Carburizing method

Abstract

This carburizing method uses a carburizing gas, and involves repeatedly supplying and halting the supplying of the carburizing gas. The start of the supplying and the halting of the supplying are each performed at a timing based on the carbon infiltration rate at which carbon enters a treatment target object by the carburizing gas.

Description

Carburizing Method

[0001] The present invention relates to a carburizing method.

[0002] A vacuum carburizing process has been known in which activated carbon penetrates and diffuses into the surface of steel by supplying carburizing gas into an evacuated furnace. For example, Patent Document 1 discloses a method for reducing the amount of carburizing gas supplied during this type of vacuum carburizing process, thereby reducing the waste of carburizing gas and reducing processing costs by decreasing the amount of carburizing gas supplied from the early to late stages of carburizing.

[0003] Japanese Patent Application Laid-Open No. 2005-350729

[0004] However, the method disclosed in Patent Document 1 has the problem that if the amount of carburizing gas supplied is reduced too much in the later stages of carburizing, the carburizing gas does not reach the entire furnace, resulting in large variations in carburizing quality.

[0005] The present invention has been made in consideration of the problems associated with the prior art as described above, and has as its object to provide a carburizing method that can reduce variation in carburizing quality without reducing the gas efficiency of the carburizing gas.

[0006] In order to achieve the above object, the present invention provides a carburizing method using a carburizing gas, in which the supply and stop of the carburizing gas are repeatedly performed, and the start and stop of the supply are performed at a timing based on the carbon penetration rate at which carbon penetrates into the workpiece by the carburizing gas.

[0007] In the present invention configured as described above, the supply of carburizing gas is started and stopped at a timing based on the rate at which carbon penetrates into the workpiece by the carburizing gas, so that the amount of carburizing gas supplied corresponds to the rate at which carbon penetrates into the workpiece. This reduces the amount of carburizing gas supplied and prevents it from becoming too small, thereby reducing variation in carburizing quality.

[0008] Further, a range of the carbon penetration rate when starting or stopping the supply may be set, and the supply may be started or stopped at a timing when the carbon penetration rate falls within the range.

[0009] The range may also be varied depending on the carburization temperature of the workpiece, in order to accommodate the fact that the higher the temperature, the faster the carbon penetrates.

[0010] Specifically, the carbon penetration rate is expressed as F [mg / m ２ ·sec] and the carburizing treatment temperature is T [K], the supply may be stopped at a timing when the carbon penetration rate F falls within the range of the following formula (1) while the supply is being performed.

[0011] 9 x 10 －１９ ×e ０．０３６Ｔ < F < 1 × 10 －１１ ×e ０．０２４３Ｔ ...Equation (1) Furthermore, the carbon penetration rate is expressed as F [mg / m ２ ·sec] and the carburizing treatment temperature is T [K], the supply may be started at a timing when the carbon penetration rate F falls within the range of the following formula (2) when the supply is started while the supply is stopped.

[0012] 3 x 10 －５ ×e ０．０１２２Ｔ <F<0.0005×e ０．０１０８Ｔ Formula (2) The workpiece may be a chromium-containing alloy steel for machine structures, and the alloy steel for machine structures may be a chromium steel.

[0013] According to the present invention, it is possible to reduce variations in carburizing quality without reducing the gas efficiency of the carburizing gas.

[0014] FIG. 1 is a diagram showing an example of an apparatus configuration for carrying out an embodiment of the carburizing method of the present invention. FIG. 2 is a diagram for explaining the action when carburizing gas is supplied in pulses. FIG. 3 is a diagram for explaining the action when carburizing gas is supplied in pulses. FIG. 4 is a flowchart for explaining the process from setting conditions to measuring the variation in carburizing quality in this embodiment. FIG. 5 is a graph showing an approximation curve connecting the set values ​​shown in Table 1. FIG. 6 is a graph showing an approximation curve connecting the set values ​​shown in Table 2. FIG. 7 is a diagram showing a calculation simulation using the set values ​​shown in Tables 1 and 2 and in FIGS. 4 and 5.

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0016] <Apparatus Configuration> FIG. 1 is a diagram showing an example of an apparatus configuration for carrying out an embodiment of the carburizing method of the present invention.

[0017] As shown in FIG. 1, this example includes a carburizing furnace 20, a mass flow controller 40, an exhaust valve 50, and a vacuum pump 60.

[0018] The carburizing furnace 20 houses the workpiece 10 to be treated. 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 outlet 22 through which components of the acetylene 30 supplied to the carburizing furnace 20 that do not penetrate the workpiece 10 are discharged as exhaust gas. Heaters 23 are arranged above and below the workpiece 10 within the carburizing furnace 20, and heat the interior of the carburizing furnace 20 to control the furnace atmosphere temperature, i.e., the carburizing temperature of the workpiece 10. A thermocouple 24 is also provided in the carburizing furnace 20 to monitor the temperature within the carburizing furnace 20. The heater 23 controls the temperature by heating the interior of the carburizing furnace 20 in accordance with the temperature monitored by the thermocouple 24.

[0019] The mass flow controller 40 controls the start / stop and supply flow rate of acetylene 30 into the carburizing furnace 20. Specifically, the mass flow controller 40 controls valve operation to start and stop the supply of acetylene 30 using a PLC (Programmable Logic Controller) or the like, and controls the acetylene 30 to be distributed into the furnace at a set flow rate when it is supplied.

[0020] The exhaust valve 50 is normally open, and discharges the components of the acetylene 30 supplied to the carburizing furnace 20 that have not penetrated into the workpiece 10 to the outside as exhaust gas.

[0021] The vacuum pump 60 draws a vacuum through the exhaust valve 50, thereby discharging the components of the acetylene 30 supplied to the carburizing furnace 20 that have not penetrated into the workpiece 10 to the outside as exhaust gas through the exhaust valve 50.

[0022] Using the apparatus configured as described above, vacuum carburizing treatment is performed on the workpiece 10. The carburizing gas supplied into the carburizing furnace 20 is not limited to acetylene 30. It may also be a hydrocarbon gas such as propane.

[0023] <Vacuum Carburizing Treatment> In the 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, the supplied acetylene 30 reacts with the workpiece 10, generating and penetrating carbon on the surface of the workpiece 10, and the carbon diffuses into the interior of the workpiece 10, hardening the surface of the workpiece 10.

[0025] On the other hand, the components of the acetylene 30 supplied to the carburizing furnace 20 that do not penetrate into the workpiece 10 are discharged to the outside as exhaust gas via the exhaust valve 50 .

[0026] 1, acetylene 30 is supplied as carburizing gas into the carburizing furnace 20 under the control of the mass flow controller 40. At this time, because the workpiece 10 is made of steel, the acetylene 30 supplied as carburizing gas reacts with the workpiece 10, causing carbon to adhere to and penetrate the surface of the workpiece 10. Meanwhile, the components of the acetylene 30 supplied to the carburizing furnace 20 that do not penetrate the workpiece 10 are discharged as exhaust gas via the exhaust valve 50.

[0027] Specifically, acetylene (C ２ H ２ ) 30 is supplied, the acetylene 30 undergoes a non-equilibrium reaction with the steel material (Fe) that becomes the workpiece 10, resulting in 2Fe + C ２ H ２ ⇒2[Fe+C]+H ２ As a result, carbon (C) is generated on the surface of the workpiece 10 and penetrates into the workpiece 10, and hydrogen (H ２ ) will be emitted as exhaust gas.

[0028] The carbon that adheres to and penetrates the surface of the workpiece 10 then diffuses inside the workpiece 10 .

[0029] This allows the surface of the workpiece 10 to be hardened.

[0030] <<Effect of Pulsed Vacuum Carburizing Treatment>> In the above-described vacuum carburizing treatment, the carburizing gas may be supplied in pulses.

[0031] 2A and 2B are diagrams for explaining the action when carburizing gas is supplied in pulses. Note that the pulse width and height in Fig. 2B are different from those shown in Fig. 2A in order to clearly show the action when carburizing gas is supplied for one pulse, but in reality, it is the same pulse as shown in Fig. 2A.

[0032] As shown in FIG. 2A, in the mass flow controller 40 shown in FIG. 1, acetylene 30 may be supplied to the carburizing furnace 20 for a certain period of time, and then the supply of acetylene 30 may be stopped for a certain period of time. This may be repeated in a pulsed manner to supply the carburizing gas.

[0033] At time t0, the pulse for supplying carburizing gas is turned ON and the carburizing gas begins 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 carburizing gas penetrates into the workpiece 10.

[0034] As carbon penetrates into the workpiece 10, the difference in carbon concentration between the surface of the workpiece 10 and the interior (near the surface) of the workpiece 10 becomes smaller, which makes it more difficult for carbon to penetrate into the workpiece 10.

[0035] Then, as it becomes more difficult for carbon to penetrate into the workpiece 10, the penetration rate of carbon into the workpiece 10 decreases, as shown by the solid line in FIG. 2B.

[0036] In this way, the process of supplying the carburizing gas with the pulse turned on to make the workpiece 10 permeate with carbon is the carburizing period.

[0037] Furthermore, the carbon that has formed on the surface of the workpiece 10 and penetrated therein diffuses inside the workpiece 10. On the other hand, when the pulse is turned OFF at time t1 and the supply of carburizing gas into the carburizing furnace 20 is stopped, the carburizing gas is no longer supplied to the workpiece 10.

[0038] When the pulse is OFF, no carburizing gas is supplied, and as shown by the solid line in FIG. 2B, the penetration rate of carbon from the surface to the interior of the workpiece 10 passes through an inflection point at time t1 and becomes "0".

[0039] In this way, the diffusion period is a process in which the carbon that has penetrated from the surface of the workpiece 10 is diffused inside the workpiece 10 without supplying carburizing gas to the workpiece 10. During the diffusion period, the carburizing gas is not supplied to the workpiece 10, and the carbon that has penetrated from the surface of the workpiece 10 diffuses inside the workpiece 10, thereby decreasing the carbon concentration inside the workpiece 10 (near the surface).

[0040] Thereafter, at time t2, the pulse is turned ON and the supply of carburizing gas into the carburizing furnace 20 is resumed. Since the carbon concentration inside (near the surface) of the workpiece 10 is low, the difference in carbon concentration between the surface of the workpiece 10 and the inside (near the surface) of the workpiece 10 becomes large.

[0041] Since the difference in carbon concentration between the surface of the workpiece 10 and the interior (near the surface) of the workpiece 10 is large, carbon easily penetrates from the surface of the workpiece 10, and as a result, the penetration rate of carbon into the workpiece 10 increases significantly at first, as shown by the solid line in Fig. 2B. Then, as carbon penetrates into the interior of the workpiece 10, the difference in carbon concentration between the surface of the workpiece 10 and the interior (near the surface) of the workpiece 10 decreases, and accordingly, the penetration rate of carbon into the workpiece 10 decreases, as shown by the solid line in Fig. 2B.

[0042] Thereafter, at time t3, the pulse is turned OFF, and by repeating this process, the effective depth of carbon in the workpiece 10 increases, as shown by the two-dot chain line in Fig. 2B. Here, the repetition of the ON and OFF states of the pulse includes the case where the pulse is turned ON once and OFF once.

[0043] As mentioned above, in conventional pulsed vacuum carburizing processes, the timing of turning on and off the pulses is uniquely determined, so the supply of carburizing gas is turned on or off regardless of the difference in carbon concentration between the surface of the workpiece 10 and the interior (near the surface) of the workpiece 10 and the resulting carbon penetration rate.

[0044] However, this may not be preferable in terms of gas efficiency for carburizing the workpiece 10 with carbon using the carburizing gas, or in terms of reducing variations in carburizing quality.

[0045] 2B, if the times t1 and t3 at which the pulses turn OFF are set to times at which the penetration rate approaches "0," then even if carburizing gas is supplied, carbon will not penetrate easily from the surface of the workpiece 10, which is undesirable in terms of gas efficiency. In this case, more carburizing gas than necessary will be adsorbed by objects other than the workpiece 10, such as jigs and the furnace inner walls, resulting in problems such as soot accumulation inside the furnace.

[0046] 2B, if the times t1 and t3 at which the pulses turn OFF are set to times when the penetration rate is not yet so low, the time during which the carburizing gas is supplied with the pulses ON will be shortened, causing the supply of carburizing gas to be stopped before the gas has permeated the entire interior of the carburizing furnace 20, resulting in greater variation in the carburizing quality between workpieces in the furnace, as will be described later.

[0047] Furthermore, in FIG. 2B, if the time t2 at which the pulse is turned 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 yet that large, then even if carburizing gas is supplied, it will be difficult for carbon to penetrate from the surface of the workpiece 10, which is undesirable in terms of gas efficiency.

[0048] On the other hand, if the pulse does not turn ON even after the difference in carbon concentration between the surface of the workpiece 10 and its interior (near the surface) becomes large enough after the pulse turns OFF (if time t2 in FIG. 2B is late), the time during which the pulse turns ON and the carburizing gas is supplied will be shortened. As a result, the supply of carburizing gas will be stopped before the carburizing gas has permeated the entire interior of the carburizing furnace 20, resulting in greater variation in the carburizing quality between workpieces in the furnace.

[0049] Therefore, in this embodiment, a range of penetration speed is set to determine the timing for supplying and stopping the supply of carburizing gas, and the supply of carburizing gas is started and stopped when the penetration speed of carbon into the workpiece 10 falls within that range.

[0050] <<Flow from Setting Conditions to Measuring Variation in Carburized Quality>> FIG. 3 is a flowchart for explaining the processing from setting conditions to measuring variation in carburized quality in this embodiment.

[0051] In the vacuum carburizing process in the above-described embodiment, first, conditions for supplying and stopping the supply of carburizing gas are set (step S1).

[0052] <Setting Conditions> As described above, in this embodiment, a range of the carbon penetration rate into the workpiece 10 is set to determine the timing for starting and stopping the supply of carburizing gas, and the supply of carburizing gas is started and stopped at times when the carbon penetration rate falls within that range. Therefore, a range of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas when the pulse is in the ON state and carburizing gas is being supplied is set, and a range of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas when the pulse is in the OFF state and carburizing gas is not being supplied is set. Note that, as described above, when no carburizing gas is being supplied, the carbon penetration rate into the workpiece 10 is "0." Therefore, the carbon penetration rate into the workpiece 10 for supplying carburizing gas when the pulse is in the OFF state and no carburizing gas is being supplied is set to the carbon penetration rate into the workpiece 10 estimated when carburizing gas is being supplied when no carburizing gas is being supplied. These set values ​​are conditions for performing the simulation described below, and the target workpiece 10 may be, for example, chromium steel SCr420, an alloy steel for mechanical structures. The workpiece 10 may be made of other materials besides chromium steel SCr420, such as chromium steel, chromium-molybdenum steel, manganese-chromium steel, nickel-chromium steel, nickel-chromium-molybdenum steel, aluminum-chromium-molybdenum steel, other alloy steels for mechanical structures, or carbon steels for mechanical structures. If the workpiece steel type used in the simulation and the actual carburizing treatment is the same, the accuracy of the carburizing treatment results calculated by the simulation matching the actual treatment results is increased. However, if the difference in structural composition is not significant, the accuracy of the simulation results reflecting the actual carburizing treatment results is considered to be increased even if the steel type used in the simulation and the actual carburizing treatment is different. In this example, chromium steel SCr420, an alloy steel for mechanical structures, was used as the target workpiece 10.

[0053] Here, the rate at which carbon penetrates into the workpiece 10 increases as the carburizing temperature of the workpiece 10 increases. Therefore, as described above, a range of the rate at which carbon penetrates into the workpiece 10 for supplying and stopping the supply of carburizing gas is set for each temperature.

[0054] First, the rate of carbon penetration into the workpiece 10 for stopping the supply of carburizing gas when the pulse is ON and carburizing gas is being supplied will be described.

[0055] Table 1 shows the carbon penetration rate, i.e., F, for turning the pulse OFF and stopping the supply of carburizing gas. ｏｆｆ 10 is a table showing the set values ​​of

[0056] The carbon penetration rate into the workpiece 10 that is expected to provide good gas efficiency and reduce variation in carburizing quality was set for each carburizing temperature of the workpiece 10. Note that this set value may be a value obtained from past cases, or a value calculated by simulation, etc.

[0057] At a temperature of 980°C (1253K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 148 (mg / m ２ At a temperature of 980° C. (1253 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 76 (mg / m ２ ・sec) was set.

[0058] At a temperature of 950° C. (1223 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 79 (mg / m ２ At a temperature of 950° C. (1223 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 31 (mg / m ２ ・sec) was set.

[0059] At a temperature of 950° C. (1223 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 77 (mg / m ２At a temperature of 950° C. (1223 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 30 (mg / m ２ ・sec) was set.

[0060] At a temperature of 950° C. (1223 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 82 (mg / m ２ At a temperature of 950° C. (1223 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 30 (mg / m ２ ・sec) was set.

[0061] At a temperature of 930° C. (1203 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 51 (mg / m ２ At a temperature of 930° C. (1203 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 17 (mg / m ２ ・sec) was set.

[0062] At a temperature of 930° C. (1203 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 53 (mg / m ２ At a temperature of 930° C. (1203 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 17 (mg / m ２ ・sec) was set.

[0063] At a temperature of 900° C. (1173 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 24 (mg / m ２ At a temperature of 900° C. (1173 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 4.3 (mg / m２ ・sec) was set.

[0064] At a temperature of 880° C. (1153 K), the upper limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse is set to 13 (mg / m ２ At a temperature of 880° C. (1153 K), the lower limit of the carbon penetration rate into the workpiece 10 for stopping the supply of carburizing gas by turning off the pulse was set to 2.3 (mg / m ２ ・sec) was set.

[0065] FIG. 4 is a graph showing an approximation curve connecting the set values ​​shown in Table 1.

[0066] As shown in FIG. 4, when the set values ​​(circles) of the upper limit values ​​shown in Table 1 are connected, an approximation curve as shown by the solid line is obtained. This approximation curve shows the rate of carbon penetration into the workpiece 10 as F [mg / m ２ sec], the carburizing temperature of the workpiece 10 is T [K], the Napier number is e, and F = 1 × 10 －１１ ×e ０．０２４３Ｔ This becomes:

[0067] 4, when the set values ​​of the lower limit values ​​(indicated by triangles in the figure) shown in Table 1 are connected, an approximation curve as shown by the broken line is obtained. This approximation curve shows the rate of carbon penetration into the workpiece 10 as F [mg / m ２ sec], the carburizing temperature of the workpiece 10 is T [K], the Napier number is e, and F = 2 × 10 －１８ ×e ０．０３６Ｔ This becomes:

[0068] As a result, when the pulse is ON and carburizing gas is being supplied, the carbon penetration rate F into the workpiece 10 is 2×10 －１８ ×e ０．０３６Ｔ < F < 1 × 10 －１１ ×e ０．０２４３Ｔ If the supply of carburizing gas is stopped at a timing within the range of the following formula (3), it is expected that gas efficiency will be good and variations in carburizing quality will be reduced.

[0069] The carbon penetration rate F into the workpiece 10 is F≧1×10－１１ ×e ０．０２４３Ｔ If the supply of carburizing gas is stopped in this case, gas efficiency will improve, but the time that the pulse is on will be shortened, and the supply of carburizing gas will be stopped before the carburizing gas has spread throughout the carburizing furnace 20, which is expected to result in greater variation in carburizing quality.

[0070] In addition, the carbon penetration rate F into the workpiece 10 is F≦2×10 －１８ ×e ０．０３６Ｔ If the supply of carburizing gas is stopped in this case, it is expected that the gas efficiency will decrease.

[0071] Next, the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas when the pulse is in the OFF state and no carburizing gas is being supplied will be described.

[0072] Table 2 shows the set values ​​of the carbon penetration rate into the workpiece for starting the supply of carburizing gas by turning on the pulse.

[0073] In this case as well, the carbon penetration rate into the workpiece 10 that is expected to have good gas efficiency and reduce the variation in carburizing quality was set for each carburizing temperature of the workpiece 10. Note that if this set value has been obtained from past cases, that value may be used.

[0074] At a temperature of 950°C (1223K), the upper limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on is set to 227 (mg / m ２ At a temperature of 950° C. (1223 K), the lower limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on was set to 127 (mg / m ２ ・sec) was set.

[0075] Furthermore, at a temperature of 930° C. (1203 K), the upper limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on is set to 183 (mg / m ２At a temperature of 930° C. (1203 K), the lower limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas by turning on the pulse was set to 103 (mg / m ２ ・sec) was set.

[0076] Furthermore, at a temperature of 900° C. (1173 K), the upper limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on is set to 132 (mg / m ２ At a temperature of 900° C. (1173 K), the lower limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on was set to 72 (mg / m ２ ・sec) was set.

[0077] Furthermore, at a temperature of 980° C. (1253 K), the upper limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on is set to 308 (mg / m ２ At a temperature of 980° C. (1253 K), the lower limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on was set to 190 (mg / m ２ ・sec) was set.

[0078] Furthermore, at a temperature of 880° C. (1153 K), the upper limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas with the pulse turned on is set to 105 (mg / m ２ At a temperature of 880° C. (1153 K), the lower limit of the carbon penetration rate into the workpiece 10 for starting the supply of carburizing gas by turning on the pulse was set to 55 (mg / m ２ ・sec) was set.

[0079] FIG. 5 is a graph showing an approximation curve connecting the set values ​​shown in Table 2.

[0080] As shown in FIG. 5, when the set values ​​(circles) of the upper limit values ​​shown in Table 2 are connected, an approximation curve as shown by the solid line is obtained. This approximation curve represents the rate of carbon penetration into the workpiece 10 as F [mg / m ２sec], the carburizing temperature of the workpiece 10 is T [K], and the Napier's number is e, F = 0.0004 × e ０．０１０８Ｔ This becomes:

[0081] 5, when the set values ​​of the lower limit values ​​(indicated by triangles in the figure) shown in Table 2 are connected, an approximation curve as shown by the broken line is obtained. This approximation curve shows the rate of carbon penetration into the workpiece 10 as F [mg / m ２ sec], the carburizing temperature of the workpiece 10 is T [K], the Napier number is e, and F = 4 × 10 －５ ×e ０．０１２２Ｔ This becomes:

[0082] As a result, when the pulse is OFF and no carburizing gas is supplied, the carbon penetration rate F into the workpiece 10 is 4×10 －５ ×e ０．０１２２Ｔ <F<0.0004×e ０．０１０８Ｔ ...It is expected that starting the supply of carburizing gas at a timing within the range of formula (4) will result in good gas efficiency and reduced variation in carburizing quality. In this case, when no carburizing gas is being supplied, the carbon penetration rate into the workpiece 10 is "0" as described above. Therefore, the carbon penetration rate into the workpiece 10 for supplying carburizing gas when the pulse is OFF and no carburizing gas is being supplied is set to the carbon penetration rate into the workpiece 10 estimated when carburizing gas is being supplied when no carburizing gas is being supplied.

[0083] The carbon penetration rate F into the workpiece 10 is F≧0.0004×e ０．０１０８Ｔ If the supply of carburizing gas is started in this case, the gas efficiency will improve, but it is expected that the processing time will simply become longer and the gas efficiency will reach saturation.

[0084] In addition, the carbon penetration rate F into the workpiece 10 is F≦4×10 －５ ×e ０．０１２２Ｔ If the supply of carburizing gas is started in this case, the rate of carbon penetration into the workpiece 10 will remain low, and it is expected that the gas efficiency will decrease.

[0085] In the vacuum carburizing process, a calculation simulation is then performed in which the supply and cessation of the carburizing gas supply is repeated until the target effective depth is reached, with the depth at which the carbon concentration is 0.35 mass % based on the set values ​​set as described above (step S2).

[0086] <Calculation Simulation> FIG. 6 is a diagram showing a calculation simulation using the set values ​​shown in Tables 1 and 2 and FIGS.

[0087] Based on the set values ​​shown in Tables 1 and 2 and Figures 4 and 5, a calculation simulation was carried out in which the supply and stop of the carburizing gas was repeated until the target effective depth was reached, with the depth at which the carbon concentration was 0.35 mass %. In this calculation simulation, when the pulse was on and carburizing gas was being supplied, the carbon penetration rate F shown by the solid line in Figure 6 was 2 x 10 －１８ ×e ０．０３６Ｔ < F < 1 × 10 －１１ ×e ０．０２４３Ｔ The supply of carburizing gas was stopped at the timing indicated by the dashed arrow in FIG.

[0088] In addition, the carbon penetration rate F into the workpiece 10, shown by the solid line in FIG. 6, estimated when carburizing gas is supplied while the pulse is OFF and no carburizing gas is supplied, is 4×10 －５ ×e ０．０１２２Ｔ <F<0.0004×e ０．０１０８Ｔ The supply of carburizing gas was started at the timing indicated by the solid arrow in FIG.

[0089] Then, for the pulses used to repeatedly supply and stop the supply of carburizing gas until the effective depth shown by the dashed line in FIG. 6 reached 0.35% or more of the surface carbon concentration, the period from the solid line arrow to the next dashed line arrow in FIG. 6 was determined as the ON time for supplying the carburizing gas, and the period from the dashed line arrow to the next solid line arrow in FIG. 6 was determined as the OFF time for stopping the supply of the carburizing gas.

[0090] In this way, the ON / OFF times of the pulses for supplying and stopping the supply of the carburizing gas were determined for each pulse (step S3).

[0091] <Measurement of Variation> Then, in the apparatus shown in FIG. 1, a vacuum carburizing process was performed using pulses having ON / OFF times actually determined by the above-mentioned simulation, and the variation in the carburizing quality during this process was measured (step S4).

[0092] To evaluate the variation in the workpieces 10, the surface carbon concentration (wt%) of the workpieces 10 placed at nine locations, eight corners and the center, in the carburizing furnace 20 was measured using an EPMA (Electron Probe Micro Analyzer), and the variation R (wt%) in the surface carbon concentration was evaluated. As a measuring instrument, for example, a JXA-8530F Field Emission Electron Probe Micro Analyzer (FE-EPMA) may be used.

[0093] Tables 3 and 4 show the experimental results of evaluating the variations in the set values ​​set in this embodiment. Note that the same numbers in Tables 3 and 4 indicate the same samples.

[0094]

[0095] In Table 3, the temperature indicates the carburizing temperature of the workpiece 10.

[0096] F ｏｎ indicates the carbon penetration rate at the moment when the supply of carburizing gas is started after the supply of carburizing gas has been stopped, from the second pulse onwards.

[0097] F ｏｆｆ indicates the rate of carbon penetration into the workpiece 10 when the supply of carburizing gas is stopped after the supply of carburizing gas is stopped.

[0098] The minimum ON time is the shortest ON time of the ON times of each pulse that is repeatedly turned ON / OFF until the target effective depth is reached. As mentioned above, if the ON time of the pulse is short, the supply of carburizing gas will be stopped before it has spread throughout the carburizing furnace, resulting in greater variation in the carburizing quality between workpieces in the furnace.

[0099] The total ON time indicates the sum of the ON times of each pulse that is repeatedly turned ON / OFF until the target effective depth is reached.

[0100] The processing time indicates the sum of the ON time and OFF time of each pulse, which is repeatedly turned ON / OFF until the target effective depth is reached.

[0101] The target effective depth indicates the target value of the effective depth at which the carbon concentration set in the carburizing treatment is 0.35 wt %.

[0102] In Table 4, the carburizing gas flow rate when on indicates the gas flow rate when the carburizing gas is being supplied.

[0103] The total carburizing gas flow rate is the amount obtained by integrating the theoretical carbon penetration rate shown by the solid line in FIG. 6 over time.

[0104] The gas efficiency relative value is calculated by dividing the total carburizing gas flow rate by the total on-time. This value indicates how much of the supplied carburizing gas was actually used to carburize the workpiece 10.

[0105] The surface carbon concentration variation refers to the variation in the surface carbon concentration at the nine points mentioned above, and is a value calculated by subtracting the lowest surface carbon concentration from the highest surface carbon concentration.

[0106] In determining the variation, if the variation in the surface carbon concentration was 0.1 wt % or less, the evaluation was rated as ◯ (good).

[0107] As shown in Table 3, for example, in sample No. 8, F ｏｆｆ to 51 (mg / m ２ · sec), the carbon penetration rate into the workpiece 10 when the carburizing gas is supplied is 51 (mg / m ２ The supply of carburizing gas was stopped at the timing when F ｏｎ to 183 (mg / m ２ · sec), the carbon penetration rate into the workpiece 10 is expected to be 183 (mg / m ２ The supply of carburizing gas was started at the timing when the temperature reached 1 / 2000 s.

[0108] Then, the variation in the surface carbon concentration was measured, and the variation was judged based on the results.

[0109] As a result, as shown in Tables 3 and 4, for Samples Nos. 1 to 6, 8 to 13, and 15 to 17, in which the carbon penetration rate into the workpiece 10 for supplying and stopping the carburizing gas was within the ranges indicated by the above-mentioned formulas 1 and 2, the variation was 0.1 wt% or less.

[0110] On the other hand, for samples Nos. 7', 14', and 18', in which the amount of carburizing gas supplied was reduced over time as in the conventional method, the variation was large, exceeding 0.1 wt %.

[0111] In this way, when the rate of carbon penetration into the workpiece 10 for starting and stopping the supply of carburizing gas is within the ranges shown in Equations 1 and 2 above, it is possible to reduce the variation in carburizing compared to when the amount of carburizing gas supplied is reduced over time.

[0112] In addition, for samples No. 7, 14, and 18, the variation was 0.1 wt% or less, which means that the carburizing variation can be reduced compared to when the amount of carburizing gas supplied is reduced over time. However, since the variation is not within the ranges shown in Equations 1 and 2 above, the gas efficiency is not good.

[0113] As described above, by supplying or stopping the supply of carburizing gas at a timing based on the penetration rate at which carbon penetrates into the workpiece 10 by the carburizing gas, it is possible to reduce variation in carburizing quality without reducing the gas efficiency of the carburizing gas.

[0114] In the examples shown so far, chromium steel SCr420, which is an alloy steel for mechanical structures, was used as the target workpiece 10. However, as described above, if the target workpiece 10 is to be applied to cases including other alloy steels for mechanical structures and carbon steels for mechanical structures, a simulation should be used to take into consideration the range of variation in the F value when applied to other steel types, and in the simulation, when the pulse is ON and carburizing gas is supplied, the carbon penetration rate F into the workpiece 10 is 9×10 －１９ ×e ０．０３６Ｔ < F < 1 × 10 －１１ ×e ０．０２４３Ｔ It has been found that if the supply of carburizing gas is stopped at a timing within the range of the following formula (1), the gas efficiency is good and the variation in carburizing quality can be reduced.

[0115] When the pulse is OFF and no carburizing gas is supplied, the carbon penetration rate F into the workpiece 10 is 3×10 －５ ×e ０．０１２２Ｔ <F<0.0005×e ０．０１０８Ｔ It was found that if the supply of carburizing gas is started at a timing within the range of the following formula (2), the gas efficiency is good and the variation in carburizing quality can be reduced.

[0116] 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.

[0117] REFERENCE SIGNS LIST 10 Workpiece 20 Carburizing furnace 21 Supply port 22 Discharge port 23 Heater 24 Thermocouple 30 Acetylene 40 Mass flow controller 50 Exhaust valve 60 Vacuum pump

Claims

1. A carburizing method using a carburizing gas, comprising repeatedly supplying and stopping the supply of the carburizing gas, the supply being started and stopped at a timing based on a carbon penetration rate at which carbon penetrates into a workpiece by the carburizing gas.

2. A carburizing method according to claim 1, wherein a range of the carbon penetration rate is set when starting or stopping the supply, and the supply is started or stopped at a timing when the carbon penetration rate falls 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 carbon penetration rate is expressed as F [mg / m ２ 2. The carburizing method according to claim 1, wherein the supply is stopped at a timing when the carbon penetration rate F falls within the range of the following formula (1) while the supply is being performed, where F is the carbon penetration rate F during the supply period and the carburizing treatment temperature is T [K]. －１９ ×e ０．０３６Ｔ <F<1×10 －１１ ×e ０．０２４３Ｔ ...Formula (1) 5. The carbon penetration rate is expressed as F [mg / m ２ 2. The carburizing method according to claim 1, wherein the supply is started at a timing when the carbon penetration rate F falls within the range of the following formula (2) when the supply is started while the supply is stopped, where the time t of the supply is [sec] and the carburizing treatment temperature is T [K]. －５ ×e ０．０１２２Ｔ <F<0.0005×e ０．０１０８Ｔ ...Formula (2) 6. The carburizing method according to claim 1, wherein the workpiece is a chromium-containing alloy steel for machine construction.

7. The carburizing method according to claim 6, wherein the mechanical structural alloy steel is a chromium steel.

Images