EP1598440B1 - Gasaufkohlungsverfahren - Google Patents

Gasaufkohlungsverfahren Download PDF

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Publication number
EP1598440B1
EP1598440B1 EP03723223A EP03723223A EP1598440B1 EP 1598440 B1 EP1598440 B1 EP 1598440B1 EP 03723223 A EP03723223 A EP 03723223A EP 03723223 A EP03723223 A EP 03723223A EP 1598440 B1 EP1598440 B1 EP 1598440B1
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EP
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Prior art keywords
carburizing
gas
temperature
treatment object
upper limit
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French (fr)
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EP1598440A4 (de
EP1598440A1 (de
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Showa Koyo Thermo Systems Co. Ltd. TACHISATO
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JTEKT Thermo Systems Corp
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Koyo Thermo Systems Co Ltd
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Priority claimed from PCT/JP2002/005767 external-priority patent/WO2003104515A1/ja
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces

Definitions

  • the present invention relates to a method for gas carburizing steel parts used, for example, in the automobile industry or machine industry.
  • the carburizing treatment time can be shortened by raising the carburizing temperature.
  • the carburizing temperature that has been conventionally used in practice was less than the eutectic point temperature (in the case of an iron-carbon equilibrium diagram shown in Fig. 1 , it is the C point temperature which is 1147°C) at which the liquid phase is transformed into ⁇ iron and cementite, and usually was not higher than 1050°C.
  • the carburizing treatment was carried out till the concentration of carbon in the surface layer of the treatment object became higher than the final target value, and then diffusion treatment which caused the carbon in the surface layer of the treatment object to diffuse was carried out in a high-temperature atmosphere with a carbon concentration less than that of the carburizing atmosphere, thereby decreasing the carbon concentration in the surface layer to the final target value (Japanese Examined Patent Publication No. 1994-45868 ).
  • the carburizing temperature is limited to a temperature less than the eutectic point temperature, the diffusion rate of carbon atoms in the treatment object is restricted. Therefore, the carburizing time cannot be shortened significantly.
  • the problem is that the diffusion treatment requires a long time, thereby reducing productivity.
  • a method of gas carburizing according to an aspect of the present invention comprises the steps of predetermining limiting carburizing conditions at which the surface layer of a sample of a steel treatment object present in a carburizing atmosphere is austenitized without melting at a carburizing temperature which is not higher than a peritectic point temperature at which ⁇ iron and liquid phase are transformed into ⁇ iron and not less than a eutectic point temperature at which liquid phase is transformed into ⁇ iron and cementite; and gas carburizing the treatment object under carburizing conditions which are set so as not to contradict the limiting carburizing conditions, at a carburizing temperature which is not higher than the peritectic point temperature and not less than the eutectic point temperature, wherein the limiting carburizing conditions comprise an upper limit value of a partial pressure of carburizing gas in the carburizing atmosphere at which the surface layer of the sample is austenitized without melting.
  • the carburizing time is shortened by gas carburizing the treatment object which is austenitized by being heated to a highest possible temperature.
  • the hardened layer of the treatment object can be prevented from becoming shallow and have sufficient thickness within a short time.
  • the treatment object is austenitized when heated to a temperature above the GS line or ES line in the iron-carbon equilibrium diagram shown in Fig. 1 . If the concentration of carbon in the surface layer of the treatment object exceeds the JE line, melting is started in the surface layer.
  • the present inventor has found out that an upper limit value of a partial pressure of carburizing gas at which the surface layer of the treatment object is not melted exists under given carburizing conditions such as carburizing temperature and carburizing time, and that this upper limit value decreases with the increase in carburizing temperature and increases with the decrease in carburizing time. Because the carburizing temperature range in the method of gas carburizing in accordance with the present invention is from not higher than the peritectic point temperature (1494°C) to not less than the eutectic point temperature (1147°C), the carburizing time can be greatly reduced.
  • the partial pressure of carburizing gas is set to not higher than the predetermined upper limit value makes it possible to carry out carburizing at a high temperature without causing melting in the surface layer of the treatment object. As a result, the amount of consumed energy can be greatly reduced and energy can be saved. Furthermore, the gas carburizing process can be carried out in series with other processes, for example, machining processes or different heat treatment processes.
  • the limiting carburizing conditions comprise an upper limit value of carburizing temperature and an upper limit value of carburizing time at which the surface layer of the sample is austenitized without melting, the relationship between the upper limit value of the partial pressure of carburizing gas, the upper limit value of carburizing temperature, and the upper limit value of carburizing time is predetermined, and the partial pressure of carburizing gas, carburizing temperature, and carburizing time are set such that the carburizing conditions of the treatment object do not contradict the limiting carburizing conditions which satisfy the predetermined relationship.
  • the upper limit value of the partial pressure of carburizing gas, the upper limit value of carburizing temperature, and the upper limit value of carburizing time serving as the limiting carburizing conditions are interrelated and one of the upper limit values can be found by fixing the other two conditions among the partial pressure of carburizing gas, carburizing temperature, and carburizing time. As a result, the conditions providing for the fastest possible carburization can be easily set within a range in which the surface layer of the treatment object is not melted.
  • a method of gas carburizing according to another aspect of the present invention is characterized in that when gas carburization of a steel treatment object is carried out, a carburizing temperature is set at a temperature which is not higher than a peritectic point temperature (it is the J point temperature which is 1494°C in the case shown in Fig. 1 ) at which ⁇ iron and liquid phase are transformed into ⁇ iron and not less than a eutectic point temperature (it is the C point temperature which is 1147°C in the case shown in Fig.
  • a target value of carbon concentration in the surface of the treatment object is set at a value which is not higher than a value at which the surface of the treatment object is not melted at the set carburizing temperature, and a partial pressure of carburizing gas in the carburizing atmosphere is set at a value at which the carbon concentration in the surface of the treatment object can reach the set target value as a result of gas carburization carried out during a preset time.
  • the present invention is based on the discovery that when the carburizing temperature is set at a temperature which is not higher than the peritectic point temperature and is not less than the eutectic point temperature and the target value of carbon concentration in the surface of the steel treatment object is set at a value which is not higher than the value at which the surface of the treatment object is not melted at the set carburizing temperature, if the partial pressure of carburizing gas in the carburizing atmosphere is set at an appropriate value, then the carbon concentration in the surface of the treatment object can reach the set target value as a result of gas carburization and a sufficient carburizing depth can be obtained.
  • the hardened layer depth that required several hours to be attained by the conventional technology can be attained within a short time. Moreover an excess increase in the concentration of carbon in the surface is prevented. As a result, the carburizing time can be shortened significantly by raising the carburizing temperature. Moreover, because the concentration of carbon in the surface layer of the treatment object does not exceed the set target value, the carbon diffusion treatment process is unnecessary. As a result, productivity can be increased. Furthermore, the gas carburizing treatment process can be carried out in series with other heat treatment processes.
  • the treatment object is cooled without carrying out a diffusion treatment after the gas carburization has been carried out. It is also preferred that the treatment object is reheated after the cooling. The reheating is carried out, for example, by induction heating. It is also preferred that quenching treatment of the reheated treatment object is carried out. Cooling for the quenching treatment is carried out, for example, by oil cooling or gas cooling. In terms of shortening the carburizing time, it is preferred that the carburizing temperature is set at 1200°C or higher.
  • limiting carburizing conditions are predetermined, those conditions are set such that the surface layer of a sample of a steel treatment object present in a carburizing atmosphere is austenitized without melting at a carburizing temperature which is not higher than the peritectic point temperature at which ⁇ iron and liquid phase are transformed into ⁇ iron and is not less than the eutectic point temperature at which liquid phase is transformed into ⁇ iron and cementite.
  • the limiting carburizing conditions comprise the upper limit value of the partial pressure of carburizing gas in the carburizing atmosphere at which the surface layer of the sample is austenitized without melting.
  • the treatment object is austenitized when heated to a temperature above the GS line or ES line in the iron-carbon equilibrium diagram shown in Fig. 1 . If the concentration of carbon in the surface layer of the treatment object exceeds the JE line, melting is started in the surface layer.
  • An upper limit value of the partial pressure of carburizing gas at which the surface layer of the treatment object is not melted exists under given carburizing conditions such as carburizing temperature and carburizing time. Predetermining the limiting carburizing conditions comprising the upper limit value of the partial pressure of carburizing gas facilitates setting the partial pressure of carburizing gas and carburizing time.
  • the total pressure of carburizing atmosphere can be a normal pressure, or can be decreased or increased with respect to the normal pressure.
  • the entire carburizing atmosphere can be a carburizing gas, or a gas mixture of a carburizing gas and a dilute gas can be used as the carburizing atmosphere.
  • dilution is preferably carried out with an inert gas such as nitrogen gas or argon gas.
  • the method of the present invention is applicable to any steel provided that it can be austenitized at a temperature which is not higher than the peritectic point temperature and not less than the eutectic point temperature.
  • This steel can be not only a carbon steel but also an alloy steel.
  • heating of the treatment object and a sample thereof is carried out with means capable of high-speed heating of the surface layer thereof.
  • the heating is preferably carried out, for example, by induction heating or laser heating.
  • heating efficiency of the carburization object can be increased.
  • the carburizing treatment is simplified, quality control is facilitated.
  • the number of factors affecting the quality is small, even if quality problems such as spots, strains, or cracks in the treatment object are encountered, the causes thereof can be easily clarified.
  • a wall covering the carburizing treatment space can be a cold wall and a waste gas combustion apparatus is unnecessary; therefore, degradation of working conditions is prevented and initial investment is reduced, moreover the method is applicable to single-item and small-scale production and can be easily incorporated into a production line, for example, in-line treatment of the individual production can be carried out. Because the conventional carburizing treatment furnace equipped with thermally insulating walls is not required, furnace heating or seasoning become unnecessary and running cost can be reduced.
  • the gas carburizing is preferably carried out, while causing a carburizing atmosphere comprising the carburizing gas at a constant partial pressure to flow.
  • a carburizing atmosphere comprising the carburizing gas at a constant partial pressure to flow.
  • the method of gas carburizing according to the present invention can greatly improve productivity.
  • the gas carburizing apparatus for performing the method of the present invention shown in Fig. 2 comprises a vacuum container 1, a heating device 2, a vacuum pump 3 for reducing the pressure inside the vacuum container 1, and a gas source 4 for supplying gas for carburizing atmosphere into the vacuum container 1.
  • the heating device 2 carries out induction heating inside the vacuum container 1 with a coil 2a connected to a power source 7.
  • the output of the power source 7 to the coil 2a is variable.
  • thermocouple 6 is welded as a sensor for temperature detection to the surface layer of the sample 5' set in the heating device 2. Then, the pressure inside the vacuum container 1 is reduced by evacuating the vacuum container 1 with the vacuum pump 3. At this time, the pressure inside the vacuum container 1 is preferably about 27 Pa or less. Means for detecting the temperature is not limited to a thermocouple.
  • a gas for carburizing atmosphere is introduced from the gas source 4 into the vacuum container 1.
  • the vacuum container 1 is filled with the carburizing atmosphere and the total pressure of the carburizing atmosphere is raised.
  • the pressure of the carburizing atmosphere inside the vacuum container 1 is raised to about 80 kPa.
  • the carburizing atmosphere is composed of a carburizing gas and a dilute gas. No specific limitation is placed on the type of the carburizing gas or dilute gas.
  • the carburizing gas of the present embodiment is methane gas and the dilute gas is nitrogen gas. Using a hydrocarbon gas as the carburizing gas makes it possible to realize a non-oxidizing carburization.
  • the carburizing gas is not limited to hydrocarbon gases.
  • the carburizing atmosphere may also be composed only of a carburizing gas.
  • the gas for carburizing atmosphere is supplied from the gas source 4 into the vacuum container 1 at a constant flow rate, and the gas for carburizing atmosphere is released by the vacuum pump 3 at a constant flow rate.
  • the gas for carburizing atmosphere flows inside the vacuum container 1 at a constant flow rate of, for example, 0.5 L/min, and the total pressure of the carburizing atmosphere is maintained at, for example, about 80 kPa.
  • the carburizing atmosphere containing a carburizing gas at a constant partial pressure flows inside the vacuum container 1.
  • the partial pressure of carburizing gas is a value obtained by multiplying the total pressure of carburizing atmosphere inside the vacuum container 1 by a molar fraction or volume percent of the carburizing gas. Therefore, the set value of the partial pressure of carburizing gas can be adjusted by changing the total pressure of carburizing atmosphere inside the vacuum container 1 or by changing the flow rate ratio of the carburizing gas and dilute gas.
  • the sample 5' is heated with the heating device 2 to a set carburizing temperature.
  • the carburizing temperature is not higher than the peritectic point temperature at which ⁇ iron and liquid phase are transformed into ⁇ iron and not less than the eutectic point temperature at which liquid phase is transformed into ⁇ iron and cementite.
  • the set value of the carburizing temperature can be adjusted by changing the output of the heating device 2 to the coil 2a.
  • the upper limit value of the partial pressure of carburizing gas is predetermined as a limit carburizing condition at which austenitization is conducted without melting the surface layer of the sample 5'.
  • a method for predetermining the upper limit value of the partial pressure of carburizing gas by fixing the carburizing temperature and carburizing time was described hereinabove, but this method can be appropriately modified without departing from the essence of the present invention.
  • the upper limit value of carburizing time can be found by fixing the partial pressure of carburizing gas and carburizing temperature, or the upper limit value of carburizing temperature can be found by fixing the partial pressure of carburizing gas and carburizing time.
  • Fig. 3 shows an example of the relationship between the upper limit value of the partial pressure of carburizing gas, the upper limit value of carburizing temperature, and the upper limit value of carburizing time predetermined in the above-described manner.
  • the concentration of methane (vol%) in carburizing atmosphere corresponding to the partial pressure of carburizing gas is plotted on the ordinate in Fig. 3 , but the partial pressure of carburizing gas may also be represented by another.
  • the partial pressure of carburizing gas is equal to the total pressure of carburizing atmosphere, and therefore the methane flow rate can be used to represent the partial pressure of carburizing gas.
  • the upper limit value of the partial pressure of carburizing gas in the carburizing atmosphere decreases with the increase in carburizing temperature and increases with the decrease in carburizing time.
  • the relationship shown in Fig. 3 is merely an example, and this relationship differs depending on the arrangement of structural components of heating device 2, material and arrangement of sample 5' to the heating device 2, type of heating device 2, type and flow rate of carburizing gas, and the like.
  • Gas carburization of a steel treatment object 5 is thereafter carried out by using the above-described apparatus for gas carburization under the carburizing conditions which are set so as not to contradict the limiting carburizing conditions satisfying the predetermined relationship.
  • Carburization of the steel treatment object 5 can be carried out in the same manner as carburization of the sample 5'.
  • the steel treatment object 5 is set to the heating device 2, the air present inside the vacuum container 1 is evacuated with the vacuum pump 3, a gas for carburizing atmosphere is introduced into the vacuum container 1 from the gas source 4 to raise the pressure of the carburizing atmosphere to the set pressure, the gas for carburizing atmosphere is supplied at a constant flow rate from the gas source 4 into the vacuum container 1, and the gas for carburizing atmosphere is released at a constant flow rate with the vacuum pump 3.
  • the partial pressure of carburizing gas in the carburizing atmosphere inside the vacuum container 1 is set at not higher than the upper limit value that was predetermined as a limiting carburization condition. Then, the steel treatment object 5 is heated to the carburizing temperature with the heating device 2.
  • the carburizing temperature is set at a value which is not higher than the aforesaid peritectic point temperature and not less than the eutectic point temperature.
  • the carburizing temperature during heating of the sample 5' can be reproduced during heating of the treatment object 5 by controlling the heating device 2 in the same manner as during heating of the sample 5', and therefore, it is not necessary to weld the thermocouple 6 to the treatment object 5.
  • Gas carburization is carried out by holding the treatment object 5 for the set carburizing time under the set partial pressure of carburizing gas and set carburizing temperature.
  • the carburizing temperature is set at a temperature which is not higher than the peritectic point temperature at which ⁇ iron and liquid phase are transformed into ⁇ iron and not less than the eutectic point temperature at which liquid phase is transformed into ⁇ iron and cementite.
  • the target value of carbon concentration in the surface of the treatment object 5 is set at a value which is not higher than a value at which the surface of the treatment object 5 is not melted at the set carburizing temperature.
  • the partial pressure of carburizing gas in the carburizing atmosphere is set at a value at which the carbon concentration in the surface of the treatment object can reach the set target value as a result of gas carburization carried out during a preset period.
  • the set value of the partial pressure of carburizing gas corresponding to the carburizing time can be predetermined by experiments.
  • the set values of the carburizing time and partial pressure of carburizing gas are less than the aforesaid upper limit values corresponding to the set temperature. Therefore, the setting of the carburizing time and partial pressure of carburizing gas is facilitated by predetermining the aforesaid upper limit values.
  • the surface of the treatment object 5 starts melting at a carbon concentration of about 1.15 wt.% at a carburizing temperature of 1573 K
  • the relationship between the carburizing treatment time required for the surface of the treatment object 5 to start melting at a carburizing temperature of 1573 K, the partial pressure of carburizing gas, and the concentration of carbon in the surface is expressed by Fig. 5 from the relationship shown in Fig. 3 .
  • the concentration of methane in the carbon atmosphere should be set at less than 10 vol% if the carburizing time is 1 min, and the concentration of methane in the carbon atmosphere should be set at less than 3 vol% if the carburizing time is 10 min.
  • the surface of the treatment object 5 starts melting at a carbon concentration of about 0.9 wt.% at a carburizing temperature of 1623 K
  • the relationship between the carburizing treatment time required for the surface of the treatment object 5 to start melting at the carburizing temperature of 1623 K, the partial pressure of carburizing gas, and the concentration of carbon in the surface is shown by Fig. 6 from the relationship shown in Fig. 3 .
  • the concentration of methane in the carbon atmosphere should be set at less than 1.8 vol% if the carburizing time is 1 min, and the concentration of methane in the carbon atmosphere should be set at less than 0.8 vol% if the carburizing time is 10 min.
  • Gas carburizing is carried out by holding the treatment object 5 for a set carburizing time under the aforesaid set partial pressure of carburizing gas and set carburizing temperature. Once the set carburizing time has elapsed, the carburizing is stopped by stopping the supply of carburizing gas or terminating the heating with the heating device 2.
  • the carburizing time can be greatly shortened because the carburizing temperature range is set between not higher than the peritectic point temperature and not less than the eutectic temperature.
  • setting the partial pressure of carburizing gas to not higher than the predetermined upper limit value makes it possible to carry out the carburization at a high temperature without melting the surface layer of the steel treatment object 5.
  • the diffusion coefficient of carbon atoms in ⁇ iron is 3.59 x 10 -5 mm 2 /sec at a temperature of 1000°C, but increases to ten or more times, that is, 43 x 10 -5 mm 2 /sec at a temperature of 1300°C.
  • the migration speed of carbon atoms at a temperature of 1300°C is not less than tenfold that at a temperature of 1000°C. Therefore, the time required to obtain the desired carburization depth can be greatly reduced and the usual carburization depth can be obtained at a carburizing time of about 1 to 10 min. Moreover, because the concentration of carbon in the surface layer of the treatment object 5 does not exceed the set target value, a carbon diffusion treatment step becomes unnecessary. As a result, the carburizing treatment time can be greatly shortened and the productivity can be increased. Furthermore, the gas carburization treatment step can be carried out in series with other heat treatment steps.
  • the carburizing temperature at not less than 1200°C is preferred from the standpoint of shortening the carburizing time, and this temperature can be set at not less than 1300°. Furthermore, because gas carburization is carried out while causing the carburizing atmosphere comprising the carburizing gas at a constant partial pressure to flow inside the vacuum container 1, a constant partial pressure of carburizing gas can be maintained and the uniformity of the quality of treatment object 5 can be improved. Moreover, no soot generation was observed in the carburizing treatment implemented according to the present invention, and in this respect, too, the present invention is greatly superior to the conventional vacuum carburizing.
  • the treatment object 5 is cooled without carrying out the diffusion treatment.
  • the cooling method No specific limitation is placed on the cooling method, and natural cooling or a variety of forced cooling methods can be used.
  • the treatment object 5 subjected to gas carburization is preferably quenched by reheating after cooling and then rapidly cooling.
  • a secondary quenching may also be carried out by employing the primary cooling as a rapid cooling.
  • the atmosphere for carrying out the quenching is preferably a neutral protective atmosphere, that is, the atmosphere in which the treatment object is neither carburized nor decarburized at this temperature, but the treatment can be carried out in another atmosphere of such as inert gas.
  • the reheating temperature for quenching is set at not less than the temperature at which at least the surface layer of the treatment object 5 is austenitized above the GS line or ES line in the equilibrium diagram shown in Fig. 1 .
  • the limiting carburizing conditions were predetermined and gas carburization was carried out under the carburizing conditions that are set so as not to contradict the limiting carburizing conditions.
  • the treatment object 5 had a shape of right cylinder with a diameter of 10 mm and a length of 52 mm made of a aickel-chromium-molybdenum steel (Japanese Industrial Standard SNCM420).
  • the carburized treatment object 5 was naturally cooled inside the vacuum container 1, hardened, polished, and finish processed with a diamond paste with a particle size of 3 micrometers, followed by hardness measurements and structure observations.
  • the vacuum container 1 was purged prior to the carburization.
  • the carburizing gas was methane and the dilute gas was nitrogen.
  • the gas for carburizing atmosphere was caused to flow inside the vacuum container 1 at a constant flow rate of 0.5 L/min.
  • Hardening was carried out by holding the treatment object 5 for 10 min inside a quartz tube furnace kept at a temperature of 860°C in which the nitrogen gas atmosphere was flowed and then quenched into oil. It goes without saying that furnaces of other types, including the induction heating furnaces, can be used for the hardening. Fig.
  • FIG. 7 shows the relationship between the hardness (Hv) and the distance (depth) (mm) from the surface of the treatment object 5 that was gas carburized under the following carburizing conditions: carburizing temperature 1250°C, carburizing time 1 min, and concentration corresponding to partial pressure of methane, which is the carburizing gas, 40 vol%.
  • the target value of carbon concentration in the surface of the treatment object 5 was set at 1.4 wt.%.
  • Fig. 8 shows the relationship between the hardness (Hv) and the distance (depth) (mm) from the surface of the treatment object that was gas carburized under the following carburizing conditions: carburizing temperature 1300°C, carburizing time 1 min, and concentration of methane, which is the carburizing gas, 10 vol%.
  • the target value of carbon concentration in the surface of the treatment object 5 was set at 1.15 wt.%.
  • Fig. 9 shows the relationship between the hardness (Hv) and the distance (depth) (mm) from the surface of the treatment object 5 that was gas carburized under the following carburizing conditions: carburizing temperature 1250°C, carburizing time 10 min, and concentration corresponding to partial pressure of methane, which is the carburizing gas, 10 vol%.
  • the target value of carbon concentration in the surface of the treatment object 5 was set at 1.4 wt.%.
  • FIG. 10 shows the relationship between the hardness (Hv) and the distance (depth) (mm) from the surface of the treatment object that was gas carburized under the following carburizing conditions: carburizing temperature 1300°C, carburizing time 10 min, and concentration of methane, which is the carburizing gas, 3 vol%.
  • the target value of carbon concentration in the surface of the treatment object 5 was set at 1. 15 wt.%.
  • Figs. 7 through 10 confirm that the carburized layer with a sufficient effective carburization depth can be obtained within a carburizing time of 1 through 10 min.
  • the partial pressure of methane, which is a carburizing gas is obtained by multiplying the total pressure of carburizing atmosphere by the methane concentration.
  • the total pressure of carburizing atmosphere was about 80 kPa.
  • the metal structure prior to quenching of the surface layer of the treatment object 5 subjected to gas carburizing at a carburizing temperature of 1300°C for a carburizing time of 1 min in accordance with the present invention is shown in Fig. 11 .
  • the metallurgical structure after quenching is shown in Fig. 12 .
  • a coarse structure that was present prior to quenching is not present after the quenching, which confirms that quenching makes the coarse structure fine.
  • the treatment object 5 had a shape of right cylinder with a diameter of 10 mm and a length of 52 mm made of a nickel-chromium-molybdenum steel (Japanese Industrial Standard SNCM420) as used in the above-described embodiment.
  • the carburized treatment object 5 was naturally cooled inside the vacuum container 1, without being subjected to diffusion treatment, then was hardened, polished, and finish processed with a diamond paste with a particle size of 3 micrometers.
  • the vacuum container 1 was purged prior to the carburization.
  • the carburizing gas was methane and the dilute gas was nitrogen.
  • Hardening was carried out by holding the treatment object 5 for 10 min inside a quartz tube furnace kept at a temperature of 860°C in which nitrogen gas atmosphere was flowed and then quenched into oil. It goes without saying that furnaces of other types, including the induction heating furnaces, can be used for the hardening.
  • the carburizing temperature was 1300°C
  • the carburizing time was 1 min
  • the concentration corresponding to the partial pressure of methane, which is the carburizing gas, in the carburizing atmosphere was 10 vol%
  • the target value of carbon concentration in the surface of the treatment object 5 was 0.74 wt.%
  • the total pressure of carburizing atmosphere was 80 kPa
  • the gas for carburizing atmosphere was flowed inside the vacuum container 1 during the carburization at a constant flow rate of 0.5 L/min.
  • Fig. 13 shows the relationship between the concentration of carbon (wt.%) and the distance (depth) (mm) from the surface of the treatment object 5 obtained in the present example.
  • the depth to which the concentration of carbon was higher than 0.2 wt.% of the base metal was 0.9 mm, thereby confirming that the sufficient effective carburizing depth can be obtained without employing a diffusion treatment step. Furthermore, the results obtained were matched well with the results on the complete carburizing depth obtained from Fig. 8 .
  • Fig. 14 shows an example of the relationship between the concentration of carbon in the surface and carburizing time in the treatment object obtained by the conventional carburizing method, in which because the concentration of carbon exceeds the target value when only the carburizing treatment is carried out, a subsequent diffusion treatment has to be carried out.
  • the present invention makes it possible to shorten significantly the carburizing time by raising the carburizing temperature. Moreover, because the concentration of carbon in the surface layer of the treatment object 5 does not exceed the set target value, the carbon diffusion treatment becomes unnecessary and productivity can be increased.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

Claims (8)

  1. Gasaufkohlungsverfahren umfassend die Schritte:
    Vorherbestimmung von limitierenden Aufkohlungsbedingungen, bei denen eine Oberflächenschicht einer Probe eines zu behandelnden Gegenstandes aus Stahl, die in einer Aufkohlungsatmosphäre vorhanden ist, ohne Schmelzen bei einer Aufkohlungstemperatur austenitisiert wird, die nicht höher als die Temperatur eines peritektischen Punktes ist, bei der ein δ-Eisen und flüssige Phase in γ-Eisen transformiert wird und nicht geringer als die Temperatur des eutektischen Punktes ist, bei der eine flüssige Phase in γ-Eisen und Zementit transformiert ist; und
    Gasaufkohlung des zu behandelnden Gegenstandes unter Aufkohlungsbedingungen, die so gesetzt sind, daß sie den limitierenden Aufkohlungsbedingungen nicht widersprechen, bei einer Aufkohlungstemperatur, die nicht höher als die Temperatur des peritektischen Punktes und nicht niedriger als die Temperatur des eutektischen Punktes ist, wobei
    die limitierenden Aufkohlungsbedindungen umfassen:
    einen oberen Grenzwert eines Partialdrucks des Aufkohlungsgases in der Aufkohlungsatmosphäre, bei der die Oberflächenschicht der Probe ohne Schmelzen austenitisiert,
    einen oberen Grenzwert einer Aufkohlungstemperatur, bei der die Oberflächenschicht der Probe ohne Schmelzen austenitisiert und einen oberen Grenzwert der Aufkohlungszeit, bei der die Oberflächenschicht der Probe ohne Schmelzen austenitisiert;
    es wird eine Beziehung zwischen dem oberen Grenzwert des Partialdrucks des Aufkohlungsgases, dem oberen Grenzwert der Aufkohlungstemperatur und dem oberen Grenzwert der Aufkohlungszeit vorherbestimmt; und
    es werden ein Partialdruck des Aufkohlungsgases, eine Aufkohlungstemperatur und eine Aufkohlungszeit als ein Satz von Aufkohlungsbedingungen des zu behandelnden Gegenstandes eingestellt, so daß sie nicht den limitierenden Aufkohlungsbedingungen widersprechen, die die vorherbestimmte Beziehung erfüllen.
  2. Gasaufkohlungsverfahren nach Anspruch 1, bei dem der obere Grenzwert des Partialdrucks des Aufkohlungsgases, der obere Grenzwert der Aufkohlungstemperatur, der obere Grenzwert der Aufkohlungstemperatur und der obere Grenzwert der Aufkohlungszeit durch Fixieren von zweien der besagten Partialdrucke des Aufkohlungsgases, der Aufkohlungstemperatur oder der Aufkohlungszeit bestimmt werden.
  3. Gasaufkohlungsverfahren nach Anspruch 2, bei dem der obere Grenzwert des Partialdrucks des Aufkohlungsgases durch Fixieren der Aufkohlungstemperatur und der Aufkohlungszeit gefunden wird.
  4. Gasaufkohlungsverfahren nach Anspruch 1, bei dem
    ein Targetwert der Kohlenstoffkonzentration in der Oberfläche des Behandlungsobjekts bei einem Wert eingestellt wird, der nicht höher als der Wert ist, bei dem die Oberfläche des zu behandelnden Gegenstandes bei der eingestellten Aufkohlungstemperatur nicht geschmolzen wird; und
    ein Partialdruck des Aufkohlungsgases in der Aufkohlungsatmosphäre wird eingestellt bei einem Wert, bei dem die Kohlenstoffkonzentration in der Oberfläche des zu behandelnden Gegenstandes den gesetzten Targetwert als ein Ergebnis der Gasaufkohlung erreichen kann, die während einer vorgegebenen Zeit durchgeführt wird.
  5. Gasaufkohlungsverfahren nach Anspruch 4, bei dem der zu behandelnde Gegenstand ohne Durchführung einer Diffusionsbehandlung gekühlt wird, nachdem die Gasaufkohlung durchgeführt wurde.
  6. Gasaufkohlungsverfahren nach Anspruch 5, bei dem der zu behandelnde Gegenstand nach dem Kühlen wieder erhitzt wird.
  7. Gasaufkohlungsverfahren nach Anspruch 6, bei dem ein Härten des wieder erhitzten zu behandelnden Gegenstandes durchgeführt wird.
  8. Gasaufkohlungsverfahren nach einem der Ansprüche 4 bis 7, bei dem die Aufkohlungstemperatur auf 1200°C und höher eingestellt wird.
EP03723223A 2002-06-11 2003-04-28 Gasaufkohlungsverfahren Expired - Lifetime EP1598440B1 (de)

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PCT/JP2002/005767 WO2003104515A1 (ja) 2002-06-11 2002-06-11 ガス浸炭方法
WOPCT/JP02/05767 2002-06-11
JP2002342505 2002-11-26
JP2002342505 2002-11-26
PCT/JP2003/005422 WO2003104516A1 (ja) 2002-06-11 2003-04-28 ガス浸炭方法

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EP1598440B1 true EP1598440B1 (de) 2010-06-16

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WO2005003401A1 (ja) * 2003-07-03 2005-01-13 Koyo Thermo Systems Co., Ltd. ガス浸炭方法
JP4569181B2 (ja) * 2004-06-10 2010-10-27 株式会社Ihi 真空浸炭方法
CN1304624C (zh) * 2005-03-29 2007-03-14 大连华锐股份有限公司 机械零件表面补碳的工艺方法
US8123872B2 (en) 2006-02-22 2012-02-28 General Electric Company Carburization process for stabilizing nickel-based superalloys
JP7189115B2 (ja) * 2019-12-05 2022-12-13 株式会社日本テクノ ガス浸炭方法およびガス浸炭装置
CN113385763B (zh) * 2021-07-14 2022-08-26 成都共益缘真空设备有限公司 一种真空回流焊正负压结合焊接工艺

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JPH06192815A (ja) * 1992-12-28 1994-07-12 Kawasaki Steel Corp 金属帯の連続浸炭設備
JP3241491B2 (ja) * 1993-06-29 2001-12-25 大同特殊鋼株式会社 高温高速回転用転がり軸受
JP3385722B2 (ja) * 1994-06-15 2003-03-10 住友金属工業株式会社 浸炭焼入方法
JPH0959756A (ja) * 1995-08-22 1997-03-04 Kobe Steel Ltd 耐高面圧浸炭部品の製法
JP3329210B2 (ja) * 1996-10-16 2002-09-30 住友金属工業株式会社 肌焼鋼の製造方法及びその方法により製造された肌焼鋼
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JPWO2003104516A1 (ja) 2005-10-06
EP1598440A4 (de) 2008-06-18
AU2003235955A1 (en) 2003-12-22
KR100592757B1 (ko) 2006-06-26
KR20040106279A (ko) 2004-12-17
WO2003104516A1 (ja) 2003-12-18
CN1257305C (zh) 2006-05-24
JP3696614B2 (ja) 2005-09-21
CN1545566A (zh) 2004-11-10
EP1598440A1 (de) 2005-11-23

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