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(11) | EP 4 800 150 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | CARBURIZING METHOD |
| (57) There is provided a carburizing method using a carburizing gas. The carburizing method
includes repeating supply of the carburizing gas and stoppage of the supply of the
carburizing gas, where initiation of the supply of the carburizing gas and the stoppage
of the supply of the carburizing gas are performed at timings based on a carbon penetration
rate at which the carburizing gas causes carbon to penetrate an article to be processed. |
TECHNICAL FIELD
BACKGROUND ART
[0002] A vacuum carburizing treatment method, in which a carburizing gas is supplied into a vacuumed furnace to allow activated carbon to permeate and diffuse into a surface of a steel material, has been known. For example, Patent Document 1 discloses a method in which a supply amount of a carburizing gas is reduced from the early stage of carburization to the late stage of carburization in such vacuum carburizing to reduce waste of the carburizing gas and the processing cost.
CITATION LIST
PATENT DOCUMENT
SUMMARY OF THE INVENTION
TECHNICAL PROBLEM
[0004] However, the method disclosed in Patent Document 1 has a problem that, if the supply amount of the carburizing gas is excessively reduced in the late stage of carburization, the carburizing gas does not spread throughout the furnace, and therefore a variation in carburization quality increases.
[0005] The present invention has been made in view of the above-described problems existing in the related art, and aims to provide a carburizing method that can reduce a variation in carburization quality without reducing gas efficiency of a carburizing gas.
SOLUTION TO THE PROBLEM
[0006] The present invention for achieving the above object is a carburizing method using a carburizing gas. The carburizing method includes repeating supply of the carburizing gas and stoppage of the supply of the carburizing gas, where initiation of the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are performed at timings based on a carbon penetration rate at which the carburizing gas causes carbon to penetrate an article to be processed.
[0007] In the present invention configured as described above, the initiation of the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are performed at timings based on the carbon penetration rate at which the carburizing gas allows carbon to penetrate the article to be processed, so that a supply amount of the carburizing gas corresponds to the penetration rate of carbon into the article to be processed. Thus, the supply amount of the carburizing gas is reduced, while the supply amount of the carburizing gas does not become excessively small, and a variation in carburization quality is reduced.
[0008] Moreover, the carburizing method may have a configuration, in which a range of the carbon penetration rate at a time of the initiation of the supply or at the time of the stoppage of the supply is set, where the initiation of the supply or the stoppage of the supply is performed at a timing at which the carbon penetration rate is within the range.
[0009] Moreover, the above range may be varied according to a carburizing treatment temperature of the article to be processed. In such a configuration, the carburizing method can correspond to increase of the penetration rate of carbon increases at a higher temperature.
[0010] Further, specifically, the stoppage of the supply may be performed at a timing at which the carbon penetration rate F is within a range represented by a formula (1) below in a state in which the supply is performed, where F [mg/m2·sec] is the carbon penetration rate, and T [K] is the carburizing treatment temperature.
[0011] In addition, the initiation of the supply may be performed at a timing at which the carbon penetration rate F is within a range represented by a formula (2) below, when the supply is performed from a state in which the stoppage of the supply is performed, where F [mg/m2·sec] is the carbon penetration rate, and T [K] is the carburizing treatment temperature.
[0012] Furthermore, the article to be processed may be machine structural alloy steel including chromium, and the machine structural alloy steel may be chromium steel.
EFFECTS OF THE INVENTION
[0013] According to the present invention, a variation in carburization quality can be reduced without reducing gas efficiency of a carburizing gas.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
[Fig. 1] Fig. 1 is a diagram illustrating an example of a configuration of a device for performing one embodiment of the carburizing method of the present invention.
[Fig. 2A] Fig. 2A is a diagram for explaining a function when a carburizing gas is supplied in pulses.
[Fig. 2B] Fig. 2B is a diagram for explaining the function when the carburizing gas is supplied in pulses.
[Fig. 3] Fig. 3 is a flowchart for explaining a process from setting of conditions to measurement of a variation in carburization quality in the present embodiment.
[Fig. 4] Fig. 4 is a graph depicting approximate curves each connecting the set values presented in Table 1.
[Fig. 5] Fig. 5 is a graph depicting approximate curves each connecting the set values presented in Table 2.
[Fig. 6] Fig. 6 is a diagram depicting a calculation simulation using the set values presented in Table 1 and Table 2, and in Fig. 4 and Fig. 5.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0015] Embodiments of the present invention will be described with reference to the drawings hereinafter.
<<Configuration of device>>
[0016] Fig. 1 is a diagram illustrating one example of a configuration of a device for performing one embodiment of the carburizing method of the present invention.
[0017] As illustrated in Fig. 1, the present example includes a carburizing furnace 20, a mass flow controller 40, an exhaust valve 50, and a vacuum pump 60.
[0018] In the carburizing furnace 20, a workpiece 10 serving as an article to be processed is accommodated. The carburizing furnace 20 includes a supply port 21, through which acetylene 30 is supplied as a carburizing gas via a mass flow controller 40, and a discharge port 22. A portion of the acetylene 30 supplied to the carburizing furnace 20, which have not penetrated the workpiece 10, is discharged from the discharge port 22 as a discharge gas. A heater 23 is disposed above and below the workpiece 10 inside the carburizing furnace 20, and is configured to heat the inside of the carburizing furnace 20 to control an internal atmospheric temperature of the furnace, i.e., a carburizing treatment temperature of the workpiece 10. Moreover, the carburizing furnace 20 is provided with a thermocouple 24 to monitor the temperature inside the carburizing furnace 20. The heater 23 is configured to heat and control the temperature inside the carburizing furnace 20 corresponding to the temperature monitored by the thermocouple 24.
[0019] The mass flow controller 40 is configured to control initiation or stoppage of supply of the acetylene 30 and a supply flow rate of the acetylene 30 into the carburizing furnace 20. Specifically, the mass flow controller 40 performs valve operation for performing initiation or stoppage of supply of the acetylene 30 by a programmable logic controller (PLC) or the like, and performs control so that, when the acetylene 30 is supplied, the acetylene 30 is supplied to the furnace at the set flow rate.
[0020] The exhaust valve 50 is normally in an opened state, and discharges, as a discharge gas, the portion of the acetylene 30 supplied into the carburizing furnace 20 and not having penetrated the workpiece 10.
[0021] The vacuum pump 60 evacuates the carburizing furnace 20 via the exhaust valve 50 to discharge, as a discharge gas, the portion of the acetylene 30 supplied into the carburizing furnace 20 and not having penetrated the workpiece 10, to the outside via the exhaust valve 50.
[0022] A vacuum carburizing treatment is performed on the workpiece 10 using the device configured as described above. The carburizing gas to be supplied to the carburizing furnace 20 is not limited to the acetylene 30. The carburizing gas may be a hydrocarbon gas, such as propane and the like.
<<Vacuum carburizing treatment>>
[0023] In a vacuum carburizing treatment, first, acetylene 30 is supplied as a carburizing gas into the carburizing furnace 20 under the control of the mass flow controller 40.
[0024] In the carburizing furnace 20, carbon is generated on and penetrates a surface of the workpiece 10 due to a reaction between the supplied acetylene 30 and the workpiece 10, and the carbon is diffused inside the workpiece 10, thereby hardening the surface of the workpiece 10.
[0025] Meanwhile, the portion of the acetylene 30 supplied into the carburizing furnace 20 and not having penetrated the workpiece 10 is discharged as a discharge gas to the outside via the exhaust valve 50.
[0026] As described above, in the vacuum carburizing treatment using the device illustrated in Fig. 1, the acetylene 30 is supplied as the carburizing gas into the carburizing furnace 20 under the control of the mass flow controller 40. At the time when the acetylene 30 is supplied, carbon is deposited on and penetrates the surface of the workpiece 10 due to the reaction between the acetylene 30 supplied as the carburizing gas and the workpiece 10 because the workpiece 10 is composed of a steel material. Meanwhile, the portion of the acetylene 30 supplied into the carburizing furnace 20 and not having penetrated the workpiece 10 is discharged as a discharge gas via the exhaust valve 50.
[0027] Specifically, once acetylene (C2H2) 30 is supplied into the carburizing furnace 20, the acetylene 30 undergoes a non-equilibrium reaction with the steel material (Fe) constituting the workpiece 10.
2Fe + C2H2 ⇒ 2[Fe + C] + H2
As a result of the non-equilibrium reaction, carbon (C) is generated on the surface of the workpiece 10 and penetrates inside of the workpiece 10, and at the same time, hydrogen (H2) is discharged as a discharge gas.
[0028] The carbon deposited on and penetrating the surface of the workpiece 10 is then diffused into the workpiece 10.
<<Function in vacuum carburizing treatment in pulses>>
[0030] In the above-described vacuum carburizing treatment, the carburizing gas may be supplied in pulses.
[0031] Fig. 2A and Fig. 2B are diagrams for explaining the function when the carburizing gas is supplied in pulses. Although the pulses in Fig. 2B are illustrated to be different in width and height from the pulses illustrated in Fig. 2A for depicting each pulse so that the function when the carburizing gas is supplied is easily understood, the pulses in Fig. 2B are in fact the same pulses as the pulses illustrated in Fig. 2A.
[0032] As illustrated in Fig. 2A, the mass flow controller 40 illustrated in Fig. 1 may supply a carburizing gas by supplying the acetylene 30 into the carburizing furnace 20 for a certain period, followed by stopping the supply of the acetylene 30 for a certain period, and repeating the above processes in pulses.
[0033] At the time t0, a pulse for supplying the carburizing gas enters an ON state, and the carburizing gas starts to be supplied into the carburizing furnace 20. Since carbon has not yet penetrated the workpiece 10, the carbon generated on the surface of the workpiece 10 by the supply of the carburizing gas starts penetrating the workpiece 10.
[0034] As the carbon penetrates the inside of the workpiece 10, a difference in carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 decreases so that carbon becomes less likely to penetrate the workpiece 10.
[0035] As the carbon becomes less likely to penetrate the workpiece 10, the penetration rate of the carbon into the workpiece 10 decreases as indicated by the solid line in Fig. 2B.
[0036] As described above, the process of causing the pulse to be in the ON state to supply the carburizing gas, thereby allowing carbon to penetrate the workpiece 10, is a carburization period.
[0037] In addition, the carbon that is generated on and has penetrated the surface of the workpiece 10 is diffused inside the workpiece 10. On the other hand, when the pulse is caused to be in an OFF state at the time t1 to stop supply of the carburizing gas into the carburizing furnace 20, the carburizing gas is not supplied to the workpiece 10.
[0038] Since the carburizing gas is not supplied when the pulse is in the OFF state, the penetration rate of the carbon from the surface of the workpiece 10 to the inside of the workpiece 10 reaches "0" after passing through the inflection point at the time tl, as indicated by the solid line in Fig. 2B.
[0039] As described above, the process of diffusing the carbon that has penetrated from the surface of the workpiece 10 to the inside of the workpiece 10 without supplying the carburizing gas to the workpiece 10 is a diffusion period. In the diffusion period, while the carburizing gas is not supplied to the workpiece 10, the carbon that has penetrated from the surface of the workpiece 10 diffuses inside the workpiece 10, and therefore the carbon concentration inside (the vicinity of the surface) of the workpiece 10 decreases.
[0040] Thereafter, as the pulse is caused to be in the ON state at the time t2 to restart supply of the carburizing gas into the carburizing furnace 20, the difference in the carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 becomes large because the carbon concentration of the inside (the vicinity of the surface) of the workpiece 10 has become low.
[0041] Since the carbon readily penetrates the surface of the workpiece 10 in the state in which the difference in the carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 is large, the penetration rate of the carbon into the workpiece 10 temporarily significantly increases as indicated by the solid line in Fig. 2B. Thereafter, as the carbon penetrates the workpiece 10, the difference in the carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 decreases, and therefore the penetration rate of the carbon into the workpiece 10 decreases along the decrease in the difference in the carbon concentration, as indicated by the solid line in Fig. 2B.
[0042] Then, the pulse is caused to be in the OFF state at the time t3. By repeating the above processes, the effective depth of the carbon with respect to the workpiece 10 is deepened as indicated by the two-dot chain line in Fig. 2B. In the present specification, the repetition of the ON state and the OFF state of the pulse also includes a case where the pulsation includes one ON state and one OFF state.
[0043] In the vacuum carburizing treatment using pulses as described above, the ON/OFF timing of the pulse is uniquely determined in the related art. Thus, a carburizing gas is supplied or supply of the carburizing gas is stopped regardless of the difference in the carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 or the penetration rate of the carbon corresponding to the difference in carbon concentration.
[0044] However, such a treatment with uniquely determined ON/OFF timing may not be preferable in view of gas efficiency of the carburizing gas to carburize the workpiece 10 with carbon, or in view of reduction of a variation in carburization quality.
[0045] For example, in the case where the times t1 and t3 at which the pulse enters the OFF state are set to times at which the penetration rate becomes close to "0" in Fig. 2B, even if the carburizing gas is supplied, the carbon is less likely to penetrate the surface of the workpiece 10, and therefore such setting is not preferable in view of gas efficiency. In this case, among the excessively supplied carburizing gas, the amount of the carburizing gas adsorbed on an object other than the workpiece 10, such as a jig and an inner wall of the furnace, increases, which causes a problem, such as deposition of soot in the furnace, or the like.
[0046] On the other hand, in the case where the times t1 and t3 at which the pulse enters the OFF state are set to times at which the penetration rate is not so low in Fig. 2B, a period of time during which the pulse is in the ON state to supply the carburizing gas becomes short. Thus, the supply of the carburizing gas is stopped before the carburizing gas sufficiently spreads throughout the carburizing furnace 20, and therefore a variation in carburization quality between workpieces within the furnace described below becomes large.
[0047] In addition, in the case where the time t2 at which the pulse enters the ON state is set to a time at which the difference in the carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 is not so large in Fig. 2B, even when the carburizing gas is supplied, the carbon is less likely to penetrate the surface of the workpiece 10, and therefore such setting is not preferable in view of gas efficiency.
[0048] On the other hand, in the case where the pulse does not enter the ON state even after the difference in the carbon concentration between the surface of the workpiece 10 and the inside (the vicinity of the surface) of the workpiece 10 becomes sufficiently large after the pulse enters the OFF state (the case where the time t2 is late in Fig. 2B), a period of time during which the pulse is in the ON state to supply the carburizing gas becomes short. Thus, the supply of the carburizing gas is stopped before the carburizing gas spreads throughout the carburizing furnace 20, and therefore a variation in carburization quality between workpieces in the furnace becomes large.
[0049] Accordingly, in the present embodiment, ranges of the penetration rate for determining the timing of performing the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are set, and the supply and the stoppage of the supply of the carburizing gas are performed at timings at which the penetration rate of the carbon into the workpiece 10 is within the respective range.
<<Flow from setting of conditions to measurement of variation in carburization quality>>
[0050] Fig. 3 is a flowchart for explaining the process from setting of conditions to measurement of a variation in carburization quality in the present embodiment.
[0051] In the above-described vacuum carburizing treatment of the present embodiment, first, conditions for performing supply and stoppage of the supply of the carburizing gas are set (Step S1).
<Setting of conditions>
[0052] As described above, in the present embodiment, ranges of the carbon penetration rate into the workpiece 10 for determining the timing of initiation of the supply and the stoppage of the supply of the carburizing gas are set, and the initiation of the supply and the stoppage of the supply of the carburizing gas are performed at timings at which the carbon penetration rate is within the respective range. Thus, the range of the carbon penetration rate into the workpiece 10 for stopping the supply of the carburizing gas when the pulse is in the ON state and the carburizing gas is supplied, and the range of the carbon penetration rate into the workpiece 10 for starting the supply of the carburizing gas when the pulse is in the OFF state and the carburizing gas is not supplied are set. In the state in which the carburizing gas is not supplied, the carbon penetration rate into the workpiece 10 is "0" as described above. Thus, as the carbon penetration rate into the workpiece 10 for supplying the carburizing gas when the pulse is in the OFF state and the carburizing gas is not supplied, the carbon penetration rate into the workpiece 10, which can be estimated when the carburizing gas is supplied from the state in which the carburizing gas is not supplied, is set. This set value is a condition for performing the below-described simulation, and as the workpiece 10 serving as an article to be processed, for example, a workpiece using chromium steel SCr420 that is machine structural alloy steel may be applied. As the workpiece 10 serving as an article to be processed, other than the above chromium steel SCr420, another chromium steel, chromium molybdenum steel, manganese chromium steel, nickel chromium steel, nickel chromium molybdenum steel, aluminum chromium molybdenum steel, another machine structural alloy steel, machine structural carbon steel, or the like can be applied. When the steel type of the workpiece is the same between the simulation and the carburizing treatment, the accuracy of the result of the carburizing treatment calculated in the simulation to match the actual treatment result increases. It is assumed that, in the case where there is not a large difference in the structural composition, the accuracy of the simulation result being reflected in the actual carburizing treatment result increases even if the steel types are different between the simulation and the actual carburizing treatment. In the present example, as the workpiece 10 serving as an article to be processed, chromium steel SCr420, which is machine structural alloy steel, is used.
[0053] The carbon penetration rate into the workpiece 10 increases, as the carburizing treatment temperature of the workpiece 10 increases. Therefore, as described above, the ranges of the carbon penetration rate into the workpiece 10 for performing supply and stoppage of supply of the carburizing gas are set for each temperature.
[0054] First, the carbon penetration rate into the workpiece 10 for stopping the supply of the carburizing gas from the state in which the pulse is in the ON state and the carburizing gas is supplied will be described.
[0055] Table 1 is a table presenting the carbon penetration rate for causing the pulse to enter the OFF state to stop supply of the carburizing gas, i.e., setting values of Foff.
[Table 1]
| Temperature (°C) | Temperature (K) | Maximum Foff (mg/m2▪sec) | Minimum Foff (mg/m2▪sec) |
| 980 | 1253 | 148 | 76 |
| 950 | 1223 | 79 | 31 |
| 950 | 1223 | 77 | 30 |
| 950 | 1223 | 82 | 30 |
| 930 | 1203 | 51 | 17 |
| 930 | 1203 | 53 | 17 |
| 900 | 1173 | 24 | 4.3 |
| 880 | 1153 | 13 | 2.3 |
[0056] The carbon penetration rate into the workpiece 10, which is assumed to achieve excellent gas efficiency and reduce a variation in the carburization quality, is set for each carburizing treatment temperature of the workpiece 10. For the set values, the set values that have been obtained in the previous experiments may be used, or values calculated by simulation or the like may be used.
[0057] At a temperature of 980°C (1,253 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 148 (mg/m2·sec). In addition, at the temperature of 980°C (1,253 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 76 (mg/m2·sec).
[0058] Moreover, at a temperature of 950°C (1,223 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 79 (mg/m2·sec). In addition, at the temperature of 950°C (1,223 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 31 (mg/m2·sec).
[0059] Moreover, at the temperature of 950°C (1,223 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 77 (mg/m2·sec). In addition, at the temperature of 950°C (1,223 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 30 (mg/m2·sec).
[0060] Further, at the temperature of 950°C (1,223 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 82 (mg/m2·sec). In addition, at the temperature of 950°C (1,223 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 30 (mg/m2·sec).
[0061] Moreover, at a temperature of 930°C (1,203 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 51 (mg/m2·sec). In addition, at the temperature of 930°C (1,203 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 17 (mg/m2·sec).
[0062] Further, at the temperature of 930°C (1,203 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 53 (mg/m2·sec). In addition, at the temperature of 930°C (1,203 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 17 (mg/m2·sec).
[0063] Moreover, at a temperature of 900°C (1,173 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 24 (mg/m2·sec). In addition, at the temperature of 900°C (1,173 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 4.3 (mg/m2·sec).
[0064] Moreover, at a temperature of 880°C (1,153 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 13 (mg/m2·sec). In addition, at the temperature of 880°C (1,153 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the OFF state to stop the supply of the carburizing gas is set to 2.3 (mg/m2·sec).
[0065] Fig. 4 is a graph depicting approximate curves each connecting the set values presented in Table 1.
[0066] As depicted in Fig. 4, the approximate curve as depicted by the solid line is obtained by connecting the set values (○ in the figure) of the upper limit values presented in Table 1. This approximate curve is represented by:
where F [mg/m2·sec] is the carbon penetration rate into the workpiece 10, T [K] is the carburizing treatment temperature of the workpiece 10, and e is Napier's constant.
[0067] As depicted in Fig. 4, the approximate curve as depicted by the broken line is obtained by connecting the set values (△ in the figure) of the lower limit values presented in Table 1. This approximate curve is represented by:
[0068] where F [mg/m2·sec] is the carbon penetration rate into the workpiece 10, T [K] is the carburizing treatment temperature of the workpiece 10, and e is Napier's constant.
[0069] From the above, it is assumed that excellent gas efficiency is achieved, and a variation in carburization quality can be reduced by stopping the supply of the carburizing gas at the timing at which the carbon penetration rate F into the workpiece 10 is in the range represented by:
when the pulse is in the ON state and the carburizing gas is supplied.
[0070] It is assumed that excellent gas efficiency is achieved, but the period during which the pulse is in the ON state becomes short, and therefore the supply of the carburizing gas is stopped before the carburizing gas is spread throughout the carburizing furnace 20, and a variation in carburization quality becomes large, if the supply of the carburizing gas is stopped when the carbon penetration rate F into the workpiece 10 satisfies:
[0071] In addition, it is assumed that gas efficiency is reduced if the supply of the carburizing gas is stopped when the carbon penetration rate F into the workpiece 10 satisfies:
[0072] Next, the carbon penetration rate into the workpiece 10 for starting the supply of the carburizing gas from the state in which the pulse is in the OFF state and the carburizing gas is not supplied will be described.
[0073] Table 2 is a table presenting set values of the carbon penetration rate into the workpiece for causing the pulse to enter the ON state to start the supply of the carburizing gas.
[Table 2]
| Temperature (°C) | Temperature (K) | Maximum Fon (mg/m2▪sec) | Minimum Fon (mg/m2▪sec) |
| 950 | 1223 | 227 | 127 |
| 930 | 1203 | 183 | 103 |
| 900 | 1173 | 132 | 72 |
| 980 | 1253 | 308 | 190 |
| 880 | 1153 | 105 | 55 |
[0074] Also in this case, the carbon penetration rate into the workpiece 10, which is assumed to achieve excellent gas efficiency and reduce carburization quality, is set for each carburizing treatment temperature of the workpiece 10. For the set values, the set values that have been obtained in the previous experiments may be used, or values calculated by simulation or the like may be used.
[0075] At a temperature of 950°C (1,223 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 227 (mg/m2·sec). In addition, at the temperature of 950°C (1,223 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 127 (mg/m2·sec).
[0076] Moreover, at a temperature of 930°C (1,203 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 183 (mg/m2·sec). In addition, at the temperature of 930°C (1,203 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 103 (mg/m2·sec).
[0077] Moreover, at a temperature of 900°C (1,173 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 132 (mg/m2·sec). In addition, at the temperature of 900°C (1,173 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 72 (mg/m2·sec).
[0078] Moreover, at a temperature of 980°C (1,253 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 308 (mg/m2·sec). In addition, at the temperature of 980°C (1,253 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 190 (mg/m2·sec).
[0079] Furthermore, at the temperature of 880°C (1,153 K), an upper limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 105 (mg/m2·sec). In addition, at the temperature of 880°C (1,153 K), a lower limit value of the carbon penetration rate into the workpiece 10 for causing the pulse to enter the ON state to start the supply of the carburizing gas is set to 55 (mg/m2·sec).
[0080] Fig. 5 is a graph depicting approximate curves each connecting the set values presented in Table 2.
[0081] As depicted in Fig. 5, the approximate curve as depicted by the solid line is obtained by connecting the set values (○ in the figure) of the upper limit values presented in Table 2. This approximate curve is represented by:
where F [mg/m2·sec] is the carbon penetration rate into the workpiece 10, T [K] is the carburizing treatment temperature of the workpiece 10, and e is Napier's constant.
[0082] As depicted in Fig. 5, the approximate curve as depicted by the broken line is obtained by connecting the set values (△ in the figure) of the lower limit values presented in Table 2. This approximate curve is represented by:
where F [mg/m2·sec] is the carbon penetration rate into the workpiece 10, T [K] is the carburizing treatment temperature of the workpiece 10, and e is Napier's constant.
[0083] From the above, it is assumed that excellent gas efficiency is achieved, and a variation in carburization quality can be reduced by starting the supply of the carburizing gas at the timing at which the carbon penetration rate F into the workpiece 10 is within the range represented by:
when the pulse is in the OFF state and the carburizing gas is not supplied. At the initiation of the supply, as described above, the carbon penetration rate into the workpiece 10 is "0" in the state in which the carburizing gas is not supplied. Therefore, as the carbon penetration rate into the workpiece 10 for supplying the carburizing gas from the state in which the pulse is in the OFF state and the carburizing gas is not supplied, the carbon penetration rate into the workpiece 10, which is estimated when the carburizing gas is supplied from the state in which the carburizing gas is not supplied, is set.
[0084] If the supply of the carburizing gas is started when the carbon penetration rate F into the workpiece 10 satisfies:
gas efficiency improves, but it is assumed that the gas efficiency saturates merely by the increase of the treatment time.
[0085] If the supply of the carburizing gas is started when the carbon penetration rate F into the workpiece 10 satisfies:
the carbon penetration rate into the workpiece 10 remains low, and therefore it is assumed that gas efficiency decreases.
[0086] In the vacuum carburizing treatment, next, a calculation simulation, in which the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are repeated until an intended effective depth is obtained is performed based on the set values, which have been set in the above manner (step S2). The effective depth is a depth at which the carbon concentration becomes 0.35 percent by mass.
<Calculation simulation>
[0087] Fig. 6 is a diagram depicting a calculation simulation using the set values presented in Table 1 and Table 2, and in Fig. 4 and Fig. 5.
[0088] Based on the set values presented in Table 1 and Table 2, and in Fig. 4 and Fig. 5, a calculation simulation, in which the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are repeated until an intended effective depth is obtained, is performed. The effective depth is a depth at which the carbon concentration becomes 0.35 percent by mass. In this calculation simulation, the supply of the carburizing gas is stopped at the timing indicated by the broken line arrows in Fig. 6, at which the carbon penetration rate F represented by the solid line in Fig. 6 satisfies:
when the pulse is in the ON state and the carburizing gas is supplied.
[0089] In addition, the supply of the carburizing gas is started at the timing indicated by the solid line arrows in Fig. 6, at which the estimated carbon penetration rate F into the workpiece 10 represented by the solid line in Fig. 6 is within the range represented by:
when the pulse is in the OFF state and the carburizing gas is not supplied.
[0090] Then, for the pulses by which the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are repeated until the effective depth, which is indicated by the broken line in Fig. 6, becomes 0.35% or greater of the carbon concentration, each gap from the solid line arrow to the broken line arrow in Fig. 6 is determined as an ON time for supplying the carburizing gas, and each gap from the broken line arrow to the solid line arrow in Fig. 6 is determined as an OFF time for stopping the supply of the carburizing gas.
[0091] Thus, the ON/OFF time of the pulses for performing the supply and the stoppage of the supply of the carburizing gas is determined for each pulse (step S3).
<Measurement of variation>
[0092] Then, a vacuum carburizing treatment is performed by the device illustrated in Fig. 1 with pulses having the ON/OFF times that have actually been determined by the above-described simulation, and the variation in carburizing quality in the vacuum carburizing treatment is measured (step S4).
[0093] As evaluation of the variation of the workpieces 10, the surface carbon concentration (wt%) of the workpieces 10 disposed at nine locations, which included eight corners and the center in the carburizing furnace 20, was measured using an Electron Probe Micro Analyzer (EPMA), and the variation R (wt%) in the surface carbon concentration was evaluated. As a measuring instrument, for example, JXA-8530F field emission electron probe microanalyzer (FE-EPMA) may be used.
[0094] Table 3 and Table 4 are tables presenting the experiment results of the variation evaluation based on the set values set in the present embodiment. The same No. in Table 3 and Table 4 indicates the same sample.
[Table 3]
| No. | Temperature (K) | Temperature (°C) | Fon (mg/m2·sec) | Foff (mg/m2·sec) | Minimum ON time (sec) | Total ON time (sec) | Treatment time (sec) | Intended effective depth (mm) |
| 1 | 1223 | 950 | 227 | 79 | 9 | 140 | 14400 | 0.8 |
| 2 | 1223 | 950 | 175 | 77 | 5 | 182 | 11097 | 0.8 |
| 3 | 1223 | 950 | 127 | 82 | 89 | 227 | 13332 | 0.8 |
| 4 | 1223 | 950 | 227 | 31 | 59 | 398 | 13625 | 0.8 |
| 5 | 1223 | 950 | 174 | 30 | 36 | 508 | 10070 | 0.8 |
| 6 | 1223 | 950 | 129 | 30 | 20 | 651 | 10213 | 0.8 |
| 7 | 1223 | 950 | - | 3.9 | 6242 | 6242 | 19544 | 0.8 |
| 7' | 1223 | 950 | - | - | - | 19544 | 19544 | 0.8 |
| 8 | 1203 | 930 | 183 | 51 | 14 | 165 | 11132 | 0.6 |
| 9 | 1203 | 930 | 139 | 51 | 8 | 221 | 8001 | 0.6 |
| 10 | 1203 | 930 | 104 | 53 | 5 | 264 | 9123 | 0.6 |
| 11 | 1203 | 930 | 183 | 17 | 80 | 574 | 9226 | 0.6 |
| 12 | 1203 | 930 | 138 | 17 | 78 | 718 | 7359 | 0.6 |
| 13 | 1203 | 930 | 103 | 17 | 46 | 841 | 8729 | 0.6 |
| 14 | 1203 | 930 | - | 3.75 | 4699 | 4699 | 13325 | 0.6 |
| 14' | 1203 | 930 | - | - | - | 13325 | 13325 | 0.6 |
| 15 | 1173 | 900 | 132 | 24 | 34 | 236 | 6499 | 0.4 |
| 16 | 1173 | 900 | 96 | 24 | 19 | 471 | 10912 | 0.4 |
| 17 | 1173 | 900 | 72 | 24 | 11 | 368 | 5725 | 0.4 |
| 18 | 1173 | 900 | - | 4.3 | 1961 | 1961 | 4802 | 0.4 |
| 18' | 1173 | 900 | - | - | - | 4802 | 4802 | 0.4 |
[Table 4]
| No. | Carburizing gas flow rate for ON (L/min) | Total carbon flow rate (g/m2) | Gas efficiency relative value | Surface carbon concentration variation (wt%) | Variation evaluation |
| 1 | 16 | 22.8414 | 13.7 | 0.07 | ○ |
| 2 | 16 | 31.2383 | 14.4 | 0.06 | ○ |
| 3 | 16 | 39.093 | 14.5 | 0.04 | ○ |
| 4 | 16 | 24.7121 | 5.2 | 0.04 | ○ |
| 5 | 16 | 31.3686 | 5.2 | 0.04 | ○ |
| 6 | 16 | 40.1656 | 5.2 | 0.05 | ○ |
| 7 | 16 | 48.8094 | 0.7 | 0.04 | ○ |
| 7' | 16⇒4 | 48.8094 | 0.7 | 0.54 | × |
| 8 | 16 | 17.5208 | 8.9 | 0.05 | ○ |
| 9 | 16 | 23.9277 | 9.1 | 0.06 | ○ |
| 10 | 16 | 28.2861 | 9.0 | 0.04 | ○ |
| 11 | 16 | 20.4086 | 3.0 | 0.06 | ○ |
| 12 | 16 | 24.3176 | 2.8 | 0.05 | ○ |
| 13 | 16 | 27.9815 | 2.8 | 0.05 | ○ |
| 14 | 16 | 35.3061 | 0.6 | 0.03 | ○ |
| 14' | 16⇒4 | 35.3061 | 0.6 | 0.48 | × |
| 15 | 16 | 11.6235 | 4.1 | 0.04 | ○ |
| 16 | 16 | 23.383 | 4.2 | 0.05 | ○ |
| 17 | 16 | 18.1263 | 4.1 | 0.04 | ○ |
| 18 | 16 | 17.135 | 0.7 | 0.04 | ○ |
| 18' | 16⇒4 | 17.135 | 0.7 | 0.32 | × |
[0096] Fon refers to the carbon penetration rate at the time when the supply of the carburizing gas is started with the second or subsequent pulse, from the state in which the supply of the carburizing gas has been stopped.
[0097] Foff refers to the carbon penetration rate into the workpiece 10 when the supply of the carburizing gas is stopped from the state in which the carburizing gas has been supplied.
[0098] Minimum ON time refers to the shortest ON time among the ON times of the pulses performed by repeating ON and OFF until the intended effective depth is obtained. As described above, in the case where the ON time of the pulse is short, the supply of the carburizing gas is stopped before spreading the carburizing gas throughout the carburizing furnace, and therefore the variation in the carburization quality between the workpieces in the furnace becomes large.
[0099] Total ON time refers to the total duration of the ON times of the pulses performed by repeating ON and OFF until the intended effective depth is obtained.
[0100] Treatment time refers to the total duration of the ON times and OFF times of the pulses performed by repeating ON and OFF until the intended effective depth is obtained.
[0101] Intended effective depth refers to an intended value of the effective depth at which the carbon concentration set in the carburizing treatment becomes 0.35 wt%.
[0102] Moreover, in Table 4, Carburizing gas flow rate for ON refers to a gas flow rate when the carburizing gas is supplied.
[0103] Total carburizing gas flow rate refers to an amount determined by integrating the theoretical carbon penetration rate indicated by the solid line in Fig. 6 with respect to time.
[0104] Gas efficiency relative value refers to a value determined by the total carburizing gas flow rate/(carburizing gas flow rate for ON × total ON time). This is a value indicating how much carburizing gas is actually used for carburization of the workpiece 10 within the supplied carburizing gas.
[0105] Surface carbon concentration variation refers to the variation in the surface carbon concentration among the above-described nine positions. This is a value determined by (the maximum surface carbon concentration) - (the minimum surface carbon concentration).
[0106] As a variation determination, the variation was evaluated as ○ (good) when the variation in the surface carbon concentration was 0.1 wt% or less.
[0107] As presented in Table 3, for Sample No. 8, for example, Foff was set to 51 (mg/m2·sec) so that the supply of the carburizing gas was stopped at the timing at which the carbon penetration rate into the workpiece 10 became 51 (mg/m2·sec) in the state in which the carburizing gas was supplied. Moreover, Fon was set to 183 (mg/m2·sec) so that the supply of the carburizing gas was started at the timing at which the estimated carbon penetration rate into the workpiece 10, when the carburizing gas was supplied from the state in which the supply of the carburizing gas was stopped, became 183 (mg/m2·sec).
[0108] Then, the variation in the surface carbon concentration was measured, and the variation was determined based on the measurement results.
[0109] As a result, as presented in Table 3 and Table 4, Sample Nos. 1-6, 8-13, and 15-17, with which the carbon penetration rates into the workpiece 10 for performing the supply and the stoppage of the supply of the carburizing gas were in the ranges represented by the formulae 1 and 2 had the variations of 0.1 wt% or less.
[0110] Conversely, Sample Nos. 7', 14', and 18' with which the supply amount of the carburizing gas was reduced as the time passed as in the related art had the variation, which greatly exceeded 0.1 wt%, indicating a large variation.
[0111] As described above, in the case where the carbon penetration rate into the workpiece 10 for performing the initiation of the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are within the ranges represented by the above-described formulae 1 and 2, the variation in carburization can be reduced compared to the case where the supply amount of the carburizing gas is reduced as the time passes.
[0112] In addition, Sample Nos. 7, 14, and 18 had variations of 0.1 wt% or less, and could reduce the variation in carburization compared to the case where the supply amount of the carburizing gas was reduced as the time passed. However, the carbon penetration rates were not within the ranges represented by the above-described formulae 1 and 2, and therefore the gas efficiency was not desirable.
[0113] As described above, a variation in carburization quality can be reduced without reducing gas efficiency of a carburizing gas by performing supply or stoppage of the supply of the carburizing gas at timings based on a penetration rate at which the carburizing gas allows carbon to penetrate the workpiece 10.
[0114] In the examples described above, chromium steel SCr420 that was machine structural alloy steel was used as the workpiece 10 serving as an article to be processed. However, as described above, in the case where another machine structural alloy steel or machine structural carbon steel is used as the workpiece 10 to be processed, a variation width of the F value upon application of other steel types is considered using a simulation. It has been found that excellent gas efficiency is achieved and a variation in carburizing quality can be reduced when the supply of the carburizing gas is stopped at timings at which the carbon penetration rate F into the workpiece 10 is within the range represented by:
in the above simulation, in the case where the pulse is in the ON state and the carburizing gas is supplied.
[0115] In addition, it has been found that excellent gas efficiency is achieved and a variation in carburization quality can be reduced when the supply of the carburizing gas is started at timings at which the carbon penetration rate F into the workpiece 10 is within the range represented by:
in the case where the pulse is in the OFF state and the carburizing gas is not supplied.
[0116] The present international application claims priority based on Japanese Patent Application No. 2023-182280, filed on October 24, 2023, and the entire contents of Japanese Patent Application No. 2023-182280 are incorporated herein by reference.
DESCRIPTION OF REFERENCE NUMERALS
1. A carburizing method using a carburizing gas, the carburizing method comprising:
repeating supply of the carburizing gas and stoppage of the supply of the carburizing gas, where initiation of the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are performed at timings based on a carbon penetration rate at which the carburizing gas causes carbon to penetrate an article to be processed.
repeating supply of the carburizing gas and stoppage of the supply of the carburizing gas, where initiation of the supply of the carburizing gas and the stoppage of the supply of the carburizing gas are performed at timings based on a carbon penetration rate at which the carburizing gas causes carbon to penetrate an article to be processed.
2. The carburizing method according to claim 1, further comprising:
setting a range of the carbon penetration rate at a time of the initiation of the supply or at the time of the stoppage of the supply, where the initiation of the supply or the stoppage of the supply is performed at a timing at which the carbon penetration rate is within the range.
setting a range of the carbon penetration rate at a time of the initiation of the supply or at the time of the stoppage of the supply, where the initiation of the supply or the stoppage of the supply is performed at a timing at which the carbon penetration rate is within the range.
3. The carburizing method according to claim 2,
wherein the range varies according to a carburizing treatment temperature of the article to be processed.
wherein the range varies according to a carburizing treatment temperature of the article to be processed.
4. The carburizing method according to claim 1,
wherein the stoppage of the supply is performed at a timing at which the carbon penetration rate F is within a range represented by a formula (1) below in a state in which the supply is performed,
where F [mg/m2·sec] is the carbon penetration rate, and T [K] is the carburizing treatment temperature.
wherein the stoppage of the supply is performed at a timing at which the carbon penetration rate F is within a range represented by a formula (1) below in a state in which the supply is performed,
where F [mg/m2·sec] is the carbon penetration rate, and T [K] is the carburizing treatment temperature.
5. The carburizing method according to claim 1,
wherein the initiation of the supply is performed at a timing at which the carbon penetration rate F is within a range represented by a formula (2) below, when the supply is performed from a state in which the stoppage of the supply is performed,
where F [mg/m2·sec] is the carbon penetration rate, and T [K] is the carburizing treatment temperature.
wherein the initiation of the supply is performed at a timing at which the carbon penetration rate F is within a range represented by a formula (2) below, when the supply is performed from a state in which the stoppage of the supply is performed,
where F [mg/m2·sec] is the carbon penetration rate, and T [K] is the carburizing treatment temperature.
6. The carburizing method according to claim 1,
wherein the article to be processed is machine structural alloy steel including chromium.
wherein the article to be processed is machine structural alloy steel including chromium.
7. The carburizing method according to claim 6,
wherein the machine structural alloy steel is chromium steel.
wherein the machine structural alloy steel is chromium steel.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description