METHOD OF STRENGTHENING BINDER METAL PHASE OF SINTERED BODY

Spherical shaped ejection particles are ejected against a surface of a sintered body including hard particles and a binder metal phase bonding the hard particles together, with a compressed gas at an ejection pressure of from 0.2 MPa to 0.6 MPa or at an ejection velocity of from 80 m/s to 200 m/s and the spherical ejection particles having a hardness not less than the hardness of the binder metal phase and that is a hardness of 1000 HV or less and being particles having an average particle diameter from 20 μm to 149 μm. Thus, plastic deformation resulting from such impact and the instantaneous temperature rise and cooling occurring at the impact sites micronizes the structure of the binder metal phase, causes a change to a dense structure, and imparts compressive residual stress thereto. This results in strengthening, and enables prevention of brittle fracture in the sintered body.

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Description
BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for strengthening a phase of a binder metal (referred to as a “binder metal phase” in the present invention) in a sintered body in which hard particles of a carbide, oxide, nitride, boride, silicide, or the like are sintered together with a binder metal such as Fe, Ni, or Co, such as in a cemented carbide, a cermet, or cBN.

2. Description of the Related Art

Taking an example of a cemented carbide as an example of such a sintered body, the cemented carbide is configured by fine particles (normal particles of cemented carbide have a particle diameter of a few μm, and ultrafine particles of cemented carbide have a particle diameter of from about 0.5 μm to about 0.8 μm) of a carbide (WC, TiC, TaC) of a metal such as tungsten (W), titanium (Ti), Tantalum (Ta) sintered together using as a binder a metal such as iron (Fe), nickel (Ni), or cobalt (Co). As narrowly defined, cemented carbide sometimes refers to only WC—Co based alloys configured from particles of tungsten carbide (WC) sintered together using a cobalt (Co) binder.

Such cemented carbides are materials have remarkable hardness, in a hardness range of from 1000 HV to 1800 HV, and excellent wear resistance, and are accordingly employed as the material for tools, machine components, and the like where wear resistance is demanded, such as cutting tools.

However, although cemented carbides have high hardness, they have the disadvantage of being brittle, and brittle fracture is liable to occur. This means that, for example, cracks, nicks, and the like are liable to occur at the cutting-edge of cutting tools made from cemented carbide. This reduces productivity due to the need to either replace cutting tools partway through a job when such cracks or nicks have occurred, or to perform a regrinding operation or the like to regenerate the cutting tools cutting-edge.

There is accordingly a desire for the provision of a cemented carbide that, while having high hardness, also has excellent toughness and is not susceptible to brittle fracture such as cracks or nicks.

The mechanical characteristics, such as hardness and toughness, of cemented carbides are known to vary according to the particle diameter of the hard particles and the addition amount of the binder metal.

Accordingly, it might be supposed that the particle diameter of the hard particles and the addition amount of the binder metal should be changed to obtain a cemented carbide having the targeted hardness and toughness.

However, as illustrated in FIG. 1, the relationships of hardness and toughness against the particle diameter of the hard particles are relationships in which the hardness of the cemented carbide increases but the toughness decreases as the average particle diameter of the hard particles decreases, and conversely the fracture toughness increases but the hardness decreases as the average particle diameter of the hard particles increases.

Moreover, as illustrated in FIG. 2, the relationships of hardness and toughness against the addition amount of binder metal are relationships in which the hardness of the cemented carbide increases but the toughness decreases as the addition amount of the binder metal is decreased, and the toughness of the cemented carbide increases but the hardness decreases as the addition amount of the binder metal is increased.

The hardness and toughness of the cemented carbide accordingly have conflicting relationships in that increasing one causes a decrease in the other. This means that a cemented carbide possessing the two conflicting properties of having excellent toughness while also having high hardness and is accordingly difficult to obtain by adjusting the particle diameter of the hard particles and adjusting the addition amount of the binder metal.

Proposed methods to improve toughness without reducing the hardness of cemented carbides accordingly include, for example: a method of coating a surface of a base body made from a cemented carbide with a hard coating layer including a toughened zone of excellent toughness (see abstract of Japanese Patent KOKAI (LOPI) No. 2000-246509 (JP2000-246509A); and a method to raise the fracture toughness of only a surface portion while maintaining the overall hardness of a cemented carbide, which is achieved by providing a surface layer of a toughness that has been raised by increasing the WC particle diameter and/or increasing the Co concentration at the surface of a cemented carbide (see abstract of Japanese Patent KOHYO (LOPI) No. 2004-514790 (JP2004-514790A)).

Note that although not directed toward raising the toughness of a sintered body such as a cemented carbide, the inventors of the present invention have proposed an instantaneous heat treatment method for a metal article directed toward forming micro-structures, dimples, and the like on a surface by shot peening. In this instantaneous heat treatment method, substantially spherical shaped shot, having a higher hardness than the base material hardness of a workpiece and including three or more different approximate ranges of grain size lying in a range of from 100 grit to 800 grit (average particle diameter: 20 μm to 149 μm), are mixed together and a mixed fluid of the shot combined with compressed air is ejected intermittently, at from 0.1 seconds to 1 second and intervals of from 0.5 seconds to 5 seconds, onto the workpiece. This ejection is performed at an ejection pressure of from 0.3 MPa to 0.6 MPa, at an ejection velocity of from 100 m/s to 200 m/s, and with an ejection distance of 100 mm to 250 mm, so as to form numerous random fine indentations having substantially circular bottom faces and a diameter of from 0.1 μm to 5 μm on the surface of the workpiece (claim 1 of Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A)). Note that an example is described in Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A) in which a “carbide” is employed as the workpiece (see Table 11-1 in Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A)).

In the related art described above, in a configuration in which a hard coating layer including a toughened zone is provided on the surface of a cemented carbide, as in the configuration described in Japanese Patent KOKAI (LOPI) No. 2000-246509 (JP2000-246509A), forming the hard coating layer provided with the toughened zone of high toughness on the surface while maintaining the hardness of the cemented carbide unaffected, enables toughness to be imparted while maintaining the characteristics of a cemented carbide i.e. high hardness.

However, this method requires an operation to form the hard coating layer provided with the toughened zone on the surface of the cemented carbide using a method such as physical vapor deposition (PVD), chemical vapor deposition (CVD), or the like. Forming the hard coating film in this manner requires extensive investment in equipment etc., such as the need for a costly vacuum deposition system.

Moreover, the reason high toughness is achieved in this method is that a hard coating film is formed on the surface, and not because the toughness is increased of the cemented carbide itself, which means that the toughness is lost if the hard coating film detaches.

However, a configuration such as that described in Japanese Patent KOHYO (LOPI) No. 2004-514790 (JP2004-514790A), in which a surface layer of high toughness is provided on a cemented carbide by increasing the WC particle diameter and/or increasing the Co concentration, enables the toughness to be raised locally for only a surface layer portion without lowering the hardness within the cemented carbide.

However, a surface layer having increased WC particle diameter and/or increased Co concentration in this manner has a hardness that is decreased as a result of increasing the toughness. The wear resistance thereof is accordingly decreased (see FIG. 1 and FIG. 2), and wear readily occurs when employed in an application in which direct contact or sliding occurs against other members.

Thus in the treatment described in Japanese Patent KOHYO (LOPI) No. 2004-514790 (JP2004-514790A), in cases in which there is a further wear resistant coating film formed on the surface layer described above, preparatory treatment is performed to prevent detachment of the wear resistant coating film (Japanese Patent KOHYO (LOPI) No. 2004-514790 (JP2004-514790A), [0001]). However, forming the surface layer in this manner does not enable both toughness and hardness to be obtained in the cemented carbide itself.

Thus, even though there is a strong desire to impart a cemented carbide with both hardness and toughness, none of the related art listed above is able to provide a solution to such a desire.

Thus the inventors of the present invention have performed diligent investigations into what is required to enable the toughness of a cemented carbide itself to be raised without forming a hard coating film or the like as described above.

As a result, the inventors have considered whether the occurrence of brittle fracture such as cracks or nicks can be suppressed if the binder metal phase can be strengthened at least in the vicinity of the surface of a cemented carbide 1.

Namely, as illustrated in FIG. 3, the cemented carbide 1 has a structure in which hard particles 10, such as WC, are bonded together by a binder metal phase 20, such as Co, having a higher ductility than that of the hard particles 10.

The hard particles 10 therein have extremely high hardness, for example 1780 HV for WC, 3200 HV for TiC, and 1800 HV for TaC, and hardly deform. Any plastic deformation occurring when an external force is imparted to the cemented carbide 1 can accordingly be logically inferred to have occurred mainly in the portion where the binder metal phase 20, such as the Co, is present. This provides support as to why the overall toughness (deformability) of the cemented carbide 1 is raised by increasing the addition amount of the binder metal (see FIG. 2).

In this manner, the deformation of the cemented carbide 1 is thought to mainly occur in the binder metal phase 20 portion, and brittle fracture, such as cracks or nicks occurring in the cemented carbide 1, is thought to be generated by cracking of the binder metal phase 20 due to strain accompanying deformation, which grows as more strain is imparted, and which eventually leads to fracturing occurring.

Following on from the above prediction, if the binder metal phase 20 portion of the cemented carbide 1 could be strengthened, and in particular the binder metal phase 20 in the vicinity of the surface of the workpiece where fractures tend to originate could be strengthened, then this should enable the ability to withstand brittle fracture such as cracks or nicks, namely the fracture toughness, to be raised.

Moreover, strengthening the binder metal phase 20 is thought to contribute to making brittle fracture less liable to occur and to raising the toughness of a sintered body, not only for the cemented carbide 1, but also for sintered bodies in general having a similar structure of the hard particles 10 bonded together with the binder metal phase 20, such as a cermet, cBN, or the like.

Note that Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A) discloses an instantaneous heat treatment method performed by ejecting beads made from high-speed steel (HSS) onto a treatment subject for an Example of a draw punch made from cemented carbide (Table 11-1 of Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A)).

However, Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A) is significantly different from the present invention in that an essential element is that such treatment should be performed with ejection particles harder than the treatment subject (claim 1 of Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A)).

Moreover, Japanese Patent KOKAI (LOPI) No. 2012-135864 (JP2012-135864A) includes the advantageous effects of increasing hardness by micronization of the surface structure using the instantaneous heat treatment method, and preventing seizing and the like by dimples formed thereby functioning as oil reservoirs. There is also a reference to “wear resistance” being increased, however there is no reference whatsoever to raising the ability to withstand nicking and cracking such as chipping, called “brittle fracture”, namely no reference whatsoever to increasing toughness.

Following on from the prediction by the inventors, the present invention is directed towards solving the disadvantages in a sintered body such as a cemented carbide mentioned above of low fracture toughness, and proposes a method to strengthen the binder metal phase 20 in the vicinity of the surface of the sintered body 1 using a comparatively simple method. An object of the present invention is to make brittle fracture less liable to occur (to impart toughness) while maintaining the characteristic high hardness of sintered bodies, such as cemented carbides, cermets, and cBN.

SUMMARY OF THE INVENTION

The following description of means for solving the problem is appended with reference signs employed in embodiments for implementing the invention. These reference signs are employed to clarify correspondence between the recitation of the scope of patent claims and the description of embodiments for implementing the invention, and obviously do not limit the interpretation of the technological scope of the present invention.

In order to achieve the object of the present invention, in a method of strengthening a binder metal phase of a sintered body, the method of strengthening a binder metal phase 20 of a sintered body 1 comprises:

ejecting spherical shaped ejection particles 30 against a surface of a sintered body 1 such as cemented carbide that includes hard particles 10 such as tungsten carbide (WC) and a binder metal phase 20 such as cobalt (Co) bonding the hard particles 10 together, by ejecting the spherical shaped ejection particles 30 together with a compressed gas at an ejection pressure of from 0.2 MPa to 0.6 MPa or at an ejection velocity of from 80 m/s to 200 m/s and the spherical ejection particles 30 having a hardness that is not less than the hardness of the binder metal phase 20 and that is a hardness of not more than 1000 HV and being particles of from 100 grit to 800 grit (having an average particle diameter of from 20 μm to 149 μm).

In the strengthening method, a sintered body 1 employed as a treatment subject is a sintered body 1 having a hard coating film (not illustrated in the drawings) coated on at least a portion of surface at a thickness of not more than 5 μm, and the ejection particles 30 may be ejected against the sintered body 1 at the portion of the surface coated with the hard coating film.

Moreover, the ejection particles 30 may be any of metal particles, ceramic particles, or a mixture of metal particles and ceramic particles, and a hardness of the ceramic particles employed is preferably not more than 800 HV.

EFFECT OF THE INVENTION

The following significant advantageous effects can be obtained by strengthening the binder metal phase 20 of the sintered body 1 using the configuration of the present invention and the method of the present invention as described above.

Ejection particles 30 ejected against the surface of the sintered body 1 impact the surface of the sintered body 1. The sintered body 1 is configured by the hard particles 10 made from WC, TiC, or TaC, and by the binder metal phase 20 such as a Co phase bonding between the hard particles 10 (see FIG. 3).

The hard particles 10, such as WC (1780 HV), TiC (3200 HV), or TaC (1800 HV), have higher hardness than the ejection particles 30, which have a hardness of not more than 1000 HV. When the ejection particles 30 having a hardness not less than the hardness of the binder metal phase 20 impact the surface of the sintered body 1 serving as the workpiece, as illustrated in FIG. 4B, although there is no deformation of the hard particles 10 in the sintered body 1, the binder metal phase 20 present between the hard particles 10 undergoes plastic deformation and moves the hard particles 10, causing the surface of the sintered body 1 to deform.

Plastic deformation resulting from such impact and the instantaneous temperature rise and cooling (instantaneous heat treatment) occurring at the impact sites micronizes the structure of the binder metal phase 20 in the vicinity of the surface of the sintered body 1, causes a change to a dense structure, and also imparts compressive residual stress thereto. This results in strengthening.

In this manner, the method of the present invention enables the binder metal phase 20 in the vicinity of the surface of the sintered body 1 to be strengthened, and enables good prevention of the occurrence of brittle fracture such as cracks or nicks in the sintered body 1, which arise from cracking and breaking occurring at the grain boundaries of the hard particles 10.

Strengthening the binder metal phase 20 in this manner may be similarly performed in cases in which a hard coating film (not illustrated in the drawings) of 5 μm or less is formed on the surface of the sintered body 1, enabling the binder metal phase 20 of the sintered body below the hard coating film to be strengthened even after the hard coating film has been formed on the surface of the sintered body 1.

Moreover, the cohesion strength of the hard coating film can be increased and detachment made less liable to occur by strengthening the binder metal phase 20 in this manner.

Moreover, the micronization and densification occurring in the structure of the binder metal phase 20, and the compressive residual stress that has been imparted thereto by the ejection of the ejection particles 30 might be lost by heating the sintered body 1. Thus film forming of the hard coating film by a method involving heating the sintered body 1 is not able to be performed after the binder metal phase 20 has been strengthened by ejecting the ejection particles 30. However, the sintered body 1 after film forming a hard coating film in this manner can be employed as the treatment subject, and so this does not provide a limitation to the method of forming the hard coating film.

Furthermore, metal particles, ceramic particles, and a mixture of both metal particles and ceramic particles may all be employed as the ejection particles 30. In cases in which ceramic particles are employed, making the hardness of such ceramic particles not more than 800 HV enables the toughness to be increased more certainly.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will become understood from the following detailed description of preferred embodiments thereof in connection with the accompanying drawings in which like numerals designate like elements, and in which:

FIG. 1 is a graph to explain relationships of hardness and toughness of a cemented carbide against particle diameter of hard particles therein;

FIG. 2 is a graph to explain relationships of hardness and toughness of a cemented carbide against addition amount of binder metal therein;

FIG. 3 is a schematic diagram to explain a structure of a sintered body (a WC—Co based cemented carbide); and

FIG. 4 is an explanatory diagram of states of deformation arising when ejection particles have impacted a workpiece of higher hardness than the ejection particles, FIG. 4A is for a general workpiece other than a sintered body, and FIG. 4B is for a sintered body workpiece including a binder metal phase having a hardness not more than the hardness of the ejection particles.

 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Explanation follows regarding a method of the present invention to strengthen a binder metal phase 20 of a sintered body 1.

Treatment Subject

In the present invention, a sintered body configured by the hard particles 10 sintered together with a binder metal is employed as a treatment subject. The hard particles 10 are not limited to being a single type of hard particle, and plural types of hard particle may be mixed together and employed therefor. Similarly, the binder metal is also not limited to being a single type of metal, and an alloy may be employed therefor.

Examples of such a sintered body 1 include a cemented carbide, a cermet, and cBN. All of these have a structure such as that schematically illustrated in FIG. 3 in which the hard particles 10 are bonded together by the binder metal phase 20.

The “cemented carbide” of the sintered body 1 is configured by the hard particles 10 made from a carbide (WC, TiC, TaC) of a metal such as tungsten (W), titanium (Ti), Tantalum (Ta) sintered together with a binder of a metal such as iron (Fe), nickel (Ni), or cobalt (Co). As narrowly defined, cemented carbide sometimes refers to only a WC—Co based alloy of particles of tungsten carbide (WC) sintered together using a binder of cobalt (Co). The present invention is not limited to a WC—Co based alloy, and a cemented carbide containing any of the above carbide particles may be employed as the treatment subject.

Moreover, such WC—Co based alloys encompass, in addition to a WC—Co alloy, alloys containing carbide particles other than WC, such as a WC—TiC—Co alloy, a WC—TiC—TaC(NbC)—Co alloy, or a WC—TaC(NbC)—Co alloy. Moreover, the binder metal is not limited to being a single metal such as Fe, Ni, or Co, and another metal such as an alloy of these metals may also be employed.

The “cermet” of the sintered body 1 is a sintered body configured by the hard particles 10 of a ceramic such as a carbide, oxide, nitride, boride, or silicide, bonded together with a binder metal, and within a wide definition may include the cemented carbides listed above.

Examples of such cermets include a TiC—Mo—Ni cermet, and also a TiC based cermet with the addition of TiN, TaN thereto, an A12O3—Cr cermet, and the like. Any of these may be employed as the treatment subject of the present invention.

Furthermore, the “cBN” of the sintered body is a sintered body of hard (fine) particles 10 of cubic boron nitride of which hexagonal boron nitride is modified by ultrahigh pressure and high temperature, that is sintered using a binder metal such as Co.

The sintered body 1 may be employed in various forms and applications, such as in cutting tools such as a milling cutter or drill, shaping tools such as a wire drawing die or a centering tool, wear resistant components such as a roller, gage or dot pin of a printer, a corrosion-resistant tool in a mining application such as a rock cutter or coal cutter, as well as a mold or the like. These may variously be employed as the treatment subject, irrespective of form and application thereof.

Moreover, the above tools and components do not need to be formed entirely of a sintered body, and, for example, a sintered body may be attached to a portion of the tool or component, such as in a cutting tool or the like in which, for example, a sintered body is attached as the cutting-edge portion alone by brazing.

Moreover, the treatment subject may be a sintered body in which the surface of the sintered body serving as the treatment subject has a hard coating film (ceramic coating film) of, for example, TiN, TiCN, TiAlN, DLC, TiCrN, CrN, or the like formed thereon at a film thickness of not more than 5 μm by physical vapor deposition (PVD) or chemical vapor deposition (CVD).

Note that for cases in which attachment by brazing or hard coating film forming accompanied by heating is performed, then such treatment is preferably completed on the sintered body 1 prior to performing the strengthening method of the present invention, since the advantageous effects of strengthening by the micronization and densification of structure, imparting of compressive residual stress to the binder metal phase 20, and the like, are sometimes lost if heat is applied after the treatment by the method of the present invention.

Treatment Content

Dry ejection of the ejection particles 30 together with compressed gas is performed on the surface of the sintered body 1 serving as the above treatment subject.

There is no particular limitation to the material employed for the ejection particles 30 as long as the material lies within the hardness range described below. As well as the ejection particles 30 being made from a metal, those made from a ceramic (including glass) may also be employed. Moreover, not only ejection particles 30 made from a single type of material, but also ejection particles 30 made from a mixture of plural materials may also be employed.

The objective of ejecting the ejection particles 30 is to perform micronization and densification of the structure by plastically deforming the binder metal phase 20, and to impart compressive residual stress and the like thereto, i.e. the objective thereof is to obtain the advantageous effects of what is referred to as “shot peening”, and so spherical shaped (spherical shaped particles) are employed therefor.

Note that reference to “spherical shaped” in the present invention need not refer strictly to a “sphere”, and includes a wide range of non-angular rounded shapes, such as spheroid shapes or barrel shapes.

Such spherical shaped ejection particles 30 may be obtained by an atomizing method for metal based materials, and may be obtained by crushing and then melting for ceramic based materials.

The hardness of the ejection particles 30 employed is a hardness not less than the hardness of the binder metal phase 20 and ejection particles of not more than 1000 HV are employed. Moreover, when the ejection particles 30 are ceramic particles, then preferably those of not more than 800 HV are employed therefor.

For example, the respective melting points for Co, Mo, Ni that may be employed as the binder metal are 1495° C., 2625° C., 1455° C. Sintering is performed at a high temperature in the vicinity of the melting point of the binder metal, and a hardness of the binder metal phase 20 after sintering is from 500 HV to 800 HV (for example, about 500 HV for Ni, and about from 700 HV to 800 HV for Co).

Thus for the sintered body 1 having a Co phase as the binder metal phase 20, alumina-silica beads (792 HV), HSS beads (1000 HV), or the like are for example suitably employed as the ejection particles 30. However, for the sintered body 1 having a Ni phase as the binder metal phase 20, preferably glass beads (565 HV) are employed as the ejection particles 30.

Note that in cases in which the same metal is employed as the binder, differences in the hardness of the binder metal phase 20 arise according to the sintering conditions (heating temperature, pressure, etc.), and so the hardness of the ejection particles 30 is selected based on the respective hardness of the binder metal phase 20.

In cases in which the hardness of the binder metal phase 20 is not known, for example, trials are performed, in which plural types of ejection particles 30 of different hardness of not more than 1000 HV are actually ejected against the surface of the sintered body 1. The ejection particles 30 capable of rendering a matt (or satin) finish on the surface of the sintered body 1 in such trials may then be employed as ejection particles 30 having a hardness not less than that of the binder metal phase 20.

Note that even in cases in which ejection particles 30 having a hardness of not more than 1000 HV are employed, sometimes considerable damage is inflicted on the surface of the sintered body 1 and the toughness thereof is actually lowered when ceramic based (including glass) ejection particles 30 with a hardness exceeding 800 HV thus toughness are lowered. Thus ejection particles 30 having a hardness of not more than 800 HV are preferably employed for ceramic based ejection particles 30.

Furthermore, the ejection particles 30 employed have a particle diameter in the range of from 100 grit to 800 grit for grain size distributions as defined by JIS R 6001(1987) (an average particle diameter of from 20 μm to 149 μm). As long as the particle diameter falls within this grain size range, a mixture of plural types of ejection particles 30 of different particle diameter may be employed.

The method for ejecting such ejection particles 30 against the sintered body 1 which is the workpiece may employ various known dry type blasting treatment apparatuses capable of ejecting particles, and an air blasting treatment apparatus is preferably employed therefor because this enables comparatively easy adjustment of ejection velocity and ejection pressure.

Various examples of such air blasting treatment apparatuses include direct pressure type, gravity suction type, and other types of blasting treatment apparatus. Any of these types of blasting treatment apparatus may be employed, and the type thereof is not particularly limited as long as the blasting treatment apparatus has the performance capable of ejecting the ejection particles at an ejection pressure of from 0.2 MPa to 0.6 MPa, or at an ejection velocity of from 80 m/sec to 200 m/sec.

Advantageous Effects Etc.

In the above manner, ejecting the ejection particles 30 and causing the ejection particles 30 to impact the surface of the sintered body 1 enables the sintered body 1 to be improved by making brittle fracture not liable to occur and by achieving excellent toughness properties.

Although the mechanism by which such advantageous effects are obtained is not entirely clear, it is thought that strengthening the binder metal phase 20 in the following manner enables the toughness to be raised without decreasing the hardness of the sintered body 1.

Namely, in cases in which the ejection particles 30 of lower hardness than the workpiece are ejected against the workpiece and the workpiece is an ordinary workpiece rather than a sintered body, then normally plastic deformation as illustrated in FIG. 4A occurs when the ejection particles 30 impact, with the plastic deformation mainly occurring on the side of the ejection particles 30 of lower hardness.

As a result, when ejection particles 30 of lower hardness than the workpiece are employed, the surface of the workpiece is not able to be plastically deformed, and the advantageous effects that accompany plastic deformation, of micronization and densification of structure, imparting of compressive residual stress, and the like, are not able to be imparted to the workpiece.

However, in a sintered body 1 having a structure in which the hard particles 10 are bonded together by the binder metal phase 20, for example a WC—Co cemented carbide, although the hardness of the WC particles configuring the hard particles 10 is a high hardness of 1780 HV, the hardness of the Co phase configuring the binder metal phase 20 is about 700 HV, giving a combined overall hardness of about 1450 HV.

Thus although the hardness of the ejection particles 30 of not more than 1000 HV is a hardness lower than the overall hardness of the sintered body 1 (hardness of the WC—Co based cemented carbide: 1450 HV) and lower than the hardness of the hard particles 10 (hardness of the WC particles: 1780 HV), the hardness of the ejection particles 30 is not less than the hardness of the binder metal phase 20 (hardness of the Co phase: 700 HV).

Moreover, the average particle diameter of the hard particles in the sintered body 1 is generally a few μm or so, and for fine hard particles is from about 0.5 μm to about 0.8 μm, and this is sufficiently smaller than the particle diameter of the ejection particles 30 at from 100 grit to 800 grit (an average particle diameter of from 20 μm to 149 μm).

As a result, when the ejection particles 30 are caused to impact the surface of the sintered body 1, as illustrated in FIG. 4B, even though no deformation can be achieved of the hard particles 10 having a higher hardness than the ejection particles 30, the hard particles can be moved by deforming the binder metal phase 20, and this is thought to deform the surface of the sintered body 1 so as to enable processing to a slight matt finish.

Moreover, at the sites impacted by the ejection particles 30, localized heating and cooling instantaneously occurs at the impact sites due to the heat generated when impact occurs, and this is thought to result in fine crystallization of the binder metal phase 20 by the instantaneous heat treatment performed thereby.

As a result, work hardening by the fine crystallization and densification is accordingly thought to be induced in the binder metal phase 20, at least in the vicinity of the surface of the sintered body 1, with the hardness thereof raised thereby. Moreover, the binder metal phase 20 is thought to be strengthened by being imparted with compressive residual stress that suppresses the generation and growth of cracks.

Such strengthening of the binder metal phase 20 is not only obtained in cases in which the ejection particles 30 are caused to directly impact the surface of the sintered body 1, and is also obtained in cases in which the ejection particles 30 are caused to impact a sintered body 1 having a hard coating film (not illustrated in the drawings) such as a ceramic coating film or the like coated on a surface thereof, by impacting from above the hard coating film. The cohesion strength of the hard coating film is improved thereby, enabling detachment etc. thereof to be made less liable to occur.

It is thought that as a result, by suppressing breaks (breaks in the binder metal phase 20) at the grain boundaries of the hard particles 10, brittle fracture is less liable to occur even when external force and strain is imparted to the sintered body 1, and the toughness of the sintered body 1 can accordingly be increased.

EXAMPLES

Next explanation follows regarding results of durability tests on sintered bodies subjected to strengthening of the binder metal phase with the method of the present invention.

Test Example 1: Cold Forging Punch (Carbide) (1) Test Method

Ejection particles were ejected under the conditions listed in Table 1 below against a cold forging punch (diameter 20 mm, length 150 mm) made from a WC—Co cemented carbide (1450 HV).

The hardness of the Co phase that is the binder metal phase is approximately 700 HV.

TABLE 1 Comparative Comparative Treatment Conditions Example 1 Example 1 Example 2 Blasting Device Gravity Type Gravity Type Gravity Type Ejection Material HSS (SKII) Glass FeCrB particles Hardness Approximately 534 HV Approximately 1000 HV 1200 HV Average particle Approximately Approximately Approximately diameter 40 μm 40 μm 40 μm Shape Substantially Substantially Substantially spherical shaped spherical shaped spherical shaped Ejection Pressure 0.6 MPa 0.6 MPa 0.6 MPa conditions Nozzle 9 mm diameter - 9 mm diameter - 9 mm diameter - diameter long long long Ejection 100 mm to 100 mm to 100 mm to distance 150 mm 150 mm 150 mm Ejection Approximately Approximately Approximately duration 30 seconds 30 seconds 30 seconds

The state of the surface of cold forging punches was observed with the naked eye after ejection of the ejection particles and on an un-processed cold forging punch. Each of the cold forging punches of Example 1 and Comparative Examples 1 and 2 was employed to perform repeated cold forging (punching 20 mm diameter holes), and the number of cycles (shot number) at the time when chipping (nicking) occurred in the respective cold forging punch was employed to evaluate the lifespan of the cold forging punch.

(2) Test Results

The test results of Test Example 1 are illustrated in Table 2 below.

TABLE 2 Comparative Comparative Example 1 Example 1 Example 2 Unprocessed (HSS) (Glass) (FeCrB) Surface Smooth Slight matt Smooth Matt state finish (no change) finish No. of punches 30,000 90,000 30,000 20,000 (lifespan) (no change)

(3) Interpretation

The above results enabled confirmation that in Example 1, employing ejection particles of 1000 HV which is a higher hardness than the hardness of the Co phase (approximately 700 HV), deformation was induced of the surface of the treatment subject to give a slight matt finish, and a lifespan of three times the untreated case was achieved.

However, in the Comparative Example 1 employing ejection particles of 534 HV which is a lower hardness than the hardness of the Co phase (approximately 700 HV), the surface state of the treatment subject was not changed and remained smooth, and there was also hardly any change in the lifespan compared to the untreated case.

Furthermore, in the Comparative Example 2 employing ejection particles of 1200 HV i.e. higher hardness than the hardness of the Co phase (approximately 700 HV) and also a higher hardness than the ejection particles of Example 1, although plastic deformation was induced in the surface of the treatment subject and a matt finish could be achieved, the lifespan actually reduced relative to the untreated case.

The ejection particles made from HSS employed in Example 1 had a hardness of approximately 1000 HV and a lower hardness than the hardness of the cemented carbide (1450 HV) of the material configuring the cold forging punch serving as the treatment subject. Thus for the case of an ordinary workpiece as the treatment subject, deformation occurring at the time of impact of the ejection particles would occur at the ejection particle side having lower hardness, and as a result hardly any plastic deformation would be induced on the treatment subject side (see FIG. 4A). The advantageous effects of micronization and densification of the surface structure of the workpiece, imparting of compressive residual stress, and the like would accordingly not be obtained.

However, the sintered body 1 serving as the treatment subject in the present invention, as illustrated in FIG. 3, has a structure in which the WC particles 10 of high hardness, i.e. 1780 HV, are bonded together with the Co phase 20 having a lower hardness of approximately 700 HV. This means that even when ejection particles having a lower hardness than the overall hardness (1450 HV) of the sintered body (carbide tool) are employed as the ejection particles 30, due to employing ejection particles of the hardness of the Co phase 20 (approximately 700 HV) or greater, as explained with reference to FIG. 4B, although the impact of the ejection particles 30 is not able to deform the WC particles 10, the Co phase 20 bonding the WC particles 10 together is deformed, moving the WC particles 10. This enables the surface of the sintered body 1 to be deformed, and the Co phase 20 to be strengthened by forming fine crystals and imparting compressive residual stress accompanying such deformation. This is thought to be the reason improvements can be achieved in making brittle fracture, such as chipping and the like, less liable to occur, and in imparting excellent toughness characteristics.

However, in the Comparative Example 1 employing the ejection particles 30 of lower hardness than the Co phase, plastic deformation of the WC particles is obviously not achieved, and plastic deformation of the Co phase is also not achievable. This is thought to be why, as a result, no change was obtained in both appearance and lifespan compared to the untreated case.

Furthermore, in Comparative Example 2 employing the ejection particles having a hardness of 1200 HV i.e. a lower hardness than the sintered body 1 but a higher hardness than the Co phase, plastic deformation can be induced in the Co phase. This could be confirmed in the test results illustrated in Table 2 by a change of the surface of the sintered body to a matt finish.

However, in the sintered body 1 treated under the conditions of Comparative Example 2, a reduction in the lifespan was confirmed relative to the untreated case, and brittle fracture, such as chipping, was confirmed to actually be more liable to occur.

This confirmed that ejection particles having a hardness of not less than the hardness of the binder metal phase (Co phase) need to be employed as the ejection particles in order to increase the toughness of the sintered body (cemented carbide), and that ejection particles having a lower hardness than 1200 HV, and more specifically preferably employed ejection particles have a hardness of not more than 1000 HV such as those confirmed to strengthen the Co phase in Example 1.

Test Example 2: Header Processing Die (Carbide)

Ejection particles were ejected under the conditions listed in Table 3 below against a header processing die (outer diameter 50 mm, inner diameter 15 mm, height 30 mm) made from a WC—Co cemented carbide (1150 HV).

Note that the hardness of the Co phase that is the binder metal phase is approximately 700 HV.

TABLE 3 Treatment Conditions Example 2 Blasting Device Gravity Type Ejection Material HSS (SKII) particles Hardness Approximately 1000 HV Average particle Approximately diameter 40 μm Shape Substantially spherical shaped Ejection Pressure 0.5 MPa conditions Nozzle diameter 9 mm diameter - long Ejection distance 100 mm to 150 mm Ejection duration Approximately 40 seconds

The state of the surface of the header processing die was observed with the naked eye after ejection of the ejection particles 30. An un-processed header processing die, and the header processing die treated under the above conditions (Example 2), were each employed to perform repeated header processing (cold heading) of SCM435, and the number of cycles (shot number) at the time when damage occurred on the inner peripheral face of the die was employed to evaluate the lifespan of the respective header processing die.

(2) Test Results

The test results of Test Example 2 are illustrated in Table 4 below.

TABLE 4 Unprocessed Example 2 Surface state Smooth Slight matt finish No. of cycles 300,000 900,000 (lifespan)

(3) Interpretation

The above results enabled confirmation that in Example 2 employing ejection particles having a hardness of 1000 HV, which is a higher hardness than the hardness of the Co phase (approximately 700 HV), plastic deformation was induced of the surface of the treatment subject to give a slight matt finish. The lifespan was also able to be extended to three times that of the untreated case. Employing the ejection particles within the hardness range stipulated by the present invention was confirmed to be effective in increasing the toughness of the sintered body.

Test Example 3: Drill (Carbide) (1) Test Method

Ejection particles were ejected under the conditions listed in Table 5 below against a drill (5 mm diameter) made from a WC—TiC—TaC—Co cemented carbide (91.5HRA (1600 HV)).

Note that the hardness of the Co phase that is the binder metal phase is approximately 700 HV.

TABLE 5 Comparative Treatment Conditions Example 3 Example 3 Blasting Device Fine powder Fine powder suction type suction type Ejection particles Material Alumina-silica Zirconia-Silica Hardness 792 HV Approximately (Approximately 1000 HV 800 HV) Average particle Approximately <50 μm diameter 38 μm Shape Substantially Substantially spherical shaped spherical shaped Ejection Pressure 0.4 MPa 0.6 MPa conditions Nozzle 7 mm diameter - 7 mm diameter - diameter long long Ejection 100 mm to 100 mm to distance 150 mm 150 mm Ejection Approximately Approximately duration 20 seconds 20 seconds

Holes were bored in ductile cast iron (FCD400) using the drills that had been subjected to ejection of the ejection particles.

(2) Test Results

In an untreated drill, regrinding of the cutting-edges was needed due to chipping when 500 holes had been bored, however the drill treated according to the method of the present invention was able to bore up to 1300 holes without performing regrinding, enabling the lifespan of the drill to be greatly extended.

Moreover, holes formed using the drill of Example 3 were confirmed to have improved smoothness of inner peripheral faces compared to cases in which the untreated drill was employed.

Moreover, in the example in which ejection particles were ejected under the processing conditions of Comparative Example 3, the lifespan of the drill was shortened by the occurrence of chipping compared to an untreated drill.

The above results are thought to arise because, in cases employing ejection particles made from a ceramic of lower toughness than ejection particles made of metal, considerable damage is imparted to the surface of the treatment subject compared to cases employing ejection particles made from metal.

These results are thought to show that even in cases employing the same ejection particles of 1000 HV, different results are obtained for the same treatment subject in cases (Examples 1, 2) employing ejection particles made from metal (high-speed steel), to cases (Comparative Example 3) employing the ejection particles are made from ceramic (zirconia-silica).

Thus in cases employing ejection particles made from a ceramic, preferably employed ejection particles have a hardness of not more than 792 HV (approximately 800 HV) such as those for which the advantageous effect of strengthening the binder metal phase (Co phase) is confirmed in the Example.

Test Example 4: Cylinder Inner Diameter Turning Insert (Cermet) (1) Test Method

Ejection particles were ejected under the conditions listed in Table 6 below against a diamond shaped insert made from a TiCN—NbC—Ni cermet (93HRA (1900 HV)) for turning the inner diameter of a cylinder made from SUS304.

Note that the hardness of the Ni phase that is the binder metal phase is approximately 500 HV.

TABLE 6 Treatment Conditions Example 4 Blasting Device Fine powder suction type Ejection Material Glass particles Hardness 565 HV Average particle Approximately diameter 38 μm Shape Substantially spherical shaped Ejection Pressure 0.4 MPa conditions Nozzle diameter 7 mm diameter - long Ejection distance 100 mm Ejection duration Approximately 1 second on each cutting-edge (each corner of diamond shaped insert)

The state of the surface of the insert was observed with the naked eye after ejection of the ejection particles under the conditions of Example 4. An un-processed insert and the insert of Example 4 were each employed to turn the inner diameter of cylinders made from SUS304.

(2) Test Results

The surface of the cutting-edge portions of the untreated insert was smooth, and the cutting-edges of the insert after treatment under the treatment conditions of Example 4 was a slight matt finish. This confirmed that ejection of the ejection particles enables plastic deformation to be induced in the cutting-edge surfaces of the insert.

Moreover, although a lifespan of 1000 cycles of cylinder processing was achieved with the untreated insert, 3000 cylinders could be processed with the insert whose Ni phase had been strengthened by the treatment conditions of Example 4, greatly increasing the lifespan by a multiple of three.

Moreover, the finish on the inner diameter finished surface was better on cylinders machined using the insert of Example 4 than on cylinders machined using the untreated insert.

For the WC—Co cemented carbide illustrated in Table 1, when glass beads of 565 HV were employed as the ejection particles in Comparative Example 1, the binder metal phase (Co phase) was not able to be strengthened due to the hardness of the binder metal phase (Co phase) being 700 HV. However, in the Example 4 in which the treatment subject was the TiCN—NbC—Ni cermet having a binder metal phase (Ni phase) of approximately 500 HV, a greatly increased lifespan was obtained by employing such glass beads of 565 HV as the ejection particles. The present test results have been able to confirm that the lower limit to a hardness of the ejection particles capable of strengthening the binder metal phase is decided in relation to the hardness of the binder metal phase.

Test Example 5: TiC Coated Cutting Insert (Carbide) (1) Test Method

Ejection particles were ejected under the conditions listed in Table 7 below against a diamond shaped cutting insert made from a WC—TiC—TaC—Co cemented carbide (91.5HRA (1600 HV)) that had been coated with a TiC film at a film thickness of approximately 3 μm using a CVD method.

Note that the hardness of the Co phase that is the binder metal phase is approximately 700 HV.

TABLE 7 Treatment Conditions Example 5 Blasting Device Fine powder suction type Ejection Material Alumina-silica particles Hardness 792 HV (Approximately 800 HV) Average particle Approximately diameter 38 μm Shape Substantially spherical shaped Ejection Pressure 0.4 MPa conditions Nozzle diameter 7 mm diameter - long Ejection distance 100 mm Ejection duration Approximately 1 second on each cutting-edge (each corner of diamond shaped insert)

Compressive residual stress values were measured in the vicinity of the surface of an untreated insert and an insert on to which ejection particles had been ejected under the conditions of Example 5. Each of the insert was also employed to machine a shaft made from SCM440.

(2) Test Results

The results of the above tests are illustrated in Table 8.

TABLE 8 Example 5 Untreated Residual stress at 5 μm from −1050 MPa +130 MPa base material surface Number of shafts machined 120 shafts 60 shafts (lifespan)

(3) Interpretation

With the untreated insert, the TiC coating detached when 60 shafts had been machined, and a replacement was needed due to chipping occurring in the base material made from cemented carbide. However, with the insert treated as Example 5, detachment of the TiC film was prevented, enabling 120 shafts to be machined and greatly increasing the lifespan.

Such an increase in the cohesion strength of the TiC film is thought to be obtained by the increased toughness of the cemented carbide serving as the base material.

Moreover, in the results of measurements of compressive residual stress values for the residual stress at a position 5 μm from the base material surface, although a tensile stress (+130 MPa) remained in the untreated case, which is thought to arise from heating when forming the TiC film using CVD, this changed to a compressive stress (−1050 MPa) when the treatment of Example 5 had been performed thereon.

These results have confirmed that employing the method of the present invention enables the mechanical characteristics of the sintered body base material in a layer below the hard coating film to be changed without causing the hard coating film etc. to detach, even in cases in which a sintered body coated with a hard coating film such as TiC is the treatment subject.

Note that in Example 5, even with the TiC coating film formed to a film thickness of 3 μm, compressive residual stress was confirmed to be imparted to at least a depth of 5 μm in the base material below (a total depth of 8 μm when the 3 μm thickness of the hard coating film is included).

Thus the logical inference therefrom is that if the hard coating film formed on the surface had a film thickness of up to about 5 μm, then compressive residual stress can be imparted at least to a depth of about 3 μm from the base material surface (a total depth of 8 μm when the 5 μm thickness of the hard coating film is included), and the binder metal phase in the vicinity of the surface of the sintered body can be strengthened.

Test Example 6: Cutting Insert (cBN) (1) Test Method

Ejection particles were ejected under the conditions listed in Table 9 below against a diamond shaped cutting insert made from cBN (4700 HV) configured from cubic crystals of boron nitride sintered together with a Co binder.

Note that in the cBN that has been sintered under ultrahigh pressure, the hardness of the Co phase binder is higher than in a carbide tool, and the hardness of the Co phase in the cBN of the present Test Example is approximately 800 HV.

TABLE 9 Treatment Conditions Example 6 Comparative Example 6 Blasting Device Gravity type Gravity type Ejection Material HSS(SKH) Alumina-silica particles Hardness Approximately 1000 HV 792 HV Average particle Approximately Approximately diameter 40 μm 38 μm Shape Substantially Substantially spherical shaped spherical shaped Ejection Pressure 0.4 MPa 0.4 MPa conditions Nozzle 9 mm diameter - 9 mm diameter - diameter long long Ejection 100 mm 100 mm distance Ejection Approximately 1 second Approximately 1 second duration from each of 4 directions from each of 4 directions at each cutting-edge at each cutting-edge (each acute angled (each acute angled corner of diamond corner of diamond shaped insert) shaped insert)

An untreated insert, and the inserts treated under the conditions of Example 6 and Comparative Example 6 were each employed to machine shafts of carburized and quenched steel, and the differences in lifespan therebetween confirmed.

(2) Test Results

The results of the above tests were then that whereas an untreated insert has a lifespan of machining 200 carburized and quenched shafts, the insert against which ejection particles had been ejected under the conditions of Example 6 was able to machine double that amount at 400 carburized and quenched shafts.

The above results have confirmed that strengthening of the binder metal phase can be performed not only for a cemented carbide and cermet, but also for cBN. A logical inference therefrom is that the method of the present invention applicable to sintered bodies in general that have a structure in which hard particles are bonded together by a binder metal phase.

Note that although strengthening of the Co phase could be performed by employing ejection particles that were alumina-silica beads of 792 HV in Example 3, in which a drill made from a cemented carbide is the treatment subject, an increased lifespan was not achieved for a sintered body having the same Co binder metal as the treatment subject in the Comparative Example 6, in which a sintered body of cBN is the treatment subject, even when ejection particles of alumina-silica beads at 792 HV were ejected thereon, and strengthening of the Co phase could not be achieved.

Such a difference is thought to be due to the cBN being sintered under ultrahigh pressure as described above, making the hardness of the Co phase, at about 800 HV, about 100 HV higher than in a cemented carbide such that sufficient plastic deformation could not be imparted to the Co phase by alumina-silica beads of 792 HV. This is thought to result in not being able to achieve strengthening through work hardening from micronization of the crystal structure and imparting compressive residual stress.

The present tests have accordingly confirmed that even in cases in which the metal employed as the material for the binder is the same, if the hardness of the binder metal phase is different due to differences in the sintering conditions or the like, then there is a need to select ejection particles to match the relevant hardness.

Thus, the broadest claims that follow are not directed to a machine that is configured in a specific way. Instead, said broadest claims are intended to protect the heart or essence of this breakthrough invention. This invention is clearly new and useful. Moreover, it was not obvious to those of ordinary skill in the art at the time it was made, in view of the prior art when considered as a whole.

Moreover, in view of the revolutionary nature of this invention, it is clearly a pioneering invention. As such, the claims that follow are entitled to very broad interpretation so as to protect the heart of this invention, as a matter of law.

It will thus be seen that the objects set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Now that the invention has been described;

DESCRIPTION OF REFERENCE NUMERALS

1 Sintered body (cemented carbide)

10 Hard particles

20 Binder metal phase

30 Spherical shaped ejection particles

Claims

1. A method of strengthening a binder metal phase of a sintered body, the binder metal phase strengthening method comprising:

ejecting spherical shaped ejection particles against a surface of a sintered body that includes hard particles and a binder metal phase bonding the hard particles together, by ejecting the spherical shaped ejection particles together with a compressed gas at an ejection pressure of from 0.2 MPa to 0.6 MPa or at an ejection velocity of from 80 m/s to 200 m/s and the spherical ejection particles having a hardness that is not less than the hardness of the binder metal phase and that is a hardness of not more than 1000 HV and being particles of from 100 grit to 800 grit, having an average particle diameter of from 20 μm to 149 μm.

2. The sintered body binder metal phase strengthening method of claim 1, wherein a sintered body employed as a treatment subject is a sintered body having a hard coating film coated on at least a portion of surface at a thickness of not more than 5 μm, and the ejection particles are ejected against the sintered body at the portion of the surface coated with the hard coating film.

3. The sintered body binder metal phase strengthening method of claim 1, wherein the ejection particles are metal particles, ceramic particles, or a mixture of metal particles and ceramic particles.

4. The sintered body binder metal phase strengthening method of claim 1, wherein a hardness of the ceramic particles employed is not more than 800 HV.

5. The sintered body binder metal phase strengthening method of claim 2, wherein the ejection particles are metal particles, ceramic particles, or a mixture of metal particles and ceramic particles.

6. The sintered body binder metal phase strengthening method of claim 2, wherein a hardness of the ceramic particles employed is not more than 800 HV.

7. The sintered body binder metal phase strengthening method of claim 3, wherein a hardness of the ceramic particles employed is not more than 800 HV.

8. The sintered body binder metal phase strengthening method of claim 5, wherein a hardness of the ceramic particles employed is not more than 800 HV.

Patent History
Publication number: 20210230729
Type: Application
Filed: Aug 24, 2020
Publication Date: Jul 29, 2021
Inventor: Yoshio MIYASAKA (Aichi)
Application Number: 17/001,351
Classifications
International Classification: C22F 1/16 (20060101); C21D 7/06 (20060101); B24C 1/10 (20060101);