Method of producing high strength sucker rod coupling

A sucker rod coupling 10 having a high ultimate tensile strength, resistance to corrosion, and resistance to surface cracking arising out of a method of making the sucker rod coupling 10, which employs a five-step process of forming a coupling 10. First, a hollow cylindrical core 12 from a heat treatable steel is formed. Second, a thin coating 18 of metallic alloy is applied to the outer surface of the core 12. Third, the core 12 is heat treated. Fourth, threads 20 are partially cut in the inner surface of the core 12. Fifth, the threads 20 are cold worked to transform the partially cut threads 20 into finished threads 20 and to place the thread roots 22 in compression.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to couplings used to connect pipe 
segments to form a string, and more particularly relates to couplings 
adapted to connect sucker rods together to form a sucker rod string for 
use in a producing well. 
2. Description of the Related Art 
Sucker rod couplings which connect individual sucker rods to form a string 
are a key component in the successful performance of a sucker rod string. 
By minimizing the wear breakage and corrosion of couplings, the life of 
the sucker rod string is increased, which can substantially decrease 
service and repair costs. However, sucker rod couplings can be the weakest 
link in the sucker rod string, limiting the amount of stress that the 
sucker rod string can absorb without failure. 
The general industry practice has been to design sucker rod strings so that 
stress range requirements of the particular well application are within 
the allowable limits of the sucker rod body. This practice, however, 
erroneously assumes that the sucker rod couplings will provide the same or 
greater service life. This assumption is not valid for certain 
combinations of rods and couplings and results in derating the allowable 
coupling loads to give satisfactory coupling stress levels and 
corresponding service life. 
Derating refers to the determination of the ultimate tensile strength of a 
particular coupling and sucker rod combination by multiplying the ultimate 
tensile strength of the sucker rod by a suitable derating factor. The 
derating factor normally has a value less than 1 and varies depending on 
the particular combination of coupling size and sucker rod size, and 
material strength. 
There are two common situations involving derating. The first case involves 
the use of American Petroleum Institute (hereinafter "API") slimhole 
couplings with standard API or high strength grades of sucker rod. 
Although the ultimate tensile strength of the coupling and rod may be 
similar, the reduced wall thickness of slimhole couplings reduces the 
effective stress area thus creating an increase in stress in the coupling 
for any given load. For example, a typical derating factor for a 1 inch 
(approximately 0.025 m) diameter API slimhole coupling, when used with an 
API grade D sucker rod, is approximately 0.89. Thus, the combination of a 
1 inch (approximately 0,025 m) diameter API slimhole coupling with an API 
grade D rod can typically withstand only about 89% of the stress that the 
API grade D rod could sustain alone. 
The second scenario involves the use of a full-size API coupling with a 
high strength sucker rod which typically has an ultimate tensile strength 
in excess of 115,000 pounds per square inch (hereinafter "psi"). Here the 
ultimate tensile strength of the high strength sucker rod exceeds the 
ultimate tensile strength of the full-size API coupling. For example, a 
derating factor for a 7/8 inch nominal size (approximately 0.019 m) 
diameter full-size API coupling in combination with a high strength sucker 
rod is approximately 0.85. Thus, the combination of a 7/8 inch 
(approximately 0.019 m) full-size API coupling with a high strength sucker 
rod can withstand only about 85% of the stress that the high strength 
sucker rod could withstand alone. 
Corrosive well environments further complicate the problem. Sour gas wells 
may cause sulfide cracking in coupling cores, or ordinary corrosion may 
cause the cores to fail. A thin nickel based metallic alloy is typically 
applied to the outer surface of a sucker rod coupling to protect the 
coupling from corrosion and sulfide cracking. However, such coatings are 
themselves susceptible to stress cracking when applied to coupling core 
materials that have a higher hardness and ultimate tensile strength, or 
when applied to a coupling core material that is to be heat treated. The 
surface coating tends to become brittle during the subsequent heat 
treatment. 
Conventional fabrication techniques have failed to produce a sucker rod 
coupling of sufficient fatigue strength, ultimate tensile strength, and 
resistance to corrosion and surface cracking to allow operators to fully 
utilize the capability of existing sucker rods. 
SUMMARY OF THE INVENTION 
The present invention includes a new method for making a new sucker rod 
coupling which can match the fatigue and ultimate tensile strength of 
modern sucker rods while providing a crack and corrosion resistant 
coating. In a preferred embodiment, the high strength sucker rod coupling 
includes a cylindrical core which has an ultimate tensile strength in 
excess of about 146,000 psi and a Rockwell hardness (hereinafter HRC) in 
excess of about 32. A smooth coating surrounds the exterior of the 
cylindrical core. The coating has a hardness exceeding about 45 HRC and is 
designed to resist corrosion and wear. The cylindrical core also has a 
plurality of internal threads which are designed to engage the external 
threads of a typical sucker rod. 
In another preferred embodiment adapted for mild well environments, the 
high strength sucker rod coupling includes a cylindrical core which has an 
ultimate tensile strength in excess of about 146,000 psi and a hardness in 
excess of about 32 HRC. The cylindrical core also has a plurality of 
internal threads which are designed to engage the external threads of a 
typical sucker rod. 
The present invention also includes a new method of producing a high 
strength sucker rod coupling which entails forming or utilizing a hollow 
cylindrical core from a heat treatable steel, heating the core above about 
1800.degree. F., and adding a thin layer of metallic alloy to the core. 
The alloy preferably contains nickel, chromium, silicon, and iron. The 
core is then cooled slowly to below about 150.degree. F. The core is then 
heat treated, salt quenched, and air cooled. The core is then tempered and 
tapped. The final step involves cold working the core threads to place the 
thread roots in a state of compression.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, therein is depicted an exemplary high strength sucker 
rod coupling 10 in accordance with the present invention. The coupling 10 
includes a core 12, end surfaces 14, 16, an external coating 18, and 
threads 20. 
In a preferred embodiment, the core 12 is drawn into a hollowed 
substantially cylindrical form. The core 12 is preferably fashioned from 
AISI 4140 or 4142 steel, though any steel capable of obtaining required 
strength/hardness through heat treatment may be suitable. After 
application of a suitable heat treatment, the core 12 preferably has a 
minimum ultimate tensile strength of approximately 117,000 psi and a 
hardness greater than about 23 HRC. It is especially preferred that the 
core 12 has a minimum ultimate tensile strength of about 146,000 psi and a 
minimum hardness of between about 32 and about 36 HRC. The end surfaces 
14, 16 should be machined smooth by grinding or lapping or machining to 
ensure that any preload applied to the coupling 10 and the sucker rods is 
retained. 
The coating 18 is preferably a nickel based alloy applied to the core 12 by 
a metal spray technique to be more fully disclosed below. In a 
particularly preferred embodiment, the coating 18 is composed of a minimum 
thickness of 0.010 inches (approximately 0.00025 m) of Colmonoy #5 spray 
powder, which contains nickel, chromium, silicon, and iron. Other coatings 
may be suitable as well such as SCM 76-M-50 (M) metal powder, or cobalt 
based powder which also contains iron, nickel, carbon, silicon, boron, 
chromium, and molybdenum. Other metal based powders, or other coatings, 
such as for example, ceramic or plastic coatings may be suitable as well, 
though plastic or other coatings which have a relatively low melting point 
will have to be applied after the core 12 is heat treated. The temperature 
at which the coating 18 is fused to the core 12 will depend upon the 
particular coating 18 material. The coating 18 preferably has a minimum 
hardness of between about 45 and about 53 HRC, but the hardness may be 
less than 45 HRC or exceed 53 HRC. To ensure that there is minimum 
friction and wear for both the coupling 10 and the well tubing through 
which it is inserted, the coating 18 should be ground to a smooth finish, 
or preferably, about 63 R.sub.a. 
Since the core 12 has a much higher hardness than lower strength cores, 
formation of the threads 20 in the core 12 requires a slightly different 
procedure than that conventionally used. Ordinarily, the threads 20 would 
be full-formed rolled in a cold working operation to place the roots 22 of 
the threads 20 in a state of compression and provide resistance to fatigue 
stress. However, owing to the hardness of the core 12, in this application 
the threads 20 should first be partially cut with an existing tap such 
that sufficient space remains in the thread roots 22 for metal 
displacement during a subsequent cold working operation. The threads 20, 
and particularly the thread roots 22, are then cold worked using a cold 
form tap to place the thread roots 22 in a state of compression. 
Experimentation has shown that by using this technique of forming the 
threads 20, the effect of the cold working extends to a depth of 
approximately 0.0030 inches (approximately 0.000076 m), which is 
approximately 75% of the cold working depth achieved on a lower strength 
coupling wherein the threads 20 are formed by the conventional full-formed 
rolling process. 
In another embodiment more suitable for mild well environments, the sucker 
rod coupling does not have an external coating. In all other aspects, this 
embodiment of the sucker rod coupling is structurally identical to, and 
has the same physical properties as, the sucker rod coupling 10 shown in 
FIG. 1. 
A preferred method for fabricating the sucker rod coupling 10 shown in FIG. 
1 includes application of a suitable corrosion resistant coating 18 to the 
core 12, heat treating the coupling 10 to increase the ultimate tensile 
strength of the core 12 to above about 117,000 psi, and preferably above 
about 146,000 psi, and the core 12 hardness preferably above about 23 HRC 
to between about 32 and about 36 HRC, and forming the threads 20 by a 
combination of a partial tapping operation and a cold working operation. 
Since the coating 18 should be able to withstand the heat treatment of the 
core 12 after the coating 18 is applied to the core 12, the spray metal 
technique for applying the coating 18 should be modified. Metal spray 
powder, preferably Colmonoy #5, is applied to the core 12 and fused 
between 1840.degree. F. and 1860.degree. F. The coating 18 formed thereby 
should have a minimum thickness of about 0.010 inches (approximately 
0.00025 m) and have a minimum hardness between about 45 and about 53 HRC. 
It is preferred that immediately following application of the coating 18, 
the coupling 10 be slow cooled to prevent the formation of cracks in the 
coating 18. This is preferably done by immersing the coupling 10 in a 
suitable insulating material which will prevent rapid heat loss from the 
coupling 10 by either conduction, convection, or radiation. 
Experimentation has shown that Vermiculite in about 1/4 inch 
(approximately 0.0064 m) granular size is particularly suitable as an 
insulating material. The coupling 10 should be cooled in the Vermiculite 
or other suitable material to below about 150.degree. F. at a cooling rate 
not exceeding: about 41.degree. F./min from about 1400.degree. F. down to 
about 1200.degree. F.; about 10.degree. F./min from about 1200.degree. F. 
to about 700.degree. F.; and about 4.degree. F./min from about 700.degree. 
F. down to about 200.degree. F., before removal from the Vermiculite or 
other material. After the coupling 10 is slow cooled, it should be checked 
for cracks in the coating 18 and the hardness of both the core 12 and the 
coating 18 should be checked. 
Following application of the coating 18 to the core 12, the coupling 10 
should be heat treated to increase the ultimate tensile strength of the 
core 12. Aside from achieving high ultimate tensile strength, the goal of 
the heat treatment is to create in combination with martensite grain 
structure with limited grain growth, such as bainitic in combination with 
martensite. 
A preferable heat treatment, well known to those skilled in the art, 
comprises the following steps. The coupling 10 should be heated to about 
900.degree. F. and held at that temperature for about thirty minutes. The 
coupling 10 is then raised to between about 1200.degree. F. to 
1225.degree. F. and held in that temperature range for about one hour. The 
coupling 10 is then heated to about 1550.degree. F., held at that 
temperature for about one hour and simultaneously exposed to a 0.40 carbon 
potential. The coupling 10 is next quenched in salt at about 525.degree. 
F. and held at that temperature for about one hour. The coupling 10 is 
then air cooled to below about 150.degree. F. and the core 12 hardness is 
again checked. The core 12 should then be tempered to achieve a hardness 
of between about 32 and about 36 HRC. 
Following heat treatment, the threads 20 are formed in the core 12 by a 
combination of cutting and cold working. The threads 20 are first 
partially cut by a suitable tap which will leave space at the thread roots 
22 for metal displacement during a subsequent cold working operation. 
Threads may be formed in a conventional lower strength sucker rod coupling 
by a pure full-formed rolling operation which does not remove any material 
from the coupling but rather displaces it, particularly at the roots, thus 
increasing the fatigue strength at the roots. However, because the 
ultimate tensile strength of the core 12, in accordance with the present 
invention, is substantially greater than about 100,000 psi, the core 12 is 
simply too hard for a pure cold working operation to form the threads 20 
therein. 
After the threads 20 have been partially tapped, they should be cold worked 
using a cold form tap to displace, but not remove, some material at the 
thread roots 22, with the goal of placing the thread roots 22 in a state 
of compression to give the thread roots 22 a heightened resistance to 
fatigue stress cracking. For example, experimentation has shown that a 1 
inch nominal size (approximately 0,025 m) sucker rod coupling 10 in 
accordance with the present invention has endured about 107 cycles of a 
stress load alternating between about 13,006 psi to about 54,904 psi 
without failure. 
Subsequent to the cold working of the threads 20, the end surfaces 14, 16 
should be machined smooth and perpendicular to the threads 20 to minimize 
bending moment on the threads 20, and to preserve any preload that may be 
placed on the coupling 10 after rod insertion. In addition, the coating 18 
should be ground to a smoothness of a 63 R.sub.a finish to minimize 
friction and wear for both the coupling 10 and the particular tubing 
through which the coupling 10 is inserted. 
For mild well environments, the above method need not include the spray 
metal application. The strength and hardness of the core 12 and the 
threads 20 may be increased using either the heat treatment and subsequent 
threading operation disclosed above or by using a pure full-formed rolling 
operation with a core strength in excess of about 23 HRC and at the 
required strength level, approximately 32 to approximately 36 HRC. 
Many modifications and variations may be made in the techniques and 
structures described and illustrated herein without departing from the 
spirit and scope of the present invention. Accordingly, the techniques and 
structures described and illustrated herein should be understood to be 
illustrative only and not limiting upon the scope of the present 
invention.