JP3960697B2 - Carburizing and carbonitriding methods - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carburizing and carbonitriding method used to harden the surface of steel machine parts such as gears, shafts, and cams, and to improve wear resistance and fatigue strength.
[0002]
[Prior art]
As a carburizing method as described above, the vacuum carburizing method is a method in which a workpiece is heated up to a predetermined carburizing temperature in a vacuum furnace and kept soaked, and then carburizing gas such as methane, propane or butane is used. Such a gaseous saturated hydrocarbon is introduced into the heating chamber, thermally decomposed, and the generated activated carbon is infiltrated into the surface of the steel part to be processed. Compared to conventional gas carburization, It has advantages such as high temperature and short time treatment, no need for shift furnace, easy atmosphere control, high concentration carburization, little intergranular oxidation, and intermittent operation.
[0003]
On the other hand, instead of saturated hydrocarbon gas such as methane or propane, etc. for the purpose of eliminating the harmful effects of soot generation (sooting), which is a problem of vacuum carburizing method using saturated hydrocarbon gas as described above. A vacuum carburizing method using a chain unsaturated hydrocarbon gas such as acetylene or ethylene has been proposed in JP-A-8-325701.
[0004]
That is, conventionally, chain unsaturated hydrocarbon gases such as acetylene and ethylene are more unstable than saturated hydrocarbon gases and are more actively decomposed than carburizing reactions. However, it has been recognized that it only generates soot and is completely unsuitable for carburizing gas. However, in saturated hydrocarbon gas, activated carbon decomposed in the furnace space other than the workpiece surface remains sooted. On the other hand, unsaturated hydrocarbon gas has the property of preferentially adsorbing on the workpiece surface, and since the number of hydrogen atoms is smaller than the number of carbon atoms, hydrogen when contacting the workpiece as carburized gas molecules In addition to less interference by gas molecules, the carburized gas molecules are unstable and chemically active, so they can easily react and decompose in a short time to supply atomic carbon to the workpiece surface. Contrary to, but rather is considered that the generation of soot can be prevented.
[0005]
[Problems to be solved by the invention]
However, in the vacuum carburizing method described in the above publication, the heating chamber is heated and carburized in a vacuum state of 1 kPa or less, so that an advanced sealing structure is required for the furnace body, and the equipment cost is increased. At the same time, since convection of atmospheric gas does not occur in the furnace, the heating rate is slow, especially when the workpiece charging density is high, and the workpiece is heated only by radiation from the heating element. Therefore, there is a problem that temperature unevenness is likely to occur. In addition to unsaturated hydrocarbon gas as carburizing gas, by supplying ammonia gas as nitrogen source, it is not impossible to perform carbonitriding treatment to infiltrate nitrogen simultaneously with carbon into the work surface. Not only is the pressure inside the furnace too low, it is technically difficult to supply ammonia gas, but under vacuum, after carbonitriding, the temperature is lowered to the quenching temperature and kept at the quenching temperature. Since nitrogen that has penetrated is lost, there is a problem that simultaneous treatment of carburizing and nitriding is practically difficult, and the solution of such a problem is a problem in vacuum carburizing treatment using the unsaturated hydrocarbon gas described above. It was.
[0006]
OBJECT OF THE INVENTION
The present invention has been made paying attention to the above problems in vacuum carburizing treatment using unsaturated hydrocarbon gas such as acetylene and ethylene as carburizing gas, and does not require a high-grade sealing structure as a vacuum furnace, An object of the present invention is to provide a carburizing process and a carbonitriding method that are excellent in temperature rise efficiency and that can simultaneously perform carburizing and nitriding.
[0007]
[Means for Solving the Problems]
In the carburizing method according to claim 1 of the present invention, the work housed in the heating chamber is heated to a predetermined carburizing temperature in a nitrogen atmosphere, and is 0.06 to 0.3% of the heating chamber capacity in a standard state. It is characterized by having a structure in which a quantity of chain unsaturated hydrocarbon gas is intermittently supplied into the heating chamber at intervals of 2 to 10 minutes and carburized, and such a structure in the carburizing treatment method is the conventional problem described above. As a means to solve the problem.
[0008]
In the carburizing method according to claim 2 as an embodiment of the carburizing method according to the present invention, the nitrogen atmosphere pressure during temperature raising and carburizing is set to 100 Torr or more. Similarly, the carburizing method according to claim 3 is used as an embodiment. Is characterized in that the diffusion after carburizing, the temperature lowering to the quenching temperature, and the soaking are maintained in a vacuum of 1 Torr or less.
[0009]
In the carbonitriding method according to claim 4 of the present invention, the work housed in the heating chamber is heated to a predetermined carbonitriding temperature in a nitrogen atmosphere, and then the chain unsaturated hydrocarbon gas and ammonia gas are intermittently supplied to the heating chamber. In the standard state, when the carbon nitride is supplied, the chain unsaturated hydrocarbon gas in an amount of 0.06 to 0.3% of the heating chamber capacity is intermittently supplied at intervals of 2 to 10 minutes . Such a structure in the carbonitriding method is a means for solving the above-described conventional problems.
[0010]
In the processing method according to claim 5 as an embodiment of the carbonitriding method according to the present invention, the nitrogen atmosphere pressure at the time of temperature rising and carbonitriding is 100 Torr or more. Similarly, the carbonitriding according to claim 6 is used as an embodiment. The processing method is characterized in that ammonia gas in an amount of 0.06 to 0.3% of the heating chamber capacity is intermittently supplied at intervals of 2 to 10 minutes in a standard state.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
In the carburizing treatment method according to the present invention, a work (object to be treated) in a heating chamber is heated to a predetermined carburizing temperature in a nitrogen atmosphere, and then a chain unsaturated hydrocarbon gas is intermittently supplied as a carburizing gas. In addition, ethylene, propylene, acetylene, methylacetylene, or the like can be used as the chain unsaturated hydrocarbon gas. Of the chain unsaturated hydrocarbon gases, acetylene is particularly preferable because it has a triple bond, is more active and is easily decomposed, in addition to being easily available. Moreover, it is possible to use not only a single type of gas but also a mixed gas of two or more types, for example, a mixed gas of ethylene and acetylene.
[0012]
FIG. 1 (a) shows an example of a heat pattern in the carburizing method according to the present invention. After the workpiece is charged into the heating chamber, the air in the heating chamber is purged and replaced with nitrogen. Then, the temperature rise to the carburizing temperature T1 is started. At this time, since the workpiece is heated in a nitrogen atmosphere, convection of nitrogen occurs, and forced stirring of the atmosphere by a fan in the furnace is also possible, so that the workpiece can be quickly charged even if the amount of workpiece charged in the heating chamber is large. The temperature can be raised, and the temperature unevenness due to the charging position can be eliminated.
[0013]
The carburizing temperature T1 can be shortened by setting the carburizing temperature T1 higher, but is generally set in the range of 850 ° C to 1030 ° C.
[0014]
When the furnace atmosphere temperature and the workpiece temperature reach a predetermined carburizing temperature T1, for example, acetylene gas is intermittently supplied as a chain unsaturated hydrocarbon gas for carburizing. As a supply amount at this time, acetylene gas (unsaturated hydrocarbon gas) in an amount of 0.06 to 0.3% of the heating chamber capacity is supplied at intervals of 2 to 10 minutes. This is because when the supply amount per one time is less than 0.06% of the heating chamber capacity or the supply interval exceeds 10 minutes, the amount of carburizing gas is insufficient and the carburization varies. Conversely, if the supply amount per time exceeds 0.3% of the heating chamber capacity or the supply interval is less than 2 minutes, the carburizing gas becomes excessive and sooting tends to occur. Because there is.
[0015]
At this time, the carburizing gas is intermittently supplied because the intrusion of carbon into the workpiece surface and the diffusion of carbon that has penetrated into the workpiece into the workpiece are alternately repeated, so that the abrupt carbon concentration on the surface is increased. This is because precipitation of cementite due to enrichment can be avoided.
[0016]
The carburizing time t1 is appropriately selected according to the intended carburized layer depth. Generally, a processing time of 10 hours or more is set when a large carburized layer depth is required, particularly 1 to 5 hours. Sometimes. Further, the pressure P1 of the nitrogen atmosphere in the temperature raising and carburizing process is basically atmospheric pressure (760 Torr), but it is set to an arbitrary pressure of 100 Torr or more as described in claim 2 in consideration of the subsequent process. Can be set. This is because if the nitrogen atmospheric pressure P1 at the time of temperature rise and carburization is less than 100 Torr, the effect of stirring the atmosphere by convection or a fan becomes insufficient and the temperature rise efficiency is lowered or temperature unevenness is eliminated. It can be difficult. In order to avoid air contamination, it is possible to set the pressure above atmospheric pressure, for example, about 860 Torr.
[0017]
When the carburization is completed, the process proceeds to the diffusion process, the lowering to the quenching temperature, and the holding process at the temperature, but the diffusion time t2 is set to be approximately the same time as the normal carburizing time t1 or slightly shorter. In addition, when the target carburizing depth is shallow, the temperature may be lowered to the quenching temperature without going through a diffusion process.
[0018]
The quenching temperature T2 is usually set to a temperature of 800 to 900 ° C. in consideration of the hardenability of the workpiece material.
[0019]
After completion of carburizing, the pressure P2 of the nitrogen atmosphere in the diffusion and quenching temperature and holding processes is most easily set to atmospheric pressure as in the heating and carburizing processes. As described, after carburizing, the nitrogen atmosphere pressure P2 is reduced to a vacuum of 1 Torr or less, so that foreign matters such as oxides and oil attached to the surface of the workpiece can be volatilized and removed. Can be made good.
[0020]
By maintaining the quenching temperature, the work whose temperature is uniformed is quenched in oil. The atmospheric pressure P3 at the time of quenching is normally performed at atmospheric pressure. However, quenching (reducing quenching) at an appropriate pressure lower than atmospheric pressure can change the cooling characteristics of the quenching oil. This makes it possible to adjust the hardness distribution of the hardened layer and to reduce burning distortion.
[0021]
That is, as for the atmospheric pressure in the carburizing treatment method according to the present invention, it is the simplest and most basic to carry out everything from the temperature rise to carburizing and quenching at atmospheric pressure (760 Torr). It is not limited only to atmospheric pressure, and there is no problem even if it is performed at a lower atmospheric pressure or a slightly higher atmospheric pressure. When performing the above-described reduced pressure quenching, the atmospheric pressures P1 and P2 from the temperature rise to the quenching can all be made to coincide with the quenching pressure P3. When moving to the quenching chamber, it is not necessary to adjust the pressure in the atmosphere of both chambers. Furthermore, there is no need to process under the same pressure from temperature rise to quenching. For example, by removing only the pressure P2 of the nitrogen atmosphere in the diffusion, temperature lowering, and holding processes, the deposits on the workpiece are volatilized and removed. Will be able to.
[0022]
In the carbonitriding method according to the present invention, as shown in FIG. 1 (b), an example of the heat pattern includes ammonia gas as a nitrogen source in addition to acetylene gas as a chain unsaturated hydrocarbon gas for carburization. Except for intermittent supply, there is essentially no difference from the carburizing method shown in FIG.
[0023]
That is, the carbonitriding temperature T3 is usually set in a range of 800 ° C. to 950 ° C., which is slightly lower than the carburizing temperature T1, and the quenching temperature T4 is selected from a temperature of 750 ° C. to 900 ° C. as in the carburizing process. .
[0024]
After charging the workpiece into the heating chamber and replacing the air in the heating chamber with nitrogen, the temperature rise to the carbonitriding temperature T3 is similarly started.
[0025]
When the furnace atmosphere temperature and the workpiece temperature reach a predetermined carbonitriding temperature T3, for example, acetylene gas as a chain unsaturated hydrocarbon gas for carburization and ammonia gas as a nitrogen source are intermittently supplied. . The supply amount of the carburizing gas at this time is not different from that in the carburizing process. As the supply amount of ammonia gas, as set forth in claim 6, like the carburizing gas, supplying amount of 0.06% to 0.3% of the heating chamber volume at 2-10 minute intervals To do. This is because when the ammonia gas supply amount per time is less than 0.06% of the heating chamber capacity, or when the supply interval exceeds 10 minutes, the nitrogen amount is insufficient and the nitridation amount varies. When the supply amount per unit exceeds 0.3% of the heating chamber capacity or the supply interval is less than 2 minutes, the amount of nitriding becomes excessive and voids are easily generated. In addition, the ammonia gas is intermittently supplied compared to the case where the same amount of ammonia gas is continuously supplied, because the ammonia concentration is temporarily increased at the time of supply, so that the nitriding rate is improved and the variation is less. By becoming. In addition, the intermittent supply of ammonia gas does not necessarily coincide with the supply amount and supply timing of the carburizing gas.
[0026]
The carbonitriding time t4 is normally set to 1 to 5 hours. Further, the pressure P4 of the nitrogen atmosphere in the temperature raising and carbonitriding process is basically atmospheric pressure (760 Torr) as in the case of the carburizing process, but as described in claim 5 , any pressure of 100 Torr or higher Can be set to Thus, since the nitrogen atmosphere pressure P4 during carbonitriding is relatively high, ammonia gas can be easily supplied into the heating chamber, and nitrogen that has entered the work surface is not released.
[0027]
When carbonitriding is completed, the temperature is lowered to the quenching temperature and is maintained at that temperature to eliminate the uneven temperature of the workpiece, but ammonia gas is also intermittently supplied during this period so that nitrogen that has entered the workpiece does not escape. It is desirable to do.
[0028]
By maintaining the quenching temperature, the workpiece having a uniform temperature is quenched in oil. As for the atmospheric pressure P5 at the time of quenching, similarly to the case of the carburizing treatment, it is possible to perform a reduced pressure quenching at a pressure lower than the atmospheric pressure. In this case, the atmospheric pressure from the temperature rise to the quenching holding is also possible. P4 may be made to coincide with the reduced pressure quenching pressure P5.
[0029]
【Example】
Hereinafter, the present invention will be described more specifically based on examples.
Example 1
FIG. 3 shows the structure of a furnace used in an embodiment of a carburizing and carbonitriding method according to the present invention. The carbonitriding furnace 1 shown in the figure includes a heating chamber 2 forming a furnace body and quenching. The front chamber 3 is provided with an oil tank 4 for use in the furnace. The front chamber 3 has an in-furnace transfer device 5 for moving the work placed on the tray between the front chamber 3 and the heating chamber 2, and carburizing or carburizing. An elevating device 6 for immersing the workpiece after nitriding in the quenching oil in the oil tank 4 is provided. Further, the front chamber 3 is provided with a tube heater 7 for heating the quenching oil in the oil tank 4 and a stirring fan 8 for circulating the quenching oil. A water-cooling device for cooling and a thermocouple for detecting the temperature of the quenching oil are provided.
[0030]
On the other hand, the heating chamber 2 has a capacity of 3 m ³ in this embodiment, and a radiant tube heater 10 as a heating source is detected inside the heating chamber 2 lined by the refractory 9 and the temperature in the furnace is detected. Thermocouple 11 and a fan 12 for forcibly agitating the atmosphere (nitrogen) in the furnace, so that temperature unevenness in the furnace can be eliminated and the temperature of the workpiece can be raised quickly. ing.
[0031]
Furthermore, the heating chamber 2 and the front chamber 3 are each provided with an evacuation device outside the figure, and can control the atmospheric pressure independently, and also through a gas control device (not shown) Source, acetylene source and ammonia source.
[0032]
Using the carbonitriding furnace 1 having such a structure, carburizing treatment was performed on a cylindrical specimen having a diameter of 15 mm and a height of 20 mm made of chromium molybdenum steel SCM415 defined in JIS G 4105, and the performance was investigated. .
[0033]
First, the entrance side vacuum door 3a of the front chamber 3 is opened, the cylindrical specimen is placed in the front chamber 3 in a state of being placed on a tray, the vacuum door 3a is closed, and the vacuum exhaust device is operated to operate the front chamber. 3 and the air in the heating chamber 2 are purged, nitrogen gas is introduced through a gas control device (not shown), and the pressure is restored to atmospheric pressure (P1 = 760 Torr), and the inside of the front chamber 3 and the heating chamber 2 is changed to nitrogen gas. Replaced. In the case of continuous operation, since the heating chamber 2 has already been replaced with nitrogen gas, it is only necessary to replace the front chamber 3.
[0034]
Next, with the inlet-side vacuum door 3a closed, the inner vacuum door 3b of the front chamber 3 and the heat-resistant door 2a of the heating chamber 2 are opened, and the in-furnace transfer device 5 is operated to remove the cylindrical specimen. After extruding together with the tray and inserting it into the heating chamber 2, the vacuum door 3b and the heat-resistant door 2a are closed, and the radiant tube heater 10 in the heating chamber 2 is energized while rotating the stirring fan 12, and the temperature rise is started. did.
[0035]
When the temperature of the heating chamber 2 reaches the carburizing temperature of 930 ° C. (T 1) in this embodiment, 6 NL of acetylene gas is supplied as a carburizing gas from the gas control device, and this is repeated 12 times every 5 minutes. (Carburization time t1 = 60 minutes). Without changing the atmospheric pressure P1 (P2 = 760 Torr), the test piece was further maintained at the carburizing temperature of 930 ° C. for 50 minutes (t 2), and then the quenching temperature in this example was lowered to 850 ° C. (T 2). This temperature was maintained for 20 minutes (t3) and waited until the temperature of the specimen became uniform.
[0036]
Then, with the inner vacuum door 3b and the heat-resistant door 2a opened, the in-furnace transfer device 5 is operated to pull out the test piece together with the tray from the heating chamber 2, and the elevating device 6 is lowered to remove the test piece from the oil tank. 4 was quenched (P3 = 760 Torr).
[0037]
As a result of measuring the hardness distribution with a 0.3 kg load using a micro Vickers hardness tester on the test piece subjected to such treatment, as shown in FIG. 3, the effective hardened layer depth (Hv550 or more) is 0.8 mm. It was found that a carburized layer with a) was formed.
Example 2
Using the carbonitriding furnace 1 shown in FIG. 3, carbonitriding was performed on a cylindrical test piece having a diameter of 16 mm and a height of 30 mm made of chromium molybdenum steel defined as SCM415 in JIS G 4105, and the hardness distribution was similarly obtained. Was measured.
[0038]
The operation procedure of the carbonitriding furnace 1 is basically the same as that of the first embodiment, the carbonitriding temperature T3 is 870 ° C., the carbonitriding time t4 is 2 hours, and the quenching temperature T4 is the same as the carbonitriding temperature T3. The temperature was set to 870 ° C., and after the carbonitriding time t 4, it was quenched in oil at 60 ° C. Note that the nitrogen atmosphere pressure P4 from the temperature rise to quenching and holding was set to atmospheric pressure (760 Torr), and the atmosphere pressure P5 at the time of quenching was reduced to 200 Torr.
[0039]
Then, after the atmospheric temperature of the heating chamber 2 reaches 870 ° C. which is the carbonitriding temperature T 3, the temperature is maintained for 30 minutes, and then, similar to the first embodiment, 6 NL of acetylene from the gas control device at a time. The gas was intermittently supplied 24 times every 5 minutes as a carburizing gas, and 5 NL ammonia gas was supplied intermittently every 5 minutes as a nitrogen source. The intermittent supply of ammonia gas was continued until just before quenching.
[0040]
FIG. 4 shows the result of measuring the hardness distribution of the test piece obtained by performing such treatment by the same method, and has an effective hardened layer depth (Hv550 or more) of 0.35 mm. It was confirmed that a carbonitriding layer was formed.
[0041]
【The invention's effect】
In the carburizing method according to claim 1 of the present invention, since the workpiece is heated to a predetermined carburizing temperature in a nitrogen atmosphere, it is not necessary to use a vacuum furnace having an advanced sealing structure, The temperature of the workpiece can be increased quickly by convection and agitation of atmospheric nitrogen, and even when the workpiece has a high density inside the furnace, uneven temperature of the workpiece can be eliminated. Carburizing gases such as acetylene and ethylene gas As the chain unsaturated hydrocarbon gas is intermittently supplied, carburization and diffusion are repeated alternately, and it is possible to effectively prevent cementite precipitation due to a temporary increase in the amount of carburization. An extremely excellent effect is possible. The unsaturated hydrocarbon gas is active and is preferentially adsorbed on the work and rapidly decomposes and reacts on the work surface, so that the generation of soot is reduced. And, in the standard state, the chain-like unsaturated hydrocarbon gas in an amount of 0.06 to 0.3% of the heating chamber capacity is intermittently supplied at intervals of 2 to 10 minutes, so carburizing unevenness and sooting occur. Carburizing treatment can be performed without causing it.
[0042]
In the carburizing method according to claim 2 as an embodiment of the carburizing method according to the present invention, since the nitrogen atmosphere pressure during temperature raising and carburizing is set to 100 Torr or more, the temperature rising rate of the workpiece is improved and temperature unevenness is eliminated. Similarly, in the carburizing treatment method according to claim 3 as an embodiment, the diffusion after carburizing, the temperature lowering to the quenching temperature, and the soaking are performed in a vacuum of 1 Torr or less. As a result, a further excellent effect is achieved in that dirt such as oil adhering to the workpiece is volatilized to improve the finished surface state.
[0043]
In the carbonitriding method according to claim 4 of the present invention, after heating the workpiece to the carbonitriding temperature in a nitrogen atmosphere, ammonia gas is intermittently supplied together with the chain unsaturated hydrocarbon gas into the heating chamber. As a result, ammonia gas can be easily supplied into the heating chamber, and nitrogen that has once entered the workpiece does not escape during cooling down and holding to the quenching temperature. Treatment and nitriding treatment can be performed at the same time , and in a standard state, the chain unsaturated hydrocarbon gas in an amount of 0.06 to 0.3% of the heating chamber capacity is intermittently supplied at intervals of 2 to 10 minutes. since it is, uneven carburization or nitriding unevenness, further results in an extremely excellent effect that Ru can be prevented the occurrence of sooting and nitrides.
[0044]
In the treatment method according to claim 5 as an embodiment of the carbonitriding method according to the present invention, the nitrogen atmosphere pressure at the time of carbonitriding is set to 100 Torr or more. As a result, the carbonitriding method according to the sixth aspect of the present invention is improved in terms of feedability and the effect of preventing nitrogen detachment. Since the ammonia gas is intermittently supplied at intervals of 2 to 10 minutes, it is possible to obtain an excellent effect of preventing the occurrence of carburizing unevenness, uneven nitriding, and sooting and nitride. it can.
[Brief description of the drawings]
FIG. 1 (a) is an explanatory diagram showing an example of a heat pattern in a carburizing method according to the present invention.
(B) It is explanatory drawing which shows an example of the heat pattern in the carbonitriding method concerning this invention.
FIG. 2 (a) is a front view showing the structure of a furnace used in an example of a carburizing and carbonitriding method according to the present invention.
(B) It is side sectional drawing of the furnace shown to Fig.2 (a).
FIG. 3 is a graph showing the hardness distribution in the vicinity of the surface of a test piece subjected to carburizing treatment according to the present invention.
FIG. 4 is a graph showing the hardness distribution in the vicinity of the surface of the test piece subjected to carbonitriding according to the present invention.

Claims (6)

After heating the workpiece stored in the heating chamber to a predetermined carburizing temperature in a nitrogen atmosphere, 2 to 10 % of chain unsaturated hydrocarbon gas in an amount of 0.06 to 0.3% of the heating chamber capacity is obtained in a standard state. A carburizing method characterized by intermittently supplying and carburizing into a heating chamber at minute intervals .   2. The carburizing method according to claim 1, wherein a nitrogen atmosphere pressure during temperature raising and carburizing is 100 Torr or more.   3. The carburizing method according to claim 1, wherein diffusion after carburizing, temperature lowering to quenching temperature, and soaking are performed in a vacuum of 1 Torr or less. After heating the workpiece housed in the heating chamber to a predetermined carbonitriding temperature in a nitrogen atmosphere, when the chain unsaturated hydrocarbon gas and ammonia gas are intermittently supplied into the heating chamber for carbonitriding , in a standard state, A carbonitriding method characterized by intermittently supplying chain unsaturated hydrocarbon gas in an amount of 0.06 to 0.3% of the heating chamber capacity at intervals of 2 to 10 minutes . 5. The carbonitriding method according to claim 4, wherein the nitrogen atmosphere pressure during temperature rising and carbonitriding is 100 Torr or more. The carbonitriding method according to claim 4 or 5 , wherein ammonia gas in an amount of 0.06 to 0.3% of the heating chamber capacity is intermittently supplied at intervals of 2 to 10 minutes in a standard state.

Images