Carburizing method

Abstract

To reduce variation in carburization quality without deteriorating gas efficiency of a carburization gas.SOLUTION: 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.SELECTED DRAWING: Figure 2

Description

[Technical field]

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

[0002] Conventionally, a vacuum carburizing process has been known in which activated carbon is infiltrated and diffused on the surface of steel by supplying carburizing gas into an evacuated furnace. In this type of vacuum carburizing, a method is disclosed in, for example, Patent Document 1, in which the amount of carburizing gas supplied is reduced from the early to late carburizing stages to reduce the waste of carburizing gas and reduce processing costs. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2005-350729 A Summary of the Invention [Problem to be solved by the invention]

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

[0005] The present invention has been made in consideration of the problems associated with the conventional techniques as described above, and has an object to provide a carburizing method that can reduce the variation in carburizing quality without reducing the gas efficiency of the carburizing gas. [Means for solving the problem]

[0006] In order to achieve the above object, the present invention provides A carburizing method using a carburizing gas, comprising the steps of: The supply and stop of the carburizing gas is repeated, and the start and stop of the supply are performed at a timing based on a 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 the amount of carburizing gas supplied 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 infiltration rate becomes.

[0010] Specifically, the carbon penetration rate is expressed as F [mg / m 2 ·sec], carburizing 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 continued.

[0011] 9×10 -19 ×e 0.036T <F<1×10 -11 ×e 0.0243T ...Equation (1) The carbon penetration rate is expressed as F [mg / m 2 ·sec], carburizing 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] 3×10 -5 ×e 0.0122T <F<0.0005×e 0.0108T ...Equation (2) The workpiece may be a chromium-containing alloy steel for machine construction, and the alloy steel for machine construction may be a chromium steel. Effect of the Invention

[0013] According to the present invention, the variation in carburizing quality can be reduced without reducing the gas efficiency of the carburizing gas. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing an example of an apparatus configuration for carrying out an embodiment of a carburization method of the present invention. [Diagram 2] FIG. 4 is a diagram for explaining an action when a carburizing gas is supplied in a pulsed manner. [Diagram 3] 4 is a flowchart for explaining a process from setting conditions to measuring the variation in carburization quality in this embodiment. [Figure 4] 1 is a graph showing an approximation curve connecting the setting values ​​shown in Table 1. [Diagram 5] 1 is a graph showing an approximation curve connecting the setting values ​​shown in Table 2. [Figure 6] FIG. 6 is a diagram showing a calculation simulation using the setting values ​​shown in Table 1 to FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0016] 《Device 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 accommodates 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 exhaust port 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. In the carburizing furnace 20, heaters 23 are arranged above and below the workpiece 10, respectively, to heat the inside of the carburizing furnace 20 and control the furnace atmosphere temperature, i.e., the carburizing temperature of the workpiece 10. In addition, the carburizing furnace 20 is provided with a thermocouple 24, which monitors the temperature inside the carburizing furnace 20. The heater 23 heats the inside of the carburizing furnace 20 in accordance with the temperature monitored by the thermocouple 24 to control the temperature.

[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 programmable logic controller (PLC) 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 exhausts components of the acetylene 30 supplied to the carburizing furnace 20 that do not penetrate 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 do not penetrate into the workpiece 10 as exhaust gas to the outside through the exhaust valve 50.

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

[0023] <Vacuum carburizing treatment> In the vacuum carburization process, first, acetylene 30 is supplied as a carburization gas into the carburization 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. As the carbon diffuses into the interior of the workpiece 10, the surface of the workpiece 10 hardens.

[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, since the workpiece 10 is made of steel, carbon adheres to and penetrates the surface of the workpiece 10 due to a reaction between the acetylene 30 supplied as the carburizing gas and 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, when acetylene (C2H2) 30 is supplied into the carburizing furnace 20, the acetylene 30 undergoes a non-equilibrium reaction with the steel material (Fe) that becomes the workpiece 10, 2Fe+C2H2⇒2[Fe+C]+H2 As a result, carbon (C) is generated on the surface of the workpiece 10 and penetrates into the workpiece 10, and hydrogen (H2) is discharged as exhaust gas.

[0028] The carbon that adheres to and penetrates the surface of the workpiece 10 will then diffuse 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-mentioned vacuum carburizing process, the carburizing gas may be supplied in pulses.

[0031] Figure 2 is a diagram for explaining the action when carburizing gas is supplied in pulses. Note that in Figure 2(b), the width and height of the pulse are different from those shown in Figure 2(a) in order to clearly show the action when carburizing gas is supplied for one pulse, but in reality, it is the same pulse as that shown in Figure 2(a).

[0032] As shown in FIG. 2(a), 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 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, and as a result, it becomes more difficult for carbon to penetrate into the workpiece 10.

[0035] Then, since 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. 2(b).

[0036] In this manner, the process in which the pulse is turned on and the carburizing gas is supplied to cause carbon to penetrate into the workpiece 10 corresponds to the carburizing period.

[0037] Furthermore, the carbon that has been generated on the surface of the workpiece 10 and has 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 in the OFF state, no carburizing gas is supplied, so that the penetration rate of carbon from the surface to the inside of the workpiece 10 becomes “0” after passing through the inflection point at time t1, as shown by the solid line in FIG. 2(b).

[0039] In this manner, 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, while no carburizing gas is supplied to the workpiece 10, the carbon that has penetrated from the surface of the workpiece 10 diffuses inside the workpiece 10, 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 inside (near the surface) of the workpiece 10 is large, carbon easily penetrates from the surface of the workpiece 10, and the penetration rate of carbon into the workpiece 10 increases significantly at first, as shown by the solid line in Fig. 2(b). Then, as carbon penetrates into the inside of the workpiece 10, the difference in carbon concentration between the surface of the workpiece 10 and the inside (near the surface) of the workpiece 10 becomes smaller, and accordingly, the penetration rate of carbon into the workpiece 10 decreases, as shown by the solid line in Fig. 2(b).

[0042] After that, at time t3, the pulse is turned OFF, and by repeating this, the effective depth of carbon in the workpiece 10 becomes deeper, as shown by the two-dot chain line in Fig. 2(b). 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 described above, in conventional vacuum carburizing treatment using pulses, the timing of turning on and off the pulses is uniquely determined, so the supply of carburizing gas is started or stopped regardless of the difference in carbon concentration between the surface of the workpiece 10 and the inside (near the surface) of the workpiece 10 and the associated carbon penetration speed.

[0044] However, this may not be preferable in terms of gas efficiency for carburizing the workpiece 10 with carbon by the carburizing gas and reduction of variation in carburizing quality.

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

[0046] 2(b), if the times t1 and t3 at which the pulses turn OFF are set to times at which the penetration speed is not yet so low, the time during which the carburizing gas is supplied while the pulses are ON will be shortened, which will cause 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 described below.

[0047] Furthermore, in FIG. 2(b), if the time t2 at which the pulse turns ON is set to a time at which 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 very large, then even if carburizing gas is supplied, carbon will have difficulty penetrating 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 the inside (near the surface) of the workpiece 10 becomes large enough after the pulse turns OFF (time t2 in FIG. 2(b) is late), the time during which the pulse turns ON and the carburizing gas is supplied will be short. As a result, the supply of the carburizing gas will be stopped before the carburizing gas has spread throughout the carburizing furnace 20, resulting in large variations in the carburizing quality between the 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 the variation in carburizing quality>> FIG. 3 is a flow chart for explaining the process from setting the conditions to measuring the variation in carburization quality in this embodiment.

[0051] In the vacuum carburization process in the above-described embodiment, first, conditions for starting 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 to start and stop the supply of the carburizing gas, and the supply of the carburizing gas is started and stopped at the timing when the carbon penetration rate falls within the range. Therefore, a 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 being supplied, and a 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 being supplied, are set. In addition, in a state in which the carburizing gas is not being supplied, the carbon penetration rate into the workpiece 10 is "0" as described above. Therefore, 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 being supplied, the carbon penetration rate into the workpiece 10 estimated when the carburizing gas is supplied in a state in which the carburizing gas is not being supplied is set. In addition, this set value is a condition for performing a simulation described later, and the target workpiece 10 may be, for example, a workpiece made of chromium steel SCr420, which is an alloy steel for mechanical structures. In addition, the workpiece 10 to be treated may be, in addition to the above-mentioned chromium steel SCr420, other chromium steel, chromium molybdenum steel, manganese chromium steel, nickel chromium steel, nickel chromium molybdenum steel, aluminum chromium molybdenum steel, other alloy steel for mechanical construction, carbon steel for mechanical construction, etc. If the workpiece steel type in the simulation and the actual carburizing treatment is the same, the probability that the result of the carburizing treatment calculated by the simulation matches the actual treatment result will increase, but if the difference in structural composition is not large, it is considered that the probability that the simulation result will be reflected in the actual carburizing treatment result will increase even if the steel type in the simulation and the actual carburizing treatment is different. In this example, chromium steel SCr420, which is an alloy steel for mechanical construction, is used as the target workpiece 10.

[0053] Here, the higher the carburizing temperature of the workpiece 10, the faster the carbon penetration rate into the workpiece 10. Therefore, as described above, the range of the carbon penetration rate into the workpiece 10 for starting and stopping the supply of carburizing gas is set for each temperature.

[0054] First, 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 will be described.

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

[0056] [Table 1] The carbon penetration rate into the workpiece 10, which is assumed to have good gas efficiency and to reduce the 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 (1253K), 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 (1223K), 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] In addition, at a temperature of 950°C (1223K), 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 2 At a temperature of 950°C (1223K), 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 (1223K), 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] In addition, at a temperature of 930°C (1203K), 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 (1203K), 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 (1203K), 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 (1173K), 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 / m 2 -sec) was set.

[0064] In addition, at a temperature of 880°C (1153K), 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 (1153K), 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, by connecting the upper limit setting values ​​(circles in the figure) shown in Table 1, an approximation curve as shown by the solid line is obtained. This approximation curve shows the carbon penetration rate into the workpiece 10 as F [mg / m 2 ·sec], the carburizing temperature of workpiece 10 is T [K], and the Napier number is e, F = 1 × 10 -11 ×e 0.0243T It becomes.

[0067] In addition, as shown in FIG. 4, by connecting the set values ​​of the lower limit values ​​(△ in the figure) shown in Table 1, an approximation curve as shown by the dashed line is obtained. This approximation curve shows the carbon penetration rate into the workpiece 10 as F [mg / m 2 ·sec], the carburizing temperature of the workpiece 10 is T [K], and the Napier's number is e, F=2×10 -18 ×e 0.036T It 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 ...Equation (3) It is expected that if the supply of carburizing gas is stopped at a timing within this range, gas efficiency will be good and variation in carburizing quality can 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, the gas efficiency will improve, but the time that the pulse is in the ON state will be shortened, and the supply of carburizing gas will be stopped before the carburizing gas has permeated the entire inside of 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 the carburizing gas is not 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] [Table 2] 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, etc., 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 was set to 227 (mg / m 2 In addition, at a temperature of 950°C (1223K), 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] In addition, at a temperature of 930°C (1203K), 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 2 In addition, at a temperature of 930°C (1203K), 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 103 (mg / m 2 -sec) was set.

[0076] In addition, at a temperature of 900°C (1173K), 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 In addition, at a temperature of 900°C (1173K), 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] In addition, at a temperature of 980°C (1253K), 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 2In addition, at a temperature of 980°C (1253K), 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] In addition, at a temperature of 880°C (1153K), 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 In addition, at a temperature of 880°C (1153K), 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 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, by connecting the upper limit setting values ​​(circles in the figure) shown in Table 2, an approximation curve as shown by the solid line is obtained. This approximation curve shows the carbon penetration rate into the workpiece 10 as F [mg / m 2 ·sec], the carburizing temperature of the workpiece 10 is T [K], and the Napier's number is e, F = 0.0004 x e 0.0108T It becomes.

[0081] In addition, as shown in FIG. 5, by connecting the set values ​​of the lower limit values ​​(△ in the figure) shown in Table 2, an approximation curve as shown by the dashed line is obtained. This approximation curve shows the carbon penetration rate into the workpiece 10 as F [mg / m 2 ·sec], the carburizing temperature of the workpiece 10 is T [K], and the Napier's number is e, F=4×10 -5 ×e 0.0122T It becomes.

[0082] As a result, when the pulse is OFF and no carburizing gas is being supplied, the carbon penetration rate F into the workpiece 10 is 4×10-5 ×e 0.0122T <F<0.0004×e 0.0108T ...Equation (4) If the supply of carburizing gas is started at a timing within this range, it is expected that the gas efficiency will be good and the variation in carburizing quality can be reduced. In this case, when carburizing gas is not being supplied, the carbon penetration rate into the workpiece 10 is "0" as described above. Therefore, as the carbon penetration rate into the workpiece 10 for supplying carburizing gas when the pulse is in the OFF state and carburizing gas is not being supplied, the carbon penetration rate into the workpiece 10 estimated when carburizing gas is supplied in a state where carburizing gas is not being supplied is set.

[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 gas efficiency will reach saturation simply because the treatment time is longer.

[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 at which carbon penetrates 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 (step S2) in which the supply and cessation of the carburizing gas supply is repeated until the 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 being the target effective depth.

[0086] <Computational Simulation> FIG. 6 is a diagram showing a calculation simulation using the set values ​​shown in Tables 1 to 5.

[0087] Based on the set values ​​shown in Table 1 to Figure 5, a calculation simulation was performed 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 speed F shown by the solid line in Figure 6 was 2×10 -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 being 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 Figure 6, within the range of

[0089] Then, for the pulse for repeatedly supplying and stopping the carburizing gas until the effective depth, shown by the dashed line in FIG. 6, becomes 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 time of the pulse for supplying and stopping the supply of the carburizing gas was determined for each pulse (step S3).

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

[0092] To evaluate the variation of the workpieces 10, the surface carbon concentration (wt%) of the workpieces 10 placed at nine locations, 8 corners and the center, in the carburizing furnace 20 was measured using an EPMA (Electron Probe Micro Analyzer) to evaluate the variation R (wt%) of the surface carbon concentration. For example, a JXA-8530F field emission electron probe microanalyzer (FE-EPMA) may be used as the measuring device.

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

[0094] [Table 3]

[0095] [Table 4] 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 is stopped from the second pulse onwards.

[0097] F off indicates the rate at which carbon penetrates into the workpiece 10 when the supply of carburizing gas is stopped from a state in which the carburizing gas is being supplied.

[0098] The minimum ON time refers to the shortest ON time among 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 the gas has permeated the entire 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 time of each pulse that is repeatedly turned ON / OFF until the target effective depth is reached.

[0100] The treatment time indicates the sum of the ON and OFF times 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 Figure 6 with respect to time.

[0104] The gas efficiency relative value is calculated by total carburizing gas flow rate / (on carburizing gas flow rate x 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. This is a value calculated by subtracting the lowest surface carbon concentration from the highest surface carbon concentration.

[0106] In terms of the variation judgment, 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 was 51 (mg / m 2 The supply of carburizing gas was stopped at the timing of F on to 183 (mg / m 2·sec), the expected carbon penetration rate into the workpiece 10 when carburizing gas is supplied while the supply of carburizing gas is stopped is 183 (mg / m 2 The supply of carburizing gas was started at the timing of 0.5 V. sec.

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

[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 due to the supply and stopping of the carburizing gas was within the ranges shown in the above-mentioned equations 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, far exceeding 0.1 wt%.

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

[0112] In addition, for samples No. 7, 14, and 18, the variation is less than 0.1 wt%, which means that the carburizing variation can be reduced compared to when the amount of carburizing gas supplied is reduced over time. However, since it does not fall within the ranges shown in the above formulas 1 and 2, 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 speed 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 construction, was used as the target workpiece 10. However, as described above, if it is intended to apply the present invention to cases in which other alloy steels for mechanical construction or carbon steels for mechanical construction are used as the target workpiece 10, a simulation is used to take into consideration the range of variation in the F value when applied to other steel types, and in the above simulation, when the pulse is ON and carburizing gas is being supplied, the carbon penetration rate F into the workpiece 10 is 9×10 -19 ×e 0.036T <F<1×10 -11 ×e 0.0243T ...Equation (1) It was found that if the supply of carburizing gas is stopped at a timing within this range, gas efficiency is good and variation in carburizing quality can be reduced.

[0115] In addition, when the pulse is OFF and no carburizing gas is being supplied, the carbon penetration rate F into the workpiece 10 is 3×10 -5 ×e 0.0122T <F<0.0005×e 0.0108T ...Equation (2) It was found that if the supply of carburizing gas is started at a timing within this range, gas efficiency is good and the variation in carburizing quality can be reduced. [Explanation of symbols]

[0116] 10 Work 20 Carburizing furnace 21 Supply port 22 Outlet 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 the steps of: 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 a carbon penetration rate at which carbon penetrates into the workpiece by the carburizing gas.

2. 2. The carburization method according to claim 1, further comprising: setting a range of the carbon penetration rate when starting or stopping the supply; and starting or stopping the supply 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 defined as F [mg / m ２ ·sec], carburizing temperature is T [K], 2. The carburization method according to claim 1, wherein the supply is stopped at a timing when the carbon penetration rate F falls within a range of the following formula (1) while the supply is being continued: 9×10 －１９ ×e ０．０３６Ｔ <F<1×10 －１１ ×e ０．０２４３Ｔ ・・・Formula (1)

5. The carbon penetration rate is defined as F [mg / m ２ ·sec], carburizing temperature is T [K], 2. The carburization method according to claim 1, wherein the supply is started at a timing when the carbon penetration rate F falls within a range of the following formula (2) when the supply is started while the supply is stopped: 3×10 －５ ×e ０．０１２２Ｔ <F<0.0005×e ０．０１０８Ｔ ・・・Formula (2)

6. 2. 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.

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