Patent Publication Number: US-2022212286-A1

Title: Welding method and welding equipment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-000205, 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 welding equipment. 
     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 wear-resistant 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 nozzle shown in  FIG. 2 . 
         FIG. 5  is a view corresponding to  FIG. 4 , showing a modification example of the welding equipment. 
     
    
    
     DETAILED DESCRIPTION 
     In an embodiment, 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 comprises: 
     a step for forming a build-up layer on the surface of the workpiece by supplying a filler metal to the surface of the workpiece along the longitudinal direction and by applying a laser beam thereto to melt the filler metal; 
     wherein a refrigerant is supplied to the surface to be built-up of the workpiece, during the step for forming the build-up layer. 
     In an embodiment, a welding equipment 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 comprises: 
     a supporter that supports the workpiece; 
     a filler-metal supply unit that supplies a filler metal to the surface to be built-up of the workpiece; 
     a laser irradiator having a laser oscillator and a laser emitter that emits a laser beam oscillated by the laser oscillator toward the workpiece supported by the supporter; 
     a longitudinal motion drive that relatively moves the laser emitter along the longitudinal direction with respect to the workpiece supported by the supporter; and 
     a refrigerant supply unit that supplies a refrigerant to the surface to be built-up of the workpiece supported by the supporter. 
     Alternatively, in an embodiment, a welding equipment 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 comprises: 
     a filler-metal-layer forming unit that forms a filler metal layer by thermally spraying or applying a filler metal to the surface of the workpiece; 
     a supporter that supports the workpiece with the filler metal layer formed thereon; 
     a laser irradiator having a laser oscillator and a laser emitter that emits a laser beam oscillated by the laser oscillator toward the workpiece supported by the supporter; 
     a longitudinal motion drive that relatively moves the laser emitter along the longitudinal direction with respect to the workpiece supported by the supporter; and 
     a refrigerant supply unit that supplies a refrigerant to the surface to be built-up of the workpiece supported by the supporter. 
     An embodiment is described with reference to the drawings.  FIG. 1  is a view showing a wear-resistant member according to an embodiment.  FIGS. 2 and 3  are views schematically showing a structure of a welding equipment for manufacturing the wear-resistant 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 wear-resistant 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 800 to 1100 nm. 
     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 . 
     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 wear-resistant 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 wear-resistant 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, the portion thereof at which the welding is ended is referred to as weld end portion, and the middle portion thereof between the weld start portion and the weld end portion is referred to as weld middle portion, the hardness of the build-up layer from the vicinity of the weld middle portion to 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 from the vicinity of the weld middle portion to the vicinity of the weld end 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. 
     In consideration of this point, the welding equipment  10  and the welding method in the embodiment are devised to prevent the increase in dilution rate of components of the filler metal  35  in the build-up layer  3  from the weld middle portion to the weld end portion, so as to increase the hardness of the build-up layer  3 . Specifically, during the period from the start to the end of welding, the build-up welding process is performed in such a manner that the workpiece  2  is cooled to decrease the temperature of the surface to be built-up  2   a . This can prevent the increase in temperature of the workpiece  2  during the build-up welding process, whereby the dilution rate of the build-up layer  3  is prevented from becoming excessively high. As a method of preventing the increase in temperature of the workpiece  2  during the build-up welding process, it may be considered that the laser beam application to the workpiece  2  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 , or that an output or the like of the laser beam  45  from the laser oscillator  41  is controlled as appropriate during the build-up welding process. 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. In addition, it is difficult to regulate welding conditions during the build-up welding process as appropriate in such a manner that the dilution rate is kept lower than a predetermined value. On the other hand, the method in which the build-up welding process is performed while the workpiece  2  is cooled makes it possible to prevent the increase in temperature of the workpiece  2  and to prevent the resulting increase in dilution rate, while performing the build-up welding process under predetermined welding conditions without interrupting the build-up welding process. 
     In order to cool the workpiece  2  during the build-up welding process as described above, the welding equipment  10  further comprises a refrigerant supply unit  80 . The refrigerant supply unit  80  has a refrigerant conduit  81  that leads a refrigerant  85  from a refrigerant supply source (not-shown) to the vicinity of the workpiece  2  supported by the supporter  20 , a nozzle  82  provided at the distal end of the refrigerant conduit  81  to eject the refrigerant  85 , and a flow-rate regulation valve  83  that regulates a flow rate of the refrigerant  85  ejected from the nozzle  82 . Since the workpiece  2  is cooled by the refrigerant  85 , the increase in temperature of the workpiece  2  due to the application of the laser beam  45  from the laser emitter  40  is suppressed. 
     In the illustrated example, the refrigerant  85  is water. The use of water as the refrigerant  85  prevents the possibility that the workpiece  2 , the build-up layer  3  and the welding equipment  10  are adversely affected by the refrigerant. The refrigerant may be tap water. In this case, the refrigerant conduit  81  may be connected to a water supply. The temperature of the refrigerant  85  is preferably 30° C. or less, more preferably 25° C. or less, and further preferably 20° C. or less. This can prevent the possibility that the workpiece  2  is not sufficiently cooled so that a dilution rate of components of the filler metal  35  becomes higher than a desired dilution rate in the build-up step described below. Hence, the possibility that the hardness of the build-up layer  3  becomes lower than a desired hardness can be prevented. The temperature of the refrigerant  85  is preferably 0° C. or more, more preferably 5° C. or more, and further preferably 10° C. or more. This can prevent the possibility that the temperature of the workpiece  2  becomes so low that the build-up layer  3  and the workpiece  2  fuse insufficiently. The temperature of tap water supplied from a water supply is generally between 5° C. and 25° C. Thus, the use of tap water as the refrigerant  85  eliminates the need for regulating the temperature of the refrigerant  85 . 
     The nozzle  82  has a tubular shape and the refrigerant  85  from it trickles down in a continuous current (in a small stream). Since the refrigerant  85  from the nozzle  82  trickles down in a continuous current (in a small stream) (in other words, since the refrigerant  85  is not sprayed from the nozzle  82 ), the refrigerant  85  is prevented from being scattered and landing on the welding equipment  10 . 
     When the refrigerant  85  is sprayed from the nozzle  82  or the flow rate of the refrigerant  85  ejected from the nozzle  82  is high so that the refrigerant  85  is scattered, it is preferable to arrange a shielding plate or cover as appropriate in order to reduce the possibility that the refrigerant  85  lands on the welding equipment  10 . The refrigerant  85  may be a liquid other than water, and it may be even a gas. When the refrigerant  85  is a gas, in order to prevent the gas from diffusing around, it is preferable to arrange a shielding plate or cover as appropriate. 
     In the illustrated example, as shown in  FIG. 3 , the distal end of the nozzle  82  is positioned above the workpiece  2  to face the surface to be built-up  2   a , and supplies the refrigerant  85  to the surface to be built-up  2   a . On the other hand, when the workpiece  2  is hollow, as shown in  FIG. 5 , the nozzle  82  may be positioned in the hollow area inside the workpiece  2 , and may supply the refrigerant  85  to the hollow area. In this case, the possibility that the refrigerant  85  scatters or diffuses is prevented by the workpiece  2  itself. 
     The nozzle  82  is provided to be relatively movable along the longitudinal direction with respect to the workpiece  2  supported by the supporter  20 . In the illustrated example, the nozzle  82  is moved, together with the welding torch  15  (laser emitter  43 ), by the longitudinal motion drive  60 , in the longitudinal direction with respect to the supporter  20 . Thus, the increase in temperature of the workpiece  2  caused by the heat applied by the laser beam  45  can be efficiently prevented during the below-described build-up step from the start to the end. 
     Further, in the illustrated example, as shown in  FIGS. 2  and  4 , the nozzle  82  is arranged forward the welding torch  15  (laser emitter  43 ) in the relative movement direction D 1  of the welding torch  15  (laser emitter  43 ) with respect to the workpiece  2 . Thus, the refrigerant  85  is supplied, in the aforementioned relative movement direction D 1 , at a position forward a position of the workpiece  2  to which the laser beam  45  is applied. Thus, each portion of the workpiece  2  can be cooled before the filler metal  35  molten by the laser beam  45  lands on the portion. In this case, as compared with a case in which a portion of the workpiece  2  is cooled immediately after the molten filler metal  35  lands on the portion, the possibility of cracking of the build-up layer  3  is reduced. 
     As shown in  FIGS. 2 and 3 , the welding equipment  10  may have a bucket  90  for receiving the refrigerant  85  having been supplied to the workpiece  2 . The bucket  90  is arranged below the workpiece  2  supported by the supporter  20 . 
     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 nozzle  82  are positioned in the vicinity of the aforementioned one end of the workpiece  2 . In the illustrated example, the nozzle  82  is positioned forward the welding torch  15  in the movement direction D 1  of them moved by the longitudinal motion drive  60 . 
     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 nozzle  82  along the longitudinal direction is started by the longitudinal motion drive  60 . The welding torch  15  and the nozzle  82  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 refrigerant  85  is supplied from the nozzle  82  to the workpiece  2  first. Then, 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 nozzle  82  are being moved by the longitudinal motion drive  60 . 
     In the illustrated example, the refrigerant  85  is supplied to a position forward the welding torch  15  in the movement direction D 1  of the welding torch  15  moved by the longitudinal motion drive  60 . Thus, each portion of the workpiece  2  is cooled before the filler metal  35  molten by the laser beam  45  lands on the portion. 
     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. 
     Since the refrigerant  85  is supplied to the workpiece  2  during the build-up step, the increase in temperature of the workpiece  2  caused by the heat applied by the laser beam  45  is suppressed. Thus, it is not necessary to interrupt the build-up step to dissipate the heat of the workpiece  2 . It is also not necessary to regulate a welding condition, such as an output of the laser beam  45  from the laser oscillator  41 , during the build-up step as appropriate in order to prevent the increase in temperature of the workpiece  2 . Further, since the supply of the refrigerant  85  is performed along the movement direction D 1  of the position on which the laser beam  45  hits the workpiece  2 , the increase in temperature of the workpiece  2  caused by the heat applied by the laser beam  45  can be efficiently prevented from the start to the end of the build-up step. Moreover, since the refrigerant  85  is supplied at a position forward the position on which the laser beam  45  hits the workpiece  2  in the movement direction D 1 , each portion of the workpiece  2  is cooled before the filler metal  35  molten by the laser beam  45  lands on the portion. Thus, as compared with a case in which a portion of the workpiece  2  is cooled immediately after the molten filler metal  35  lands on the portion, the possibility of cracking of the build-up layer  3  is reduced. 
     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 semiconductor laser was used as the laser oscillator  41 , and tap water was used as the refrigerant  85 . Then, the build-up step was performed under the following welding conditions. The welding conditions were unchanged from the start to the end of the welding. 
     &lt;Welding Conditions&gt; 
     Laser output: 2 kW to 10 kW 
     Feed rate of filler metal: 10 g/min to 60 g/min 
     Welding speed: 200 mm/min to 1000 mm/min 
     Average flow rate of refrigerant: 180 g/min 
     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 wear-resistant 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 in the same way as in Example, except that cooling of the workpiece  2  by the refrigerant  85  was not performed during build-up welding. 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 in the same way as in Example. 
     (Evaluation) 
     Table 1 shows the hardness and the dilution rates of each build-up layers  3  in Example and Comparative Example. In Table 1, the hardness is shown as a ratio (hardness ratio) of the hardness of each portion of the build-up layer  3  in Example and Comparative Example, to the hardness of the build-up layer  3  in the vicinity of the weld end portion of Example. In addition, in Table 1, the dilution rate is shown as a ratio (dilution rate ratio) of the aforementioned dilution rate of the build-up layer  3  at the weld end portion of Example, to the dilution rate of the build-up layer  3  at the weld end portion of Comparative Example. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                 Dilution rate 
               
               
                   
                 Hardness ratio 
                 ratio (to 
               
               
                   
                 (to the end portion of Example) 
                 Comp. Ex.) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Near 
                 Near 
                 Near 
                 Near 
               
               
                   
                   
                 end 
                 middle 
                 end 
                 end 
               
               
                   
                 Cooling 
                 portion 
                 portion 
                 portion 
                 portion 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 
                 Yes 
                 1.39 
                 1.08 
                 1.00 
                 0.625 
               
               
                 Comp. 
                 No 
                 1.22 
                 0.88 
                 0.82 
                 1 
               
               
                 Example 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the dilution rate of the build-up layer  3  at the weld end portion of Comparative Example was 1, while the aforementioned ratio (dilution rate ratio) of the dilution rate of the build-up layer  3  at the weld end portion of Example was 0.625. Namely, the dilution rate of the build-up layer  3  of Example 1 was lower than the dilution rate of the build-up layer  3  of Comparative Example. Namely, the increase in dilution rate of the build-up layer  3  of Example was more prevented than that of Comparative Example. In addition, as shown in Table 1, the hardness of the build-up layer in the vicinity of the weld end portion of Example was 1, while the hardness (hardness ratio) of the build-up layer  3  in the vicinity of the weld start portion of Comparative Example, in the vicinity of the weld middle portion thereof and in the vicinity of the weld end portion thereof were 1.22, 0.88 and 0.82, respectively. On the other hand, the hardness of the build-up layer  3  in the vicinity of the weld end portion of Example was 1, while the hardness (hardness ratio) of the build-up layer  3  in the vicinity of the weld start portion of Example and in the vicinity of the weld middle portion thereof were 1.39 and 1.08, respectively. Namely, the hardness of each portion of the build-up layer  3  of Example was more improved than that of Comparative Example. 
     From the above results, it can be understood that cooling of the workpiece  2  during the build-up welding prevents the increase in dilution rate of the build-up layer  3 , and improves the hardness of the build-up layer  3 . 
     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-layer 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 along a longitudinal direction of the workpiece  2 , the welding method comprising a step for 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 . A refrigerant  85  is supplied to the workpiece  2 , during the step for forming the build-up layer. Such a welding method can prevent the increase in temperature of each portion of the workpiece  2  caused by the heat applied by the laser beam  45 . Thus, the increase in dilution rate of the build-up layer  3  is prevented and the decrease in hardness of the build-up layer  3  is prevented. 
     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 for 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 is formed, a step for 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. A refrigerant  85  may be supplied to the surface to be built-up of the workpiece, during the step for forming the build-up layer. Such a welding method can also prevent the increase in temperature of each portion of the workpiece  2  caused by the heat applied by the laser beam  45 . Thus, the increase in dilution rate of the build-up layer  3  is prevented and the decrease in hardness of the build-up layer  3  is prevented. 
     In the welding method according to the embodiment, the refrigerant  85  is supplied to the workpiece  2  at a position forward an application position on the workpiece  2  to which the laser beam  45  is applied, in the relative movement direction D 1  of the application position with respect to the workpiece  2 . Thus, each portion of the workpiece  2  can be cooled before the filler metal  35  molten by the laser beam  45  lands on the portion, whereby the possibility of cracking of the build-up layer  3  can be reduced. 
     In the embodiment, the workpiece  2  may be hollow, and the refrigerant  85  may be supplied to a hollow area inside the workpiece  2 . In this case, the possibility that the refrigerant  85  scatters or diffuses is prevented by the workpiece  2  itself. 
     In the embodiment, the refrigerant  85  is water. This prevents the possibility that the workpiece  2 , the build-up layer  3  and the welding equipment  85  are eroded or so by the refrigerant  85 . 
     In the embodiment, the workpiece  2  is rotated around the rotation axis  70 X along the longitudinal direction, during the step for forming the build-up layer. Thus, the build-up layer  3  is formed spirally on the surface to be built-up  2   a . Namely, the build-up layer  3  can be formed on the entire circumference of the surface to be built-up  2   a.    
     In addition, the welding equipment  10  according to the embodiment is a welding equipment  10  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 equipment  10  comprising a supporter  20 , a filler-metal supply unit  30 , a laser irradiator  40 , a longitudinal motion drive  60 , and a refrigerant supply unit  80 . The supporter  20  supports the workpiece  2 . The filler-metal supply unit  30  supplies a filler metal  35  to the surface to be built-up  2   a  of the workpiece  2 . The laser irradiator  40  has a laser oscillator  41 , and a laser emitter  43  that emits a laser beam  45  oscillated by the laser oscillator  41  toward the workpiece  2  supported by the supporter  20 . The longitudinal motion drive  60  relatively moves the laser emitter  43  along the longitudinal direction with respect to the workpiece  2  supported by the supporter  20 . The refrigerant supply unit  80  supplies a refrigerant  85  to the surface to be built-up  2   a  of the workpiece  2  supported by the supporter  20 . Such a welding equipment  10  can prevent the increase in temperature of each portion of the workpiece  2  caused by the heat applied by the laser beam  45 , by supplying the refrigerant  85  to the workpiece  2  while the laser beam  45  is being applied to the workpiece  2 . Thus, the increase in dilution rate of the build-up layer  3  can be prevented and the decrease in hardness of the build-up layer  3  can be prevented. 
     Alternatively, the welding equipment  10  according to the embodiment is a welding equipment  10  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 equipment  10  may comprise a filler-metal-layer forming unit, a supporter  20 , a laser irradiator  40 , a longitudinal motion drive  60 , and a refrigerant supply unit  80 . The filler-metal-layer forming unit may form a filler metal layer by thermally spraying or applying a 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. The laser irradiator  40  may have a laser oscillator  41 , and a laser emitter  43  that emits a laser beam  45  oscillated by the laser oscillator  41  toward the workpiece  2  supported by the supporter  20 . The longitudinal motion drive  60  may relatively move the laser emitter  43  along the longitudinal direction with respect to the workpiece  2  supported by the supporter  20 . The refrigerant supply unit  80  may supply a refrigerant  85  to the surface to be built-up  2   a  of the workpiece  2  supported by the supporter  20 . Such a welding equipment  10  can also prevent the increase in temperature of each portion of the workpiece  2  caused by the heat applied by the laser beam  45 , by supplying the refrigerant  85  to the workpiece  2  while the laser beam  45  is being applied to the workpiece  2 . Thus, the increase in dilution rate of the build-up layer  3  can be prevented and the decrease in hardness of the build-up layer  3  can be prevented. 
     In the embodiment, the refrigerant supply unit  80  has the nozzle  82  for ejecting the refrigerant  85 . The longitudinal motion drive  60  relatively moves the nozzle  82 , together with the laser emitter  43 , along the longitudinal direction with respect to the workpiece  2  supported by the supporter  20 . Such a welding equipment  10  can efficiently prevent the increase in temperature of the workpiece  2  caused by the heat applied by the laser beam  45  from the end to the start of the build-up welding process. 
     In the embodiment, the nozzle  82  is positioned forward the laser emitter  43  in the relative movement direction D 1  of the laser emitter  43  with respect to the workpiece  2 . Such a welding equipment  10  can cool each portion of the workpiece  2  before the filler metal  35  molten by the laser beam  45  lands on the portion, whereby the possibility of cracking of the build-up layer  3  can be reduced. 
     In the embodiment, the welding equipment  10  further comprises the rotational motion unit  70  that rotates the workpiece  2  supported by the supporter  20  around the rotation axis  70 X along the longitudinal direction. Such a welding equipment  10  can form the build-up layer  3  spirally on the surface to be built-up  2   a . Namely, the build-up layer  3  can be formed on the entire circumference of the surface to be built-up  2   a.    
     According to an embodiment, a welding method and a welding equipment for subjecting a workpiece to a build-up welding process can be provided, which are capable of improving a hardness of a build-up layer. 
     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 sprit 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.