Abstract:
An inspection method for a welded joint formed between a pair of base metals  10, 20  with a groove part  12, 22  and an abutment face  14, 24  being formed on a joint surface Wc between the pair of base metals  10, 20,  includes steps of: forming a recessed groove  32  opening to a surface of the base metal  10, 20  in advance at one end of the abutment face; irradiating the joint surface Wc from an X-ray generator  34  placed on a groove part formation side (an exterior space O side) toward the joint surface Wc after at least one pass P 1  of build-up welding is performed on the groove parts  12, 22;  and determining presence or absence of incomplete penetration in the welded joint part W based on an image formed on a photosensitive film  42  by radiation penetrating the joint surface Wc.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a method of non-destructively inspecting a welded joint for incomplete penetration using radiation. 
       BACKGROUND ART 
       [0002]    In a rotor of a steam turbine, in association with the trend of using high temperature steam, a high-temperature section subjected to high temperature steam is made of material different from a low-temperature section subjected to low temperature steam so that each section is made of material appropriate for its environment and the sections are connected to each other by welding. For instance, the high-temperature section is made of high heat resisting steel, whereas the low-temperature section is made of low alloy steel.  FIG. 8  illustrates a common structure of a steam turbine. In  FIG. 8 , the steam turbine  1  is configured such that a plurality of split hollow disks  7  are fitted together between split hollow disks  5  having support shafts  3  and cylindrical ends and that abutment portions of adjacent two of the split hollow disks  5 ,  7  are connected at a welded joint part W. In this manner, in the steam turbine  1  manufactured by welding the split hollow disks  5 ,  7  together, it is important to inspect a welding state of the welded joint part W. Further, as illustrated in  FIG. 1 , the steam turbine  1  has an interior space I formed therein. 
         [0003]      FIG. 9A  and  FIG. 9B  illustrate a welding method of related art for the welded joint part W. In  FIG. 9A , the base metal  100  and the base metal  200  constitute the hollow disks that are made of different materials and on joint surfaces of the base metals  100 ,  200 , groove parts  102 ,  202  and abutment faces  104 ,  204  are respectively formed. As illustrated in  FIG. 9A , the groove parts  102 ,  202  are formed on a side that faces an exterior space O and the abutment faces  104 ,  204  are formed on a side facing the interior space I. A welding torch  300  is positioned between the groove part  102  and the groove part  202  from the exterior space O. 
         [0004]    Next, as illustrated in  FIG. 9B , the abutment faces  104 ,  204  are melted together and also build-up welding is performed on the groove parts  102 ,  202  one pass at a time using the welding torch  300 . After performing the build-up welding, a penetration part M formed between the abutment faces  104 ,  204  needs to be inspected for lack of penetration. However, the penetration part M cannot be visually checked from the exterior space O. Thus, an inspection hole  106  is formed in the base metal  100  (or the base metal  200 ) to insert a borescope (not shown) into the interior space I through the inspection hole  106  from the exterior space O in the direction of arrow a. If the penetration part M is accessible, the borescope is inserted toward the penetration part M from the interior space I in the axial direction of the interior space I (direction of arrow b). Then, using the borescope, the penetration part M is visually checked for complete penetration. This visual inspection is preferably performed immediately after welding the first pass (a root pass) to make it easier to repair the part in case that lack of penetration is found. 
         [0005]    It is described in Patent Literature 1 to perform the above inspection on the welded joint part of a steam turbine rotor. Further, Patent Literature 1 also describes that an X-ray source is inserted into the interior space I through the inspection hole and then a radiographic test of the welded joint part is carried out to inspect the welding state of the welded joint part. Further, it is described in Patent Literature 2 to visually monitor the welding part, in the case of welding a rotor of a steam turbine or the like, by means of a video system integrated in a welding torch. Furthermore, Patent Literature 3 describes that in the case of welding the steam turbine rotor or the like, the joint surface is formed with a groove and an abutment face and the abutment faces have complementary shapes to form a protrusion and a recess for orienting the joint surfaces of two base metal pieces. 
       CITATION LIST  
     Patent Literature  
       [0006]    [PTL 1] 
         [0007]    JP 09-108883 A 
         [0008]    [PTL 2] 
         [0009]    JP 2010.201507 A 
         [0010]    [PTL 3] 
         [0011]    JP 2011-177790 A 
       SUMMARY 
     Technical Problem  
       [0012]    In the inspection methods of the related art, it is necessary to form the inspection hole or the interior space I so that the borescope can be inserted to the welded joint part W from the exterior space O. However, the interior space I does not always exist. Also, by making the inspection hole in the base metal, the strength of the base metal is deteriorated and thus, structural design taking into account the strength of the base metal around the inspection hole is required. Further, there is a concern that foreign objects such as steam enter the base metal through the inspection hole, which may affect operation of a device such as a steam turbine formed by the base metals. 
         [0013]    As described in Patent Literature 1, in the radiographic test using the X-ray source inserted in the inspection hole, the remaining state of the abutment face is checked using a radiographic image projected on a photosensitive film. Based on this, it is checked whether or not penetration of the abutment face is incomplete. However, the abutment face is hard to form an image on the photosensitive film. Further, the image captured on the photosensitive film is of the part disposed on the side farther from an X-ray generator and thus it is difficult to check presence or absence of the abutment face based on the image on the photosensitive film. 
         [0014]    In view of the above issues of the related art, it is an object of the present invention to attain an inspection method for a welded joint, which enables inspection of the abutment face disposed where it cannot be visually inspected, without making an inspection hole in the base metal. 
       Solution to Problem  
       [0015]    To achieve the above object, an inspection method for a welded joint according to the present invention is an inspection method for a welded joint formed between a pair of base metals with a groove part and an abutment face being formed on a joint surface between the pair of base metals. The inspection method comprises steps of:
       forming a recessed groove opening to a surface of the base metal in advance at one end of the abutment face;   irradiating the joint surface from a groove part formation side after at least one pass of build-up welding is performed on the groove part; and   determining presence or absence of incomplete penetration in the welded joint based on an image formed on a photosensitive film by radiation penetrating the joint surface.       
 
         [0019]    In the present invention, the recessed groove is formed in advance and the step of irradiating the joint surface is performed after welding. Then, presence or absence of the recessed groove is determined based on the image formed on the photosensitive film. The presence or absence of the recessed groove can be clearly confirmed, unlike the abutment face. If the recessed groove is confirmed, it is determined there is incomplete penetration. If the recessed groove is not confirmed, it is determined that the abutment faces are penetrated. As a result, it is no longer necessary to provide the inspection hole and the above-mentioned issues regarding the inspection hole can be solved. Further, the irradiation step may be performed immediately after welding the first pass (a root pass) so that it is easier to repair the part in case that incomplete penetration is found. 
         [0020]    In the present invention, the recessed groove preferably has a cross section area whose dimension has a lower limit so that the recessed groove is distinguishable using the image obtained in the determining step and an upper limit so that the recessed groove is fillable with the at least one pass of build-up welding. As a result, presence or absence of the recessed groove after welding can be confirmed by irradiating the joint surface and the recessed groove can be filled by build-up welding. Therefore, the adverse effect on strength, etc. of the base metal can be eliminated. 
         [0021]    A shape of the recessed groove in cross section may be arc, preferably rectangular, so that the image obtained by irradiation can be clearly formed on the film. In the case where the recessed groove has a rectangular shape in cross section, the recessed groove has, for example, an opening width of 0.3 mm or more to 0.5 mm or less and a depth of 0.2 mm or more to 1.0 mm or less to satisfy the above conditions. 
         [0022]    In the present invention, the recessed groove is formed preferably on the joint surface of only one of the pair of base metals. This makes it easier to form the recessed groove. Further, on the joint surface, a protrusion and a recess are preferably formed adjacent to the recessed groove, the protrusion and the recess being configured to complementarily fit to each other. This makes it easier to position the joint surfaces of the pair of base materials with respect to each other. 
         [0023]    In such a case that the pair of base metals has a hollow cylindrical shape and are welded to each other in an axial direction via the joint surfaces, with application of the present invention to this, presence or absence of incomplete penetration in the abutment face formed facing the interior space of a hollow cylindrical shape can be inspected without making the inspection hole. For instance, by applying the present invention to a welded rotor for a turbine which is formed by the hollow cylindrical bodies welded to one another in the axial direction via the joint surfaces, presence or absence of incomplete penetration in the abutment face formed in the joint surface can be inspected without making the inspection hole. 
       Advantageous Effects  
       [0024]    According to the present invention, it is possible to check presence or absence of incomplete penetration in the abutment faces of the welded joint surfaces where they cannot be visually checked directly, without making the inspection hole in the base metal. This saves extra work for making the inspection hole and also prevents decline in strength of the base metal. Further, the interior space of the base metal is maintained tightly sealed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a cross-sectional view of a welded joint part in relation to a first embodiment of the present invention. 
           [0026]      FIG. 2  is an enlarged cross-sectional view of a part of  FIG. 1 . 
           [0027]      FIG. 3  is an enlarged cross-sectional view of section A of  FIG. 2 . 
           [0028]      FIG. 4  is a cross-sectional view of a photosensitive film used in the first embodiment. 
           [0029]      FIG. 5  is a cross-sectional view of a welded joint part in relation to a second embodiment of the present invention. 
           [0030]      FIG. 6A  is a cross-sectional view of a pre-welding joint surface which is the joint surface before welding in relation to a third embodiment of the present invention. 
           [0031]      FIG. 6B  is a cross-sectional view of a post-welding joint surface which is a joint surface after welding in relation to the third embodiment of the present invention. 
           [0032]      FIG. 7A  is an image of the pre-welding joint surface captured on a photosensitive film in relation to the third embodiment. 
           [0033]      FIG. 7B  is an image of the post-welding joint surface captured on the photosensitive film in relation to the third embodiment. 
           [0034]      FIG. 7C  is an image of a pre-welding joint surface in which a recessed groove is not formed (Comparison Example). 
           [0035]      FIG. 8  is a cross-sectional view of a steam turbine rotor taken from the front. 
           [0036]      FIG. 9A  is a cross-sectional view of a pre-welding joint surface according to a conventional welding method. 
           [0037]      FIG. 9B  is a cross-sectional view of a joint surface on which one pass is welded according to the conventional welding method. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The present invention will now be described in detail using embodiments shown in the accompanying drawings. It is intended, however, that unless particularly specified in these embodiments, dimensions, materials, and shapes of components, their relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present invention. 
       First Embodiment 
       [0039]    A first embodiment of the present invention is explained in reference to  FIG. 1  to  FIG. 4 .  FIG. 1  illustrates the state where split hollow disks  10 ,  20  forming a steam turbine rotor are welded together at a welded joint part W. The steam turbine rotor is formed by a plurality of split hollow disks including the split hollow disks  10 ,  20  that are welded together in the axial direction. Inside the welded joint part W, an interior space I is formed.  FIG. 2  and  FIG. 3  show a joint surface We before welding. 
         [0040]    In  FIG. 2  and  FIG. 3 , the joint surface We has groove parts  12 ,  22  formed on an exterior space O side and abutment faces  14 ,  24  formed on the interior space I side. The abutment faces  14 ,  24  contact with each other. 
         [0041]    As illustrated in  FIG. 3 , a circular recessed groove  32  opening to the interior space I is formed at one end of the abutment faces  14 ,  24  in the circumferential direction of the split hollow disks  10 ,  20 . The end of the abutment faces  14 ,  24  where the recessed groove  32  is formed is on an opposite side of the abutment faces  14 ,  24  from the groove parts  12 ,  22 . The recessed groove  32  is formed by a rectangular notch  16  formed in the abutment face  14  of the split hollow disk  10  and another rectangular notch  26  formed in the abutment face  24  of the split hollow disk  20 , that are disposed facing each other. The recessed groove  32  has a symmetrical shape with respect to the abutment faces  14 ,  24 . In this embodiment, the recessed groove  32  has an opening width X in the axial direction, X being 0.3 mm to 0.5 mm and a depth Y being 0.2 mm to 1.0 mm. 
         [0042]    As illustrated in  FIG. 2 , the joint surface We is welded by placing a welding torch  30  in a groove formed by the groove parts  12 ,  22  from the exterior space  0 , directing the welding torch  30  toward the abutment faces  14 ,  24 , and then rotating the split hollow disks  10 ,  20  in such a state that the split hollow disks  10 ,  20  are positioned with respect to each other. In this state, build-up welding is performed by the welding torch  30  on the groove formed by the groove parts  12 ,  22 . By laying one pass (a root pass), a penetration part is formed in the abutment faces  14 ,  24  and the recessed groove  32  is filled. Next, ten to twenty passes of build-up welding are performed to fill the groove formed by the groove parts  12 ,  22 . 
         [0043]    After laying one pass, an X-ray generator  34  is placed in the exterior space O to check presence or absence of incomplete penetration in the abutment faces  14 ,  24  as illustrated in  FIG. 1 . Further, a high-temperature film cassette  36  is attached to outer circumferential surfaces of the split hollow disks  10 ,  20  which are 180° out of phase with the X-ray generator  34 , at a position to extend over the welded joint part W. 
         [0044]      FIG. 4  illustrates the configuration of the high-temperature film cassette  36 . The high-temperature film cassette  36  includes a Teflon™ plate  38 , a heat-insulating plate  40  made of heat insulating material, and a photosensitive film  42  on which radiation having passed through the welded joint part W is exposed. The Teflon™ plate  38 , the heat-insulating plate  40  and the photosensitive film  42  are stacked in this order from a side nearer to the welded joint part W so as to form the high-temperature film cassette  36 . When irradiating a specimen, the radiation penetrates the specimen and gradually weakens due to interaction with the specimen. In the case of a welded part, the radiation penetrates well in a region with defects such as blowholes compared to a region without defects. As a result, the region with defects such as blowholes is detected as a dark image on the photosensitive film  42 . 
         [0045]    In this configuration, X-rays are emitted toward the welded joint part W from the X-ray generator  34  and the photosensitive film  42  is exposed to the X-rays that have penetrated the welded joint part W to form an image on the photosensitive film  42 . The image formed on the photosensitive film  42  is the welded joint part W on a side farther from the X-ray generator  34 . Thus, if there is incomplete penetration in the abutment faces  14 ,  24 , an image of the recessed groove  32  is formed clearly on the photosensitive film  42 . 
         [0046]    According to this embodiment, when there is incomplete penetration in the abutment faces  14 ,  24 , the recessed groove  32  is shown on the photosensitive film  42  clearly as a dark image. Thus, it is possible to precisely acknowledge presence or absence of incomplete penetration in the abutment faces  14 ,  24 . Therefore, it is no longer necessary to drill an inspection hole in the split hollow disks  10 ,  20 . This saves extra work for making the inspection hole and also avoids decline in the strength of the split hollow disks  10 ,  20 . 
         [0047]    Further, the interior space I can be maintained airtight, so that steam can be prevented from entering the interior space I and affecting the operation of the steam turbine or deteriorating the operation efficiency. Further, as the cross section area of the recessed groove  32  is rectangular, a shape of the recessed groove  32  can be clearly formed on the photosensitive film  42 . Further, as the cross section area of the recessed groove  32  has the above described dimensions, the image of the recessed groove  32  can be formed distinctly on the photosensitive film  42  and the recessed groove  32  can be surely filled with one pass of build-up welding. Furthermore, the welded joint part W is irradiated after one pass of build-up welding. This makes it easier to repair the part after confirming incomplete penetration. 
       Second Embodiment 
       [0048]    Next, a second embodiment of the present invention is explained in reference to  FIG. 5 . In this embodiment, similarly to the first embodiment, a circular recessed groove  44  opening to the interior space I is formed at one end of the abutment faces  14 ,  24  in the circumferential direction of the split hollow disks  10 ,  20 . The end of the abutment faces  14 ,  24  where the recessed groove  44  is formed is on an opposite side of the abutment faces  14 ,  24  from the groove parts  12 ,  22 . The recessed groove  44  is formed by arranging the split hollow disk  10  and a rectangular notch  46  formed only in the abutment face  24  of the split hollow disk  20  to face each other. The rest of the configuration is the same as that of the first embodiment. According to this embodiment, the recessed groove  44  can be formed by machining only the abutment face  24  and thus formation of the recessed groove  44  is easy. 
       Third Embodiment 
       [0049]    Next, a third embodiment of the present invention is explained in reference to  FIG. 6A ,  FIG. 6B ,  FIG. 7A ,  FIG. 7B , and  FIG. 7C . In this embodiment, similarly to the second embodiment, the recessed groove  44  opening to the interior space I is formed by arranging the split hollow disk  10  and the rectangular notch  46  formed only in the abutment face  24  of the split hollow disk  20  to face each other. In addition to the recessed groove  44 , a protrusion  48  and a protrusion  50  are formed adjacent to the recessed groove  44  behind the recessed groove  44  (the exterior space O side) in the abutment faces  24 ,  14  of the split hollow disks  20 ,  10 , respectively. These protrusions  48 ,  50  have shapes to complementarily fit to each other. 
         [0050]      FIG. 6B  illustrates the joint surface We after the first pass P 1 , the second pass P 2  and the third pass P 3  of build-up welding are performed. By the first pass P 1 , the penetration part M is formed in the abutment faces to fill the recessed groove  44 .  FIG. 7A  is an image of the pre-welding joint part W captured on the photosensitive film  42 .  FIG. 7B  is an actual image of the welded joint part W captured on the photosensitive film  42  after the first pass of build-up welding is performed. In  FIG. 7A , the image S 44  of the recessed groove  44  is clearly shown. In contrast, in  FIG. 7B , no image is formed on the photosensitive film  42  and thus it can be seen that the penetration part M is formed in the abutment faces and the recessed groove  44  is successfully filled. 
         [0051]      FIG. 7C  is an image of the joint part W shown as a comparison example. In this comparison example, the welded joint part W is not formed with the recessed groove  44  and the image of the welded joint part W is formed on the photosensitive film  42 . In  FIG. 7C , the line of the abutment face is vaguely confirmed. However, it is hard to distinguish from the rest. This makes it difficult to determine whether or not the penetration part M is formed normally after the first pass of build-up welding. 
         [0052]    According to this embodiment, the same function effects as the second embodiment can be obtained. Further, as the protrusions  48 ,  50  that complementarily fit to each other are formed in the abutment face of the split hollow disks  10 ,  20 , sealing effect of the interior space I in the welded joint part W can be enhanced. Furthermore, as the protrusions  48 ,  50  are formed, it is made easier to position the split hollow disks  10 ,  20  with respect to each other when welding. 
       INDUSTRIAL APPLICABILITY  
       [0053]    According to the present invention, inspection of the welded joint part can be performed on the abutment faces of the welded joint surfaces located where they cannot be visually inspected, without making an inspection hole in a base metal. 
       REFERENCE SIGNS LIST  
       [0000]    
       
           1  Steam turbine 
           3  Support shaft 
           5 ,  7 ,  10 ,  20  Split hollow disk 
           12 ,  22 ,  102 ,  202  Groove part 
           14 ,  24 ,  104 ,  204  Abutment face 
           16 ,  26 ,  46  Notch 
           30 ,  300  Welding torch 
           32 ,  44  Recessed groove 
           34  X-ray generator 
           36  High-temperature film cassette 
           38  Teflon™ plate 
           40  Heat-insulating plate 
           42  Photosensitive film 
           48 ,  50  Protrusion 
           100 ,  200  Base metal 
           106  Inspection hole 
         I Interior space 
         M Penetration part 
         O Exterior space 
         P 1 , P 2 , P 3  Build-up welding 
         W Welded joint part 
         W C  Joint surface