Note: Descriptions are shown in the official language in which they were submitted.
CA 02414822 2002-12-18
HYDROGEN-INDUCED-CRACKING RESISTANT AND SULPHIDE-STRESS-
CRACKING RESISTANT STEEL ALLOY
FIELD OF THE INVENTION
This invention relates in general to a steel alloy and more particularly to a
steel alloy that resists hydrogen-induced cracking and sulfide stress
cracking. This alloy
is particularly suitable for use in casing for use with sour oil and gas
wells. The invention
also comprises casing made from such alloy.
BACKGROUND TO THE INVENTION
HIC and SSC are two types of hydrogen embrittlement problems of particular
concern to steel casing operating in sour gas environments. HIC is a hydrogen
sulfide
(HZS) related hydrogen embrittlement phenomenon that manifests itself by
surface blisters
and/or internal cracking. HIC occurs whenever the outside H2S concentration is
sufficiently
large; external stress is not always necessary for HIC to occur. Hydrogen
atoms are
formed on the surface of the steel when the H2S reacts with iron from the
steel. The
hydrogen atoms then diffuse easily through the metal lattice. Then, the
hydrogen atoms
combine preferentially to form molecular hydrogen in hydrogen traps such as
inclusionlmatrix interfaces, voids, impurities, dislocations, laminations or
micro-cracks.
Once hydrogen molecules have farmed in any of these structural defects, the
internal
pressure increases. Once a certain threshold pressure is exceeded, cracking
occurs. The
cracking can assume a straight or stepwise form. If the stepwise cracking
occurs beneath
the steel's surface, blistering will occur on the surface. It has been
observed that HIC
initiates especially at elongated manganese sulfide (MnS) inclusions.
SSC differs from HIC in that SSC manifests itself when an external load is
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CA 02414822 2002-12-18
applied to the steel in a sour gas environment. When the load is applied
parallel to an
elongated MnS inclusion, HIC cracks will not generate the final rupture.
Instead, final
rupture of the steel occurs through cracks perpendicular to the applied load
and localized
in a shear band induced by HIC. If the applied load is perpendicular to the
elongated MnS
inclusion, hydrogen induced cracks nucleated at the inclusion tips will
directly lead to final
rupture. Structural defects other than MnS inclusions will have their critical
concentration
(Ck) for crack initiation lowered by the applied load, and will then compete
with the MnS
inclusions for crack nucleation. Such other structural defects include grain
boundaries,
carbide-ferrite interfaces, oxides, and dislocation tangles.
Oil and gas well steel casing is commonly exposed to highly acidic conditions.
Many new wells contain significant concentrations of HzS while older wells
become
increasingly sour over their production lifetime. It is well known in the oil
and gas industry
that the presence of HZS will act to promote H1C in well casing as HIC is
exacerbated when
well casing is used in low pH environments. In addition to resisting HIC,
casing in mature
oil wells may also be required to bear severe mechanical stresses; thus,
susceptibility to
SSC must also be considered.
As gas and oil reserves are depleted, improved methods of extracting gas
and oil from mature reservoirs have developed apace. In particular, horizontal
well
technology has developed rapidly over the past decade as a means of enhancing
recovery
from mature gas and oil reserves. Well casing used in horizontal well
technology for
mature oil and gas wells is subjected to severe mechanical stresses, SSC, and
HiC.
Two types of horizontal well technology are Steam Assisted Gravity Drainage
(SAGD) and Cyclic Steam Simulation (CSS); while both applications subject
casing to
mechanical stresses, there are particularly severe cyclical tensile and
compressive
stresses in CSS applications. In CSS, steam is pumped into the well during the
initial well
stimulation portion of the horizontal well cycle, thereby heating the casing
wall to
temperatures as high as 350°C for prolonged periods of time up to
around one month.
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CA 02414822 2002-12-18
However, the casing is unable to expand due to the physical constraints placed
on the
casing by the environment. Accordingly, a significant compressive stress is
developed
during this period of the cycle. This compressive stress will gradually relax
through stress
relaxation over an extended period of time. At the conclusion of the
stimulation cycle, the
well cools and the resultant thermal contraction of the material results in a
tensile load.
Again, this load may gradually relax over an extended period of time, so that
when the next
stimulation cycle is initiated, the casing again goes into compression. This
cycling will be
repeated several times through the life of the well and places severe fatigue
stresses on
the casing.
As was well known in the art prior to the time of the invention, HIC- and SSC-
resistant casing alloys are generally made from seamless casing. In order to
produce a
pipe the alloy is cast as a billet and rolled into a solid round piece that is
then pierced.
Unfortunately, seamless casing is very expensive to manufacture. Further to
this, the wall
thickness may be irregular and segregation may occur on the centreline. which
becomes
the inner wall upon piercing. On the other hand, rolling and welding produces
a casing that
has a more uniform wall thickness. It is also less expensive to produce.
Consequently,
it would be preferable to manufacture the casing by rolling and welding the
alloy. AS would
be apparent to one skilled int the art, the development of HIC- and SSC
resistant steel
alloys suitable for rolling and welding requires a synergy between the method
of production
and the alloy chemistry employed.
Casing for horizontal oil wells must be made using alloys that can resist
severe mechanical stresses, HIC and SSC. The IK55 alloy of Ipsco Inc.
described
hereinbelow, is one such alloy that is at present used to make horizontal well
casing.
The IK55 alloy is a medium-carbon quench-and-temper steel to which boron
has been added to ensure hardening of the full thickness of the steel during
the quenching
treatment. To be effective, boron must be retained in solid solution
throughout the
processing schedule; however, boron interacts strongly with nitrogen to form
boron nitrides
-,
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CA 02414822 2002-12-18
which render boron additions ineffective. To prevent boron nitride formation,
titanium is
added to react with nitrogen in advance of any reaction between boron and
nitrogen,
thereby ensuring that boron is retained in solid solution.
Although steel alloys such as the IK55 alloy perform well in laboratory tests
in which the pH is greater than about 4.25, if such alloys are exposed to low
pH
environments, environments having a pH lower than about 4.25, over a prolonged
period
of time, such alloys are subject to HIC. Accordingly, there is a need for a
HIC resistant
steel alloy suitable for environments having a pH of less than 4.25. However,
even alloys
that demonstrate good HIC resistance in laboratory trials may perform poorly
when stress
is applied. Thus, there is a need for alloys that demonstrate both good HIC
resistance and
good SSC resistance.
SUMMARY OF THE INVENTION '
An object of one aspect of the present invention is to provide a quench-and
temper-steel alloy that provides resistance to HIC and SSC in low pH
environments and
that has use especially in oil and gas industry applications such as casing.
~ther objects
are for this steel alloy to have good corrosion resistance and stability at
elevated
temperature in the order of 335°C.
In accordance with one aspect of the invention there is provided a quench
and temper steel alloy characterized in that the alloy has, by weight, a
carbon (C) range
of 0.15% to 0.35%,a manganese range of 0.60% to 1.10%, a molybdenum (Mo) range
of
at least 0.15%, and a sulfur (S) range of less than 0.002%, a chromium (Cr)
range of less
than or equal to 0.50%, an aluminum (AI) range of less than or equal to
0.080%, a calcium
(Ca) range of less than or equal to 0.0045%, a silicon (Si) range of less than
or equal to
0.40%, and the substantial balance of the alloy being iron and unavoidable
impurities. The
steel alloy is further characterized in that the alloy has a quench-and-temper
micro-
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CA 02414822 2002-12-18
structure and has precipitated spheroidal Mo carbides in Mn and C rich bands.
The
precipitated Mo carbides result from sustained tempering at high temperatures.
The
precipitation reduces the carbon content in the matrix of the Mn and C rich
segregation
bands, and decreases the hardness of the matrix.
J
Mo is included in the alloy to harden the alloy, so as to enable boron (B) and
titanium (Ti) to be substantially excluded from the alloy and to reduce the Mn
content in the
steel. This substantially precludes the formation of boron nitride and
titanium nitride~which
may play a role in the formation of HIC and SSC in the steel. As HIC and SSC
are found
particularly at MnS inclusions in the steel, the relatively low S and Mn
content reduce the
presence of such inclusions in this steel alloy. Mo has the additional
beneficial effect of
retarding stress relaxation in steel at elevated temperature; this contributes
to the steel
alloy being able to withstand stresses for prolonged periods at elevated
temperatures. Mo
also contributes to slow the rate of corrosion of the steel alloy. As hydrogen
is a corrosion
product, hydrogen formation in the steel is thus slowed.
In various preferred embodiments of the invention, the range of each element
of the alloy is more narrowly defined. The selection of the particular alloy
chemistry, within
the above specified limits, depends on trade-offs between a number of
different factors
such as the cost of the various alloying elements, as well as the HIC and SSC
resistance
required. In a first preferred alloy chemistry, the alloy has a carbon range
by weight of 0.20 w
to 0.3%, a manganese range by weight of 0.60% to 1.10%, a molybdenum range by
weight
of 0.45 to 0.55%, and a sulfur range by weight of less than 0.001 %. The above-
defined
preferred alloy chemistry is further definable to have a calcium range, by
weight, of
0.0020% to 0.0045%, and an aluminum range of 0.030% to 0.050%, The alloy may
also
comprise silicon in a range of 0.15 to 0.25% by weight. Chromium in a range of
0.2 to
0.3% may further be added to the alloy.
In another preferred alloy chemistry, the carbon range by weight is 0,18% to
0.27%, the manganese range by weight is 0.70% to 0.95%, the molybdenum range
by
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CA 02414822 2002-12-18
weight is 0.35% to 0.55%, and the sulfur range by weight lis less than 0.001
%. The steel
alloy has a calcium range, by weight, of 0.0020% to 0.0045%, and an aluminum
range of
0.030% to 0.050%. This alloy is cheaper and somewhat less resistant to SSC
than an
alloy of the first chemistry.
Both preferred chemistries are further characterized by having a quench-and-
temper micro-structure and having precipitated spheroidal Mo carbides in Mn
and C rich
bands, the precipitation resulting from a sustained high tempering treatment
of the alloy
during manufacture.
In accordance with another aspect of the invention, there is provided a
quench-and-temper casing for transporting fluids such as oil, gas and steam
having good
HIC and SSC resistance, corrosion resistance and stability at elevated
temperature in the
order of 335°C. The casing has an alioy chemistry, by weight, a carbon
range of 0.15%
to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at feast
0.15%,
a sulfur range of less than 0.002%, a chromium (Cr) range of less than or
equal to 0.50%,
an aluminum (AI) range of less than or equal to 0.080%, a calcium (Ca) range
of less than
or equal to 0.0045%, a silicon (Si) range of less than or equal to 0.40% and
the substantial
balance of the alloy being iron and unavoidable impurities. The casing
substantially
excludes boron and titanium. The casing is further characterized by having a
quench-and-
temper micro-structure and having precipitated spheroidal Mo carbides in Mn
and C rich
bands, the precipitation resulting from a sustained high tempering treatment
of the alloy
during manufacture.
In accordance with a further aspect of the invention, there is provided a
method of extracting oil and gas from oil wells. In accordance with this
method, a casing
is installed to carry the oil and gas. The casing is made from a steel alloy
that has ari alloy
chemistry, by weight of a carbon range of 0.15% to 0.35%, a manganese range of
d.60%
to 1.10%, a molybdenum range of at least 0.15%, a sulfur range of less than
0.002%, a
chromium (Cr) range of less than or equal to 0.50%, an aluminum (AI) range of
less than
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CA 02414822 2002-12-18
or equal to 0.080%, a calcium (Ca) range of less than or equal to 0.0045%, a
silicon (Si)
range of less than or equal to 0.40% and the substantial balance of the alloy
being iron and
unavoidable impurities. The casing substantially excludes boron and titanium.
The casing
is further characterized by having a quench-and-temper micro-structure and
having
precipitated spheroidal Mo carbides in Mn- and C-rich bands, the precipitation
resulting
from a sustained high tempering treatment of the alloy during manufacture.
The scope of the invention contemplates adding other known alloying
elements to the alloy, provided that such known alloying elements do not
substantially
affect the HIC and SSC resistance of the alloy. The effect of such additional
alloying
elements may be tested empirically by a person skilled in the art. These known
alloying
elements may be added to provide new properties to the alloy, or enhance
certain
properties already present in the alloy. These known alloying elements may be
added to
provide new properties to the alloy or enhance certain properties already
present in the
alloy. For example, Ca is added to desulphurize the alloy, Si is added as a de-
oxidant and
to strengthen the steel, Cr is added to enhance the hardenability and increase
the
corrosion resistance and AI is added to deoxidize the steel. Additionally,
minor alloying
elements including a nickel (Ni) niobium (Nb), vanadium(, and copper (Cu)may
be
present. Cu and Ni can be added to improve the hardenability of the alloy.
They can also .
be expected to enhance the corrosion resistance. Nb and 'V can be added to
increase the
strength of the alloy. ' .
BRIEF DESCRIPTION OF THE DRAWINGS
A detailed description of the preferred embodiments are provided herein
below with reference to the following drawings, in which:
Figure 1 is a plane view of a hydrogen-induced crack in the IK55 casing
product which clearly shows boron nitride and titanium nitride inclusions on
the surface of
7
CA 02414822 2002-12-18
the crack.
Figure 2 is a graph plotting the compressive stress applied to a casing made
from alloy A of the invention against the resulting percentage compression of
the casing.
Figure 3 is a graph plotting the tensile stress applied to a casing made from
alloy A of the invention against the resulting percentage elongation of the
casing. .
Figure 4 is a graph plotting HIC behaviour of heat treated IK55 casing.
Figure 5 is a graph plotting SSC test results for alloys B, X and Y, as
defined
in Table 5.
Figure 6 is a graph plotting segregation for alloys B, X and Y, as defined in
Table 5.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
It was well known prior to the present invention that other benign alloying
elements might be added to alloys of the present general type without
interfering with the
metallurgical objectives of the present invention.
It was well known prior to the present invention that trace amounts of
miscellaneous elements might be found in typical charges of scrap steel to the
melt
furnaces, without serious damage to the alloying objectives of the present
invention.
Examples of the foregoing are Cu at approximately 0.1 %, Ni at less than
approximately
0.10%, AI and Si. The present invention as described and claimed does not take
into
account the possible presence of such trace amounts of miscellaneous elements.
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CA 02414822 2002-12-18
It was well known prior to the present invention that small amounts of some
elements having a potentially deleterious effect on the desired metallurgical
objectives of
the invention could be present in the scrap charge. Such elements include
phosphorus,
tin, arsenic, boron, titanium, lead and tungsten. It was also previously known
that the
following countermeasures could be taken if such elements were present in the
steel
beyond an acceptable trace level. The present invention as described and
claimed does
not take into account the possible presence of such potentially deleterious
amounts of
harmful elements nor of the countermeasures that might conventionally be
taken; it is
presumed that if a steelmaker encounters a problem of the foregoing sort, the
steelmaker
will take effective countermeasures; in the worst case, a particular batch of
steel may be
scrapped or put to a less demanding end use.
It is also well known that a number of minor alloying elements may be present
in quantities sufficient to impart an effect on the physical properties of the
resultant alloy
steel. For example, silicon is well known to be present in the scrap charge.
Other alloying
elements that can frequently be present in typical charges of scrap steel
include aluminum,
copper and nickel.
Accordingly, the description below is directed to the chemistry of the
invention
without referencing the possible presence of benign allaying elements,
miscellaneous
elements, deleterious elements, methods to remove deleterious elements or
minor alloying
elements.
The applicant has developed an alloy steel having good HIC and SSC
resistance performance, and that is particularly useful for use in steel well
casing for oil and
gas operations wherein the environment is acidic and may be at elevated
temperatures for
pralonged periods of time.
This alloy steel was designed to meet the following target design criteria:
9
CA 02414822 2002-12-18
Class: Quench and temper (Q&T) steel alloy, 0.15 to 0.35 wt.
carbon content
Mechanical properties: API 5CT L80
HIC: MACE TM 0284: <10% crack length ratio in acidified brine solution pH=2:68
SSC: NACE TM 0177; SSC threshold test > 90% actual yield stress
Other: good corrosion resistance ' .
stability at elevated temperatures around 335°C
As discussed below, the target design criteria was achieved by an alloy steel
having a particular chemical composition and having a particular micro-
structure and
physical properties that are obtained from a selected method of manufacture.
1. CHEMICAL COMPOSITION
Through experimentation, the applicant has gained additional insight into
factors that contribute to the susceptibility of steel alloys to HIC and SSC.
These factors
include the presence of large or elongated inclusions in the alloy, as well
the presence of
titanium nitride and boron nitride precipitates, and the degree of alloy
segregation. These
factors were considered when selecting the particular chemical composition of
the steel
alloy.
a. Large or elongated inclusions
Large or elongated inclusions act as sinks for atomic hydrogen, which diffuse
into the metal from corrosion reactions at the casing surface. At the
inclusion-matrix
interface, atomic hydrogen combines to form molecular H 2. Gradually, the
hydrogen
pressure will increase until a crack is initiated and propagates through the
metal.
Elongated MnS inclusions are particularly susceptible to this form of attack.
CA 02414822 2002-12-18
Manganese typically serves as a hardening agent in alloy steels. To reduce
the MnS inclusion content in such a steel alloy, one or both of Mn and S
content can
be reduced. To reduce sulfur content, the alloy of the present invention
employs
a clean scrap melting practice to reduce sulfur to less than 0.002 wt
percentage and
preferably around 0.001 wt percentage. As well, Ca powder is injected into the
molten alloy. Calcium powder preferably combines with sulfur remaining in the
steel
alloy to form globular CaS particles which float out of the bath. Calcium
powder is
no longer added when the concentration of calcium reaches a range of 0.0020%
to
0.0045%; at that point, sufficient sulfur has been removed to lower the sulfur
concentration to an acceptable range. Silicon (Si) of between 0% and 0.40% by
weight may also be added to the alloy as a deoxidant, to enhance
desulphurization
and to increase the strength of the steel alloy end product.
By reducing the Mn content in the alloy, MnS inclusion formation should be
suppressed. As the presence of Mn desirably promotes hardenability of the
alloy
steel, i.e. the achievement of martensite through the thickness of the steel,
another
element must be provided that will provide comparable hardenability.
Molybdenum
(Mo) is such an element; Mo in the order of at least 0.15 wt. % is thus added.
to the
alloy composition to enable the Mn content to be reduced to between 0.60 and
1.0
wt. %. Mo has also the beneficial effect of retarding stress relaxation in
steel at
elevated temperatures, and particularly at temperatures in the range which an
oil
and gas casing would be exposed to during a steaming cycle . Mo has a further
beneficial effect of slowing the rate of corrosion of the steel. As hydrogen
is a
corrosion product, the formation of molecular hydrogen in the steel when the
steel
is exposed to a sour gas environment should be slowed.
Mo has the additional benefit of segregating to the Mn- and C-rich bands in
the steel. Carbon tends to segregate during casting and the carbon content in
the
C-rich band may be significantly higher than the bulk analysis, perhaps as
high as
0.8%. The segregated Mo combines with the C to form spherical molybdenum
11
CA 02414822 2002-12-18
carbides in the bands, thereby relieving the high carbon content in the matrix
of the
bands. This effectively decreases the hardness of the matrix. Further,
the'local
tolerance to hydrogen is improved, as the precipitated carbides act as small
hydrogen traps.
It has been found that adding beyond 0.65 wt.% Mo is not cost-effective for
typical casing applications given the relative expense of Mo and that most of
the
beneficial effects of Mo are achieved with less than 0.65 wt.%.
Chromium (Cr) may be added to the alloy to promote hardenability, thereby
enabling the Mn content to be reduced. The amount of Cr to be added depends
on the percentage of Mn and Mo in the composition; the percentage of Cr varies
inversely with the percentage of Mn and Mo. Cr is a relatively expensive
alloying
element, and it has been found not to be cost-effective to exceed a Cr content
of
0.50 wt.% for typical casing applications.
b. Titanium and boron nitride precipitates
From experimentation, and as illustrated in Figure 1, the applicant observed
that titanium and boron nitride (BN and TiN) precipitates are frequently
present on
the crack surtaces of alloy samples after HIC testing, and appear to
contribute to
the fracture behaviour. Accordingly, the alloy of the present invention
excludes
titanium (Ti) and boron (B); the selected Mo and Cr content provides hardening
that
would have been provided by the boron, and thus enables the exclusion of B
and.
Ti.
c. Alloy segregation
The presence of hard bands associated with alloy segregation appears to
contribute to HIC and SSC. The most direct way to minimize segregation is to
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CA 02414822 2002-12-18
reduce the percentage of the segregating elements, particularly carbon,
manganese
and phosphorous. Accordingly, the carbon, and manganese contents have been
kept relatively low in the alloy of the invention. Through experimentation
with
various alloys in which titanium and boron have been substantially excluded
and
molybdenum has been added to provide hardening, it has been found that the
increased alloy segregation resulting from increases in the carbon content can
be
ofFset by reductions in the manganese content. The chemistries of alloys A and
B
specified below have been determined, in part, by this insight.
The below is a summary of the elements that are important to achieving the
design target properties of the iron-based steel alloy, and the acceptable
content o~ each
of these elements in the alloy:
1. The acceptable percentage range of carbon is 0.15 wt% to about 0.35 wt%
(significant digits = 2).
2. The acceptable percentage range of manganese is 0.60 wt% to 1.10 wt%
(significant digits = 2).
3. The acceptable minimum percentage of molybdenum is 0.15 wt% (significant
digits
- 2); note that while the alloy can be made using greater than 0.65 wt%
molybdenum, to do so would probably be uneconamic; accordingly, the preferred
range would not exceed about 0.65 wt %.
4. The acceptable percentage range of sulfur is up to 0.002 wt% (significant
digits =3);
sulfur should preferably be eliminated to the extent economically possible.
While all of the alloy chemistries falling within the above-specified ranges
will
enjoy, to some extent, the advantages of the invention, not all chemistries
falling within
these ranges are desirable as there are trade-offs that must be borne in
mind~'when
13
CA 02414822 2002-12-18
selecting an alloy chemistry. For example, while an alloy having a carbon
content of
0.35%, a manganese content of 1.0%, and acceptable ranges of sulphur,
molybdenum,
boron and titanium, would fall within fihe above-specified range and would
enjoy the
general advantages of the invention in terms of the reduced susceptibility to
HIC due to the
exclusion of titanium and boron, the relatively high carbon and manganese
content of such
an alloy would lead to increased alloy segregation. This increased alloy
segregation would
reduce HIC and SSC resistance. Accordingly, increases in one of carbon or
manganese
should be offset by decreases in the other of carbon or manganese. If, on the
other hand,
the manganese content is kept towards the low end of the range, it will be
possible to raise
the carbon content to near the upper end of the range, but it will also be
necessary to raise
the molybdenum content of the alloy to compensate for some of the contribution
to
hardening that would otherwise be afforded by the manganese. In this case, the
need to
increase the molybdenum content to substitute for the manganese, can be offset
by
increasing the chromium content to substitute for the manganese and
molybdenum. Thus,
while the above-specified ranges are stated in absolute terms, there are
complicated
interrelationships between the elements that must be borne in mind when
selecting a
suitable alloy chemistry. However, through reasonable experimentation, those
skilled in
the art could determine many different suitable alloy chemistries that fall
within the above-
specified ranges. Two such alloy chemistries are described in detail below.
Alloy A
This is a high performance alloy that offers HIC resistance combined with
superior performance in laboratory SSC tests. Generally, the following ranges
of
acceptable chemistries are suitable for alloy A of the invention:
Essential alloying elements:
1. The percentage range of carbon in alloy A is 0.20 to 0.30 wt%(significant
digits = 2).
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CA 02414822 2002-12-18
2. The percentage range of manganese in alloy A is 0.70 wt% to 0.85 wt%
(significant
digits = 2).
3. The percentage range of molybdenum in alloy A is 0.45 wt% to 0.55 wt%
(significant digits = 2).
4. Chromium is in the range of 0.20 wt% to 0.30 wt% (significant digits =2).
Within the
above-defined ranges, the percentage of chromium varies inversely with the
percentage of manganese and molybdenum.
4. Calcium is in the range of 0.0020 to 0.0045 wt% (significant digits = 4).
(As
mentioned above, calcium can be used to combine with trace sulfur to form CaS
globules that float out of the bath.
Optional alloying elements:
1. Aluminum is in the range of 0.030 to 0.050 wt% and is preferably 0.040 wt%
(significant digits = 3). Aluminum is added to deoxidize the alloy.
2. Silicon is in the range of 0 to 0.40 wt% (significant digits = 2). Si is
added to
deoxidize the steel, enhance the desulphurization process, and improve the
strength of the alloy steel end product.
3. Copper is in the range of 0.010 to 0.80%.
.
4. Nickel is in the range of less than or equal to 0.50%.
5. Niobium is in the range of less than or equal to 0.10%.
6. Vanadium is in the range of less than or equal to 0.10%:
IS
CA 02414822 2002-12-18
Undesirable alloying elements:
1. The percentage of sulfur is s 0.002 wt% and preferably less than 0.001 wt.%
(significant digits = 3).
2. Boron and titanium should be present in no more than trace amounts; they
should
be eliminated to the extent economically feasible.
Alloy B
Alloy B represents a cheaper alternative to Alloy A as it has a lower content
of
alloying elements such as chromium. Alloy B has good HIC resistance but is
less resistant
to SSC than alloy A. Alloy B has the following chemistry: '
Essential alloying elements:
1. The percentage range of carbon in alloy B is 0.18 to 0.22 wt%; the optimum
amount
is 0.20 wt% (significant digits = 2).
2. The percentage range of manganese in alloy B is 0.6% to 1.10% (significant
digits
= 2).
3. The percentage range of molybdenum in alloy B is 0.28 wt% to 0.32 wt%; the
optimum amount is 0.30 wt% (significant digits = 2).
3. Calcium is in the range of 0.0020 to 0.0045 wt% (significant digits = 4).
(As
mentioned above, calcium can be used to combine with trace sulfur to form CaS
globules that float out of the bath.)
16
CA 02414822 2002-12-18
Optional alloying elements:
1. Aluminum is in the range of 0.030 to 0.050 wt% and is preferably 0.040 wt%
(significant digits = 3). Aluminum is added to deoxidize the alloy.
2. Silicon is in the range of 0 to 0.50 wt% (significant digits = 2). Si is
added to
deoxidize the steel, enhance the desulphurization process, and improve the
strength of the alloy steel end product.
3. Copper is in the range of 0.010 to 0.50%.
4. Nickel is in the range of less than or equal to 0.50%.
5. Niobium is in the range of less than or equal to 0.10%.
6. Vanadium is in the range of less than or equal to 0.10%.
Undesirable alloying elements:
1. The percentage of sulfur is <_ 0.001 wt% (significant digits = 3).
2. Boron and titanium should be present in no more than trace amounts; they
should
be eliminated to the extent economically feasible.
2. MANUFACTURE
Once the chemical composition of the alloy has been selected, the alloy is
manufactured for the most part according conventional steelmaking practice.
The steel is
prepared by melting in an electric arc furnace, employing a clean scrap
practice to
minimize sulphur content. The steel is then transferred to a ladle metallurgy
furnace for
17
CA 02414822 2002-12-18
final alloy addition and calcium treatment. The steel is then cast; casting
speed can be
optimized to permit flotation of calcium sulfides. The steel is then hot
rolled and coiled.
Such processes steps are conventional and known to a skilled operator in the
art.
Hot rolling is considered to be generally superior to the production of
seamless casing as it is more cost effective and produces a pipe with a
greater uniformity
of wall thickness. It is, however, not generally applied to thae production of
SSC- and HIC-
resistance steel alloys prior to the present invention. The Applicants expect
that hot rolling
was not, to their knowledge, employed because the appropriate alloy chemistry
was not
available.
To manufacture casing, the rolled steel is cut to the desired width (depending
on the diameter of the casing to be made) and formed into pipe by utilizing an
Electric
Resistance Welding (ERW) process.
The steel is then austenized at around 925°C and quenched. The
steel is
then tempered; the tempering temperatures are dependant upon the desired final
properties. It has been found that tempering for two (2) hours at 700°C
achieves the
design targets.
It was well known prior to the present invention that conventional quench-
and-temper technology could produce the desired micro-structure of the present
invention.
Quench-and-tempering treatment is carried out to produce a high proportion of
martensite
and hence a tempered martensite micro-structure. The quench-and-tempering
treatment
of the alloy in the present invention is adjusted to allow for the slower
tempering kinetics
of molybdenum-bearing steel. As would be known to someone skilled in the art
at the time
of the invention, a relatively high temperature for an extended time period
would be
required to temper the steel in order to achieve the desired hardness. If the
desired
hardness was achieved, it would also have been implicit that molybdenum
carbide
precipitation would occur. However, one skilled in the art of metallurgy would
not have
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CA 02414822 2002-12-18
assumed that the treatment would produce not only a martensite micro-
structure, but would
promote the formation of precipitated molybdenum carbides in manganese- and
carbon-
rich bands of the alloy micro-structure.
Both alloys manufactured by this method exhibit a quenched and tempered
martensite structure through the full wall thickness, with diffuse bands of
segregation.
Unlike non-tempered quenched martensite, tempered quenched martensite has iron
carbide precipitates throughout the micro-structure, that serve to lower the
hardness of the
steel. Also as a result of the tempering step, spherical molybdenum carbides
will be
present and especially in the segregation bands rich in Mn and C. Tensile
properties may
be adjusted by adjustment of the tempering treatment.
Alloy A manufactured according to the above process has a Rockwell B
hardness of 96 (Vickers (10 Kg load) of 227) and enjoys the properties listed
in Table 1
below.
Table 1 - Mechanical properties at elevated temperattares:
Temperature Yield StrengthUltimate TensileYdT
(C) MPa) Stren th (MPa
628 695 0.904
20 150 522 604 0.865
270 501 689 0.727
335 481 661 0.728
* 25.5. mm gauge
Alloy B manufactured according to the above process has been found to
resist HIC in laboratory environments with a pH as low as 2.68, and in
addition, possesses
the mechanical properties necessary to resist cracking due to thermal
expansion and
compression experienced in the horizontal well environment. Specifically, the
applicant's
19
CA 02414822 2002-12-18
alloy B has a thermal expansion coefficient of 14.2 x 10-6 °c-' enjoys
the properties listed
in Tables 2 and 3 below:
Table 2 - Mechanical properties of Alloy B:
Heat TreatmentYS UTS YIT Elong. Hardness
/~ VHN
I Q & T 602 MPa 662 MPa 0.91 32 245
~
Table 3 - Mechanical properties at elevated temperatures:
Test YS UTS YtT Elong.*
(MPa) MPa %)
Compression (350C) 468 - -
Tension (350C) 457 642.10.71 33.8
Tension (190C) after 1 % compression520 657 0.79 42.8
* 25.5. mm gauge
Table 4 - Properties of a production run of Alloy B casing:
Product YS UTS YIT Elong.Hardness
(MPa) (MPa) (%j RB HVSOo
0.395 wall, 9 5/8" dia 603 698 0.86 33 96.9 228
API 5CT L-80 Specification552 655 - 18.5 23 max _
- min min R
655
In the following table, Table 5, double cantilever beam SSC resistance data
obtained for Alloy B are outlined. The double cantilever beam accetJtance
criterion i~
currently 35 Mpa.m 'h or greater. Note, however, that there is evidence that
the use of
subsize samples significantly lowers Kissc - the difference may be as much as
37%, and
the samples of Alloy B were subsized.
CA 02414822 2002-12-18
Table 5 - Double Cantilever Beam SSC resistance for Alloy B and IK55 casing:
Sample Grade Kiscc- Ave Kissc- Overall
Mpa.m'/Z Mpa.m'/2 Ave
1 Kissc-
K55 27.2 28.9 Mpa.m'/2
30.25
1 IK55 27.7
1 IK55 31.7
2 IK55 29.7 31.6
2 IK55 37.3
2 IK55 27.9
3 Alloy B 27.9 31.1
4 Alloy B 30.7
4 Alloy B 31.2
5 Alloy B 36.2 32.3
5 Alloy B 35.2
5 Alloy B 25.4
Figure 4 shows the HIC behaviour of heat treated 1K55 casing. A threshold
exists at a pH of 4.25, above which, no cracking was observed. The extent of
cracking is
somewhat dependent upon time, but very significant cracking was evident at pH
< 3.5. In
comparison, HIC testing of Alloy B was carried out at pH 2.68. The crack
length ratio was
usually zero at this pH, although some cracking was occasionally observed. If
cracking
was observed, it was less than 6%.
Figure 5 shows the SSC resulfis for three alloys: Alloy B of the present
invention, Alloy X and Alloy Y, which are IK55 with boron and titanium
eliminated and
employing a clean steel melting practice to minimize sulphur content. The
alloy chemistry
21
CA 02414822 2002-12-18
of the three alloys is shown in Table 6. The effect of lowering the manganese
content can
be seen. Figure 6 shows segregation data for alloys B, X and Y, as defined in
Table 5.
The data were generated by tracing across a segregation band with a micro-
probe. Alloy
Y shows manganese peaks of approximately 2.1 % while Alloy B only shows peaks
manganese of approximately 1.1 %. These data therefore show the beneficial
effect of
lowering the manganese content.
Table 6 - Chemistry of the alloys employed to determine the SSC results of
Figure
5:
Alloy C Mn Mo Cr AI S Other
B 0.26 0.71 0.512 0.223 0.03 < 15 Ca
ppm
X 0.18 1.14 0.449 0.293 0.03 < 15 Ca
ppm
Y 0.21 1.31 0.291 0.073 0.037 <15 Ca
ppm
In a further preferred embodiment of the invention, the invention is embodied
as a well casing made from an alloy as described below. The casing is
manufactured in
the manner described above. The quench and tempering treatment of the alloy is
adjusted
to allow for the slower tempering kinetics of molybdenum-bearing steel. A well
casing in
accordance with this embodiment of the invention and made from alloy B as
defined above
enjoys the mechanical properties listed in Table 5 below. The API 5CT L-80
specification
is provided for comparison purposes.
The invention may also be implemented as a method of extracting oil and gas
from oil wells in which the casing of the above embodiment of the invention is
installed to
carry oil and gas or in which a well casing is installed made of the alloy of
the invention.
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CA 02414822 2002-12-18
Other variations and modifications of the invention are possible. All such
modifications or variations are believed to be within the sphere and scope of
the invention
as defined by the claims appended hereto.
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