THREAD JOINT AND ROCK DRILL ELEMENT

Abstract: The present invention relates to a thread joint of steel for percussive drilling wherein flushing water is used, comprising at least one substantially cylindrical external thread as well as a substantially cylindrical internal thread (12). The external thread is provided on a spigot (13) intended to constitute an integral part of a first drill string component. The threads (12) comprise thread flanks (16, 17; 18, 19) and thread roots (20; 21) provided between the flanks. The thread roots (20) of the cylindrical external thread are provided substantially distant from associated crests (22) of the cylindrical internal thread (12). By coating the thread roots (20) of the cylindrical external thread with at least one layer of a material of lower electrode potential than the steel, said layer being made from metals like aluminum, zinc and magnesium or alloys of these, an increased life length for the threaded connection is obtained.

Description: THREAD JOINT AND ROCKDRILL ELEMENT Background of the invention The present invention relates to a thread joint and a drill element for rock drilling in accordance with the preambles of the appended independent claims. Prior art During percussive rock drilling the drill elements, i.e. bits, rods, tubes, sleeves and shanks adapters, are subjected to corrosive attack. This applies in particular to underground drilling where water is used as flushing medium and where the environment is humid. The corrosive attacks are particularly serious in the most stressed parts, i.e. thread bottoms and thread clearances. In combination with pulsating stress, caused by shock waves and bending loads, so-called corrosion fatigue arises. This is a common cause for failure of the drill element. Today low-alloyed, case hardened steels are normally used in the drill elements. The reason for this is that abrasion and wear of the thread parts have generally been limiting for life lengths. As the drill machines and the drill elements have become more efficient, these problems have however diminished and corrosion fatigue has become a limiting factor. The case hardening gives compressive stresses in the surface, which gives certain effects against the mechanical part of the fatigue. The resistance to corrosion of a low- alloyed steel is, however, poor and for that reason corrosion fatigue still happens easily. In US-A-4,872,515 or 5,064,004 a drill rod is shown wherein a threaded portion is provided with a metallic material, which is softer than the steel of the drill element. Thereby is intended to solve the problem of pitting in the threads by covering at least the parts of the thread of the drill element that cooperate with other parts of the threaded connection. Objects of the invention One object of the present invention is to substantially improve the resistance against corrosion fatigue of a drill element for percussive rock drilling. Another object of the present invention is to substantially improve the resistance against corrosion fatigue in sections of reduced cross-section in a drill element for percussive rock drilling. Still another object of the present invention is to substantially improve the resistance against corrosion fatigue in the roots of the thread in a threaded portion in a drill element for percussive rock drilling. Brief description of the drawings These and the other objects have been achieved by means of a thread joint and a drill element which have obtained the features in accordance with the characterizing portions of the appended independent claims with reference to the drawings. Fig. 1 shows a drill element according to the present invention in a side view, partly in cross-section. Fig. 2 shows one end of the drill element in a side view. Fig. 3 shows an axial cross- section of the end. Fig. 4 shows an axial cross-section of an embodiment of a thread joint according to the present invention. Fig. 5 shows an axial cross-section of an alternative embodiment of a thread joint according to the present invention. Fig. 6 shows an axial cross-section of an alternative embodiment of a drill element according to the present invention. Detailed description of the invention The drill element, the drill tube or the first drill string component 10 for percussive drilling shown in Figs. 1 to 4 is provided at one end with a sleeve portion or a female portion 1 1 with a cylindrical female thread or cylindrical internal thread 12. The female portion 1 1 constitutes an integral part of the drill tube 10. At its other end the drill tube 10 is formed with a spigot or male portion 13 according to the present invention provided with a cylindrical male thread or cylindrical external thread 14. The shown thread is a so- called trapezoid thread but other thread shapes can be used, for example a rope thread. Furthermore, the drill element has a through-going flush channel 15, through which a flush medium, usually air or water, is transferred. The male thread 14 comprises the thread flanks 16, 17 and thread roots 20 arranged between the flanks. The female thread 12 comprises the thread flanks 18, 19 and thread roots 21 arranged between flanks. At a tightened joint according to Fig. 4 the thread roots 20 of the male thread 14 are provided substantially distant from the associated crests 22 of the female thread. According to the present invention the thread roots 20 of the drill element of the male portion are provided with a coating consisting of at least one surface modifying, corrosion protective layer. Only the most exposed portions, that is sections of reduced cross-section such as thread roots 20, restrictions 24 and clearances are coated. The greatest layer thickness is 0.002-5 mm, preferably 0.02-2 mm. The thread root has a first width, Wl, and the thread, that is the thread crest 23 and the uncoated part of the tread flanks 16, 17, has a second width, W2 (Fig. 3), where the ratio W1/W2 is 0.02-1.2, preferably 0.3 - 0.8. For example a rope thread (R35) was covered by a 5 mm thick coating (Wl). The thread pitch was 12.7 mm, which gave W2=12.7-5=7.7 and the ratio Wl/W2=0.65. Said corrosion protective layer in the coating of the drill element according to the invention is less noble than the carrying or underlying steel, that is the layer has a more negative electrode potential, at least 50 mV, preferably at least 100 mV in the actual environment. That difference in electrode potential then functions as a cathode protection where the coating constitutes a galvanic anode (sacrificial anode). Examples of such protective materials are aluminum, zinc and magnesium as well as alloys of these, preferably zinc alloys. The remaining layers can be constituted of binder layers in order to increase the bond between the coating and the steel. A number of different coating methods can be used to apply the layer, for example hot dipping, chemical or electrolytic plating or thermal spraying. In case the coating process gives a coating which covers more than some of the sections of reduced cross- section, such as the thread root, the restriction or the clearance, this part of the coating is removed by means of machining before use or through wear after short period of use. With the latter is meant that coating of impact transferring surfaces is not advantageous. In the independent claims is consequently indicated that "at least one of the sections having a reduced cross-section comprises partly a layer of a material with lower electrode potential" wherein the word "partly" also comprises the cases when a possible layer on the impact transferring surfaces is worn away very quickly. Example During so-called drifter drilling of long holes about 4 m long drill tubes are used. The weak parts of the tubes are the bottoms 20 of the external threads 14 (Fig. 2) where flushing water and pulsating tensile stresses lead to corrosion fatigue that frequently results in fracture. Eight tubes of case hardened, low-alloyed steel were coated with a layer of zinc with a thickness of about 0.2 mm by dipping in a bath of molten zinc, so-called dip galvanizing. Zinc has an electrode potential of about -860mV in seawater at 20°C, which shall be compared to -500mV for low-alloyed steel. The zinc layer was removed from the thread flanks by means of a rotating brush. Then drilling was performed in a rig for drifter drilling underground until fracture or the tubes were worn-out. Following life lengths, measured in drilled meter, were obtained: No. Drilled meter 1 4297 2 2489 3 3210 4 2041 5 3933 6 4268 7 3085 8 2608 Normal life length for uncoated drifter tubes of conventional type steel is about 2000 m at the actual test place where the rock substantially consists of granite, which shows that the use of a drill steel according to the invention gives a striking improvement. In an alternative embodiment of a thread joint according to the present invention also the thread 12' of the female portion 1 1 ' is coated with a layer of a material of lower electrode potential than steel, Fig. 5. Consequently also sections of the female portion 1 1 ' of reduced cross-section are provided with a coating consisting of at least one layer constituting a sacrificial anode. Only the most exposed portions, that is sections of reduced cross-section such as thread roots 21 ', restrictions and clearances are coated. What has been stated above about coating applies also to the case when the coating is applied in the female portion 11 * . In an alternative embodiment of a drill element according to the present invention only the most stressed parts of the thread root, for example one (to the right in Fig. 6) or both (to the left) transitions from the thread root to the flank of a trapezoid thread are coated, that is where the drill element has the smallest radius, Fig. 6. The invention consequently relates to a thread joint and a drill element for percussive drilling with a restricted portion which is coated by a layer in order to substantially improve the resistance to corrosion fatigue. The layer is made preferably discontinuous in the axial direction to avoid deposition on and softening of the thread flanks.

Claims: Claims 1. A thread joint of steel for percussive drilling wherein flushing water is used, comprising sections (20;21 ';24) of reduced cross-section, at least one substantially cylindrical external thread (14) as well as a substantially cylindrical internal thread (12), said external thread (14) being provided on a spigot (13) intended to constitute an integral part of a first drill string component (10), said threads (12,14) comprising thread flanks (16, 17; 18, 19) and thread roots (20;21 ;21 ') provided between the flanks, said thread roots (20) of the cylindrical external thread (14) being provided substantially distant from associated crests (22) of the cylindrical internal thread (12), c h a r a c t e r i z e d i n that at least one of the sections (20;21 ';24) of reduced cross- section partly comprises a layer of a material of lower electrode potential than steel, said layer being made from aluminum, zinc and magnesium or alloys of these. 2. The thread joint according to claim 1, wherein the layer is applied substantially only in the thread root (20;21 ') of the thread (14;12'), said thread root having a first width, Wl, and the crest of the thread having a second width, W2, where the ratio W1/W2 is 0.02- 1.2, preferably 0.3 - 0.8. 3. The thread joint according to claim 1 or 2, wherein the material in the layer has at least 50 mV lower electrode potential than steel, preferably at least 100 mV lower electrode potential than steel. 4. The thread joint according to anyone of the preceding claims, wherein the greatest layer thickness is 0.002-5 mm, preferably 0.02-2 mm. 5. The thread joint according to anyone of the preceding claims, wherein one of the following coating methods has been used to apply the layer: hot dipping, chemical or electrolytic plating or thermal spraying. 6. A drill element for a thread joint for percussive drilling of the type according to claim 1, wherein flushing water is used, said drill element comprising sections (20;21 ';24) of reduced cross-section, at least one substantially cylindrical external thread (14), said external thread (14) being provided on a spigot (13) intended to constitute an integral part of a first drill string component (10), said thread (12,14) comprising thread flanks (16,17;18,19) and thread roots (20;21 ;21 ') provided between the flanks, said thread roots (20) of the cylindrical external thread (14) being substantially distant from associated crests (22) of a cylindrical internal thread (12), c h a r a c t e r i z e d i n that at least one of the sections (20;21 ';24) of reduced cross- section partly comprises a layer of a material of lower electrode potential than steel, said layer being made from metals like aluminum, zinc and magnesium or alloys of these. 7. The drill element according to claim 6, wherein the layer is applied substantially only in the thread root (20;21 ') of the thread (14;12'), said thread root having a first width, Wl , and the crest of the thread has a second width, W2, where the ratio W1/W2 is 0.02- 1.2, preferably 0.3 - 0.8. 8. The drill element according to claim 6 or 7, wherein the material in the layer has at least 50 mV lower electrode potential than steel, preferably at least 100 mV lower electrode potential than steel. 9. The drill element according to anyone of the claims 6, 7 or 8, wherein the greatest layer thickness is 0.002-5 mm, preferably 0.02-2 mm. 10. The drill element according to anyone of the claims 6, 7, 8, or 9, wherein one of the following coating methods has been used to apply the layer: hot dipping, chemical or electrolytic plating or thermal spraying.

Inventors: LINDEN JOHAN, LINDEN, JOHAN
Applicants: SANDVIK AB
Application year: 2001
Main classification: E21B17
IPC codes: B23B45, E21B17, F16B33, F16B37