Carburizing method

Abstract

A carburizing method using carburizing gas, wherein the supply and cessation of the aforementioned carburizing gas are repeatedly performed, and the supply is started and stopped at a time based on the carbon penetration rate of carbon penetrating into the workpiece through the aforementioned carburizing gas.

Description

Technical Field

[0001] This invention relates to a carburizing method. Background Technology

[0002] Previously, vacuum carburizing methods were known, in which activated carbon was diffused into and diffused onto the surface of steel by supplying carburizing gas into a vacuum furnace. In such vacuum carburizing, for example, Patent Document 1 discloses a method to reduce waste of carburizing gas and reduce processing costs by gradually reducing the supply of carburizing gas from the early stage to the later stage of carburizing.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2005-350729 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] However, in the method disclosed in Patent Document 1, if the supply of carburizing gas is too small in the later stage of carburizing, there is a problem that the carburizing gas cannot be distributed throughout the entire furnace, resulting in a larger deviation in carburizing quality.

[0008] The present invention was made in view of the problems of the prior art as described above, and its object is to provide a carburizing method that can reduce the deviation of carburizing quality without reducing the gas efficiency of carburizing gas.

[0009] Solution for solving the problem

[0010] To address the aforementioned issues, this invention provides a carburizing method utilizing carburizing gas, wherein...

[0011] The supply and cessation of the aforementioned carburizing gas are repeated, with the supply being started and stopped at the timing based on the carbon penetration rate of carbon into the treated material through the aforementioned carburizing gas.

[0012] In the present invention configured as described above, the supply of carburizing gas is started and stopped at the timing based on the carbon diffusion rate of carbon into the workpiece via the carburizing gas, thereby ensuring that the amount of carburizing gas supplied corresponds to the carbon diffusion rate into the workpiece. As a result, the amount of carburizing gas supplied is reduced, and the deviation in carburizing quality is reduced without the situation where the amount of carburizing gas supplied becomes too small.

[0013] Alternatively, the following configuration may be adopted: a range of carbon penetration rates is set when the aforementioned supply begins or when the aforementioned supply stops, and the aforementioned supply begins or stops when the aforementioned carbon penetration rate is within the aforementioned range.

[0014] Alternatively, the aforementioned range can vary depending on the carburizing temperature of the material being treated. In this configuration, the higher the temperature, the faster the carbon penetration rate.

[0015] Alternatively, specifically, the aforementioned carbon penetration rate can be set as F [mg / m]. 2 ·sec], set the carburizing treatment temperature to T[K],

[0016] While the aforementioned supply is in progress, the aforementioned supply may be stopped when the aforementioned carbon penetration rate F is within the range of the following formula (1).

[0017] 9×10 -19 ×e 0.036T <F<1×10 -11 ×e 0.0243T …Formula (1)

[0018] Alternatively, the aforementioned carbon penetration rate can be set as F[mg / m]. 2 ·sec], set the carburizing treatment temperature to T[K],

[0019] If the aforementioned supply is carried out while the aforementioned supply is in a state of cessation, the aforementioned supply can be started when the aforementioned carbon penetration rate F is within the range of the following formula (2).

[0020] 3×10 -5 ×e 0.0122T <F<0.0005×e 0.0108T …Formula (2)

[0021] In addition, the material being treated can be chromium-containing alloy steel for mechanical structures, and the aforementioned alloy steel for mechanical structures can also be chromium steel.

[0022] The effects of the invention

[0023] According to the present invention, deviations in carburizing quality can be reduced without reducing the gas efficiency of the carburizing gas. Attached Figure Description

[0024] Figure 1 This is a diagram illustrating an example of the apparatus configuration for carrying out one embodiment of the carburizing method of the present invention.

[0025] Figure 2A This diagram illustrates the effect of supplying carburizing gas via pulses.

[0026] Figure 2B This diagram illustrates the effect of supplying carburizing gas via pulses.

[0027] Figure 3This is a flowchart illustrating the process from setting the conditions to measuring the deviation in carburizing quality in this embodiment.

[0028] Figure 4 It is a graph showing an approximate curve connecting the set values ​​shown in Table 1.

[0029] Figure 5 It is a graph showing an approximate curve of the set values ​​shown in Table 2.

[0030] Figure 6 This shows the use of Tables 1 and 2, and Figure 4 and Figure 5 The graph shows the calculation simulation of the set values. Detailed Implementation

[0031] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0032] Device Composition

[0033] Figure 1 This is a diagram illustrating an example of the apparatus configuration for carrying out one embodiment of the carburizing method of the present invention.

[0034] This example Figure 1 As shown, it includes a carburizing furnace 20, a mass flow controller 40, an exhaust valve 50, and a vacuum pump 60.

[0035] The workpiece 10, which is to be processed, is housed in a carburizing furnace 20. The carburizing furnace 20 has a supply port 21, through which acetylene 30 is supplied as carburizing gas via a mass flow controller 40; and an exhaust port 22, through which un-carburized components of the acetylene 30 supplied to the carburizing furnace 20 are discharged as waste gas. Inside the carburizing furnace 20, heaters 23 are respectively arranged above and below the workpiece 10 to heat the interior of the furnace and control the furnace atmosphere temperature, i.e., the carburizing treatment temperature of the workpiece 10. Furthermore, a thermocouple 24 is installed in the carburizing furnace 20 to monitor the temperature inside the furnace. The heaters 23 heat the interior of the carburizing furnace 20 based on the temperature monitored by the thermocouples 24, thereby controlling the temperature.

[0036] The mass flow controller 40 controls the start / stop and flow rate of the acetylene 30 supplied to the carburizing furnace 20. Specifically, the mass flow controller 40 controls the valve operation required to start and stop the supply of acetylene 30 via a PLC (Programmable Logic Controller) or similar device, and controls the supply of acetylene 30 to the furnace at a set flow rate.

[0037] The exhaust valve 50 is always open, and the components of the acetylene 30 supplied to the carburizing furnace 20 that have not penetrated into the workpiece 10 are discharged to the outside as waste gas.

[0038] Vacuum pump 60 draws a vacuum through exhaust valve 50, thereby discharging the un-permeated components of acetylene 30 supplied to carburizing furnace 20 as waste gas to the outside through exhaust valve 50.

[0039] The workpiece 10 is subjected to vacuum carburizing using the apparatus configured as described above. It should be noted that the carburizing gas supplied to the carburizing furnace 20 is not limited to acetylene 30. Hydrocarbon gases such as propane can also be used.

[0040] Vacuum carburizing treatment

[0041] In the vacuum carburizing process, firstly, acetylene 30 is supplied as carburizing gas to the carburizing furnace 20 under the control of the mass flow controller 40.

[0042] Inside the carburizing furnace 20, carbon is generated and diffused into the surface of the workpiece 10 by the reaction of the supplied acetylene 30 with the workpiece 10. The carbon gradually diffuses into the interior of the workpiece 10, thereby hardening the surface of the workpiece 10.

[0043] On the other hand, the components of the acetylene 30 supplied to the carburizing furnace 20 that have not been incorporated into the workpiece 10 are discharged to the outside as waste gas through the exhaust valve 50.

[0044] As mentioned above, when using Figure 1 In the vacuum carburizing process of the apparatus shown, acetylene 30 is supplied as carburizing gas to the carburizing furnace 20 under the control of the mass flow controller 40. Since the workpiece 10 is made of steel, carbon adheres to and penetrates the surface of the workpiece 10 through the reaction between the acetylene 30 supplied as carburizing gas and the workpiece 10. On the other hand, the components of the acetylene 30 supplied to the carburizing furnace 20 that have not penetrated the workpiece 10 are discharged as waste gas through the exhaust valve 50.

[0045] Specifically, if acetylene (C2H2)30 is supplied into the carburizing furnace 20, the acetylene 30 will undergo a non-equilibrium reaction with the steel (Fe) that becomes the workpiece 10.

[0046] 2Fe+C2H2 2[Fe+C]+H2

[0047] As a result, carbon (C) is generated on the surface of workpiece 10 and penetrates into the interior of workpiece 10, while hydrogen (H2) is discharged as waste gas.

[0048] The carbon that adheres to and penetrates the surface of workpiece 10 then gradually diffuses into the interior of workpiece 10.

[0049] This allows the surface of workpiece 10 to be hardened.

[0050] The role of pulse-based vacuum carburizing process

[0051] In the vacuum carburizing process described above, carburizing gas can also be supplied in pulses.

[0052] Figure 2A and Figure 2B This diagram illustrates the effect of pulsed supply of carburizing gas. It should be noted that... Figure 2B To facilitate understanding the effect of a pulse on the supply of carburizing gas, its width and height are... Figure 2A The pulses shown are different from those displayed, but are actually the same as... Figure 2A The pulse shown is the same pulse.

[0053] like Figure 2A As shown, in Figure 1 The mass flow controller 40 shown can also supply acetylene 30 to the carburizing furnace 20 for a certain period of time, and then stop the supply of acetylene 30 for a certain period of time, repeating this pulse-like process to supply carburizing gas.

[0054] When the pulse for supplying carburizing gas becomes ON at time t0 and begins to supply carburizing gas into the carburizing furnace 20, since carbon has not yet penetrated into the workpiece 10, the carbon generated on the surface of the workpiece 10 through the supply of carburizing gas gradually penetrates into the workpiece 10.

[0055] As carbon gradually penetrates into the interior of workpiece 10, the difference in carbon concentration between the surface of workpiece 10 and the interior (near the surface) of workpiece 10 gradually decreases, thus making it increasingly difficult for carbon to penetrate into workpiece 10.

[0056] Then, as carbon gradually becomes less able to penetrate the workpiece 10, thus... Figure 2B As shown by the solid line, the rate of carbon penetration into workpiece 10 gradually decreases.

[0057] Thus, the process of setting the pulse to the ON state and supplying carburizing gas to allow carbon to penetrate into the workpiece 10 is called the carburizing period.

[0058] Furthermore, the carbon generated and diffused on the surface of workpiece 10 gradually diffuses into the interior of workpiece 10. On the other hand, when the pulse at time t1 becomes OFF and stops supplying carburizing gas into the carburizing furnace 20, carburizing gas is no longer supplied to workpiece 10.

[0059] When the pulse is OFF, since no carburizing gas is supplied, therefore... Figure 2B As shown by the solid line, the rate at which carbon penetrates from the surface of workpiece 10 into the interior becomes "0" at the inflection point at time t1.

[0060] Thus, the process in which carbon diffuses into the interior of workpiece 10 without supplying carburizing gas is called the diffusion period. During the diffusion period, carburizing gas is not supplied to workpiece 10, and on the other hand, carbon diffuses into the interior of workpiece 10 from the surface, thereby gradually reducing the carbon concentration inside (near the surface) of workpiece 10.

[0061] Subsequently, when the pulse at time t2 becomes ON and carburizing gas is supplied to the carburizing furnace 20 again, the carbon concentration inside (near the surface) of the workpiece 10 becomes lower, resulting in a large difference in carbon concentration between the surface of the workpiece 10 and the inside (near the surface) of the workpiece 10.

[0062] Then, due to the large difference in carbon concentration between the surface of workpiece 10 and its interior (near the surface), carbon easily penetrates from the surface of workpiece 10, thus... Figure 2B As shown by the solid line, the rate of carbon penetration into workpiece 10 increases dramatically instantaneously. Then, the carbon gradually penetrates into the interior of workpiece 10, thereby gradually reducing the difference in carbon concentration between the surface and interior (near the surface) of workpiece 10. Figure 2B As shown by the solid line, the rate of carbon penetration into workpiece 10 gradually decreases.

[0063] Then, at time t3, the pulse becomes OFF. This process is repeated, as follows: Figure 2B As shown by the double-dotted line, the effective depth of the carbon in the workpiece 10 gradually increases. Here, the repetition of the ON and OFF states of the pulse also includes the case where the pulse is in the ON and OFF states once each.

[0064] Here, as described above, in pulse-based vacuum carburizing, the timing of the ON / OFF pulse is uniquely determined. Therefore, regardless of the difference in carbon concentration between the surface of workpiece 10 and its interior (near the surface), and the associated carbon penetration rate, carburizing gas will be supplied or the supply of carburizing gas will be stopped.

[0065] However, in terms of gas efficiency and reducing deviations in carburizing quality, the carburizing gas is sometimes not preferred.

[0066] For example, in Figure 2B In this case, when the pulse is in the OFF state at times t1 and t3, which are when the penetration rate is close to "0", even if carburizing gas is supplied, carbon is difficult to penetrate from the surface of the workpiece 10, which is not optimal in terms of gas efficiency. In addition, in this case, the amount of carburizing gas adsorbed on objects other than the workpiece 10, such as fixtures and furnace walls, increases, which is the cause of undesirable conditions such as carbon buildup in the furnace.

[0067] On the other hand, Figure 2B In this process, when the pulse becomes OFF at times t1 and t3, which are set to be times when the carburizing rate is not too low, the time for supplying carburizing gas when the pulse is ON is shortened. As a result, the supply of carburizing gas stops before the carburizing gas permeates the entire carburizing furnace 20, thereby increasing the deviation in carburizing quality between workpieces in the furnace, as described later.

[0068] In addition, Figure 2B In the case where the moment t2 when the pulse becomes ON is set to a time when the difference in carbon concentration between the surface of the workpiece 10 and the interior (near the surface) of the workpiece 10 is not too large, even if carburizing gas is supplied, carbon is difficult to penetrate from the surface of the workpiece 10, which is not preferred in terms of gas efficiency.

[0069] On the other hand, even after the pulse becomes OFF, if the difference in carbon concentration between the surface of workpiece 10 and the interior (near the surface) of workpiece 10 becomes sufficiently large, the pulse does not become ON. Figure 2B In the case of a delayed t2, the time for the pulse to turn ON and supply carburizing gas will be shorter. As a result, the supply of carburizing gas will be stopped before the carburizing gas can spread throughout the entire carburizing furnace 20, and the deviation in carburizing quality between workpieces in the furnace will increase.

[0070] Therefore, in this embodiment, a range of carburizing gas infiltration rate is set to determine the timing of supplying and stopping the carburizing gas, and the supply and stopping of the carburizing gas are performed when the carburizing gas infiltration rate into the workpiece 10 is within this range.

[0071] The process from setting conditions to determining carburizing quality deviations

[0072] Figure 3 This is a flowchart illustrating the process from setting the conditions to measuring the deviation in carburizing quality in this embodiment.

[0073] In the vacuum carburizing process described above, firstly, conditions for supplying and stopping the carburizing gas are set (step S1).

[0074] <Condition Setting>

[0075] As described above, in this embodiment, a range of carburizing speeds to the workpiece 10 is set to determine the timing of starting and stopping the supply of carburizing gas. The supply of carburizing gas is started and stopped when the carburizing speed is within this range. Therefore, the following ranges are set: the range of carburizing speeds to the workpiece 10 for stopping the supply of carburizing gas when the pulse is ON and carburizing gas is supplied, and the range of carburizing speeds to the workpiece 10 for starting the supply of carburizing gas when the pulse is OFF and carburizing gas is not supplied. It should be noted that the carburizing speed to the workpiece 10 is "0" as described above when carburizing gas is not supplied. Therefore, the carburizing speed to the workpiece 10 for supplying carburizing gas when the pulse is OFF and carburizing gas is not supplied is set to the estimated carburizing speed to the workpiece 10 when carburizing gas is supplied when it is not supplied. It should be noted that this setting value serves as a condition for the simulation described later. The workpiece 10, which is the object of the simulation, can, for example, be a workpiece using SCr420 chromium steel, an alloy steel for mechanical structures. It should also be noted that, in addition to SCr420 chromium steel, other chromium steels, chromium-molybdenum steels, manganese-chromium steels, nickel-chromium steels, nickel-chromium-molybdenum steels, aluminum-chromium-molybdenum steels, other alloy steels for mechanical structures, and carbon steels for mechanical structures can also be used as the workpiece 10 to be processed. If the simulated and actual carburizing processes use the same type of steel, the carburizing results obtained through simulation calculations will be highly consistent with the actual processing results. Furthermore, it is assumed that even if the steel types used in the simulation and actual carburizing processes are different, the simulation results will highly reflect the actual carburizing results, provided that the differences in structural composition are not significant. In this example, SCr420 chromium steel, an alloy steel for mechanical structures, is used as the workpiece 10.

[0076] Here, the higher the carburizing temperature of workpiece 10, the faster the carburization rate into workpiece 10. Therefore, as described above, a range is set for the carburization gas supply and the carburization rate into workpiece 10 at which the supply stops for each temperature.

[0077] First, the carbon diffusion rate to the workpiece 10, which is used to stop the supply of carburizing gas when the pulse is ON and carburizing gas is supplied, will be explained.

[0078] Table 1 shows the carburization rate, F, used to stop the supply of carburizing gas by setting the pulse to OFF. off The table of set values.

[0079] [Table 1]

[0080]

[0081] For each carburizing treatment temperature of workpiece 10, a carburizing rate to workpiece 10 is set that is designed to ensure good gas efficiency and reduce deviations in carburizing quality. It should be noted that this setting value can be used if it is obtained based on past examples, etc., or a value calculated through simulation, etc.

[0082] At a temperature of 980℃ (1253K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to OFF, is set to 148 (mg / m³). 2 •sec). Furthermore, at a temperature of 980°C (1253K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 76 (mg / m³). 2 ·sec).

[0083] Furthermore, at a temperature of 950°C (1223K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 79 (mg / m³). 2 •sec). Furthermore, at a temperature of 950°C (1223K), the lower limit for the carburizing gas supply to the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 31 (mg / m³). 2 ·sec).

[0084] Furthermore, at a temperature of 950°C (1223K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 77 (mg / m³). 2 •sec). Furthermore, at a temperature of 950°C (1223K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 30 (mg / m³). 2 ·sec).

[0085] Furthermore, at a temperature of 950°C (1223K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 82 (mg / m³). 2 •sec). Furthermore, at a temperature of 950°C (1223K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 30 (mg / m³). 2 ·sec).

[0086] Furthermore, at a temperature of 930°C (1203K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 51 (mg / m³). 2 •sec). Furthermore, at a temperature of 930°C (1203K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 17 (mg / m³). 2 ·sec).

[0087] Furthermore, at a temperature of 930°C (1203K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 53 (mg / m³). 2 •sec). Furthermore, at a temperature of 930°C (1203K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 17 (mg / m³). 2 ·sec).

[0088] Furthermore, at a temperature of 900°C (1173K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 24 (mg / m³). 2 •sec). Furthermore, at a temperature of 900℃ (1173K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to OFF, is set to 4.3 (mg / m³). 2 ·sec).

[0089] Furthermore, at a temperature of 880°C (1153K), the upper limit for the carburizing rate of the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to the OFF state, is set to 13 (mg / m³). 2 •sec). Furthermore, at a temperature of 880℃ (1153K), the lower limit for the carburizing rate into the workpiece 10, used to stop the supply of carburizing gas by setting the pulse to OFF, is set to 2.3 (mg / m³). 2 ·sec).

[0090] Figure 4 It is a graph showing an approximate curve connecting the set values ​​shown in Table 1.

[0091] like Figure 4 As shown, if the upper limit values ​​shown in Table 1 (0 in the figure) are connected to each other, an approximate curve as shown by the solid line is obtained. This approximate curve is obtained when the carbon penetration rate into the workpiece 10 is set to F [mg / m]. 2·sec], with the carburizing temperature of workpiece 10 set to T[K], and the Napier constant set to e, satisfying F=1×10 -11 ×e 0.0243T .

[0092] In addition, such as Figure 4 As shown, if the lower limit values ​​shown in Table 1 (△ in the figure) are connected to each other, an approximate curve as shown by the dashed line is obtained. This approximate curve is obtained when the carbon penetration rate into the workpiece 10 is set to F [mg / m]. 2 When the carburizing temperature of workpiece 10 is set to T [K], and the Napier constant is set to e, F = 2 × 10 -18 ×e 0.036T .

[0093] Therefore, when the pulse is ON and carburizing gas is supplied, if the supply of carburizing gas is stopped when the carburizing rate F to the workpiece 10 is within the range of equation (3), it can be expected that the gas efficiency will be good and the deviation in carburizing quality can be reduced.

[0094] 2×10 -18 ×e 0.036T <F<1×10 -11 ×e 0.0243T …Formula (3)

[0095] It should be noted that if the carbon diffusion rate F into workpiece 10 satisfies F≥1×10 -11 ×e 0.0243T If the supply of carburizing gas is stopped under these circumstances, it is expected that the gas efficiency will become better. However, because the time when the pulse is in the ON state is shortened, the supply of carburizing gas will be stopped before the carburizing gas can spread throughout the entire carburizing furnace 20, and the deviation in carburizing quality will become larger.

[0096] Furthermore, if the carbon diffusion rate F into workpiece 10 satisfies F≤2×10 -18 ×e 0.036T If the supply of carburizing gas is stopped under these circumstances, a decrease in gas efficiency can be expected.

[0097] Next, the carbon diffusion rate to the workpiece 10 for starting the supply of carburizing gas when the pulse is OFF and no carburizing gas is supplied will be explained.

[0098] Table 2 shows the set values ​​for the carburizing rate of the workpiece when the pulse is set to ON to start the supply of carburizing gas.

[0099] [Table 2]

[0100]

[0101] In this case, a carburizing rate for carburizing workpiece 10 is also set for each carburizing treatment temperature, which is designed to ensure good gas efficiency and reduce deviations in carburizing quality. It should be noted that this setting value can also be used in cases obtained based on past examples, etc.

[0102] At a temperature of 950℃ (1223K), the upper limit value for the carburizing rate of the workpiece 10, used to supply carburizing gas to initiate the carburizing process by setting the pulse to the ON state, is set to 227 (mg / m³). 2 •sec). Furthermore, at a temperature of 950°C (1223K), the lower limit for the carburizing rate of the workpiece 10, used as the supply of carburizing gas to initiate the carburizing process by turning the pulse ON, is set to 127 (mg / m³). 2 ·sec).

[0103] Furthermore, at a temperature of 930°C (1203K), the upper limit of the carburizing rate of the workpiece 10, which is used to supply carburizing gas to the workpiece 10 to enable the pulse to be ON, is set to 183 mg / m³. 2 •sec). Furthermore, at a temperature of 930°C (1203K), the lower limit for the carburizing rate of the workpiece 10, used as the supply of carburizing gas to initiate the carburizing process by turning the pulse ON, is set to 103 (mg / m³). 2 ·sec).

[0104] Furthermore, at a temperature of 900°C (1173K), the upper limit of the carburizing rate of the workpiece 10, which is used to supply carburizing gas to the workpiece 10 to enable the pulse to be ON, is set to 132 (mg / m³). 2 •sec). Furthermore, at a temperature of 900°C (1173K), the lower limit for the carburizing rate of the workpiece 10, used to supply the carburizing gas to initiate the carburizing process by turning the pulse ON, is set to 72 (mg / m³). 2 ·sec).

[0105] Furthermore, at a temperature of 980°C (1253K), the upper limit of the carburizing rate of the workpiece 10, which is used to supply carburizing gas to the workpiece 10 to enable the pulse to be ON, is set to 308 mg / m³. 2 •sec). Furthermore, at a temperature of 980°C (1253K), the lower limit for the carburizing rate of the workpiece 10, used to supply the carburizing gas to initiate the carburizing process by turning the pulse ON, is set to 190 (mg / m³). 2 ·sec).

[0106] Furthermore, at a temperature of 880°C (1153K), the upper limit of the carburizing rate of the workpiece 10, which is used to supply carburizing gas to the workpiece 10 to enable the pulse to be ON, is set to 105 mg / m³.2 •sec). Furthermore, at a temperature of 880°C (1153K), the lower limit for the carburizing rate of the workpiece 10, used to supply the carburizing gas to initiate the carburizing process by turning the pulse ON, is set to 55 (mg / m³). 2 ·sec).

[0107] Figure 5 It is a graph showing an approximate curve of the set values ​​shown in Table 2.

[0108] like Figure 5 As shown, if the upper limit values ​​shown in Table 2 (0 in the figure) are connected to each other, an approximate curve as shown by the solid line is obtained. When the carbon penetration rate into workpiece 10 is set to F [mg / m³],... 2 When the carburizing temperature of workpiece 10 is set to T [K], and the Napier constant is set to e, the approximate curve satisfies F = 0.0004 × e. 0.0108T .

[0109] In addition, such as Figure 5 As shown, if the lower limit values ​​shown in Table 2 (△ in the figure) are connected to each other, an approximate curve as shown by the dashed line is obtained. When the carbon penetration rate into workpiece 10 is set to F [mg / m³],... 2 When the carburizing temperature of workpiece 10 is set to T [K], and the Napier constant is set to e, the approximate curve satisfies F = 4 × 10 -5 ×e 0.0122T .

[0110] Therefore, when the pulse is OFF and no carburizing gas is supplied, if the supply of carburizing gas is started at a time when the carburizing rate F to the workpiece 10 is within the range of equation (4), it can be expected that the gas efficiency will be good and the deviation in carburizing quality can be reduced.

[0111] 4×10 -5 ×e 0.0122T <F<0.0004×e 0.0108T …Formula (4)

[0112] At this time, when no carburizing gas is supplied, the carburizing rate into the workpiece 10 is "0" as described above. Therefore, the carburizing rate into the workpiece 10 when carburizing gas is supplied in the OFF state and no carburizing gas is supplied is set to the carburizing rate into the workpiece 10 estimated when carburizing gas is supplied in the state of no carburizing gas supply.

[0113] It should be noted that if the carbon diffusion rate F in workpiece 10 satisfies F≥0.0004×e 0.0108TIf the supply of carburizing gas is started under these conditions, it can be expected that the gas efficiency will become good, and the gas efficiency will tend to saturate as the processing time increases.

[0114] Furthermore, if the carbon diffusion rate F into workpiece 10 satisfies F≤4×10 -5 ×e 0.0122T If the supply of carburizing gas is started under these conditions, it is foreseeable that the gas efficiency will decrease because the carburizing rate into the workpiece 10 will remain low.

[0115] In the vacuum carburizing process, based on the set values ​​as described above, the depth at which the carbon concentration reaches 0.35% by mass is taken as the effective depth, and the calculation simulation of the supply and cessation of carburizing gas is performed repeatedly until the target effective depth is reached (step S2).

[0116] <Computational Simulation>

[0117] Figure 6 This shows the use of Tables 1 and 2, and Figure 4 and Figure 5 The graph shows the calculation simulation of the set values.

[0118] Based on Tables 1 and 2 and Figure 4 and Figure 5 The setpoints shown indicate that a depth with a carbon concentration of 0.35% by mass was taken as the effective depth. Repeated simulations of carburizing gas supply and supply interruption were performed until the target effective depth was reached. In this simulation, with the pulse on and carburizing gas supplied, Figure 6 The carbon penetration rate F shown by the solid line satisfies 2 × 10 -18 ×e 0.036T <F<1×10 -11 ×e 0.0243T of Figure 6 The timing indicated by the dashed arrow is when the supply of carburizing gas is stopped.

[0119] Furthermore, the assumption is that carburizing gas is supplied when the pulse is OFF and no carburizing gas is supplied. Figure 6 The carbon penetration rate F into workpiece 10, as shown by the solid line, is at 4 × 10⁻⁶. -5 ×e 0.0122T <F<0.0004×e 0.0108T Within the scope Figure 6 The solid arrow in the middle indicates the timing when the carburizing gas supply begins.

[0120] Then, the supply of carburizing gas for repeated processes is stopped until... Figure 6 The effective depth indicated by the dashed line represents a pulse with a surface carbon concentration of 0.35% or higher. Figure 6The time from the solid arrow to the next dashed arrow is defined as the ON time for supplying carburizing gas. Figure 6 The interval between the dashed arrow and the next solid arrow is defined as the OFF time for stopping the supply of carburizing gas.

[0121] Therefore, the ON / OFF time of the pulse for supplying and stopping the carburizing gas is determined for each pulse (step S3).

[0122] <Determination of Deviation>

[0123] Then, in Figure 1 In the apparatus shown, vacuum carburizing is performed using a pulse with an ON / OFF time that is actually determined by the above simulation, and the deviation in carburizing quality at this time is measured (step S4).

[0124] To evaluate the deviation of workpiece 10, the surface carbon concentration (wt%) of workpiece 10 at the corners 8 and the center 9 locations within the carburizing furnace 20 is measured using an EPMA (Electron Probe Micro Analyzer), and the deviation R (wt%) of the surface carbon concentration is evaluated. For example, a JXA-8530F field emission electron probe microanalyzer (FE-EPMA) can also be used as the measuring instrument.

[0125] Tables 3 and 4 are tables showing the experimental results evaluating the deviations from the set values ​​set in this embodiment. It should be noted that the same No. in Tables 3 and 4 represents the same sample.

[0126] [Table 3]

[0127]

[0128] [Table 4]

[0129]

[0130] In Table 3, temperature refers to the carburizing temperature of workpiece 10.

[0131] F on This indicates the carbon penetration rate at the instant the carburizing gas supply begins from the state of cessation after the second pulse.

[0132] F off This indicates the rate at which the carburizing gas supply is stopped when the supply of carburizing gas is interrupted from the state of supplying carburizing gas into the workpiece 10.

[0133] The shortest ON time refers to the shortest ON time among the ON times of each pulse that repeats ON / OFF until the effective depth of the target is reached. As mentioned above, when the ON time of the pulse is short, the supply of carburizing gas will be stopped before the carburizing gas can spread throughout the entire carburizing furnace, thereby increasing the deviation in carburizing quality between workpieces in the furnace.

[0134] Total ON time represents the sum of the ON times of each pulse that repeats ON / OFF until the target's effective depth is reached.

[0135] Processing time represents the sum of the ON and OFF times of each pulse that repeats ON / OFF until the target's effective depth is reached.

[0136] The target effective depth refers to the target value set in this carburizing process to achieve an effective depth of 0.35 wt% carbon concentration.

[0137] Additionally, in Table 4, the carburizing gas flow rate at "on" refers to the gas flow rate during the carburizing gas supply process.

[0138] Total carburizing gas flow rate refers to the total carburizing gas flow rate. Figure 6 The amount obtained by integrating the theoretical carbon penetration rate, represented by the solid line in the middle, over time.

[0139] The relative gas efficiency is a value calculated as total carburizing gas flow rate / (carburizing gas flow rate during ON time × total ON time). This indicates the extent to which the supplied carburizing gas is actually used for carburizing workpiece 10.

[0140] The surface carbon concentration deviation refers to the deviation of the surface carbon concentration at the above 9 locations. This value is calculated by subtracting the lowest surface carbon concentration from the highest surface carbon concentration.

[0141] As a deviation criterion, if the deviation of surface carbon concentration is less than 0.1 wt%, it is rated as 0 (good).

[0142] As shown in Table 3, in, for example, sample No. 8, by using F off Set to 51 (mg / m 2 •sec), under the condition that the carburizing gas supply is in progress, the carburizing rate into the workpiece 10 is 51 (mg / m). 2 The supply of carburizing gas is stopped at a time of ·sec. Additionally, by using F... on Set to 183 (mg / m³) 2 •sec), assuming the carburizing gas supply is stopped and then supplied, the estimated carburizing rate into workpiece 10 is 183 (mg / m³). 2 The supply of carburizing gas begins at a time when the timing is ·sec.

[0143] Then, the deviation of the surface carbon concentration was measured, and a deviation determination was made based on the results.

[0144] The results are shown in Tables 3 and 4. For samples No. 1 to 6, 8 to 13, and 15 to 17, which are within the range shown in Equations 1 and 2 above, the carburization rate to workpiece 10 during the supply and cessation of carburizing gas is less than 0.1 wt%.

[0145] On the other hand, for samples No.7', 14', and 18', where the supply of carburizing gas gradually decreases over time as in the past, the deviation exceeds 0.1wt%, which is a large deviation.

[0146] Thus, in the example where the carbon diffusion rate of the workpiece 10 at the start and stop of the supply of carburizing gas is within the range shown in Equations 1 and 2 above, compared with the example where the supply of carburizing gas is gradually reduced over time, carburizing deviation can be reduced.

[0147] Furthermore, for samples No. 7, 14, and 18, the deviation also became less than 0.1 wt%, which is a reduction in carburizing deviation compared to examples where the supply of carburizing gas gradually decreases over time. However, since these samples are outside the ranges shown in Equations 1 and 2 above, the gas efficiency becomes poor.

[0148] As described above, by supplying or stopping the carburizing gas at the timing of the penetration rate of carbon into the workpiece 10 based on the carburizing gas penetration rate, deviations in carburizing quality can be reduced without reducing the gas efficiency of the carburizing gas.

[0149] It should be noted that in the example shown so far, the workpiece 10, which is the object of the simulation, uses SCr420 chromium steel, which is an alloy steel for mechanical structures. However, as mentioned above, if the workpiece 10 is also used in cases where other alloy steels or carbon steels for mechanical structures are also used, the simulation is used to consider the variation of the F value when applied to other steel types. In the simulation, it is found that when the pulse is ON and carburizing gas is supplied, if the supply of carburizing gas is stopped when the carburizing rate F to the workpiece 10 is within the range of Equation (1), the gas efficiency is good and the deviation of carburizing quality can be reduced.

[0150] 9×10 -19 ×e 0.036T <F<1×10 -11 ×e 0.0243T …Formula (1)

[0151] Furthermore, it is known that when the pulse is OFF and no carburizing gas is supplied, if the supply of carburizing gas is started at a time when the carburizing rate F to the workpiece 10 is within the range of Equation (2), the gas efficiency is good and the deviation in carburizing quality can be reduced.

[0152] 3×10 -5 ×e 0.0122T <F<0.0005×e 0.0108T …Formula (2)

[0153] This application claims priority based on Japanese Patent Application No. 2023-182280, filed on October 24, 2023, the entire contents of which are incorporated herein by reference.

[0154] Explanation of reference numerals in the attached figures

[0155] 10 workpieces

[0156] 20 Carburizing Furnace

[0157] 21 Supply Port

[0158] 22 Discharge outlets

[0159] 23 Heater

[0160] 24 thermocouples

[0161] 30 Acetylene

[0162] 40 Mass Flow Controller

[0163] 50 Exhaust valve

[0164] 60 Vacuum pump.

Claims

1. A carburizing method that uses carburizing gas, The carburizing method repeatedly supplies and stops the carburizing gas, starting and stopping the supply at times based on the carbon penetration rate of carbon through the carburizing gas into the workpiece.

2. The carburizing method according to claim 1, wherein, A range is set for the carbon penetration rate at which the supply begins or stops, and the supply is started or stopped when the carbon penetration rate is within the range.

3. The carburizing method according to claim 2, wherein, The range varies depending on the carburizing temperature of the workpiece.

4. The carburizing method according to claim 1, wherein, Let the carbon penetration rate be F [mg / m 2 ·sec], set the carburizing treatment temperature to T[K], While the supply is in progress, the supply is stopped when the carbon penetration rate F is within the range of the following formula (1). 9×10 -19 ×e 0.036T <F<1×10 -11 ×e 0.0243T …Formula (1).

5. The carburizing method according to claim 1, wherein, Let the carbon penetration rate be F [mg / m 2 ·sec], set the carburizing treatment temperature to T[K], In the case where the supply is being carried out while the supply is in progress, the supply is initiated when the carbon penetration rate F is within the range of the following equation (2). 3×10 -5 ×e 0.0122T <F<0.0005×e 0.0108T …Formula (2).

6. The carburizing method according to claim 1, wherein, The material being processed is chromium-containing alloy steel for mechanical structures.

7. The carburizing method according to claim 6, wherein, The alloy steel used in the mechanical structure is chromium steel.

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