CARBURIZATION METHOD

Abstract

 A carburization method includes: a carburizing step of performing a carburization process on an object to be processed that is made of carbon steel; and a diffusing step of performing a diffusion process to diffuse carbon that is supplied onto a surface of the object to be processed at the carburizing step into an inner portion of the object to be processed. In the carburization method, at the diffusing step, as a surface carbon concentration of the object to be processed decreases as a result of internal diffusion of the carbon, a diffusion processing temperature is increased continuously or in stages within a temperature range that exceeds a carburization processing temperature and falls below a solidus temperature of yFe when the surface carbon concentration is reduced in an equilibrium diagram of carbon steel.

Description

[Cross-Reference to Related Applications]

[0001] The present application is based on on and claims the benefit of priority from Japanese Patent Application No. 2017-209610, filed October 30, 2017, 2017, the description of which is incorporated herein by reference.

[Technical Field]

[0002] The present disclosure relates to a carburization method.

[Background Art]

[0003] Conventionally, for example, as a carburization method for an object to be processed that is composed of carbon steel, an atmospheric gas carburization method, a vacuum carburization method, an atmospheric pressure direct carburization method, and the like are known.

[0004] In the atmospheric gas carburization method, a carburization process is performed using mixed gas and raw material gas that are generated in a conversion furnace under atmospheric pressure. In the vacuum carburization method, a carburization process is performed using only raw material gas under reduced pressure. In the atmospheric pressure direct carburization method, a carburization process is performed using raw material gas and diluting gas under atmospheric pressure.

[0005] In these carburization methods, ordinarily, a diffusion process is performed after a carburizing step to diffuse carbon that has been supplied onto the surface of the object to be processed into an inner portion of the object to be processed.

[0006] For example, PTL 1 discloses a carburization method in which, after the carburizing process is performed for a predetermined amount of time, in a state in which a processing temperature is maintained, a diffusing process in which the carburized carbon is diffused into the inner portion of the object to be processed is performed.

[Citation List]

[Patent Literature]

[0007] [PTL 1] JP-A-2012-25998

[Summary of Invention]

[0008] The carburization methods described above are generally restricted by temperature. Therefore, processing time of a certain amount or more is required. Consequently, in terms of improving productivity and the like, shortening of the processing time is desired. In the above-described carburization methods, carbon is solid-soluted and diffused from the surface of the object to be processed. Therefore, processing time can be shortened by a carburization processing temperature and a diffusion processing temperature being increased. However, when the processing temperature becomes equal to or higher than a eutectic temperature of carbon steel, the object to be processed may melt.

[0009] Therefore, in actual production, the carburization processing temperature and the diffusion processing temperature are both ordinarily set to be equal to or lower than the eutectic temperature of carbon steel. In addition, the diffusion process is performed in a state in which the carburization processing temperature is maintained as is. Therefore, shortening of the diffusion processing time is also difficult.

[0010] An object of the present disclosure is to provide a carburization method that is capable of shortening processing time while suppressing melting of an object to be processed that is composed of carbon steel.

[0011] An aspect of the present disclosure provides a carburization method including: a carburizing step of performing a carburization process on an object to be processed that is composed of carbon steel; and a diffusing step of performing a diffusion process to diffuse carbon that is supplied onto a surface of the object to be processed at the carburizing step into an inner portion of the object to be processed, wherein: at the diffusing step, as a surface carbon concentration of the object to be processed decreases as a result of internal diffusion of the carbon, a diffusion processing temperature is increased continuously or in stages within a temperature range that exceeds a carburization processing temperature and falls below a solidus temperature of yFe when the surface carbon concentration is reduced in an equilibrium diagram of the carbon steel.

[0012] At the diffusion step, as the surface carbon concentration of the object to be processed decreases as a result of internal diffusion of the carbon, a melting point of a surface layer portion of the object to be processed increases. In the above-described carburization method, advantage is taken of this phenomenon, and the diffusion processing temperature is increased within the above-described specific temperature range as the surface carbon concentration of the object to be processed decreases as a result of internal diffusion of the carbon.

[0013] Consequently, in the above-described carburization method, shortening of the processing time through shortening of the diffusion processing time can be achieved while suppressing melting of the object to be processed that is made of carbon steel.

[Brief Description of Drawings]

[0014] The above-described object, other objects, characteristics, and advantages of the present disclosure will be further clarified through the detailed description herebelow, with reference to the accompanying drawings. The drawings are as follows:

Fig. 1 is an explanatory diagram schematically showing a heat treatment pattern in a carburization method according to a first embodiment;

Fig. 2 is an explanatory diagram schematically showing an equilibrium diagram of carbon steel that composes an object to be processed that is used in the carburization method according to the first embodiment;

Fig. 3 is a graph of a relationship between a carbon concentration and a distance in a depth direction from a surface of the object to be processed after a carburization process during fabrication of sample 1;

Fig. 4 is a diagram of a heat treatment pattern during fabrication of sample 1;

Fig. 5 illustrates, by (a), an outer appearance photograph of sample 1 and, by (b), a cross-sectional photograph of sample 1;

Fig. 6 is a diagram of a heat treatment pattern during fabrication of sample 1C; and

Fig. 7 illustrates, by (a) is an outer appearance photograph of sample 1C and, by (b), a cross-sectional photograph of sample 1C.

[Description of Embodiments]

(First embodiment)

[0015] A carburization method according to a first embodiment will be described with reference to Fig. 1 and Fig. 2. As shown in Fig. 1, the carburization method according to the present embodiment includes a carburizing step and a diffusing step. The carburizing step is a step of performing a carburization process on an object to be processed that is made of carbon steel. The diffusing step is a step of diffusing carbon that is supplied onto the surface of the object to be processed in the carburizing step into an inner portion of the object to be processed.

[0016] Specifically, the carburizing step and the diffusing step can be a carburizing step and a diffusing step in a vacuum carburization method or an atmospheric pressure direct carburization method. In the vacuum carburization method, the carburization process is performed using only raw material gas under reduced pressure. In addition, in the atmospheric pressure direct carburization method, the carburization process is performed using raw material gas and diluting gas under atmospheric pressure.

[0017] In an atmospheric gas carburization method, to maintain the atmosphere, improving a carburization processing temperature and a diffusion processing temperature is difficult. In this regard, in the vacuum carburization method and the atmospheric pressure direct carburization method, the carburization processing temperature and the diffusion processing temperature can be more easily improved compared to that in the atmospheric gas carburization method. Therefore, in the above-described configuration, shortening of the processing time can be more easily achieved.

[0018] For example, as the raw material gas that is used in the vacuum carburization method and the atmospheric pressure direct carburization method, hydrocarbon gas, such as CH₄, C₂H₂, C₃H₈, and C₄H₁₀, and the like can be used. As the diluting gas used in the atmospheric pressure direct carburization method, N₂ and the like can be given. Here, the above-described diluting gas serves a purpose of keeping the raw material gas at or below exploding pressure, suppressing generation of soot, ensuring a flow amount for supplying the raw material gas to the object to be processed, and the like.

[0019] For example, in terms of an austenite formation temperature and the like, a lower limit of a carburization temperature at the carburizing step can preferably be set to an A3-line temperature or higher. An upper limit of the carburization temperature at the carburizing step may be selected from temperatures that are lower than a eutectic temperature of the carbon steel composing the object to be processed, so that decomposition of the raw material gas does not occur. The upper limit of the carburization temperature at the carburizing step can preferably be set to be equal to or lower than 1100 °C and, more preferably, equal to or lower than 1050 °C.

[0020] Here, for example, in a temperature increasing step that is performed before the carburizing step, the object to be processed can be increased in temperature by heater heating or the like.

[0021]  Here, in the carburization method according to the present embodiment, at the diffusing step, the diffusion processing temperature is increased continuously or in stages as a surface carbon concentration of the object to be processed decreases as a result of internal diffusion of carbon. At this time, the diffusion processing temperature is a temperature that exceeds the carburization processing temperature. A reason for this is that, as a result of the diffusion processing temperature being a temperature that exceeds the carburization processing temperature, a diffusion processing speed is improved and overall processing speed is improved.

[0022] In addition, in an equilibrium diagram (Fe-C phase diagram) of the carbon steel composing the object to be processed, such as that in an example shown in Fig. 2, the diffusion processing temperature is a temperature that is lower than a solidus JQ temperature of yFe when the surface carbon concentration is reduced described above.

[0023] That is, as shown in Fig. 2, the diffusion processing temperature at the surface carbon concentration during diffusion is a temperature that is lower than a temperature Td at point D intersecting with a solidus JQ of yFe at a surface carbon concentration Cd during diffusion. A reason for this is that, at the surface carbon concentration that has decreased as a result of diffusion, when a sudden temperature increase such as that which exceeds the of yFe is performed, melting of the object to be processed made of carbon steel occurs.

[0024] In the carburization method according to the present embodiment in which the carburizing step and the diffusing step are performed in this manner, shortening of the processing time through shortening of the diffusion processing time can be achieved while suppressing melting of the object to be processed that is made of carbon steel.

[0025] Here, the equilibrium diagram of the carbon steel composing the object to be processed is determined based on quality of the carbon steel. In addition, in Fig. 2, J denotes a peritectic reaction point, Tp denotes a peritectic temperature, Q denotes a maximum solid-solubility point of carbon of yFe, E denotes a eutectic point, Te denotes a eutectic temperature, S denotes a eutectoid point, and Ts denotes a eutectoid temperature.

[0026] In the carburization method according to the present embodiment, specifically, a maximum value of the diffusion processing temperature can be set to be equal to or higher than the eutectic temperature of the carbon steel composing the object to be processed. However, as described above, the maximum value of the diffusion processing temperature is selected so as not to exceed the solidus temperature of yFe.

[0027] In this configuration, the diffusion process is performed at a high temperature that is equal to or higher than the eutectic temperature of carbon steel. In addition, in this configuration, temperature increase in the diffusion processing temperature to a temperature near the peritectic temperature of carbon steel can be performed. Therefore, in this configuration, shortening of the processing time through shortening of the diffusion processing time can be easily performed.

[0028] In the carburization method according to the present embodiment, the diffusion processing temperature can be controlled by a temperature increase speed being controlled. In this configuration, the diffusion processing temperature can be easily increased continuously or in stages. Therefore, in this configuration, the above-described working effects can be reliably achieved.

[0029] In the carburization method according to the present embodiment, the diffusion processing temperature can be increased by high-frequency heating. In this configuration, compared to heater heating, the temperature can be promptly increased to a predetermined diffusion processing temperature. In addition, control of the temperature increase speed can be easily performed. Therefore, in this configuration, shortening of the processing time by shortening of the diffusion processing time can be easily performed.

[0030] When the carburization method according to the present embodiment is used, as a result of a quenching step for quenching the object to be processed being performed after the above-described diffusing step, a hardened layer can be formed on a surface layer of the object to be processed. Here, publicly known conditions can be applied as quenching conditions and the like.

(Experiment examples)

<Fabrication of sample 1>

[0031] An object to be processed that is made of carbon steel was prepared. The carbon steel contains 0.15% by weight C; 0.2% by weight Si; 0.75% by weight Mn; 1.05% by weight Cr; 0.2% by weight Mo; and a remaining part made of Fe and inevitable impurities. Here, the eutectic temperature of the above-described carbon steel is 1143 °C. In addition, the peritectic temperature of the above-described carbon steel is 1485 °C.

[0032] The carburization process was performed on the prepared object to be processed under the following carburization process conditions.

-Carburization processing conditions-

[0033] 

Atmospheric pressure: atmospheric pressure

Gas type: N₂ gas (99% by volume) + C₂H₂ gas (1% by volume)

Heat treatment conditions: hold for 250 seconds after increasing temperature to 1050 °C

[0034] The diffusion process was performed on the object to be processed after the above-described carburization process under the following diffusion processing conditions:

-Diffusion processing conditions-

[0035] 

Atmospheric pressure: atmospheric pressure

Gas type: N₂ gas

Heat treatment conditions: hold for 26 seconds after increasing temperature from 1050 °C to 1275 °C at temperature increase speed of 11 °C/second

[0036]  The quenching process was performed on the object to be processed after the above-described diffusion process by water-cooling.

[0037] Sample 1 was fabricated in the manner described above. Here, the carburization method of the present example is the atmospheric pressure direct carburization method. In addition, the surface carbon concentration of the object to be processed after the carburization process during fabrication of sample 1 is 1.55% by weight.

[0038] Fig. 3 shows a relationship between a distance in a depth direction from the surface of the object to be processed after the carburization process during fabrication of sample 1 and carbon concentration. In addition, Fig. 4 shows a heat treatment pattern during fabrication of sample 1.

[0039] Here, in the fabrication of sample 1, as a result of the temperature increase speed being controlled, the diffusion processing temperature is continuously increased within a temperature range that exceeds the carburization processing temperature and falls below the solidus temperature of yFe when the surface carbon concentration is reduced in the equilibrium diagram of carbon steel, as the surface carbon concentration of the object to be processed decreases as a result of internal diffusion of carbon.

[0040] Upon observation of a cross-section that is perpendicular to the surface of sample 1 (position of line L in Fig. 5 by (a)) under a metallurgical microscope, as shown in Fig. 5 by (b), melting of the object to be processes cannot be confirmed. Here, based on this result, it can be said that, in the carburization method of sample 1, shortening of the processing time can be performed while suppressing melting of the object to be processed that is made of carbon steel.

<Fabrication of sample 1C>

[0041] Sample 1C was fabricated in a manner similar to the fabrication of sample 1, other than the temperature increase speed in the heating treatment conditions of the diffusion processing conditions being 16 °C/second, and the holding time at the diffusing step being 29 seconds. Here, in the present example, the holding time at the diffusing step is longer than that in the fabrication of sample 1 because diffusion time is required since the temperature is increased at a high speed.

[0042] Fig. 6 shows a heat treatment pattern during fabrication of sample 1C. Here, in the fabrication of sample 1C, as a result of intentionally excessive rapid temperature increase being performed at the diffusion processing step, the diffusion processing temperature is set to a temperature that exceeds the solidus temperature of yFe at the surface carbon concentration that has decreased as a result of internal diffusion of carbon in the equilibrium diagram of carbon steel.

[0043] Upon observation of a cross-section that is perpendicular to the surface of sample 1C (position of line L in Fig. 7 by (a)) under a metallurgical microscope, as shown in Fig. 7 by (b), a melting portion M is confirmed in a surface layer portion of the object to be processed. Based on this result, it can be said that, in the carburization method of sample 1C, even if shortening of the processing time is achieved, suppression of melting of the object to be processed that is made of carbon steel is difficult.

[0044] The present disclosure is not limited to the above-described embodiments and experiment examples. Various modifications are possible without departing from the spirit of the disclosure. That is, while the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure covers various modification examples and modifications within the range of equivalency.

[0045] In addition, various combinations and configurations, and further, other combinations and configurations including more, less, or only a single element thereof are also within the spirit and scope of the present disclosure. For example, in the above-described experiment examples, the diffusion processing temperature is continuously increased within the above-described specific temperature range as a result of the temperature increase speed being controlled. However, in addition, the diffusion processing temperature may be increased in stages within the above-described specific temperature range as a result of the temperature increase speed being controlled.

Claims

1. A carburization method comprising:

a carburizing step of performing a carburization process on an object to be processed that is made of carbon steel; and

a diffusing step of performing a diffusion process to diffuse carbon that is supplied onto a surface of the object to be processed at the carburizing step into an inner portion of the object to be processed, wherein:

at the diffusing step,

as a surface carbon concentration of the object to be processed decreases as a result of internal diffusion of the carbon,

a diffusion processing temperature is increased continuously or in stages within a temperature range that exceeds a carburization processing temperature and falls below a solidus temperature of yFe when the surface carbon concentration is reduced in an equilibrium diagram of the carbon steel.

2. The carburization method according to claim 1, wherein:
a maximum value of the diffusion processing temperature is equal to or higher than a eutectic temperature of the carbon steel.

3. The carburization method according to claim 1 or 2, wherein:
the diffusion processing temperature is controlled by a temperature increase speed being controlled.

4. The carburization method according to any one of claims 1 to 3, wherein:
the diffusion processing temperature is increased by high-frequency heating.

5. The carburization method according to any one of claims 1 to 4, wherein:
the carburizing step and the diffusing step are a carburizing step and a diffusing step in a vacuum carburization method or an atmospheric pressure direct carburization method.

Drawing