Method and apparatus for controlling the atmosphere in a heat treatment furnace

Abstract: In a method of and apparatus for controlling an atmosphere in a heat treatment furnace according to the present invention, a carburizing is carried out while supplying a hydrocarbon series gas and an oxidization gas into the furnace(1), and the supply of the hydrocarbon series gas is stopped either when the quantity of a residual CH 4 in the furnace(1) is changed to increasing from decreasing, or when a partial pressure of oxygen in the furnace(1) reaches a predetermined value.

Description: This invention relates to a method of and apparatus for controlling an atmosphere in a heat treatment furnace, and more particularly relates to a control method of and apparatus for an atmosphere in a heat treatment furnace for carrying out a gas carburizing, carbonitriding or bright controlled atmosphere heat treatment, etc. In the conventional heat treatment methods, such as a gas carburizing of metals, a mixture of a hydrocarbon series gas with air is generated into a converted gas (endothermic gas) by using an endothermic type converted gas generator, the endothermic gas is introduced into a furnace, and a hydrocarbon series gas (enriched gas) is added to the furnace in order to obtain a predetermined carbon potential. However, recently, in order to enhance the quality, and to reduce the treatment time and running cost, such a method that the gas generator is not used, but a hydrocarbon series gas and an oxidizing gas are introduced directly into the furnace to carry out the carburizing in-the furnace has been proposed. Such method is described in Japanese Patent Applications Laid-Open Nos. 54931/1979, 159567/1986 and 63260/1992. UK Patent No. 1543510 describes a process for carburizing a metal workpiece in a gaseous atmosphere containing oxygen -and methane wherein the carburization is controlled by controlling the proportion of methane in the carburizing atmosphere. US Patent No. 4372790 describes a method and apparatus for controlling the carbon level of a gas mixture reacting in a heat treatment furnace. Japanese Patent Application No. 62243754 describes a control device for controlling the carbon potential in a carburization furnace atmosphere by controlling carbon concentration to a prescribed value by the fluctuation of the content of CO in an atmospheric gas. However, the carburizing speed in the direct carburizing method is varied on a large scale according to the carburizing time and the diffusion time. In the carburizing time, the main effect is the direct decomposition of the hydrocarbon series gas, etc. (raw gas) and in 2 the diffusion time, the main effect is the Boundouard reaction. In the carburizing time, the degree of the decomposition is different due to the quantity of the hydrocarbon series gas to be introduced directly into the furnace and the temperature of the atmosphere in the furnace as well as the type of goods to be treated in the furnace. As a result, the hydrocarbon series gas in excess of the amount required to the carburizing is piled as a soot in the furnace or the goods to be treated are sooted. If the heat treatment is carried out in the sooting range, the service life of the oxygen sensor becomes short. An object of the present invention is to obviate the above defects. Further object of the present invention is to provide a method of controlling an atmosphere in a heat treatment furnace comprising the steps of carrying out a carburizing while supplying a hydrocarbon series gas and an oxidization gas into a furnace, and stopping the supply of the hydrocarbon series gas when the quantity of a residual CH 4 in the furnace is changed to increasing from decreasing. Another object of the present invention is to provide a method of controlling an atmosphere in a heat treatment furnace comprising the steps of carrying out a carburizing while supplying a hydrocarbon series gas and an oxidization gas into a furnace, and stopping the supply of the hydrocarbon series gas when the partial pressure of oxygen in the furnace reaches a predetermined value. Further object of the present invention is to provide a control apparatus for controlling an atmosphere in a furnace comprising a furnace, a heater for heating the inside of the furnace, means for measuring a partial pressure of oxygen and a partial pressure of CH 4 in the furnace, means for introducing a hydrocarbon series gas and an oxidization gas into the furnace, and means for controlling the quantities of the hydrocarbon series gas and the oxidization gas to be introduced into the furnace. In the present invention, a liquid containing carbon atoms such as alcohol, gas such as acetylene, methane, propane or butane containing hydrocarbon for its main ingredient, preferably, methane, propane or butane is used as the hydrocarbon series gas. In the present invention, the oxidization gas is air or CO 2 gas. The forgoing and other objects, features, and advantages of the present invention will become apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. Fig. 1 is a view illustrating a control method and apparatus of an atmosphere in a heat treatment furnace in accordance with the present invention. Fig. 2 is a graph explaining the relationship between the effective case depth and the carburizing time according to the carbon potential. Fig. 3 is a graph explaining the relationship between the quantity of residual CH 4 and the carburizing time according to the quantity of added enriched gas. Fig. 4 is a graph explaining the relationship between the quantity of undecomposed residual CH 4 and the quantity of added C 4 M 10 according to the carburizing time. Fig. 1 shows a control apparatus for a heat treatment furnace according to the present invention. In Fig. 1, reference numeral 1 denotes a shell of furnace, 2 denotes a refractory brick forming the shell of furnace 1, 3 denotes a fun for recirculating the atmosphere in the furnace, 4 denotes a heater, 5 denotes a thermocouple for controlling the temperature in the furnace, 6 denotes a zirconian type sensor for sensing the partial pressure of a solid electrolyte oxygen, for example, which is inserted directly into the furnace, 8 denotes a tube for measuring the partial pressure of CH 4 , 10 denotes an analyzer for analyzing the partial pressure of CH 4 , 11 denotes a pipe for introducing hydrocarbon series gas into the furnace, 12 denotes a control valve inserted into the pipe 11, 13 denotes a pipe for introducing oxidization gas into the furnace, 14 denotes a control valve inserted into the pipe 13, 15 denotes an operating apparatus for the carbon potential, and 16 denotes a controller for supplying control signals to the valves 12 and 14. Fig. 2, shows the relationship between the effective case depth and the carburizing time according to the carbon potential. As shown in Fig. 2, it is publicly known that if the carbon potential in the carburizing time is higher, the carburizing can be completed with a shorter time period and that it is not suitable to carry out the heat treatment in the hatched sooting region of the Fe-C series equilibrium diagram shown in Fig. 2. It is better to add a large quantity of enriched gas (hydrocarbon series gas) in order to increase the carbon potential.- As shown In Fig. 3, in each of cases that if the goods to be treated is 150kg and C 4 H 10 gas of 2.5 liter/minute is introduced (case A), C 4 H 10 gas of 1.4 liter/minute is introduced (case B), and C 4 H 10 gas of 1.0 liter/minute Is introduced (case C), the quantity of residual CH 4 is decreased and then increased with time, so that the goods are sooted. However, in case that C 4 H 10 gas of 0.5 liter/minute is introduced (case D), the quantity of residual CH 4 is constant substantially, so that the goods are not sooted. It is considered that in the cases of (A), (B)and (C), the quantity of added C 4 H 10 gas is large and accordingly some carbon cannot be absorbed by the steel, so that the quantity of undecomposed residual CH 4 is increased, but in case of (D), entire carbon can be absorbed by the steel. Accordingly, the sooting can be prevented from occurring by analyzing the quantity of residual CH 4 and controlling it. In the present invention, therefore, the quantity of residual CH 4 is analyzed by the analyzer 10 and when the quantity of residual CH 4 is changed to increasing from decreasing the control valve 12 is closed to stop the supply of the hydrocarbon series gas C x H y , so that the quantity of residual CH 4 is prevented from being increased. As apparent from the Fe-C series equilibrium diagram, the sooting can be prevented from occurring by measuring the partial pressure of oxygen corresponding to the maximum carbon solid solution, because the maximum carbon solid solution is constant at a specific temperature. In the present invention, accordingly, the output value of the sensor 6 for sensing the partial pressure of oxygen is measured to know the partial pressure of oxygen, and the control valve 12 is closed when the partial pressure of oxygen reaches a predetermined value, in order to prevent the sooting from occurring. Further, in the present invention, the control valve 12 can be closed at an earlier time either when the partial pressure of oxygen reaches a predetermined value or the partial pressure of CH 4 reaches a predetermined value by carrying out the measurements of the partial pressure of oxygen and the measurement of the partial pressure of CH 4 at the same time. (Embodiment 1) A batch furnace is used, the goods to be treated of 150kg are introduced into the furnace, and the carburizing operation is carried out for four hours at 930°C by using C 4 H 10 gas as a hydrocarbon series gas and CO 2 gas as an oxidization gas. As shown in Fig. 3, the quantity of CH 4 increases with time in case that more than 1.0 liter/minute of butane is added as the hydrocarbon series gas. This means that the residual CH 4 is undecomposed and accumulated in the furnace, so that the sooting is accelerated. Fig. 4 shows the relationship between the quantity of residual CH 4 in the furnace and the quantity of added C 4 H 10 according to the carburizing time, in case that no sooting is occurred. It is apparent from Fig. 4 that the sooting is occurred when the quantity of added hydrocarbon series gas is 2.5 liter/minute, but the sooting can be prevented from occurring if the introduction of the hydrocarbon series gas is stopped according to the present invention. As the hydrocarbon series gas, a liquid containing carbon atoms, such as alcohol, or gas such as acetylene, methane, propane or butane gas containing a hydrocarbon for its main ingredient, preferably methane, propane or butane gas is used. Air or CO 2 gas is used as the oxidization gas. As stated above,according to the present invention, the sooting can be prevented from occurring in advance by controlling the quantity of hydrocarbon series gas to be added according to the partial pressure of CH 4 and partial pressure of oxygen in the atmosphere of the heat treatment for the gas carburizing, carbonitriding or bright heat treatment. While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: A method of controlling an atmosphere in a heat treatment furnace comprising the steps of carrying out a carburizing while supplying a hydrocarbon series gas and an oxidation gas into a furnace, and stopping the supply of the hydrocarbon series gas when the quantity of a residual CH 4 in the furnace is changed to increasing from decreasing. A method of controlling an atmosphere in a heat treatment furnace comprising the steps of carrying out a carburizing while supplying a hydrocarbon series gas and an oxidation gas into a furnace, and stopping the supply of the hydrocarbon series gas when the partial pressure of oxygen in the furnace reaches a predetermined value. The method of controlling an atmosphere in a heat treatment furnace as claimed in claim 1 or claim 2, further comprising the step of stopping the supply of the hydrocarbon series gas according to whichever occurs earlier of the residual CH 4 changing to increasing from decreasing or the partial pressure of oxygen in the furnace reaching a predetermined value. The method of controlling an atmosphere in a heat treatment furnace as claimed in claim 1, 2 or 3, wherein a liquid containing carbon atoms such as alcohol, gas such as acetylene, methane, propane or butane containing hydrocarbon for its main ingredient, preferably, methane, propane or butane is used as the hydrocarbon series gas. The method of controlling an atmosphere in a heat treatment furnace as claimed in claim 1, 2, 3 or 4, wherein the oxidation gas is air or CO 2 gas. Use of an apparatus for controlling an atmosphere in a furnace, the apparatus comprising a furnace (1), a heater (4) for heating the inside of the furnace (1), means (6,8) for measuring a partial pressure of oxygen and a partial pressure of CH 4 in the furnace (1), means (11,13) for introducing a hydrocarbon series gas and an oxidation gas into the furnace (1), and means (12,14) for controlling the quantities of the hydrocarbon series gas and the oxidation gas to be introduced into the furnace (1), wherein said means to control the quantities of the hydrocarbon series gas and the oxidation gas comprises means to stop the supply of hydrocarbon series gas and/or oxidation gas when the quantity of residual CH 4 changes to increasing from decreasing or the partial pressure of oxidation gas reaches a predetermined value.

Inventors: INOUE HIDEKI, NAITO TAKESHI, NAKAHIRO TADANORI, NAKASHIMA YOSHIO, OGIHARA KOUICHI, WAKATSUKI AKIHIRO, INOUE, HIDEKI, NAITO, TAKESHI, NAKAHIRO, TADANORI, NAKASHIMA, YOSHIO, OGIHARA, KOUICHI, WAKATSUKI, AKIHIRO, Inoue, Hideki, c/o Dowa Mining Co., Ltd., Naito, Takeshi, c/o Dowa Mining Co., Ltd., Nakahiro, Tadanori, c/o Dowa Mining Co., Ltd., Nakashima, Yoshio, c/o Dowa Mining Co., Ltd., Ogihara, Kouichi, c/o Dowa Mining Co., Ltd., Wakatsuki, Akihiro, c/o Dowa Mining Co., Ltd.
Applicants: DOWA MINING CO, DOWA MINING CO., LTD.
Application year: 1998
Main classification: C23C8
IPC codes: C23C8, C21D1