Cemented carbide for oil and gas applications

The present invention relates to a cemented carbide with excellent properties for oil and gas applications including resistance to the combined erosion and corrosion synergistic effects at temperatures between -50 and 300.degree. C., preferably 0-100.degree. C. The cemented carbide contains, in wt %, 2.5-4.5 (Co+Ni) with a weight ratio Co/Ni of about 3, 0.25-0.6 Cr and 0.1 Mo wherein essentially all of the WC grains have a size <1 .mu.m and wherein the total carbon content is in the interval of 6.13-(0.061.+-.0.008).times.binder phase (Co+Ni) content (wt-%).

FIELD OF THE INVENTION 
The present invention relates to a cemented carbide grade with special 
properties for oil and gas applications. Moreover the invention refers to 
a corrosion and erosion resistant grade for choke valves to control the 
flow of multimedia fluid (e.g., gas, liquid and sand particles). 
BACKGROUND OF THE INVENTION 
Cemented carbide for applications such as seal rings, bearings, bushings, 
hot rolls, etc., should have a certain degree of corrosion resistance. A 
corrosion resistant cemented carbide generally has a binder phase 
consisting of Co, Ni, Cr and Mo where the Cr and/or Mo act as corrosion 
inhibiting additions. An example of such a cemented carbide with a medium 
WC grain size is disclosed in EP 28 620. EP 568 584 discloses the use of a 
corrosion resistant cemented carbide with submicron WC grain size with 
excellent properties particularly for tools used in the wood industry. 
A critical component of subsea oil/gas production systems is the choke trim 
components, the primary function of which is to control the pressure and 
flow of well products. Under severe conditions of multi flow media, these 
components may suffer from extreme mass loss by exposure to solid particle 
erosion, acidic corrosion erosion-corrosion synergy and cavitation 
mechanisms even when fitted with cemented carbide trims. 
The opportunity to maintain or replace such equipment in the field 
especially in offshore deep water sites is limited by weather conditions. 
It is therefore essential that reliable and predictable products form part 
of the subsea system. 
The composition of the cemented carbide grades presently used for 
withstanding conditions of service in this type of environment generally 
consist of tungsten carbide (WC) as the hard component and cobalt (Co) or 
nickel (Ni) as the binder material to cement together the WC crystals. 
To meet the demands of hardness and toughness, the amount of binder and/or 
the WC grain size are varied and cobalt is generally accepted as the 
optimum binder constituent. Where corrosion resistance is the predominant 
consideration then the binder material is usually of a nickel or a 
nickel+chromium (Ni+Cr) composition. 
Analogous to stainless steels, Cr and Ni alloys have improved passivity by 
reducing the critical currents involved in corrosion, however (Cr+Ni) are 
not so resistant to halides (seawater) or inorganic acids. For these 
conditions the addition of molybdenum gives improved corrosion resistance 
in addition to improving binder strength of Ni. 
Recent experimental work, including field trial evaluation, has proven that 
under high erosion conditions including a corrosion medium, the mechanism 
of mass loss is due not only to a combination of each individual corrosive 
condition, but the combination of corrosive conditions is synergistic. 
SUMMARY OF THE INVENTION 
The present invention relates to cemented carbides with excellent 
properties regarding resistance to the synergistic combined erosion and 
corrosion effects at temperatures between -50 and 300.degree. C., 
preferably 0-100.degree. C. 
According to the principles of the present invention, a cemented carbide 
for oil and gas applications having resistance to erosion and corrosion at 
temperatures between -50.degree. and 300.degree. C. comprising: in wt %, 
2.5-4.5% (Co+Ni) with a weight ratio Co/Ni of about 3.0, 0.25-0.6% Cr and 
0.1% Mo wherein essentially all of the WC grains have a size &lt;1 .mu.m and 
wherein the total carbon content is in the interval of 
6.13-(0.061.+-.0.008).times.binder phase (Co+Ni) content (wt. %). 
Further according to the present invention, a choke trim component for use 
in oil/gas production systems is formed, at least in part, by the cemented 
carbide described above. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Resistance to particle erosion under corrosive environments has been 
achieved by using a specifically optimized multi-alloy binder sintered 
with a submicron grain size WC (i.e. essentially all of the WC grains have 
a size &lt;1 .mu.m). The cemented carbide according to the invention has a 
composition including 2.5-4.5 wt. % (Co+Ni) with a weight ratio Co/Ni of 
about 3, 0.25-0.6 wt. % Cr and about 0.1 wt. % Mo. 
In one preferred embodiment the cemented carbide has the composition, in 
wt. %, 3.3% Co, 1.1% Ni, 0.52% Cr, 0.1% Mo with the balance of WC with an 
average grain size of 0.8 .mu.m. 
In another preferred embodiment the composition, in wt. %, is 1.9% Co, 0.7% 
Ni, 0.3% Cr, 0.1% Mo with the balance of WC of 0.8 .mu.m grain size. 
The carbon content within the sintered cemented carbide must be kept within 
a narrow band in order to retain a high resistance to corrosion and wear 
as well as toughness. The total carbon content shall be in the interval of 
6.13-(0.061.+-.0.008).times.binder phase (Co+Ni) content (wt-%), 
preferably 6.13-(0.061.+-.0.005). 
The hardness of the cemented carbide according to the invention shall be 
&gt;1875 HV30, preferably &gt;1900 HV30 and the transverse rupture strength 
(TRS) as determined according to ISO 3327 (type B test pieces) shall be 
&gt;2100 N/mm.sup.2, preferably &gt;2200 N/mm.sup.2. 
The cemented carbide of this invention can be manufactured by conventional 
powder metallurgical methods such as milling, pressing, shaping, sintering 
and hipping. 
The cemented carbide according to the invention is particularly applicable 
for the choke trim components used in oil and gas industry where 
components are subjected to high pressures of a multi-media fluid where 
there is a corrosive environment including seawater.

EXAMPLE 1 
A cemented carbide according to the invention had the composition, in wt. 
%, 3.3% Co, 1.1% Ni, 0.6% Cr.sub.3 C.sub.2, 0.1% Mo with the balance of 
WC, a hardness of 1900 HV30 and transverse rupture strength (TRS) of 2350 
N/mm.sup.2 with a mean WC grainsize of 0.6 .mu.m. It was tested against 
commercially available cemented carbide grades one made from 6% Co and the 
other from 6% Ni both with the balance of WC (0.8 .mu.m grain size) under 
the following simulated test conditions: 
synthetic seawater 
sand 18 m/s 
CO.sub.2 1 Bar 
temp 54.degree. C. 
The following results were obtained. 
______________________________________ 
corrosion erosion synergistic 
total 
(material (material (material (material 
loss loss loss loss 
Grade in mm/year) in mm/year) in mm/year) in mm/year) 
______________________________________ 
WC 6% Co 
0.02 0.09 0.35 0.46 
WC 6% Ni 0.015 0.265 0.17 0.45 
invention 0.015 0.06 0.025 0.10 
______________________________________ 
EXAMPLE 2 
Cemented carbides were made according to the invention with the composition 
3.3% Co, 1.1% Ni, 0.6% Cr.sub.3 C.sub.2, 0.1% Mo with the balance of WC 
having a grain size on the order of 0.8 .mu.m. A similar alloy with 1.9% 
Co, 0.7% Ni, 0.35% Cr.sub.3 C.sub.2, 0.1% Mo with the balance of WC was 
also made. These alloys are referred to as grades 1 and 2 of the 
invention, respectively. These materials had hardness values of 1900HV30 
and 1910HV30 and transverse rupture strength (TRS) of 2350 N/mm.sup.2 and 
2350 N/mm.sup.2, respectively, each with a mean WC grainsize of 0.6 .mu.m. 
They were tested against commercially available cemented carbide grades 
under the following simulated test conditions of seawater and sand. 
data 
Flow rate: 90 m/sec and impingement angles of 30.degree. and 90.degree.. 
The following results were obtained. 
______________________________________ 
erosion erosion 
(mm.sup.3 /kg sand) (mm.sup.3 /kg sand) 
angle of impingement = angle of impingement = 
Grade 30.degree. 90.degree. 
______________________________________ 
WC 6% Co 1.6 1.4 
WC 6% Ni 2.1 1.6 
invention 1 0.5 0.3 
invention 2 0.25 0.15 
______________________________________ 
EXAMPLE 3 
A cemented carbide according to the invention with the composition 3.3% Co, 
1.1% Ni, 0.6% Cr.sub.3 C.sub.2, 0.1% Mo, with the balance of WC and a 
hardness of 1900HV30 and transverse rupture strength (TRS) of 2350 
N/mm.sup.2 with a mean WC grainsize of 0.6 .mu.m was tested against 
commercially available cemented carbide grades. Test conditions of air and 
sand at 200 m/s: 
Flow rate: 200 m/s Air. 
The following results were obtained. 
______________________________________ 
erosion erosion 
(mm.sup.3 /kg sand) (mm.sup.3 /kg sand) 
angle of impingement = angle of impingement = 
Grade 30.degree. 90.degree. 
______________________________________ 
WC 6% Co 2.5 4.0 
WC 6% Ni 2.6 5.6 
invention 0.8 1.4 
______________________________________ 
The cemented carbide according to the invention shows significant reduction 
in wear as measured by volume loss. 
While the present invention has been described by reference to specific 
examples, it is to be understood that numerous modifications and 
variations will be evident to those skilled in the art. The scope of the 
present invention being limited only by the spirit and scope of the 
appended claims.