AND METHODS FOR USING AND PRODUCING SAME

CROSS-REFERENCE TO RELATED PATENT APPLICATION

[0001] This application claims priority to U.S. Provisional Patent Application No.

61/921,861, filed December 30, 2013, which is incorporated by reference herein in its entirety.

FIELD

[0002] This invention relates to drill rods that are selectively hardened and/or coated with corrosion-resistant material to be resistant to fatigue, wear, and/or corrosion.

BACKGROUND

[0003] Conventional drill rods regularly experience stress corrosion, which leads to corrosion pitting and fatigue failures. These fatigue failures frequently occur in threaded portions of the drill rods. Conventionally, the threads of drill rods are completely carburized or otherwise hardened to make the threads wear-resistant. However, these hardened threads are often brittle and susceptible to fatigue cracking. For example, in conventional threads with case-hardened thread roots, the male ends of the threads often experience fatigue failures caused by thread stresses in the thread roots. In these threads with case-hardened thread roots, fatigue failure cracks (not tensile overload failures) typically propagate from the thread rood. Moreover, carburized drill rods are susceptible to rust. Conventionally, corrosion pitting can cause both midbody and male end fatigue failures. Oil cuttings are conventionally used to reduce the possibility of rust in carburized drill rods; however, there are significant environmental costs associated with this approach.

[0004] Thus, there is a need in the pertinent art for drill rods that are resistant to both wear and fatigue. There is a further need in the pertinent art for drill rods that are resistant to corrosion.

SUMMARY

[0005] Described herein, in one aspect, is a drill rod having a longitudinal axis. The drill rod can comprise an elongate body having a first end portion, a second end portion, a medial portion positioned between the first and second end portions, an outer surface, and an inner surface. The inner surface can define a central bore of the elongate body. At least a portion of the outer surface of the elongate body can be non-carburized. At least a portion of the outer surface of the elongate body can be coated with a corrosion-resistant material.

Optionally, at least a portion of the inner surface of the elongate body can be non-carburized. At least a portion of the inner surface of the elongate body can optionally be coated with a corrosion-resistant material. Drilling systems including the disclosed drill rods are also described.

[0006] In another aspect, a drill rod can have a longitudinal axis and comprise an elongate body. The elongate body can have a first end portion, a second end portion, a medial portion positioned between the first and second end portions, an outer surface, and an inner surface. The inner surface can define a central bore of the elongate body. At least a portion of the outer surface of the elongate body can be non-carburized. At least a portion of the outer surface of the elongate body can be coated with a corrosion-resistant material. The corrosion-resistant material can be chemically bonded to the outer surface of the elongate body. Optionally, at least a portion of the inner surface of the elongate body can be non- carburized. At least a portion of the inner surface of the elongate body can optionally be coated with a corrosion-resistant material. The corrosion-resistant material can be chemically bonded to the inner surface of the elongate body. The outer surface of the first end portion of the elongate body can define a threaded portion, the threaded portion comprising a plurality of threads and a plurality of roots, each thread having a crest, each root being positioned between adjacent threads relative to the longitudinal axis of the drill rod, the crest of each thread being spaced radially outwardly from adjacent roots relative to the longitudinal axis of the drill rod. Each crest of the plurality of crests can have a respective hardness, each root of the plurality of roots can have a respective hardness, and the hardness of the crest of each thread can be greater than the hardness of the roots adjacent to the thread. Drilling systems including the disclosed drill rods are also described.

[0007] In a further aspect, a method for forming a drill rod is described. The method can comprise induction hardening a selected portion of an outer surface of the drill rod. A plurality of threads can then be defined in the selected portion of the outer surface of the drill rod using a hard turning process.

[0008] Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

DETAILED DESCRIPTION OF THE FIGURES

[0009] These and other features of the preferred embodiments of the invention will become more apparent in the detailed description in which reference is made to the appended drawings wherein:

[0010] Figure 1 is perspective view of an exemplary drill rod as disclosed herein.

[0011] Figure 2A is a side perspective view of another exemplary drill rod as disclosed herein. Figure 2B is a partially transparent side perspective view of the drill bit of Figure 2A. Figure 2C is a close-up perspective view of the threaded portion of a first end portion of the drill rod of Figure 2A. Figure 2D is a close-up perspective view of the receptacle of a second end portion of the drill rod of Figure 2A.

[0012] Figure 3 is a cross-sectional image of the threaded outer portion of an exemplary drill rod as disclosed herein.

[0013] Figure 4 is an isolated perspective view of an exemplary receptacle positioned at an end of a drill rod as disclosed herein.

[0014] Figure 5A is a partially transparent schematic view of an exemplary drill rod as disclosed herein, shown prior to formation of the threaded portion of the first end portion. Figure 5B depicts a cross-section of the medial portion of the drill rod, taken at line A-A.

DETAILED DESCRIPTION

[0015] The present invention can be understood more readily by reference to the following detailed description, examples, drawings, and claims, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this invention is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.

[0016] The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.

[0017] As used throughout, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a thread" can include two or more such threads unless the context indicates otherwise.

[0018] Ranges can be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0019] As used herein, the terms "optional" or "optionally" mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0020] The word "or" as used herein means any one member of a particular list and also includes any combination of members of that list.

[0021] Disclosed herein are drill rods that overcome the male-end fatigue failures caused by thread stresses in conventional case-hardened thread roots. The disclosed drill rods also overcome the fatigue failures that conventionally are initiated during corrosion pitting, which has been shown to be a root cause of both mid-body and male-end failures.

[0022] Applicant has discovered that during conventional case-hardening of drill rods, removal of a carburized case can cause mid-body tension overload failures (as opposed to male-end failures) without any compensation for the decrease in strength. Conversely, in the same process, carburized rods can suffer male-end fatigue failures at the thread root. The newly discovered tension overload failures are surprising in that they occur consistently in a mid-body cross section that is significantly larger than that of any other portion of the drill rod, i.e. there are weaker, smaller cross-sections in the threaded ends and in an adjacent portion of the mid-body that is reduced for friction-weld flash removal. In view of these tension overloads, Applicant has studied distribution of dynamic loading across a drill rod to evaluate solutions to address the issues of conventional drill rots set forth herein. In particular, Applicant has studied dynamic tension waves in a drill string as a result of a) compressive impact waves initiating from a 'top-hammer' drill and b) reflecting waves from the drill bit impacting the ground.

[0023] In case-hardened drill rods, it is known that a hardened case has significantly higher strength than the core (the portion of the drill rod surrounded by the hardened case). Further, the average tensile strength of any case hardened cross-section is significantly higher than the same cross-section without the case (given equal core material properties).

Conventionally, the entire rod, inside and out is affected by carburizing, and the depth of the case depends on the time during which the drill rod is exposed to a flow of carbon in the atmosphere. However, Applicant has discovered that the exposure time and costs associated with production of the drill rod can be substantially reduced while still providing a sufficiently hardened case and a desired strength increase.

[0024] Described herein with reference to Figures 1-5B are drill rods 10 that are at least partially non-carburized, thru-hardened (e.g., by furnace or low-frequency induction), and/or at least partially coated with a corrosion-resistant material. It is contemplated that the drill rods disclosed herein can be configured to prevent and/or limit corrosion pitting in the threaded portions of the drill rods. It is further contemplated that selective induction hardening of the drill rods as disclosed herein can prevent and/or reduce fatigue failure (e.g., stress corrosion fatigue failure) while maintaining sufficient wear resistance to extend the life of the drill rods, thereby enhancing productivity and efficiency of drilling operations. It is still further contemplated that the use of corrosion-resistant coatings as disclosed herein can provide improved corrosion and rust protection for the drill rods, thereby improving the appearance of the drill rods. It is still further contemplated that the elimination of carburization in the production of the disclosed drill rods can make the production of the drill rods less labor-intensive, less expensive, and more environmentally friendly than

conventional drill rod manufacturing processes.

[0025] In exemplary aspects, as shown in Figures 1-2D and 5A-5B, each drill rod 10 can be one of a coring drill rod, a percussive drill rod, and a shank. Each drill rod 10 has a longitudinal axis 11 and an elongate body 12. The elongate body 12 can have a first end portion 14, a second end portion 16, a medial portion 18 positioned between the first and second end portions, an outer surface 20, and an inner surface 22. The inner surface 22 can define a central bore 24 of the elongate body 12. Optionally, as shown in Figure 5A, which depicts the drill rod prior to formation of threads in the outer surface of the first end portion 14 as disclosed herein, the elongate body 12 can have a first transition portion 15 between the first end portion 14 and the medial portion 18. In use, it is contemplated that at least one drill rod 10 can be operatively connected with a drill head and/or other conventional drill string components to form a drilling system. It is further contemplated that the drill bits disclosed herein can be configured for at least one of surface drilling operations (with air flushing), underground drilling operations (with flushing by water or other drilling fluid), and coring applications (with flushing by water or other drilling fluid).

[0026] In exemplary aspects, it is contemplated that the drills rods disclosed herein can be formed from one or more steel materials, such as, for example and without limitation, alloy steels. In these aspects, it is contemplated that the steel materials used to form the drill rods can provide high levels of at least one of the following mechanical properties: yield strength, fatigue strength, and hardness. It is contemplated that high levels of these mechanical properties can be required to allow the drill rods to transmit drilling loads and withstand abrasion against drill hole walls while still being easily formable into tubular members with highly productive threaded connections and other mechanical properties. For example, it is contemplated that conventional plastic, conventional stainless steel, and conventional aluminum materials can lack sufficient mechanical properties, and conventional engineered composite materials cannot withstand the abrasion. It is further contemplated that an acceptable cost level is required to operate within industry drilling costs. Thus, it is contemplated that aircraft-grade alloy steels such as Titanium alloys can be cost prohibitive. Exemplary alloy steel grades for the drill rods disclosed herein can be variations of the chromium-molybdenum-nickel families, such as, for example and without limitation, AISI 4340 or AISI 9315. In exemplary aspects, it is contemplated that the amount of carbon in the grade of alloy steel can determine how receptive the steel is to various forms of surface hardening. For example, in these aspects, it is contemplated that a low amount of carbon can allow for absorption of gaseous carbon in a carburizing process, whereas a comparatively higher level of carbon can be required to respond to induction hardening.

[0027] It is contemplated that conventional carbon-based steels form a matrix of chemically dissimilar carbide and ferrite that forms microgalvanic cells, which are significant drivers of corrosion. Optionally, in exemplary aspects, the drill rods disclosed herein can comprise steel materials produced by MMFX Steel Corporation (Irvine, CA), such as, for example and without limitation, the steel materials described in U.S. Patent Nos. 6,273,968; 6,646,427; 6,709,534; and 7, 118,637, each of which is hereby incorporated herein by reference in its entirety. In these aspects, it is contemplated that these steel materials can have a microstructure configured to form a "lathe" matrix with substantially no carbides, thereby avoiding the formation of microgalvanic cells.

[0028] In exemplary aspects, it is contemplated that drill rods formed from high- strength, corrosion-resistant steels can achieve the objectives disclosed herein without the need for an anti-corrosion coating and while allowing thru-hardening to thereby increase mid- body section strength. In these aspects, it is further contemplated that such drill rods can also increase section strength by case-hardening only the outer diameter of the drill rod using induction methods as further disclosed herein. Thus, it is contemplated that the use of such drill rods can avoid the difficulties associated with applying coatings to the small inner- diameter bore of a drill rod and with wear/degradation of anti-corrosion coatings.

Additionally, the use of such drill rods can eliminate the need for inner-diameter hardening using induction, during which it is difficult to run sufficient quenchant through the drill rod to achieve hardening. The high-strength, corrosion-resistant steels used to form the drill rods can comprise, for example and without limitation, precipitation-hardened stainless steels and low-carbon nanocomposite martensitic steels as are known in the art, including those described in U.S. Patent Nos. 6,273,968; 6,646,427; 6,709,534; and 7, 118,637.

[0029] In one exemplary aspect, and with reference to Figures 1-3, the outer surface 20 of the first end portion 14 of the elongate body 12 can optionally define a threaded portion 30. In this aspect, the threaded portion 30 can comprise a plurality of threads 32 and a plurality of roots 34, with each thread extending radially outwardly relative to the longitudinal axis 1 1 of the drill rod 10 and each root being positioned between adjacent (consecutive) threads relative to the longitudinal axis of the drill rod. It is contemplated that each thread 32 of the plurality of threads can have a crest 36. It is further contemplated that the crest 36 of each respective thread 32 can be spaced radially outwardly from adjacent roots 34 relative to the longitudinal axis 1 1 of the drill rod 10. Thus, it is contemplated that each root 34 can cooperate with adjacent threads 32 to define a respective groove of the threaded portion 30. It is contemplated that the specific geometric configuration of the threaded portion 30 can be modified using methods for manufacturing drill rods further disclosed herein. In exemplary aspects, the plurality of threads 32 of the threaded portion 30 can be a plurality of male threads. During formation of the threads 32 of the threaded portion 30, the threads can extend into at least a portion of transition portion 15. In exemplary aspects, the transition portion 15 can have an outer diameter that varies moving away from the first end portion 14. In these aspects, it is contemplated that the threads 32 can terminate at a location in the transition portion 15 where a minor diameter of the threaded portion 30 (the outer diameter defined by the roots 34 of the threaded portion) is flush with and/or is substantially equal to the outer diameter of the transition portion. It is further contemplated that the outer diameter of the transition portion 15 can gradually increase along at least a portion of the transition portion, moving away from the location where the threaded portion 30 terminates into the transition portion. It is still further contemplated that the transition portion 15 can optionally define a fixed-outer-diameter portion proximate the medial portion 18 of the elongate body 12, thereby providing a low-stress transition from the minor diameter of the threaded portion 30 into the medial portion of the elongate body.

[0030] In another exemplary aspect, and with reference to Figures 2A, 2B, 2D, 4, and 5 A, it is contemplated that the second end portion 16 of the elongate body 12 can define a receptacle 40 configured to receive a drill string component. Optionally, in one aspect, it is contemplated that the receptacle 40 can be configured to receive a threaded portion of a second drill rod (or other drill string component). In this aspect, it is contemplated that the inner surface 22 of the second end portion 16 of the elongate body 12 can define a plurality of threads 42. It is further contemplated that the plurality of threads 42 can be configured for complementary engagement with the threaded portion 30 of the second drill rod. In additional aspects, the inner surface 22 of the second end portion 16 of the elongate body 12 can further define a plurality of roots 44, with each thread 42 extending radially outwardly relative to the longitudinal axis 11 of the drill rod 10 and each root being positioned between adjacent (consecutive) threads relative to the longitudinal axis of the drill rod. It is contemplated that each thread 42 of the plurality of threads can have a crest 46. It is further contemplated that the crest 46 of each respective thread 42 can be spaced radially outwardly from adjacent roots 44 relative to the longitudinal axis 11 of the drill rod 10. Thus, it is contemplated that each root 44 can cooperate with adjacent threads 42 to define a respective groove of the inner surface 22 of the second end portion 16. It is contemplated that the specific geometric configuration of the threads 42 of the inner surface 22 of the second end portion 16 can be modified using methods for manufacturing drill rods further disclosed herein. In exemplary aspects, the plurality of threads 42 of the second end portion 16 can be a plurality of female threads. In exemplary aspects, as shown in Figure 5A, the receptacle 40 can comprise a second transition portion 41 extends from the medial portion 18 of the elongate body 12 toward the second end portion 16. In these aspects, the transition portion 41 can have a variable outer diameter that increases moving away from the medial portion 18 relative to the longitudinal axis 11 of the drill rod 10.

[0031] In a further aspect, it is contemplated that the medial portion 18 and the second end portion 16 of the elongate body 12 can have respective inner diameters defined by the inner surface 22 of the elongate body. In this aspect, it is further contemplated that the inner diameter of at least a portion of the second end portion 16 can be greater than the inner diameter 25 of the medial portion 18. In another aspect, the medial portion 18 and the second end portion 16 of the elongate body 12 can have respective outer diameters defined by the outer surface 20 of the elongate body. In this aspect, it is contemplated that at least a portion of the outer diameter of the second end portion 16 can be greater than the outer diameter 26 of the medial portion 18. The inner and outer diameters of the medial portion 18 are depicted in Figure 5B. In an additional aspect, it is contemplated that the first end portion 14 can have an inner diameter and an outer diameter. In this aspect, it is contemplated that the inner diameter of the first end portion 14 can be substantially equal to the inner diameter 25 of the medial portion 18. It is further contemplated that the outer diameter of the first end portion 14 can substantially correspond to the inner diameter of the second end portion 16. It is still further contemplated that the outer diameter of the first end portion 14 can optionally be substantially the same as the outer diameter of the medial portion 18. Optionally, in exemplary aspects, the inner diameter 25 of the medial portion 18 can range from about 10 mm to about 40 mm, and more preferably from about 15 mm to about 30 mm. Optionally, in further exemplary aspects, the outer diameter 26 of the medial portion (and optionally the outer diameter of the first end portion 14 and the inner diameter of the second end portion 16) can range from about 20 mm to about 100 mm, and more preferably from about 40 mm to about 80 mm. Optionally, in still further exemplary aspects, the outer diameter of the second end portion 16 can range from about 30 mm to about 160 mm.

[0032] In exemplary aspects, at least a portion of the outer surface 20 of the elongate body 12 can be non-carburized. In one aspect, it is contemplated that at least a portion of the outer surface of the first end portion of the elongate body can be non-carburized. Optionally, in this aspect, it is contemplated that substantially the entire outer surface of the first end portion of the elongate body can be non-carburized. In another aspect, it is contemplated that at least a portion of the outer surface of the second end portion of the elongate body can be non-carburized. Optionally, in this aspect, it is contemplated that substantially the entire outer surface of the second end portion of the elongate body can be non-carburized. In a further aspect, it is contemplated that at least a portion of the outer surface of the medial portion of the elongate body can be non-carburized. Optionally, in this aspect, it is contemplated that substantially the entire outer surface of the medial portion of the elongate body can be non-carburized. In one exemplary aspect, it is contemplated that substantially the entire outer surface of the elongate body can be non-carburized. In another exemplary aspect, when the first end portion of the elongate body comprises a threaded portion 30, it is contemplated that at least the roots 34 of the threaded portion can be non-carburized, thereby imparting the roots with decreased hardness and higher load stresses. In this aspect, it is contemplated that the crests 36 of the threaded portion 30 can be carburized (case-hardened), thereby imparting the crests 36 with increased hardness and lower load stresses. During the production of at least partially non-carburized drill rods as disclosed herein, it is

contemplated that selective carburizing can be achieved by using a high-temperature resistant "stop-off paint that is configured to mask the non-carburized portions of the drill rods by preventing exposure of those portions of the drill rods to carbon gas during heat treatment.

[0033] In exemplary aspects, at least a portion of the inner surface 22 of the elongate body 12 can be non-carburized. In one aspect, it is contemplated that at least a portion of the inner surface of the first end portion of the elongate body can be non-carburized. Optionally, in this aspect, it is contemplated that substantially the entire inner surface of the first end portion of the elongate body can be non-carburized. In another aspect, it is contemplated that at least a portion of the inner surface of the second end portion of the elongate body can be non-carburized. Optionally, in this aspect, it is contemplated that substantially the entire inner surface of the second end portion of the elongate body can be non-carburized. In a further aspect, it is contemplated that at least a portion of the inner surface of the medial portion of the elongate body can be non-carburized. Optionally, in this aspect, it is contemplated that substantially the entire inner surface of the medial portion of the elongate body can be non-carburized. In one exemplary aspect, it is contemplated that substantially the entire inner surface of the elongate body can be non-carburized. In another exemplary aspect, when the second end portion of the elongate body comprises a receptacle 40, it is contemplated that at least the roots 44 within the receptacle of the second end portion can be non-carburized, thereby imparting the roots with decreased hardness and higher load stresses. In this aspect, it is contemplated that the crests 46 within the receptacle 40 can be carburized (case-hardened), thereby imparting the crests 46 with increased hardness and lower load stresses. During the production of at least partially non-carburized drill rods as disclosed herein, it is contemplated that selective carburizing can be achieved by using a high- temperature resistant "stop-off paint that is configured to mask the non-carburized portions of the drill rods by preventing exposure of those portions of the drill rods to carbon gas during heat treatment.

[0034] In further aspects, each crest 36 of the plurality of crests of the threaded portion 30 can have a respective hardness. In these aspects, it is contemplated that each root 34 of the plurality of roots can have a respective hardness. It is further contemplated that the hardness of the crest 36 of each thread 32 can be greater than the hardness of the roots 34 adjacent to the thread (on opposing sides of the thread relative to the longitudinal axis 11 of the drill rod 10). In exemplary aspects, the hardness of the crest 36 of each thread 32 can optionally range from about 50 to about 60 on the Rockwell Hardness "C" Scale (HRC). In these aspects, it is contemplated that the hardness of the crest 36 of each thread 32 can optionally be at least about 50 HRC. It is further contemplated that the hardness of the crest 36 of each thread 32 can optionally be at least about 55 HRC. It is still further contemplated that the hardness of the crest 36 of each thread 32 can optionally be at least about 60 HRC. It is still further contemplated that the hardness of the crest 36 of each thread 42 can optionally be about 55 HRC. In further exemplary aspects, the hardness of each root 34 can optionally range from about 35 to about 45 on the Rockwell Hardness "C" Scale (HRC). In these aspects, it is contemplated that the hardness of each root 34 can optionally be less than about 35 HRC. It is further contemplated that the hardness of each root 34 can optionally be less than about 40 HRC. It is still further contemplated that the hardness of each root 34 can optionally be less than about 45 HRC. It is still further contemplated that the hardness of each root 34 can optionally be about 40 HRC. In use, it is contemplated that the hardness of each crest can impart advantageous wear resistance to the thread portion, while the hardness of each root can impart advantageous toughness to the thread portion.

[0035] In still further aspects, each crest 46 of the plurality of crests of the second end portion 16 (optionally, within receptacle 40) can have a respective hardness. In these aspects, it is contemplated that each root 44 of the plurality of roots can have a respective hardness. Optionally, in some aspects, it is contemplated that the hardness of the crest 46 of each thread 42 can be substantially equal to the hardness of the roots 44 adjacent to the thread (on opposing sides of the thread relative to the longitudinal axis 1 1 of the drill rod 10). In these aspects, the hardness of each root 44 (and the hardness of each crest 46) can optionally range from about 35 to about 45 HRC. It is contemplated that the hardness of each root 44 (and the hardness of each crest 46) can optionally be less than about 35 HRC. It is further contemplated that the hardness of each root 44 (and the hardness of each crest 46) can optionally be less than about 40 HRC. It is still further contemplated that the hardness of each root 44 (and the hardness of each crest 46) can optionally be less than about 45 HRC. It is still further contemplated that the hardness of each root 34 (and the hardness of each crest 46) can optionally be about 40 HRC. In exemplary aspects, it is contemplated that the hardness of the crest 46 of the plurality of threads 42 can be less than the hardness of the crest 36 of the plurality of threads 32 by about 5 HRC to about 10 HRC. In use, it is contemplated that, for a given contact pressure and given amount of slide between the mating surfaces of the threads 32, 42, the difference in hardness between the threads can substantially eliminate or reduce adhesion wear (e.g., galling or micro-welding) and promote abrasion wear (e.g., polishing) primarily on the softer of the two mating surfaces. It is further contemplated that, for the threaded joint between threads 32, 42, there is only slide and wear during making or breaking of the joint, with the rate of wear depending on the hardness of the softer surface (i.e., threads 42). In exemplary aspects, it is contemplated that the disclosed hardness range for the threads 42 of the inner surface 22 can provide sufficient toughness to the threads while minimizing notch sensitivity, thereby achieving ideal strength and life characteristics.

[0036] Optionally, in further aspects, it is contemplated that the hardness of the crest 46 of each thread 42 can optionally be greater than the hardness of the roots 44 adjacent to the thread (on opposing sides of the thread relative to the longitudinal axis 11 of the drill rod 10).

[0037] In other exemplary aspects, it is contemplated that at least a portion of the threaded portion 30 of the drill rod 10 can be induction hardened using known induction hardening techniques. For example, it is contemplated that the crest 36 of each thread 32 of the threaded portion 30 can be induction hardened. It is further contemplated that, in exemplary aspects, the roots 34 of the threaded portion 30 are not induction hardened.

[0038] In additional exemplary aspects, at least a portion of the outer surface 20 of the elongate body 12 can optionally be coated with a corrosion-resistant material. In these aspects, the corrosion-resistant material can be chemically bonded to the outer surface 20 of the elongate body 12. In one aspect, it is contemplated that at least a portion of the outer surface of the first end portion of the elongate body can be coated with the corrosion-resistant material. Optionally, in this aspect, it is contemplated that substantially the entire outer surface of the first end portion of the elongate body can be coated with the corrosion-resistant material. In another aspect, it is contemplated that at least a portion of the outer surface of the second end portion of the elongate body can be coated with the corrosion-resistant material. Optionally, in this aspect, it is contemplated that substantially the entire outer surface of the second end portion of the elongate body can be coated with the corrosion- resistant material. In a further aspect, it is contemplated that at least a portion of the outer surface of the medial portion of the elongate body can be coated with the corrosion-resistant material. Optionally, in this aspect, it is contemplated that substantially the entire outer surface of the medial portion of the elongate body can be coated with the corrosion-resistant material. In one exemplary aspect, it is contemplated that substantially the entire outer surface of the elongate body can be coated with the corrosion-resistant material.

[0039] In additional aspects, it is contemplated that at least a portion of the inner surface 22 of the elongate body 12 can optionally be coated with the corrosion-resistant material. In these aspects, the corrosion-resistant material can be chemically bonded to the inner surface 22 of the elongate body. In one aspect, it is contemplated that at least a portion of the inner surface of the first end portion of the elongate body can be coated with the corrosion-resistant material. Optionally, in this aspect, it is contemplated that substantially the entire inner surface of the first end portion of the elongate body can be coated with the corrosion-resistant material. In another aspect, it is contemplated that at least a portion of the inner surface of the second end portion of the elongate body can be coated with the corrosion-resistant material. Optionally, in this aspect, it is contemplated that substantially the entire inner surface of the second end portion of the elongate body can be coated with the corrosion- resistant material. In a further aspect, it is contemplated that at least a portion of the inner surface of the medial portion of the elongate body can be coated with the corrosion-resistant material. Optionally, in this aspect, it is contemplated that substantially the entire inner surface of the medial portion of the elongate body can be coated with the corrosion-resistant material. In one exemplary aspect, it is contemplated that substantially the entire inner surface of the elongate body can be coated with the corrosion-resistant material. In exemplary aspects, at least a portion of the outer surface 20 of the elongate body 12 and at least a portion of the inner surface 22 of the elongate body 12 can be coated with the corrosion-resistant material. In further exemplary aspects, substantially the entire outer surface of the elongate body and substantially the entire inner surface of the elongate body can be coated with the corrosion-resistant material.

[0040] In exemplary aspects, it is contemplated that the corrosion-resistant material can be an Aquence® coating (Henkel Corporation), such as, for example and without limitation, the Aquence® 930 series coating or the Aquence® 830 series coating. It is contemplated that the Aquence® 930 coating, which includes UV protection, can be used in applications where drill rods will be stored outdoors. It is further contemplated that the Aquence® 830 series coating, which does not include UV protection, can be used in applications with less outdoor exposure, such as, for example percussive drilling applications. In further exemplary aspects, it is contemplated that the corrosion-resistant material can be a "self-building" and self- terminating chemical coating, with the coating having a substantially constant thickness that does not "build" or "run" excessively on or in geometric corners, grooves, and/or ridges like traditional paint coatings do. In these aspects, it is contemplated that these characteristics of the corrosion-resistant material are compatible with the tight fits and clearances of conventional drill rod threads.

[0041] In other exemplary aspects, it is contemplated that the corrosion-resistant material can comprise a thin coating comprising nano-scale particles and/or nano-fibers configured to induce super-hydrophobic and oleophobic properties by trapping a layer of air on the surface. An example of such a corrosion-resistant material is the Ultra Ever-Dry™ coating (UltraTech International, Inc.).

[0042] In further exemplary aspects, it is contemplated that the corrosion-resistant material can be provided using a surface preparation process followed by an application of an oil coating or similar rust preventative. In these aspects, it is contemplated that the surface preparation can optionally comprise at least one of an acid or phosphate process, a nitriding process, and a steam oxidizing process that creates micro-voids in the surface. It is further contemplated that the micro-voids in the surface of the drill rod can be receptive to molecular bonds or capillary action with a rust preventative oil coating, which can optionally comprise a base or carrier grease such as calcium sulfonate.

[0043] Methods of forming the disclosed drill rods are also provided. In view of the issues disclosed herein, it is desirable that selected portions of the medial portion of the disclosed drill rods can be strengthened. [0044] Optionally, in exemplary aspects, a method of forming a drill rod can comprise furnace carburizing selected portions of the inner surfaces of the elongate body. Optionally, in these aspects, it is contemplated that substantially the entire axial length of the inner and outer surfaces of the elongate body can be furnace carburized.

[0045] In other exemplary aspects, the method of forming a drill rod can optionally comprise induction hardening a selected portion of an outer surface of the drill rod, such as, for example and without limitation, the threaded portion of the drill rod and/or the medial portion of the drill rod. In these aspects, the step of induction hardening can be performed using a high-frequency induction system, such as, for example and without limitation, a Tocco #OL-204 generator. It is contemplated that the high-frequency induction system can be configured to output power of up to at least 300 kW at frequencies of up to about 10 kHz. In operation, it is contemplated that the high-frequency induction system can be configured to apply power to an induction coil positioned circumferentially around the selected portion of the outer surface of the drill rod. It is further contemplated that the high-frequency induction system can be configured to generate heat by reversing a magnetic field induced by the high frequency of the alternating current within the coil. It is still further contemplated that, following heating of the selected portion of the outer surface, the selected portion can be rapidly quenched by water and a selected polymer to harden the steel of the drill rod.

[0046] Optionally, in further exemplary aspects, it is contemplated that the method of forming a drill rod can comprise induction thru- wall hardening at least a portion of the elongate body of the drill rod. In still further exemplary aspects, the method of forming the drill rod can optionally comprise quench-and-temper hardening at least a portion of the outer surface of the elongate body of the drill rod using a furnace. In these aspects, it is contemplated that the method of forming the drill rod can comprise quench-and-temper hardening substantially the entire axial length of the outer surface of the elongate body of the drill rod.

[0047] Regardless of the method utilized to increase mid-body strength in the disclosed drill rods, it is contemplated that fatigue failures conventionally caused by a hardened case in the thread roots can be avoided using various methods, such as, for example, (a) machining the thread after carburizing ("hard turning"), (b) selectively applying a carbon-impervious "stop-off coating, or (c) friction-welding threaded ends onto a pre-treated medial portion of a drill rod. [0048] In exemplary aspects, the method can comprise defining a plurality of threads in the selected portion of the outer surface of the drill rod using a carburizing (hard turning) process. It is contemplated that the hard turning process can optionally comprise heat treating at least the selected portion of the outer surface of the drill rod using conventional methods. It is further contemplated that the hard turning process can comprise rotating the drill rod about the longitudinal axis of the drill rod. It is still further contemplated that the hard turning process can comprise advancing a work piece at a selected orientation to the longitudinal axis during rotation of the drill rod to define the plurality of threads in the selected portion of the outer surface of the drill rod. In exemplary aspects, it is contemplated that the excessive heat generated during the hard turning process can require the use of conventional ceramic cutting "inserts" in place of conventional carbide or "high-speed-steel" coated inserts, which are often used for medium-low hardness steels. In these aspects, it is contemplated that the ceramic cutting "inserts" can provide improved wear life and cutting performance compared to the conventional carbide or "high-speed-steel" coated inserts.

[0049] In further aspects, it is contemplated that the geometry of the threads of the outer and/or inner surfaces of the elongate body of the drill rods can be selectively defined during the hard turning process by selectively adjusting the case hardening depth to be less than the depth of the machined thread. In these aspects, it is contemplated that the currents induced during the hard turning process can comprise eddy current effects that collect in the geometric features defined in the surfaces of the drill rod depending upon how the induction coil passes over the corresponding portion of the drill rod. For example, for a non-symmetric coring thread profile, it is contemplated that the case pattern can be deeper on the load flank than the stab flank to ensure that the case pattern completely covers the crest of the thread.

[0050] In exemplary aspects, it is contemplated that the hardness of the crest of the male threads on the outer surface of the drill rod can be maximized through a conventional case hardening method, and the female threads of the inner surface of the drill rod can be formed to have a hardness less than the hardness of the crest of the male threads. Optionally, in these aspects, it is contemplated that the female threads of the inner surface of the drill rod can be hardened using a case hardening method and then tempered back to achieve the desired hardness. Alternatively, it is contemplated that the desired hardness of the female threads can be achieved using a thru-wall hardening method as is known in the art. It is contemplated that it can be advantageous to avoid performance of a difficult secondary female thread case hardening operation and to instead elevate the hardness of the core (parent) material and to machine the female thread into that core material.

[0051] Optionally, it is contemplated that the method of forming the drill rod can comprise coating at least a portion of at least one of the outer surface and the inner surface of the drill rod with the corrosion-resistant material.

Exemplary Aspects

[0052] In one exemplary aspect, a drill rod having a longitudinal axis is provided. The drill rod can comprise an elongate body having a first end portion, a second end portion, a medial portion positioned between the first and second end portions, an outer surface, and an inner surface. The inner surface can define a central bore of the elongate body. At least a portion of the outer surface of the elongate body can be non-carburized. At least a portion of the outer surface of the elongate body can be coated with a corrosion-resistant material.

[0053] In another exemplary aspect, the corrosion-resistant material can be chemically bonded to the outer surface of the elongate body.

[0054] In another exemplary aspect, the outer surface of the first end portion of the elongate body can define a threaded portion, the threaded portion comprising a plurality of threads and a plurality of roots, each thread having a crest, each root being positioned between adjacent threads relative to the longitudinal axis of the drill rod, the crest of each thread being spaced radially outwardly from adjacent roots relative to the longitudinal axis of the drill rod.

[0055] In another exemplary aspect, each crest of the plurality of crests can have a respective hardness, each root of the plurality of roots can have a respective hardness, and the hardness of the crest of each thread can be greater than the hardness of the roots adjacent to the thread.

[0056] In another exemplary aspect, the hardness of the crest of each thread can range from about 50 to about 60 HRC, and the hardness of each root can range from about 35 to about 45 HRC.

[0057] In another exemplary aspect, each root of the plurality of roots can be non- carburized.

[0058] In another exemplary aspect, the crest of each thread of the threaded portion can be induction hardened. [0059] In another exemplary aspect, the second end portion of the elongate body can define a receptacle configured to receive a threaded portion of a second drill rod.

[0060] In another exemplary aspect, the inner surface of the second end portion of the elongate body can define a plurality of threads.

[0061] In another exemplary aspect, the medial portion and the second end portion of the elongate body can have respective inner diameters, and the inner diameter of the second end portion can be greater than the inner diameter of the medial portion.

[0062] In another exemplary aspect, the first end portion can have an inner diameter and an outer diameter, the inner diameter of the first end portion can be substantially equal to the inner diameter of the medial portion, and the outer diameter of the first end portion can substantially correspond to the inner diameter of the second end portion.

[0063] In another exemplary aspect, the medial portion and the second end portion of the elongate body can have respective outer diameters, and the outer diameter of the second end portion can be greater than the outer diameter of the medial portion.

[0064] In another exemplary aspect, at least a portion of the inner surface can be coated with the corrosion-resistant material.

[0065] In another exemplary aspect, the corrosion-resistant material can be chemically bonded to the inner surface of the elongate body.

[0066] In one exemplary aspect, a drill rod having a longitudinal axis is provided. The drill rod can comprise an elongate body having a first end portion, a second end portion, a medial portion positioned between the first and second end portions, an outer surface, and an inner surface. The inner surface can define a central bore of the elongate body. At least a portion of the outer surface of the elongate body can be non-carburized. At least a portion of the outer surface of the elongate body can be coated with a corrosion-resistant material. The corrosion-resistant material can be chemically bonded to the outer surface of the elongate body. The outer surface of the first end portion of the elongate body can define a threaded portion, the threaded portion comprising a plurality of threads and a plurality of roots, each thread having a crest, each root being positioned between adjacent threads relative to the longitudinal axis of the drill rod, the crest of each thread being spaced radially outwardly from adjacent roots relative to the longitudinal axis of the drill rod. Each crest of the plurality of crests can have a respective hardness, each root of the plurality of roots can have a respective hardness, and the hardness of the crest of each thread can be greater than the hardness of the roots adjacent to the thread.

[0067] In another exemplary aspect, the hardness of the crest of each thread can range from about 50 to about 60 HRC, and the hardness of each root can range from about 35 to about 45 HRC.

[0068] In another exemplary aspect, each root of the plurality of roots can be non- carburized.

[0069] In another exemplary aspect, the crest of each thread of the threaded portion can be induction hardened.

[0070] In another exemplary aspect, the second end portion of the elongate body can define a receptacle configured to receive a threaded portion of a second drill rod.

[0071] In another exemplary aspect, the inner surface of the second end portion of the elongate body can define a plurality of threads.

[0072] In another exemplary aspect, the medial portion and the second end portion of the elongate body can have respective inner diameters, and the inner diameter of the second end portion can be greater than the inner diameter of the medial portion.

[0073] In another exemplary aspect, the first end portion can have an inner diameter and an outer diameter, the inner diameter of the first end portion can be substantially equal to the inner diameter of the medial portion, and the outer diameter of the first end portion can substantially correspond to the inner diameter of the second end portion.

[0074] In another exemplary aspect, the medial portion and the second end portion of the elongate body can have respective outer diameters, and the outer diameter of the second end portion can be greater than the outer diameter of the medial portion.

[0075] In another exemplary aspect, at least a portion of the inner surface can be coated with the corrosion-resistant material.

[0076] In another exemplary aspect, the corrosion-resistant material can be chemically bonded to the inner surface of the elongate body.

[0077] In one exemplary aspect, a drilling system can be provided in which a drill rod as disclosed herein can be operatively coupled with a drill head. [0078] In one exemplary aspect, a method is provided for forming a drill rod as disclosed herein.

[0079] In one exemplary aspect, a method for forming a drill rod can comprise induction hardening a selected portion of an outer surface of the drill rod. The method can further comprise defining a plurality of threads in the selected portion of the outer surface of the drill rod using a hard turning process.

[0080] Although several embodiments of the invention have been disclosed in the foregoing specification, it is understood by those skilled in the art that many modifications and other embodiments of the invention will come to mind to which the invention pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the invention is not limited to the specific embodiments disclosed hereinabove, and that many modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although specific terms are employed herein, as well as in the claims which follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described invention, nor the claims which follow.