Patent Publication Number: US-2022212285-A1

Title: Welding method and welded member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-000213, filed Jan. 4, 2021; the entire contents of which are incorporated herein by reference. 
     FIELD 
     An embodiment described herein relates to a welding method and a welded member. 
     BACKGROUND 
     Reduction of the emission of greenhouse gases, which is typified by carbon dioxide, is required for environmental protection. In order to reduce carbon dioxide emissions, a thermal power plant, which uses a large amount of fossil fuel is desired to improve power generation efficiency. 
     In order to improve the power generation efficiency of a thermal power plant, it is effective to increase a temperature of steam flowing through the thermal power plant. Thus, components used in a thermal power plant need to have superior high-temperature resistance and improved wear resistance more than ever before. 
     For example, a steam valve which controls the flow rate of steam flowing into a steam turbine opens and closes while being exposed to a high temperature and high pressure steam. A valve stem which is a part of the steam valve is required to be escaped from wear caused by sliding and from generation of an oxidized scale. This is due to the following reasons. Namely, when the valve stem is worn by sliding, an amount of steam leaking from a gap between the valve stem and a valve chest increases, which lowers the thermal efficiency of the thermal power plant. In addition, the valve stem reacts with high temperature steam to form an oxidized scale on its surface. The formation of the oxidized scale increases an external diameter of the valve stem. Then, the oxidized scale peels off and accumulates around the valve stem. When the external diameter of the valve stem increases and/or when the oxidized scale accumulates between the valve stem and the valve chest, the valve stem cannot move as desired. 
     In terms of this point, JPH6-174126 discloses a method of forming a build-up layer by welding a cobalt-base alloy to a surface of a valve stem base material to form a hardened layer (build-up layer) in order to improve wear resistance of the valve stem and to prevent generation of an oxidized scale. 
     However, it was found that a portion of the valve stem manufactured by the method described in JPH6-174126 did not have sufficient wear resistance to withstand use in a thermal power plant where high temperature steam flows. More specifically, it was found that the hardness of the build-up layer formed on the valve stem was sufficient in the vicinity of a weld start portion, but was insufficient from the vicinity of a weld middle portion to the vicinity of a weld end portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing a welded member according to an embodiment of the present invention, wherein a build-up layer is formed on a workpiece. 
         FIG. 2  is a side view showing a welding equipment for forming the build-up layer shown in  FIG. 1 . 
         FIG. 3  is a view showing a cross-section of the welding equipment shown in  FIG. 2  along the III-III line in the figure. 
         FIG. 4  is a partially enlarged view showing the welding torch and the temperature sensor shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     A welding method in an embodiment is a welding method for subjecting a surface to be built-up of an elongated workpiece to a build-up welding process along a longitudinal direction of the workpiece, the welding method comprising: 
     a step of forming a build-up layer on the surface to be built-up by supplying a filler metal to the surface to be built-up along the longitudinal direction and by applying a laser beam thereto to melt the filler metal; 
     wherein, in the step of forming the build-up layer, information on a dimension of a molten pool formed by the filler metal and the workpiece molten by the laser beam is obtained, and an output of the laser beam is controlled based on the information. 
     A welded member in an embodiment is a welded member with a build-up layer on a surface to be built-up of an elongated workpiece, the build-up layer being formed by welding a filler metal to the surface to be built-up, wherein 
     a dilution rate of components of the filler metal in the build-up layer is 10% or more and 40% or less. 
     Alternatively, a welded member in an embodiment is a welded member with a build-up layer on a surface to be built-up of an elongated workpiece, the build-up layer being formed by welding cobalt-base alloy to the surface to be built-up, wherein 
     a Vickers hardness of the build-up layer is Hv 320 or more and Hv 500 or less. 
     An embodiment is described with reference to the drawings.  FIG. 1  is a view showing a welded member according to an embodiment.  FIGS. 2 and 3  are views schematically showing a structure of a welding equipment for manufacturing the welded member shown in  FIG. 1 .  FIG. 3  is a view showing a cross-section of the welding equipment shown in  FIG. 2  along the III-III line in the figure. In  FIG. 3 , illustration of a filler-metal supply unit  30 , a laser irradiator  40  and a shielding-gas supply unit  50  described below is partially omitted for the sake of simplicity of illustration.  FIG. 4  is a view partially showing the welding equipment shown in  FIGS. 2 and 3  in enlargement. 
     A welded member  1  shown in  FIG. 1  is manufactured by a build-up welding process for forming a build-up layer  3  on a surface to be built-up  2   a  of a workpiece  2 . A welding equipment  10  shown in  FIGS. 2 and 3  subjects the surface to be built-up  2   a  of the workpiece  2  to the build-up welding process. More specifically, the welding equipment  10  forms the build-up layer  3  on the surface to be built-up  2   a  of the workpiece  2  with a filler metal  35  that is molten by means of a laser beam  45 . 
     As shown in  FIG. 1 , the workpiece  2  has an elongated shape with a longitudinal direction. In the illustrated example, the workpiece  2  is formed in a solid cylindrical shape, and has a cylindrical surface (surface to be built-up  2   a ). In the illustrated example, the workpiece  2  is a forged bar of nickel (Ni)-base alloy. It goes without saying that the shape of the workpiece  2  and the material forming the workpiece  2  are not limited thereto. For example, the workpiece  2  may be formed in a hollow cylindrical shape. In addition, the material forming the workpiece  2  may be an iron (Fe)-base alloy. In this specification, the nickel-base alloy refers to a material having the highest fractions by weight of nickel elements, and the iron-base alloy refers to a material having the highest fractions by weight of iron elements. 
     As shown in  FIG. 2 , the welding equipment  10  has a supporter  20  that supports the workpiece  2 , a filler-metal supply unit  30  that supplies the filler metal  35  to the workpiece  2 , a laser irradiator  40  that applies the laser beam  45  to the workpiece  2 , a shielding gas supply unit  50 , a longitudinal motion drive  60  that relatively moves the laser irradiator  40  with respect to the supporter  20 , and a rotational motion unit  70  that rotates the workpiece  2  supported by the supporter  20 . 
     The supporter  20  supports both the ends of the elongated workpiece  2 . The supporter  20  supports both the longitudinal ends of the workpiece  2  such that the workpiece  2  is rotatable around a rotation axis  70 X along the longitudinal direction. 
     As shown in  FIG. 4 , the filler-metal supply unit  30  has a filler-metal housing  31  that houses powder of the filler metal  35 , a filler-metal supply tube  32  that leads the powder of the filler metal  35  derived from the filler-metal housing  31  to the vicinity of the workpiece  2  supported by the supporter  20 , and a filler-metal ejection hole  33  provided at the distal end of the filler-metal supply tube  32  to eject the filler metal  35 . The filler metal  35  may be a cobalt (Co)-base alloy, a nickel-base alloy and an iron-base alloy, for example. The filler-metal supply unit  30  may further have a carrier-gas supply unit (not shown) that supplies a carrier gas to the filler-metal supply tube  32 . In this case, when the filler metal  35  is ejected from the filler-metal ejection hole  33 , the filler metal  35  accompanied by a carrier gas can be supplied to the workpiece  2 . 
     As shown in  FIG. 4 , the laser irradiator  40  applies the laser beam  45  to the workpiece  2  supported by the supporter  20 . The laser irradiator  40  has a laser oscillator  41 , an optical fiber  42  that guides the laser beam  45  oscillated by the laser oscillator  41  to the vicinity of the workpiece  2  supported by the supporter  20 , and a laser emitter  43  provided at the distal end of the optical fiber  42  to emit the laser beam  45  guided by the optical fiber  42  toward the workpiece  2  supported by the supporter  20 . The laser oscillator  41  may be an oscillator using any laser such as a semiconductor laser or a solid-state laser, for example. The laser oscillator  41  is preferably capable of oscillating the laser beam  45  in a wavelength range of from 400 to 1100 nm. The powdery filler metal  35  ejected from the filler-metal ejection hole  33  is molten by the laser beam  45  emitted from the laser emitter  43 . In addition, the workpiece  2  is partially molten by the laser beam  45  emitted from the laser emitter  43 . Then, components of the molten filler metal  35  are dissolved in the molten workpiece  2 , and components of the workpiece  2  are dissolved in the molten filler metal  35 . The molten filler metal  35  and the molten workpiece  2  form a molten pool  4  on the surface to be built-up  2   a . Thereafter, the molten pool  4  solidifies to become the build-up layer  3 . 
     As shown in  FIG. 4 , the shielding gas supply unit  50  has a shielding gas housing  51  that houses a shielding gas  55 , a gas supply tube  52  that leads the shielding gas  55  derived from the shielding gas housing  51  to the vicinity of the workpiece  2  supported by the supporter  20 , and a shielding gas ejection hole  53  provided at the distal end of the gas supply tube  52  to eject the shielding gas  55 . The shielding gas  55  may be an inert gas such as helium, argon or nitrogen, for example. 
     In the illustrated example, the welding equipment  10  has a welding torch  15 . The welding torch  15  has a distal end surface  15   a  where the aforementioned filler-metal ejection hole  33 , the laser emitter  43  and the shielding gas ejection hole  33  are formed. The welding torch  15  is positioned such that its distal end surface  15   a  faces the surface to be built-up  2   a  of the workpiece  2  supported by the supporter  20 . The welding torch  15  is provided to be relatively movable in the aforementioned longitudinal direction with respect to the supporter  20 . 
     The longitudinal motion drive  60  shown in  FIGS. 2 and 3  relatively moves the laser emitter  43  along the longitudinal direction with respect to the workpiece  2  supported by the supporter  20 . In the illustrated example, the longitudinal motion drive  60  moves the welding torch  15  in the longitudinal direction with respect to the supporter  20 . The longitudinal motion drive  60  moves the welding torch  15  in the direction D 1  from one end of the workpiece  2  supported by the supporter  20  toward the other end thereof. It goes without saying that the longitudinal motion drive  60  may move the supporter  20  in the longitudinal direction with respect to the welding torch  15 . 
     The rotational motion unit  70  rotates the workpiece  2  supported by the supporter  20  around the rotation axis  70 X. In the illustrated example, the rotation axis  70 X corresponds to the axis  2 X of the workpiece  2  (the central axis of the cylindrical surface to be built-up  2   a ). The rotational motion unit  70  rotates the workpiece  2  at a predetermined rotating speed. In the example shown in  FIG. 3 , the rotational motion unit  70  rotates the workpiece  2  clockwise in  FIG. 3 , but the present invention is not limited thereto. The rotational motion unit  70  may rotate the workpiece  2  counterclockwise in  FIG. 3 . 
     Since the welding torch  15  is relatively moved by the longitudinal motion drive  60  in the longitudinal direction with respect to the workpiece  2 , and the workpiece  2  is rotated by the rotational motion unit  70  around the rotation axis  70 X, the welding torch  15  draws a spiral trajectory around the surface to be built-up  2   a  of the workpiece  2 . Due to the movement of the welding torch  15  and the rotation of the workpiece  2 , the molten pool  4  formed on the surface to be built-up  2   a  moves away from the laser bean  45 , and solidifies to become the build-up layer  3 . 
     In recent years, in order to improve the power generation efficiency of a thermal power plant, it has been desired to improve a wear resistance of a component used in the thermal power plant and to suppress formation of an oxidized scale on the component. For example, when the welded member shown in  FIG. 1  is used as a valve stem of a steam valve, an opening and closing action of the steam valve with the movement of the valve stem can be more reliable by improving the wear resistance of the build-up layer. Namely, when the build-up layer is worn as the valve stem slides, the amount of steam leaking from a gap between the valve stem and the valve chest increases, which lowers the thermal efficiency of the thermal power plant. In addition, an oxidized scale formed on the valve stem increases an external diameter of the valve stem. Alternatively, the oxidized scale peels off and accumulates around the valve stem. When the external diameter of the valve stem increases and/or when the oxidized scale accumulates between the valve stem and the valve chest, the valve stem cannot move as desired. In terms of this point, JPH6-174126 discloses a method of forming a build-up layer by welding a cobalt-base alloy to a surface to be built-up of a workpiece in order to improve wear resistance of a valve stem and to suppress an generation of oxidized scale. 
     However, the inventors found that, when a welded member is manufactured by the method described in JPH6-174126, the hardness of the build-up layer differs along the longitudinal direction of the workpiece. Specifically, when the portion of the build-up layer at which the welding is started is referred to as weld start portion and the portion thereof at which the welding is ended is referred to as weld end portion, the hardness of the build-up layer in the vicinity of the weld end portion was found to be lower than the hardness of the build-up layer in the vicinity of the weld start portion. After having conducted extensive studies, the inventors discovered the cause of the decrease in hardness of the build-up layer in the vicinity of the weld end portion compared to the hardness in the vicinity of the weld start portion. Namely, from the start to the end of the build-up welding process, heat is applied to the workpiece by a laser beam. Thus, the temperature of each portion of the workpiece increases along a welding direction (the direction in which the welding torch travels). As a result, the dilution rate of components of the filler metal in each portion of the build-up layer increases along the welding direction (in other words, from the weld start portion toward the weld end portion). When the dilution rate becomes excessively high, the hardness of the build-up layer becomes insufficient for use in a thermal power plant where high temperature steam flows. It goes without saying that characteristics of the build-up layer other than the hardness differ between the weld start portion and the weld end portion. Specifically, compared to the characteristics of the build-up layer at the weld start portion, the characteristics of the build-up layer at the weld end portion more deviate from characteristics of the filler metal before the welding. 
     In consideration of these points, the welding equipment  10  and the welding method in the embodiment are devised to prevent a dilution rate of components of the filler metal from increasing from the weld start portion toward the weld end portion so as to improve the characteristics of the build-up layer  3  (make them closer to the characteristics of the filler metal  35 ). Namely, the welding equipment  10  comprises means for controlling, during the build-up welding process, an output of the laser beam  45  oscillated from the laser oscillator  41  so as to prevent the increase in temperature of the workpiece  2  caused by the heat input of the laser beam  45 . 
     As a method of preventing the increase in temperature of the workpiece  2  during the build-up welding process, it can be considered that application of the laser beam to the workpiece is interrupted during the build-up welding process (i.e., the build-up welding process is interrupted) to dissipate the heat of the workpiece  2 . However, when the build-up welding process is interrupted, it takes longer to complete the build-up welding process from the start to the end. Thus, the build-up welding cannot be efficiently performed. On the other hand, the method in which the build-up welding process is performed while an output of the laser beam  45  oscillated from the laser oscillator  41  is being controlled makes it possible to prevent the increase in temperature of the workpiece  2  and to prevent the resulting increase in dilution rate, without interrupting the build-up welding process. 
     Further, it can be considered that, when the output of the laser beam  45  oscillated from the laser oscillator  41  is controlled, the temperature of the workpiece  2  is directly measured, and the output of the laser beam  45  is controlled based on the measured temperature. However, it is necessary to measure temperatures of plural points on the workpiece in order to control the output of the laser beam  45 . To control the output of the laser beam based on plural measured temperatures requires time and effort. 
     In this regard, under favor of the fact that a dimension of the molten pool  4  becomes larger as the temperature of the workpiece  2  increases, the increase in temperature of the workpiece  2  can be reliably and efficiently prevented by controlling the output of the laser beam  45  based on the dimension of the molten pool  4 . 
     Specifically, the welding equipment  10  in the embodiment comprises a dimension measuring unit  80  that obtains information on the dimension of the molten pool  4 , and a controller  90  that controls the laser irradiator  40  based on the information. 
     The dimension measuring unit  80  obtains information on the dimension of the molten pool  4  as follows. Namely, the dimension measuring unit  80  obtains temperatures of respective points in an area including the molten pool  4  on the workpiece  2  which is being subjected to the build-up welding process, and performs temperature mapping of the area including the molten pool  4  to create a temperature distribution map. Then, on the assumption that points having a predetermined temperature or more in the aforementioned area including the molten pool  4  correspond to an area including only the molten pool  4 , the dimension measuring unit  80  obtains information on the dimension of this area based on the temperature distribution map so as to obtain information on the dimension of the molten pool  4 . 
     The predetermined temperature used for determining an area including only the molten pool  4  is determined in consideration of melting points of the workpiece  2  and the filler metal  35 . 
     In order that the molten pool  4  is measured in this manner, in the illustrated example, the dimension measuring unit  80  comprises a non-contact radiation temperature sensor  81 , and a processer  82  that creates a temperature distribution map by performing temperature mapping of an area including the molten pool  4  based on temperatures obtained by the temperature sensor  81 . The temperature sensor  81  detects an infrared ray radiated from an area including the molten pool  4  on the workpiece  2  to obtain temperatures of respective points in the area. The dimension measuring unit  80  may include a reflection element (e.g., a dichroic mirror) for reflecting the infrared ray toward the temperature sensor  81 . 
     The temperature sensor  81  is relatively movable along the longitudinal direction with respect to the workpiece  2  supported by the supporter  20 . The temperature sensor  81  is moved, together with the welding torch  15 , by the longitudinal motion drive  60  in the longitudinal direction. Thus, a temperature of the aforementioned area including the molten pool  4  can be continuously measured from the start of the below-stated build-up step to the end thereof. In the illustrated example, the temperature sensor  81  is fixed on the welding torch  15 . 
     The processor  82  creates information on a dimension of the molten pool  4  based on a temperature distribution map, and sends it to the controller  90 . Since the temperature sensor  81  measures a temperature of the aforementioned area including the molten pool  4  from the start of the below-stated build-up step to the end thereof, the processer  82  can continuously send information on a dimension of the molten pool  4  to the controller  90  from the start of the build-up step to the end thereof. 
     The controller  90  controls the laser oscillator  41  based on the information on a dimension of the molten pool  4  received from the processor  82 . Specifically, the controller  90  controls the laser oscillator  41  in such a manner that, the larger a dimension of the molten pool  4  is, the lower an intensity of the laser beam  45  emitted from the laser emitter  43  becomes. In other words, the controller  90  controls the laser oscillator  41  in such a manner that, the smaller a dimension of the molten pool  4  is, the higher an intensity of the laser beam  45  emitted from the laser emitter  43  becomes. Thus, a dimension of the molten pool  4  can be controlled within a predetermined range. This means that a temperature of the workpiece  2  can be controlled as desired during the build-up welding process. Further, this means that a dilution rate of components of the filler metal  35  in the build-up layer  35  can be controlled as desired. As a result, the build-up layer  3  can have desired characteristics. Since information on a dimension of the molten pool  4  is continuously sent from the processer  82  to the controller  90  from the start of the build-up step to the end thereof, the controller  90  can continuously control the laser oscillator  41  from the start of the build-up step to the end thereof. For this reason, the increase in temperature of the workpiece  2  and the resulting increase in dilution rate can be prevented without interrupting the build-up step. 
     In the illustrated example, an output of the laser beam  45  is controlled in such a manner that the aforementioned dilution rate is 10% or more and 40% or less, preferably 15% or more and 35% or less. This is because, when the dilution rate is less than 10%, an insufficiently fused portion may be formed between the build-up layer  3  and the workpiece  2 . On the other hand, when the dilution rate exceeds 40%, a hardness of the build-up layer may become insufficient for use in a thermal power plant where high temperature steam flows. When an output of the laser beam  45  is controlled in such a manner that the aforementioned dilution rate is 15% or more and 35% or less, a Vickers hardness of the build-up layer  3  can be more reliably within the range of Hv 320 or more and Hv 500 or less. 
     Next, an operation of the embodiment as structured above will be described. Herein, a welding method using the aforementioned welding equipment  10  is described. 
     First, as shown in  FIG. 2 , the both ends of the workpiece  2  are supported by the supporter  20 . 
     Following thereto, the welding torch  15  and the temperature sensor  81  are positioned in the vicinity of the aforementioned one end of the workpiece  2 . 
     Then, the workpiece  2  is rotated by the rotational motion unit  70  around the rotation axis  70 X. In addition, the movement of the welding torch  15  and the temperature sensor  81  along the longitudinal direction is started by the longitudinal motion drive  60 . The welding torch  15  and the temperature sensor  81  are moved in the aforementioned movement direction D 1 . 
     Next, the build-up step for forming the build-up layer  3  on the workpiece  2  is performed. In this build-up step, the filler metal  35  is supplied from the welding torch  15  to the surface to be built-up  2   a  of the workpiece  2 , and the laser beam  45  is applied thereto. The build-up step of forming the build-up layer is performed in such a manner that the workpiece  2  is being rotated by the rotational motion unit  70  while the welding torch  15  and the temperature sensor  81  are being moved by the longitudinal motion drive  60 . 
     In the illustrated example, the laser beam  45  is emitted from the laser emitter  43  of the welding torch  15 . For this while, the powdery filler metal  35  is supplied from the filler-metal ejection hole  33  of the welding torch  15 . The filler metal  35  is supplied from around the laser beam  45  along the laser beam  45 . Thus, the powdery filler metal  35  is molten by the laser beam  45 . In addition, the workpiece  2  is partially molten by the laser beam  45 . The molten filler metal  35  and the molten portion of the workpiece  2  form a molten pool  4  on the surface to be built-up  2   a  of the workpiece  2 . Then, components of the molten filler metal  35  are dissolved in the molten portion of the workpiece  2 , and components of the molten portion of the workpiece  2  are dissolved in the molten filler metal  35 . The powdery filler metal  35  ejected from the filler-metal ejection hole  33 , and the molten pool  4  formed by the molten filler metal  35  and the molten portion of the workpiece  2  are surrounded by the shielding gas  55  supplied from the shielding gas ejection hole  53  and thus are prevented from being oxidized by the atmosphere. 
     During the build-up step, the welding torch  15  is moved with respect to the workpiece  2 , so that the molten pool  4  formed on the surface to be built-up  2   a  of the workpiece  2  becomes away from the laser beam  45  and solidifies to become the build-up layer  3 . In addition, during the build-up step, the welding torch  15  draws a spiral trajectory around the surface to be built-up  2   a  of the workpiece  2 , so that the molten pool  4  and the build-up layer  3  are formed on the surface to be built-up  2   a  along the above spiral trajectory. 
     During the build-up step, the temperature sensor  81  measures a temperature of an area including the molten pool  4  on the workpiece  2 . The position of the molten pool  4  on the workpiece  2  moves along the spiral trajectory, and the position of the temperature sensor  81  on the workpiece  2  also moves following to the molten pool  4 . The processor  82  creates a temperature distribution map based on temperatures measured by the temperature sensor  81 . Then, the processor  82  creates information on a dimension of the molten pool  4  based on the temperature distribution map. The controller  90  controls the laser oscillator  41  based on the information on a dimension of the molten pool  4  created by the processor  82 . 
     After the build-up step, a surface treatment step for machining the surface of the build-up layer  3  to smooth the surface (into a cylindrical surface) may be performed. 
     Next, the present invention will be described more specifically by means of an example, but the present invention is not limited to the following example, as long as they are within the scope of the invention. 
     Example 
     The workpiece  2  was subjected to build-up welding using the aforementioned welding equipment  10 , whereby one build-up layer  3  was formed on the surface to be built-up  2   a  of the workpiece  2 , as shown in  FIG. 1 . A cylindrical forged bar of nickel-base alloy was used as the workpiece  2 , and powder of cobalt-base alloy was used as the filler metal  35 . An oscillator using a solid laser was used as the laser oscillator  41 . Then, the build-up step was performed under the following welding conditions. 
     &lt;Welding Conditions&gt;
         Feed rate of filler metal: 10 g/min to 60 g/min   Welding speed: 200 mm/min to 1000 mm/min   Laser output: 2 kW to 10 kW       

     The term “welding speed” means here the speed of the welding torch  15  with respect to the surface to be built-up  2   a  of the workpiece  2  (the speed of the welding torch  15  along the spiral trajectory described above). 
     The surface treatment step was performed by machining the build-up layer  3  formed on the workpiece  2  under the aforementioned conditions so that the build-up layer  3  had a thickness of 0.5 mm. The thickness of the build-up layer  3  was measured here with reference to the original position of the surface to be built-up  2   a  of the workpiece  2  before performing build-up welding. In other words, the thickness of the build-up layer  2  is a difference T between a radius  3 R of the outer peripheral surface  3   a  of the build-up layer  3  formed on the workpiece  2  and the radius  2 R of the outer peripheral surface (surface to be built-up  2   a ) of the workpiece  2  before the workpiece  2  is subjected to build-up welding. 
     After the surface treatment step had been performed, a sectional sample of the obtained welded member  1  was made, and the hardness of the build-up layer  3  and the dilution rate of the filler metal  35  in the build-up layer  3  were measured. The hardness is measured by using the Vickers hardness test. 
     Comparative Example 
     The build-up step and the surface treatment step were performed similarly to Example, except that the output control of the laser beam  45  was not performed during the build-up welding process. Then, the hardness of the build-up layer  3  and the dilution rate of components of the filler metal  35  in the build-up layer  3  were measured. 
     (Evaluation) 
     Table 1 shows the hardnesses and the dilution rates of each build-up layers  3  in Example and Comparative Example. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Core 
                   
                   
               
               
                   
                 Max. 
                 Hardness (Hv) 
                 Dilution (%) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Output 
                 TEMP 
                 Start 
                 End 
                 Start 
                 End 
               
               
                   
                 control 
                 (° C.) 
                 portion 
                 portion 
                 portion 
                 portion 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 
                 Yes 
                 303 
                 492 
                 353 
                 10.0 
                 21.8 
               
               
                 Comp. 
                 No 
                 369 
                 418 
                 319 
                 18.0 
                 41.3 
               
               
                 Example 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the dilution rates of components of the filler metal  35  in the build-up layer  3  of Comparative Example were 18.0% at the weld start portion and 41.3% at the weld end portion. Namely, in Comparative Example, the aforementioned dilution rate was higher at the weld end portion than at the weld start portion. The dilution rate of the build-up layer  3  of Comparative Example exceeded 40% at the weld end portion. In addition, the hardnesses of the build-up layer  3  of Comparative Example were Hv 418 at the weld start portion and Hv 319 at the weld end portion. Namely, in Comparative Example, the hardness of the build-up layer  3  was lower at the weld end portion than at the weld start portion. The hardness of the build-up layer  3  of Comparative Example was less than Hv 320 at the weld end portion. 
     On the other hand, the dilution rates of components of the filler metal  35  in the build-up layer  3  of Example were 10.0% at the weld start portion and 21.8% at the weld end portion. Namely, in Example, the aforementioned dilution rate was higher at the weld end portion than at the weld start portion. The dilution rate of the build-up layer  3  of Example was 10% or more and 40% or less both at the weld start portion and the weld end portion. In addition, the hardnesses of the build-up layer  3  of Example were Hv 492 at the weld start portion and Hv 353 at the weld end portion. Namely, in Example, the hardness of the build-up layer  3  was lower at the weld end portion than at the weld start portion. The hardness of the build-up layer  3  of Example was Hv 320 or more both at the weld start portion and the weld end portion. 
     From the above results, it can be understood that the characteristics of the build-up layer  3  can be controlled by controlling the output of the laser beam  45  based on information on the dimension of the molten pool  4 . 
     In the aforementioned embodiment and Example, the build-up layer  3  is formed in the build-up step by supplying the filler metal  35  to the surface to be built-up  2   a  along the longitudinal direction, and by applying the laser beam  45  thereto. However, the present invention is not limited thereto. The build-up layer  3  may be formed by applying the laser beam  45  to a filler metal layer formed by thermally spraying or applying the filler metal  35  to the surface to be built-up  2   a . In this case, the welding equipment  10  may comprise, instead of the filler-metal supply unit  30 , a filler-metal-layer forming unit that forms a filler metal layer by thermally spraying or applying the filler metal  35  to the surface to be built-up  2   a . The supporter  20  may support the workpiece  2  with the filler metal layer formed thereon, and the laser irradiator  40  may apply a laser beam to the workpiece  2  with the filler metal layer formed thereon. In addition, in this case, the welding method may comprise a filler-metal-layer forming step of forming a filler metal layer by thermally spraying or applying the filler metal  35  to the surface to be built-up  2   a . In the build-up step, the laser beam  45  may be applied to the filler metal layer after the filler-metal-forming step to melt again the filler metal  35  of the filler metal layer so as to form the build-up layer  3  on the surface to be built-up  2   a.    
     As described above, the welding method according to the embodiment is a welding method for subjecting a surface to be built-up  2   a  of an elongated workpiece  2  to a build-up welding process along a longitudinal direction of the workpiece  2 , the welding method comprising a step of forming a build-up layer  3  on the surface to be built-up  2   a  by supplying a filler metal  35  to the surface to be built-up  2   a  along the longitudinal direction and by applying a laser beam  45  thereto to melt the filler metal  35 . In the step of forming the build-up layer, information on a dimension of a molten pool formed by the filler metal and the workpiece molten by the laser beam is obtained, and an output of the laser beam is controlled based on the information. Such a welding method can control the temperature of the workpiece  2  and the dilution rate of components of the filler metal  35  in the build-up layer  3  during the build-up welding process as desired. As a result, the build-up layer  3  can have desired characteristics. 
     Alternatively, the welding method according to the embodiment is a welding method for subjecting a surface to be built-up  2   a  of an elongated workpiece  2  to a build-up welding process along a longitudinal direction of the workpiece  2 , which welding method may comprise: a step of forming a filler metal layer by thermally spraying or applying a filler metal  35  to the surface to be built-up  2   a ; and after the filler-metal-layer forming, a step of forming a build-up layer  3  on the surface to be built-up  2   a  by applying a laser beam  45  to the filler metal layer along the longitudinal direction to again melt the filler metal  35  of the filler metal layer. In the step of forming the build-up layer, information on a dimension of a molten pool  4  formed by the filler metal  35  and the workpiece  2  molten by the laser beam  45  may be obtained, and an output of the laser beam  45  may be controlled based on the information. Such a welding method can also control the temperature of the workpiece  2  and the dilution rate of components of the filler metal  35  in the build-up layer  3  during the build-up welding process as desired. As a result, the build-up layer  3  can have desired characteristics. 
     In the welding method according to the embodiment, the information on a dimension of the molten pool  4  is obtained using a non-contact radiation temperature sensor  81 . 
     The welded member  1  according to the embodiment is a welded member  1  with a build-up layer  3  of a filler metal  35  welded to a surface to be built-up  2   a  of an elongated workpiece  2 , wherein a dilution rate of components of the filler metal  35  in the build-up layer  3  is 10% or more and 40% or less, preferably, 15% or more and 35% or less. Such a welded member  1  can prevent the possibility that an insufficiently fused portion is formed between the build-up layer  3  and the workpiece  2 , and can make it possible that the build-up layer  3  has a hardness suitable for use in a thermal power plant where high temperature steam flows. 
     The welded member  1  according to the embodiment is a welded member  1  with a build-up layer  3  of a cobalt-base alloy  35  welded to a surface to be built-up  2   a  of an elongated object  2 , wherein a Vickers hardness of the build-up layer  3  is Hv 320 or more and Hv 500 or less. Such a welded member  1  can make it possible that the build-up layer  3  has a hardness suitable for use in a thermal power plant where high temperature steam flows. 
     The embodiment can provide a weld method for subjecting a workpiece to a build-up welding process, which is capable of improving characteristics such as a hardness of a build-up layer, and a welded member wherein characteristics of a build-up layer are improved. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fail within the scope and spirit of the invention. In addition, it goes without saying that these embodiments and modifications can be partially combined as appropriate, within the range of the scope of the present invention.