Patent Publication Number: US-6657152-B2

Title: Torch head for plasma spraying

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a torch head for plasma spraying and, more particularly, to a torch head which is inserted into a tube member having a very small diameter to form a film by complete spraying on the inner surface of the tube member. 
     2. Prior Art 
     As torch heads for forming coatings by plasma splaying on inner surfaces of tube members, various torch heads have been proposed already. For example, in U.S. Pat. No. 4,877,937, a “plasma spray torch” as shown in FIG. 4 is proposed. This spray torch, according to the brief of the above publication, is described as follows 
     “a plasma spray torch comprises a spray nozzle which forms an electrode and which includes a nozzle duct, and a second electrode associated therewith, in a portion of a torch arm, which is electrically insulated from the spray nozzle. The torch arm has flow passages for working gas and for a cooling agent, the latter flowing in one of the flow ducts to the nozzle and being removed after producing its cooling effect from another flow duct. A powder feed conduit opens into the nozzle duct. The working gas flow duct is connected to a duct which passes through the second electrode while at least in the region of its mouth opening, the nozzle duct is inclined relative to the longitudinal axis of the torch arm or the flow duct therein. In a method of internally coating a tube by plasma spraying, the torch is introduced into the tube which is then rotated and moved axially relative to the torch during the spray operation”. 
     In a conventional torch head as shown in FIG. 4, since working gas (changed into a plasma by a discharge arc and heated to such a temperature that a powder can be melted) passage must be formed in a cathode, a cooling agent for cooling the cathode side cannot be formed in the cathode. 
     In the conventional torch head shown in FIG. 4, since the nozzle duct is inclined relative to the longitudinal axis of a flow duct, melted spraying material cannot perpendicularly collide with the inner wall surface of a tube material. For this reason, the spraying material is partially scattered without forming a coating, and it is considered that an excess of material must be used to form a satisfactory coating. 
     For this reason, for example, a “plasma spray gun” is proposed in Japanese Patent Publication No. 3-57833. This spray gun, according to FIG.  5  and “Claims” in the above publication, 
     “is a plasma spray gun which is inserted into a pipe or an object to be processed and which includes a cooled electrode  10  and a burner nozzle  12  for coating the inner surface of the object to be processed, and is characterized in that 10 kw can be obtained at the most.” 
     A satisfactory coating cannot be obtained when a plasma energy is small for the following reason. Since a spraying material is supplied into plasma working gas together with gas, the spraying material is a powder having an average grain diameter of 5 to 45 μm to make it easy to supply the spraying material. When the spraying material has a grain diameter of 5 μm or smaller, not only is the spraying material very expensive, but the spraying material may also combine with oxygen and nitrogen in the air and thus fail to form an expected coating. When the spraying material has a grain diameter of 45 μm or more, the spraying material is not sufficiently melted by the plasma working gas. When the spraying material comprises the powder is to be melted, and an arc is small and short, the working gas is not sufficiently changed into a plasma and a high temperature is not achieved, and the spraying material is not sufficiently melted. In addition, since the injection speed of the working gas cannot be considerably high, the kinetic energy of the spraying material must be small, and a collision energy sufficient to form a coating cannot be obtained. 
     For this reason, the present inventor investigated a torch head shown in FIG. 6 or  7 . In the torch head shown in FIG. 6, a plasma generation chamber is perpendicular to the longitudinal axis of the torch body, and a cathode is coaxially arranged in the plasma generation chamber. Although a high-energy plasma can be generated, it is difficult to set the diameter of the entire torch head such that the torch head can be inserted into a tube member having an inner diameter of about 50 mm. This is because, when the torch head is to be reduced in size, the distance between the cathode and the anode member must be reduced, and a high voltage cannot be applied across these electrodes. In addition, the cooling passage is limited, and a high-energy plasma cannot be generated. 
     On the other hand, in the torch head shown in FIG. 7, a cathode is coaxially arranged in a torch body, and the distance between the cathode and the anode member can be increased such that a high-energy plasma can be generated. However, since the passage of a plasma gas is bent at an angle of 90°, the anode member is considerably worn. This is because, a high-temperature working gas changed into a plasma by an arc generated between the cathode and the anode member collides with the wall of the passage which is formed in the anode member and which is bent at an angle of 90° to heat the wall portion and to wear the wall portion within a short period of time. 
     In addition, the present inventor devised a torch head shown in FIGS. 8 and 9 to improve the above torch head. The torch head shown in FIGS. 8 and 9 has a plasma gas supply chamber located in an anode member along the longitudinal axis of the anode member. A cathode is coaxially arranged in the plasma gas supply chamber, and a mouth opening to be perpendicular to the longitudinal axis of the plasma gas supply chamber is formed on the side surface of the anode member. In this manner, it is considered that an arc toward the mouth opening is generated. In fact, at the beginning of the use of the torch head, “distorted arcs” indicated by reference numerals  21  in FIGS. 8 and 9 are generated, and it is understood that the anode member is quickly worn by the distorted arcs. 
     Therefore, the present inventor evaluated various torch heads configurations of this type in order to: 
     1) spray a plasma gas into a narrow tube member (diameter of 30 mm to 300 mm), 
     2) use a powder having an average grain diameter of 5 to 45 m as a spraying material, 
     3) increase the plasma energy to about 30 kw to 45 kw, and 
     4) suppress distorted arcs from being generated to elongate the lifetime of a positive electrode (anode). 
     SUMMARY OF THE INVENTION 
     The present invention has been made on the basis of the above circumstances. It is an object of the present invention to provide a coating that can be satisfactorily formed in plasma spraying in a narrow tube member to make it possible to elongate the lifetimes of electrodes. 
     In order to achieve this objective as a means which passes through the orifice  16  at a high speed. Since the plasma generation chamber  17  located at the position of the outlet of the orifice  16  is bent at an angle of 90° with respect to the longitudinal axis of the cathode  12 , the working gas generates a small turbulent flow and has not been sufficiently thinned at this point. The working gas is gradually thinned while forming a stationary flow between the inner bottom of the plasma generation chamber  17  and the mouth opening  18 . This thinning is maximum in the plasma generation chamber  17  located immediately near the mouth opening  18 . This is because the region outside of the mouth opening  18  has the atmospheric pressure, and the atmospheric pressure is remarkably lower than the pressure in the plasma gas supply chamber  15 . 
     The working gas in the plasma generation chamber  17  which is immediately near the mouth opening  18  is thinned because the orifice  16  exists. In the orifice  16 , the opening area is set to be ⅓ to {fraction (1/10)} the opening area of the mouth opening  18 . This is because when the opening area of the orifice  16  is larger than ⅓ of the opening area of the mouth opening  18 , the working gas cannot be effectively thinned immediately near the mouth opening  18 . When the opening area of the orifice  16  is smaller than {fraction (1/10)} of the opening area of the mouth opening  18 , it cannot be expected to smoothly inject the working gas. 
     When a DC voltage is applied across the cathode  12  and the anode member  13 , the arc  20  is generated between the cathode  12  and the anode member  13 . This arc  20  extends from the cathode  12  to a region where the working gas of the plasma generation chamber  17  is maximally thinned, i.e., a region near the mouth opening  18  of the plasma generation chamber  17  in the torch head  10 . More specifically, the arc  20 , as shown in FIGS. 1 to  3 , is generated from the dial end of the cathode  12  at an angle of about 90°. 
     One the anode member  13  side at which the arc  20  arrives, as described above, cooling is performed from the outside by the cooling water which enters from the anode cooling water passage  13   a  into the cooling chamber  13   b . In the plasma generation chamber  17  in the anode member  13 , since the working gas which has not been heated stationarily flows, cooling by the working gas is stationarily performed. As a matter of course, no parts are heated by the arc  20 , and no parts are worn by the arc  20 . 
     As described above, the arc  20  is generated between the cathode  12  and the inner wall of the plasma generation chamber  17  near the mouth opening  18 , i.e., a region near the mouth opening  18  of the anode member  13 . When the working gas passes through the plasma generation chamber  17 , the working gas is changed into a high-temperature gas plasma by the arc  20 . At this time, since the arc  20  extends from the cathode  12  to a position immediately near the mouth opening  18 , the working gas is sufficiently changed into a plasma and heated to a high temperature. More specifically, the torch head  10  generates a plasma gas having a high energy. 
     When the spraying material  30  is supplied, through the spraying material supply tube  14 , to the plasma gas discharged from the mouth opening  18 , the spraying material  30  goes toward the inner surface of the tube member  40  together with the plasma gas flow. At the same time, energy is given from the high- temperature plasma gas to the spraying material  30  to soften or melt the spraying material  30 . When the spraying material  30  collides with the inner surface of the tube member  40 , the spraying material  30  is further heated by the kinetic energy. The spraying material  30  sufficiently adheres to the inner surface of the tube member  40  without being reflected or rebounded from the inner surface, and the coating  31  is formed without wasting the spraying material  30 . 
     A torch head  10  according to the first aspect can achieve the following operations or advantages: 
     1) Since the arc  20  is generated from the distal end of the cathode  12  at an angle of about 90°, the arc  20  can be sufficiently long, and the plasma energy of the plasma working gas can be made high, i.e., about 30 to 45 kw. 
     2) Since the above high energy can be obtained, an oxide or a metal oxide having a size of about 5 to 45 μm can be used as the spraying material  30 , and the coating  31  having a sufficient thickness and a sufficient function can be formed. 
     3) For this reason, although the tube member  40  is narrow, the coating  31  facing an open wall and having a sufficient thickness and a sufficient function can be formed. 
     4) Since the disturbed arc  21  or a high-temperature plasma is not in direct contact with the anode member  13  constituting the plasma generation chamber  17 , the anode member  13  is not worn early, and, consequently, the lifetime of the anode member  13  is long. In the embodiment to be described later, the lifetime is 200 hours. 
     In order to solve the above problems, as a means according to the second aspect of the invention, in the torch head  10  according to the first aspect, 
     “the longitudinal axes of an orifice  16 , a cathode  12  stored in the orifice  16 , and a cathode tube  12   a  supporting the cathode  12  are spaced apart from the center of the torch body  11  by a distance which is 5 to 15% the size of the torch body  11  on the opposite side of the mouth opening  18 ”. 
     More specifically, in the torch head  10  according to the second aspect, the longitudinal axis of the orifice  16 , the cathode  12 , and the cathode tube  12   a  are spaced apart from the mouth opening  18  as far as possible. In this manner, the arc generated between the cathode  12  and the anode member  13  is elongated. 
     As a matter of course, “keeping away” of the respective members from the mouth opening  18  must be performed in the torch body  11  having only a limited space. For this reason, the actual distance between the mouth opening  18  and the respective members must be about 10 to 15% the size (outer diameter) of the torch body  11 . More specifically, when the distance of the “keeping away” from the center of the torch body  11  is smaller than 5% the diameter of the torch body  11 , a substantial advantage cannot be obtained. In contrast to this, it is almost impossible that the distance is larger than 15% in the limited space of the torch body  11 , and spraying on the inner surface of the narrow tube member  40  cannot be performed. 
     Therefore, the torch head  10  according to the second aspect can achieve the same function as that of the torch head  10  according to the first aspect, as a matter of course, can more elongate the arc  20 , can increase a plasma energy even on the inner surface of the narrow tube member  40 , and, consequently, can increase and improve the thickness and the function of the coating  31 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an enlarged sectional view of a torch head according to the present invention when the torch head is inserted into a tube member subjected to spraying. 
     FIG. 2 is a more enlarged sectional view of the torch head. 
     FIG. 3 is a cross-sectional view along a  1 — 1  line in FIG.  2 . 
     FIG. 4 is a partially sectional view showing a prior art. 
     FIG. 5 is a partially sectional view showing another prior art. 
     FIGS. 6A and 6B show Sample  1  made by the present inventor, in which FIG. 6A is a partially sectional view and FIG. 6B is a cross-sectional view along a  2 — 2  line in FIG.  6 A. 
     FIGS. 7A and 7B show Sample  2  made by the present inventor, in which FIG. 7A is a partially sectional view and FIG. 7B is a cross-sectional view along a  3 — 3  line in FIG.  7 A. 
     FIG. 8 is a partially sectional view showing Sample  3  made by the present inventor. 
     FIG. 9 is a cross-sectional view along a  4 — 4  line in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
     A torch head  10  according to an embodiment in which the present invention is illustrated will be described below. The torch head  10  substantially includes the aspects of the present invention. 
     FIG. 1 is a sectional view of the torch head  10  which is to perform spraying in the tube member  40 . The tube member  40  is set for the torch head  10  according to this embodiment such that the tube member  40  itself is repeatedly reciprocated and rotated. As a matter of course, to the torch head  10  shown in FIG. 1, a supply of cooling water and a spraying material  30  which is a powder, a power supply, and a supply of working gas are processed from the right in FIG.  1 . 
     The torch head  10  includes a cylindrical torch body  11  having such a diameter (25 to 45 mm in this embodiment) that the torch body  11  can be inserted into the tube member  40 , a cathode tube  12   a  accommodated in the torch body  11 , an anode cooling water passage  13   a , and a plasma supply tube  19 . The distal end (the left end in FIG. 1) of the torch body  11  is integrated with an anode member  13  having a mouth opening  18 . A spraying material supply tube  14  opening toward the mouth opening  18  of the anode member  13  is arranged outside the torch body  11 . 
     In the torch head  10  according to this embodiment, the longitudinal axis of the cathode tube  12   a  is spaced apart from the center of the torch body  11  by a distance which is about 10% of the diameter of the torch body  11  on the opposite side of the mouth opening  18 , arid a cooling water tube  12   b  is inserted into the cathode tube  12   a , and the cathode  12  is attached to the distal end of the cathode tube  12   a . As a matter of course, the cathode tube  12   a , as shown in FIGS. 1 and 2, is insulated from the torch body  11  and the anode member  13  through an insulator  11   a.    
     The distal end of the cathode tube  12   a  is stored in a plasma gas supply chamber  15  formed in the anode member  13  in a non-contact state, and the cathode  12  arranged at the distal end of the cathode tube  12   a  is stored in a non-contact state in an orifice  16  formed deep in the plasma gas supply chamber  15 . The distal end of the cathode  12  projects into the plasma generation chamber  17  communicating with the orifice  16 , and the projection position of the distal end is substantially set at the center of the plasma generation chamber  17 . The longitudinal axis of the plasma generation chamber  17  is bent at an angle of 90° with respect to the longitudinal axis of the orifice  16 , so that the direction of the flow of working gas flowing from the orifice  16  is bent at an angle of 90°. The distal end of the plasma generation chamber  17  serves as the mouth opening  18  facing the inner surface of the tube member  40 . 
     The plasma generation chamber  17  according to this embodiment has a diameter of about 6 mm. The diameter is about four times the opening area of the orifice  16  into which the cathode  12  is inserted. The longitudinal axis extending from the bent portion of the plasma generation chamber  17  is perpendicular to the direction of the longitudinal axis of the torch body  11  as described above. 
     To the mouth opening  18  at the distal end of the plasma generation chamber  17 , the spraying material  30  which is a powder is supplied by the spraying material supply tube  14  in the transverse direction. The spraying material  30  used in the torch head  10  according to this embodiment is alumina having an average grain size of 20 μm. 
     The anode member  13  according to this embodiment, as indicated by a dotted line in FIG. 2, supplies cooling water into the cooling chamber  13   b  formed at the distal end of the anode member  13  through the forward anode cooling water passage  13   a  arranged in the torch body  11 . The cooling water which exhibits a cooling function is exhausted to the outside through the backward anode cooling water passage  13   a  communicating with the cooling chamber  13   b.    
     As a result of the above configuration, in the torch head  10 , an arc  20  between the cathode  12  and the anode member  13  is generated substantially perpendicular to the longitudinal axis of the cathode  12 . For this reason, as shown in FIGS. 1 to  3 , the arc  20  is generated such that the arc  20  long extends from the cathode  12  to a position immediately near the mouth opening  18 , a change from working gas into a plasma and an increase in energy of the working gas are achieved. When the spraying material  30  is injected into the plasma gas, the spraying material  30  is changed into droplets by the heat or the like of the plasma gas, and the coating  31  having a relatively large thickness is efficiently formed on the inner surface of the tube member  40 . 
     Spraying is performed by using the torch head  10  according to this embodiment under the following conditions: 
     Material and average grain size of spraying material: alumina, 20 μm; 
     Supply of cooling water: 20 m/min; 
     Applied voltage and current value: 60 volts, 700 ampere (42 kw); 
     Material tube and inner diameter of tube member  40 : cast-iron tube, 50 mm; and 
     Diameter of torch body 11:26 to 32 mm. 
     The thickness of the coating  31  formed by the above items was 500 μm or more, an impurity such as nitride was rarely detected on the surface of the coating  31 . In addition, when the torch head  10  is used under the above conditions, the endurance time of the coating  31  was about 200 hours. 
     As has been described above, as illustrated in the above embodiment, the present invention has the following characteristic feature, 
     “the torch head  10  for plasma spraying which is inserted into the tube member  40  to form the coating  31  on the inner surface of the tube member  40  by plasma spraying including the torch body  11  which is inserted into the tube member  40 , the cathode tube  12   a  which is arranged in the torch body  11  such that the longitudinal axis of the cathode tube  12   a  is aligned to the longitudinal axis of the torch body  11  and which has the cathode  12  at the distal end of the cathode tube  12   a , the anode member  13  which is arranged on the distal end side of the cathode tube  12   a , and the spraying material supply tube  14  which opens toward the mouth opening  18  formed in the anode member  13  and which is arranged outside the torch body  11 , 
     wherein, in the anode member  13 , the plasma gas supply chamber  15  in which the front end of the cathode tube  12   a  is stored in a non-contact state, the orifice  16  which communicates with the plasma gas supply chamber  15  and in which the cathode  12  is stored in a non-contact state, and the plasma generation chamber  17  which communicates with the orifice  16 , which has a longitudinal axis substantially perpendicular to the longitudinal axis of the torch body  11 , and which has the mouth opening  18  are formed, 
     the opening area of the orifice  16  when the anode is inserted is made ⅓ to {fraction (1/10)} the opening areas of the plasma generation chamber  17  and the mouth opening  18  so that the arc  20  from the distal end of the cathode  12  is generated within a range of 0° to 40° with respect to the longitudinal axis of the plasma generation chamber  17  perpendicular to the longitudinal axis of the cathode  12 . In this manner, when spraying in a narrow tube member is performed, a satisfactory coating can be obtained, and the lifetimes of electrodes can be elongated. 
     In the torch head  10 , when 
     “the longitudinal axes of the orifice  16 , the cathode  12  stored in the orifice  16 , and the cathode tube  12   a  supporting the cathode  12  are spaced apart from the center of the torch body  11  by a distance which is 5 to 15% of the size of the torch body  11  on the opposite side of the mouth opening  18 ″, 
     in addition to the above advantages, the arc  20  can be more elongated, and a high energy can be obtained. The coating  31  can be more effectively formed.