A drill rod and a method of producing such a drill rod

Abstract

The disclosure relates to a drill rod (10) comprising an internal opening (15) along its entire length, which internal opening (15) is configured to convey a fluid though the drill rod (10) The drill rod (10) comprises a pipe section (11), with a cold-worked internal surface (14) that radially defines the internal opening (15) through the pipe section (11). The disclosure also relates to a method of producing such a drill rod (10)

Description

[0001] A DRILL ROD AND A METHOD OF PRODUCING SUCH A DRILL ROD

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a drill rod and to a method of producing such a drill rod. Specifically, the disclosure relates to a drill rod with an internal fluid channel and with a cold-worked internal surface defining this fluid channel.

[0004] BACKGROUND

[0005] In the technical field of underground percussive rock drilling, water is used as a flushing medium inside a steel drill string comprised of at least one steel drill rod. The flushing medium provides efficient cleaning of the drill hole by transporting away the produced drill chipping, while maintaining a dust free environment. In some applications, such as drifter drilling, the drill string is comprised of a single drill rod, whilst in other applications the drill string is comprised of a plurality of interconnected drill rods.

[0006] Water in contact with the inside of the drill rod will cause corrosion, which will strongly reduce the usable life of the drill rod.

[0007] There are currently no cost-efficient corrosion protective coatings available that hold over time, surviving the expected lifetime of the drill rod. Up until now the method of achieving an acceptable usable lifetime of drill rods has been dependent on the use of case carburized products in which a surface layer of the drill rods will obtain an elevated carbon content. For drill rods it is particularly the inside surface that is the subject for such case carburization.

[0008] Case carburization of the drill rod provides for an improved steel composition to withstand the continuous contact with water in combination with intermittently high level of alternating stress, both tensile and compressive, which are caused by the incident compressive stress waves delivered by the hammer piston and the resulting tensile and compressive stress waves created as said incident waves are reflected from cross sectional area transitions in the drill rod and at the respective ends of the drill rod. . Specifically, case carburization provides a residual compressive stress at the internal surface of the drill rod which may extend up to at least 0.5 mm into the material. This surface region with an inherent residual stress is useful as it prevents the propagation of cracks caused e.g., by fatigue. However, case carburization of long products such as drill rods does not only produce desired residual stresses on the internal surface but also produces stresses that will almost inevitably cause the drill rods to bend such that a straightening process will be needed.

[0009] Further, the case carburization is a somewhat unpredictable process in that the achieved hardening may not always be sufficient. In such eventualities, complete batches of case carburized rods need to be re-hardened. In addition to the time, energy, and cost consumption, the re-hardening process may also lead to distortion of the thread dimensions such that drill rods may not be fit to use.

[0010] It would be advantageous to achieve a method to produce drill rods overcoming, or at least alleviating, one or some of the drawbacks of the prior art.

[0011] SUMMARY

[0012] It is an object of the disclosure to provide drill rods produced in a reliable and costefficient manner without compromising their quality.

[0013] According to a first aspect, the disclosure relates to a drill rod comprising an internal opening along the length of the drill rod, which internal opening is configured to convey a fluid though the drill rod, wherein the drill rod comprises a pipe section of a length of at least half a metre, said pipe section having a cold-worked internal surface that encloses the internal opening of the pipe section.

[0014] With the inventive drill rod the production of it will be more reliable and cost-efficient. Furthermore, the inventive drill rod may be made of steel of a lower initial cost than conventional drill rods in view of that the steel need not be composed to withstand a case carburization process. For example, a steel material containing a lower portion of alloying elements, such as nickel, chromium and molybdenum may be used.

[0015] The cold worked surface will typically withstand the continuous contact with water in combination with the intermittently high level of alternating stress, both tensile and residual, which originate from the percussive stress wave during operation and reflections thereof. Specifically, the cold working will provide a residual stress at the internal surface of the drill rod which extends into the material. This surface region with inherent residual stress is useful as it prevents the propagation of cracks caused, e.g., by fatigue without the use of a case carburization process. In embodiments of the drill rod, the pipe section is formed of a non-carburized steel, preferably a through hardened steel.

[0016] In embodiments of the drill rod, the pipe section has compressive residual stresses of at least 200 MPa at depths from 50 to 300 pm, preferably from 20 to 500 pm into the pipe section from the internal surface thereof.

[0017] With such compressive residual stresses, the drill rod will have a good resistance to fatigue, such that a satisfactory useful life of the drill rod may be achieved.

[0018] Further, the pipe section may advantageously have compressive residual stresses in the axial direction of the drill pipe of at least 650 MPa at depths from 20 to 30 pm, preferably from 20 to 100 pm into the pipe section from the internal surface thereof.

[0019] With these properties the compressive residual stress in the pipe section will extend into a sufficient depth inside of the internal surface to allow the pipe section to withstand important high operational stress levels, especially in the axial direction of the drill rod.

[0020] In embodiments of the drill rod, the drill rod comprises a first threaded portion at a first end of the drill rod and a second threaded portion at a second end of the drill rod, opposite the first end, the first threaded portion comprising an internal thread and the second threaded portion comprising an external thread.

[0021] This is a useful manner of producing a drill rod, making it possible to attach the drill rod to other drill rods so as to provide a drill string of desired length and to attach the outermost part of the outermost drill rod to a drilling equipment, such as a drill bit.

[0022] In embodiments of the drill rod, the first threaded portion is made of a material with a different composition than that of the pipe section and is welded to the pipe section at a weld section.

[0023] This allows the main part of the drill rod to be produced separately from the threaded portion with an internal thread such that each part may be produced with the most optimal properties.

[0024] For example, the first threaded portion may be comprised of a carburized steel, so as to obtain the desired residual stresses in this part as well, in view of that it may not be desirable to cold work the threaded internal surface of the first threaded portion. In embodiments of the drill rod, the weld section has an inner surface, which is also cold worked.

[0025] Cold working of the inner surface of the weld section provides both a smoot inner surface for optimal conveyance inside the drill rod and also produces useful residual compressive stresses in the weld section.

[0026] When hardening a surface with cold working it is possible to limit the hardening process to only the portions and surfaces of the drill rod that will benefit from the process and to exclude parts and surfaces, such as threaded parts, in which such a process is not desired. In specific embodiments it may, however, be desired to cold work all surfaces, including inner and outer threads.

[0027] In embodiments of the drill rod, the second threaded portion is an integrated portion of the pipe section of the drill rod, the cold-worked internal surface extending into the second threaded portion, without affecting the external thread.

[0028] In embodiments of the drill rod, at least one of the first threaded portions and the second threaded portion is induction hardened.

[0029] This is another advantageous manner of hardening a steel that makes it possible to limit the hardening to specific portions, typically portions that may be difficult to harden in other ways.

[0030] In embodiments of the drill rod, the pipe section of the drill rod has a length of at least one metre, preferably at least two metres.

[0031] In embodiments of the drill rod, the drill rod is a mining drill rod configured to be connected to a rock drilling machine and to convey water in its internal opening.

[0032] Specifically, the drill rod is well adapted to withstand high stresses in combination with corrosive water contact such that it may advantageously be used as a mining drill rod.

[0033] In embodiments of the drill rod, the internal surface of the pipe section is cold worked by means of shot peening.

[0034] The shot peening has proven to be very advantageous way of obtaining the desired properties in the material. According to a second aspect the disclosure relates to a method of producing a drill rod as described above, which method comprises a step of cold working the internal surface of the pipe section of the drill rod by means of shot peening.

[0035] Specifically, the shot peening may be performed by entering a nozzle into the pipe section of the drill rod, the nozzle forming part of a shot peening tool for projecting shots and being configured to redirect the shots to an angle of between 25° and 55° with respect to an axial extension of the pipe section.

[0036] The use of such a nozzle offers an advantageous manner of obtaining the desired properties in desired portions of the drill rod.

[0037] Other embodiments of the disclosure according to the three aspects and advantages thereof will be apparent from the detailed description and the appended drawings.

[0038] BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Below, specific embodiments of the disclosure will be described with reference to the appended drawings, of which:

[0040] Fig. 1 is a very schematic drawing of a drill rod according to an embodiment of the disclosure,

[0041] Fig. 2 is a perspective view of a nozzle configured to redirect shots towards the internal surface of drill rod,

[0042] Fig. 3 is a front view of the nozzle in Fig 2,

[0043] Fig. 4 is a longitudinal sectional view of the nozzle in Fig 2,

[0044] Fig. 5 shows a diagram illustrating residual stresses in various conventional steel rods as a function of the depth from the surface, and

[0045] Fig. 6 shows a diagram illustrating the residual stress at different positions of a drill rod according to an embodiment of the disclosure as a function of the depth from the cold-worked surface.

[0046] DETAILED DESCRIPTION

[0047] In Fig. 1, a drill rod according to an exemplary embodiment of the disclosure is illustrated in a very schematic manner. The drill rod 10 comprises an internal opening 15 along its complete length in the axial direction A of its extension. The internal opening 15 is configured to convey a fluid though the drill rod 10. Preferably, the internal opening 15 is configured to convey water during a drilling operation. Namely, the drill rod 10 is typically a mining drill rod configured for conveying water in its internal opening 15.

[0048] In an embodiment of the disclosed drill rod, the drill rod is used in underground percussive rock drilling, where water is used as a flushing medium inside the drill string. In some applications, such as drifter drilling, the drill string is comprised of a single drill rod, whilst in other embodiments the drill string may be comprised of a plurality interconnected drill rods.

[0049] The drill rod 10 may be used both at surface drilling and in underground drilling, although it is especially advantageous in underground drilling applications where water is used to as cleaning fluid and is conveyed inside the drill rod.

[0050] The drill rod 10 comprises a pipe section 11 with a cold-worked internal surface 14 that encloses the internal opening 15 of the pipe section 11. The cold working process creates useful residual stresses as will be described further below. Typically, the internal surface 14 of the pipe section 11 is cold worked by means of shot peening.

[0051] The pipe section 11 of the illustrated drill rod 10 may be formed of a non-carburized steel, such as through hardened steel. The pipe section 11 may for example be produced from a low carbon steel, with C below 0,50 weight-%, Cr below 5,0 weight-% and Ni less than 5,0 weight-%. For example, the steel may be a 24CrMo13-6 steel, which is a through-hardened alloy steel with good wear resistance in the as-rolled condition.

[0052] The drill rod 10 comprises a first threaded portion 12 at a first end 20 of the drill rod 10 and a second threaded portion 18 at a second end 21 of the drill rod 10, opposite the first end 20, the first threaded portion 12 comprising an internal thread 13 and the second threaded portion 18 comprising an external thread 19.

[0053] Specifically, the internal thread 13 of the first threaded portion 12 is configured to mate with the external thread 19 of the second threaded portion 18, such that the drill rod 10 may be attached by threading to at least one other similar drill rod to provide a drill string comprised of a plurality interconnected drill rods as is conventional in the art. Further, of course, the drill rod may be attached to various types of drilling equipment, such as for example drill bits, shank adapters, couplings and other drilling equipment.

[0054] In the shown embodiment, the first threaded portion 12 is made of a material with a different composition than that of the pipe section 11 and is welded to the pipe section 11 at a weld section 16. Specifically, the first threaded portion 12 may by comprised of a carburized steel, such as 22NiCrMo12-5, which is a group of carburized steel with high hardenability.

[0055] The first threaded portion 12 is typically about 20 centimetres long and is therefore of a length short enough to withstand a case carburization process without problems of bending. Hence, normally no after-process - or a very limited one - will be needed after subjecting a piece such as the first threaded portion 12 to a case carburization process. As an alternative, the first threaded portion 12 may be induction hardened.

[0056] Typically, the welding between the first threaded portion 12 and the pipe section 11 is performed by friction welding the first threaded portion 12 to the pipe section 11, wherein the material in the weld section 16 will have the same composition as the materials in first threaded portion 12 and the pipe section 11 , or, if these materials have different compositions, the composition in the weld section 16 will gradually vary from one side of the weld section 16 to the other. At the weld section 16 a weld seam will appear, on both the inside and the outside surface of the drill rod 10.

[0057] The inner surface 17 of the weld section 16 should preferably be processed to smoothen the weld seam and provide a smooth and fatigue resistant inner surface. To trim the inner surface 17 and remove protruding parts of the weld seam, the inner surface 17 is preferably cold worked in a similar way as the internal surface 14 of the pipe section 11, specifically by shot peening. As will be illustrated below with reference to Fig. 6, this cold working will create a residual compressive stress in the weld section in a similar way as in the internal surface 14 of the pipe section 11.

[0058] In the shown embodiment, the second threaded portion 18 is an integrated portion of the pipe section 11 of the drill rod 10, such that the cold-worked internal surface 14 of the pipe section 11 extends into the second threaded portion 18.

[0059] The pipe section 11 of the drill rod 10 has a length of at least half a metre. The drill rod 10 typically has a length of 0.9 - 6.4 metres depending on the application, of which the pipe section 11 constitutes the major part, specifically about 0.7 - 6.2 metres. In Fig. 2 a nozzle 31 of a shot peening tool is illustrated in a perspective view and in Figs. 3 and 4 the nozzle is shown in front view and longitudinal sectional view, respectively. The shot peening tool (not shown) is arranged to project shots for shot peening and the nozzle 31 is configured to redirect the projected shots to an angle a of between 25° and 55° with respect to the axial extension A of the pipe section 11. To minimize the risk of a recoiling effect the angle a should not be too high, and to make sure that the impact energy against the internal surface 14 is sufficiently high to achieve the desired creation of residual stresses beneath said internal surface 14, it should not be too low. A person skilled in the art will be able to find a configuration that will meet the demanded criteria of generated residual compressive stress.

[0060] The nozzle includes an inlet channel 32 which is arranged to be connected to an outlet end of the shot peening tool so as to receive the emitted shots. Further, the nozzle comprises an angled outlet 33 arranged to deflect the shots at an angle a with respect to the axial extension A of the inlet channel 32. The nozzle 31 may advantageously include a circumferential rim 34 arranged to fit tightly inside the drill rod and make sure that the shots will not recoil backwards inside the drill rod.

[0061] Specifically, a 1.6 mm steel cut wire of a hardness of HV640 may be used. A usable size range of the shots may be from about 1 mm to about 2 mm. In conventional shot peening, 0.8 mm steel cut wire is often used. In addition to the relatively bigger size of the steel cut wire, the velocity at which they are delivered is also decisive for the result. Tests have indicated that a velocity of about 75 - 125 m / s is suitable for obtaining residual compressive forces in the specimens into a desired depth beneath the shot peened surface.

[0062] To achieve these desired velocities, the air pressure for accelerating the shots will need to be higher than the conventional 3 - 6 bars, typically about 7 - 10 bars

[0063] In a method of producing the drill rod 10, shot peening is performed on the internal surface 14 of the pipe section 11. The shot peening is performed by entering the nozzle 31 into the pipe section 11 of the drill rod 10, from either side of the drill rod 10, and continuously or intermittently advancing and rotating the nozzle 31 so as to shot peen all desired parts of the internal surface 14 of the pipe section 11 and, if desired, the internal thread 13 of the first threaded portion 12. Fig. 5, shows a diagram of measured residual stresses in MPa in different conventional specimens as a function of the distance in millimetres from the surface of the specimens.

[0064] The residual stresses have been measured in the axial direction of the specimens, which is the most important stress direction for a drill rod. The residual stresses may also be measured in other directions, for example in the radial and circumferential direction, or in directions for example extending 45 degrees with respect to the axial direction. The residual stresses in these directions will however be similar in magnitude to the ones in the axial direction, therefore the representation of the axial stresses is sufficient to illustrate the advantageous qualities of the disclosure.

[0065] It should be noted that in the diagrams a compressive stress is illustrated as a negative residual stress.

[0066] The first curve (A) illustrates the residual stress in a state-of-the-art shot peened low alloyed steel as a function of the distance from its surface, at a position close to a friction weld.

[0067] The second curve (B) illustrates the same relation for a shot peened low alloyed steel at a position unaffected by friction welding.

[0068] The third curve (C) illustrates the same relation for a case-hardened steel, close to a friction weld.

[0069] The fourth curve (D) illustrates the same relation for a case-hardened steel at a position unaffected by friction welding.

[0070] The fifth curve (E) illustrates the same relation for a low alloyed steel at a position unaffected by friction welding.

[0071] As a comparison, in Fig. 6 the residual stress in a steel rod in accordance with an embodiment of the disclosure as a function of the depth from the shot peened surface is shown. The steel is a low alloyed steel, specifically a 24CrMo13-6 steel.

[0072] The first curve (i) in Fig. 6 shows the residual stress as a function of the depth from the shot peened surface of the low alloyed steel at 40 mm from a friction weld.

[0073] The second curve (ii) shows the same relation in the same steel at 30 mm from the friction weld. The third curve (iii) in Fig. 6 shows the same relation in the same steel at a position unaffected by the friction weld.

[0074] As is apparent, the steel rod in accordance with the specific embodiment of the disclosure as illustrated in Fig. 6 has higher compressive stresses, i.e. negative residual stresses, than the conventional steel rods illustrated in Fig. 5 already at a small depth from the surface and inwards. Further, as is apparent from first curve (i) and the second curve (ii) in Fig. 6, the effect of the residual stress is clearly visible also at positions close to the friction weld.

[0075] In the diagrams, a compressive stress is illustrated as a negative residual stress.

[0076] The stresses are preferably measured using X-ray diffraction (XRD). XRD is a surface sensitive analysis which only penetrates the surface to a depth of <20 pm (depending on the x-ray source). Therefore, electrolytic polishing is used for material removal to measure the residual stresses at depths up to 1 mm. In Figs. 5 and 6 the illustrated stresses are measured in the axial direction of the rods, but as mentioned above the residual stresses in other directions will, at least roughly, be in the same magnitude.

[0077] After shot peening, the residual compressive stress reaches its maximum value at approximately 0.1 - 0.2 mm depth from the surface, which then gradually decreases with an increasing depth.

[0078] The inner surface of the pipe section 11 has a residual stress profile, as measured from the surface and into the material, which profile shows a compressive stress of at least 100 MPa at the very surface, from there between 200 - 500 MPa dependent of the depth, and a peak compressive stress of at least 500 MPa, but possibly up to 800 - 1050 MPa further into the material.

[0079] The stress peak can be situated at a depth of 20 - 100 pm, and compressive stress levels of about 500 MPa may extend to a depth of at least 300 pm from the surface.

[0080] Specifically, the pipe section of the drill rod in accordance with an embodiment of the disclosure has compressive residual stresses F of at least 200 MPa at depths from 50 to 300 pm, preferably from 20 to 500 pm into the pipe section from the internal surface thereof. The above relationship is also valid for positions close to the weld section, i.e. at positions between 30 and 40 mm from the interface of the weld section, as is apparent from first and second curves (i) and (ii), respectively, in Fig. 6. Further, the pipe section of the drill rod has compressive residual stresses F of at least 650 MPa at depths from 20 to 30 pm, preferably from 20 to 100 pm into the pipe section 11 from the internal surface 14 thereof, for the steel unaffected by the friction weld shown in in curve (iii), and at positions between 30 and 40 mm from the interface of the weld section as is apparent from first and second curves (i) and (ii), respectively, in Fig. 6.

[0081] In the illustrated example the steel unaffected by the friction weld shown in in curve (iii) has compressive residual stresses F of at least 800 MPa at depths from 20 to 200 pm.

[0082] For depths in the range of 100 - 500 pm into the pipe section 11 from its internal surface 14, the pipe section 11 has compressive residual stresses F that is dependent of said depth D in accordance with the following formula:

[0083] F < 715 - D where

[0084] F is the residual stresses in MPa, and

[0085] D is the depth in pm.

[0086] It should be noted that, at least for depths in the range of 200 - 500 pm into the pipe section 11 from its internal surface 14, the above relationship is also valid for positions close to the weld section, i.e. at positions between 30 and 40 mm from the interface of the weld section.

[0087] These high compressive residual stress levels are achieved by means of a process where shots of a size of about 1.6 mm are projected towards the surface at a speed of about 75-125 m / s.

[0088] Above, the disclosure has been described with reference to specific embodiments. The disclosure is however not limited to these embodiments. It is obvious to a person skilled in the art that other embodiments are possible within the scope of the following claims.

Claims

CLAIMS1. A drill rod (10) comprising an internal opening (15) along the length of the drill rod, which internal opening (15) is configured to convey a fluid though the drill rod(10), wherein the drill rod (10) comprises a pipe section (11) of a length of at least half a metre, said pipe section (11) having a cold-worked internal surface (14) that encloses the internal opening (15) of the pipe section (11).

2. The drill rod (10) according to claim 1 , wherein the pipe section (11) has compressive residual stresses (F) of at least 200 MPa at depths from 50 to 300 pm, preferably from 20 to 500 pm into the pipe section (11) from the internal surface (14) thereof.

3. The drill rod (10) according to claim 1 or 2, wherein the pipe section (11) has compressive residual stresses (F) in the axial direction of the drill pipe of at least 650 MPa at depths from 20 to 30 pm, preferably from 20 to 100 pm into the pipe section(11) from the internal surface (14) thereof.

4. The drill rod (10) according to any one of claims 1 - 3, wherein, for depths in the range of 100 - 500 pm into the pipe section (11) from the internal surface (14) of the pipe section (11), the pipe section (11) has compressive residual stresses (F) that is dependent of said depth in accordance with the following formula:F > 715 - D whereF is the compressive residual stress in MPa, and D is the depth in pm.

5. The drill rod (10) according to any one of the preceding claims, wherein the pipe section (11) is formed of a non-carburized steel.

6. The drill rod (10) according to any one of the preceding claims, wherein the drill rod (10) comprises a first threaded portion (12) at a first end (20) of the drill rod (10) and a second threaded portion (18) at a second end (21) of the drill rod (10), opposite the first end (20), the first threaded portion (12) comprising an internal thread (13) and the second threaded portion (18) comprising an external thread (19).7 The drill rod (10) according to claim 6, wherein the first threaded portion (12) is made of a material with a different composition than that of the pipe section (11) and is welded to the pipe section (11) at a weld section (16).

8. The drill rod (10) according to claim 7, wherein the first threaded portion (12) is comprised of a carburized steel.

9. The drill rod (10) according to the claims 7 or 8, wherein the weld section (16) has an inner surface (17), which inner surface (17) is also cold worked.

10. The drill rod (10) according to 9, wherein, at a position between 30 - 40 mm from the weld section (16) and for depths in the range of 200 - 500 pm into the pipe section (11) from the internal surface (14) of the pipe section (11), the pipe section (11) has compressive residual stresses (F) that is dependent of said depth in accordance with the following formula:F > 715 - D whereF is the compressive residual stress in MPa, and D is the depth in pm.

11. The drill rod (10) according to any one of the claims 6 - 10, wherein the second threaded portion (18) is an integrated portion of the pipe section (11) of the drill rod, the cold-worked internal surface (14) extending into the second threaded portion (18).

12. The drill rod (10) according to any one of the claims 6 - 11 , wherein at least one of the first threaded portion (12) and the second threaded portion (18) is induction hardened.

13. The drill rod (10) according to any one of the preceding claims, wherein the pipe section of the drill rod (10) has a length of at least one metre, preferably at least two metres.

14. The drill rod (10) according to any one of the preceding claims, wherein the drill rod (10) is a mining drill rod configured to be connected to rock drilling machine and to convey water in its internal opening (15).

15. The drill rod (10) according to any one of the preceding claims, wherein the internal surface (14) of the pipe section (11) is cold worked by means of shot peening.

16. A method of producing a drill rod (10) according to any one of the preceding claims, wherein the method comprises a step of cold working the internal surface (14) of the pipe section (11) of the drill rod (10) by means of shot peening.

17. The method according to claim 16, wherein the shot peening is performed by entering a nozzle (31) into the pipe section (11) of the drill rod (10), the nozzle (31) forming part of a shot peening tool for projecting shots and being configured to redirect the shots to an angle (a) of between 25° and 55° with respect to an axial extension of the pipe section (11).

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