion of this invention.

[0009] The following are examples of such aspects of the present invention. [1] A reduced-pressure carburizing method, comprising: a first supply step of supplying carburizing gas into a heating chamber containing a workpiece; a first stop step of stopping the supply of carburizing gas during the first supply step; a second supply step of resuming the supply of carburizing gas after the first stop step; and a second stop step of stopping the supply of carburizing gas during the second supply step, to repeatedly carry out a carburizing gas supply pattern for carburizing the workpiece; and evacuating the heating chamber during either the first or second supply step, the first stop step, and the second stop step. [2] The reduced-pressure carburizing method described in [1], in which evacuation of the heating chamber is stopped during the first supply step, and evacuating the heating chamber during the second supply step. [3] The reduced-pressure carburizing method described in [1], in which evacuation of the heating chamber is stopped during the first supply step, and evacuating the heating chamber during the second supply step. [4] A reduced-pressure carburizing treatment method, comprising: a first supply step of supplying carburizing gas into a heating chamber in which a workpiece is placed; a first stop step of stopping the supply of carburizing gas in the first supply step; a second supply step of resuming the supply of carburizing gas after the first stop step; and a second stop step of stopping the supply of carburizing gas in the second supply step, wherein the carburizing gas supply pattern is repeatedly executed to carburize the workpiece; in each of the first supply step and the second supply step, the heating chamber is switched between an exhaust state and an exhaust-stop state; and in each of the first stop step and the second stop step, the heating chamber is evacuated.[5] A reduced-pressure carburizing furnace comprising: a heating chamber in which carburizing treatment of workpieces is performed; a gas supply mechanism for supplying carburizing gas into the heating chamber; an exhaust mechanism for evacuating the inside of the heating chamber; and a control device for controlling the gas supply mechanism and the exhaust mechanism, wherein the control device carburizes the workpiece by repeatedly executing a carburizing gas supply pattern consisting of a first supply step for supplying carburizing gas into the heating chamber; a first stop step for stopping the supply of the carburizing gas in the first supply step; a second supply step for resuming the supply of the carburizing gas after the first stop step; and a second stop step for stopping the supply of the carburizing gas in the second supply step, and is configured to execute control to evacuate the inside of the heating chamber during either the first supply step or the second supply step, the first stop step, and the second stop step. [6] The reduced-pressure carburizing furnace according to [5], wherein the control device is configured to execute control to stop evacuation from the heating chamber in the first supply step and to perform evacuation from the heating chamber in the second supply step. [7] The reduced-pressure carburizing furnace according to [5], wherein the control device is configured to execute control to perform evacuation from the heating chamber in the first supply step and to stop evacuation from the heating chamber in the second supply step. [8] A reduced-pressure carburizing furnace comprising: a heating chamber in which carburizing treatment of workpieces is performed; a gas supply mechanism for supplying carburizing gas into the heating chamber; an exhaust mechanism for evacuating the inside of the heating chamber; and a control device for controlling the gas supply mechanism and the exhaust mechanism, wherein the control device carburizes the workpiece by repeatedly executing a carburizing gas supply pattern consisting of a first supply step for supplying carburizing gas into the heating chamber; a first stop step for stopping the supply of the carburizing gas in the first supply step; a second supply step for resuming the supply of the carburizing gas after the first stop step; and a second stop step for stopping the supply of the carburizing gas in the second supply step, and is configured to switch between an exhaust state and an exhaust stop state in the heating chamber in each of the first supply step and the second supply step, and to execute control to evacuate the inside of the heating chamber in each of the first stop step and the second stop step.

[0010] According to the present invention, when a plurality of workpieces are carburized simultaneously, it is possible to reduce variations in carburization of the plurality of workpieces processed in the same lot.

[0011] FIG. 1 is an explanatory diagram showing the schematic configuration of a reduced-pressure carburizing furnace according to one embodiment of the present invention. FIG. 2 is a perspective view illustrating the schematic configuration of a heating chamber. FIG. 3 is a diagram illustrating the pressure history and temperature history in the heating chamber from the process of loading workpieces into the heating chamber to the process of quenching the workpieces in the cooling chamber. FIG. 4 is a diagram illustrating the operation of a scissor lifter in the process of loading workpieces into the heating chamber. FIG. 5 is a diagram illustrating the operation of loading workpieces into the heating chamber and the operation of transporting workpieces from the heating chamber to the cooling chamber. FIG. 6 is a diagram illustrating the position of test pieces relative to a jig used in a carburizing treatment test. FIG. 7 is a diagram illustrating measurement points for surface hardness of test pieces. FIG. 8 is a diagram illustrating the measurement results of surface hardness of each test piece in Example 1. FIG. 9 is a diagram illustrating the measurement results of surface hardness of each test piece in Comparative Example 1. FIG. 10 is a diagram illustrating the measurement results of surface hardness of each test piece in Comparative Example 2.

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals, and redundant description will be omitted.

[0013] <Reduced-pressure carburizing furnace> Fig. 1 is an explanatory diagram showing the general configuration of a reduced-pressure carburizing furnace according to this embodiment. Fig. 2 is a perspective view showing the general configuration of a heating chamber. Note that the X direction shown in the drawing is the depth direction of the reduced-pressure carburizing furnace 1, the Y direction is the width direction of the reduced-pressure carburizing furnace 1, and the Z direction is the height direction of the reduced-pressure carburizing furnace 1.

[0014] Reduced-pressure carburizing is carburizing performed under a pressure lower than atmospheric pressure, and the reduced-pressure carburizing furnace 1 according to this embodiment is a furnace that is also capable of vacuum carburizing. The reduced-pressure carburizing furnace 1 comprises a heating chamber 10 in which the carburizing process is performed on a plurality of workpieces W, such as automobile parts or machine parts, made of steel, and a cooling chamber 40 disposed adjacent to the heating chamber 10. While the workpieces W are shown as a single block in the drawing, multiple workpieces W are actually placed on a jig shaped like, for example, a tray or basket. The number of workpieces W to be carburized in one lot is, for example, 10 or more and 100,000 or less.

[0015] (Heating Chamber) The heating chamber 10 is a substantially cylindrical container whose axial direction faces the X direction, and a heat insulating material 11 is provided on the inner surface of the heating chamber 10. A plurality of heaters 12 are installed inside the heating chamber 10, extending downward from the ceiling, and the heaters 12 are spaced apart in the X direction. Known heating devices such as ceramic heaters such as SiC heaters, electric burners, and gas burners are used as the heaters 12, but from the viewpoint of suppressing heater deterioration due to oxidation at high temperatures, it is preferable to use ceramic heaters as the heaters 12. As shown in FIG. 1 , a stirring fan 13 for stirring the atmosphere inside the heating chamber 10 is attached to the center of the ceiling of the heating chamber 10.

[0016] Note that, within the heating chamber 10, a processing space for performing a carburizing process in which carbon is dissolved in the surface (component surface) of the workpiece W, and a processing space for performing a diffusion process in which carbon is diffused into the interior (inside the component surface) of the workpiece W may be provided separately. When these processing spaces are provided within the heating chamber 10, partition doors (not shown) are installed within the heating chamber 10 to separate the processing spaces. However, when such partition doors are installed, it becomes difficult to control the atmosphere in each processing space when the partition doors are open, so it is preferable that the processing space for performing the carburizing process and the processing space for performing the diffusion process be the same.

[0017] A transfer opening 14 for transferring the workpiece W from the heating chamber 10 to the cooling chamber 40 is formed in the side wall of the heating chamber 10 on the cooling chamber 40 side (positive side in the X direction). A liftable door 15 is provided between the heating chamber 10 and the cooling chamber 40, and the transfer opening 14 is opened or closed by the door 15.

[0018] A pusher 16 is provided on the side wall of the heating chamber 10 opposite to the side wall where the transfer port 14 is formed (the negative side in the X direction) to push the workpiece W from the heating chamber 10 into the cooling chamber 40. The pusher 16 has a linear movement mechanism (not shown), such as a linear guide, and is configured to be movable in the X direction.

[0019] As shown in FIG. 2, an opening 17 for the pusher 16 is formed in the side wall of the heating chamber 10, and when the pusher 16 pushes out the workpiece W, the pusher 16 passes through the opening 17.

[0020] As shown in Fig. 1, the pusher 16 is surrounded by a housing 18. This housing 18 is fixed to the side wall of the heating chamber 10 so as to cover the opening 17 shown in Fig. 2 (the housing 18 is not shown in Fig. 2). The housing 18 has a sealed structure to prevent outside air from entering the heating chamber 10 during the carburizing treatment of the workpiece W.

[0021] An exhaust pipe 19 is provided above the housing 18 to exhaust the atmosphere inside the heating chamber 10. The exhaust pipe 19 is connected to a vacuum pump 20. A valve (not shown) is also provided in the exhaust pipe 19, and the amount of exhaust inside the heating chamber 10 is adjusted by adjusting the opening / closing amount of the valve or by controlling the operating state of the vacuum pump 20. That is, in this embodiment, the exhaust pipe 19, the vacuum pump 20, and the valve (not shown) constitute an exhaust mechanism that evacuates the heating chamber 10.

[0022] A pressure gauge 21 is attached to the exhaust pipe 19, and the pressure inside the heating chamber 10 is measured by the pressure gauge 21. Note that the installation position of the pressure gauge 21 is not particularly limited as long as it is possible to measure the pressure inside the heating chamber 10. Furthermore, from the viewpoint of suppressing carbon dioxide emissions, a carbon dioxide capture device (not shown) may be attached to the exhaust pipe 19.

[0023] 2, a gas inlet 22 serving as a gas supply port is provided on the outer circumferential surface of the cylindrical portion of the heating chamber 10. The gas inlet 22 supplies gases such as carburizing gases (e.g., acetylene gas, ethylene gas, propane gas, butane gas, or mixtures of these gases), inert gases (e.g., nitrogen gas, argon gas), and oxidizing gases (e.g., air, oxygen, carbon dioxide) into the heating chamber 10.

[0024] A plurality of gas inlets 22 are provided along the X and Y directions, and the gas inlets 22 are arranged at intervals from each other. Although hidden by the heating chamber 10 and not shown in Fig. 2, a similar gas inlet is also provided on the outer circumferential surface of the cylindrical portion of the heating chamber 10 at a position opposite the gas inlet 22 shown in Fig. 2. In other words, the pair of gas inlets 22 arranged opposite each other are provided so as to sandwich the heating chamber 10 therebetween, and gas supplied to the heating chamber 10 is supplied from both sides of the heating chamber 10 in the width direction toward the center of the heating chamber 10.

[0025] A gas supply pipe (not shown) is connected to each gas inlet 22. Of the plurality of gas supply pipes, for example, some gas supply pipes are connected to a cylinder (not shown) storing a carburizing gas, other gas supply pipes are connected to a cylinder (not shown) storing an inert gas, and the remaining gas supply pipes are connected to a cylinder (not shown) storing an oxidizing gas or an air compressor (not shown).

[0026] Each of the cylinders serving as gas supply sources is provided with a valve (not shown). By controlling the opening and closing of these valves or the operating state of the air compressor, it is possible to adjust the amount of gas supplied into heating chamber 10 or change the type of gas.

[0027] In this embodiment, the gas supply mechanism for supplying gases such as carburizing gas, inert gas, oxidizing gas, etc. is configured by the gas inlet 22, gas supply pipe (not shown), cylinder (not shown), air compressor (not shown), etc. This gas supply mechanism can change the atmosphere inside the heating chamber 10 to a carburizing gas atmosphere, an inert gas atmosphere, an oxidizing gas atmosphere, or a mixed atmosphere of these gases.

[0028] It is not necessary to provide a plurality of gas inlets 22. For example, the gas supply mechanism may be configured so that a carburizing gas, an inert gas, and an oxidizing gas are supplied in an appropriate mixed state from a single gas inlet 22.

[0029] As shown in Figure 1, a loading port 23 for loading the workpiece W is formed in the bottom 10a of the heating chamber 10. A scissor lifter 30 is provided below the heating chamber 10 as an elevator for loading the workpiece W into the heating chamber 10 through the loading port 23.

[0030] Above the scissor lifter 30, there are provided a support table 31 that supports the workpiece W, a heat insulating material 32 provided below the support table 31, and a lid 33 disposed below the heat insulating material 32. The lid 33 is a member that closes the loading opening 23 and functions as the bottom wall of the heating chamber 10, and is shaped to cover the entire loading opening 23. The underside of the lid 33 is connected to the upper end of the scissor lifter 30, and the lid 33 is configured to move up and down in conjunction with the lifting and lowering operation of the scissor lifter 30.

[0031] (Cooling Chamber) The workpiece W is cooled in the cooling chamber 40 disposed adjacent to the heating chamber 10. The cooling chamber 40 shown in Fig. 1 is an oil-cooled cooling chamber, and includes an oil tank 41 that stores oil for quenching.

[0032] Above this oil tank 41, there is a transport space for the workpiece W. An elevator rack 42 is provided in the transport space to raise and lower the workpiece W between the transport space and the oil tank 41. A transport opening 43 for transporting the workpiece W into the cooling chamber 40 is formed in the side wall of the cooling chamber 40 on the heating chamber 10 side (negative side in the X direction). Meanwhile, a carry-out opening 44 for transporting the workpiece W from the cooling chamber 40 is formed in the side wall opposite the side wall in which the transport opening 43 is formed (positive side in the X direction). Furthermore, a lift-up door 45 for closing the carry-out opening 44 is provided on the outside of the side wall in which the carry-out opening 44 is formed.

[0033] The cooling method of the cooling chamber 40 is not limited to oil cooling, and may be other cooling methods such as gas cooling. Furthermore, the cooling chamber 40 does not have to be located adjacent to the heating chamber 10. For example, the apparatus may be configured such that the workpiece W carried out from the heating chamber 10 is loaded into the cooling chamber 40 located at an interval from the heating chamber 10.

[0034] (Control Device) The above-mentioned reduced pressure carburizing furnace 1 is equipped with a control device 100. The control device 100 is, for example, a computer equipped with a CPU, memory, etc., and has a program storage unit (not shown). The program storage unit stores various programs that control the series of processes carried out in the reduced pressure carburizing furnace 1. For example, the program storage unit stores programs that control the operation of a gas supply mechanism for supplying gas into the heating chamber 10 and the operation of an exhaust mechanism for evacuating the heating chamber 10. Note that the above programs may be recorded on a computer-readable storage medium and installed into the control device 100 from the storage medium.

[0035] The reduced-pressure carburizing furnace 1 according to this embodiment is configured as described above. Although not described in this specification, the reduced-pressure carburizing furnace 1 also has components required for general carburizing furnaces, such as a temperature sensor that measures the temperature inside the heating chamber 10.

[0036] <Reduced-Pressure Carburizing Treatment Method> Next, a description will be given of an example of a reduced-pressure carburizing treatment method for the workpiece W performed in the reduced-pressure carburizing furnace 1. Fig. 3 is a diagram showing the pressure history and temperature history within the heating chamber 10 from the process of loading the workpiece W into the heating chamber 10 to the process of quenching the workpiece W performed in the cooling chamber 40.

[0037] The reduced-pressure carburizing method described below is performed automatically by controlling each operation of the reduced-pressure carburizing furnace 1 with the control device 100, but some operations may also be performed manually by an operator. Note that the "pressure" used in the following description is absolute pressure.

[0038] (Charging Process) The charging process is a process up to charging the workpiece W into the heating chamber 10 of the reduced-pressure carburizing furnace 1. In the charging process, before the workpiece W is charged into the heating chamber 10 (before the charging port 23 is opened), an inert gas (e.g., nitrogen gas) is supplied into the heating chamber 10 from the gas inlet 22, thereby creating an inert gas atmosphere within the heating chamber 10. At this time, the pressure within the heating chamber 10 is maintained at, for example, 10 to 150 kPa, and the temperature within the heating chamber 10 is maintained at, for example, 750 to 1000°C.

[0039] 4(A), the lid 33 is lowered to open the loading opening 23 provided in the bottom 10a of the heating chamber 10. Then, as shown in FIG. 4(B), the work W transported from outside the furnace by a transport means such as a roller conveyor (not shown) is supported on the support table 31.

[0040] Thereafter, the control device 100 outputs a signal to the scissor lifter 30 to instruct it to rise, and the lid body 33 rises as shown in Figure 5(A). This brings the bottom 10a of the heating chamber 10 and the lid body 33 into close contact, closing the loading port 23, and the workpiece W is loaded into the heating chamber 10.

[0041] (Heating Process) The heating process is a process in which the atmosphere in the heating chamber 10 is heated to the carburizing temperature at which the carburizing process described below is carried out. In the heating process, after the workpiece W is loaded into the heating chamber 10 in the loading process described above, the heating chamber 10 is first evacuated to a vacuum. After the pressure in the heating chamber 10 drops to, for example, 1.0 kPa or less due to this evacuation, the evacuation is stopped. Next, an inert gas (e.g., nitrogen gas) is supplied into the heating chamber 10, and the heater 12 and stirring fan 13 are activated, and the temperature in the heating chamber 10 begins to rise.

[0042] Thereafter, when the pressure inside the heating chamber 10 reaches, for example, 30 to 99 kPa, the supply of inert gas is stopped, and the heating chamber 10 is heated until it reaches a predetermined carburizing temperature. In the temperature-raising process, since the inside of the heating chamber 10 is an inert gas atmosphere rather than a vacuum atmosphere, the temperature inside the heating chamber 10 rises easily, and the temperature-raising time can be shortened. The carburizing temperature, which is the target temperature for the temperature-raising process, is a temperature that is set appropriately depending on the steel type of the workpiece W and the structure inside the furnace, and is set to, for example, 730 to 1200°C.

[0043] After the temperature inside the heating chamber 10 reaches a predetermined temperature, evacuation of the heating chamber 10 begins. The predetermined temperature mentioned here is a temperature that is, for example, 10 to 40°C lower than the predetermined carburizing temperature described above. During this evacuation, the workpiece W is heated to the carburizing temperature.

[0044] (Primary Heat-Soaking Step) The primary heat-soaking step is a step of soaking the workpiece W that has been heated to the carburizing temperature. In the primary heat-soaking step, after the temperature inside the heating chamber 10 has been raised to the predetermined carburizing temperature in the temperature-raising step, evacuation is continued so that the pressure inside the heating chamber 10 is maintained at, for example, 1.0 kPa or less. The workpiece W is soaked during this evacuation. Note that this primary heat-soaking step does not necessarily have to be performed.

[0045] (Carburizing Process) The carburizing process is a process in which the workpiece W is held in the presence of carburizing gas in the heating chamber 10, and the carburizing gas is subjected to a thermal decomposition reaction to form a solid solution of carbon on the surface of the workpiece W. In the carburizing process, after the heating chamber 10 has been created into a vacuum atmosphere in the above-mentioned primary soaking process, the supply and stop of the carburizing gas, and the exhaust and stop of the exhaust from the heating chamber 10 are repeatedly carried out in a certain pattern, as shown in Table 1 below.

[0046]

[0047] In Table 1 above, the on state of the carburizing gas supply means that the carburizing gas is being supplied from the gas inlet 22 into the heating chamber 10, and the off state of the carburizing gas supply means that the supply of the carburizing gas from the gas inlet 22 into the heating chamber 10 has stopped. Also, in Table 1 above, the on state of the vacuum exhaust means that the atmosphere inside the heating chamber 10 is being exhausted from the exhaust pipe 19, and the off state of the vacuum exhaust means that exhaust of the atmosphere inside the heating chamber 10 from the exhaust pipe 19 has stopped.

[0048] Here, with regard to the repeated supply and stop of the carburizing gas, two supply cycles and two supply stop cycles are defined as one supply pattern. More specifically, one supply pattern is defined as on (first time), off (first time), on (second time), off (second time) of the carburizing gas. Then, in the carburizing process, this supply pattern is executed multiple times (e.g., five times), thereby switching between the supply state and the supply stop state of the carburizing gas. In this embodiment, the supply state of the carburizing gas in the first supply cycle within the above supply pattern is referred to as the first supply step A, the supply stop state of the carburizing gas in the first cycle is referred to as the first stop step B, the supply state of the carburizing gas in the second cycle is referred to as the second supply step C, and the supply stop state of the carburizing gas in the second cycle is referred to as the second stop step D.

[0049] In other words, the supply pattern of carburizing gas into heating chamber 10 is composed of a first supply step A in which carburizing gas is supplied into heating chamber 10 in which workpiece W has been placed, a first stop step B in which the supply of carburizing gas in first supply step A is stopped, a second supply step C in which the supply of carburizing gas is resumed after first stop step B, and a second stop step D in which the supply of carburizing gas in second supply step C is stopped. In the following explanation, for convenience of explanation, these first supply step A, first stop step B, second supply step C, and second stop step D may be referred to simply as step A, step B, step C, and step D.

[0050] On the other hand, as shown in Table 1, in this embodiment, evacuation is turned off only in step A in one supply pattern, and is turned on during steps B to D. In other words, of the two supply steps A and C, evacuation is stopped only in the first supply step A, in which the carburizing gas is supplied the first time, and evacuation is performed in the second supply step C, in which the carburizing gas is supplied the second time.

[0051] As will be shown in the examples below, when carburizing is performed using the above exhaust pattern for one supply pattern, the carburizing variation of each workpiece W can be reduced more than when carburizing is performed with the vacuum exhaust always on, as shown in Table 2 below.

[0052]

[0053] Furthermore, as will be shown in the examples below, when the carburizing treatment is carried out using the exhaust pattern according to this embodiment shown in Table 1, the variation in carburizing of each workpiece W can be suppressed more than when the carburizing treatment is carried out using the exhaust pattern shown in Table 3 below.

[0054]

[0055] The exhaust pattern shown in Table 3 is a pattern in which, within one supply pattern, evacuation is stopped in step A, evacuation is performed in step B, evacuation is stopped in step C, and evacuation is performed in step D. As described above, steps A and C are steps in which carburizing gas is supplied into heating chamber 10, and steps B and D are steps in which the supply of carburizing gas into heating chamber 10 is stopped. For this reason, the exhaust pattern shown in Table 3 is a pattern in which evacuation is always stopped when carburizing gas is being supplied, and evacuation is always performed when the supply of carburizing gas is stopped.

[0056] As mentioned above, when carburizing is performed using the exhaust pattern shown in Table 3, the carburizing variation of each workpiece increases compared to when carburizing is performed using the exhaust pattern shown in Table 1. For this reason, simply stopping the vacuum exhaust when supplying carburizing gas does not have the effect of suppressing the carburizing variation.

[0057] The reason why the exhaust control according to this embodiment shown in Table 1 can suppress variations in carburizing is presumably because the carburizing gas can be made to flow in various directions within the heating chamber 10. The reason for this will be explained in detail below.

[0058] First, when the supply of carburizing gas is on, i.e., when carburizing gas is being supplied into the heating chamber 10, the carburizing gas flows in a substantially linear manner along the direction of the carburizing gas supply. In the case of the reduced-pressure carburizing furnace 1 according to this embodiment, the carburizing gas supplied from the gas inlet 22 flows in a substantially linear manner along a direction parallel to the Y-axis. On the other hand, when the vacuum exhaust is on, i.e., when the atmosphere inside the heating chamber 10 is being exhausted, the atmosphere flows in a direction different from the direction of the carburizing gas supply. In the case of the reduced-pressure carburizing furnace 1 according to this embodiment, because the exhaust pipe 19 is provided on the wall surface on the negative side of the X-axis of the heating chamber 10, when the vacuum exhaust is on, the atmosphere inside the heating chamber 10 flows mainly along a direction parallel to the X-axis.

[0059] In the carburizing process according to this embodiment shown in Table 1, there are three or more combinations of on / off carburizing gas supply and on / off vacuum exhaust within one supply pattern. Specifically, there are three combinations: "carburizing gas supply: on, vacuum exhaust: off," "carburizing gas supply: on, vacuum exhaust: on," and "carburizing gas supply: off, vacuum exhaust: on."

[0060] Considering the flow of the carburizing gas and the atmosphere within the heating chamber 10 described above, in the "carburizing gas supply: on, vacuum exhaust: off" state, the carburizing gas flows from the gas inlet 22 toward the center of the heating chamber 10 in a direction parallel to the Y axis and then diffuses within the heating chamber 10. Also, in the "carburizing gas supply: on, vacuum exhaust: on" state, the carburizing gas flows from the gas inlet 22 toward the center of the heating chamber 10 in a direction parallel to the Y axis, and also flows in a direction toward the exhaust pipe 19. Also, in the "carburizing gas supply: off, vacuum exhaust: on" state, no flow of carburizing gas occurs from the gas inlet 22 toward the center of the heating chamber 10, and the entire atmosphere within the heating chamber 10, including the carburizing gas, flows in a direction toward the exhaust pipe 19.

[0061] That is, in the carburizing process according to this embodiment shown in Table 1, three or more combinations of on or off carburizing gas supply and on or off vacuum exhaust are present in one supply pattern, so that the carburizing gas can be caused to flow in various directions within the heating chamber 10.

[0062] Furthermore, during carburizing, multiple workpieces and jigs on which they are placed are present within the heating chamber 10, and the shapes of the workpieces and jigs vary depending on the product specifications. In the heating chamber 10 containing such workpieces and jigs, if exhaust control is performed so that there are three or more combinations of on / off carburizing gas supply and on / off vacuum exhaust within a single supply pattern, the randomness of the carburizing gas flow direction within the heating chamber 10 tends to increase. This makes it easier for the carburizing gas to diffuse throughout the heating chamber 10, and when carburizing multiple workpieces W simultaneously, it is possible to suppress variations in carburizing of each workpiece W carburized within the same lot.

[0063] In this embodiment, the carburizing gas is supplied in a direction different from the direction of exhaust, but the carburizing gas may be supplied in the same direction. Even in this case, if exhaust control is performed so that three or more combinations of on / off carburizing gas supply and on / off vacuum exhaust are present in one supply pattern, the flow of the carburizing gas tends to become random, coupled with the presence of workpieces and jigs in the heating chamber 10. In other words, regardless of the direction of supply and exhaust of the carburizing gas, the carburizing gas tends to diffuse throughout the heating chamber 10, suppressing variations in carburizing of each workpiece W.

[0064] In order to obtain the effect of suppressing the above-mentioned variations in carburization, the carburization process may be carried out using the exhaust pattern shown in Table 4 below, for example.

[0065]

[0066] The exhaust pattern shown in Table 4 differs from the exhaust pattern shown in Table 1 in the state of vacuum exhaust in process A and process C, with vacuum exhaust being on in process A and off in process C. Even in this exhaust pattern, there are three or more combinations of on / off carburizing gas supply and on / off vacuum exhaust within one supply pattern, so the carburizing gas flows in various directions within the heating chamber 10. This makes it easier for the carburizing gas to diffuse throughout the heating chamber 10, suppressing variations in carburizing of each workpiece W carburized within the same lot.

[0067] In order to obtain the effect of suppressing variations in carburization, the carburization process may be carried out using the exhaust pattern shown in Table 5 below, for example.

[0068]

[0069] The exhaust pattern shown in Table 5 differs from the exhaust pattern shown in Table 1 in the state of vacuum exhaust in steps A and C. Specifically, the vacuum exhaust switches between on and off during step A, and the vacuum exhaust switches between on and off during step B. Even in this exhaust pattern, there are three or more combinations of on or off carburizing gas supply and on or off vacuum exhaust within one supply pattern, so the carburizing gas flows in various directions within the heating chamber 10. This makes it easier for the carburizing gas to diffuse throughout the heating chamber 10, suppressing variations in carburizing of each workpiece W carburized within the same lot.

[0070] The pressure inside the heating chamber 10 during the carburizing process is maintained at, for example, 100 kPa or less. The time (processing time) for maintaining each of the above-mentioned processes A to D is set appropriately depending on the type of steel of the workpiece W and the required level of carburizing quality. The processing time for process A is, for example, 30 to 100 seconds, the processing time for process B is, for example, 30 to 100 seconds, the processing time for process C is, for example, 30 to 100 seconds, and the processing time for process D is, for example, 70 to 2500 seconds.

[0071] The surface carbon concentration of the workpiece W at the end of the carburizing process is higher than the surface carbon concentration of the workpiece W at the end of the diffusion process described below. The carburizing conditions are appropriately set so as to obtain a desired surface carbon concentration depending on the application of the workpiece W, but in the carburizing process, it is preferable to perform the carburizing treatment so that the surface carbon concentration is 0.8% or more so as to obtain, for example, a eutectoid composition. Alternatively, the carburizing treatment may be performed so that the surface carbon concentration is 6.67% or less or 3% or less.

[0072] (Diffusion Process) The diffusion process is a process for diffusing carbon into the interior (inside the surface of the component) of the workpiece W. Whether or not carbon has diffused into the interior of the workpiece W can be determined by analyzing the carbon concentration distribution in the cross section of the workpiece before and after the diffusion process, and by confirming that the carbon concentration on the workpiece surface after the diffusion process is performed is lower than the carbon concentration on the workpiece surface without the diffusion process.

[0073] In the diffusion process, the heating chamber 10 is evacuated while the temperature inside the heating chamber 10 is maintained at, for example, 730 to 1200°C. The temperature inside the heating chamber 10 is changed as appropriate depending on the type of steel of the workpiece W and the structure of the heating chamber 10. The diffusion process ends when the pressure inside the heating chamber 10 reaches, for example, 0.1 kPa and the atmosphere inside the heating chamber 10 is sufficiently exhausted.

[0074] (Temperature-reducing and secondary soaking steps) In the temperature-reducing and secondary soaking steps, an inert gas (e.g., nitrogen gas) is supplied after the diffusion step is completed. After that, the pressure in the heating chamber 10 reaches, for example, 100 kPa, and the supply of the inert gas is stopped. This state is maintained for a certain period of time, and the workpiece W is soaked in heat.

[0075] (Transportation Process) In the transportation process, after the secondary soaking process is completed, the door 15 installed on the side wall of the heating chamber 10 is opened as shown in Fig. 5(B) . Thereafter, the workpiece W is transported from the heating chamber 10 to the cooling chamber 40, and the door 15 is closed.

[0076] (Quenching Process) In the quenching process, the workpiece W transported to the cooling chamber 40 is immersed in the oil tank 41 and subjected to quenching treatment. Thereafter, the workpiece W is lifted out of the oil tank 41 and carried out of the cooling chamber 40.

[0077] The above series of steps completes the reduced-pressure carburizing treatment of one lot of workpieces W. When the next lot of workpieces W is loaded into the heating chamber 10, the above-described loading step is carried out again.

[0078] The above has described the reduced-pressure carburizing method according to this embodiment. In the carburizing process, this reduced-pressure carburizing method repeatedly supplies and stops the supply of carburizing gas in a supply pattern consisting of a first supply step A, a first stop step B, a second supply step C, and a second stop step D. In addition, control is performed to switch between an exhaust state and an exhaust-stop state in the heating chamber 10 so that three or more combinations of on / off states of the carburizing gas supply and on / off states of the vacuum evacuation exist within one supply pattern.

[0079] According to the reduced pressure carburizing method having such a carburizing step, the carburizing gas flows in various directions within the heating chamber 10, making it easier for the carburizing gas to diffuse throughout the heating chamber 10. This makes it possible to suppress variations in carburizing of each workpiece W carburized within the same lot.

[0080] While one embodiment of the present invention has been described above, the present invention is not limited to such an embodiment. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that such modifications also fall within the technical scope of the present invention.

[0081] For example, in the above embodiment, the supply pattern is executed five times, but the supply pattern may be executed four times or less, or six times or more.

[0082] Furthermore, for example, the components of the above-described embodiments can be combined in any manner, and such combinations will naturally provide the functions and advantages of each of the components in the combination, as well as other functions and advantages that will be apparent to those skilled in the art from the description herein.

[0083] In the above embodiment, one supply pattern includes two sets of "one carburizing gas supply step and one stop step." However, even if one supply pattern includes three or more sets of carburizing gas supply steps and stop steps, it is presumed that it is possible to make the diffusion state of the carburizing gas within the heating chamber uniform and reduce variations in carburizing by evacuating the heating chamber during some of the carburizing gas supply steps, or by switching the heating chamber between an evacuated state and an evacuated state during the carburizing gas supply steps.

[0084] Eleven round bar test pieces (φ18 mm x total length 40 mm) made of SCR420 steel were prepared for the evaluation of carburizing results, and a reduced-pressure carburizing test was conducted on these eleven test pieces simultaneously. In addition to the test pieces, several automotive gear parts were placed in the jig as dummy workpieces. The total weight of these parts was 400 kg, and the total surface area of ​​the parts was 8.7 m. 2 The total weight of the jig was 630 kg.

[0085] Figure 6 is a schematic diagram showing the placement positions of each test piece relative to the jig used in the test. The jig has a rectangular parallelepiped shape with dimensions of 760 mm in length, 610 mm in width, and 200 mm in height. The placement positions of the test pieces will be described in detail below.

[0086] First, one test piece is placed at each of the vertices P1 to P8 of the rectangular parallelepiped, and one test piece is also placed at the center of gravity P9 of the rectangular parallelepiped. Another test piece is also placed at position P10, which is vertically above position P9 and is the intersection of the diagonal line between positions P1 and P7 and the diagonal line between positions P3 and P5. Another test piece is also placed at position P11, which is vertically below position P9 and is the intersection of the diagonal line between positions P2 and P8 and the diagonal line between positions P4 and P6.

[0087] The jig with the test pieces arranged as described above was placed in a reduced-pressure carburizing furnace, and reduced-pressure carburizing treatment was carried out under the following conditions: The reduced-pressure carburizing furnace had the same configuration as the reduced-pressure carburizing furnace 1 of the embodiment described above with reference to Figures 1 and 2.

[0088] Example 1 First, the processing conditions of Example 1 will be described.

[0089] (Loading Step) The temperature in the heating chamber is adjusted to 850° C., and the pressure in the heating chamber is adjusted to 100 kPa, and the jig with each test piece placed therein is loaded into the heating chamber.

[0090] (Heating process) After loading the jig into the heating chamber, the heating chamber is evacuated to a vacuum within 5 minutes so that the pressure inside the heating chamber becomes 0.5 kPa. After completion of evacuation, the supply of nitrogen gas to the heating chamber is started, and after the pressure inside the heating chamber reaches 90 kPa, the supply of nitrogen gas is stopped. The temperature inside the heating chamber is then raised to 900°C over 60 minutes. After the temperature inside the heating chamber reaches 900°C, the supply of inert gas is stopped and evacuation is started. During this evacuation, the temperature inside the heating chamber reaches the carburizing temperature of 930°C.

[0091] (Primary Heat-Soaking Step) The temperature inside the heating chamber is maintained at 930° C., and evacuation is performed for 10 minutes until the pressure inside the heating chamber reaches 0.1 kPa.

[0092] (Carburizing Process) After creating a vacuum atmosphere inside the heating chamber, the supply and cessation of acetylene gas into the heating chamber are switched, and the vacuum evacuation and cessation of evacuation are switched, according to the pattern shown in Table 6 below. The pattern shown in Table 6 is the same as the pattern shown in Table 1 above. The time allocation for each of the processes A to D in the pattern shown in Table 6 is set to a well-balanced condition taking into consideration the results of various tests, the efficiency of carburizing, and the state of soot deposition on the workpiece surface. This time allocation is the same condition as in the comparative example described below, but the time allocation for each process is determined appropriately depending on the capacity of the heating chamber, the surface area of ​​the workpiece, the arrangement of the workpiece, and other factors.

[0093]

[0094] Acetylene gas is supplied into the heating chamber from each gas inlet at a flow rate of 14 L / min. During the carburizing process, the pressure inside the heating chamber fluctuates depending on the on / off pattern of the acetylene gas supply and the on / off pattern of the vacuum exhaust shown in Table 6, but the maximum pressure is limited to 2 kPa. The carburizing process ends when the supply pattern shown in Table 6 has been performed five times.

[0095] (Diffusion Process) The heating chamber 10 is evacuated to a vacuum while the temperature inside the heating chamber 10 is maintained at, for example, 930° C. Thereafter, the diffusion process is completed when the pressure inside the heating chamber 10 is reduced to 0.1 kPa.

[0096] (Temperature Decreasing, Second Soaking Step) After the diffusion step is completed, the supply of nitrogen gas to the heating chamber is started, and the supply of nitrogen gas is stopped when the pressure in the heating chamber reaches 90 kPa. Thereafter, the temperature in the heating chamber is decreased to 870°C, and the state is maintained for soaking.

[0097] (Quenching Process) The jig on which the soaked test piece was placed was immersed for 10 minutes in an oil bath at an oil temperature of 130° C. Thereafter, the jig was lifted out of the oil bath and carried out of the reduced pressure carburizing furnace.

[0098] The reduced pressure carburizing treatment in Example 1 is completed through the above steps. Next, the treatment conditions for Comparative Examples 1 and 2 will be explained. The treatment conditions for Comparative Examples 1 and 2 differ from those of Example 1 only in the carburizing step conditions, and therefore, in the following explanation, only the differences from Example 1 will be explained.

[0099] <Comparative Example 1> After creating a vacuum atmosphere inside the heating chamber, the supply and stop states of acetylene gas into the heating chamber were switched, and the vacuum evacuation and stop states were switched, according to the patterns shown in Table 7. The patterns shown in Table 7 are the same as the patterns shown in Table 2 above.

[0100]

[0101] <Comparative Example 2> After creating a vacuum atmosphere inside the heating chamber, the supply and stop states of acetylene gas into the heating chamber were switched, and the vacuum evacuation and stop states were switched, according to the patterns shown in Table 8. The patterns shown in Table 8 are the same as the patterns shown in Table 3 above.

[0102]

[0103] Next, carburizing performance was evaluated using each of the test pieces of Example 1 and Comparative Examples 1 and 2 that had been subjected to reduced-pressure carburizing treatment under the above conditions. Carburizing performance was evaluated by evaluating the surface hardness and its variation of 11 test pieces in each of the Example and Comparative Examples, and the differences in effect resulting from different carburizing conditions were compared from the evaluation results. Note that the surface hardness of the test piece varies depending on the amount of carbon dissolved in the test piece surface, so the variation in surface hardness serves as an index of carburizing variation.

[0104] The surface hardness of the test specimen (round bar test specimen) was measured as follows. First, the test specimen was suspended using a wire with its axial direction oriented vertically. Then, at the axial center of the 40 mm long test specimen, i.e., at a position 20 mm away, the surface hardness was measured at four points spaced apart along the circumferential direction of the test specimen, as shown in Figure 7. The specific measurement points were 0°, 90°, 180°, and 270°, with the positive side of the transport direction from the heating chamber to the cooling chamber (positive side of the X direction) being defined as the 0° direction. At these measurement points 1 to 4, Rockwell hardness C scale (HRC) evaluation was performed in accordance with JIS Z 2245 to measure the surface hardness of the test specimen.

[0105] The above-described surface hardness measurements were performed on each of the 11 test pieces, and the surface hardness measurement results were evaluated in Example 1 and Comparative Examples 1 and 2. The measurement results of Example 1 are shown in Fig. 8, the measurement results of Comparative Example 1 in Fig. 9, and the measurement results of Comparative Example 2 in Fig. 10.

[0106] 8 to 10, the variation in surface hardness due to differences in the placement position of the test specimens in Comparative Example 1 and Comparative Example 2 is greater than the variation in surface hardness in Example 1. Specifically, in Comparative Example 1, the surface hardness of measurement point 2 of the test specimen placed at position P9 is significantly lower than the surface hardness of the other measurement points 1, 3, and 4 at position P9, and is also significantly lower than the surface hardness of each of the test specimens placed at the other placement positions P1 to P8, P10, and P11. Furthermore, in Comparative Example 2, the surface hardness of the test specimens placed at positions P7 and P8 is significantly lower than the surface hardness of the test specimens placed at the other positions P1 to P6 and P9 to P11.

[0107] Therefore, the results of this carburizing test show that it is possible to suppress the variation in carburizing of each test piece within the same lot by performing the carburizing treatment under the conditions in Table 6. In particular, the variation in carburizing of Comparative Example 2 is significantly greater than that of Example 1, which shows that the exhaust control carried out in Comparative Example 2, in which the evacuation was simply stopped at the timing of the acetylene supply, actually worsens the variation in carburizing.

[0108] It should be noted that the effects described in this specification are merely descriptive or exemplary and are not limiting. In other words, the technology according to the present disclosure may achieve other effects that are apparent to those skilled in the art from the description of this specification, in addition to or in place of the above-described effects.

[0109] The present invention can be applied to reduced pressure carburizing of a workpiece.

[0110] DESCRIPTION OF SYMBOLS 1 Reduced-pressure carburizing furnace 10 Heating chamber 10a Bottom of heating chamber 11 Heat insulating material 12 Heater 13 Stirring fan 14 Transport port 15 Door 16 Pusher 17 Opening 18 Housing 19 Exhaust pipe 20 Vacuum pump 21 Pressure gauge 22 Gas inlet 23 Loading port 30 Scissor lifter 31 Support base 32 Heat insulating material 33 Lid 40 Cooling chamber 41 Oil tank 42 Elevator rack 43 Transport port 44 Discharge port 45 Door 100 Control device W Work

Claims

1. A reduced-pressure carburizing treatment method, comprising: a first supply step of supplying carburizing gas into a heating chamber containing a workpiece; a first stopping step of stopping the supply of carburizing gas in the first supply step; a second supply step of resuming the supply of carburizing gas after the first stopping step; and a second stopping step of stopping the supply of carburizing gas in the second supply step, to repeatedly execute the carburizing gas supply pattern to carburize the workpiece; and evacuating the inside of the heating chamber during either the first supply step or the second supply step, the first stopping step, and the second stopping step.

2. The reduced pressure carburizing method according to claim 1, wherein evacuation of the heating chamber is stopped in the first supplying step, and evacuation of the heating chamber is carried out in the second supplying step.

3. The reduced pressure carburizing method according to claim 1, wherein the heating chamber is evacuated in the first supplying step, and the evacuation of the heating chamber is stopped in the second supplying step.

4. A reduced-pressure carburizing treatment method, comprising: a first supply step of supplying carburizing gas into a heating chamber in which a workpiece is placed; a first stop step of stopping the supply of carburizing gas in the first supply step; a second supply step of resuming the supply of carburizing gas after the first stop step; and a second stop step of stopping the supply of carburizing gas in the second supply step, wherein the carburizing gas supply pattern is repeatedly executed to carburize the workpiece; and during each of the first supply step and the second supply step, the heating chamber is switched between an exhaust state and an exhaust-stop state; and the heating chamber is evacuated during each of the first stop step and the second stop step.

5. A reduced-pressure carburizing furnace comprising: a heating chamber in which carburizing treatment of workpieces is performed; a gas supply mechanism for supplying carburizing gas into the heating chamber; an exhaust mechanism for evacuating the interior of the heating chamber; and a control device for controlling the gas supply mechanism and the exhaust mechanism, wherein the control device carburizes the workpiece by repeatedly executing a carburizing gas supply pattern consisting of a first supply step for supplying carburizing gas into the heating chamber; a first stop step for stopping the supply of carburizing gas in the first supply step; a second supply step for resuming the supply of carburizing gas after the first stop step; and a second stop step for stopping the supply of carburizing gas in the second supply step, and is configured to execute control to evacuate the interior of the heating chamber during either the first supply step or the second supply step, the first stop step, and the second stop step.

6. The reduced-pressure carburizing furnace according to claim 5, wherein the control device is configured to execute control to stop evacuation from within the heating chamber during the first supply step and to start evacuation from within the heating chamber during the second supply step.

7. The reduced-pressure carburizing furnace according to claim 5, wherein the control device is configured to execute control to evacuate the heating chamber in the first supply step and to stop evacuating the heating chamber in the second supply step.

8. A reduced-pressure carburizing furnace comprising: a heating chamber in which carburizing treatment of workpieces is performed; a gas supply mechanism for supplying carburizing gas into the heating chamber; an exhaust mechanism for evacuating the interior of the heating chamber; and a control device for controlling the gas supply mechanism and the exhaust mechanism, wherein the control device carburizes the workpiece by repeatedly executing a carburizing gas supply pattern consisting of a first supply step for supplying carburizing gas into the heating chamber; a first stop step for stopping the supply of carburizing gas in the first supply step; a second supply step for resuming the supply of carburizing gas after the first stop step; and a second stop step for stopping the supply of carburizing gas in the second supply step, and is configured to switch between an exhaust state and an exhaust stop state in the heating chamber in each of the first supply step and the second supply step, and to execute control to evacuate the interior of the heating chamber in each of the first stop step and the second stop step.

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