JP2009114480A - Direct carburization method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct carburization method which prevents cementite from precipitating in the corner without adding an extra treatment step and can efficiently manufacture steel parts having superior toughness. <P>SOLUTION: This carburization method includes: placing steel parts 1 of an object to be treated in a closed vessel 2 into which a hydrocarbon-based gas of a carburization gas is introduced; and selectively heating a portion except a corner 1a of the steel parts 1 with high-frequency coils 3 and 4 integrated into the closed vessel 2, while heating the corner 1a by thermal conduction occurring in the periphery to control the temperature lower than that of peripheral flat portions. Thereby, the decomposition reaction on the surface of the corner 1a is inhibited, as a result, the carburization quantity at the corner 1a is reduced, and the precipitation of the cementite at the corner 1a is retarded. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a carburizing method for carburizing a steel part, and more particularly to a direct carburizing method in which a hydrocarbon gas is brought into contact with a steel part at a predetermined temperature.

  A direct carburizing method in which a hydrocarbon-based gas is introduced into a furnace, the gas is brought into contact with a steel part at a predetermined temperature to cause a decomposition reaction, and carbon generated by the decomposition reaction is infiltrated into the surface layer portion of the steel part. Has been known for some time. According to this direct carburizing method, carbon infiltrates into the surface layer portion of the steel part by a non-equilibrium reaction. is there.

  However, according to the direct carburizing method, since a non-equilibrium reaction is used, a certain amount of carbon permeates from the surface of the steel part per hour, and the steel part has a large surface area per unit volume. The amount of carbon intruding into the corners of this is larger than the surrounding flat part. As a result of an increase in the amount of carbon entering the corners, cementite is excessively deposited at the corners, and a large amount of the cementite remains in the hardened structure, leading to a reduction in the toughness of the steel part. In addition, as a direct carburizing method, there are atmospheric pressure carburizing performed under atmospheric pressure and vacuum carburizing performed under reduced pressure. In either case, cementite is easily deposited at the corners.

So, for example, in the thing of patent document 1, after performing a carburizing process (vacuum carburizing process), decarburizing gas is introduce | transduced in a furnace, a decarburizing process is performed, and the surface of a workpiece | work (steel part) is carried out. Cementite is reduced or eliminated.
JP 2004-115893 A

  However, as described in the above-mentioned Patent Document 1, the decarburization treatment after the carburization treatment requires an extra decarburization treatment step, and thus the special advantage of the direct carburization method that the treatment time is short is lost. As a result, a decline in production efficiency is inevitable.

  The present invention has been made in view of the above-described conventional problems, and the problem is to suppress the precipitation of cementite at the corners without adding an extra processing step, and to provide a steel part with excellent toughness. The object is to provide a direct carburizing method that can be produced efficiently.

In order to solve the above-described problems, the present invention is characterized in that the carburizing process is performed directly by lowering the temperature of the corners of the steel part lower than the other parts. In the direct carburization method performed in this way, the decomposition reaction on the surface of the corner is suppressed by lowering the temperature of the corner than the other portions, and the amount of carbon entering the corner is suppressed.
In the following, some aspects of the present invention will be illustrated and described in terms of items.

  (1) Heat the surface layer of a steel part in a sealed container into which hydrocarbon-based gas is introduced so that the corners are at a lower temperature than the part other than the corners. A direct carburizing method characterized in that the hydrocarbon gas is brought into contact to cause a decomposition reaction, and carbon generated by the decomposition reaction is infiltrated into a surface layer portion of a steel part.

  In the direct carburizing method described in the item (1), since the corner of the steel part is heated so as to have a lower temperature than the other portions, the decomposition reaction on the surface of the corner is suppressed, and the corner The amount of carbon permeating into the surface is suppressed. Thereby, precipitation of cementite at the corners is suppressed, and as a result, the amount of cementite remaining in the quenched structure is reduced, and a steel part having excellent toughness can be obtained.

  (2) The direct carburizing method according to claim 1, wherein a portion other than the corner portion of the steel part is selectively heated, and the corner portion is heated by heat transfer from the periphery.

  (3) The direct carburizing method according to claim 1, wherein the steel part is heated by changing heating energy between the corner portion and a portion other than the corner portion.

  In the present invention, as a method of heating the surface layer portion of the steel part so that the corner portion is at a lower temperature than the portion other than the corner portion, the corner portion is selected as described in the above (2). In particular, a method of heating a steel part, or a method of heating by changing heating energy between a corner and a portion other than the corner as described in the above (3) can be adopted. In the former case, the temperature of the corners only rises due to heat conduction from the surroundings, so the temperature of the corners is lower than the other parts, and in the latter case, the temperature of the corners depends on the choice of heating energy. Can be heated lower than the rest.

  (4) The direct carburizing method according to any one of (1) to (3), wherein high-frequency heating or laser heating is used for heating the steel part.

  In the present invention, the method for heating the surface layer portion of the steel part is arbitrary, but high-frequency heating by a high-frequency coil or laser heating by a laser beam can be used as described in the item (4).

  According to the direct carburizing method according to the present invention, since it is possible to suppress the precipitation of cementite in the corners without adding an extra processing step, it is possible to suppress the residual cementite in the hardened structure, and the steel has excellent toughness. Parts can be produced efficiently.

  In carrying out the direct carburizing method according to the present invention, the surface layer portion of the steel part is lower than the portion other than the corner portion in the sealed container into which the hydrocarbon-based gas as the carburizing gas is introduced. The steel part heated in this way is brought into contact with the hydrocarbon-based gas to cause a decomposition reaction, and carbon generated by this decomposition reaction is allowed to enter the surface layer of the steel part. .

  The grade of the steel parts to be treated according to the present invention is arbitrary, such as a low carbon steel having a carbon content of 0.5% or less and a machine such as a low alloy steel in which an alloy element such as chromium, molybdenum, nickel is added thereto. Structural hardened steel can be selected.

  Examples of the hydrocarbon gas used in the present invention include unsaturated hydrocarbon gases such as propane gas, acetylene gas, and ethylene gas. The carburizing treatment is performed by introducing these gases into a sealed container. In this case, the furnace pressure may be a normal pressure condition substantially equal to the atmospheric pressure or a reduced pressure condition lower than the atmospheric pressure. .

  FIG. 1 shows an example of a processing apparatus for carrying out this direct carburizing method. In the figure, 1 is a steel part to be treated, and 2 is a sealed container in which the steel part 1 can be stored. The steel part 1 has a cylindrical shape here, and in the sealed container 2, a first high-frequency coil 3 that heats the outer peripheral surface of the steel part 1 and a second that heats both ends of the steel part 1. High frequency coils 4 and 4 are incorporated. The length of the first high-frequency coil 3 is set so that a portion excluding the predetermined range L1 on both ends of the steel part 1 can be heated, and the second high-frequency coil 4 is formed on the outer peripheral side of the steel part 1. The length is set so that the part excluding the predetermined range L2 can be heated. Moreover, each high frequency coil 3 and 4 has a structure which has the injection nozzle for injecting a cooling liquid with respect to the steel component 1 here. On the other hand, in the sealed container 2, the gas pipe 5 for introducing hydrocarbon gas, the exhaust pipe 6 for discharging the air in the sealed container 2, and the coolant injected from the high-frequency coils 3 and 4 are discharged. For this purpose, a drainage pipe 7 is connected.

  In direct carburizing treatment of the steel part 1, the steel part 1 is placed in the sealed container 2 as shown in FIG. At this time, the steel part 1 is positioned with respect to the high frequency coils 3 and 4 so that the non-heating ranges L1 and L2 are left on both ends of the outer peripheral surface of the steel part 1 and on the outer peripheral side of the end face. In this state, a hydrocarbon-based gas as a carburizing gas is introduced into the sealed container 2 through the gas pipe 5, and the hydrocarbon-based gas is introduced into the sealed container 2 while discharging the air in the sealed container 2 from the exhaust pipe 6. Fill with gas.

  Thereafter, a high-frequency current is supplied to the high-frequency coils 3 and 4 to heat the steel part 1 at a high frequency. By this high-frequency heating, as shown in gray scale in FIG. 1, the surface layer portion S1 of the steel part 1 facing the first high-frequency coil 2 and the surface layer portion S2 facing the second high-frequency coil 3 are selectively selected. Heated, the corner 1a of the steel part 1 becomes a non-heated part. In this case, the corner portion 1a of the steel part 1 only rises in temperature due to heat conduction from the periphery, and therefore the temperature of the corner portion 1a is lower than the other portions.

  By heating the steel part 1 under the hydrocarbon-based gas described above, a decomposition reaction occurs on the surface of the steel part 1, and the carbon (C) generated by this decomposition reaction as shown in FIG. Infiltrate. At this time, since the temperature of the corner 1a of the steel part 1 is lower than the other portions, the decomposition reaction on the surface of the corner 1a is suppressed. As a result, the carbon intrusion amount (concentration) with respect to the corner portion 1a is suppressed, and precipitation of cementite at the corner portion 1a is suppressed. However, since this corner 1a has a larger surface area per unit volume than the peripheral flat part, it is balanced with the peripheral flat part when viewed as the absolute value of the amount of carbon intrusion, and on the surface layer part of the steel part 1 The carburized layer S having a substantially uniform carbon concentration is formed.

  In the present embodiment, after the carburizing process described above, the coolant is sprayed from the nozzles of the high-frequency coils 3 and 4 toward the steel part 1 and the water distribution pipe 7 is opened to drain the coolant. As a result, the surface of the steel part 1 is rapidly cooled, and the surface portion S of the steel part 1 undergoes martensitic transformation and hardens. That is, although the surface layer part of the steel part 1 is quenched, in the present invention, precipitation of cementite at the corner part 1a of the steel part 1 is suppressed during the above carburizing treatment, so that the corner part 1a is quenched. The amount of cementite remaining in the structure (martensite) is also reduced, and as a result, steel parts having excellent toughness can be obtained.

  In the above embodiment, the high-frequency coils 3 and 4 are arranged and heated so that portions other than the corner 1a of the steel part 1 can be selectively heated, but only the corner 1a of the steel part 1 is heated. A high-frequency coil that can be heated may be further provided, and the applied current (heating energy) to the high-frequency coil may be set smaller than the applied current to the high-frequency coils 3 and 4 to heat the entire steel part 1 at high frequency. . Also in this case, since the heating of the corner portion 1a can be suppressed, the amount of carbon entering the excessive portion 1a can be suppressed.

  Moreover, in the said embodiment, although the high frequency heating by the high frequency coils 3 and 4 was used for the heating of the steel component 1, laser heating can be used instead of this in this invention. In this case, for example, by incorporating the laser diode array and the cooling nozzle into the sealed container 1, it is possible to heat the corner 1a so that the temperature of the corner 1a is lower than the other portions.

  Here, as the type of the steel part to be processed as described above, a part in which the toughness of the corner portion is particularly important is selected. Examples of such steel parts include a gear 10 shown in FIG. 3 and a sliding sheave 15 for an automatic transmission (CVT) shown in FIG. Among these, in the case of the gear 10, if there is cementite in the tooth tip portion 11, defects easily occur. Therefore, the heating temperature of the tooth tip portion 11 is suppressed to suppress cementite precipitation. In the case of the CVT sliding sheave 15, the opening edge 15b of the shaft hole 15a, which is the sliding surface, is damaged by the moment generated by the repulsive force from the belt 17 interposed between the shaft sheave 16 and the shaft sheave 16. Since it becomes a problem, the heating temperature of the opening edge 15b is suppressed to suppress the precipitation of cementite.

  A test piece TP having a diameter D = 18 mm and a length L = 50 mm as shown in FIG. 5 is manufactured using the material of SCM420, and this is placed in a sealed container 2 similar to that shown in FIG. The parts other than the corners were selectively heated by 4 and carburized. Carburizing treatment uses propane gas as the carburizing gas (normal pressure), the maximum heating temperature by the high frequency coils 3 and 4 is 1100 ° C., and after carburizing treatment, the coolant is injected from the high frequency coils 3 and 4 onto the test piece TP. Then, quenching was performed, and then the process was shifted to perform tempering treatment at 180 ° C. Then, after all the treatments are completed, a spectroscopic sample (present invention material) A is taken from the corner of the test piece TP as shown by the dotted line in FIG. 5, and subjected to predetermined polishing and corrosion. The microstructure was observed with a microscope. For comparison, a microscopic sample (comparative material) was collected from the same part of a similar test piece that was carburized in the furnace as in the prior art, and the microstructure was observed in the same manner.

  FIG. 6 shows the observation result of the above-mentioned Miroku structure. Thus, in the material of the present invention in which the parts other than the corners are selectively heated and carburized, the presence of cementite is not recognized and a uniform martensite structure is obtained. In contrast, in the comparative material, a large number of coarse cementite F was observed, and it was confirmed that the method of the present invention was extremely useful for preventing precipitation of cementite at the corners.

It is a schematic diagram which shows an example of the apparatus structure used for implementation of the direct carburizing method which concerns on this invention. It is a schematic diagram which shows the model of the decomposition reaction by this direct carburizing method. It is a perspective view which shows the shape of the gearwheel which is one of the process targets of this invention. It is sectional drawing which shows the shape of the sliding king sheave for CVT which is one of the process targets of this invention. It is a schematic diagram which shows the implementation state of the Example of this invention. It is a microscope picture which shows the microstructure of the corner | angular part of the test piece obtained in the Example of this invention as contrasted with a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel part 1a Corner part of steel part 2 Airtight container 3, 4 High frequency coil S1, S2 High frequency heating part S Carburizing layer

Claims (4)

  The surface layer portion of the steel part is heated in a sealed container into which the hydrocarbon-based gas is introduced so that the corner portion has a lower temperature than the portion other than the corner portion, and the heated steel part is subjected to the hydrocarbon. A direct carburizing method characterized in that a decomposition reaction is caused by bringing a gas into contact, and carbon generated by the decomposition reaction is infiltrated into a surface layer portion of a steel part.   The direct carburizing method according to claim 1, wherein a portion other than the corner portion of the steel part is selectively heated, and the corner portion is heated by heat transfer from the periphery.   The direct carburizing method according to claim 1, wherein the steel part is heated by changing heating energy between a corner portion and a portion other than the corner portion.   The direct carburizing method according to any one of claims 1 to 3, wherein high-frequency heating or laser heating is used for heating the steel part.