Method 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). The quantity of a residual CH 4 , a partial pressure of the oxidization gas and a partial pressure of C0 are measured. The quantity of each gas to be supplied into the furnace(1) is controlled according to either one of the values of the partial pressures.

Description: This invention relates to a method of controlling an atmosphere in a heat treatment furnace, and more particularly relates to a control method of 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.159567/1986 and 63260/1992. However, in the method shown in the Japanese Patent Application Laid-Open No.159567/1986, the oxidization gas to be added in the furnace is oxygen, the partial pressure of CO is approximately 29% in case that CH 4 is used as the hydrocarbon series gas, and the partial pressure of CO is approximately 38% in case that C 4 H 10 is used as the hydrocarbon series gas. In the method shown in the Japanese Patent Application Laid-Open No.63260/1992, the partial pressure of CO is approximately 40% in case that CO 2 is used and butane is used as the hydrocarbon series gas. According to the conventional methods, the carburizing time can be shortened, because the partial pressure of CO is higher than that in the other normal method, however, the oxidization at the grain boundary layer of the goods to be treated is promoted. Further, the partial pressure of CO in the atmosphere in the furnace is fluctuated because a large quantity of air is introduced into the furnace when the goods to be treated are inserted into and taken out of the furnace. In the method shown in the Japanese Patent Application Laid-Open No. 63260/1992, the quantity of the hydrocarbon series gas to be supplied into the furnace is controlled so that the carbon potential in the atmosphere becomes constant. However, in fact, the atmosphere is varied in the large extent according to the change of the type (weight and surface area) of goods to be treated, and accordingly the fluctuation of the carbon potential becomes large, so that the fluctuation in the surface carbon contents of steel becomes large. Further, 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 the diffusion time, the main effect is the Boudouard 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. European Patent Application No. 0 738 785 discloses a method for controlling the CO content of furnace atmosphere for carburizing and carbonitriding metallic workpieces in a furnace. The method includes the step of feeding a mixture of an oxidizing reagent and a hydrocarbon-containing fuel into the furnace for producing a CO-containing furnace atmosphere. The CO content of the furnace atmosphere is measured and compared to a preset minimal CO value. A CO-forming substance is introduced into the furnace atmosphere when the measured CO content is no longer greater than the preset minimal CO value. US Patent No. 4 950 334 discloses a gas carburizing method in which a hydrocarbon and air are supplied to a furnace that carburizes a workpiece. The flow rate of the hydrocarbon gas is maintained at a predetermined value. The flow rate of air to be supplied can be altered by adjusting the carbon potential of the atmospheric gas generated in the furnace. Japanese Patent Application Publication No. 03193863 discloses a method for obtaining adequate carburization depth by determining a carbon potential from the detected values of the carburizable gas quantity, oxygen quantity and temperature of zones in a carburization chamber and comparing the detected values with set values thereby controlling the carbon potential to the optimum value. US Patent No. 4 372 790 discloses a method and device for control of the carbon level of a gas mixture reacting in a heat treatment furnace. The controlled condition is determined by a measurement of the portion of the gas component CO present in the furnace chamber, a measurement of the electrical voltage of an oxygen-ion conducting solid body electrolyte and a measurement of the furnace chamber temperature. An object of the present invention is to obviate the above defects. The invention provides a method of controlling an atmosphere in a heat treatment furnace in accordance with claim 1 of the appended claims. 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 the furnace, and stopping the supply of the oxidization gas when the partial pressure of CO in the furnace reaches a predetermined value. 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 the furnace, stopping the supply of the oxidization gas when the partial pressure of CO in the furnace reaches a predetermined value, and controlling the supply quantity of the hydrocarbon series gas so that the carbon potential in the furnace reaches a predetermined value. In the present invention, the hydrocarbon series gas is butane, and the predetermined value for the partial pressure of CO is approximately 30%. In the present invention, the hydrocarbon series gas is propane, and the predetermined value for the partial pressure of CO is approximately 27%. In the present invention, the hydrocarbon series gas LPG, and the predetermined value for the partial pressure of CO is approximately 29%. In the present invention, the hydrocarbon series gas is methane, and the predetermined value for the partial pressure of CO is approximately 24%. Still 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 the furnace, : and controlling the supply quantity of the hydrocarbon series gas so that the carbon potential in the furnace reaches a predetermined value. In the present invention, the supply of the hydrocarbon series gas is stopped 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 provied 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 CO in the furnace, means for operating a carbon potential 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 partial pressure of CO in the furnace and the carbon transfer coefficient. Fig. 5 is a graph explaining the relationship between CO% and the depth of the grain boundary oxidization layer. Fig. 6 is a graph explaining the relationship between CO 2 /CH 4 and CO%. Fig. 7 is a graph explaining the relationship between the present invention and the conventional method with respect to the changes in CO%, the surface carbon contents, and the effective case depth. Fig. 8 is a graph explaining the relationship between the changes in the quantity of undecomposed residual CH 4 , the quantity of added C 4 H 10 , and the quantity of added CO 2 , according to the carburizing time. Fig. 9 is microphotographs of structure showing the grain boundary layer oxidization of the present invention and the conventional method. Fig. 10 is a graph explaining the relationship between the carburizing time and the effective case depth in each of the present invention and the conventional method. Fig. 11 is a table for explaining the quantity of consumed gas in each of the present invention and the endothermic method. 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 fan 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 , 9 denotes an analyzer for analyzing the partial pressure of CO, 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 of 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. 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. As shown in Fig. 4, the carburizing speed is varied according to the carbon transfer factor β and becomes maximum when the partial pressure of CO in the carburizing furnace atmosphere is 50%. Further, if the partial pressure of CO is increased, the partial pressure of CO 2 is also increased. Fig. 5 shows the relationship between the depth of the grain boundary oxidization layer from the surface and the partial pressure of CO (the partial pressure of CO is in proportion to the partial pressure of CO 2 ). It is publicly known that the depth of grain boundary oxidization layer is limited to 13.5µm in consideration of the affection on the figure strength of materials. Accordingly, an optimum partial pressure of CO is determined by the value of the partial pressure of CO corresponding to the depth is 13.5 µm. The optimum value is approximately 30% CO in case that the hydrocarbon series gas is butane. Accordingly, in the present invention, when the partial pressure of CO in the furnace reaches approximately 30%, the optimum partial pressure of CO is judged from the analyzing result of the analyzer 9 and the control valve 14 for the oxidization gas is closed. As apparent from the experimental result shown in Fig. 6, CH 4 and CO 2 are reacted in the ratio 1:1 stoichiometrically, so that the control valve 14 is adjusted so that the hydrocarbon series gas is varied centering around approximately 30% CO, in case that the hydrocarbon series gas is butane. However, In actual, the ratio of CO 2 and CH 4 becomes other than 1:1, because air is entered into the furnace when the goods to be treated are introduced into the furnace and the hermetical seal of the furnace is disordered. Accordingly, each of valves 12 and 14 is controlled according to the result of measurement of the partial pressure of CO. Further, the same effect can be obtained by controlling the quantity of the hydrocarbon series gas to be introduced into the furnace while maintaining the quantity of the oxidization gas constant. As stated above, when CO is controlled to approximately 30%, the following formula can be obtained: a c = Pco/(Kp · Po 2 1/2 ) where a c is the activity of carbon in austenite, Kp is the equilibrium constant obtained from <C>+1/2. O 2 →CO, and PO 2 is the partial pressure of oxygen. Accordingly, a c can be expressed by the function of PO 2 1/2 , because Kp is constant if the temperature and CO are constant. In order to obtain a required carbon potential, the valve 12 for the hydrocarbon series gas is opened if the value of the electromotive force of oxygen is less than a required value, and the valve 12 is closed if the value of the electromotive force of oxygen is more than the required value. The carbon potential can be obtained if CO and O 2 are operated by substituting the analyzing result of CO into the formula (1). When the temperature is varied, the change of Kp is calculated automatically (for example, by the formula, log Kp=5840.6/T+4.583) and the operation is carried out by substituting the value of change of Kp into the formula (1). (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. Fig.7 shown the differences between the present invention and the methods shown in the Japanese Patent Applications Laid-Open Nos. 159567/1986 and 63260/1992 with respect to the partial pressure of CO, the surface carbon contents of goods to be treated and the effective case depth when the heat treatment is carried out. As shown in Fig. 7, according to the present invention the fluctuation of CO with respect to CO% can be reduced in the range of 28.5-31.5% (30%±1.5%) in case that the hydrocarbon series gas is butane and the desired value of CO% is 30%, whereas according to the conventional methods the fluctuation of CO is in the range of 23-40%. Further, according to the present invention the fluctuation of surface carbon contents can be reduced in the range of 1.10-1.30% in case that the desired value is 1.20%, whereas according to the conventional methods the fluctuation of surface carbon contents is in the range of 0.7-1.70%. Similarly, according to the present invention the fluctuation of effective case depth can be reduced in the range of 0.6-0.8mm in case that the desired value of effective case depth is 0.7mm, whereas according to the conventional methods the fluctuation of effective case depth is in the range of 0.55-0.85mm. Fig. 8 shows the relationship between the change in quantity of added gases with time and the change of the partial pressure of CO with time, where the maximum quantity of C 4 H 10 gas passing through the value 12 is set to 2.5liter/minute, and the maximum quantity of CO 2 gas passing through the valve 14 is set to 2.0 liter/minute. Each quantity of added C 4 H 10 and CO 2 becomes maximum at approximately 930°C, however, each of the valves 12 and 14 is controlled directly according to the analyzing result of CO, so that the quantity of CO is controlled with the precision of 30%±1.05%. 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. As apparent from Fig. 8 in case that butane of 2.5 liter/minute is added as the hydrocarbon series gas at 930°C, the sooting is occurred. However, the sooting can be prevented from occurring, because the quantity of the hydrocarbon series gas to be introduced is reduced gradually in the present invention. Further, according to the present invention as shown in Fig. 8, the fluctuation of CO in the atmosphere could be controlled to 30% ± 1.50%, in cases that butane was added as the hydrocarbon series gas while the weight of goods introduced into the furnace was varied from 150kg÷2 to 150kg×2, or the weight was set to a predetermined value and the surface area was reduced by one half or increased by six times. (Embodiment 2) Fig. 9 shows the microphotographs of structure of carburizing in case that butane is used as the hydrocarbon series gas according to the present invention (CO is approximately 30%) and according to the conventional method using an endothermic gas (CO is approximately 23%). The microphotograph of right side shows that of the present invention, whereas the left side shows that of the conventional method. In each of microphotographs, the left side is the surface where the grain boundary oxidization is occurred. The depth of grain boundary oxidization layer is about 10 µm in both cases. This means that the grain boundary oxidization is not accelerated, because CO is controlled to approximately 30%. (Embodiment 3) Fig. 10 shows the relationship between the carburizing time and effective case depth in case that the goods of 150kg are carburized at 930°C according to the method of the present invention and the conventional method. It is apparent from Fig. 10 that according to the present invention, effective case depth becomes larger by approximately 19% during a predetermined carburizing time than in case that the endothermic gas is used. Accordingly, in the present invention, the carburizing time can be shortened in case that effective case depth is set to a predetermined value, compared with the conventional method. (Embodiment 4) Fig. 11 shows the comparison of consumed gas in the present invention wherein C 4 H 10 gas and CO 2 gas are used and in the conventional method wherein endothermic gas as the raw gas and C 4 H 10 gas as the enriched gas are used, in case that the carburizing is carried out to obtain a depth of effective hardened layer of 1mm (corresponding to 0.4% C)under the state that the heat treating temperature is 930 °C, and the carbon potential is fixed to 1.0%. As a result, according to the present invention the quantity of C 4 H 10 gas to be used for obtaining the depth of effective hardened layer of 1mm can be reduced by 69% compared with that in the conventional endothermic gas method. 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. Further, according to the present invention, the sooting can be prevented from occurring by closing the control valve 12 in accordance with the analyzing result of the analyzer 10 when the quantity of the residual CH 4 is changed to increasing from decreasing to stop the introduction of hydrocarbon series gas C x H y and to prevent the residual CH 4 from increasing. Furthermore, in the present invention, the partial pressure of oxygen is measured by measuring the electromotive force of the sensor 6, and the control valve 12 is closed when the partial pressure of oxygen reaches a predetermined value, so that the sooting can be prevented from occurring. As stated above, according to the present invention, even if the type (weight and surface area) of goods to be treated or the empty furnace holding time is changed the carbon potential can be maintained constant and the quality of goods to be treated can be stabilized, by controlling the quantities of hydrocarbon series gas and oxidization gas to be added to maintain the partial pressure of CO in the atmospher constant, in the heat treatment, such as gas carburizing, carbonitriding or the bright controlled atmosphere heat treatment. Further, 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. 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 oxidization gas into the furnace; and stopping the supply of the oxidization gas when the partial pressure of CO in the furnace reaches a predetermined value; characterised in that : the hydrocarbon series gas is selected from butane, propane, LPG and methane; and the predetermined value of the partial pressure of CO in the furnace is approximately 30% if the hydrocarbon series gas is butane, approximately 27% if the hydrocarbon series gas is propane, approximately 29% if the hydrocarbon series gas is LPG and approximately 24% if the hydrocarbon series gas is methane. The method of controlling an atmosphere in a heat treatment furnace as claimed in claim 1, characterised in that the method further comprises the step of controlling the supply quantity of the hydrocarbon series gas so that the carbon potential in the furnace reaches a predetermined value. The method of controlling an atmosphere in a heat treatment furnace as claimed in claim 1 or 2, characterised in that the supply of the hydrocarbon series gas is stopped when the quantity of a residual CH 4 in the furnace is changed to increasing from decreasing. The method of controlling an atmosphere in a heat treatment furnace as claimed in claim 1, 2 or 3, characterised in that the oxidization gas is air or CO 2 gas.

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
Applicants: DOWA MINING CO, DOWA MINING CO., LTD.
Application year: 1998
Main classification: C23C8
IPC codes: C23C8, C21D1