Patent Publication Number: US-2019186108-A1

Title: Earth and sand abrasion resistant component and method for producing the same

Description:
TECHNICAL FIELD 
     The present invention relates to an earth and sand abrasion resistant component and a method for producing the component. 
     BACKGROUND ART 
     Hydraulic excavators, bulldozers, wheel loaders, and other work machines that operate in an environment where earth and sand exist have earth and sand abrasion resistant components such as teeth or ripping tips as their constituent components. In such an earth and sand abrasion resistant component, an overlay may be formed in a region requiring particularly high earth and sand abrasion resistance. As the overlay, for example, one having a matrix made of steel, with hard particles dispersed therein, may be adopted. Such an overlay can be formed by overlaying welding, for example. As the material constituting the hard particles, cemented carbide having tungsten carbide (WC) as its main component, for example, may be adopted (see, for example, Japanese Patent Application Laid-Open No. H8-47774 (Patent Literature 1), Japanese Patent Application Laid-Open No. 2007-268552 (Patent Literature 2), Japanese Patent Application Laid-Open No. 2008-762 (Patent Literature 3), and Japanese Patent Application Laid-Open No. 2008-763 (Patent Literature 4)). When the above-described cemented carbide is adopted as the material constituting the hard particles, WC having a high hardness is included in the overlay, and further, tungsten (W) and carbon (C) are eluted into the matrix, leading to an increased hardness of the matrix. As a result, an overlay having high earth and sand abrasion resistance can be formed. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-Open No. H8-47774 
     Patent Literature 2: Japanese Patent Application Laid-Open No. 2007-268552 
     Patent Literature 3: Japanese Patent Application Laid-Open No. 2008-762 
     Patent Literature 4: Japanese Patent Application Laid-Open No. 2008-763 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, even the component having an overlay for which cemented carbide has been adopted as the material constituting the hard particles may not be able to offer sufficient resistance to earth and sand abrasion. 
     An object of the present invention is to provide an earth and sand abrasion resistant component that offers excellent earth and sand abrasion resistance. 
     Solution to Problem 
     An earth and sand abrasion resistant component according to the present invention includes: a base; a first overlay layer disposed in contact with the base so as to cover a covered region being a part of a surface of the base; and a second overlay layer disposed on the first overlay layer. The first overlay layer and the second overlay layer each include a matrix made of iron or steel, and hard particles made of cermet and dispersed in the matrix. 
     As used herein, cermet refers to a composite material having particles of one or more ceramics selected from the group consisting of titanium carbide (TiC), titanium nitride (TiN), and titanium carbonitride (TiCN) sintered with metal as a binder, with the ceramic particles accounting for at least 50% by mass. It should be noted that the cermet herein does not include a composite material having WC particles sintered with metal as a binder, with the WC particles accounting for at least 50% by mass. As used herein, cemented carbide refers to a composite material having WC particles sintered with metal as a binder, with the WC particles accounting for at least 50% by mass. 
     The present inventors studied how to improve earth and sand abrasion resistance when such abrasion resistance cannot be obtained sufficiently even when an overlay is formed adopting cemented carbide as the material constituting the hard particles. As a result, they have found the following and reached the present invention. 
     In the case where sufficient earth and sand abrasion resistance cannot be obtained even by forming an overlay adopting cemented carbide as the material constituting the hard particles, a plurality of overlay layers may be formed by overlaying welding, for example, to increase the thickness of the overlay. However, earth and sand abrasion resistance does not improve with this approach, conceivably for the following reasons. 
     When cemented carbide is adopted as the material constituting the hard particles, the constituent elements would likely be eluted into the matrix, as described above, leading to an increased hardness of the matrix. In this case, although the matrix having the cemented carbide adopted as the material constituting the hard particles may become high in hardness, the matrix becomes brittle. Particularly in the case where a plurality of overlay layers are formed and stacked by overlaying welding, the boundary region between the lower overlay layer and the upper overlay layer is heated again during the formation of the upper overlay layer. This increases the amount of W and C eluted into the matrix, making the matrix harder and more brittle. 
     Further, with W and C eluted into the matrix, the hardness of the hard particles decreases. For example, while the hard particles made of cemented carbide originally have a hardness of about 1500 to 2000 HV, the hardness of the particles decreases to about 1000 HV. Therefore, in the case where a plurality of overlay layers are formed adopting cemented carbide as the material constituting the hard particles, while the hardness of the matrix increases, the matrix becomes brittle and the hardness of the hard particles decreases. As a result, although the matrix having a high hardness may more contribute to earth and sand abrasion resistance, the overlay would more likely suffer chipping or the like during the use of the earth and sand abrasion resistant component. Further, with the hard particles having a decreased hardness, contribution of the hard particles to earth and sand abrasion resistance becomes small. Still further, cracking may occur in the overlay during the formation of the overlay due to W and C eluted into the matrix. As explained above, forming a plurality of overlay layers adopting cemented carbide as the material constituting the hard particles would not lead to improved earth and sand abrasion resistance. 
     In contrast, in the earth and sand abrasion resistant component in the present invention, a plurality of overlay layers (first and second overlay layers) are formed and stacked, and cermet is adopted as the material constituting the hard particles. In the case where cermet is adopted as the material constituting the hard particles, the hard cermet particles are dispersed in the overlay, and the amount of the constituent elements eluted into the matrix becomes smaller than in the case where cemented carbide is adopted. Accordingly, the matrix of the overlay with cermet adopted as the material constituting the hard particles offers superior toughness, although it is smaller in hardness than the matrix of the overlay with cemented carbide adopted. 
     Further, as the amount of the constituent elements (Ti, C, and N) eluted into the matrix is small, the decrease in hardness of the hard particles is small. For example, the hard particles made of cermet originally having a hardness of about 1500 to 2000 HV maintain a hardness of about 1500 HV. Therefore, in the case of forming a plurality of overlay layers adopting cermet as the material constituting the hard particles, while the increase in hardness of the matrix is small, the matrix becomes excellent in toughness, and the decrease in hardness of the hard particles is restricted. As a result, although the contribution of the matrix to the earth and sand abrasion resistance does not increase considerably, occurrence of chipping of the overlay or the like during the use of the earth and sand abrasion resistant component is suppressed. With the decrease in hardness prevented, the hard particles contribute significantly to earth and sand abrasion resistance. Furthermore, cracking of the overlay during the formation of the overlay due to elution of the constituent elements into the matrix is also suppressed. In this manner, earth and sand abrasion resistance can be improved by forming a plurality of overlay layers adopting cermet as the material constituting the hard particles. Further, the matrix is prevented from becoming brittle, so it is readily possible to further improve the earth and sand abrasion resistance by increasing the number of overlay layers stacked. 
     As described above, according to the earth and sand abrasion resistant component of the present invention, it is possible to provide an earth and sand abrasion resistant component that is highly resistant to earth and sand abrasion. 
     In the earth and sand abrasion resistant component described above, in a region including an interface between the first overlay layer and the second overlay layer, the matrix may have a Vickers hardness that is not more than a half of a Vickers hardness of the hard particles. By restricting the Vickers hardness of the matrix to a half or less of that of the hard particles, the toughness of the matrix can further be improved. 
     In the earth and sand abrasion resistant component described above, the earth and sand abrasion resistant component may be a tooth, and the covered region may be located in a region in the base corresponding to a distal end portion of the tooth. 
     The tooth is an earth and sand abrasion resistant component that is attached to a bucket of a hydraulic excavator or the like and penetrates into earth and sand. The distal end is used in an extremely harsh environment where it is subject to earth and sand abrasion. Applying the inventive earth and sand abrasion resistant component to such a tooth makes it possible to provide a tooth excellent in earth and sand abrasion resistance. 
     A method for producing an earth and sand abrasion resistant component according to the present invention includes the steps of: preparing a base; forming a first overlay layer to cover a covered region being a part of a surface of the base; and forming a second overlay layer on the first overlay layer. The step of forming the first overlay layer and the step of forming the second overlay layer include forming, by overlaying welding, the first overlay layer and the second overlay layer each including a matrix made of iron or steel and hard particles made of cermet and dispersed in the matrix. 
     In the method for producing an earth and sand abrasion resistant component in the present invention, cermet is adopted as the material constituting the hard particles, and a plurality of overlay layers are formed and stacked by overlaying welding. Thus, the overlay layers having the hard cermet particles dispersed therein and offering excellent toughness are formed in a stacked manner. As a result, an earth and sand abrasion resistant component excellent in earth and sand abrasion resistance can be produced. 
     Effects of Invention 
     As is clear from the above description, according to the earth and sand abrasion resistant component of the present invention, it is possible to provide an earth and sand abrasion resistant component that is highly resistant to earth and sand abrasion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view showing a structure of a bucket of a hydraulic excavator; 
         FIG. 2  is a schematic plan view showing a structure of a tooth; 
         FIG. 3  is a schematic cross-sectional view taken along the line in  FIG. 2 ; 
         FIG. 4  is a schematic cross-sectional view showing, in an enlarged view, a portion of the tooth including its distal end; 
         FIG. 5  is a flowchart schematically illustrating steps for producing a tooth; 
         FIG. 6  is a schematic cross-sectional view illustrating a method for producing an overlay; 
         FIG. 7  includes photographs showing an appearance of test pieces each formed by stacking three overlay layers including cermet particles; 
         FIG. 8  includes photographs showing a cross section of the test pieces each formed by stacking three overlay layers including cermet particles; 
         FIG. 9  illustrates hardness distribution in a thickness direction of the overlays each including cermet particles and formed with three layers stacked; 
         FIG. 10  is a photograph showing an appearance of a test piece formed by stacking three overlay layers including cemented carbide particles; 
         FIG. 11  illustrates hardness distribution in a thickness direction of the overlay including cemented carbide particles and formed with three layers stacked; 
         FIG. 12  includes an optical micrograph of a cermet particle and a matrix surrounding the particle, and a diagram illustrating hardness distribution in a region corresponding to the field of view of the optical micrograph; and 
         FIG. 13  includes an optical micrograph of a cemented carbide particle and a matrix surrounding the particle, and a diagram illustrating hardness distribution in a region corresponding to the field of view of the optical micrograph. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will now be described. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated. 
     An earth and sand abrasion resistant component according to the present embodiment will be described by giving as an example a bucket tooth of a hydraulic excavator.  FIG. 1  is a schematic perspective view showing the structure of a bucket of a hydraulic excavator.  FIG. 2  is a schematic plan view showing the structure of a tooth.  FIG. 3  is a schematic cross-sectional view taken along the line in  FIG. 2 . 
     Referring to  FIG. 1 , a bucket  1  is attached to a distal end of an arm (not shown) of a hydraulic excavator and operative to excavate earth and sand. The bucket  1  includes: a main body  10 , made up of a plate-like member and having an opening; a plurality of (in the present embodiment, four) adapters  40  disposed so as to partially protrude from a periphery  12  of the opening of the main body  10  on its excavating side; a plurality of (in the present embodiment, four) teeth  20  attached to the corresponding adapters  40  to protrude from the periphery  12  of the opening on its excavating side; and a mounting portion  30  disposed on a side of the main body  10  opposite to the side where the teeth  20  are attached. 
     The adapters  40  are attached to the periphery  12  of the opening on its excavating side by welding, for example. Each tooth  20  is fitted onto the protruding portion of the corresponding adapter  40  and secured by a pin  42 . Each adapter  40  has a through hole  41  formed to extend in a direction along the periphery of the opening, perpendicular to the protruding direction of the adapter. Each tooth  20  has a through hole  29  formed in a region corresponding to the through hole  41  of the adapter  40 . The tooth  20  is secured to the adapter  40  as the pin  42  is inserted to penetrate through the through holes  41  and  29 . 
     The bucket  1  is supported by the arm of the hydraulic excavator via the mounting portion  30 . When the bucket  1  is used for excavation, the teeth  20  penetrate into earth and sand first. The teeth  20  are thus required to have high earth and sand abrasion resistance. The teeth  20  are earth and sand abrasion resistant components that are machine components used for applications where they come into contact with earth and sand. 
     Referring to  FIGS. 2 and 3 , a tooth  20  includes a distal end  21  and a proximal end  22 . The tooth  20  has a cavity  23  formed on the proximal end  22  side. The tooth  20  is secured to the adapter  40  in the state where the protruding portion of the adapter  40  is inserted into the cavity  23 . This causes the distal end  21  side to protrude from the periphery  12  of the opening of the bucket  1 . The bucket  1  penetrates into earth and sand from the distal end  21  side of the teeth  20 . The distal end  21  side of the teeth  20  therefore requires particularly high earth and sand abrasion resistance. 
     A tooth  20  includes a base  25  and an overlay  27 . The base  25  is made of steel or cast iron and has a wedge shape. At a distal end of the base  25 , a distal end face  25 A is formed as a covered region. The overlay  27  is formed on the distal end face  25 A. Of the surface of the base  25 , the region other than the distal end face  25 A constitutes an exposed region on which no overlay  27  is formed. For the material constituting the base  25 , carbon steel for machine structural use or alloy steel for machine structural use specified in JIS standard (for example, S45C or SCM435, as well as manganese steel (SMn), chromium steel (SCr), chromium molybdenum steel (SCM), or nickel chromium molybdenum steel (SNCM) containing an equivalent amount of carbon), tool steel, or cast steel, for example, can be adopted. 
     The overlay  27  will now be described in detail with reference to  FIG. 4 . The overlay  27  formed on the distal end face  25 A of the base  25  includes a first overlay layer  271 , a second overlay layer  272 , and a third overlay layer  273 . The first overlay layer  271  is disposed directly on the distal end face  25 A of the base  25 . The second overlay layer  272  is disposed directly on the first overlay layer  271 . As seen from the first overlay layer  271 , the second overlay layer  272  is disposed on a side opposite to the base  25 . The third overlay layer  273  is disposed directly on the second overlay layer  272 . As seen from the second overlay layer  272 , the third overlay layer  273  is disposed on a side opposite to the first overlay layer  271 . A region of the third overlay layer  273  on a side opposite to the second overlay layer  272  constitutes the distal end  21  of the tooth  20 . 
     The first overlay layer  271 , the second overlay layer  272 , and the third overlay layer  273  each include a matrix  95  made of iron or steel, and cermet particles  91  (hard particles) dispersed in the matrix  95 . The cermet particles  91  may be, for example, crushed particles obtained by crushing used throwaway tips made of cermet. The cermet particles  91  may have a particle diameter of not less than 0.2 mm and not more than 3.5 mm, for example. 
     In the tooth  20  as the earth and sand abrasion resistant component in the present embodiment, the overlay  27  is made up of a plurality of stacked layers (first overlay layer  271 , second overlay layer  272 , and third overlay layer  273 ), and cermet particles  91  are adopted as the hard particles. With the cermet particles  91  being adopted as the hard particles, the hard cermet particles  91  are dispersed in the overlay  27 , and the amount of the constituent elements (Ti, C, N) eluted into the matrix  95  is reduced as compared to the case where cemented carbide particles are adopted as the hard particles. Accordingly, the matrix  95  of the overlay  27 , including cermet particles  91  adopted as the hard particles, is superior in toughness, although its hardness is low as compared to the matrix of the overlay including cemented carbide particles adopted as the hard particles. 
     Further, as the amount of the constituent elements eluted into the matrix  95  is small, the decrease in hardness of the cermet particles  91  is small. Specifically, in the overlay  27 , the cermet particles  91  maintain a hardness of about 1500 HV. Therefore, in the case of forming a plurality of layers of overlay  27  (first overlay layer  271 , second overlay layer  272 , and third overlay layer  273 ) adopting cermet particles  91  as the hard particles, although the increase in hardness of the matrix  95  is small, the matrix  95  offers excellent toughness and the decrease in hardness of the cermet particles  91  as the hard particles is restricted. As a result, while the contribution of the matrix  95  to the earth and sand abrasion resistance does not increase considerably, occurrence of chipping of the overlay  27  or the like during the use of the tooth  20  is suppressed. With the decrease in hardness prevented, the cermet particles  91  as the hard particles contribute significantly to earth and sand abrasion resistance. Furthermore, cracking of the overlay  27  during the formation of the overlay  27  due to elution of the constituent elements into the matrix is also suppressed. 
     As described above, the tooth  20  in the present embodiment is an earth and sand abrasion resistant component that offers improved earth and sand abrasion resistance by virtue of the cermet particles  91  adopted as the hard particles and the overlay  27  made up of a plurality of layers (first overlay layer  271 , second overlay layer  272 , and third overlay layer  273 ). With embrittlement of the matrix  95  being prevented, the number of layers constituting the overlay  27  can readily be increased to further improve the earth and sand abrasion resistance. As the increase in hardness of the matrix  95  is restricted, alloy components may be added to the matrix  95 , for example, whereby the hardness of the matrix  95  can readily be controlled to a desired level. Such addition of alloy elements to the matrix  95  may be implemented, for example, by changing the component composition of the welding wire used in overlaying welding (described later), or by adding alloy components from the outside during the overlaying welding. 
     The tooth  20  according to the present embodiment is therefore an earth and sand abrasion resistant component that is highly resistant to earth and sand abrasion. 
     It should be noted that in a region of the tooth  20  including an interface between neighboring overlay layers (region including the interface between the first overlay layer  271  and the second overlay layer  272 , and region including the interface between the second overlay layer  272  and the third overlay layer  273 ), the matrix  95  preferably has a Vickers hardness that is not more than a half of the Vickers hardness of the cermet particles  91 . By restricting the Vickers hardness of the matrix  95  to a half or less of that of the cermet particles  91 , the toughness of the matrix  95  can further be improved. 
     A method for producing a tooth  20  as the earth and sand abrasion resistant component in the present embodiment will now be described with reference to  FIGS. 2 to 6 .  FIG. 5  is a flowchart schematically illustrating the tooth producing method.  FIG. 6  is a schematic cross-sectional view illustrating an overlay forming method. 
     Referring to  FIG. 5 , in the method for producing a tooth  20  in the present embodiment, firstly, a base preparing step is carried out as a step S 10 . In this step S 10 , referring to  FIGS. 2 and 3 , a base  25  of the tooth  20  is prepared. For preparing the base  25 , a steel member made of a steel constituting the base  25  is formed through forging, casting, cutting, or other processing, and then subjected to appropriate heat treatment (for example, quenching and tempering) as necessary. 
     Next, a first overlay layer forming step, a second overlay layer forming step, and a third overlay layer forming step are carried out successively as steps S 20 , S 30  and S 40 . In the step S 20 , the first overlay layer  271  is formed on a distal end face  25 A of the base  25  by overlaying welding. In the step S 30 , the second overlay layer  272  is formed on the first overlay layer  271  by overlaying welding. In the step S 40 , the third overlay layer  273  is formed on the second overlay layer  272  by overlaying welding. 
     The overlay  27  can be formed, for example, by overlaying welding using a metal inert gas (MIG) welding method as follows. Although a way of forming the first overlay layer  271  will be described below, the second overlay layer  272  and the third overlay layer  273  can be formed in a similar manner as the first overlay layer  271 . 
     Firstly, an overlay forming device will be described. Referring to  FIG. 6 , the overlay forming device includes a welding torch  70  and a hard particles supplying nozzle  80 . The welding torch  70  includes a welding nozzle  71  having a hollow cylindrical shape, and a contact tip  72  disposed inside the welding nozzle  71  and connected to a power source (not shown). A welding wire  73 , while being in contact with the contact tip  72 , is supplied continuously to the tip end side of the welding nozzle  71 . For the welding wire, JIS YGW12, for example, may be adopted. A gap between the welding nozzle  71  and the contact tip  72  is a flow path of shielding gas. The shielding gas flowing through the flow path is discharged from the tip end of the welding nozzle  71 . The hard particles supplying nozzle  80  has a hollow cylindrical shape. Inside the hard particles supplying nozzle  80 , cermet particles  91  are supplied, which are discharged from the tip end of the hard particles supplying nozzle  80 . 
     This overlay forming device can be used to form an overlay  27  (first overlay layer  271 ) in the following manner. With the base  25  as one electrode and the welding wire  73  as another electrode, voltage is applied across the base  25  and the welding wire  73 . This generates an arc  74  between the welding wire  73  and the base  25 . The arc  74  is shielded from the ambient air by the shielding gas discharged from the tip end of the welding nozzle  71  along the arrows β. For the shielding gas, argon, for example, may be adopted. The heat in the arc  74  melts a part of the base  25  and also melts the tip end of the welding wire  73 . The tip end of the welding wire  73  thus molten forms droplets, which transfer to the molten region of the base  25 . This forms a molten pool  92  which is a liquid region where the molten base  25  and the molten welding wire  73  are mixed together. The cermet particles  91  discharged from the hard particles supplying nozzle  80  are supplied to this molten pool  92 . 
     As the welding torch  70  and the hard particles supplying nozzle  80  constituting the overlaying welding device move relatively in the direction shown by the arrow α with respect to the base  25 , the position where the molten pool  92  is formed moves accordingly. The molten pool  92  previously formed solidifies, resulting in a first overlay layer  271 . The first overlay layer  271  includes a matrix  95  formed by solidification of the molten pool  92 , and cermet particles  91  dispersed in the matrix  95 . Through the above procedure, the first overlay layer  271  is formed to cover the distal end face  25 A which is the covered region on the surface of the base  25 . The surface of the base  25  on which no first overlay layer  271  has been formed is an exposed region  25 B. It should be noted that overlaying welding may be carried out, for example, under the following conditions: welding current of 250 A, welding voltage of 26.5 V, hard particles feed rate of 80 g/min, and welding speed of 2.0 mm/sec. 
     Following the formation of the first overlay layer  271  as described above, the second overlay layer  272  and the third overlay layer  273  are formed one on another in a similar manner, whereby the tooth  20  in the present embodiment is completed. After the first overlay layer  271 , the second overlay layer  272 , and the third overlay layer  273  are formed, heat treatment such as quenching may be performed. 
     EXAMPLES 
     A device similar to the overlay forming device explained in the above embodiment was used to form a first overlay layer  271 , a second overlay layer  272 , and a third overlay layer  273  in a stacked manner, and an experiment was conducted to examine the properties. The experimental procedure was as follows. 
     A plate of SS400 (mild steel) was prepared as a base member, and a first overlay layer  271 , a second overlay layer  272 , and a third overlay layer  273  were formed, stacked on the plate. As the welding wire  73 , JIS YGW12 (with a diameter of 1.2 mm) was adopted. As the hard particles, cermet particles  91  (crushed particles of waste cermet tips, with a particle diameter of 0.71 to 2.36 mm) were adopted. The welding current was 225 A, the welding voltage was 26 V, the shielding gas was argon (Ar), and the cermet particles were supplied at a rate of 80 g/min. The welding speeds of three levels of 3.8 mm/sec, 2.9 mm/sec, and 2.3 mm/sec were adopted. These welding speeds correspond to heat inputs of 15. 4 kJ/cm, 20.2 kJ/cm, and 25.4 kJ/cm, respectively. The obtained samples were observed in terms of appearance and cross section, and checked for cracking or the like. Further, hardness distribution was examined in the direction (thickness direction) of stacked layers of the overlay. A sample for comparison was prepared adopting cemented carbide particles instead of the cermet particles  91  (at a welding speed of 2.3 mm/sec), and the appearance and hardness distribution of the sample were examined in a similar manner. Further, hardness of the hard particles and that of the matrix surrounding the particles, in the case of the welding speed of 2.3 mm/sec, were measured. 
       FIG. 7  includes photographs showing an appearance of the overlays in the case where cermet particles  91  were adopted as the hard particles.  FIG. 8  includes photographs showing a cross section along a thickness direction of the overlays when the cermet particles  91  were adopted as the hard particles. Referring to  FIGS. 7 and 8 , in the case of adopting the cermet particles  91  as the hard particles, even when three layers of overlay were formed, no cracking or the like was confirmed irrespective of the welding speed. It is thus confirmed that a favorable overlay can be formed. 
       FIG. 9  illustrates hardness distribution in the thickness direction of the overlays. In  FIG. 9 , the horizontal axis represents distance from an interface between the base and the overlay, with the overlay side indicated by positive values. In  FIG. 9 , the vertical axis represents Vickers hardness. 
     Referring to  FIG. 9 , marks in the region constituting the baseline (delimited by the broken line) correspond to the hardness of the matrix. As is apparent from  FIG. 9 , in the case where cermet particles  91  were adopted as the hard particles, the hardness of the matrix has become about 500 HV. On the other hand, marks in the region where the hardness is high in  FIG. 9  correspond to the hardness of the cermet particles as the hard particles. As is apparent from  FIG. 9 , the cermet particles have maintained a hardness of about 1500 HV or higher. The Vickers hardness of the matrix is a half or less of the Vickers hardness of the hard particles (cermet particles). 
       FIG. 10  is a photograph showing an appearance of the overlay in the case where cemented carbide particles were adopted as the hard particles.  FIG. 11 , corresponding to  FIG. 9  above, illustrates hardness distribution in the thickness direction of the overlay when the cemented carbide particles were adopted as the hard particles. 
     Referring to  FIG. 10 , in the case where three overlay layers were formed adopting cemented carbide particles as the hard particles, occurrence of cracking was confirmed. Further, referring to  FIG. 11 , when the cemented carbide particles were adopted as the hard particles, while the hardness of the matrix has increased to about 700 to 800 HV, the hardness of the cemented carbide particles as the hard particles has decreased to about 1000 to 1300 HV. 
       FIGS. 12 and 13  each show, in combination, an optical micrograph of a hard particle and a matrix surrounding the particle and hardness distribution in the region corresponding to the field of view of the optical micrograph, the figures corresponding respectively to the cases where cermet particles and cemented carbide particles were adopted as the hard particles. In the optical micrographs in  FIGS. 12 and 13 , a cermet particle and a cemented carbide particle are respectively observed at the center. In the hardness distribution diagrams in  FIGS. 12 and 13 , the broken lines represent the interface between the hard particle and the matrix. 
     Referring to  FIG. 12 , the cermet particle within the overlay formed by overlaying welding as described above maintains a hardness of about 1400 to 1500 HV. The hardness of the matrix surrounding the cermet particle is 400 HV or less. This is conceivably because the amount of the constituent elements eluted from the cermet particle into the matrix is small. 
     Referring to  FIG. 13 , the hardness of the cemented carbide particle within the overlay formed by overlaying welding as described above has decreased to about 900 to 1200 HV. The hardness of the matrix surrounding the cemented carbide particle has increased to 600 HV or more. This is conceivably because the amount of the constituent elements eluted from the cemented carbide particle into the matrix is large as compared to the eluted amount of the constituent elements of the cermet particle into the matrix. 
     The above experimental results confirm that, in the case where cemented carbide is adopted as the material constituting the hard particles, the constituent elements would likely be eluted into the matrix, causing an increase in hardness of the matrix and a decrease in hardness of the hard particles. Therefore, even when two or more overlay layers are formed adopting cemented carbide as the material constituting the hard particles, it would not lead to improved earth and sand abrasion resistance. Further, it is considered that the constituent elements eluted into the matrix in a large amount have caused cracking in the overlay. 
     In contrast, it is confirmed that, in the case where cermet is adopted as the material constituting the hard particles, elution of the constituent elements into the matrix is reduced, so toughness of the matrix is maintained and a decrease in hardness of the hard particles is prevented. Therefore, forming two or more overlay layers adopting cermet as the material constituting the hard particles leads to improved earth and sand abrasion resistance. Further, with elution of the constituent elements into the matrix being suppressed, it is readily possible to form a favorable overlay with no cracking. 
     The above experimental results confirm that, according to the earth and sand abrasion resistant component and its producing method of the present invention, it is possible to provide an earth and sand abrasion resistant component that is highly resistant to earth and sand abrasion. 
     While a tooth used for a bucket of a hydraulic excavator or the like has been described as an example of the earth and sand abrasion resistant component of the present invention in the above embodiment, the earth and sand abrasion resistant component according to the present invention is not limited to such a tooth. The earth and sand abrasion resistant component according to the present invention is applicable to various earth and sand abrasion resistant components requiring high resistance to earth and sand abrasion, which for example include: a shoe constituting a tracked undercarriage; a cutting edge and protector of a bucket; a variety of wear plates and liners; as well as components constituting underground construction equipment (shield machines, tunnel boring machines, etc.), crushing machines, rock splitters, cement machines, steelmaking machines, casting machines, and so on. 
     It should be understood that the embodiment and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     INDUSTRIAL APPLICABILITY 
     The earth and sand abrasion resistant component and its producing method according to the present invention are applicable particularly advantageously to an earth and sand abrasion resistant component requiring high earth and sand abrasion resistance and to its producing method. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1 : bucket;  10 : main body;  12 : periphery of opening;  20 : tooth;  21 : distal end;  22 : proximal end;  23 : cavity;  25 : base;  25 A: distal end face;  25 B: exposed region;  27 : overlay;  271 : first overlay layer;  272 : second overlay layer;  273 : third overlay layer;  29 : through hole;  30 : mounting portion;  40 : adapter;  41 : through hole;  42 : pin;  70 : welding torch;  71 : welding nozzle;  72 : contact tip;  73 : welding wire;  74 : arc;  80 : hard particles supplying nozzle;  91 : cermet particle;  92 : molten pool; and  95 : matrix.