JP4614359B2 - Method for detecting oxygen partial pressure in heating chamber - Google Patents

Description

  The present invention relates to a device for detecting the partial pressure of oxygen in a heating chamber of a vacuum carburizing furnace that decomposes a hydrocarbon-based gas under reduced pressure and high temperature to carburize the surface of an iron-based work (member) such as a steel product.

  Conventionally, in the vacuum carburizing method, carbon that did not react with iron is hatched, and a polymer of hydrocarbon gas accumulates inside the heat insulating material, so that the heat insulating property deteriorates with time, excessive energy, time, etc. Therefore, so-called burnout or the like is performed in which air is periodically introduced into the soot and hydrocarbon gas polymer accumulated in the heat insulating material and burned (Patent Document 1 or the like). Also, the heat insulating material is replaced with a new one due to deterioration of the heat insulating material or the like. Also, repairs and suspensions due to vacations are performed regularly or irregularly. In such a case, the amount of oxygen in the heating chamber changes. Further, even in the operation state, the amount of residual / adsorbed oxygen varies depending on the amount of air brought in by the work and the base, the surface area of the work, the degree of oxidation of the work surface, and the material and state of the in-furnace structure.

  However, carburizing conditions change depending on the amount of oxygen in these furnaces, the amount of air (oxygen) leaks, the amount of oxygen volatilized from the furnace structure, and the amount of oxygen brought in with the workpiece. Cannot be secured.

Therefore, in Patent Document 2, the amount of oxygen is measured while measuring the amount of oxygen in the atmosphere gas and the thermal conductivity of the atmosphere gas in an atmosphere gas containing a carbon-containing compound under a reduced pressure of 13 to 4000 Pa, for example. Carburizing is carried out while adjusting the composition of the atmospheric gas so that the degree of decomposition of the carbon-containing compound is maintained at a predetermined value. The oxygen concentration is measured by a zirconia oxygen sensor, and the degree of decomposition of the carbon compound is measured by the thermal conductivity of the atmospheric gas. Further, it is disclosed that this zirconia oxygen sensor can accurately measure a slight difference in oxygen concentration in a very small concentration range of oxygen partial pressure of about 1 × 10 ⁻²⁰ atm.

If the output value of the zirconia oxygen sensor suddenly increases and reaches 1200 mV in 1 to 2 minutes when butane gas is introduced into the heating chamber using such a zirconia oxygen sensor, the amount of oxygen in the carburizing chamber However, the carburizing process is continued, assuming that the carburizing process is not hindered. Conversely, when the output value of the zirconia sensor does not increase rapidly and is about 1150 mV or less, the carburizing process is stopped because normal carburizing process cannot be performed even if butane gas is introduced. Moreover, without stopping the carburizing process, the introduction of butane gas is stopped and the introduction of hydrogen is switched to reduce the amount of oxygen in the carburizing chamber. It is stated that the carburizing process will continue. In this way, the status of each sensor is grasped, and the carburization status is monitored and controlled.
JP-A-2-115357 JP 2004-59959 A

  However, in such a conventional method, since the determination is performed while carburizing, if the oxygen amount does not decrease even if the amount of oxygen is switched to hydrogen gas, the work is returned to the outside of the heating chamber, the cause is removed, and then again. Since the workpiece is loaded and carburized while judging the amount of oxygen, the influence of the heat load on the workpiece is inevitable. Further, there is a problem that consumption of hydrocarbon gas, hydrogen gas, etc. increases, and raw material gas, heating energy, etc. are wasted. Furthermore, a large amount of oxygen is brought into the furnace at the time of heat insulation replacement or burnout. In this case, several times or a long period of time are required until a dummy work is inserted and the oxygen amount falls below a predetermined amount. There has been a problem that a wasteful operation such as entering a regular (normal operation) operation is required after the operation.

  In particular, the zirconia oxygen sensor has a problem that it deteriorates early when it is exposed to a hydrocarbon gas for a long time, requires frequent replacement, and is expensive.

  In view of the above-described problems, an object of the present invention is to extend the life of a zirconia oxygen sensor and reduce the replacement frequency.

  The inventors of the present application conducted various studies on the amount of oxygen in the heating chamber in addition to mechanical causes due to leaks, etc., and found that oxygen entering the heating chamber along with workpieces, jigs, transport devices, etc., and opening / closing of the intermediate door Oxygen in the inert gas that intrudes can be easily reduced by introducing ordinary vacuum exhaust and oxygen replacement gas such as hydrogen gas, hydrocarbon gas or nitrogen gas, and the amount of oxygen can be reduced in a short time. Less impact on carburizing quality. On the other hand, after the heating chamber was opened to the outside for a long time due to repairs and holidays, the carburizing quality was found to be very poor after replacing the heat insulating material or after burning out with oxygen. This is because the oxygen adhering to and absorbed in the furnace structure, especially the heat insulating material, that is, the adsorbed oxygen, is not easily exhausted. I knew that.

  Therefore, the inventors of the present application bring an iron-based workpiece into the heating chamber, decompose the hydrocarbon gas under reduced pressure and high temperature, cause carbon to react on the iron surface of the workpiece, and carburize the workpiece surface, Without bringing the workpiece into the heating chamber, the intermediate door of the heating chamber is closed and sealed, the heating chamber is heated and evacuated, and the furnace structure in the heating chamber is kept until the oxygen partial pressure in the heating chamber becomes a predetermined value or less. A pretreatment method for the heating chamber of the vacuum carburizing furnace is proposed. This makes it possible to obtain a small amount of adsorbed oxygen that affects carburization and to obtain a stable carburizing quality without being affected by the amount of oxygen even immediately after entering normal operation, and the amount of oxygen in the carburizing process can be reduced. Constant monitoring is no longer necessary.

Based on this knowledge, it is not necessary to measure the oxygen concentration during the carburizing process. Therefore, in the present invention, in a vacuum carburizing furnace in which an iron-based work is carried into a heating chamber, hydrocarbon gas is decomposed under reduced pressure and high temperature, carbon is reacted on the iron surface of the work, and the work surface is carburized. A detection pipe having one end opened in the heating chamber, a first opening / closing valve connected to the other end opening of the detection pipe and one connection port outside the heating chamber, and the other of the first opening / closing valve A sensor attachment tube having a tip connected to the connection port, an opening opening on a side opposite to the tip of the sensor attachment tube, a second opening / closing valve connected at one connection port, and the second A third on-off valve connected to the other connection port of the on-off valve via one pipe, a vacuum pump connected to the other connection port of the third on-off valve at the intake port, and The detector is positioned inside the sensor mounting tube. And said zirconia oxygen sensor attached to the anti-tip side of the sensor mounting tube to provided, the first, second, open the third switching valve to measure the oxygen partial pressure, the heating chamber of the furnace It is determined that the discharge of the adsorbed oxygen of the structure is completed, and at least during vacuum carburization, the first on-off valve is closed, and either the second or third or both are closed, and zirconia The above-described problems have been solved by providing a method for detecting the partial pressure of oxygen in vacuum carburization of a heating chamber, which has an oxygen oxygen sensor and a hydrocarbon gas that do not come into contact with each other.

  In other words, open / close valves are arranged at both ends of the zirconia oxygen sensor, and when the oxygen partial pressure in the heating chamber is measured, the open / close valve is opened and the open / close valve is opened during the carburizing period in which carburizing gas is supplied to the heating chamber. It obstructs and eliminates the influence of hydrocarbon gases on the zirconia oxygen sensor. Further, since the exhaust can be performed by the vacuum pump, the oxygen partial pressure in the heating chamber can be accurately measured. In addition, since open / close valves are provided on both sides between the zirconia oxygen sensor and the vacuum pump, the piping between the valves is kept in a vacuum state, such as vacuum breakage on the heating chamber side, hydrocarbon gas, nitrogen gas, etc. There is little influence at the time of exhaust. In addition, the degree of vacuum during maintenance can be secured.

Also, at least during vacuum carburization, the first open / close valve is closed, and the second or third or both are closed, and the oxygen partial pressure is detected in the heating chamber. Prevent contact with hydrogen gas.

  In the present invention, opening and closing valves are arranged at both ends of the zirconia oxygen sensor, the influence of hydrocarbon gas on the zirconia oxygen sensor is eliminated, the oxygen partial pressure in the heating chamber can be accurately measured, and the degree of vacuum during maintenance Therefore, the zirconia oxygen sensor has a long service life, and there is no need to replace it, making maintenance easier.

Further, since the contact between the zirconia oxygen sensor and the hydrocarbon gas is prevented, the zirconia oxygen sensor is hardly deteriorated and a long life can be secured.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional explanatory view of a vacuum carburizing furnace showing an embodiment of the present invention, and FIG. 2 is a partial cross-sectional view showing a state in which a zirconia oxygen sensor is attached to a heating chamber. The vacuum carburizing furnace 1 includes a cooling chamber 4 having a furnace door 3 in which a workpiece (not shown) can be carried in and out, a heating chamber 6 connected via an intermediate door 5, and a lower part of the cooling chamber 4. It is comprised from the oil tank 7 arrange | positioned. The cooling chamber 4 and the heating chamber 6 are each made to have a sealed structure by closing the intermediate door 5 and the furnace door 3, and the cooling chamber can be evacuated by a vacuum exhaust device (not shown). A vacuum pump 15 is connected to the heating chamber 6 through on-off valves 10 and 12, a pneumatic operation valve 11, and an in-furnace pressure control valve 13 operated by air pressure, so that the heating chamber can be evacuated. The on-off valves 10 and 12 are for maintenance, the on-off valves 10 and 11 are normally open, 12 is normally closed, and the heating chamber 6 and the vacuum pump 15 are opened and closed by a pneumatically operated valve 11 and a furnace pressure control valve 13. The In addition, a gas supply device (not shown) is provided so that carburizing gas, nitrogen gas, air, and the like can be supplied into the heating chamber, and nitrogen gas and air can be supplied into the cooling chamber.

  Quenching oil is stored in the oil tank 7, and oil quenching is enabled by immersing the work that has been carburized by heating. In the heating chamber 6, the heat insulating material 16 is pasted in the furnace wall 17 that forms the outer wall of the heating chamber, a space 37 is provided, the heat insulating frame 33 is disposed, and the heat insulating material 16 is further provided for vacuum heating. To ensure furnace temperature and vacuum. As the heat insulating material 16, for example, an alumina / silica ceramic heat insulating material or the like is used. The same heat insulating material 16 is also used for the intermediate door 5. A furnace pressure gauge 8 and a furnace thermometer 9 are provided in the heating chamber 6. In addition to the heat insulating material 16, the furnace internal structure includes an oxygen adsorbing material such as a ceramic tube, a ceramic hearth, and a ceramic rail.

  In carburizing and quenching, the work is carried into the cooling chamber 4 from the furnace door 3. The furnace door 3 is closed and the cooling chamber is evacuated. After the predetermined exhaust of the cooling chamber 4 is performed, the intermediate door 5 is opened, the work is carried into the heating chamber 6, and the intermediate door is closed. In this state, the workpiece is heated, and after the soaking, carburizing, diffusion, and temperature lowering steps, the intermediate door 5 is opened, the workpiece is transferred to the cooling chamber 4, the intermediate door is closed, and the workpiece is immersed in the oil tank 7. Pull up the workpiece after quenching, open the furnace door and carry the workpiece out of the furnace. Since such a vacuum carburizing furnace is known, a detailed description thereof will be omitted.

In particular, in the present invention, as shown in FIGS. 1 and 2, the detection pipe 22 is inserted through the furnace wall 17 and the heat insulating materials 16 and 16 of the heating chamber 6, and the first on-off valve 20 and the sensor mounting pipe. 21, second and third on-off valves 23, 24 are connected to the vacuum pump 15 via the order. That is, the detection pipe 22 having one end 22a opened through the furnace wall 17 and the heat insulating materials 16 and 16 in the heating chamber 6, and the other end opening 22b of the detection pipe and one connection port 20a outside the heating chamber One open / close valve 20 is connected via a flange joint 35 having a different diameter. The different-diameter flange joint 35 is fixed to the furnace wall 17 via a pedestal 34 and supports the detection pipe. The detection pipe 22 is a ceramic tube having an outer diameter of 15 mm and an inner diameter of 10 mm, and the second opening / closing valve 20 is an angle type opening / closing valve having a diameter of 1 inch.

  A sensor attachment tube 21 having a tip 21a connected to the other connection port 20b of the first opening / closing valve 20 is flange-connected. A second opening / closing valve 23 is connected to an opening 21d that opens to a side 21c on the side opposite to the tip 21b of the sensor mounting tube 21 through one connection port 23a. The third opening / closing valve 24 is connected to the other connection port 23b of the second opening / closing valve 23 via the pipe 36 by the one connection port 24a. The second and third on-off valves 23 and 24 are ball valves having a diameter of 3/8 inch. The other connection port 24b of the third on-off valve 24 is connected to the intake port 10a of the vacuum pump 10 together with other piping.

  A zirconia oxygen sensor 30 is attached to the opposite end 21 b of the sensor attachment tube 21, and a detection unit 31 for detecting the oxygen partial pressure is positioned inside the sensor attachment tube 21 so as to contact the gas in the attachment tube 21. It is made to be able to react. The detection unit 31 is preferably matched with an exhaust passage formed in the sensor attachment tube 21 by vacuum exhaust. The zirconia oxygen sensor 30 is provided with a measurement control unit (not shown) to display the oxygen partial pressure. A signal is output when the oxygen partial pressure reaches a predetermined value. Since the function, control, measurement method, and the like of the zirconia oxygen sensor are known, detailed description thereof is omitted.

In such a vacuum carburizing apparatus 1, the pretreatment of the heating chamber of the vacuum carburizing furnace of the present invention is performed as follows. The furnace door 3 and the intermediate door 5 of the heating chamber are closed and sealed while the work chamber is not carried in and the heating chamber 6 is left empty. The on-off valves 20, 23, 24 are opened so that the oxygen partial pressure of the heating chamber 6 acts on the zirconia oxygen sensor. While monitoring the furnace temperature with a furnace thermometer 9 using a radiant tube (not shown), the furnace is heated to a predetermined temperature of 850 ° C. to 1050 ° C. At the same time, the electromagnetic on-off valve 13 is opened, the vacuum pump 15 is operated, and the pressure in the furnace is monitored by the furnace pressure gauge 8, while the oxygen partial pressure in the heating chamber is a predetermined value, 1 × 10 ^−19.6 to 1 × 10 Apply vacuum until ^-20 atm. Since the zirconia oxygen sensor is attached to the sensor attachment tube 21 and disposed between the heating chamber 6 and the vacuum pump 15, the oxygen partial pressure in the heating chamber can be accurately measured.

When the oxygen partial pressure in the heating chamber becomes a predetermined value, 1 × 10 ^−19.6 to 1 × 10 ⁻²⁰ atm or less, it is determined that the adsorption of adsorbed oxygen in the furnace structure in the heating chamber is completed, and the on-off valve 20 To finish the preparation. When the vacuum pump is stopped, the on-off valves 23 and 24 are closed to prevent the backflow of the vacuum pump oil. Thereafter, the work is carried into the cooling chamber from the furnace door as in the conventional case, and carburizing and quenching is performed by a predetermined operation. Note that when there is almost no decrease in oxygen partial pressure, a mechanical problem such as a leak may be considered. In this case, needless to say, it is necessary to stop the pretreatment and remove the cause.

  By performing such pretreatment, even if the heating chamber is not used for two days or more, after burnout, or after replacement of the internal structure of the furnace, the normal operation operation can be promptly started, and carburization is performed. Quality is also stable. Further, at the time of vacuum carburizing, at least the on-off valves 20 and 23 are closed, so that the zirconia oxygen sensor and the hydrocarbon gas in the heating chamber do not react with each other. The open / close valves 20, 23, and 24 are manual valves. This is because the oxygen partial pressure is measured after burnout or the like, and thus is used less frequently. Instead of pneumatic valves, solenoid valves, and the like. Automatic measurement is also possible.

It is a section explanatory view of a vacuum carburizing furnace showing an embodiment of the invention. It is a fragmentary sectional view which shows the state which attached the zirconia type oxygen sensor which shows embodiment of this invention to the heating chamber.

Explanation of symbols

1 Vacuum carburizing furnace 5 Intermediate door 6 Heating chamber
15 Vacuum pump 10a Vacuum pump inlet 16 Structure in furnace (heat insulating material, ceramic heater, ceramic rail)
20 First open / close valve 20a One connection port 20b of the first open / close valve The other connection port 21 of the first open / close valve 21 Sensor mounting tube 21a Tip 21b of sensor mounting tube Anti-tip side 21c of sensor mounting tube Side 21d Side opening 22 Detection pipe 22a Detection pipe one end 22b Detection pipe other end opening 23 Second open / close valve 23a Second open / close valve one connection port 23b Second open / close valve other connection Port 24 Third open / close valve 24a One connection port 24b of the third open / close valve Other connection port 30 of the third open / close valve 30 Zirconia-type oxygen sensor 31 Detector 36 Piping

Claims (1)

In a vacuum carburizing furnace that carries an iron-based workpiece into a heating chamber, decomposes hydrocarbon-based gas under reduced pressure and high temperature, reacts carbon on the iron surface of the workpiece, and carburizes the workpiece surface.
A detection pipe having one end opened in the heating chamber;
A first on-off valve connected to the other end opening of the detection pipe and one connection port outside the heating chamber;
A sensor mounting pipe having a tip connected to the other connection port of the first on-off valve;
A second opening / closing valve connected at one connection port with an opening that opens on the side opposite to the tip of the sensor mounting tube;
A third on-off valve connected at one connection port via a pipe with the other connection port of the second on-off valve;
A vacuum pump connected to the other connection port of the third on-off valve via an intake port, and a zirconia type attached to the opposite end side of the sensor mounting tube so that a detection portion is located inside the sensor mounting tube An oxygen sensor ,
Measuring the oxygen partial pressure by opening the first, second and third on-off valves, and determining that the adsorption of the adsorbed oxygen in the furnace structure in the heating chamber is complete; and
At least during vacuum carburization, the first opening / closing valve is closed and either the second or third or both are closed so that the zirconia oxygen sensor and the hydrocarbon gas do not come into contact with each other. A method for detecting oxygen partial pressure of vacuum carburizing in a heating chamber.

Images