Patent Grant
8435626
Cemented tungsten carbide materials are known in the industry. This material constitutes a composite material of tungsten carbide (WC) that is embedded in a cobalt matrix. (Cemented tungsten carbide materials are sometimes abbreviated as “WC—Co”.) Typical compositions of the cobalt metal ranges from 3 to 30 percent by weight, although other percentages of cobalt may be used. For background information regarding cemented tungsten carbide materials, the reader is invited to consult U.S. Patent Application Publication No. 2005/0276717 A1 (which publication is expressly incorporated herein by reference).
Cemented tungsten carbide materials have unique properties compared to steel, metal alloys, or ceramic materials. For example cemented tungsten carbide materials have higher hardness, wear resistance and strength, as compared to steel. However, cemented tungsten carbide materials have been known to more easily fracture than steel. In the industry, this property of the material is referred to as “fracture toughness.” Fracture toughness refers to the propensity of the material to chip or fracture during use as a result of the service under mechanical loading. Thus, cemented tungsten carbide materials have less fracture toughness than steel. When compared to ceramic materials, WC—Co materials have higher fracture toughness and have equivalent or better hardness and wear resistance than do the comparable ceramic materials. Because of these unique properties, many cemented tungsten carbide materials are used in a wide range of industrial applications including metal cutting tools, mining tools, oil and gas exploration tools, and many other applications requiring extreme wear resistance.
Many WC—Co materials have a relatively low fracture toughness which limits their effectiveness in some potential applications. This low fracture toughness means that the material has a propensity to chip or fracture during use. This chipping and fracturing is especially prevalent when the WC—Co material is used as a cutter on a rock drill bit. Chipping and fracturing are the leading causes of degradation or premature failure or cemented tungsten carbide tools.
Another type of material that is used for metal cutting tools, mining tools, oil and gas exploration tools, and other similar applications is polycrystalline diamond (“PCD” or “Cd”) materials. PCD materials are composed of diamond crystallite particles with a small percent of cobalt residing in the inter-particle spaces formed by the diamond crystallite particles. In many instances, PCD materials have many of the same advantageous properties that are generally associated with WC—Co materials. At the same time, many PCD materials suffer from many of the same deficiencies as WC—Co materials.
As noted above, one of the common modes of failure of tools made of WC—Co and/or PCD materials is that they tend to crack, chip, or fracture during use. Such cracking or failure of these materials is especially common when the tool become hot (as a result of the friction caused by contact with another surface). This cracking of the tool as a result of frictional heat is sometimes referred to as “heat checking.” (The cracks formed by frictional heat may also be referred to as “thermal fatigue cracks”).
The formation of heat checking cracks will now be described. The tool (cutter) engages the surface of a rock formation or a work piece. For example, if the tool is a rock drill mining tool or a construction tool, the tool may engage the rock formation when it is being used to cut the rock formation. Likewise, the tool may be used with a work piece in applications such as metal removal or in the formation of engineered wear parts (e.g., abrasive nozzles and seal rings). When the tool engages the surface of the rock formation or work piece, the temperature at the contact point and the adjacent surface areas increases dramatically from ambient temperature. This increase in temperature is due to intense frictional heating.
For example, in the case of cutting inserts on a lathe, the peak temperature at the tool's cutting tip can reach temperatures above 1000° C. The temperature at the contact between a rock drilling cutter and rock formations can reach temperatures above 800° C. The actual peak temperature at the cutting edge of the tool is obviously a function of surface speed (i.e., the speed at which the tool contacts the surface), the depth of cut being performed, the load (or force) applied to the surface, and, more importantly, the thermal conductivity of the cutting tool material. The thermal conductivity of the tool refers to the ability of the material to dissipate heat by conducting the heat out of the cutting contact area into other areas of the tool. When the thermal conductivity of the tool is “low,” the temperature of the cutting area will be higher because the tool is not able to dissipate the heat away from the cutting area. Alternatively, if the thermal conductivity of the tool is “high,” the temperature of the cutting area will be lower because some of the heat associated with the cut will be dissipated to other areas of the tool.
In order to reduce the frictional heat that is imparted to the cutting area, most cutting and drilling operations will use coolant or a cooling system to cool the tool and/or the cutting area. Then, when a coolant or cooling system is used, the cutting tool may experience drastic and sudden changes in temperature, namely, sudden heating caused by the frictional contact and then sudden cooling caused by the cooling system. These sudden changes in temperature may cause the thermal fatigue and thermal cracking of the tool.
It should also be noted that cemented tungsten carbide, and polycrystalline diamond (PCD) are essentially composites of the primary phase WC (or polycrystalline diamond) and the matrix or binder material. (Generally, the matrix or binder material is cobalt). The hard particles of the primary phase (WC or Cd) have much lower coefficients of expansion than that of the matrix ductile metal matrix phase (e.g., Co). Accordingly, the high temperature associated with tool usage also creates cyclic residual stresses between the hard particles and matrix phase. Such cyclic residual stresses can be an additional cause of heat checking cracks (thermal fatigue) of the material.
Of course, heat-checking cracks in a tool surface can propagate and get larger over time. When such cracks exceed a “critical size,” the tool can no longer be safely used. At this time, the tool must be replaced and discarded, thereby increasing the overall costs of the cutting process.
Thus, there is a need in the industry for a new type of WC—Co or PCD material that may be used in industry as part of a cutting tool. In generally, this new type of WC—Co/PCD material should have high thermal conductivity such that it is capable of readily dissipating the heat caused by frictional contact with the cutting surface. Such heat dissipation would reduce the temperature at the cutting edge, thereby resulting is lower residual thermal stresses. Such dissipation of heat would also reduce the amount of heat cracking of the tool. Such new materials (as well as the tools that incorporate these materials) are disclosed herein.
The present invention is designed to provide a new type of cemented tungsten carbide (or polycrystalline diamond) material that will address the above-mentioned problems associated with heat checking or thermal fatigue cracks. This new material will be designed by having a gradient in the grain size of the particles. Specifically, top layer (or layers) of the material will have coarser grain sizes whereas the inner layers will have finer grain sizes. This means that the size of the particles at the material's top layer is larger than the size of the particles in the inner layer(s) of the material. In many embodiments, there will be a gradient in the particle size, meaning that the particle size will gradually decrease from the larger, coarser grains at the top layer to the smaller, finer grains in the inner layer(s). (The inner layer(s) may also be referred to as the “bulk substrate”). The top layer with the coarser grain sizes essentially acts as a coating for the material. The top layer, with the coarser grain sizes, increases the thermal conductivity of the material, thereby allowing the material to more easily dissipate the frictional heat caused during use of the tool. Thus, the material is well-suited to avoid heat cracking and/or thermal fatigue. At the same time, the finer grain-sized particles throughout the inner layers of the material impart a sufficient hardness and wear resistance to the material. Accordingly, the materials of the present embodiments have the desired properties, namely high strength and wear resistance, and at the same time, resist heat cracking and/or thermal fatigue.
Exemplary embodiments of the invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only exemplary embodiments and are, therefore, not to be considered limiting of the invention's scope, the exemplary embodiments of the invention will be described with additional specificity and detail through use of the accompanying drawings in which:
The embodiments of the present invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the present invention, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of embodiments of the invention.
The present embodiments involves the understanding of how the conductivity and mechanical properties of WC—Co and PCD materials may be affected by the composition (i.e., amount of cobalt) and the microstructure (i.e., the grain size of the particles) of the material. This relationship is described herein.
The thermal conductivity of WC—Co or polycrystalline diamond materials may be a function of the composition and microstructure. (Polycrystalline diamond materials may be abbreviated as “Cd—Co” to indicate that cobalt (or another metal phase) is also present in the material). The higher the cobalt (metal) content, the lower the thermal conductivity of the composite because WC and Cd have higher thermal conductivities than the cobalt (metal) phase. (It should be noted that other transition metal binder materials, such as Fe or Ni, or their combinations, may also be used as the metal phase. Like cobalt, these other metals also have lower thermal conductivities than WC or Cd). Thus, if the cobalt (or metal) content is increased, the thermal conductivity of the material decreases because the cobalt (metal) simply cannot dissipate/transfer as much heat as the WC or Cd material.
The thermal conductivities of WC—Co or Cd—Co materials are also functions of the grain sizes. As the grain size increases, the thermal conductivity of the composite material also increases. As an individual constituent, tungsten carbide (WC) has sufficiently good thermal conductivity. In fact, the thermal conductivity of WC is higher than that of cobalt or any other transition metal binder (Fe, Ni, or their combinations) that may be used. However, a well-grown, large WC grain has better thermal conductivity than a WC grain that does not have well developed facets. Similarly, a well-grown, large Cd grain has better thermal conductivity than a Cd grain that does not have well developed facets. Therefore, a WC—Co or Cd—Co composite that has larger grain size tends to have better thermal conductivity (and hence better thermal fatigue resistance) than does a composite with smaller grains sizes or less developed facets. The reason for this is that a composite with larger grain sizes has less grain boundaries (per unit volume of the material) than a composite with finer grain sizes.
Thus, the present embodiment relies on the principles that (1) the thermal conductivity of a WC—Co or Cd—Co material may be increased by increasing the grain size of the material and (2) decreasing the cobalt content (or the amount of the metal matrix phase) will likewise increase the thermal conductivity of the material.
However, if the grain size of the WC—Co or Cd—Co material is increased, the hardness and wear resistance of the WC—Co or Cd—Co material will generally decrease. At the same time, increasing the grain size will also increase of the fracture toughness of the WC—Co materials. (This comparison assumes that the cobalt content for both materials is the same.) Similarly, using finer grain sizes will in general increase the hardness and wear resistance of the WC—Co material, but will decrease the fracture toughness of the material.
Changing the cobalt (or metal phase) content of the WC—Co or Cd—Co material will generally have a similar effect as changing the grain sizes. Higher cobalt content will increase the fracture toughness of a WC—Co or Cd—Co composite while decreasing its hardness and wear resistance.
For many industrial cutting tool applications, the hardness of the material at high temperatures (i.e., the cutting temperature) is an important feature. In general, if a material exhibits a high (acceptable) hardness at room temperature, it also exhibits a similar high (acceptable) level of hardness at the elevated cutting temperature. As noted above, such high levels of hardness are associated with the use of finer grain sizes rather than coarser grain sizes. Thus, with respect to the hardness of the material, using finer grain size would be beneficial (whereas for purposes of thermal conductivity, using coarser grain sizes is desired). Having the tool have a high (acceptable) hardness level at the cutting temperature means the tool will maintain its geometry without deforming during the extreme conditions associated with the cutting process. Deformations of the tool surface are undesirable because it may cause the tool coatings (which are often used on metal cutting tools) to fail.
In the case of PCD (assuming a constant cobalt percentage), larger diamond grain size also yields better thermal conductivity of the composite material. Larger grain size or higher cobalt content will also lead to higher impact resistance, or fracture toughness. For PCD, there are two other detrimental effects of increasing the percentage of cobalt used in the material. First, cobalt has a rather high solubility for carbon which increases as temperature increases. Thus, if the cobalt content is increased, the cobalt is more likely to dissolve the Cd structure and results in a weakened PCD structure. Second, cobalt is also prone to oxidation and corrosion at high temperatures, which would further weaken the PCD composite material. Accordingly, increasing the cobalt content may not be desirable for many PCD materials and may detrimentally affect the performance of the PCD composite material.
Based on the above-recited factors, it is clear that the effects of grain size and cobalt content are two fold. If the grain size and/or the cobalt content is increased, the thermal conductivity and fracture toughness (i.e., cracking resistance) of the WC—Co or Cd—Co material will increase. At the same time, the wear resistance and hardness of the WC—Co or Cd—Co material will decrease. Thus, in order to get high levels of hardness, a WC—Co material could be designed with a low cobalt content and coarse grain combination. However, because a coarse grain structure reduces hardness, this approach is limited in terms of the maximum hardness that can be achieved for a specific cobalt content. Assuming the cobalt content is maintained constant, to increase the hardness, finer grain sizes can be used. However, the use of finer grain sizes decreases the thermal conductivity and hence thermal crack resistance of the material. In other words, if finer grain sizes were used, that would increase the material's hardness but would decrease the material's resistance to thermal cracking. If coarser grain sizes were used, the material's resistance to thermal cracking would increase, but the material's hardness would decrease.
Therefore, in order to achieve a tool with high (acceptable) hardness without sacrificing the material's resistance to thermal cracking, or vice versa, a functionally graded material (“FGM”) may be constructed. An example of a functionally graded material 13 is illustrated in
It should be noted that the above-recited example is given with respect to WC—Co materials. Similar types of materials may be designed using PCD materials. For example, a Cd—Co material may be made in which the top surface has coarser grains and the bulk substrate has finer grains, thereby achieving the same type of properties discussed above.
The exact grain sizes used (for both the top layer with “coarse” grains and the bulk substrate with “fine” grains) may be selected according to the particular embodiment. Any number of grain sizes may be used, as appropriate. One skilled in the art would appreciate how to change the grain sizes of both the top layer and the bulk substrate.
It should also be noted that the above-recited embodiments are designed in which the top layer (at the surface) has coarser grains whereas the bulk substrate has finer grains. Those skilled in the art would appreciate that other embodiments may be designed in which the top layer has finer grains and the bulk layer has coarser grains. Obviously, changing the layers in this manner would affect the properties of the WC—Co or Cd—Co material.
It should also be noted that the above-recited embodiments may be designed in which the coarse grains are a layer on top of the bulk substrate. Other embodiments may be designed in which the transition between coarse grains to fine grains is not a sudden, discrete change. Rather, embodiments may be formed in which a “gradient” in the grain size is formed, meaning that the size of the grains will gradually change in accordance with the depth of the material.
Those skilled in the art will appreciate that a variety of different embodiments may be constructed in which the top layer or area has coarser grains whereas the bulk substrate has finer grains. The following is a description of one or more of the embodiments that may be constructed.
It should be noted that coarse WC or Cd grains could range in size from 1 micron to 40 microns. Fine WC or Cd grains could range in size from 0.1 microns to 10 microns. In other embodiments, fine grains may range from about 0.5 to about 3 microns, and coarse grain may range from about 5 to about 30 microns. In one embodiment, grain size means “average grain size.” Actual materials may have a grain size distribution. Thus, in one embodiment, the surface layer grain size is larger than that of the bulk of the material. For embodiments involving PCD, the average size of the coarse grains may be from about bout 20 to about 60 microns whereas the average size of the fine grains may range in average size from about 0.5 or 1 to about 30 microns.
There are a variety of different manufacturing methods that may be used to construct the functionally graded materials described herein. Both WC—Co and PCD may be made by powder metallurgy processes. The first step may involve preparing powders of desired compositions. The second step may be to compact and form the powder to a desired geometric shape. The third step may be to sinter the compact into a fully densified solid material. Depending on the specific material being constructed, the specific details of the above-recited steps may vary, as appreciated by those skilled in the art. For example, PCD materials may be sintered under extremely high pressure (5-7 GPa) at 1400-1500° C., while WC—Co materials are typically sintered in vacuum at temperatures from 1360 to 1500° C. Sinter-HIP processes involving vacuum sintering followed by low pressure hot isostatic pressing with 1 to 10 MPa pressure in one single furnace run are also common in modern manufacturing operations.
To manufacture the functionally graded WC—Co (or PCD) materials containing a top surface layer with large grains and a bulk substrate with finer grains, several manufacturing methods can be used. One method may include the following steps:
1. Prepare two separate powder mixtures, one with the top surface layer composition and grain size, and the other with the bulk composition and grain size;
2. If using die compaction to fabricate green compact, apply and spread the top surface layer powder on the inner wall of the die cavity first, then fill the die with the bulk substrate material powder; and
3. Press and sinter.
Another alternative method may include the following steps:
1. Prepare the top surface layer powder into a slurry;
2. Compact the bulk substrate powder separately;
3. Dip coat the substrate in the slurry of the top surface layer powder; and
4. Sinter.
The additional step of pressing may optionally be used.
Another approach may involve creating a surface with large grains in-situ during sintering may be formed as follows:
1. Prepare one powder mixture,
2. Sinter in a controlled and custom engineered atmosphere that promotes grain growth.
However, during the process of this step (step 2.), the depth of enhanced grain growth may be controlled to obtain a desired thickness of the top surface layer. For example, the starting powder mixture of WC and Co may be made to have lower than stoichiometric carbon content. During sintering, the compact may be allowed to densify first to near full density or until all pores that are not connected to the surface are closed. Then, the material may be introduced into the furnace a carburizing atmosphere. Due to the high carbon content in the atmosphere, carbon may diffuse into the material from the surface. During that process, the surface layer may have higher carbon content than the interior, thus there will be more pronounced grain growth at and near the surface layer that in the interior. By controlling the kinetics of the diffusion process, a differential grain growth may be achieved between the surface layer and the interior bulk of the material.
The patent application referenced above (namely, U.S. Patent Application Publication No. 2005/0276717 A1) discloses methods for creating a gradient in the amount of cobalt used in the material. It should be noted that the methods of this application may be combined with the present methods, thereby creating a material that has both a cobalt gradient and a gradient in the grain size. For example, the cobalt may migrate in the direction of finer grain sizes and/or in the direction of carbon diffusion. Therefore, one can prepare two powders with different grain sizes. The coarse grain size powder may be made to have higher carbon content than the powder with finer grain sizes. Using one of the disclosed green compaction methods, the coarse-grain powder may be compacted as a surface layer on the top of a substrate made of the finer-grain powder. When subjected to sintering, due to the initial difference in both the grain size and the carbon content, cobalt may migrate to the interior of the part because it has finer grain size and because carbon is diffusion from the carbon rich surface layer to the carbon deficient interior. The final microstructure may therefore consists of a coarse grained surface layer with lower cobalt content than the bulk substrate, which has a finer grain size and higher cobalt content. The hardness of the top surface layer, as a result of the microstructure, may therefore be either lower, or higher, or equal to that of the bulk substrate.
Those skilled in the art will appreciate that there are a variety of different applications for the materials disclosed herein. They include rock drilling tools, metal cutting tools, mining tools, oil and gas exploration tools, and any other application of WC—Co/PCD materials (including other applications requiring extreme wear resistance).
The present invention may be embodied in other specific forms without departing from its structures, methods, or other essential characteristics as broadly described herein and claimed hereinafter. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.
This application claims the benefit of U.S. Provisional Patent Application No. 61/034,842 filed on Mar. 7, 2008. This application is expressly incorporated herein by reference.
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