U.S. patent application number 09/796441 was filed with the patent office on 2001-10-18 for corrosion-resistant thread joint for percussion drill element and method of achieving such resistance.
Invention is credited to Linden, Johan.
Application Number | 20010029807 09/796441 |
Document ID | / |
Family ID | 20278676 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010029807 |
Kind Code |
A1 |
Linden, Johan |
October 18, 2001 |
Corrosion-resistant thread joint for percussion drill element and
method of achieving such resistance
Abstract
A percussive drilling component includes a cylindrical male
screw thread formed of a steel material. The thread includes thread
crests and thread roots interconnected by thread flanks. To protect
the thread against corrosion, the thread is coated with a material
having a lower electrode potential than the steel material. The
coating is situated at least in regions located radially inwardly
of the thread flanks. The male screw thread can be attached to a
female screw thread of another percussive drilling component, the
female screw thread also being coated with the material.
Inventors: |
Linden, Johan; (Gavle,
SE) |
Correspondence
Address: |
Ronald L. Grudziecki, Esq.
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
20278676 |
Appl. No.: |
09/796441 |
Filed: |
March 2, 2001 |
Current U.S.
Class: |
75/228 |
Current CPC
Class: |
Y10T 403/5746 20150115;
E21B 17/0426 20130101 |
Class at
Publication: |
75/228 |
International
Class: |
C22C 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 2, 2000 |
SE |
0000702-1 |
Claims
What is claimed is:
1. In combination, percussive drilling components connected
together by a thread joint defined by substantially cylindrical
male and female screw threads formed integrally on respective ones
of the components, the components and their respective screw
threads formed by a steel material; each of the threads comprising
thread crests and thread roots interconnected by thread flanks; the
thread roots of the male thread being spaced from and facing
respective crests of the female thread; the male thread being
coated with a material having a lower electrode potential than the
steel material of the male thread, the coating on the male thread
being situated at least in regions thereof located radically
inwardly of the thread flanks on the male thread.
2. The combination according to claim 1 wherein the coating is
aluminum, zinc, magnesium, or alloys thereof.
3. The combination according to claim 1, wherein the coating is
situated only in the root of each thread; each coating having a
first width W1, and separated from the next coating of the same
thread by a second width W2, wherein the ratio of W1 to W2 being
from 0.2 to 1.2.
4. The combination according to claim 3, wherein the ratio is from
0.3 to 0.8.
5. The combination according to claim 1, wherein the electrode
potential of the coating is lower than that of the steel material
by at least 50 mV.
6. The combination according to claim 1, wherein the electrode
potential of the coating is lower than that of the steel material
by at least 100 mV.
7. The combination according to claim 1, wherein the coating has a
maximum thickness in the range of from 0.002 to 5.0 mm.
8. The combination according to claim 1, wherein the coating has a
maximum thickness in the range of from 0.02 to 2.0 mm.
9. The combination according to claim 1, wherein each coating
comprises a hot dipped coating.
10. The combination according to claim 1, wherein each coating
comprises a chemical plating.
11. The combination according to claim 1, wherein each coating
comprises an electrolytic plating.
12. The combination according to claim 1, wherein each coating
comprises a thermally sprayed-on coating.
13. The combination according to claim 1 wherein the female thread
is coated with the same material as the male thread, the coating on
the female thread being situated at least in regions thereof
located radially outwardly of the female thread flanks.
14. A percussive drilling component including a substantially
cylindrical male screw thread; the component and the thread formed
of a steel material; the thread comprising thread crests and thread
roots interconnected by thread flanks; the thread being coated with
a material having a lower electrode potential than the steel
material of the drilling component, the coating being situated at
least in regions located radially inwardly of the thread
flanks.
15. The percussive component according to claim 14, wherein the
coating is aluminum, zinc, magnesium, or alloys thereof.
16. The percussive drilling component according to claim 14,
wherein the coating is situated only in the root of the thread;
each coating having a first width W1, and separated from the next
coating of the same thread by a second width W2, wherein the ratio
of W1 to W2 being from 0.2 to 1.2.
17. The percussive drilling component according to claim 16,
wherein the ratio is from 0.3 to 0.8.
18. The percussive drilling component according to claim 14,
wherein the electrode potential of the coating is higher than that
of the steel material by at least 50 mV.
19. The percussive drilling component according to claim 14,
wherein the electrode potential of the coating is higher than that
of the steel material by at least 100 mV.
20. The percussive drilling component according to claim 14,
wherein the coating has a maximum thickness in the range of from
0.002 to 5.0 mm.
21. The percussive drilling component according to claim 14,
wherein the coating has a maximum thickness in the range of from
0.02 to 2.0 mm.
22. The percussive drilling according to claim 14, wherein each
coating comprises a hot dipped coating.
23. The percussive drilling according to claim 14, wherein each
coating comprises a chemical plating.
24. The percussive drilling according to claim 14, wherein the
coating comprises an electrolytic plating.
25. The percussive drilling according to claim 14, wherein the
coating comprises a thermally sprayed-on coating.
26. The percussive drilling component according to claim 14,
wherein each thread root forms two transitions with respective
thread flanks, the coating being situated at only one of the
transitions.
27. The percussive drilling component according to claim 14,
wherein each thread root from two transitions with respective
thread flanks, the coating being situated at both of the
transitions.
28. A method of protecting a threaded end of a percussive drilling
component against corrosion, the component formed of a steel
material, the threaded end comprising a substantially cylindrical
screw thread, the method comprising coating the entire threaded end
with a coating material having a lower electrode potential than the
steel material of the drilling component, wherein the coating is
disposed on impact regions of the thread where the coating will be
worn off during percussive drilling.
Description
RELATED INVENTION
[0001] This application claims priority under 35 U.S.C.
.sctn..sctn. 119 and/or 365 to Patent Serial No. 0000702-1 filed in
Sweden on Mar. 2, 2000, the entire content of which is incorporated
by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a drill element for rock
drilling, and to a thread joint for interconnecting the drill
element to other drill elements, wherein the thread joint is
protected against corrosion. The invention also pertains to a
method of protecting a threaded end of a drill element from
corrosion.
PRIOR ART
[0003] During percussive rock drilling, the drill elements, i.e.
bits, rods, tubes, sleeves and shanks adapters, are subjected to
corrosive attacks. This applies in particular to underground
drilling where water is used as a 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.
[0004] Today low-alloyed, case hardened steels are normally used in
the drill element. The reason for this is that abrasion and wear of
the thread parts have generally limited the life of the drill
element. As the drill machines and the drill elements have become
more efficient, problems due to abrasion and wear have diminished,
and corrosion fatigue has become a major factor in limiting the
life of the drill element.
[0005] The case hardening produces compressive stresses in the
surface, which gives certain beneficial effects against the
mechanical part of the fatigue. The resistance to corrosion in a
low-alloyed steel is however poor and for that reason corrosion
fatigue still happens easily.
[0006] In U.S. Pat. No. 4,872,515 or 5,064,004 a drill element is
shown wherein a threaded portion is covered with a metallic
material, which is softer than the steel of the drill element.
Thus, it 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
[0007] One object of the present invention is to substantially
improve the resistance against corrosion fatigue of a drill element
for percussive rock drilling.
[0008] Another object of the present invention is to substantially
improve the resistance against corrosion fatigue in sections of
reduced cross-sections in a drill element for percussive rock
drilling.
[0009] 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.
SUMMARY OF THE INVENTION
[0010] The present invention relates to a percussive drilling
component which has a male screw thread, as well as to the
combination of that drilling component attached to another drilling
component which has a female screw thread.
[0011] The percussive drilling component is formed of a steel
material and includes an integral substantially cylindrical male
screw thread. The thread comprises thread crests and thread roots
interconnected by thread flanks. The thread is coated with a
material having a lower electrode potential than the steel
material. The coating on the thread is situated at least in regions
located radially inwardly of the thread flanks.
[0012] In the case of the combination wherein the above-described
drilling component is attached to another drilling component having
a female screw thread, the female screw thread could also be coated
with the lower potential material, with the coating situated in
regions located radially outwardly of the female thread flanks.
[0013] The invention also pertains to a method of protecting a
threaded end of a steel percussive drilling component against
corrosion, by coating the entire threaded end with a coating
material having a lower electrode potential than the steel material
of the drilling component. The coating will thus be disposed on
impact regions of the thread where the coating will be worn off
during percussive drilling.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The objects and advantages of the invention will become
apparent from the following detailed description of preferred
embodiments thereof in connection with the accompanying drawings,
in which like numerals designate like elements, and in which:
[0015] FIG. 1 shows a drill element according to the present
invention in a side view, partly in cross-section;
[0016] FIG. 2 shows one end of the drill element of FIG. 1 in a
side view;
[0017] FIG. 3 shows an axial cross-section of a fragment of the end
shown in FIG. 2;
[0018] FIG. 4 shows an axial cross-section of a first embodiment of
a thread joint according to the present invention;
[0019] FIG. 5 shows an axial cross-section of a second embodiment
of a thread joint according to the present invention; and
[0020] FIG. 6 shows an axial cross-section of an alternative
embodiment of a drill element according to the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] A drill element or first drill string component 10 for
percussive drilling shown in FIGS. 1 to 4 is in the form of a drill
tube provided at one end with a sleeve or female portion 11 having
a cylindrical female (internal) screw thread 12. The female portion
11 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
provided with a cylindrical male screw thread or cylindrical
external screw 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 central
flush channel 15, through which a flush medium, usually air or
water, is transferred.
[0022] In use, a plurality of the components 10 are screwed
together, i.e., the male portion 13 of one component 10 is screwed
into the female portion 11 of another, identical component 10, as
depicted in FIG. 4.
[0023] The male thread 14 comprises 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. In a tightened joint shown in FIG. 4 the thread
roots 20 of the male thread 14 are provided substantially distant
from the associated crests 22 of the female thread.
[0024] According to the present invention, regions of reduced
cross-section of the male portion, e.g., the thread roots 20,
restrictions 24, and clearances, are provided with a coating formed
of at least one surface-modifying, corrosion-resistant layer L. The
greatest layer thickness is 0.002-5 mm, preferably 0.02-2 mm. The
thread root has a first width, W1 (measured in a direction parallel
to the axis of the component 10). The thread, that is the thread
crest 23 and the uncoated part of the thread flanks 16, 17 have a
second width, W2 (FIG. 3), wherein the ratio W1/W2 is 0.02-1.2,
preferably 0.3-0.8. For example, a rope thread (of designation R35)
was covered by a 5 mm thick coating (W1). The thread pitch was 12.7
mm, resulting in W2 being 7.7 mm (i.e., 12.7 minus 5). Also,
W1/W2=0.65.
[0025] Said corrosion-resistant layer L in the coating of the drill
element according to the invention is less noble than the carrying
or underlying steel of the component 10. That is, the layer has a
more negative electrode potential by at least 50 mV, preferably by
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.
[0026] 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 produces a
coating which cover more than some of the sections of reduced
cross-section, e.g., the entire thread, the excess coating portions
can be machined off before the tube is used. Alternatively, the
excess coating portions could be allowed to wear of during use. In
that regard, it will be appreciated that after the steel tube 10
has been screwed together with the female thread of another tube
during the formation of a string, parts of the two threads will be
in contact with one another. During a percussive drilling
operation, the coating at those contact or impact regions, i.e.,
the excess coating portions which are not needed, will be quickly
worn-off, leaving the coating intact at the regions where corrosion
protection is especially needed, i.e., at the regions of reduced
cross section that are exposed to corrosive attacks.
[0027] A number of different coating methods can be used to apply
the layer L, for example hot dipping, chemical or electrolytic
plating, or thermal spraying. Tube 10 has been screwed together
with the female thread of another tube during the formation of a
string, parts of the two threads will be in contact with one
another. During a percussive drilling operation, the coating at
those contact or impact regions, i.e., the excess coating portions
which are not needed, will be quickly worn off, leaving the coating
intact at the regions where corrosion protection is especially
needed, i.e., at the regions of reduced cross section that are
exposed to corrosive attacks.
[0028] Thus, it is possible within the scope of the invention to
coat most or all of the drilling component, whereafter the coating
portions disposed at regions where the drilling component contacts
an adjacent drilling component will wear away quickly.
EXAMPLE
[0029] During so-called production drilling of long holes a drill
tube 10 of about 4 m long is used, FIG. 1, which is combined with
others to form a long string, i.e., eight tubes of case-hardened
low-alloy steel were employed in the string. The critical parts of
the tubes from a corrosion standpoint are the bottoms (roots) 20 of
the external threads 14 (FIG. 2). Flushing water and pulsating
tensile stresses lead to corrosion fatigue (galvanic corrosion)
that frequently results in fracture.
[0030] The 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 -860 mV in seawater at
20.degree. C., which shall be compared to -500 mV for low-alloyed
steel. The zinc layer was machined 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 spans for the eight tubes, measured in
drilled meter, were obtained:
1 Test No. Drilled meters 1 4297 2 2489 3 3210 4 2041 5 3933 6 4268
7 3085 8 2608
[0031] Normal life spans for uncoated drifter tubes of conventional
type steel are about 2000 m at the actual test place where the rock
substantially consists of granite, which shows that the use of a
drill steel coated according to the invention gives a striking
improvement.
[0032] In an alternative embodiment of a thread joint according to
the present invention shown in FIG. 5 also the thread 12' of the
female portion 11' would be coated with a layer of a material of
lower electrode potential than the low-alloy steel, FIG. 5. In
other words, sections of the female portion 11' of reduced
cross-section would be provided with a coating constituting a
sacrificial anode. Only the most exposed portions, that is,
sections of reduced cross-section such as thread roots 21',
restrictions and clearances would preferably be coated. Everything
stated above about the coating L, including all of the thickness
and width characteristics, applies also to the case where the
coating is applied to the female portion 11'. For example, the
entire female thread could be dipped in a bath of coating material,
whereupon the coating at the impact regions would wear away during
drilling.
[0033] In another alternative embodiment of a drill element
according to the present invention only the most stressed parts of
the thread root would be coated. For example, as shown in the right
half of FIG. 6, only one of the two transitions 30,32 between the
thread root 20 and the flank of a trapezoidal thread would be
provided with a layer L. Alternatively, as shown in the left half
of FIG. 6, both of the transitions 30,32 could be provided with
layers L.
[0034] The invention consequently relates to a thread joint and a
drill element for percussive drilling with a restricted portion
which is coated by a corrosion-resistant layer in order to
substantially improve the resistance to corrosion fatigue. The
layer is preferably discontinuous in the axial direction of the
tube to avoid deposition on and softening of the thread flanks.
[0035] Although the present invention has been described in
connection with preferred embodiments thereof, it will be
appreciated by those skilled in the art that additions, deletions,
modifications, and substitutions not specifically described may be
made without departing from the spirit and scope of the invention
as defined in the appended claims.
* * * * *