Hydrogen-induced-cracking resistant and sulphide-stress-cracking resistant steel alloy

Abstract

The invention relates to a quench-and-temper steel alloy for use in casing for oil and gas wells wherein such casing is exposed to low pH environments. The steel alloy has a carbon range by weight of 0.15% to 0.35%, a manganese range by weight of 0.60% to 1.10%, a molybdenum range by weight of 0.15% to 0.65%, and a sulphur range by weight of less than 0.002%. The steel alloy has a quench-and-temper micro-structure and features precipitated spheroidal molybdenum carbides in manganese- and carbon-rich bands. The steel alloy also has, by weight, a chromium range of less than 0.50%, an aluminum range of less than or equal to 0.08% and a calcium range of less than or equal to 0.0045%.

Description

FIELD OF THE INVENTION

[0002] 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

[0003] 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 (H2S) 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 inclusion/matrix interfaces, voids, impurities, dislocations, laminations or micro-cracks. Once hydrogen molecules have formed 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.

[0004] SSC differs from HIC in that SSC manifests itself when an external load is 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.

[0005] Oil and gas well steel casing is commonly exposed to highly acidic conditions. Many new wells contain significant concentrations of H2S while older wells become increasingly sour over their production lifetime. It is well known in the oil and gas industry that the presence of H2S will act to promote HIC 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.

[0006] 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.

[0007] 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. 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.

[0008] 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.

[0009] 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.

[0010] 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 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.

[0011] 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

[0012] 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. Other objects are for this steel alloy to have good corrosion resistance and stability at elevated temperature in the order of 335° C.

[0013] 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 (Al) 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-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.

[0014] 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.

[0015] 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 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.

[0016] 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 weight is 0.35% to 0.55%, and the sulfur range by weight is 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.

[0017] 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.

[0018] 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 alloy 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 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 (Al) 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.

[0019] 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 an alloy chemistry, by weight of a carbon range of 0.15% to 0.35%, a manganese range of 0.60% 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 (Al) 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.

[0020] 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 Al is added to deoxidize the steel. Additionally, minor alloying elements including a nickel (Ni) niobium (Nb), vanadium (V), 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

[0021] A detailed description of the preferred embodiments are provided herein below with reference to the following drawings, in which:

[0022] FIG. 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 the crack.

[0023] FIG. 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.

[0024] FIG. 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.

[0025] FIG. 4 is a graph plotting HIC behaviour of heat treated IK55 casing.

[0026] FIG. 5 is a graph plotting SSC test results for alloys B, X and Y, as defined in Table 5.

[0027] FIG. 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

[0028] 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.

[0029] 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%, Al and Si. The present invention as described and claimed does not take into account the possible presence of such trace amounts of miscellaneous elements.

[0030] 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.

[0031] 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.

[0032] Accordingly, the description below is directed to the chemistry of the invention without referencing the possible presence of benign alloying elements, miscellaneous elements, deleterious elements, methods to remove deleterious elements or minor alloying elements.

[0033] 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 prolonged periods of time.

[0034] This alloy steel was designed to meet the following target design criteria:

[0035] Class: Quench and temper (Q&T) steel alloy, 0.15 to 0.35 wt. % carbon content

[0036] Mechanical properties: API 5CT L80

[0037] HIC: NACE TM 0284: <10% crack length ratio in acidified brine solution pH=2.68

[0038] SSC: NACE TM 0177: SSC threshold test>90% actual yield stress

[0039] Other: good corrosion resistance stability at elevated temperatures around 335° C.

[0040] 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.

[0041] 1. Chemical Composition

[0042] 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.

[0043] a. Large or Elongated Inclusions

[0044] 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 H2. 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.

[0045] 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.

[0046] 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.

[0047] 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 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.

[0048] 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. %.

[0049] 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.

[0050] b. Titanium and Boron Nitride Precipitates

[0051] From experimentation, and as illustrated in FIG. 1, the applicant observed that titanium and boron nitride (BN and TiN) precipitates are frequently present on the crack surfaces 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.

[0052] c. Alloy Segregation

[0053] 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 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.

[0054] 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 of each of these elements in the alloy:

[0055] 1. The acceptable percentage range of carbon is 0.15 wt % to about 0.35 wt % (significant digits=2).

[0056] 2. The acceptable percentage range of manganese is 0.60 wt % to 1.10 wt % (significant digits=2).

[0057] 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 uneconomic; accordingly, the preferred range would not exceed about 0.65 wt %.

[0058] 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.

[0059] 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 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 the 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.

[0060] Alloy A

[0061] 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:

[0062] Essential Alloying Elements:

[0063] 1. The percentage range of carbon in alloy A is 0.20 to 0.30 wt % (significant digits=2).

[0064] 2. The percentage range of manganese in alloy A is 0.70 wt % to 0.85 wt % (significant digits=2).

[0065] 3. The percentage range of molybdenum in alloy A is 0.45 wt % to 0.55 wt % (significant digits=2).

[0066] 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.

[0067] 5. 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.

[0068] Optional Alloying Elements:

[0069] 1. Aluminium 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.

[0070] 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.

[0071] 3. Copper is in the range of 0.010 to 0.80%.

[0072] 4. Nickel is in the range of less than or equal to 0.50%.

[0073] 5. Niobium is in the range of less than or equal to 0.10%.

[0074] 6. Vanadium is in the range of less than or equal to 0.10%.

[0075] Undesirable Alloying Elements:

[0076] 1. The percentage of sulfur is ≦0.002 wt % and preferably less than 0.001 wt. % (significant digits=3).

[0077] 2. Boron and titanium should be present in no more than trace amounts; they should be eliminated to the extent economically feasible.

[0078] Alloy B

[0079] 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:

[0080] Essential Alloying Elements:

[0081] 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).

[0082] 2. The percentage range of manganese in alloy B is 0.6% to 1.10% (significant digits=2).

[0083] 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).

[0084] 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.)

[0085] Optional Alloying Elements:

[0086] 1. Aluminium 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.

[0087] 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.

[0088] 3. Copper is in the range of 0.010 to 0.50%.

[0089] 4. Nickel is in the range of less than or equal to 0.50%.

[0090] 5. Niobium is in the range of less than or equal to 0.10%.

[0091] 6. Vanadium is in the range of less than or equal to 0.10%.

[0092] Undesirable Alloying Elements:

[0093] 1. The percentage of sulfur is ≦0.001 wt % (significant digits=3).

[0094] 2. Boron and titanium should be present in no more than trace amounts; they should be eliminated to the extent economically feasible.

[0095] 2. Manufacture

[0096] 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 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.

[0097] 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 the 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.

[0098] 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.

[0099] 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.

[0100] 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 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.

[0101] 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.

[0102] 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 temperatures:
	Yield Strength	Ultimate Tensile
Temperature (° C.)	(MPa)	Strength (MPa)	Y/T
25	628	695	0.904
150	522	604	0.865
270	501	689	0.727
335	481	661	0.728
* 25.5. mm gauge

[0103] 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 alloy B has a thermal expansion coefficient of 14.2×10−6° c−1 enjoys the properties listed in Tables 2 and 3 below:

TABLE 2
Mechanical properties of Alloy B:
				Elong.	Hardness
Heat Treatment	YS	UTS	Y/T	(%)	(VHN)
Q & T	602 MPa	662 MPa	0.91	32	245

[0104]

TABLE 3
Mechanical properties at elevated temperatures:
	YS	UTS		Elong. *
Test	(MPa)	(MPa)	Y/T	(%)
Compression (350° C.)	468	—	—
Tension (350° C.)	457	642.1	0.71	33.8
Tension (190° C.) after 1%	520	657	0.79	42.8
compression
* 25.5. mm gauge

[0105]

TABLE 4
Properties of a production run of Alloy B casing:
	YS	UTS		Elong.	Hardness
Product	(MPa)	(MPa)	Y/T	(%)	RB	HV500
0.395 wall, 9 5/8″ dia	603	698	0.86	33	96.9	228
API 5CT L-80 Specification	552-655	655	—	18.5	23 max	—
		min		min	Rc

[0106] In the following table, Table 5, double cantilever beam SSC resistance data obtained for Alloy B are outlined. The double cantilever beam acceptance criterion is currently 35 Mpa.m {fraction (1/2)} 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.

TABLE 5
Double Cantilever Beam SSC resistance for Alloy B and
IK55 casing:
				Overall
			Ave	Ave
		Kiscc-	Kissc-	Kissc-
Sample	Grade	Mpa · m ½	Mpa · m ½	Mpa · m ½
1	IK55	27.2	28.9	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

[0107] FIG. 4 shows the HIC behaviour of heat treated IK55 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%.

[0108] FIG. 5 shows the SSC results 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 of the three alloys is shown in Table 6. The effect of lowering the manganese content can be seen. FIG. 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 FIG. 5.
Alloy	C	Mn	Mo	Cr	Al	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

[0109] 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.

[0110] 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.

[0111] 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.

Claims

1. A steel alloy characterized by a quench-and-temper microstructure and by an alloy chemistry comprising by weight of a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of 0.15% to 0.65%, a sulfur range of less than 0.002%, an aluminum range of less than or equal to 0.080%, a calcium range of less than or equal to 0.0045%, and the substantial balance of the alloy being iron and unavoidable impurities; the alloy being further characterized by precipitated molybdenum carbides in manganese- and carbon-rich bands of the alloy micro-structure.

2. The steel alloy as defined in claim 1 wherein molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy, thereby substantially precluding the formation of boron nitride and titanium nitride.

3. The steel alloy as defined in claim 2, further characterized in that the carbon range by weight is 0.2% to 0.3%, the manganese range by weight is 0.65% to 0.75%, the molybdenum range by weight is 0.45% to 0.55%, and the sulfur range by weight is less than 0.001%.

4. The steel alloy as defined in claim 3, further characterized in that the aluminum range by weight is 0.020% to 0.040%, and the calcium range by weight is 0.0020% to 0.0045%.

5. The steel alloy as defined in claim 4 further characterized in that the alloy comprises by weight a silicon range of less than or equal to 0.40%.

6. The steel alloy as defined in claim 5 further characterized in that the alloy comprises by weight a silicon range of 0.15 to 0.25%.

7. The steel alloy as defined in claim 2, further comprising a chromium range by weight of less than or equal to 0.50%.

8. The steel alloy as defined in claim 7, further characterized in that the chromium range by weight is 0.20% to 0.30%.

9. The steel alloy as defined in claim 8 further comprising a silicon range of 0.15 to 0.25%.

10. The steel alloy as defined in claim 9 further comprising a nickel range of less than or equal to 0.50%, a copper range of 0.01% to 0.50% a niobium range of less than or equal to 0.10%, and a vanadium range of less than or equal to 0.10%.

11. A method of manufacturing a steel alloy in a steel-making mill involving melting, casting, and hot rolling, the method comprising (a) selecting an alloy chemistry by weight comprising a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of 0.15% to 0.65%, an aluminum range of less than or equal to 0.080%, and the substantial balance of the alloy being iron and unavoidable impurities; (b) applying a clean scrap melting practice to reduce the sulphur content in the alloy to less than 0.002 wt. %; (c) adding up to 0.0045 wt. % calcium to the alloy to combine with the remaining sulfur to form globular CaS particles and after casting and hot rolling; (d) austenizing the steel; (e) quenching the steel; then, (f) tempering the steel plate for a sustained period at an elevated temperature such that precipitation of molybdenum carbides is promoted in manganese and carbon rich bands of the alloy.

12. The method of manufacture as defined in claim 11 wherein molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy, thereby substantially precluding the formation of boron nitride and titanium nitride.

13. The method of manufacture as claimed in claim 11 wherein silicon of up to 0.40 wt. % is added to the alloy to deoxidize, desulfurize, and strengthen the steel alloy.

14. The method of manufacture as claimed in claim 12 wherein silicon of up to 0.40 wt. % is added to the alloy to deoxidize, desulfurize, and strengthen the steel alloy.

15. The method of manufacture as claimed in claim 12 wherein the steel is austenized at about 925° C.

16. The method of manufacture as claimed in claim 14 wherein the steel is tempered for about two hours at about 700° C.

17. The method of manufacture as claimed in claim 15 wherein after hot rolling, the steel is slit to a selected width, formed into a pipe shape then welded to form casing.

18. The method of manufacture as claimed in claim 11 further comprising adjusting the content of the following minor alloying elements to nickel in a range of less than or equal to 0.50%, copper in a range of 0.01% to 0.5%, chromium in a range of less than or equal to 0.50%, niobium in a range of less than or equal to 0.10%, and vanadium in a range of less than or equal to 0.10%.

19. A casing for transporting fluids including oil, gas and steam, wherein the casing is made from a steel alloy characterized by a quench-and-temper micro-structure and by an alloy chemistry comprising by weight of a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, a sulfur range of less than 0.002%, an aluminum range of less than or equal to 0.080%, a calcium range of less than or equal to 0.0045%, the substantial balance of the alloy being iron and unavoidable impurities; wherein molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy, thereby substantially precluding the formation of boron nitride and titanium nitride, the alloy being further characterized by precipitated molybdenum carbides in manganese and carbon rich bands of the alloy.

20. The casing as defined in claim 19 further characterized in that the alloy comprises by weight an aluminum range of 0.020% to 0.040%; and a calcium range of 0.0020% to 0.0045%.

21. The casing as claimed in claim 20, further characterized in that the carbon range by weight is 0.20% to 0.30%; the manganese range by weight is 0.70% to 0.85%; the molybdenum range by weight is 0.45% to 0.55%; and the sulfur range by weight is less than 0.002%.

22. The casing of claim 21 further comprising an alloy chemistry comprising a chromium range of 0.20% to 0.30% by weight and a silicon range of less than or equal to 0.40% by weight.

23. The casing of claim 22 further comprising an alloy chemistry comprising by weight of a copper range of 0.010% to 0.50%, a nickel range of less than or equal to 0.50%, a chromium range of less than or equal to 0.50%, a niobium range of less than or equal to 0.10%, and a vanadium range of less than or equal to 0.10%.