Abstract:
A fabrication method of a steam turbine blade equipped with an erosion shield includes the steps of preparing constituent elements including the steam turbine blade having a blade part, the erosion shield to be joined to a leading edge part of the blade part on the tip side thereof, and a shim to be disposed between the blade part and the erosion shield, any of the constituent elements having a backing part to serve as a backing for preventing burn through of molten metal at the time of the electron beam welding; assembling the constituent elements; performing electron beam welding to the leading edge part of the blade part, the erosion shield and the shim while utilizing the backing; and machining including removal of the backing part, after the electron beam welding, thereby forming the steam turbine blade in the shape of a final product thereof.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese Patent application serial no. 2013-234681, filed on Nov. 13, 2013, the content of which is hereby incorporated by reference into this application. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a fabrication method of a steam turbine blade equipped with an erosion shield, and in particular, to a method of fabricating a steam turbine blade by joining an erosion shield to a steam turbine blade by means of welding. 
       BACKGROUND ART 
       [0003]    With a steam-power steam turbine or a nuclear power generation steam turbine, an erosion protection material is jointed to the leading edge of a steam turbine blade, on a side thereof, adjacent to steam-inflow with a shim material interposed therebetween through GTAW (Gas Tungsten Arc Welding) or electron beam welding in order to prevent erosion from occurring at the leading edge of the steam turbine blade for use in wet steam, as described in Patent Literatures 1 through 3. 
         [0004]    In general, in the case of joining executed by single-layer welding, using an electron beam, a welding condition, such as an acceleration voltage, an electron beam current, a welding speed, a focal length, etc., is finely adjusted against the type and the board thickness of a constituent material to thereby select an optimum condition for preventing occurrence of burn through before the joining is executed. 
       CITATION LIST 
       [0005]    [Patent Literature 1] Japanese Unexamined Patent Application Publication No. S62(1987)-250124 
         [0006]    [Patent Literature 2] Japanese Unexamined Patent Application Publication No. S63(1988)-97802 
         [0007]    [Patent Literature 3] Japanese Unexamined Patent Application Publication No. Hei 05(1993)-23920 
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0008]    As higher efficiency of the steam turbine has been attained in recent years, there have been advances in trends toward a longer length of the blade of a turbine blade, and rendering of the turbine blade in a three-dimensional shape, for the purpose of attaining flow optimization, resulting in an increase of the board thickness of the erosion protection material. In the case where the single-layer welding by use of downward electron beam welding is applied to a blade material, an erosion protection material, and a shim material, each of which is larger in thickness, metal melted by heat of the electron beam is caused to flow downward due to empty weight, concurrently with the electron beam penetrating through the board, thereby causing occurrence of burn through of molten metal. A resultant occurrence of an undercut of a bead surface will pose an important issue in the joining of the erosion protection material from a fabrication point of view. 
         [0009]    Further, with welding using an electron beam, the larger the thickness of a weldment as a target is, the greater will be the need for increasing a welding current value, that is, an output. Still further, with a low-voltage electron beam welding machine, there is the need for rendering a working current value larger than that for a high-voltage electron beam welding machine. As the output is increased, so does a deviation of an electron beam output, and the range of an optimum welding condition for the single-layer welding will become narrower. The deviation in a welding current will become the cause of a defect due to the burn through of the molten metal and incomplete fusion, thereby causing the joining of the erosion protection material through the single-layer welding using the electron beam welding to be extremely difficult. 
         [0010]    There is conceivably a method whereby a stiffening plate is placed on the back side of a groove, in a joint geometry, to thereby prevent occurrence of the burn through in order to cope with the burn through occurring at the time of application of the electron beam welding. With this method, however, there arises the need for preparing a backing material aside from those materials for use in fabrication, resulting in occurrence of a supply cost of the backing material, so that the method has demerits in terms of a fabrication cost. 
         [0011]    It is therefore an object of the present invention to provide a fabrication method of a steam turbine blade equipped with an erosion shield, whereby an erosion shield can be welded to the leading edge of a steam turbine blade by electron beam welding without separately preparing a backing material, while preventing occurrence of burn through. 
       Solution to Problem 
       [0012]    According to one aspect of the present invention, there is provided a method of fabricating a steam turbine blade equipped with an erosion shield. The method includes the steps of preparing constituent elements including the steam turbine blade having a blade part, the erosion shield, and a shim, wherein any of the constituent elements has a backing part to serve as a backing for preventing burn through of molten metal at the time of the electron beam welding; assembling the constituent elements so that the backing part is arranged on the back side of a groove; performing electron beam welding to the leading edge part of the blade part, the erosion shield and the shim while utilizing the backing; and applying a machining work including removal of the backing part after the electron beam welding so as to be finished up in the final shape of the blade part as a target. 
       Advantageous Effects of Invention 
       [0013]    With the present invention, an erosion shield can be welded to the leading edge of a steam turbine blade without separately preparing a backing material, while preventing occurrence of burn through. 
         [0014]    Other problems, configurations, and effects of the invention will be apparent from the following detailed description of the preferred embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIGS. 1 ( a )  through  1  ( d ) each are a view illustrating the flow of a joining process for an erosion protection material, according to an embodiment of the present invention (a first embodiment); 
           [0016]      FIGS. 2 ( a )  through  2  ( d ) each are a view illustrating the flow of a joining process for an erosion protection material, according to another embodiment of the present invention (a second embodiment); 
           [0017]      FIGS. 3 ( a )  through  3  ( d ) each are a view illustrating the flow of a joining process for an erosion protection material, according to still another embodiment of the present invention (a third embodiment); and 
           [0018]      FIG. 4  is a general view of a steam turbine blade as an example of the steam turbine blade to which each embodiment of the present invention is applied. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    Embodiments of the present invention are described below with reference to the accompanied drawings. 
         [0020]      FIG. 4  is a general view of a steam turbine blade, as an example of a steam turbine blade to which each of the embodiments of the present invention is applied.  FIG. 4  illustrates the turbine blade in the final stage of the steam turbine in the case of a low pressure turbine. The steam turbine blade has a blade part  1 , a shroud cove  7 , a coupling part  8  to be coupled with a turbine rotor, and an erosion shield  2  provided at a leading edge part of the blade part (on the steam inflow side of the steam turbine blade), on the tip side thereof. In each of  FIGS. 1( a ) through 3( d )  to be referred to later on, there is shown the blade part of the steam turbine blade, in cross section, taken on line A-A of  FIG. 4 . 
       First Embodiment 
       [0021]      FIGS. 1 ( a )  through  1  ( d ) each illustrate a fabrication process of a steam turbine blade, according to a first embodiment of the present invention. A leading edge part of the tip of the steam turbine blade is shown in the respective figures. Constituent materials (constituent elements) that constitute the steam turbine blade are composed of the blade part  1  of the steam turbine blade, the erosion shield  2 , and a shim  3  disposed between the blade part  1  and the erosion shield  2 , as shown in  FIG. 1 ( a ) . For the turbine blade, use is made of a Ti alloy (for example, a Ti alloy containing 16% Al, and 4% V), 12-Cr stainless steel, etc. For the erosion shield, use is made of an erosion-resistant Ti alloy (for example, a Ti alloy containing 15% Mo, 5% Zr, and 13% Al) if the turbine blade is made of the Ti alloy, while use is made of a Co alloy if the turbine blade is made of 12-Cr stainless steel. For the shim, use is made of a Ti-made shim or an Ni alloy-made shim, both lower in hardness than the turbine blade and the erosion shield. 
         [0022]    The blade part  1 , the erosion shield  2 , and the shim  3  are assembled, as shown in  FIG. 1 ( b ) . With the present embodiment, a mechanism for prevention of burn through occurring at the time of the electron beam welding is provided in the shim  3 . More specifically, a part of the shim  3  is used to serve as the mechanism for prevention of the burn through, provided on the back side of a groove, that is, on the outlet side of the electron beam. With the present embodiment, the shim  3  is in a sectional shape resembling the letter T as inverted. And tack welding  9  using GTAW is applied to respective back surfaces of the blade part  1 , the erosion shield  2 , and the shim  3 , opposite from an incidence side of the electron beam, (on the upper side in the figure), and the blade part  1 , the erosion shield  2 , and the shim  3  are attached to each other in such a way as to minimize a gap therebetween so as to have no opening in the gap. 
         [0023]    Thereafter, the single-layer welding by use of the electron beam welding is applied ( FIG. 1 ( c ) ). With the electron beam welding according to the present embodiment, the single-layer welding is applied to the blade part  1 , the erosion shield  2 , and the shim  3  by use of low-voltage electron beam welding (for example, up to 60 KW) using the low-voltage electron beam welding machine. At this point in time, occurrence of burn through at a weld metal part  4  is prevented by means of the mechanism for prevention of the burn through, composed of the part of the shim  3 . By so doing, it is possible to expand tolerance of an electron beam condition in the case of the low-voltage electron beam welding. 
         [0024]    Subsequently, portions of the respective constituent materials, including the mechanism for prevention of the burn through, are removed by a machining work so as to be finished up in the shape of the blade part as a target ( FIG. 1 ( d ) ). In this machining work, removal of the mechanism for prevention of the burn through, provided in the shim  3 , including removal of portions denoted by reference sign  10  shown in  FIG. 1 ( c )  is executed. The removal of the portions denoted by the reference sign  10  is executed so that the blade part of the turbine blade can have the three-dimensional shape for the purpose of flow optimization. A finish processing is executed as appropriate after the machining work. 
         [0025]    In the machining work shown in  FIG. 1 ( d ) , a weld tip at the time of the electron beam welding, are also removed. The weld tip is susceptible to formation of a blowhole, however, since the portions denoted by reference sign  10 , including the weld tip, are removed, the soundness of a welded joint is secured. 
         [0026]    With the present embodiment, as a backing function is imparted to a constituent material (the shim in the case of the present embodiment) by making use of the constituent material, joining of the erosion shield to the leading edge of the steam turbine blade (joining of an erosion protection plate to the board of the blade material as a target by means of one-time welding) is enabled by the single-layer welding of the low-voltage electron beam welding without separately preparing the backing material, while preventing occurrence of the burn through. Accordingly, a cost for preparing a separate backing material is saved, and a fabrication cost can be reduced. In the case of an increase in board thickness with respect to the blade part, the erosion shield, and the shim, respectively, in particular, (at the time of an increase in the board thickness, burn through is liable to occur), the joining of the erosion shield can be easily executed. Accordingly, it is possible to fabricate a steam turbine blade designed to suit for a longer turbine blade, and a more complex three-dimensional shape by use of the electron beam welding. 
         [0027]    Further, with the present embodiment, the steam turbine blade excellent in strength can be obtained owing to lack of an unwelded part. Still further, since the groove shape of the blade part  1  as well as the erosion shield  2  will be linear, it is also possible to obtain advantageous effects in that the groove shape can be easily formed. 
       Second Embodiment 
       [0028]    A second embodiment of the present invention is described below with reference to  FIGS. 2 ( a )  through  2  ( d ). Description of parts in the second embodiment, identical to those in the first embodiment, is omitted. 
         [0029]    With the present embodiment, a part of the constituent material of a blade part  1 , is used as the mechanism for prevention of burn through, provided on the back face of a groove, that is, on the outlet side of an electron beam. With the present embodiment, a joining area between the part of the constituent material of the blade part  1 , and an erosion shield  2  is formed in a shape resembling the letter L. A shim  3  and the erosion shield  2  are fitted to a protrusion (pedestal) of the blade part  1 , in a shape resembling the letter L. The protrusion in the shape resembling the letter L acts as the mechanism for prevention of the burn through. Otherwise, the present embodiment is similar to the first embodiment, and after the electron beam welding, a machining work including removal of the protrusion of the blade part  1 , in the shape resembling the letter L, is executed so as to be finished up in the shape of a turbine blade as a target. 
         [0030]    With the present embodiment as well, advantageous effects basically identical to those of the first embodiment are obtained. 
       Third Embodiment 
       [0031]    A third embodiment of the present invention is described below with reference to  FIGS. 3 ( a )  through  3  ( d ). Description of parts in the third embodiment, identical to those in the first embodiment, is omitted. 
         [0032]    With the present embodiment, part of the constituent material of an erosion shield  2  is used as the mechanism for prevention of burn through, provided on the back of a groove, that is, on the outlet side of an electron beam. With the present embodiment, a joining area between the part of the constituent material of the erosion shield  2  and a blade part  1  is formed in a shape resembling the letter L when viewed from the back side of the drawing in  FIGS. 3( a ) and 3( b ) . A shim  3  and the erosion shield  2  having a protrusion (pedestal) in the shape resembling the letter L are fitted to the blade part  1 . The protrusion in the shape resembling the letter L acts as the mechanism for prevention of the burn through. Otherwise, the present embodiment is similar to the first embodiment, and after the electron beam welding, a machining work including removal of the protrusion of the erosion shield  2 , in the shape resembling the letter L, is executed so as to be finished up in the shape of a turbine blade as a target. 
         [0033]    With the present embodiment as well, advantageous effects basically identical to those of the first embodiment are obtained. 
         [0034]    Now, it is to be pointed out that the present invention be not limited to the embodiments described as above and that the invention may include various changes and modifications. For example, the embodiments described as above are explained about in detail simply for the purpose of assisting easy understanding of a configuration with respect to the respective embodiments of the invention, and it is to be understood that the invention is not necessarily limited to the embodiments having all the configurations as described. Further, a part of the configurations of a certain embodiment can be replaced with a configuration of another embodiment. Still further, the configuration of another embodiment can be added to part of the configuration of a certain embodiment. Furthermore, addition·deletion·replacement with the use of another configuration can be applied to part of the configuration of each of the embodiments described as above.