Patent Publication Number: US-2023148212-A1

Title: Processing method and processing apparatus for workpiece

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from Japanese Patent Application No. 2021-182416 filed on Nov. 9, 2021, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a processing method and a processing apparatus for a workpiece. 
     BACKGROUND 
     Metal part such as mold or turbine is formed with hole, such as water cooling hole. Since an inner surface of the water cooling hole is always exposed to cooling water flowing through the water cooling hole, corrosion is likely to occur on the inner surface of the water cooling hole. 
     In addition, thermal stress is repeatedly generated on the inner surface of the metal part due to a temperature difference between the outer surface of the metal part and the inner surface of the cooling hole cooled by the cooling water. Therefore, the inner surface of the water cooling hole is prone to stress corrosion cracking and other damage. 
     In order to prevent damage to the metal part, a technique is known to increase fatigue strength of the inner surface of the water cooling hole by applying compressive residual stress to the inner surface. For example, Patent Literature 1 describes that a nozzle is inserted into a water cooling hole of a mold and injecting shot media from a tip of the nozzle to perform shot peening on an inner surface of the water cooling hole. In this method, compressive residual stress is applied to the inner surface of the water cooling hole by collision of the shot media with the inner surface. 
     Further, Patent Literature 2 describes a burnishing tool for performing burnish treatment on an inner surface of a machined hole. The burnishing tool includes a cylindrical retainer, a plurality of rollers that are rotatable about an axis and are capable of protruding from the retainer in a radial direction, and a mandrel that rotates about the axis and causes the plurality of rollers to protrude from the retainer. In this burnishing tool, the mandrel is rotated inside the machined hole to strike and roll the inner surface of the machined hole with the rollers. As a result, the inner surface of the machined hole is plastically deformed, and compressive residual stress is applied to the inner surface of the machined hole. 
     PRIOR ART DOCUMENT 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 6036704 
         Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2003-159648 
       
    
     SUMMARY 
     In the method described in Patent Literature 1, for example, when the water cooling hole having a complicated shape such as being curved in the depth direction is formed in the mold, it is required to use a special nozzle that can be inserted into the water cooling hole having the complicated shape. Further, since the burnishing tool described in Patent Literature 2 extends linearly in the axial direction, it is difficult to insert the burnishing tool into the machined hole having a complicated shape. Therefore, it is difficult to uniformly process the inner surface of a machined hole having a complicated shape using this burnishing tool. 
     Therefore, an object of the present disclosure is to provide a processing method and a processing apparatus for a workpiece capable of applying compressive residual stress to an inner surface of a hole regardless of the shape of the hole. 
     A processing method according to one aspect includes preparing a metal workpiece having a hole that opens to a surface thereof, filling the hole with processing media, and applying an external force to the processing media to apply a compressive residual stress to an inner surface of the hole. 
     According to the present disclosure, compressive residual stress can be applied to an inner surface of a hole regardless of the shape of the hole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a flowchart illustrating a processing method for a workpiece according to one embodiment. 
         FIG.  2    is a cross-sectional view of an exemplary workpiece. 
         FIG.  3    is a perspective view schematically illustrating a processing apparatus according to one embodiment. 
         FIG.  4 A  is a side view of an exemplary pressing member. 
         FIG.  4 B  is a bottom view of an exemplary pressing member. 
         FIG.  5    is a diagram schematically illustrating a force acting on an inner surface of a water cooling hole. 
         FIG.  6    is a flowchart illustrating a processing method for a workpiece according to another embodiment. 
         FIG.  7 A  is a side view of another variation of a pressing member. 
         FIG.  7 B  is a bottom view of another variation of a pressing member. 
         FIG.  8    is a diagram schematically illustrating a force acting on an inner surface of a water cooling hole. 
         FIG.  9    is a graph showing the relationship between the number of hammering times and residual stress. 
         FIG.  10    is a graph showing the relationship between the number of stirring times and residual stress. 
     
    
    
     DETAILED DESCRIPTION 
     Summary of Embodiments of the Present Disclosure 
     First, a summary of embodiments of the present disclosure will be described. 
     (Clause 1) A processing method, comprising preparing a metal workpiece having a hole that opens to a surface thereof, filling the hole with processing media, and applying an external force to the processing media to apply a compressive residual stress to an inner surface of the hole. 
     In the above aspect, when an external force is applied to the processing media filled in the hole, the external force is transmitted to the inner surface of the hole via the processing media. At this time, the external force substantially uniformly acts on the inner surface of the hole, and compressive residual stress is applied to the inner surface of the hole. Therefore, in this processing method, by applying the external force to the inner surface of the hole via the processing media, compressive residual stress can be applied to the inner surface of the hole regardless of the shape of the hole. 
     (Clause 2) The processing method according to clause 1, further comprising, inserting a pressing member into the hole, and applying a load to the pressing member inserted into the hole to apply the external force to the processing media. 
     In the above aspect, the external force applied to the pressing member is transmitted to the inner surface of the hole via the processing media. By this external force, compressive residual stress can be applied to the inner surface of the hole. 
     (Clause 3) The processing method according to clause 2, further comprising dropping a hammer against the pressing member to apply the load to the pressing member. 
     In the above aspect, since the load is applied from the hammer to the pressing member and the external force acts on the inner surface of the hole via the processing media, compressive residual stress can be applied to the inner surface of the hole. 
     (Clause 4) The processing method according to any one of clauses 1 to 3, comprising stirring the processing media within the hole. 
     In the above aspect, by stirring the processing media within the hole, the compressive residual stress applied to the inner surface of the hole can be increased. 
     (Clause 5) The processing method according to any one of clauses 1 to 4, further comprising filling the hole with a powder containing a metal material to be attached to the inner surface of the hole before applying the external force to the processing media. 
     In the above aspect, by applying the external force to the processing media after filling with the powder containing the metal material, the powder is pressed against the inner surface of the hole, and the film containing the metal material can be formed on the inner surface of the hole. 
     (Clause 6) A processing apparatus for processing an inner surface of a hole formed in a metal workpiece, the processing apparatus comprising, a pressing member inserted into the hole filled with processing media, and an external force applying device to apply an external force to the processing media through the pressing member. 
     In the above aspect, by applying the external force to the pressing member, the external force is applied from the processing media to the inner surface of the hole. At this time, the external force substantially uniformly acts on the inner surface of the hole, and compressive residual stress is applied to the inner surface of the hole. Therefore, according to this processing apparatus, compressive residual stress can be applied to the inner surface of the hole regardless of the shape of the hole. 
     (Clause 7) The processing apparatus according to clause 6, wherein the external force applying device includes a hammer disposed above the pressing member, and a lifting unit to move the hammer in a vertical direction, wherein when the hammer is lowered, the hammer collides with the pressing member to apply a load toward a depth direction of the hole to the pressing member. 
     In the above aspect, since the load is applied from the hammer to the pressing member, the external force is applied to the inner surface of the hole via the processing media. Therefore, compressive residual stress can be applied to the inner surface of the hole. 
     (Clause 8) The processing apparatus according to clause 6 or 7, wherein the pressing member includes a cylindrical main body and a stirring pin extending parallel to a central axis of the main body from the main body, and wherein the stirring pin is coupled to the main body at a position offset from the central axis of the main body. 
     In the above aspect, since the stirring pin is coupled to the main body at a position offset from the central axis of the main body, when the main body rotates around the central axis, the stirring pin revolves inside the hole along the circumference around the central axis. As a result, the processing media filled in the hole is agitated and stirred by the stirring pin. By stirring the processing media, the compressive residual stress applied to the inner surface of the hole can be increased. 
     Exemplary Embodiments of the Present Disclosure 
     Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the description based on the drawings, the same elements or elements having the same functions are denoted by the same reference numerals, and redundant description is omitted. In the drawings, some parts may be simplified or exaggerated for easy understanding, and dimensional ratios, angles, and the like are not limited to those described in the drawings. 
     A processing method according to the present disclosure applies compressive residual stress to an inner surface of a hole formed in a workpiece. Examples of the workpiece include metal part that requires high fatigue strength and wear resistance, such as a mold or a turbine. Although not limited thereto, the metal part is formed of an iron-based alloy containing iron as a main component, an aluminum alloy, or the like. The iron-based alloy is, for example, a steel material. Typically, quenched and tempered martensitic steel is used for the metal member made of steel, but raw material which have not been subjected to heat treatment may be used. 
     The workpiece may be a block-shaped metal part, or may be a multilayer molded article formed three dimensionally by laminating a plurality of metal layers. A hole such as a water cooling hole is formed in the workpiece. The hole formed in the workpiece is not limited to the water cooling hole. In the following description, an example in which a mold having a water cooling hole is adopted as a workpiece will be described. 
       FIG.  1    is a flow chart illustrating a processing method MT 1  according to one embodiment. In the processing method MT 1  illustrated in  FIG.  1   , a mold  10  which is an example of the workpiece is prepared (Step ST 1 ). 
       FIG.  2    is a cross-sectional view of an example of the mold  10  prepared in the step ST 1 . The mold  10  is, for example, a steel mold for die casting. Die casting is a type of mold casting method in which molten metal is pressed into a mold to mass-produce high-precision castings. A nitriding treatment may be applied on the surface of the mold  10 . The nitriding treatment is a method of heating the mold  10  in a nitriding gas such as ammonia gas to form a nitrided layer on the surface of the mold  10 . By applying nitriding treatment on the mold  10 , distortion of the mold  10  due to heat is suppressed. 
     As shown in  FIG.  2   , the mold  10  is a block-shaped steel material having a first surface  11  and a second surface  12 . A cavity, which is a space corresponding to a product shape, is formed on the first surface  11  of the mold  10 . A plurality of water cooling holes  13  that open to the second surface  12  are formed in the mold  10 . The plurality of water cooling holes  13  are bottomed holes, and the mold  10  is cooled by cooling water flowing through the water cooling holes  13  during casting using the mold  10 . Each of the plurality of water cooling holes  13  has a side surface  14  and a bottom surface  15 . The side surface  14  and the bottom surface  15  constitute an inner surface  16  defining the water cooling hole  13 . Since thermal stress is repeatedly generated on the inner surface  16  of the water cooling hole  13  due to a temperature difference between the first surface  11  of the mold  10  and the inner surface  16  of the water cooling hole  13 , stress corrosion cracking is particularly likely to occur on the inner surface  16  of the water cooling hole  13 . 
     As shown in  FIG.  2   , the plurality of water cooling holes  13  extend from the second surface  12  of the mold  10  to a position before the first surface  11 . The water cooling hole  13  has a circular cross-sectional shape and has a substantially constant diameter in the entire region in a depth direction of the water cooling hole  13 . The water cooling hole  13  may have an any cross-sectional shape such as an elliptical shape or a polygonal shape, and the cross-sectional area of the water cooling hole  13  may change continuously or stepwise in the depth direction. In the embodiment shown in  FIG.  2   , the water cooling hole  13  extends linearly in the thickness direction of the mold  10 , but the water cooling hole  13  may be curved inside the mold  10 . 
     Next, a processing apparatus used in the processing method MT 1  will be described with reference to  FIGS.  3 ,  4 A and  4 B .  FIG.  3    is a perspective view schematically illustrating a processing apparatus  1  according to an embodiment.  FIGS.  4 A and  4 B  show a pressing member  30  which is part of the processing apparatus  1 . The processing apparatus  1  includes a base plate  21 , an external force applying device  28 , and a pressing member  30 . 
     The base plate  21  provides a support surface for placing the mold  10  thereon. The mold  10  is placed on and fixed to the base plate  21  with the openings of the plurality of water cooling holes  13  facing upward. 
     The external force applying device  28  applies an external force to processing media  25  via a pressing member  30 , and includes a lifting unit  22  and a hammer  23 . The lifting unit  22  is a linear motion mechanism that moves the hammer  23  in the up-down direction (vertical direction). The lifting unit  22  has a spline shaft  24  and a ball spline  26 . The spline shaft  24  stands on the base plate  21  and extends in the up-down direction. The ball spline  26  has an annular shape surrounding the spline shaft  24 . The ball spline  26  has a plurality of rotatable rolling elements, and is movable along the spline shaft  24  by rolling of the plurality of rolling elements. The lifting unit  22  may further include a stopper that fixes the position of the ball spline  26  in the up-down direction. The lifting unit  22  holds the hammer  23  above the mold  10  in an initial state. 
     The hammer  23  is a metal plate, for example, and functions as a weight that applies a load to the pressing member  30  in the depth direction of the water cooling hole  13 . A portion of the hammer  23  is located above the plurality of water cooling holes  13  of the mold  10 . The hammer  23  is connected to the ball spline  26  so as to be movable in the up-down direction above the mold  10 . When the fixing of the ball spline  26  is released, the hammer  23  falls (descends) toward the mold  10  together with the ball spline  26 . 
       FIG.  4 A  is a side view of an exemplary pressing member  30 , and  FIG.  4 B  is a bottom view of the pressing member  30 . The pressing member  30  is a metal member having a substantially cylindrical shape and extends along the central axis AX between one end  31  and the other end  32 . The length of the pressing member  30  is formed to be longer than the depth of the water cooling hole  13  (the distance between the second surface  12  of the mold  10  and the bottom surface  15  of the water cooling hole  13  in the thickness direction of the mold  10 ). The pressing member  30  has a diameter smaller than that of the water cooling hole  13 . In order to efficiently apply force to the processing media  25 , the diameter of the pressing member  30  may be 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more of the diameter of the water cooling hole  13 . Accordingly, the pressing member  30  may be inserted into the water cooling hole  13 . 
     Reference is again made to  FIG.  1   . The mold  10  prepared in step ST 1  is fixed on the base plate  21  with the openings of the plurality of water cooling holes  13  facing upward. Then, the plurality of water cooling holes  13  of the mold  10  are filled with the processing media  25  (Step ST 2 ). The processing media  25  is abrasive grains such as an abrasive that applies residual stress to the mold  10 . The material of the processing media  25  is selected according to the workpiece from various materials such as metal (for example, iron, zinc or stainless steel), ceramic (for example, alumina, silicon carbideor zircon), glass, and resin (for example, nylon resin, melamine resin or urea resin). The shape of the abrasive is selected from various shapes such as a spherical shape, a polygonal shape, and a cylindrical shape. Typically, steel balls having diameters of about 0.2 mm to 1.2 mm are used as the processing media  25 . 
     In one embodiment, the plurality of water cooling holes  13  of the mold  10  may be filled with the processing media  25  and the powder containing a metal material to be attached to the inner surfaces of the water cooling holes  13  (Step ST 3 ). For example, when a zinc film is required to be formed on the inner surface  16  of the plurality of water cooling holes  13 , the plurality of water cooling holes  13  of the mold  10  may be filled with powdered zinc. 
     Next, the pressing member  30  is inserted into each water cooling hole  13  filled with the processing media  25  (Step ST 4 ). At this time, the pressing member  30  is inserted into the water cooling hole  13  in a state in which the other end  32  of the water cooling hole  13  faces the bottom surface  15  of the water cooling hole  13 . Accordingly, the one end  31  of the pressing members  30  is disposed above the second surface  12  while being exposed from the mold  10 , and the other end  32  of the pressing member  30  is disposed inside the water cooling holes  13  while being in contact with the processing media  25 . 
     Next, the fixing of the ball spline  26  is released so that the hammer  23  disposed above the mold  10  is dropped toward the pressing member  30  (Step ST 5 ). As shown in  FIG.  5   , the dropped hammer  23  collides with the one end  31  of the pressing member  30  to apply a load F toward the bottom surface  15  of the water cooling hole  13  to the pressing member  30 . The load F applied to the pressing member  30  is transferred to the processing media  25 , and the processing media  25  is pressurized in the water cooling hole  13 . As a result, the inner surface  16  of the water cooling hole  13  is pressed by the processing media  25 . When the external force is applied from the processing media  25 , the inner surface  16  of the water cooling hole  13  is plastically deformed, and compressive residual stress is applied to the inner surface  16 . At this time, since the force transmitted from the processing media  25  substantially uniformly acts on the inner surface  16  of the water cooling hole  13 , the compressive residual stress is applied to the inner surface  16  with high uniformity. 
     In addition, in a case where the plurality of water cooling holes  13  of the mold  10  are filled with powder containing zinc together with the processing media  25 , the powder is pressed against the inner surface  16  of the water cooling hole  13  when the force is applied from the processing media  25  to the inner surface  16 , and the powdered zinc is attached to the inner surface  16 . As a result, a zinc film is formed on the inner surface  16 . By forming the zinc film on the inner surface  16  of the water cooling hole  13 , corrosion of the inner surface  16  due to the cooling water circulating through the water cooling hole  13  can be suppressed. As a result, it is possible to suppress disappearance of the compressive residual stress layer from the inner surface  16  due to erosion of the inner surface  16 . 
     Next, the hammer  23  is lifted by the lifting unit  22  (step ST 6 ). The position of the lifted hammer  23  is determined according to the compressive residual stress to be applied to the inner surface  16  of the water cooling hole  13 . Next, it is determined whether or not the drop of the hammer  23  has been repeated a predetermined number of times (step ST 7 ). When the number of the drops of the hammer  23  is less than the predetermined number of times, the step ST 5  and the step ST 6  are repeated until the number of the drops reaches the predetermined number to apply the load F to the pressing member  30 . When the number of the drops of the hammer  23  reaches the predetermined number, the processing method MT 1  according to an embodiment is ended. 
     As described above, in the processing method MT 1 , an external force is applied to the processing media  25  filled in the water cooling hole  13  of the mold  10  through the pressing member  30 . The external force applied to the processing media  25  substantially uniformly acts on the inner surface  16  of the water cooling hole  13  to apply compressive residual stress to the inner surface  16  of the water cooling hole  13  with high uniformity. Therefore, according to this processing method MT 1 , it is possible to apply compressive residual stress to the inner surface  16  of the water cooling hole  13  regardless of the shape of the water cooling hole  13 . 
     In general shot peening, when shot media collide with a workpiece, a damage such as a dent called a tool mark may be generated. When the tool mark is formed on the inner surface  16  of the water cooling hole  13 , stress is concentrated on a portion where the tool mark is formed, and the portion may become a starting point of cracking of the mold  10 . In the processing method MT 1 , a large surface pressure may be applied to the inner surface  16  of the water cooling hole  13  by repeatedly applying the load F to the pressing member  30 . Accordingly, the unevenness of the inner surface  16  of the water cooling hole  13  may be leveled and the tool mark causing the damage of the mold  10  may be removed. 
     Next, a processing method of a workpiece according to another embodiment will be described. Hereinafter, differences from the above-described processing method MT 1  will be mainly described, and redundant description will be omitted.  FIG.  6    is a flowchart illustrating a processing method MT 2  of a workpiece according to another embodiment. As shown in  FIG.  6   , the processing method MT 2  differs from the processing method MT 1  shown in  FIG.  2    in that it further includes a step of stirring the processing media  25 . 
     In the processing method MT 2 , the pressing member  40  is used to apply an external force to the processing media  25 .  FIG.  7 A  is a side view of the pressing member  40 , and  FIG.  7 B  is a bottom view of the pressing member  40 . As shown in  FIGS.  7 A and  7 B , the pressing member  40  includes a main body  41  and a stirring pin  42 . The main body  41  is a metallic member having a substantially cylindrical shape and extends along the central axis AX between one end  43  and the other end  44 . The length of the pressing member  40  in the direction along the central axis AX is longer than the depth of the water cooling hole  13 . The pressing member  40  has a diameter smaller than that of the water cooling hole  13 . Accordingly, the pressing member  40  may be inserted into the water cooling hole  13 . 
     The stirring pin  42  has a substantially cylindrical shape and has a diameter smaller than that of the main body  41 . The stirring pin  42  extends from the other end  44  of the main body  41  in a direction parallel to the central axis AX, and is coupled to the main body  41  at a position offset from the central axis AX in the radial direction. Therefore, when the main body  41  rotates around the central axis AX, the stirring pin  42  revolves along a circumference around the central axis AX. 
     As shown in  FIG.  6   , the processing method MT 2  includes a step of stirring the processing media  25  filled in the plurality of water cooling holes  13  (Step ST 8 ). The stirring of the processing media  25  is performed, for example, by rotating the main body  41  of the pressing member  40  about the central axis AX after lifting the hammer  23 . In the step ST 8 , the operator may manually rotate the pressing member  40 , or may rotate the pressing member  40  using a power source such as a motor. When the main body  41  rotates around the central axis AX, the stirring pin  42  revolves along a circumference around the central axis AX. As a result, the processing media  25  is agitated and stirred in the water cooling hole  13 . 
     In one embodiment, as shown in  FIG.  8   , the pressing member  40  may be rotated about the central axis AX in a state where the load F is applied to the pressing member  40  by the hammer  23  or the like. By rotating the pressing member  40  to stir the processing media  25  in the water cooling hole  13 , the processing media  25  may be uniformly brought into contact with the inner surface  16  of the water cooling hole  13 , thereby improving the uniformity of the compressive residual stress applied to the inner surface  16 . Further, by applying an external force to the inner surface  16  while stirring the processing media  25 , the unevenness of the inner surface  16  of the water cooling hole  13  is leveled, and the tool mark can be removed from the inner surface  16 . 
     In the processing method MT 2 , it is determined whether or not dropping of the hammer  23  is repeated a predetermined number of times after stirring of the processing media  25  (step ST 7 ). When the number of the drops of the hammer  23  is less than the predetermined number, the step ST 5 , the step ST 6  and the step ST 8  are repeated until the predetermined number is reached. When the number of the drops of the hammer  23  reaches the predetermined number, the processing method MT 2  is ended. 
     As described above, according to the processing method MT 2 , the compressive residual stress may be applied to the inner surface  16  of the water cooling hole  13  regardless of the shape of the water cooling hole  13 . The method of stirring the processing media  25  is not limited to the above-described example, and the processing media  25  may be stirred in the water cooling hole  13  by applying vibration, for example. Even when the processing media  25  is stirred by vibration, the uniformity of the compressive residual stress applied to the inner surface  16  can be improved. 
     Next, effects of the processing method for a workpiece described above will be described based on examples, but the processing method of the present disclosure is not limited to the following examples. 
     Example 1 
     First, in Example 1, a test piece made of alloy steel specified by JIS (Japanese Industrial Standards) SKD61 was prepared, and a plurality of holes each having a circular cross-section with a diameter of 6 mm and a depth of 25 mm were formed in the test piece. The test piece was then subjected to quenching and tempering. 
     In Example 1, the plurality of holes of the test piece were filled with steel balls made of JIS SUJ2 as processing media. The diameter of the steel balls was 1.0 mm, and the Vickers hardness of the steel balls was 62. Then, the pressing member  30  shown in  FIGS.  4 A and  4 B  was inserted into the plurality of holes filled with the steel balls, and the hammer  23  was dropped toward the pressing member  30  using the processing apparatus  1  shown in  FIG.  3   . The dropping height of the hammer  23  was set to 100 mm. The sum of the masses of the hammer  23  and the ball spline  26  was 1.34 kg. 
     In Example 1, the mold  10  was processed by changing the number of drops of the hammer  23  (i.e., the number of hammering times), and the residual stress applied to the inner surface of the plurality of holes was measured by an X-ray diffraction method. Residual stress was measured at a position on the side surface 12.5 mm away from the bottom surface of the hole. 
     The measurement conditions of the residual stress by the X-ray diffraction method were as follows.
         Residual stress analysis method: cos a method   Characteristic X-ray: Cr-Kα   Diffraction surface: Fe, 211   Diffraction angle: 156.396 [deg]   Voltage: 30 [kV]   Current: 1 [mA]   X-ray elastic constant: 224 [GPa]   Poisson&#39;s ratio: 0.28   X-ray incident angle: 35 [deg]   X-ray irradiation diameter: 3 [mm]   X-ray irradiation time: 30 [s]       

       FIG.  9    is a graph showing the relationship between the number of hammering times and the residual stress applied to the inner surface of the hole of the test piece. In  FIG.  9   , the tensile residual stress is represented as a positive value, and the compressive residual stress is represented as a negative value. As shown in  FIG.  9   , in Example 1, it was confirmed that compressive residual stress was applied to the inner surface of the hole regardless of the number of hammering times. In addition, it was confirmed that the compressive residual stress applied to the inner surface of the hole increases as the number of hammering increases. 
     Example 2 
     In Example 2, the pressing member  40  shown in  FIGS.  7 A and  7 B  was inserted into the plurality of holes filled with the same steel balls as in Example 1, and the processing apparatus  1  shown in  FIG.  3    was used to cause the hammer  23  to collide with the pressing member  40  a predetermined number of times. In addition, in Example 2, the pressing member  40  was rotated once or a plurality of times to stir the steel balls inside the hole while the hammer  23  collided with the pressing member  40  the predetermined number of times. Except for this point, the test piece was processed under the same conditions as in Example 1, and the residual stress applied to the inner surface of the plurality of holes was measured by the X-ray diffraction method. 
       FIG.  10    is a graph showing the relationship between the number of stirring times of the steel balls and the residual stress applied to the inner surface of the hole. As shown in  FIG.  10   , it was confirmed that by making the hammer  23  collide with the pressing member  40  while stirring the steel balls, a larger compressive residual stress can be applied as compared with the case where the steel balls are not stirred. 
     Although the processing methods for a workpiece according to various embodiments have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention. 
     For example, although the compressive residual stress is applied to the inner surface  16  of the water cooling hole  13  formed in the mold  10  in the above-described embodiment, the compressive residual stress may be applied to the inner surface of a hole other than the water cooling hole. Although the water cooling hole  13  described above is a bottomed hole, the water cooling hole  13  may be a through hole. For example, the processing media  25  may be filled into a through hole which is placed on the base plate  21  and whose bottom opening is closed, and an external force may be applied to the processing media  25 . 
     Further, in the above-described processing methods MT 1  and MT 2 , the powder is filled in the plurality of water cooling holes  13 , but the powder does not necessarily have to be filled. When a load is applied to the pressing member  30  or  40  in a state in which the processing media  25  is filled, compressive residual stress may be applied to the inner surface  16  of the water cooling hole  13 . Further, as long as the external force can be applied to the processing media  25 , the shape of the pressing members  30  and  40  is not limited to the shape shown in  FIGS.  4 A and  4 B  or  FIGS.  7 A and  7 B , and may have any shape. For example, the pressing member may have a conical shape of which the diameter decreases toward the bottom surface  15 . 
     In the above-described embodiment, the external force applying device  28  drops the hammer  23  to apply a load to the pressing member  30 . However, the external force applying device  28  may apply an external force to the processing media  25  without using the hammer  23 . For example, the external force applying device  28  may press the pressing member  30  toward the bottom surface  15  of the water cooling hole  13  by using a hydraulic actuator such as a hydraulic cylinder. Further, in the processing methods MT 1  and MT 2 , an external force is applied to the processing media  25  via the pressing member  30  or  40 , but the pressing member  30  or  40  may not necessarily be used. For example, external force may be applied to the inner surface  16  of the water cooling hole  13  by applying vibration or the like to the processing media  25  filled in the water cooling hole  13  to cause the processing media  25  to flow inside the water cooling hole  13 . Even in this case, compressive residual stress may be applied to the inner surface  16  of the water cooling hole  13 . 
     REFERENCE NUMERALS 
       1 : processing apparatus,  12 : second surface (surface of workpiece),  13 : water cooling hole,  16 : inner surface,  22 : lifting unit,  23 : hammer,  25 : processing media,  28 : external force applying device,  30  and  40 : pressing member,  41 : main body,  42 : stirring pin, AX: central axis, F: load.