METHOD OF MANUFACTURING A HEATING FURNACE TUBE WITH AN INTERMEDIATE ELEMENT

Abstract: A heating furnace tube, a method of using the same and a method of manufacturing the same which have been developed with a view to eliminating inconveniences occurring when a carbon-containing fluid is made to flow in the heating furnace tube. The heating furnace tube which comprises a rare earth oxide particle distributed iron alloy containing 17-26 wt.% of Cr and 2-6 wt.% of Al. The method of manufacturing this heating furnace tube which comprises the steps of forming or inserting an insert metal on or into at least one of a joint end portion of one heating furnace tube element and that of the other heating furnace tube element, bringing these two joint end portions into pressure contact with each other directly or via an intermediate member, and diffusion welding the two heating furnace tube elements to each other by heating the insert metal.

Description: The present invention relates to a method of manufacturing a heating furnace tube, and more particularly to a method of manufacturing a heating furnace tube whose problems are coking and carburization during operation at high temperature, such as a cracking tube of an ethylene plant. BACKGROUND ART In a cracking tube of an ethylene plant used as a heating furnace tube, for example, the coking is a problem, where carbon precipitates and deposits on the inner surface of the tube in the atmosphere of gas including carbon, such as, hydrocarbon, when the temperature is in the temperature range where carbon precipitation occurs. If coking is generated on the inner surface of the heating furnace tube, such serious problems as overheating and plugging may occur during the operation of the plant. Therefore decoking to remove deposited carbon by burning in a high temperature steam atmosphere must be performed frequently, and to perform decoking, operation of the plant must temporarily be stopped, which causes a considerable drop in productivity. Technologies which have been developed to solve this problem include the construction of a heating furnace tube made of material with Al-contained oxide film on the surface created by adding 1 - 10% Al to ferric alloy, or material with a high Al-contained layer created by aluminizing the surface of base alloy. In these conventional art, however, coking resistance is actually improved, but it is not yet sufficient to use the tube as a heating furnace tube in an actual industrial furnace. EP-A-0 418 606, which is considered to represent the most relevant state of the art, discloses a method of manufacturing a assembly of two tubes made of corrosion resistance material being joined via an insert material by diffusion bonding, in which prior to diffusion bonding the insert material is inserted between the bonding interface of the two tubes to be joined. During the diffusion bonding achieved by heating the inser element at least, the joint side edges of the two tubes a pressure is applied on the parts to be joined. With the foregoing in view, it is an object of the present invention to provide a method of manufacturing a heating furnace tube which has good coking resistance and can prevent a drop in productivity due to decoking. In the case of a heating furnace tube which causes carburization, such as the case of a cracking tube of an ethylene plant, the depth of carburization on the inner surface of the heating furnace tube is measured regularly to prevent damage on the heating furnace tube caused by carburization, and since operation of the plant must be stopped at each measurement, productivity drops considerably. Oxide dispersion strengthened (ODS) ferrous alloy, which is ferritic high chrome alloy (20Cr-5 Al-Fe) where rare earth oxide is dispersed, is known to have an extremely excellent high temperature strength and carburization resistance compared with conventional materials, and is therefore applied to a heating furnace tube, including joining method using fusion welding, friction welding, brazing or mechanical joint method. With the fusion welding method by TIG welding and electronic beam welding, which involves the fusion of joint areas, however, oxide particles float up and the dispersion strengthening function, which is a feature of rare earth ODS ferrous alloy, is lost, dropping high temperature to half or less. With the friction welding method, which does not involve the fusion of material, high temperature strength does not drop very much but the high jointing pressure generates a big burr at the joint area of the heating furnace tube, which will block the flow of fluid in the heating furnace tube. With brazing, high heat resistance cannot be expected because the melting point of the brazing filler metal is much lower than that of the base material, and with the mechanical joint method, such as riveting and screwing, maintaining air tightness at high temperature is extremely difficult, therefore both methods are inappropriate for jointing the heating furnace tubes. In this way, conventional jointing methods have difficulties to meet requirements for the joint areas of heating furnace tubes, including carburization, air tightness and high reliability, in addition to extremely high temperature strength and carburization resistance. A method of manufacturing a heating furnace tube according to the present invention is defined in claim 1. With this configuration, at least one heating furnace tube element is manufactured to be a heating furnace tube made of rare earth ODS ferrous alloy with good carburization resistance, therefore the interval of heating furnace replacement can be extended longer than a conventional heating furnace tube when used in an ethylene plant, and as a consequence a drop in productivity can be prevented. A further aspect of the invention is that the insert metal is formed by plating. With this configuration, the insert metal can be simply and surely disposed between joint side edges of the heating furnace tube element on one side and the heating furnace tube element on the other side and the coupling short pipe. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a table on the result after examining coking resistance, high temperature oxidation resistance and mechanical characteristics of various compositions of alloys including rare earth ODS ferrous alloy of the heating furnace tube; Fig. 2 is a graph which indicates the coking resistance comparison among Fe-20Cr-5 Al-Y 2 O 3 alloy which is one of the materials of the heating furnace tube, austenitic heat resistant alloy, which is a conventional material, and Fe-20Cr-5 Al alloy; Fig. 3 is a table on the result after examining the influence of S on coking resistance for rare earth ODS ferrous alloy of the heating furnace tube, and for austenitic heat resistant alloy of a conventional heating furnace tube; Fig. 4 is a perspective view of the main portion of the heating furnace tube; Fig. 5 is a perspective view depicting components and the tightener of the heating furnace tube; Fig. 6 is a cross-sectional side view of the main portion depicting the manufacturing mode of the heating furnace tube according to the present invention; Fig. 7 is a perspective view of the main portion of the heating furnace tube; Fig. 8 is a perspective view depicting components and the tightener of the heating furnace tube; Fig. 9 is a cross-sectional side view depicting the manufacturing mode of the heating furnace tube according to the present invention; Fig. 10 is a cross-sectional side view of the main portion depicting another embodiment of the heating furnace tube; Fig. 11 is a cross-sectional side view of the main portion depicting another embodiment of the heating furnace tube; and Fig. 12 is a cross-sectional side view of the main portion depicting another embodiment of the heating furnace tube. The embodiments disclosed in Figures 1-4, 7 and 11-12 and the corresponding parts of the description do not form part of the invention but represent background art and are useful for understanding the invention. BEST MODE FOR CARRYING OUT THE INVENTION Referring now to the drawings, best mode for carrying out the invention related to a method of manufacturing the heating furnace tube is described. In the drawings the same numerals and signs are used for the same components so as to omit redundant explanations. A heating furnace tube is a heating furnace tube used to flow fluid containing hydrocarbon or carbon monoxide, such as a cracking tube of an ethylene plant, and is made of rare earth ODS ferrous alloy using Al-added ferritic high chrome ferrous alloy as a base, more particularly yttrium ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight. Fig. 1 shows the result after examining coking resistance, higher temperature oxidation resistance and the mechanical characteristic (high temperature strength, ductility) for various compositions of alloys including yttrium ODS ferrous alloy used for the heating furnace tube of the present invention (Claim 1). Coking resistance here was judged based on a known carbon precipitation test. In other words, a sample (4 x 10 x 45 mm) for each alloy which surface has been finished with emery paper (600 grid) and has been oxidized under 950°C steam is prepared, each sample is buried into a solid carburizer, is carburized at 1100°C, is oxidized in a 1100°C atmosphere, and then coking and decoking are repeated 10 times, then weight changes before and after coking is checked for each sample, and coking resistance is judged depending on the degree of weight change. Coking test has been conducted under the following conditions. Source gas: benzene (0.5g / h), carrier gas: argon (16 Nml/min), adding quantity of S: 1 ppm or less, temperature: 800°C and time: 8 hours. High temperature oxidation resistance, high temperature strength and ductility are criteria to judge whether each alloy material can be practically used for a heating furnace tube considering state where the heating furnace tube is set, that is, the outer surface is heated by a burner, high temperature fluid flows through, and a mechanical strength sufficient enough to be a part of plant is required. In Fig. 1, indicates practically sufficient, ○ indicates possible, Δ indicates practically insufficient and * indicates impossible, and in the table of Fig. 1, the samples No. 8 - No. 15, that is, yttrium ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight is suitable for a heating furnace tube in terms of coking resistance and other characteristics. Fig. 2, on the other hand, shows the result of comparing austenitic heat resistant alloy (high Ni high Cr steel), Fe-20Cr-5Al alloy, and Fe-20Cr-5Al-Y 2 O 3 alloy, which is one of the materials of the heating furnace tube of the present invention for coking resistance, more specifically weight change before and after coking. While both high Ni high Cr steel and Fe-20Cr-5Al alloy are manufactured by melting process, Fe-20Cr-5Al-Y 2 0 3 alloy of the heating furnace tube is a dispersion-strengthened alloy manufactured by a powder metallurgy. A sample (4 x 10 x 45 mm) for each alloy which surface has been finished with emery paper (600 grid) and has been oxidized under a 950°C steam is prepared, each sample is buried into a solid carburizer, is carburized at 1100°C, is oxidized in a 1100°C atmosphere, and then coking and decoking are repeated 10 times, then weight changes before and after coking is checked for each sample. Coking test has been conducted under the following conditions. Source gas: benzene (0.5g / h), carrier gas: argon (16 Nml/min), adding quantity of S: 1 ppm or less, temperature: 800°C and time: 8 hours. As shown in Fig. 2, the weight change before and after coking of Fe-20Cr-5Al - Y 2 O 3 alloy of the heating furnace tube is still less than Fe-20Cr-5Al alloy with excellent coking resistance, which make it clear that the addition of yttrium oxide to Fe-C -Al alloy contributes to a major improvement of coking resistance. A comparison result between samples No. 5 and No. 8 in Fig. 1 also shows that the addition of yttrium oxide to Fe-Cr-Al alloy contributes to a major improvement of coking resistance. The table in Fig. 1 clearly shows that ferritic high chrome ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight present a relatively good coking resistance, and particularly yttrium ODS ferrous alloy of samples No. 8 - No. 15 causes extremely low weight changes before and after coking, approximately 1 mg / cm 2 , indicating good coking resistance, as seen in Fe-20Cr-5 Al-Y 2 O 3 alloy in Fig. 2 compared with the high Ni High Cr steel and Fe-20Cr-5Al alloy. Material which contains 28% or more Cr by weight and 8% or more Al by weight is low in ductility, more specifically in breaking elongation, and material which contains 14% or less Cr by weight is poor in oxidation resistance at a high temperature area, therefore both of these materials have problems in practical use as material for heating furnace tubes, and material where yttrium oxide has not been added is low in high temperature strength, therefore it is difficult to apply this material to actual heating furnace tubes. Yttrium ODS ferrous alloys of samples No. 8 - No. 15 where yttrium oxide has been added, on the other hand, have better coking resistance and high temperature strength, and as a result, it became clear that rare earth ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight is suitable for the material of heating furnace tubes. The heating furnace tube is made of rare earth ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight. And since rare earth ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight has extremely good coking resistance, as mentioned above, the heating furnace tube can minimize the coking caused by operation of plant, and the interval of decoking can be extended much longer than conventional types. The extension of the interval of decoking in this manner improves the productivity of plant considerably, and the economical effect becomes enormous, including a decrease in cost for decoking, a decrease of thermal fatigue due to operation stops and restarts of the plant at decoking, and an increase of life. In the case of cracking tubes of ethylene plant, where the tube wall temperature exceeds 1000°C, even reaching 1100°C at some locations, accurate control is required for the operation of the plant with current austenitic heat resistant alloy, since the melting point is only about 150 - 200°C higher than 1100°C, but if rare earth ODS ferrous alloy which has the melting point of 1480°C is used for heating furnace tubes, safety will further improve, and operation of plant will be even simpler. Since the heating furnace tube of ethylene plant is extremely long, it is advisable to use the heating furnace tube only for locations where coking problems occur, using the heating furnace tube made of conventional materials for other locations, and connecting these tubes to construct an entire heating furnace tube from an economical point of view. It is well known that adding S (sulfur) compound to fluid which flows through the heating furnace tube is effective to control coking. Fig. 3 shows the result of examining the influence of S on coking resistance for rare earth ODS ferrous alloy of the heating furnace tube that is, rare earth ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight, and austenitic heat resistant alloy (25Cr-35Ni steel) of a conventional heating furnace tube. Coking test has been conducted under the following conditions. Source gas: 10% methane + hydrogen, additive: DMS (dimethyl sulfide), 0 ppm, 200 ppm (S = 100 ppm in atomic weight units), temperature: 900°C, 1000°C, 1100°C and time: 5 hours. Fig. 3 indicates a comparison of coking resistance depending on material, the amount of adding S compound, and temperature when the amount of weight change caused by coking of rare earth ODS ferrous alloy when S compound is added. Under 900°C and 1000°C temperature conditions, weight change caused by coking is decreased by adding S compound for both the rare earth ODS ferrous alloy and austenitic heat resistant alloy which is the comparison material (conventional material), as the table in Fig. 3 shows. Under a 1100°C temperature condition, on the other hand, weight change caused by coking is decreased by adding S compound in the case of the rare earth ODS ferrous alloy, but weight change caused by coking tends to be increased by adding S compound in the case of austenitic heat resistant alloy, which is the comparison material (conventional material). In this way, it is clear that adding 100 ppm of S in atomic weight units improves the coking resistance of both rare earth ODS ferrous alloy and austenitic heat resistant alloy under a 1000°C or less temperature condition. So in a heating furnace tube created by connecting the heating furnace tube of rare earth ODS ferrous alloy, and the heating furnace tube made of austenitic heat resistant alloy, good coking resistance is acquired by adding 100 ppm or less of S in atomic weight units to fluid that flows through the tube and by using the tube under a temperature condition of 1000°C or less and 550°C or more where the rare earth ODS ferrous alloy does not cause brittle fractures (at 475°C), and as a consequence the heating furnace tube can be suitably used in a hydrocarbon environment where coking easily occurs. Adding the amount of adding S exceeds 100 ppm (in atomic weight units) is not suitable for industrial purposes because problems related to corrosion and problems related to S removal in downstream process occur. To add S (sulfur) to fluid that flows through the heating furnace tube, DMS (dimethyl sulfide) and DMDS (dimethyl disulfide) can be suitably used, and a sufficient effect can also be expected by adding hydrogen sulfide gas. As described above, the cost of plant can be considerably decreased if the entire heating furnace tube of an ethylene plant is created by connecting the heating furnace tube made of rare earth ODS ferrous alloy and the heating furnace tube made of austenitic heat resistant alloy, in other words, using the heating furnace tube made of austenitic heat resistant alloy for part of the entire heating furnace tube. If fluid containing 100 ppm or less of S in atomic weight units flows through the heating furnace tube constructed by connecting the heating furnace tube made of rare earth ODS ferrous alloy and a heating furnace tube made of austenitic heating resisting alloy, using the heating furnace tube in a 550°C to 1000°C temperature range, coking on rare earth ODS ferrous alloy and austenitic heat resistant alloy can be minimized, and therefore the interval of decoking can be extended much longer than conventional types. The extension of the interval of decoking in this manner improves the productivity of plant considerably, and the economical effect becomes enormous, including a decrease in cost for decoking, a decrease of thermal fatigue due to operation stops and restarts of the plant at decoking, and an increase of life. Fig. 4 to Fig. 6 show the heating furnace tube created by connecting the heating furnace made of rare earth ODS ferrous alloy and the heating furnace tube made of conventional material, and the method of manufacturing the heating furnace tube according to the present invention. The heating furnace tube 1 shown in Fig. 4 is created by joining a heating furnace tube element 10 on one side made of the heating furnace tube made of rare earth ODS ferrous alloy which contains 19 - 26% of Cr by weight and 3 - 6% of Al by weight with the heating furnace tube element on the other side 20 made of austenitic heat resistant alloy, which are joined via insert metal by diffusion bonding. The heating furnace tube element on one side 10 and the heating furnace tube element on the other side 20 of the heating furnace tube 1 are joined via the coupling short pipe 30 made of austenitic heat resistant alloy which is the same material as the heating furnace tube element on the other side 20. For the heating furnace tube element on the other side 20, not only austenitic heat resistant alloy but also rare earth ODS ferrous alloy which is the same material as the heating furnace tube element on one side 10 can be used. For the coupling short pipe 30 as well, not only the above mentioned austenitic heat resistant alloy but also rare earth ODS ferrous alloy which is the same material as the heating furnace tube element on one side 10 can be used. The above-described heating furnace tube 1 is manufactured by the following processes according to the present invention. At first, the outer surface of the joint side edges of the heating furnace tube element on one side 10 (outer diameter: 70 mm, wall thickness: 5 mm) and the heating furnace tube element on the other side 20 which size is the same as the heating furnace tube element on one side 10 (outer diameter: 70 mm, wall thickness: 5 mm) are ground in a 30 mm range from the edge of the respective tube component to be a surface roughness of 25S. Then Ni - 4% B alloy film M used as insert metal is formed to 50 µm thick by electroplating on the outer surface of the joint side edge of the heating furnace tube element on one side 10, and on the outer surface of the joint side edge of the heating furnace tube element on the other side 20, which are finished as above. For the insert metal, a regular amorphous metal product, such as B Ni metal for brazing, can be used. Then the joint side edge of the heating furnace tube element on one side 10 and the joint side edge of the heating furnace tube element on the other side 20 are inserted to each edge of the coupling short pipe 30 for 30 mm respectively. Here, the coupling short pipe 30 has a 70-mm internal diameter, 8-mm wall thickness, and 60-mm length, and the inner surface 30a has been finished to a 25S surface roughness. Both ends of the outer surface of the coupling short pipe 30 have approximately 10° of tapered surfaces 30t and 30t where the diameter decreases toward the edge. It is possible to form the film of insert metal not only on the joint side edges of the above mentioned heating furnace tube elements 10 and 20, but also on the inner surface 30a of the above mentioned coupling short pipe 30, and it is also possible to form the film of insert metal only on the inner surface of the above mentioned coupling short pipe 30. After inserting the joint side edge of the heating furnace tube element on one side 10 and the joint side edge of the heating furnace tube element on the other side 20 to the coupling short pipe 30 respectively, the coupling short pipe 30 is contacted in the radius direction by the tighteners 40 and 41 attached to each tapered surface 30t and 30t of the coupling short pipe 30, so that the joint side edges of the heating furnace tube elements 10 and 20 and inner surface 30a of the coupling short pipe 30 are pressed sandwiching the plating layers M and M of the insert metal, and are contact with pressure for joining. The tighteners 40 and 41 here have a ring shape respectively, of which the inner surface has tapered surface 40t and 41t, similar to the tapered surface 30t and 30t of the above mentioned coupling short pipe 30, and the coupling short pipe 30 is contacted in the radius direction by moving the tighteners attached to each tapered surface 30t and 30t of the coupling short pipe 30 in the direction where they approach each other. The pressurization means P to contact with pressure the coupling short pipe 30 to the heating furnace tube element on one side 10 and the heating furnace tube element on the other side 20 is made of the tapered faces 30t and 30t of the above mentioned coupling short pipe 30 and the above mentioned tighteners 40 and 41. After joining the heating furnace tube element on one side 10 and the heating furnace tube element on the other side 20 with the coupling short pipe 30, the inside of each heating furnace tube element 10 and 20 is exhausted until the degree of vacuum becomes 0.001 Torr or less. 10A and 20A of Fig. 6 are barrier plates attached to seal openings of the edges of the heating furnace tube elements 10 and 20 when the inside of each one of the heating furnace tube elements 10 and 20 is exhausted. Exhausting inside each one of the heating furnace tube elements 10 and 20 prevents oxidation of the film M of the insert metal, and allows checking the joining state between each one of the heating furnace tube elements 10 and 20 and the coupling short pipe 30. Joining each one of the heating furnace tube elements 10 and 20 via the coupling short pipe 30 makes centering (aligning center axis) of the heating furnace tube element on one side 10 and the heating furnace tube element on the other side 20 easier, and implements air tightness inside and outside each heating furnace tube elements 10 and 20. It is acceptable to fill inert gas inside each one of the heating furnace tube element 10 and 20 after exhausting to prevent oxidation of the film M of insert metal, and it is also acceptable to fill inert gas inside each one of the heating furnace tube element 10 and 20 without exhausting. After exhausting inside each one of the heating furnace tube element 10 and 20, a heater H inserted in the above mentioned heating furnace tube elements 10 and 20 increase the temperature by induction heating up to the temperature at which the film M of the insert metal melts, and this temperature is held for one hour to progress diffusion bonding (liquid phase diffusion bonding). It is also possible to melt the film M of insert metal and progress diffusion bonding by infrared heating, instead of using induction heating. When each one of the heating furnace tube elements 10 and 20 is heated from inside, each heating furnace tube element 10 and 20 thermally expands in the radius direction, which prevents a drop of joining pressure between each heating furnace tube element 10 and 20 and the coupling short pipe 30 during heating along with the joining force by the above mentioned pressurization means P. After diffusion bonding is completed by holding high temperature for one hour, heating furnace tube 1 is cooled down to room temperature, then the tighteners 40 and 41 are removed from the coupling short pipe 30 and the manufacturing process of the heating furnace tube 1 ends. As Fig. 4 shows, the completed heating furnace tube 1 has an appearance that the coupling short pipe 30 is attached at the connection part between the heating furnace tube element on one side 10 and the heating furnace tube element on the other side 20. In the case of the heating furnace tube 1 that has the above mentioned configuration, the cost of plant can be considerably decreased by using the heating furnace tube element on the other side 20 which is partially made of austenitic heating resisting alloy. If the heating furnace tube 1 that has the above mentioned configuration allows fluid containing 100 ppm or less of S in atomic weight units to flow through and is used in a 550°C to 1000°C temperature range, coking at the heating furnace tube element on one side 10 made of rare earth ODS ferrous alloy and at the other heating furnace tube element on the other side 20 made of austenitic heat resistant alloy can be minimized, and as a consequence the interval of decoking can be extended much longer than a conventional heating furnace tube, which generates enormous high economical effect. Needless to say, the heating furnace tube and its use can be effectively applied not only to the cracking tubes of an ethylene plant, but also to various heating furnace tubes which are subject to coking problems, such as CCR plant in a petroleum refining plant. The heating furnace tube of which one objective is to solve the problem of carburization is also manufactured by exactly the same process as the heating furnace tube 1 shown in Fig. 4 to Fig. 6. As Fig. 7 to Fig. 9 show, the heating furnace tube 1' is created by joining the heating furnace tube element on one side 10' made of yttrium ODS ferrous alloy which contains 20% of Cr by weight and 4.5% of Al by weight, and the heating furnace tube element on the other side 20' made of austenitic heat resistant alloy (25Cr - 35Ni-Fe) by diffusion bonding via insert metal. The above mentioned heating furnace tube 1' has a coupling short pipe (intermediate member) 30' made of an austenitic heat resistant alloy (25Cr-35Ni-Fe) which is the same material as the heating furnace tube element on the other side 20', and the heating furnace tube element 10' on one side and the heating furnace tube element on the other side 20' are joined via the above mentioned coupling short pipe 30'. For the heating furnace tube element on the other side 20', not only austenitic heat resistant alloy tube, such as Hpmod centrifugal casting tube, Sumitomo Metal HPM, and Inco Alloy 803, but also ODS ferrous alloy tube which has the same material as the heating furnace tube element on one side 10' can be used. For the coupling short pipe 30', not only the above mentioned austenitic heat resistant alloy tube but also an ODS ferrous alloy tube which has the same material as the heating furnace tube element on one side 10' and ferritic heat resistant alloy tube can be used. The above mentioned heating furnace tube 1' is manufactured through the following process according to the present invention. At first, the outer surface of the joint side edges of the heating furnace tube element on one side 10' (outer diameter: 70 mm, wall thickness: 5 mm) and the heating furnace tube element on the other side 20' which size is the same as the heating furnace tube element on one side 10' (outer diameter: 70 mm, wall thickness: 5 mm) are ground in a 30 mm range from the edge of the respective tube component to be a surface roughness of 25S. Then Ni - 4% B alloy film M used as insert metal is formed to 5 µm thick by electroplating on the outer surface of the joint side edge of the heating furnace tube element on one side 10' and the outer surface of the joint side edge of the outer heating furnace tube element on the other side 20', which are finished as above. For insert metal, a regular amorphous metal product, such as BNi metal for brazing, can be used. Then the joint side edge of the heating furnace tube element on one side 10' and the joint side edge of the heating furnace tube element on the other side 20' are inserted to each edge of the coupling short pipe 30' for 30 mm respectively. Here the coupling short pipe 30' has a 70-mm inner diameter, an 8-mm wall thickness and 60-mm length, and the inner surface 30a' has been finished to be a 25S surface roughness. Both ends of the outer surface of the coupling short pipe 30' have approximately 10° of tapered surfaces 30t' and 30t' where the diameter decreases toward the edge. It is possible to form the film of the insert metal not only on the joint side edges of the above mentioned heating furnace tube elements 10' and 20', but also on the inner surface 30a' of the above mentioned coupling short pipe 30', and it is also possible to form the film of the insert metal only on the inner surface of the above mentioned coupling short pipe 30'. After inserting the joint side edge of the heating furnace tube element on one side 10' and the joint side edge of the heating furnace tube element on the other side 20' to the coupling short pipe 30' respectively, the coupling short pipe 30' is contacted in the radius direction by the tighteners 40' and 41' attached to each tapered surface 30t' and 30t' of the coupling short pipe 30', so that the joint side edges of the heating furnace tube elements 10' and 20' and the inner surface 30a' of the coupling short pipe 30' are pressed sandwiching the plating layers M and M of the insert metal, and are contact with pressure for joining. The tighteners 40' and 41' here have a ring shape respectively, of which the inner surface has tapered surface 40t' and 41t' similar to the tapered surfaces 30t' and 30t' of the above mentioned coupling short pipe 30', and the coupling short pipe 30' is contacted in the radius direction by moving the tighteners attached to each tapered surface 30t' and 30t' of the coupling short pipe 30' in the direction where they approach each other. The pressurization means P to contact with pressure the coupling short pipe 30' to the heating furnace tube element on one side 10' and the heating furnace tube element on the other side 20' is made of the tapered faces 30t' and 30t' of the above mentioned coupling short pipe 30' and the above mentioned tighteners 40' and 41'. After joining the heating furnace tube element on one side 10' and the heating furnace tube element on the other side 20' with the coupling short pipe 30, the inside of each heating furnace tube element 10' and 20' is exhausted until the degree of volume becomes 0.001 Torr or less. 10A' and 20A' in Fig. 9 are barrier plates attached to seal openings of the edges of the heating furnace tube elements 10' and 20' when the inside of each one of the heating furnace tube elements 10' and 20' is exhausted. Exhausting inside each one of the heating furnace tube element 10' and 20' prevents oxidation of the film M of the insert metal, and allows checking the joining state between each one of the heating furnace tube elements 10' and 20' and the coupling short pipe 30'. Joining each one of the heating furnace tube elements 10' and 20' via the coupling short pipe 30' makes centering (aligning center axis) of the heating furnace tube element on one side 10' and the heating furnace tube element on the other side 20' easier and implements air tightness inside and outside each heating furnace tube element 10' and 20'. It is acceptable to fill inert gas inside each one of the heating furnace tube element 10' and 20' after exhausting to prevent oxidation of the film M of the insert metal, and it is also acceptable to fill inert gas inside each one of the heating furnace tube element 10 and 20 without exhausting. After exhausting the inside of each one of the heating furnace tube element 10' and 20', a heater H inserted in the above mentioned heating furnace tube elements 10' and 20' increases the temperature by induction heating up to the temperature at which the film M of the insert metal melts, and this temperature is held for one hour to progress diffusion bonding (liquid phase diffusion bonding). It is also possible to melt the film M of insert metal and progress diffusion bonding by infrared heating, instead of induction heating. When each one of the heating furnace tube elements 10' and 20' is heated from the inside, each heating furnace tube element 10' and 20' thermally expands in the radius direction, which prevents a drop of joining pressure between each heating furnace tube element 10' and 20' and the coupling short pipe 30' during heating along with the joining force by the above mentioned pressurization means P. After diffusion bonding is completed by holding high temperature for one hour, the heating furnace tube 1' is cooled down to room temperature, then the tighteners 40' and 41' are removed from the coupling short pipe 30', and the manufacturing process of the heating furnace tube 1' ends. As Fig. 7 shows, the completed heating furnace tube 1' has an appearance that the coupling short pipe on one side 30' is attached at the connection part between the heating furnace tube element on one side 10' and the heating furnace tube element on the other side 20'. It has been confirmed that the heating furnace tube 1' manufactured in this way has a practically sufficient performance where the joint section can endure a 100 barometric pressure during hydraulic tests. In the heating furnace tube 1' manufactured through the above mentioned processes, at least the heating furnace tube element on the one side 10' is made of yttrium ODS ferrous alloy which has good carburization resistance, therefore the interval of heating furnace tube replacement can be extended longer than a conventional heating furnace tube. This brings out a considerable economical effect since the expense of replacing heating furnace tubes can be decreased, and an extension of the interval of operations stops of the plant due to carburization depth measurement prevents a drop in productivity, and an extension of the interval of operation stops of the plant due to carburization depth measurement also decreases thermal fatigue caused by the stop in furnace operation and restart of operation, which extends the operating life of the plant. The strength of the joint section of each heating furnace tube element 10' and 20' of the heating furnace tube 1' is lower than an ODS ferrous alloy tube and an austenitic heat resistant alloy tube which are base materials. The high temperature strength and the carburization resistance of an austenitic heat resistant alloy tube, which is used for a part of the heating furnace tube 1', is lower than those of an ODS ferrous alloy tube, and it is known that an ODS ferrous alloy tube also exhibits an embrittlement phenomena, depending on the operating temperature range. As a result, experiments were conducted in terms of structural change and carburization resistance to confirm the optimum operating temperature range of the heating furnace tube 1', and the following results were obtained. Joint Structural change Carburization resistance ODS ferrous alloy / ODS ferrous alloy 475°C brittleness 1200°C or less ODS ferrous alloy / ferritic heat resistant alloy 475°C brittleness 1100°C or less ODS ferrous alloy / austenitic heat resistant alloy 475°C brittleness 1100°C or less For an evaluation of carburization resistance, a specimen was set in a quartz tube and heated in an electric furnace where hydrogen and methane gas flowed through, then the weight change of the specimen was measured. As the above result shows, the operating temperature range of the heating furnace tube 1' should be set to 550°C or more where 475°C embrittlement does not occur, and to 1200°C or less where carburization resistance is sufficient. In other words, using the heating furnace tube 1' in a 550°C to 1200°C temperature range prevents brittle fractures caused by 475°C embrittlement and implements sufficient carburization resistance. In the case of a conventional heating furnace tube made of austenitic heat resistant alloy, on the other hand, considerable carburization occurs in a temperature range that exceeds 1100°C. This carburization problem can be solved by using ODS ferrous alloy tubes for all the heating furnace tube elements located near a 1100°C temperature environment, and using austenitic heat resistant alloy tubes for heating furnace tube elements located near a 1000°C temperature environment, such as the exit of the furnace. In this way, the method of manufacturing the heating furnace tube related to the present invention allows using different materials for heating furnace tube elements depending on the operating temperature conditions, and especially using an austenitic heat resistant alloy tube for a part of the heating furnace tube can decrease plant cost. The heating furnace tube 100 in Fig. 10 is made of the heating furnace tube element on one side 110 made of an ODS ferrous alloy tube and the heating furnace tube element on the other side 120 made of an austenitic heat resistant alloy which are joined via insert metal by diffusion bonding. For the heating furnace tube element on the other side 120, not only an austenitic heat resistant alloy tube but also an ODS ferrous alloy tube which is the same material as the heating furnace tube element on one side 110 can be used. Film M of insert metal is formed on the outer surface of the joint side edge of heating furnace tube element on one side 110 and the joint side edge of the heating furnace tube element on the other side 120 is expanded so as to engage with the joint side edge of the heating furnace tube element on one side 110. To manufacture the heating furnace tube 100, the joint side edge of the heating furnace tube element on one side 110 is engaged with the joint side edge of the heating furnace tube element on the other side 120 first, then the tightener 140 of which the inner surface has the tapered surface 140t is attached to the tapered surface 120t of the heating furnace tube element on the other side 120, and a stopper block 141 is attached to the section 120f of the heating furnace tube element on the other side 120 where the diameter increases. Then the joint side edge of the heating furnace tube element on the other side 120 is contacted with pressure to the joint side edge of the heating furnace tube element on one side 110 sandwiching the plate layer M of the insert metal, then diffusion bonding is progressed by exhausting, increasing temperature, and by holding the high temperature, in the same manner as the manufacturing process of the above mentioned heating furnace tube 1', and the tube is cooled down to room temperature after diffusion jointing completes, then the tightener 140 and the stopper block 141 are removed and the manufacturing process of the heating furnace tube 100 ends. The film M of the insert metal can be formed not only on the heating furnace tube element on one side 110 but also on the inner surface of the heating furnace tube element on the other side 120, and can be formed only on the heating furnace tube element on the other side 120. Needless to say, the heating furnace tube 100 with the above mentioned configuration also generates an effect equivalent to the above mentioned heating furnace tube 1'. In the case of the above mentioned heating furnace tube 100, where the heating furnace tube element on one side 110 and the heating furnace tube element on the other side 120 are directly jointed via insert metal by diffusion bonding, the coupling short pipe as intermediate member used for the above mentioned heating furnace tube 1' is unnecessary. The heating furnace tubes and their manufacture disclosed in Figures 11-12 do not fall under the scope of claim 1. The heating furnace tube 200 in Fig. 11 is made of the heating furnace tube element on one side 210 made of an ODS ferrous alloy tube and the heating furnace tube element on the other side 220 made of an austenitic heat resistant alloy tube which are joined via insert metal by diffusion bonding. For the heating furnace tube element on the other side 220, not only an austenitic heat resistant alloy tube but also an ODS ferrous alloy tube which is the same material as the heating furnace tube element on one side 210 can be used. A male screw 210S is on the joint side edge of the heating furnace tube element on one side 210 and a female screw 220S is on the heating furnace tube element on the other side 220, and film M of the insert metal is formed on the outer surface of the joint side edge of the heating furnace tube element on one side 210 for the entire joining surface with the heating furnace tube element on the other side 220, by electroplating. To manufacture the heating furnace tube 200, the male screw 210S on the heating furnace tube element on one side 210 and the female screw 220S on the heating furnace tube element on the other side 220 are screwed together to mechanically join each heating furnace tube element 210 and 220, and the joint side edges of the heating furnace tube elements 210 and 220 are contacted with pressure sandwiching the plate layer M of the insert metal, then diffusion bonding is progressed by exhausting, increasing temperature, and holding the high temperature, in the same matter as the manufacturing process of the above mentioned heating furnace tube 1', then the tube is cooled down to room temperature after diffusion bonding completes, and the manufacturing process of the heating furnace tube 200 ends. The film M of the insert metal M can be formed not only on the heating furnace tube element on one side 210 but also on the heating furnace tube element on the other side 220, and can be formed only on the heating furnace tube element on the other side 220. The female screw can be on the heating furnace tube element on one side 210, and the male screw on the heating furnace tube element on the other side 220. A circular insert ring made of insert metal can be attached at the joint sections a and b of each one of the heating furnace tube elements 210 and 220. Needless to say, the heating furnace tube 200 with the above mentioned configuration also generates an effect equivalent to the above mentioned heating furnace tube 1'. In the case of the above mentioned heating furnace tube 200, where the heating furnace tube element on one side 210 and the heating furnace tube element on the other side 220 are directly joined via insert metal by diffusion bonding, the coupling short pipe as intermediate member used for the above mentioned heating furnace tube 1' is unnecessary. Also in the above mentioned heating furnace tube 200, mechanical strength and air tightness in a high temperature environment can be implemented by combining mechanical joining by screwing and diffusion bonding. The heating furnace tube 300 in Fig. 12 is made of the heating furnace tube element on one side 310 made of an ODS ferrous alloy tube and the heating furnace tube element on the other side 320 made of an austenitic heat resistant alloy tube which are joined via insert metal by diffusion bonding. For the heating furnace tube element on the other side 320, not only an austenitic heat resistant alloy tube but also an ODS ferrous alloy tube which is the same material as the heating furnace tube element on one side 310 can be used. A tapered shaped convex engaging section 310T is on the joint side edge of the heating furnace tube element on one side 310, and a tapered shaped concave engaging section 320T is on the joint side edge of the heating furnace tube element on the other side 320, and film M of insert metal is formed on the outer surface of the joint side edge of the heating furnace tube element on one side 310, for the entire joining surface with the heating furnace tube element on the other side 320 by electroplating To manufacture the heating furnace tube 300, the convex engaging section 310T on the heating furnace tube element on one side 310 and the concave engaging section 320T on the heating furnace tube element on the other side 320 are engaged to mechanically join each one of the heating furnace tube element 310 and 320, and the joint side edges of the heating furnace tube elements 310 and 320 are contact with pressure sandwiching the plate layer M of insert metal by applying 0.1 kg / mm 2 or more compression stress in the tube axis direction of each heating furnace tube element 310 and 320. Then diffusion bonding is progressed by exhausting, increasing temperature and holding the temperature in the same manner as the manufacturing process of the above mentioned heating furnace tube 1', then the tube is cooled down to room temperature after diffusion bonding completes, and the manufacturing process of the heating furnace tube 300 ends. The film M of insert metal can be formed not only on the heating furnace tube element on one side 310 but also on the heating furnace tube element on the other side 320, and can be formed only on the heating furnace tube element on the other side 320. The concave engaging section can be on the heating furnace tube element on one side 310, and the convex engaging section can be on the heating furnace tube element on the other side 320. A circular insert ring made of insert metal can be attached at the joint sections a and b of each one of the heating furnace tube elements 310 and 320. Needless to say, the heating furnace tube 300 with the above mentioned configuration also generates an effect equivalent to the above mentioned heating furnace tube 1'. In the case of the above mentioned heating furnace tube 300, where the heating furnace tube element on one side 310 and the heating furnace tube element on the other side 320 are directly joined via insert metal by diffusion bonding, and the coupling short pipe as intermediate member used for the above mentioned heating furnace tube 1' is unnecessary. Also in the above mentioned heating furnace tube 300, mechanical strength and air tightness in a high temperature environment can be implemented by combining mechanical joining by a tapered joint and diffusion bonding. The film M of insert metal used in each one of the above mentioned embodiments is formed by electroplating, but wet plating, dry plating, electroless plating, physical deposition (e.g. vacuum deposition, sputtering, ion plating) and vapor plating, including chemical deposition (e.g. high temperature CVD, plasma CVD), spraying and coating metal paste and other methods can be used to form the film M of insert metal. It is also possible to insert metal, specifically tubular or circular insert material made of a thin plate of insert metal or insert metal film into the area between the heating furnace tube element on one side and the heating furnace tube element on the other side. Needless to say, the method of manufacturing the heating furnace tube related to the present invention can be effectively applied not only to the cracking tubes of an ethylene plant but also to various heating furnace tubes which are subject to carburization problems, such as CCR plant in a petroleum refining plant. INDUSTRIAL APPLICABILITY A method of manufacturing a heating furnace tube related to the present invention can be effectively applied to heating furnace tubes which are subject to coking and carburization problems.

Claims: A method of manufacturing a heating furnace tube (1) comprising one heating furnace tube element (10) made of rare earth oxide dispersion strengthened ferrous alloy which contains 17 - 26% of Cr by weight and 2 - 6% of Al by weight; and another heating furnace tube element (20) made of the rare earth oxide dispersion strengthened ferrous alloy or heat resistant alloy which are joined via an insert metal (M) by diffusion bonding, wherein the method of manufacturing the heating furnace tube comprises the steps of: forming or inserting the insert metal (M) to at least one of joint side edges of said one heating furnace tube element (10) and said another heating furnace tube element (20); contacting with pressure the joint side edge of said one heating furnace tube element (10) with the joint side edge of said another heating furnace tube element (20) via an intermediate member (30); and diffusion bonding of said one heating furnace tube element (10) and said another heating furnace tube element (20) by heating the insert metal (M), wherein the intermediate member (30) is a coupling short pipe (30) to which the joint side edge of said one heating furnace tube element (10) and the joint side edge of said another heating furnace tube element (20) are inserted, and said one heating furnace tube element (10) and said another heating furnace tube element (20) are joined via the coupling short pipe (30) by performing diffusion bonding in the state where the joint side edges of said one heating furnace tube element (10) and said another heating furnace tube element (20) and the coupling short pipe (30) are contacted under pressure by a pressurization means (P), via the insert metal (M) isposed between the joint side edges of said one heating furnace tube element (10), said another heating furnace tube element (20) and the coupling short pipe (30), wherein the pressurization means (P) comprises a tightener (40, 41) having tapered surfaces (40t, 41t) which engages with the tapered surface (40t, 41t) and contacts the coupling short pipe (30) on tapered surfaces (30t) in a radius direction. The method of manufacturing the heating furnace tube according to Claim 1, characterized in that the insert metal (M) is formed by plating.

Inventors: HOSOYA KEIZO, ICHIMURA MUNEYASU, ISHII KUNIO, MURATA TAKEO, NAKAMURA TOSHIKAZU, SASANO RIN, SATO KENJI, YAMAMOTO KATSUMI, HOSOYA, KEIZO, ICHIMURA, MUNEYASU, ISHII, KUNIO, MURATA, TAKEO, NAKAMURA, TOSHIKAZU, SASANO, RIN, SATO, KENJI, YAMAMOTO, KATSUMI, HOSOYA, Keizo, JGC Corporation, ICHIMURA, Muneyasu, Res. &amp; Dev. Center, JGC Corp., ISHII, Kunio, JGC Corporation, MURATA, Takeo, Res. &amp; Dev. Center, JGC Corp, NAKAMURA, Toshikazu, Res. &amp; Dev. Center, JGC Corp, SASANO, Rin, Res. &amp; Dev. Center, JGC Corp., SATO, Kenji, Res. &amp; Dev. Center, JGC Corp., YAMAMOTO, Katsumi, Res. &amp; Dev. Center, JGC Corp.
Applicants: JGC CORP, JGC CORPORATION
Application year: 1998
Main classification: C22C38
IPC codes: B23K20, C22C32, C22C33, C10G9, C22C1, F27B17, C22C38