EP4023985A1

EP4023985A1 - Heat treatment furnace

Info

Publication number: EP4023985A1
Application number: EP20859246.9A
Authority: EP
Inventors: Shinichi Takahashi; Kiichi Kanda
Original assignee: Kanto Yakin Kogyo Co Ltd
Current assignee: Kanto Yakin Kogyo Co Ltd
Priority date: 2019-08-27
Filing date: 2020-08-22
Publication date: 2022-07-06
Also published as: KR20220038479A; WO2021039677A1; JP6948746B2; EP4023985A4; JPWO2021039677A1

Abstract

A purpose of the present disclosure is to provide a configuration of a heat treatment furnace for quenching that makes it possible to forgo the use of a converted gas and suitably prevent appearance of a modified layer resulting from decarburization or the like at a surface of a workpiece such as steel. A heat treatment furnace (10) according to an aspect of the present invention includes a gas supply section (26) configured to supply a neutral gas or an inert gas to a quenching heating chamber (12), and an internal structure such as a muffle (34) in the quenching heating chamber that is at least partially made from a graphite-based material.

Description

[Technical Field]

The present invention relates to a heat treatment furnace and particularly to a heat treatment furnace for quenching.

[Background Art]

Conventionally, a heat treatment furnace for quenching has been known in which a converted gas with a fuel (butane or the like) and air as raw materials is used as an in-furnace atmospheric gas, when performing bright heating for preventing formation of a modified layer at a surface due to oxidation, decarburization, cementation, or the like during quenching heating of steel as a workpiece (see, for example, PTL 1).

[Citation List]

[Patent Literature]

[PTL 1]
Japanese Utility Model Laid-open No. Sho 57-92757

[Summary]

[Technical Problem]

However, in order to use the converted gas in the heat treatment furnace for quenching, consumption of a fuel is indispensable, and there is room for improvement in terms of energy efficiency. In addition, for producing the desired converted gas, carbon potential (CP) control that requires a certain degree of skill is indispensable. Further, on a quality basis, in quenching heating of an alloy steel containing metallic components that are difficult to reduce such as silicon, manganese, and chromium, when the conventional converted gas is used as the atmospheric gas, intergranular oxidation occurs at the surface of the workpiece; therefore, it is needed to perform machining such as grinding the surface after quenching.
It is an object of the present invention to provide a configuration of a heat treatment furnace for quenching that makes it possible to forgo the use of a converted gas and suitably prevent the appearance of a modified layer resulting from decarburization or the like at the surface of a workpiece such as steel.

[Solution to Problem]

In order to achieve the above object, according to one aspect of the present invention, there is provided a heat treatment furnace including a gas supply section configured to supply a neutral gas or an inert gas to a quenching heating chamber, and an internal structure in the quenching heating chamber that is at least partially made from a graphite-based material.
Preferably, in the quenching heating chamber, a muffle made of a graphite-based material is provided so as to isolate a space in which a workpiece is conveyed from heating means. A belt for conveying a workpiece into the quenching heating chamber may be a conventional heat-resistant metal belt, but may preferably be made from a graphite-based material. A muffle defining at least a part of a dropping space for a workpiece to drop into a quenching oil tank disposed on a downstream side of the quenching heating chamber may be made of a graphite-based material, but may preferably be made of a material that is other than the graphite-based material and that has at least a predetermined level of oxidation resistance performance.
Preferably, the heat treatment furnace further includes oil flow-in preventing means configured to prevent flowing-in of oil from the quenching oil tank on the downstream side of the quenching heating chamber into the quenching heating chamber. It is favorable that the oil flow-in preventing means include a fluid curtain forming section configured to form a fluid curtain between the quenching heating chamber and the quenching oil tank. It is favorable that the fluid curtain forming section include an oil curtain forming device configured to form an oil curtain between the quenching heating chamber and the quenching oil tank, and an oil accepting section provided between the quenching heating chamber and the quenching oil tank so as to accept the oil curtain. It is favorable that the oil accepting section be connected to the quenching oil tank and includes a foam restraining section.
It is favorable that the heat treatment furnace further include an oil smoke treating device configured to treat oil smoke in the dropping space for a workpiece to drop into the quenching oil tank on the downstream side of the quenching heating chamber.
In addition, preferably, the heat treatment furnace further includes a fluid curtain forming section configured to form a fluid curtain between the quenching heating chamber and a quenching tank on the downstream side of the quenching heating chamber.

[Advantageous Effect of Invention]

According to the aspect of the present invention, configured as above, it is possible to forgo the use of a converted gas in a heat treatment furnace for quenching and suitably prevent the appearance of a modified layer due to decarburization or the like at the surface of a workpiece.

[Brief Description of Drawings]

[FIG. 1]
FIG. 1 is a schematic configuration diagram of a part of a heat treatment furnace according to a first embodiment of the present invention.
[FIG. 2]
FIG. 2 is a diagram for explaining atmospheric gas control in the heat treatment furnace of FIG. 1.
[FIG. 3]
FIG. 3 is a diagram depicting a display example of a display device in the heat treatment furnace of FIG. 1.
[FIG. 4] FIG. 4 is a diagram for explaining atmospheric gas control in the heat treatment furnace of FIG. 1.
[FIG. 5]
FIG. 5 is a schematic configuration diagram of a part of a heat treatment furnace according to a second embodiment of the present invention.

[Description of Embodiments]

Embodiments of the present invention will be described below based on the attached drawings. The same parts (or configurations) are denoted by the same signs, and the names and functions thereof are also the same. Therefore, detailed description of them will not be repeated.
A schematic configuration of a part of a heat treatment furnace 10 according to a first embodiment of the present invention is depicted in FIG. 1. The heat treatment furnace 10 includes a quenching heating chamber 12 and a quenching oil tank 14. The quenching oil tank 14 is provided on a downstream side of the quenching heating chamber 12. As is clear from FIG. 1, the quenching oil tank 14 is located below the quenching heating chamber 12 in a vertical direction. It is to be noted that, although not illustrated in FIG. 1, the heat treatment furnace 10 includes a tempering heating chamber on the downstream side of the quenching oil tank 14.
The quenching heating chamber 12 is provided therein with a mesh belt conveyor 16 as conveying means, or a conveying device, for conveying workpieces W. A mesh belt 18 of the mesh belt conveyor 16 is an endless belt, and is wound around a first roller 20, a second roller 22, and the like. The mesh belt 18 can be moved in a circulating manner within the quenching heating chamber 12. It is to be noted that, while the first roller 20 here is a driving roller and is electrically driven, the first roller 20 may be referred to as a driving drum in some cases and, in this instance, the second roller 22 as a driven roller can be referred to as a driven drum, for example.
The quenching heating chamber 12 is provided with a heater 24 as heating means, namely, a heating device. A plurality of heaters 24 are provided. Here, the heaters 24 each extend in a direction orthogonal to a conveying direction of the workpieces W, namely, in a widthwise direction. While the heaters 24 are provided on an upper side and a lower side in the quenching heating chamber 12 in FIG. 1, they may be provided also at left and right side portions in the widthwise direction in the quenching heating chamber 12. The workpieces W conveyed by the mesh belt conveyor 16 pass through a heating space (heating region or heating area) HS between the heaters 24. The heaters 24 are not limited to being provided on the upper side and the lower side in the quenching heating chamber 12 and may naturally be provided in any of various layouts or arrangements, for example, only on one of the upper side and the lower side. In addition, the heaters 24 may each be an electric heating type heating device or a combustion heating type heating device. Examples of the combustion heating type heating device include a radiant tube burner. The radiant tube burner is a burner of a system in which a fuel is burned in a radiant tube and heating is conducted by radiant heat thereof.
A gas supply section 26 configured to supply an atmospheric gas to the quenching heating chamber 12 is provided. Here, as the atmospheric gas, a nitrogen gas which is a neutral gas is used. However, the atmospheric gas is not limited to the neutral gas and may be an inert gas such as an Ar gas. The gas supply section 26 includes a supply pipe 30 having an introduction port 28 opened into the quenching heating chamber 12, and a valve 32 provided in the supply pipe 30. The supply pipe 30 is connected to a nitrogen gas tank 33. It is to be noted that, while only one introduction port 28 is depicted in FIG. 1, a plurality of introduction ports 28 may be provided. In addition, the introduction port 28 may be provided at a position different from that depicted in FIG. 1. In addition, the gas tank is not limited to the nitrogen gas tank 33 and may be a gas tank according to the gas used as the atmospheric gas, namely, a neutral gas or an inert gas.
Internal structures in the quenching heating chamber 12 are fabricated from a graphite-based material. As depicted in FIG. 1, a partition wall, namely, a muffle 34 is provided such that a conveying space in which the workpieces W are conveyed by the mesh belt conveyor 16, namely, the aforementioned heating space HS is isolated from the heaters 24. The muffle 34 is made of a graphite-based material, particularly, made of graphite here. A sheet-shaped or plate-shaped graphite, or a graphite plate, as the muffle 34 can be fabricated by the cold isostatic pressure molding method (CIP). Therefore, the workpieces W are conveyed so as to pass through a tunnel surrounded by the graphite muffle 34, in a nitrogen gas atmosphere heated by radiant heat of the graphite muffle 34. While it is favorable that the muffle 34 as an internal structure in the quenching heating chamber 12 be wholly formed from a graphite-based material, at least a part of the muffle 34 may be formed from a graphite-based material. In addition, while the tunnel defining the heating space HS here is configured by the graphite muffle 34 over the whole periphery thereof, only a part of the tunnel may be configured by the muffle 34. It is to be noted that a part or the whole part of the muffle 34 as an internal structure in the quenching heating chamber 12 may be fabricated from other graphite-based material, specifically, C/C composite which is a graphite-based material. Further, at least a part of the internal structures in the quenching heating chamber 12 other than the muffle may be fabricated from a graphite-based material.
Here, the mesh belt 18 is also made of a graphite-based material, specifically, made of graphite. Alternatively, the mesh belt 18 also, like the muffle 34, may be formed of other graphite-based material, for example, C/C composite. However, the mesh belt 18 is not limited to being made of a graphite-based material and may be, for example, a heat-resistant metal belt. It is to be noted that the conveying device is not limited to one using the mesh belt 18 or the like and may be configured as a roller conveying device.
An outside air block structure 36 is applied to a conveying-in end (at the left end of FIG. 1) of the mesh belt conveyor 16. The outside air block structure 36 has an inlet constriction section 36a at a part entering the furnace of the mesh belt 18, and an outlet constriction section 36b at a part going out of the furnace of the mesh belt 18. As depicted in FIG. 1, the inlet constriction section 36a and the outlet constriction section 36b are set close to each other, to thereby form substantially one entrance of the mesh belt 18. As a result, suction of outside air into the furnace due a pressure difference generated when the entrance of the mesh belt 18 is divided can be prevented, and stabilization of the in-furnace atmosphere can be realized.
Further, a plurality of curtain bodies 36c are provided at the conveying-in end of the mesh belt conveyor 16. The curtain bodies 36c have flexibility and are sheet-shaped here, but may have other shape, for example, may be line-shaped or string-shaped. The plurality of curtain bodies 36c are suspended so as to droop down from the upper side toward the lower side in the vertical direction. It is to be noted that the curtain bodies 36c here are formed of a nickel-based sheet material. However, the curtain bodies 36c may be produced from other material, for example, a carbon-based material, a glass-based material, a ceramic-based material, or a metallic material having sufficient strength and flexibility at a heat treatment temperature such as a steel material or a titanium-based material.
The quenching oil tank 14 on the downstream side of the quenching heating chamber 12 is provided at a conveying-out end (namely, an end portion on the second roller 22 side in FIG. 1) of the mesh belt conveyor 16. A chute 13 extending in the vertical direction is provided at a conveying-out end (on the right side in FIG. 1) of the quenching heating chamber 12. The chute 13 is located below a dropping space, described later, in the heating space HS. The chute 13 extends to the inside of the quenching oil tank 14. While the chute 13 does not reach an oil surface S in the quenching oil tank 14, it may extend into the oil in the quenching oil tank 14. The quenching oil tank 14 communicates with the inside of the quenching heating chamber 12 through the chute 13. Therefore, the atmospheric gas substantially the same as inside the quenching heating chamber 12 can be present to the oil surface S in the quenching oil tank 14. In other words, since the gas in contact with the oil surface S is different from a converted gas and is a nitrogen gas which is a neutral gas not containing hydrogen and carbon monoxide, the possibility of reactions such as oxidation of the quenching oil can be suppressed to extremely low. It is to be noted that, for further preventing oxidation of the oil surface S, a neutral gas or an inert gas, for example, a nitrogen gas, may be further supplied between a curtain C to be described later and the oil surface S. For example, it is favorable that a further supply pipe be connected to the nitrogen gas tank 33 and the nitrogen gas be supplied between the oil curtain C and the oil surface S through the supply pipe.
The heat treatment furnace 10 further includes oil flow-in preventing means OP configured to prevent flowing-in of an oil (for example, oil smoke) from the quenching oil tank 14 into the quenching heating chamber 12. The oil flow-in preventing means OP also called an oil flow-in preventing device here includes a fluid curtain forming section 40. Specifically, the fluid curtain forming section 40 configured to form a fluid curtain between the quenching heating chamber 12 and the quenching oil tank 14 is provided as the oil flow-in preventing means OP. The fluid curtain forming section 40 is provided in the chute 13. While the fluid curtain forming section 40 here is configured so as to form an oil curtain, it may be configured, for example, so as to form an air curtain by a nitrogen gas. In this case, a supply port 46 to be described later can be a gas supply port, which can be connected to a tank of nitrogen gas or the like.
The fluid curtain forming section 40 includes an oil curtain forming device 42 configured to form the oil curtain C between the quenching heating chamber 12 and the quenching oil tank 14. An oil passage 43 for scooping out the oil from the tank 14 is provided. The oil passage 43 is provided with an oil pump 44. The oil passage 43 has the oil supply port 46 provided at the position of the chute 13. The oil curtain forming device 42 includes a rectifying member 48 so as to form the oil curtain C by adjusting the flow of the oil coming out of the oil supply port 46. The rectifying member 48 is provided to extend between an oil delivery port 49 provided in the chute 13 and the oil supply port 46. The rectifying member 48 includes a smoothly recessed and curved surface 48a. The oil flowing out from the oil supply port 46 can flow along the recessed and curved surface 48a, so that the oil curtain C as depicted in FIG. 1 is formed.
Further, the fluid curtain forming section 40 includes an oil accepting section 50 provided between the quenching heating chamber 12 and the quenching oil tank 14 so as to accept the oil curtain C. The oil accepting section 50 is provided at a position facing the rectifying member 48 of the oil curtain forming device 42 so as to accept the oil curtain generated by the oil curtain forming device 42. More specifically, the oil accepting section 50 is provided to open to the chute 13. The oil accepting section 50 is provided at a predetermined position so as to accept the oil curtain C while preventing the oil curtain C from directly colliding on the oil surface S in the quenching oil tank 14. The oil accepting section 50 is connected to the quenching oil tank 14 on the lower side thereof. In addition, the oil accepting section 50 includes a foam restraining section 52. The foam restraining section 52 is provided so as to restrain foam of the oil generated attendant on the acceptance of the oil curtain C, preferably to cause the foam to disappear. Here, the foam restraining section 52 is configured as a member having an oil passage of a labyrinth structure, but may be any of various devices and structures producing such a foam-restraining effect.
It is to be noted that the quenching oil tank 14 is provided with a conveyor 54 for conveying the workpieces W from the quenching oil tank 14 toward the tempering heating chamber.
The heat treatment furnace 10 configured as above includes a controller 60. The controller 60 is what is generally called a computer including a processing section (for example, CPU), a storage section (for example, ROM, RAM), an input-output port, and the like. A temperature sensor 62 for detecting the temperature of an atmospheric gas in the quenching heating chamber 12, a first gas sensor 64 and a second gas sensor 65 for detecting gas components (concentrations) of the atmospheric gas in the quenching heating chamber 12 are connected to the controller 60. The first gas sensor 64 here is an oxygen sensor, and the second gas sensor 65 is a CO sensor; the heat treatment furnace 10 can include one or a plurality of gas sensors selected from an oxygen sensor, a CO sensor, and/or a CO₂ sensor, for example. The controller 60 controls rotation of a driving motor for the first roller 20, operation of the heaters 24, an opening degree (for example, opening and closing) of the valve 32, operation of the oil pump 44, and the like according to a predetermined program so as to keep in a preferable state the atmospheric gas in the quenching heating chamber 12, particularly, the heating space HS, and to enable preferable performance of heat treatment of the workpieces 12. Thus, the controller 60 has a control section for the driving motor for the first roller 20, a control section for the heaters 24, a control section for the valve 32 of the gas supply section 26, and a control section for the oil pump 44, as functional modules, namely, functional sections. Each functional module is realized by executing a program stored in the storage section by the processing section of the controller 60. However, these functional modules are not limited to being realized by one controller 60 and may be realized individually by a plurality of controllers, for example. It is to be noted that the controller 60 is connected to a display device 68, where results of processing by the controller 60, various values of the atmospheric gas in the quenching heating chamber 12, and the like are displayed. The description of an input device, or input means such as a keyboard, for input to the controller 60 is omitted.
The controller 60 stores therein a program and data for executing bright heat treatment of the workpieces W, during quenching heating. The workpieces W can each be of any of various materials, and here, it is made of a steel material. FIG. 2 depicts conceptually a part of an Ellingham diagram E. Here, the Ellingham diagram is a graph in which the axis of abscissas represents temperature, the axis of ordinates represents Gibbs energy of formation, and, with respect to various oxides, standard Gibbs energy of formation (ΔG⁰) at each temperature is plotted. A line L1 in the Ellingham diagram E in FIG. 2 is an approximate straight line of standard Gibbs energy of formation of iron (Fe) and iron oxide (FeO), and a line L2 is an approximate straight line of standard Gibbs energy of formation of the reaction of 2C + O₂ = 2CO. When the workpieces W are made of a steel material, a program for performing bright heat treatment or the like is specified such that the ΔG⁰ (standard Gibbs energy of formation) of the atmospheric gas in the quenching heating chamber 12 during quenching heating is located in a region GA on the lower side of both the line L1 and the line L2 in the graph in FIG. 2. Since the region GA is a reduction region for iron and also a reduction region for carbon, a problem of oxidation or decarburization of the workpieces W during quenching heating does not occur. In this instance, it is more favorable that a program or the like be specified such that the ΔG⁰ of the atmospheric gas is located in a further specific region (second predetermined region) within the region GA (first predetermined region). It is to be noted that, according to a component of the workpieces W, the line L1 or a line designated further additionally can be made to be a straight line for the oxide according to the component. Therefore, the controller 60 stores data on various oxides, and a user can designate one or a plurality of oxides by the input means.
The controller 60 further has functional sections in charge of functions of an oxygen partial pressure calculation section, a CO partial pressure calculation section, and a ΔG⁰ (standard Gibbs energy of formation) calculation section. The oxygen partial pressure calculation section calculates based on an output of the first gas sensor 64. The CO partial pressure calculation section calculates based on an output of the second gas sensor 65. The ΔG⁰ calculation section calculates the ΔG⁰ of the in-furnace atmospheric gas in the heat treatment furnace 10 during operation, based on an output of the temperature sensor 62 and referring to the results of calculation performed by each of the oxygen partial pressure calculation section and the CO partial pressure calculation section. In order to preferably perform bright heat treatment based on the results of calculation, the controller 60 controls the valve 32 and the like such that the ΔG⁰ of the calculation result is located in the aforementioned region GA, further preferably in the second predetermined region.
While there are some calculating methods for ΔG⁰, typical calculating methods will be described below. It is to be noted that, it is favorable that various sensors such as the sensors 62, 64, and 65 be selected according to the calculating method for ΔG⁰ adopted and provided at suitable places.
The ΔG⁰ can be calculated based on an oxygen partial pressure and an absolute temperature by using formula (1). In this case, P(O₂) is the oxygen partial pressure, T is the absolute temperature, and R is a gas constant. $Δ G^{0} = RTlnP (O_{2})$
In addition, paying attention to CO-O₂ reaction: $2 C + O_{2} = 2 CO$
the ΔG⁰ can be calculated based on a CO partial pressure and an absolute temperature by using formula (3). In this case, P(CO) is the CO partial pressure, T is the absolute temperature, and R is a gas constant. In addition, ΔG⁰ (2) is a value determined by a relational formula [ΔG⁰ (2) = - 221000 - 179.6T (J·mol^-1)]. $Δ G^{0} = RTlnP (O_{2}) = Δ G^{0} (2) + RTlnP (CO)$
In this way, the ΔG⁰ can be calculated based on the oxygen partial pressure by using formula (1). Besides, the ΔG⁰ can be determined based on the carbon monoxide partial pressure (CO partial pressure) by using formula (3) .
Here, for enhancing accuracy, ΔG⁰ = RTlnP(O₂) by formula (1) and ΔG⁰ = ΔG⁰(2) + 2RTlnP(CO) by formula (3) are each calculated, and, for example, an average of them is calculated as ΔG⁰, so that the calculation accuracy of ΔG⁰ is enhanced. For this purpose, the heat treatment furnace 10 includes the first gas sensor 64 which is the oxygen sensor and the second gas sensor 65 which is the CO sensor as described above. It is to be noted that the second gas sensor may not be provided, for example, in the case of calculating the ΔG⁰ by use of only formula (1). In addition, in the case of calculating the ΔG⁰ by use of formula (3) paying attention to the CO-O₂ reaction, it is sufficient to provide the second gas sensor 65 as the gas sensor.
FIG. 3 depicts an example of display on the display device 68 connected to the controller 60. The display device 68 displays a plot P of the ΔG⁰ of the atmospheric gas generated by the ΔG⁰ calculation section of the controller 60 on the displayed Ellingham diagram E. It is to be noted that the line L1 and the line L2 are the same as those in FIG. 2. As a result, a state of the atmospheric gas for heat treatment in the heat treatment furnace 10 can be visualized, and visualization can be realized.
Incidentally, quenching in the heat treatment furnace 10 configured as above, or a former-stage part of quenching and tempering, will be described. The workpieces W are each subjected, before treatment in the heat treatment furnace 10, to pretreatment for removing a film, oil, and the like deposited on its surface. Here, examples of the matter to be removed include a phosphate film and processing oil. The pretreated workpiece W is placed on the mesh belt conveyor 16 at the conveying-in end, enters the furnace, or the heating space HS of the quenching heating chamber 12, while pushing the curtain bodies 36c aside, is sent to the conveying-out end, and is put into the quenching oil tank 14 by natural dropping due to its own weight. The atmospheric gas in the quenching heating chamber 12 has been replaced with a nitrogen gas atmosphere, and is kept at a quenching temperature by operation of the heaters 24. The quenching temperature is a predetermined temperature, which may be set according to the workpieces.
In general, the atmospheric gas of the nitrogen gas contains a trace amount of oxygen. The oxygen reacts with graphite constituting the muffle 34 and the mesh belt 18 to be carbon monoxide (CO), which is discharged through gaps between members of the heat treatment furnace 10 or the like to outside of the heat treatment furnace 10, together with the atmospheric gas serving also as a carrier gas. Therefore, the nitrogen gas atmosphere comes to have a further lowered oxygen concentration, so that oxidation, decarburization, and the like of the workpieces occur with more difficulty.
Then, the workpiece W having passed through the quenching heating chamber, when put into the quenching oil tank 14, breaks through the oil curtain C to reach the oil surface S. The oil curtain C is broken by the workpiece W but is immediately returned to a film-shaped curtain state, so that oil droplets and oil smoke (hereinafter, oil smoke and the like) which can be generated by the break-through of the oil surface S by the workpiece W are shielded by the oil curtain C. As a result, oil such as the oil smoke and the like can be suitably prevented from entering the quenching heating chamber 12. Therefore, the aforementioned bright heat treatment can be suitably generated.
In addition, since the oil curtain C is accepted by the oil accepting section 50, foam of the oil can be prevented from being generated at the oil surface S by the oil curtain C. Further, by the foam restraining section 52 of the oil accepting section 50, foam of the oil generated in the oil accepting section 50 attendant on the acceptance of the oil curtain C can also be restrained, preferably caused to disappear. Therefore, the oil used for the oil curtain C can return to the oil tank 14 substantially without generating foam. Accordingly, generation of oil smoke and the like can be prevented more suitably.
It is to be noted that the workpiece W thus quenched is conveyed by the conveyor 54 from the quenching oil tank 14 toward the tempering heating chamber.
As has been described above, according to the heat treatment furnace 10 configured as above, a nitrogen gas is supplied particularly as the in-furnace atmospheric gas, and the in-furnace structures, or the internal structures, such as the muffle 34 in the quenching heating chamber 12 are made of a graphite material. Therefore, as has been described above, quenching heating can be performed in the nitrogen atmospheric gas with a lower oxygen concentration, so that appearance of a modified layer due to oxidation, decarburization, or the like at the surface of the workpiece W during quenching heating can be prevented sufficiently.
In addition, as has been described above, the oil flow-in preventing means OP such as the fluid curtain forming section 40 is provided. As a result, flowing-in of the oil into the quenching heating chamber 12 can be suitably prevented, so that the quenching heating can be performed further suitably.
It is to be noted that, while the fluid curtain forming section 40 is provided as the oil flow-in preventing means OP in the above heat treatment furnace 10, various configurations for preventing flowing-in of the oil into the quenching heating chamber 12 may be provided in addition to the fluid curtain forming section or in place of the fluid curtain forming section 40. For example, a pump for sucking oil smoke and the like may be provided at the chute 13. It is to be noted that a heat treatment furnace 10A of a second embodiment described later includes such a configuration.
Here, atmospheric gas control by the above heat treatment furnace 10 will be further described.
The steel material as the workpiece W to be subjected to quenching treatment includes those containing at least one component of carbon (C), chromium (Cr), manganese (Mn), silicon (Si), and the like in a predetermined ratio. For example, there are carbon steels for machine structure (SC material), alloy steels for machine structure, and the like. The alloy steels for machine structure are obtained by adding such an element as chromium or manganese to the carbon steels for machine structure, and examples thereof include manganese chromium steel (SMnC), chromium steel (SCr), and chromium molybdenum steel (SCM).
For example, in such an alloy steel for machine structure, an internal oxidation phenomenon such as intergranular oxidation in which crystal grain boundaries at a surface layer are oxidized by oxygen in the atmospheric gas may occur in conventional quenching heating using a converted gas. In the internal oxidation, a chromium oxide, a manganese oxide, and the like may be generated. Such internal oxidation may cause abnormal breakage or the like, and therefore, machining such as grinding the surface layer is generally conducted after the heat treatment.
In addition, in the conventional quenching heating using a converted gas, particularly when a converted gas (for example, RX gas) enhanced in reducing property by adding a large amount of reducing gas such as a hydrocarbon gas is used as the atmospheric gas, there is a fear that cementation or soot is generated, and it is difficult to stably maintain a CO partial pressure or a CO₂ partial pressure.
Further, to prevent decarburization, it is necessary to control carbon potential (CP) of the atmospheric gas. However, since the carbon potential varies according to temperature, a certain degree of skill is required for controlling the atmospheric gas.
FIG. 4 depicts conceptually a part of the Ellingham diagram E. On the Ellingham diagram, the line L1 (approximate straight line of standard Gibbs energy of formation of iron (Fe) and iron oxide (FeO)) and the line L2 (approximate straight line of standard Gibbs energy of formation of the reaction of 2C + O₂ = 2CO) are depicted. Further, on the Ellingham diagram, a straight line L3 which is an approximate straight line of standard Gibbs energy of formation of chromium (Cr) and chromium oxide (Cr₂O₃) is also depicted. When a converted gas is used as the atmospheric gas for quenching heating, though detailed explanation is omitted, the plot P of the ΔG⁰ of the atmospheric gas is required to be located in a narrow region GA1 on the lower side of line L1 and line L2 in the graph of FIG. 4, taking the aforementioned oxidation, decarburization, cementation, and the like into account. The region GA1 is a region on the upper side relative to the line L3, and by locating the plot P of the ΔG⁰ of the atmospheric gas in the region GA1, oxidation of Cr occurs. On the other hand, in the heat treatment furnace 10 according to the present embodiment, since the nitrogen gas is used as the atmospheric gas and the internal structures in the quenching heating chamber are made of a graphite material, the plot P of the ΔG⁰ of the atmospheric gas can be easily and securely located in a region GA2 on the lower side of the lines L1, L2, and L3, even taking the oxidation, decarburization, cementation, and the like at the surface of the workpiece into account. Therefore, for example, when the workpiece is chromium steel, oxidation, decarburization, and the like of the workpiece can be suitably prevented, while suitably preventing internal oxidation such as intergranular oxidation of Cr or the like.
In other words, in the above heat treatment furnace 10, a low-oxygen atmosphere using a nitrogen gas can easily be realized by the aforementioned configuration, and it is sufficient to locate the plot P of the ΔG⁰ of the atmospheric gas in the region GA2 on the lower side of the line L2 and the line L3 in the graph of FIG. 4. This facilitates atmospheric gas control and enables stable heat treatment of workpieces to be easily carried out. Thus, the heat treatment furnace 10 of the present embodiment is extremely excellent in terms of atmospheric gas control. It is to be noted that the region GA2 is an example of the second predetermined region.
Next, the heat treatment furnace 10A according to the second embodiment of the present invention will be described based on FIG. 5. Hereinafter, the heat treatment furnace 10A according to the present second embodiment will be described focusing on different points from the above heat treatment furnace 10. With respect to configurations, effects, and the like not particularly referred to below, the heat treatment furnace 10A has configurations similar to those described concerning the above heat treatment furnace 10 and produces similar effects or more effects as compared to the heat treatment furnace 10.
In the heat treatment furnace 10A of FIG. 5, partition curtains 70 and 72 are disposed between three spaces including a raised temperature section HS1, a soaking section HS2, and a dropping section HS3 inside the furnace, or inside the heating space HS. As a result, the atmosphere in each of the spaces HS1, HS2, and HS3 can be substantially shielded from the atmosphere in adjacent spaces. Particularly, with the partition curtain 72 provided, if oil such as oil smoke enters the space of the dropping section HS3 (hereinafter referred to as a dropping space), flowing-in of the oil to the further upstream side can be prevented more securely. It is to be noted that, while the partition curtains 70 and 72 of FIG. 5 are made of a nickel-based material same as the above curtain bodies 36c, they may be fabricated from a heat-resistant material such as a ceramic material other than the nickel-based material. In addition, while the partition curtains 70 and 72 are separate from the mesh belt 18 of the mesh belt conveyor 16 in FIG. 5, they may extend to such a position as to make contact with the mesh belt 18. This similarly applies to the curtain bodies 36c disposed at the conveying-in end to the heating space HS in the present second embodiment and the above first embodiment. It is to be noted that each of the partition curtains 70 and 72 is not limited to one curtain and may be a plurality of curtains.
In addition, a part where an internal structure in the furnace protrudes downward, called a drop arch, may be provided in place of or in addition to the partition curtains. In this case, it is favorable that the drop arch be made of a graphite-based material such as a C/C composite. With such a carbon-based drop arch provided, the atmospheric gas can be made to be a low-oxygen atmosphere more suitably. It is to be noted that the drop arch may be configured integral with the muffle 34, or may be configured as a body separate from the muffle 34 and disposed separate from or in contact with the muffle 34.
Further, the heat treatment furnace 10A includes an oil smoke treating device 74. The oil smoke treating device 74 has a pump 76 and an oil smoke withdrawal path 78. Here, the pump 76 is connected to the controller 60 and is controlled by the controller 60, but only a power source ON-OFF switch to be operated by an operator may be provided. The oil smoke withdrawal path 78 is connected to the pump 76 and has suction ports 80 and 82. The suction port 80 is connected to the dropping space. The suction port 82 is connected to the oil accepting section 50. As a result, even when oil smoke or the like is generated at these members, the oil smoke or the like can be sucked and discharged, so that the oil smoke or the like can be securely prevented from reaching the heating space HS. However, only either one of the suction ports 80 and 82, for example, only the suction port 80, may be provided. It is to be noted that, preferably, the oil smoke or the like sucked by the pump 76 is subjected to combustion treatment or the like. In this case, the combustion heat may be used for heating a part or the whole part of the heating space HS of the heat treatment furnace.
Further, in the heat treatment furnace 10A, a muffle 34a defining a part of the dropping space is configured from a member containing SiC as a main raw material. As the member containing SiC as a main raw material, an SiC brick is used here. The SiC brick not only has a predetermined level or more of oxidation resistance performance but also is excellent in high-temperature acid resistance/alkali corrosion resistance performance and high-temperature strength. Therefore, even if oil smoke or the like enters the dropping space located immediately on the upstream side of the chute 13, here, located directly above the chute 13, the muffle 34a is not substantially reduced by a reaction with the oil smoke or the like and can keep its heating effect as a muffle for a longer time. However, more muffles such as the muffle 34a for defining the dropping space may be fabricated by a member containing SiC as a main raw material, or may be fabricated from any of other various materials excellent in oxidation resistance performance. It is to be noted that this does not exclude the fabrication of the muffle 34a from a graphite-based material.
It is to be noted that, in the heat treatment furnace 10A, the heating space HS is substantially divided into the three spaces including the raised temperature section HS1, the soaking section HS2, and the dropping section (or dropping space) HS3 by the partition curtains 70 and 72. It is favorable that, corresponding to this, the aforementioned various sensors be provided for each of these spaces. As a result, in the heat treatment furnace 10A, atmospheric gas control can be performed more suitably. Specifically, needless to say, it is more preferable that the temperature sensor 62, the first gas sensor 64, and the second gas sensor 65 be provided for each of the spaces HS1, HS2, and HS3.
While the embodiments of the present invention and modifications thereof have been described above, the present invention is not limited to them. Various replacements and modifications are possible insofar as they do not depart from the spirit and scope of the present invention defined by the claims of the present application.
For example, various combinations of any parts of the above embodiments and modifications are possible. For example, the muffle 34a defining the dropping space in the heat treatment furnace 10A of the second embodiment may be applied to the heat treatment furnace 10 of the first embodiment. In addition, the oil smoke treating device 74 may also be applied similarly to the heat treatment furnace 10 of the first embodiment.
In the above two embodiments, the heat treatment furnaces 10 and 10A are continuous heat treatment furnaces, or continuous furnaces. However, the present invention is applicable to various type of heat treatment furnaces, and is applicable to, for example, what is generally called a semi-continuous furnace or a batch furnace, flexibly in such a range as not to generate technical contradiction. In addition, in the above two embodiments, the quenching tank is the quenching oil tank 14, and the coolant is oil. However, the coolant in the quenching tank is not limited to oil and may be water or water-soluble quenching oil. It is to be noted that, in the fluid curtain forming section 40 configured to form a fluid curtain between the quenching heating chamber and the quenching tank on the downstream side of the quenching heating chamber, a fluid curtain may be formed of the coolant. Specifically, it is favorable that, like in the above embodiments, the coolant in the quenching layer be scooped up and a fluid curtain such as the oil curtain C be formed of the coolant. In this case, the fluid curtain forming section 40 may include not only a fluid curtain forming device (corresponding to the aforementioned oil curtain forming device 42) configured to form a fluid curtain between the quenching heating chamber and the quenching tank but also a fluid accepting section (corresponding to the aforementioned oil accepting section 50) provided between the quenching heating chamber and the quenching tank so as to accept a fluid curtain. It is favorable that the fluid accepting section be connected to the quenching tank and include the aforementioned foam restraining section 52. It is to be noted that a treatment device (corresponding to the aforementioned oil smoke treating device 74) configured to treat the coolant, for example, a mist-formed coolant, in the dropping space for workpieces to drop into the quenching tank on the downstream side of the quenching heating chamber may further be provided. In this case also, a fluid curtain may be formed of a fluid other than the coolant, for example, a neutral gas such as a nitrogen gas, or an inert gas.

[Reference Signs List]

10, 10A:: Heat treatment furnace
12:: Quenching heating chamber
14:: Quenching oil tank
16:: Mesh belt conveyor
18:: Mesh belt
24:: Heater
26:: Gas supply section
34, 34a:: Muffle
40:: Fluid curtain forming section
70, 72:: Partition curtain
74:: Oil smoke treating device
HS:: Heating space
HS1:: Raised temperature section
HS2:: Soaking section
HS3:: Dropping section (dropping space)
OP:: Oil flow-in preventing means
C:: Oil curtain

Claims

A heat treatment furnace comprising:
a gas supply section configured to supply a neutral gas or an inert gas to a quenching heating chamber; and

an internal structure in the quenching heating chamber that is at least partially made from a graphite-based material.
The heat treatment furnace according to claim 1,
wherein, in the quenching heating chamber, a muffle made of a graphite-based material is provided so as to isolate a space in which a workpiece is conveyed from heating means.
The heat treatment furnace according to claim 1 or 2,
wherein a belt for conveying a workpiece into the quenching heating chamber is made from a graphite-based material.
The heat treatment furnace according to any one of claims 1 to 3,
wherein a muffle defining at least a part of a dropping space for a workpiece to drop into a quenching oil tank disposed on a downstream side of the quenching heating chamber is formed from a material that is other than the graphite-based material and that has at least a predetermined level of oxidation resistance performance.
The heat treatment furnace according to any one of claims 1 to 4, further comprising:
oil flow-in preventing means configured to prevent flowing-in of oil from a quenching oil tank disposed on a downstream side of the quenching heating chamber into the quenching heating chamber.
The heat treatment furnace according to claim 5,
wherein the oil flow-in preventing means comprises a fluid curtain forming section configured to form a fluid curtain between the quenching heating chamber and the quenching oil tank.
The heat treatment furnace according to claim 6,
wherein the fluid curtain forming section comprises
an oil curtain forming device configured to form an oil curtain between the quenching heating chamber and the quenching oil tank, and

an oil accepting section provided between the quenching heating chamber and the quenching oil tank so as to accept the oil curtain.
The heat treatment furnace according to claim 7,
wherein the oil accepting section is connected to the quenching oil tank and comprises a foam restraining section.
The heat treatment furnace according to any one of claims 1 to 8, further comprising:
an oil smoke treating device configured to treat oil smoke in a dropping space for a workpiece to drop into a quenching oil tank disposed on a downstream side of the quenching heating chamber.
The heat treatment furnace according to any one of claims 1 to 4, further comprising:
a fluid curtain forming section configured to form a fluid curtain between the quenching heating chamber and a quenching tank disposed on a downstream side of the quenching heating chamber.