Industrial Products – Martin Valdez, Jorge Mitre, Bruce A. Reichert, Tenaris Coiled Tubes LLC

Abstract for “High performance material for coil tubing applications and the process of producing it”

“Embodiments in the present disclosure relate to coiled stainless steel tubes and methods for manufacturing them. Some embodiments allow for homogeneous final microstructures across all base metal areas, weld joints and heat affected zones. The final microstructure of the coil steel tube can also be made up of bainite and tempered martensite.

Background for “High performance material for coil tubing applications and the process of producing it”

“Description of Related Art”

Coiled tubing has seen increased use in applications that require high pressure and extended reach operations. Coiled tubing must have high tensile properties to withstand: i. axial loads from hanging or pooling long strings; ii. elevated pressures during operation.

“The standard production process of coiled tubing utilizes hot rolled strips with mechanical characteristics that have been microstructure refined during rolling. This refinement can be achieved by using different microalloying additives (Ti N, V), as well as the appropriate hot rolling conditions. To achieve an ultra-fine microstructure, it is important to control material recrystalization. Because of the limited use of precipitation hardening and solid solution alloying elements, the material’s ability to simultaneously exhibit high strength and toughness is restricted.”

“This raw material must be specified by each supplier and may need varying mechanical characteristics in the hot-roiled steel to make coiled tubes of varying mechanical property. The properties of the raw material increase in value, and so does the production cost. The strip-to-strip welding used in the assembly of the “long strip” is well-known. The joining area of the coiled tubing is affected by the way it is ERW formed/welded. The performance of the strip welds will be affected by the increasing properties of the coiled tubing. This is due to the fact that welding destroys the refinement created during hot rolling and there is no simple heat treatment post weld capable of regenerating both toughness and tensile properties. This zone generally restores tensile, but it also reduces toughness and fatigue life. The current industrial route is capable of producing high-strength coiled tubing but at a higher cost and with poor relative performance to strip welds joints with respect to the pipe body.

A full body heat treatment is another option for making coiled tubing. This heat treatment is used on a material that has been made into a pipe in the “green” state. Because the heat treatment conditions are still determining its properties, this treatment is applied to a material that has been formed into a pipe in the so-called?green? state. The steel chemistry and heat treatment conditions are key factors in determining the final product’s properties. The coiled tubing can be made uniform in length by combining steel composition, welding material, and heat treatment. This eliminates the need for a strip-to-strip joint that is so critical for high-strength conventional coiled tubes. Although this general idea has been used before, it was not successfully applied to high-strength coiled tubing (yield strength between 80 and 140 ksi). This is because heat treatment at high line speeds (which is required to achieve high productivity) will often require complex and extended facilities. If the right chemistry is used and the heat treatment conditions are chosen, this process can be simplified.

“Choosing the right chemistry for an industrial heat treatment facility with reasonable dimensions is dependent on an understanding of the many variables that impact coiled tubing performance. These include: a) Axial mechanical properties, b) Uniformity in Microstructure and Properties. c) Toughness. d) Fatigue Resistance. e. Sour Resistance.

“Below is a description of chemistry that can be used to heat treat coiled tubing. This is largely outside the limits set forth by API 5ST. (Max.C0.16%, Max.Mn:1.2% (CT70-90) Max.Mn:1.65 (CT100-110), Max.P:0.02%, (CT70-90) Max.P:0.025 (CT100-CT110). Max.S:0.005, Si.Max:0.5).”

“Embodiments in this disclosure refer to a coiled-steel tube and methods for producing it. In some embodiments, the tube can have a yield strength greater than 80 Ksi. The tube’s composition can be as low as 0.16-0.35 Wt. % carbon. 0.30-2.00 wt. % manganese, 0.10-0.35 wt. % silicon, upto 0.005 wt. % sulfur, upto 0.018 wt. % phosphorus. The remainder is iron and other impurities. A final microstructure can be added to the tube. This could include a mixture of bainite and tempered martensite. The final microstructure of the tube may also contain a mixture of bainite and tempered martensite.

“Disclosed is a coil steel tube made from a plurality welded strips. The tube can contain base metal regions, weld joints and their heat affected areas and can have a yield strength greater that about 80 ksi. It can also include a composition consisting of iron and 0.17-0.35 Wt. % carbon, 0.30-2.00 wt. % manganese, 0.10-0.30 wt. % silicon, 0.010-0.040 wt. % aluminum up to 0.10 wt. % sulfur, up to 0.010 wt. % sulfur, and up to 0.015 wt.

“In some embodiments, there is additional composition up to 1.0 wt. % chrome, up to 0.5% wt. % molybdenum up to 0.0030 Wt. % Boron, up to 0.0.030 wt. % titanium, upto 0.50 wt. Copper, up to 0.50 Wt. % nickel, upto 0.1 wt. % nickel, up to 0.1% vanadium, and up to 0.0050wt. % oxygen and up to 0.05 wt. % calcium.”

“In certain embodiments, the composition may contain 0.17 to 0.30 wt. % carbon, 0.30 – 1.60 wt. % manganese from 0.10 to 0.20 weight % silicon, upto 0.7 wt. % chromium up to 0.5 wt. % molybdenum, between 0.0005 and 0.0025 wt. % Boron, 0.010 to 0.0025 wt. Titanium, 0.25 to 0.35 Wt. Copper, 0.20 to 0.35 Wt. % nickel, 0.04 to 0.35 wt. % Niobium, up 0.04 wt. % vanadium up to 0.0015wt. % oxygen, upto 0.03 wt. % calcium up to 0.003wt. % sulfur, up to 0.003 wt. % phosphorus.”

“Some embodiments allow the tube to have a minimum yield strength as low as 125 ksi. The tube may have a minimum yield strength as low as 140 ksi in some embodiments. The minimum yield strength for a tube may be between 125 and 140 ksi in some embodiments.

“In certain embodiments, the final microstructure may contain at least 95 volume% tempered martensite within the base metal regions, weld joints and heat affected zones. Some embodiments. The final grain size can be below 20 mm in the base metal areas, weld joints and heat affected zones. Some embodiments. The tube may have a final grain size below 15 in the base metal areas, the weld joints and the heat affected zone.

“Some embodiments can include bias welds in the weld joints. The fatigue life of bias welds in some embodiments can exceed 80%. Some embodiments allow for a percentage hardness of a joint that includes its heat affected zone to be as high or lower than the hardness of the base material.

“In some cases, welding strips can include bias welding. Some embodiments allow for the formation of a tube by forming a line joint. The method may also include coiling the tempered oil on a spool. Some embodiments allow for austenitizing to form a grain size of less than 20 mm in the base metal areas, weld joints, or heat affected zones.

“In certain embodiments, the composition may also contain tip up to 1.0 wt. % chrome up to 0.5 wt. % molybdenum, up to 0.0030 Wt. % Boron, up to 0.0.030 wt. % titanium, upto 0.50 wt. Copper, up to 0.50 Wt. 1% nickel, up to 0.01 wt. % Niobium, upto 0.15 wt. % vanadium up to 0.0050wt. % oxygen and up to 0.05 wt. % calcium.”

“In certain embodiments, the composition may contain 0.17 to 0.30 wt. % carbon, 0.30 – 1.60 wt. % manganese from 0.10 to 0.20 weight % silicon, upto 0.7 wt. % chromium up to 0.5 wt. % molybdenum, between 0.0005 and 0.0025 wt. % Boron, 0.010 to 0.25 wt. % titanium, 0.25 – 0.35 wt. Copper, 0.20 to 0.35 Wt. % nickel, 0.04 to 0.35 wt. % Niobium, up 0.04 wt. % vanadium up to 0.00015 Wt. % oxygen, upto 0.03 wt. % calcium up to 0.003wt. % sulfur and as high as 0.010 wt. % phosphorus.”

“The yield strength of tempered tubes can be greater than or equal 125 ksi in some embodiments. The minimum yield strength for tempered tubes can be 140 ksi in some embodiments. The minimum yield strength of tempered tubes can be between 125 to 140 ksi in some embodiments.

Coiled Tubing is a raw material that is made in a steel shop from hot-rolled strips. Microstructural refinement and controlled rolling are used to ensure high strength and toughness. For pipe production, the strips are cut longitudinally to the desired width and then joined end to end using a joining process (e.g. To form a longer strip, you can use Friction Stir Welding and Plasma Arc Welding. The ERW process is used to form the tube. Performance of the final product is measured by: a. axial mechanical properties; b. uniformity of microstructures and properties; c. toughness; d. fatigue resistance; e. sour resistance, etc. The traditional processing method results in coiled tubing’s mechanical properties. This is due to the combination of hot-rolled strip properties with modifications made during welding operations and tube formation. These properties are not as good as the coiled tube performance, which is what we have just listed. Because the welding process used for joining the strips modifies the as-rolled microstructure so that final properties can still be affected even after heat treatments are applied. Heterogeneities in microstructure, as well as the presence of brittle constituents along the welds, are associated with decreased fatigue life and poor sour performances. A new route should at minimum include a complete body heat treatment. The route has been described but not specified. This disclosure describes the raw material characteristics and chemistries that are combined with appropriated welding procedures and heat treatment conditions will produce a quenched, tempered product that is highly efficient in both strip and pipe joining welds. This material is suitable for coiled tube because it has been chosen not only for its relative cost but also to maximize fatigue life in the specific conditions of operation of coiled tubes (low cycle fatigue under simultaneous axial load and internalpressures).

This disclosure is about a high-strength coiled tubing (minimum yield strength of 80 ksi to140 ksi) that has a longer low-cycle fatigue time than standard products as defined by API5ST. This disclosure also improves resistance to Sulfide Stress Cracking. The right selection of steel chemistry, and the appropriate processing conditions are key to this remarkable combination. As disclosed in U.S.App. No. US201210186686 A1. US201210186686 A1. This disclosure focuses on the steel chemistries used to create a quenched, tempered coiled tube with the mentioned properties. While certain mechanical properties can be created by heat treatment of a base material of a particular composition are well-known, this particular application of coiled tubing uses a raw material with specific chemical to minimize the negative effect of specific variables, such as segregation patterns on the specific properties.

The coiled tube’s increased resistance to fatigue is one of its most important features. Because coiled tubes are used in standard field operations, they are frequently spooled and unspooled often. This causes cyclic plastic deformations which can eventually lead to failures. Low cycle fatigue causes deformation to be more concentrated at the microscopical level in soft material regions. Cracks can easily grow and nucleate if brittle constituents are present in these strain concentration areas. Heterogeneous microstructures, which have softer regions than localized deformation, can lead to a decrease in fatigue life. They are also associated with brittle constituents that nucleate or propagate cracks. These micro-structural features are found in the Heat Affected zone of welds (HAZ). Some types of pipe bodies microstructures also have the above-mentioned characteristics. They are made up of a mix of hard and non-hard constituents like ferrite, pearlite, and bainite. This case, strain is located in the softer ferrite near the border with bainite. Cracks can then be nucleated and propagated. This microstructure is present in high strength coiled tubes.

“To avoid strain localization in low cycle fatigue, the microstructure must be homogeneous across the pipe body and joints as well as at the microscopic scale. A microstructure made of tempered martensite is best for low carbon steels. This is essentially a ferrite matrix containing a uniform and fine distribution of carbides. The chemistry selection and processing conditions described hereafter aim to create a homogeneous microstructure in tube body, bias welding and ERW line that is at least 90% tempered and preferably more than 95% tempered.

“Moreover, tempered martensite can produce extremely high strength grades better than standard coiled-tube microstructures (composed ferrite pearlite, and bainite). For which, very expensive alloying additions must be made to achieve yield strengths greater than about 125 Ksi, highly costly alloying additives are required.”

Tempered martensite has a higher SSC resistance than structures that contain bainite.

The best steel chemistry is the one that can produce heat-treated coiled tubing with a FBHT. It can be described as: Carbon (wt% C), Manganese, (wt% Mn), Silicon(wt% Si), and Molybdenum. You can also set upper limits on certain impurities like Sulfur (w.% S), Phosphorus, and Oxygen.

“This disclosure uses tempered martensite as the final structure. The steel chemistry is different from other coiled tube art due to the higher carbon content. (See API 5ST for more information. Maximum Carbon allowed for Coiled Tubing is 0.16%). This allows for the creation of desired microstructures through a FBHT that includes at least one cycle each of austenitization and quenching.

“The terms ‘approximately? and?about? are used herein. The amounts used in this document are close to the stated amount, but still achieve a desired function. For example, the terms “approximately”, “about”, and “substantially” may be used. It could refer to an amount that is less than 10%, less than 5 percent, less than 11%, less than 0.1%, or less than 0.01% from the stated amount.

Carbon is an element that can be inexpensively added to steel. It improves hardenability and promotes carbide precipitation during heat treatment. It is possible to form large amounts of bainite during heat treatment if carbon hardenability is lower than 0.17%. It is difficult to achieve a yield strength of 80 ksi or more with the required fatigue life and SSC resistance due to the formation of bainite. The API5ST maximum carbon limit for coiled tubing is 0.16%. This makes it unsuitable for heat treatment. Conventional coiled tube microstructures contain large amounts of bainite, which can reduce toughness, fatigue life, and SSC resistance in higher strength grades. Coiled tubings that have a minimum yield strength of 110 Ksi or less.

Steels with more carbon than 0.35% will be less weldable. They are more likely to have brittle constituents, cracks, and other problems during welding, and post-weld heat treatments. Higher carbon content may lead to significant amounts of retained ausstenite that are converted into brittle constituents by tempering. These brittle constituents can reduce fatigue life and SSC resistance. The C content of steel compositions can vary from 0.1% to 0.3%, or preferably between 0.0.17% and 0.0.30%.

“Manganese addition improves hardenability, strength. Mn contributes to the steelmaking process’s deoxidation, as well as sulfur control. It may be difficult for you to achieve the desired strength level if your Mn content is below 0.30%. Large segregation patterns can be formed as Mn content rises. Segregated Mn areas may form brittle constituents in heat treatment, which can reduce toughness and fatigue. These segregated areas can also increase material susceptibility for sulfide stress cracking. The Mn content in the steel composition should be between 0.30% and 2.0%. Preferably, it should range between 0.30% and 1.60%. More preferably, it should range between 0.30% and 0.80% for applications that have an enhanced SSC resistance.

Silicon is an element that can be added to steel. It has a deoxidizing and strength-enhancing effect. The toughness of some embodiments may drop if the Si content is greater than 0.30%. Large segregation patterns can also be formed. The Si content in the steel composition can vary from 0.10% up to 0.30%, or preferably between 0.10% and 0.20%.

“Chromium addition increases the steel’s hardenability, tempering resistance and toughness. You can use Cr to partially replace Mn to increase strength and reduce fatigue life. Because of its effect on hot forming loads, Cr can be a costly component that makes coiled tubing more difficult. In some embodiments, Cr is reduced to 1.0% and preferably to 0.7%.

“Molybdenum” is an element that can be added to steel to increase its strength and prevent softening during tempering. This resistance to tempering makes it possible to produce high-strength steels with a lower Mn content, increasing fatigue life and SSC resistance. Mo additions can also be used to reduce the segregation and grain boundaries of phosphorous, which may improve resistance to inter-granular cracking. This ferroalloy can be expensive so it is desirable to lower the maximum Mo content in the steel composition. In certain embodiments, the maximum Mo content is approximately 0.5%.

“Boron” is an element that can be added to steel in a strong way to increase its hardenability. B, for example, may increase hardenability by inhibiting formation of ferrite in quenching. Some embodiments use B to improve hardenability (i.e. to quench a structure of at least 90% martensite in steels with reduced Mn content to increase fatigue life and resistance to SSC. The B content should not be less than 0.0005 wt. These embodiments may make it difficult to achieve the desired hardenability. If the B content is too high, then coarse boron carbides can form at grain boundaries, which could adversely affect toughness. In an embodiment, the B content in the composition should not be less than 0.0030%. In another embodiment, the B content ranges from 0.0005% to 0.025%.

Titanium, an element that can be added to steel to increase the steel’s effectiveness by fixing nitrogen impurities (TiN), and inhibiting the formation Boron nitrides. In some cases, it might be difficult to achieve the desired effect of Boron on steel hardenability if the Ti content is too low. However, TiC and TiN may form if the Ti content exceeds 0.03 wt.%. This can adversely affect ductility and toughness. In certain embodiments, the Ti concentration may be as low as 0.030%. Other embodiments may have a concentration of Ti ranging from 0.010% to 0.025%.

“Considering that the manufacture of coiled tubing with low mechanical properties benefits from low temperature resistance. The addition of B and Ti increases hardenability, but does not increase tempering resistance. It allows the production of 80ksi grades without the need for prolonged soaking during tempering. This results in an increase in productivity. The limit to producing coiled tubing in heat treatment lines is the length required to soak the material properly during tempering. This is why the use of Ti and B is especially important for low yield strength coiled tube production.

“Copper is an element which is not required in some embodiments of the steel alloy. Cu is sometimes required for some applications in coiled tubing to increase atmospheric corrosion resistance. In certain embodiments, the Cu content in the steel composition might be less than 0.50%. Other embodiments may have Cu concentrations ranging from 0.2% to 0.3%.

Nickel is an element that increases the strength and durability of steel. Cu can be added to the steel composition. Hot shortness, also known as hot rolling defects, can be avoided by using Ni. But Ni is expensive and in some embodiments the Ni content in the steel composition may be less than or equal 0.5%. Other embodiments may have Ni concentrations ranging from 0.2% to 0.3%.

“Niobium, an element that can be added to steel compositions may refine the austenitic grains size of the steel when it is reheated into the austenitic area. This will result in an increase in strength and toughness. Nb can also precipitate during tempering, which increases steel strength through particle dispersion toughening. An embodiment of the steel composition’s Nb content may range from 0% to 0.10%, or preferably 0% to 0.04%.

Vanadium can be added to steel in order to increase its strength by carbide precipitations during tempering. If the V content of the steel composition exceeds 0.15%, vanadium carbide particles can form in large quantities, which may result in a decrease of steel’s toughness. In certain embodiments, the steel’s V content is restricted to 0.15% or less, and preferably, to 0.10%.

Aluminum is an element that can be added to steel compositions. This has the ability to reduce the steel’s oxidizing effects and refine the steel’s grain size. An embodiment may have a lower Al content than 0.010% to make steel more susceptible to oxidation and displaying high levels of inclusions. Other embodiments may result in the formation of coarse precipitates that reduce the steel’s toughness. The Al content of the steel composition can vary from 0.010% to 0.040%.

“Sulfur is an element which causes steel’s toughness and workability to decrease. In some instances, the steel composition’s S content is restricted to 0.010% or less.

“Phosphorus, an element that reduces the steel’s toughness, is a component. The steel composition should have a minimum P content of 0.015% and preferably 0.010%.

“Oxygen could be an impurity in the steel composition, which is primarily made up of oxides. The steel’s impact properties are affected by increasing O content in an embodiment of the steel composition. In certain steel compositions, an O content that is less than or equals 0.0050 wt%, or preferably less than or equivalent to 0.0015 Wt%, is desirable.

Calcium, an element that can be added to steel compositions may increase toughness by altering the shape of sulfide additions. An embodiment may have a minimum Ca-S content ratio of Ca/S>1.5. Other embodiments of the steel mixture may not contain excessive Ca. The steel composition may have a maximum Ca content of approximately 0.05% or preferably 0.03%.

“The content of unavoidable impureties including. However, N. Pb and Sn, Ass, Sb, Bi, and the like should be kept to a minimum. The properties of steels made from the steel compositions described in the present disclosure, such as strength and toughness, may not be significantly impaired if these impurities remain below certain levels. One embodiment may have a N content less than 0.010%. Preferably, it will be less than or equal 0.008%. Another embodiment may have a Pb content less than or equaling 0.005%. A further embodiment. The Sri content of the steel composition can be less or equal to 0.02%. An additional embodiment may have the As content of steel composition less than or equaling 0.012%. Another embodiment may have a Sb content less than or equaling 0.008%. A further embodiment may have a Bi content less than or equaling 0.003%.

“The final product specification and industrial facility constraints will determine the steel chemistry that is selected for this disclosure. It is not possible to obtain large soaking times during tempering in induction heat treatments lines. Because Mn addition can reduce fatigue life and SSC resistance by forming large segregation patterns, it will be minimized whenever possible. To replace Mn, Cr and Mo will be used in a lesser extent. The body heat treatment will remain as simple as possible. These elements can increase carbide stability and softening resistivity, which could lead to long soaking times in tempering. These elements should be used for higher strength grades (for instance Grade 110 and above), where tempering resistance is desirable. They should not be used in lower grades (Grade 80), for which it would be impractical to use long industrial heat treatment lines.

“Grade 80 will have the lowest grades and it will be preferred to use B and Ti microalloyed additives, in combination with appropriate C contents. These elements can be used to achieve good hardenability, without high Mn additions. Tempering resistance is not affected by B and Ti. To achieve the desired strength, a simple and quick tempering treatment is possible.

“The industrial processing route that corresponds to this disclosure will be described in the following paragraphs. The focus is on Full Body Heat Treatment (FBHT).

Raw material for coiling tubing is made in a steel shop from hot rolled strips. The wall thickness of the strips can vary from 0.08 to 0.30 inches. The steel supplier may use controlled rolling to refine the as-rolled microstructure. An important microstructural refinement is not necessary for the as roiled strip because the disclosure microstructure and mechanical characteristics are almost entirely defined by the final FBHT. This flexibility in hot rolling allows for lower raw-material costs and allows you to use steel chemistries that are not possible with more complex hot rolling processes (in general controlled roll can only be applied to low carbon micro-alloyeds steels).

The steel strips are cut longitudinally to the required width for pipe production. The strips are then joined end-to-end by welding (e.g. To make a longer strip, you can use Friction Stir Welding (or Plasma Arc Welding). These welded strips can be used to form a pipe, such as an ERW process. The typical outer diameter of a coiled tube is between 1 and 5 inches. Pipe lengths range from 15,000 feet to 40000 feet.

The Full Body Heat Treatment (FBHT), is then applied after the pipe has been formed. This heat treatment aims to create a homogeneous microstructure that contains at least 90% tempered Martensite and the remainder being bainite. This microstructure has a uniform carbide distribution, grain size below 20 mm?preferably below 15?m? and provides good combination of strength, toughness, strength and low cycle fatigue. This microstructure can also be used to increase resistance to Sulfide Stress cracking (SSC), as it is more resistant than conventional structures made of ferrite, pearlite, and large volumes of upper bainite.

“EXAMPLES”

“Example A”: Chemistry Selection to Increase Hardenability

“As previously mentioned, this disclosure’s microstructure is composed at least 90% of tempered martensite and a homogenous distribution fine carbides. The rest is bainite. This microstructure permits the production of a coiled tub with the desired combination: high strength, increased low cycle fatigue life, and better SSC resistance.

“The tempered martensite can be obtained after the pipe has been formed using ERW. If further refinement is required to improve SSC resistance, the heat treatment can be repeated up to two times. Because subsequent austenization and quenching cycles reduce the size of prior austenitic grains, as well as martensite blocks and packets.

0.15 0.28 TinSTD2 0.14 0.80 0.33 0.55 0.10 0.27 0.27 Nb?TinSTD3 0.14 0.80 0.34 0.57 0.32 0.22 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.22 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.35 0.33 0.55 0.33 0.32 0.32 0.32 0.32 0.32 0.22 0.28 0.22 0.32 0.22 0.28 TnCMn1 1.17 2.20 0.17 2.00 0.17 2.20 0.20 0.20 0.17 2.00 0.20 0.20 0.20 0.20 0.35 0.20 0.20 0.20 0.20 0.35 0.20 0.20 0.20 0.20 0.20 0.35 0.20 0.20 0.35 0.28 0.15 0.28 Ti\nSTD2 0.14 0.80 0.33 0.55 0.10 0.17 0.27 Nb?Ti\nSTD3 0.14 0.80 0.34 0.57 0.32 0.22 0.28 Nb?Ti\nCMn1 0.17 2.00 0.20 ? ? ? ? ?\nCMn2 0.25 1.60 0.20 ? ? ? ? ?\nBTi1 0.17 1.60 0.20 ? ? ? ? B?Ti\nBTI2 0.25 1.30 0.20 ? ? ? ? B?Ti\nCrMo1 0.17 1.00 0.25 1.00 0.50 ? ? ?\nCrMo2 0.25 0.60 0.20 1.00 0.50 ? ? ?\nCrMoBTi1 0.17 0.60 0.20 1.00 0.50 ? ? B?Ti\nCrMoBTi2 0.24 0.40 0.15 1.00 0.25 ? ? B?Ti\nCrMoBTi3 0.24 0.40 0.15 1.00 0.50 ? ? B?Ti\nCrMoBTi4 0.26 0.60 0.15 0.50 0.25 B?Ti”

The following guidelines were derived from Table A2’s analysis of experimental data:

“C?Mn Steels: Hardenability is mainly dependent on Carbon and Manganese Additions. When C is below the lower limit (CMn1 Steel), about 2% Mn can be used for the desired hardenability. Mn, however, is an element that can produce strong segregation patterns which may reduce fatigue life. The Mn addition in higher carbon formulations is then decreased. When carbon concentration is around 0.25%, 1.6%Mn is sufficient to reach hardenability (CMn2 Steel).

B?Ti steels: These alloys are plain carbon and microalloyed with Boron or Titanium. Mn can be further decreased due to the Boron effect’s increase in hardenability. To achieve hardenability for Carbon below the lower limit, approximately 1.6% Mn is possible. 1.3% Mn is sufficient to attain the hardenability (BTi2 Steel) when carbon concentration is less than 0.25%.

“Cr.Mo steels: These steels contain Cr and Mo additions, which are useful for increasing tempering resistance. They can be used to produce ultra-high strength grades. Cr and Mo are elements that increase hardenability, so Mn may be reduced. Cr and Mo, however, are expensive additions that decrease the steel’s hot workability. Their maximum contents are 1% and 0.5% respectively. One example of Carbon with the lower limit is that 1% Mn can achieve the CR90 (CrMo1) if the steel has been microalloyed with Bi?Ti. A further reduction of Mn to 0.6% is possible (CrMoBTi1).

“Example B”: Chemistry Selection for Different Coiled Tube Grades

The results show that increased carbon and Cr?Mo can be used to achieve higher grades. Particularly. Low Carbon content means Grade 140 cannot be achieved using standard chemistries as described in API5ST. A lean chemistry that has low carbon is the best option to achieve Grade 80. To ensure good hardenability, B?Ti microalloying additives can be used (e.g. BTi1 chemistry).

“It is important that we mention that to produce martensitic structures using the standard steels (STD1, STD2 or STD3), it was necessary for laboratory to use higher quenching rates than those at the mill in order to achieve this. If we reduce the cooling rate to the industrially acceptable level, no coiled tube grades are possible with conventional steels through the FBHT process route.

“Example C”: Chemistry Selection to Reduce Negative Impacts of Segregation during Solidification

“During steel solidification, alloying elements tend not to be diluted in liquids due to their higher solubility than the solid (?). ferrite or austenite). Two types of non-uniform chemical composition patterns are formed by solute rich areas upon solidification: macrosegregation or microsegregation.

“Microsegregation is caused by freezing the solute-enriched fluid in the interdendritic space. It is not a problem as the microsegregation effects can be removed by subsequent hot work. Macrosegregation, on the other hand is a non-uniformity in chemical composition of the cast section at a larger scale. You cannot eliminate it by hot working or soaking at high temperatures. It produces the centerline separation band in the case of the disclosure.

“A prominent central segregation band must be avoided because:

“Brittle constituents such as non-tempered martensite can appear in this area as a result welding operations (biasweld and ERW), see FIGS. 5A-B). These unwanted constituents can be removed by the full body heat treatment. The tube can be bent between heat treatment and welding, resulting in a failure during industrial production.

“After FBHT, the remnant of central segregation bands is a region enriched substitutional solutes (as Mo, Si, and Mn) with a lower density of coarse carbides that the rest of material. This area is vulnerable to nucleate cracks in low cycle fatigue as shown in FIGS. 6-7. 6-7.

“Although macrosegregation cannot be eliminated, it can be minimized by selecting the right steel chemistry.

“Based on EDX measurements of samples corresponding to a wide variety of steel chemistries and alloying elements, the enrichment factors at central segregation bands were calculated for different alloying element. Table C1 shows the results. Table C1 shows the enrichment factors (EF). These are the ratios of each element’s concentration in the central band to that which corresponds to the average matrix. These factors are dependent on the thermodynamic partition coefficient of liquid and solid, and diffusivities during solidification.

“TABLE C1nEnrichment Factors (EF) at the central band correspondnto different substitutional alloy elements.

Table C1 clearly shows that some elements have a strong tendency for solidification to segregate, such as Si and Cu. However, Cr and Ni have low enrichment values. Ni is an expensive addition. Cr can be used when a higher hardenability or tempering resistance is required without creating strong segregation patterns.

The enrichment factors provide information on the expected increase in concentration for each element within the central segregation bands. Not all elements have the same effect on the tendency of material to form brittle constituents in heat treatment or welding. The tendency to form brittle constituents during heat treatment is higher for elements with higher hardenability. Important to note that elements with high diffusion coefficients such as Carbon or Boron can segregate during solidification but are homogenized by hot rolling. They do not form brittle constituents at the segregation bands.

“Example A shows that Manganese is the most hardenable metal. This can be derived from the analysis of CCT diagrams. This is in contrast to Carbon and Boron which have very small segregation patterns following hot rolling. Si and Cu, on the other hand, have a strong tendency towards segregation but do not play a significant role in hardenability. Due to its high enrichment factor, and significant effect on hardenability Mn addition must be decreased as much as possible to reduce the negative effects of macrosegregation.

Because of its impact on hardenability, high Mn content is usually added to steel compositions. This disclosure reveals that hardenability is mainly achieved by higher Carbon additions, so Mn concentrations can generally be reduced. Boron and/or Chromium can also be used to reduce Manganese. You can see examples in Table C2, which displays the critical cooling rate (CR90), for various steel compositions based on CCT diagrams (data taken directly from Example A). To achieve hardenability in steels with about 0.25 percent Carbon, Mn can reduce from 1.6% to 1.3% by adding Boron. If Cr?Mo is added, it can further be reduced to 0.4%.

“Example D”: Homogenization in Microstructure

“As mentioned previously, the fatigue life for coiled tubing depends on microscopical characteristics as well as microstructural heterogeneities. Combining soft and hard micro-constituents can lead to plastic strain localization which is the driving force behind crack nucleation. This section compares the microstructures of coiled tubing obtained using the API 5ST standard production method with those that correspond to the chemistry and processing conditions described in this disclosure.

“A standard coiled tube grade 110 (yield strength between 110 Ksi and 120 Ksi) was used as a reference material. It is listed in Table A1 with the chemistry STD2 which is within API 5ST specifications. This standard material was compared with a coiled tubing of the same quality made using chemistry BTi2and using the FBHT.

Summary for “High performance material for coil tubing applications and the process of producing it”

“Description of Related Art”

Coiled tubing has seen increased use in applications that require high pressure and extended reach operations. Coiled tubing must have high tensile properties to withstand: i. axial loads from hanging or pooling long strings; ii. elevated pressures during operation.

“The standard production process of coiled tubing utilizes hot rolled strips with mechanical characteristics that have been microstructure refined during rolling. This refinement can be achieved by using different microalloying additives (Ti N, V), as well as the appropriate hot rolling conditions. To achieve an ultra-fine microstructure, it is important to control material recrystalization. Because of the limited use of precipitation hardening and solid solution alloying elements, the material’s ability to simultaneously exhibit high strength and toughness is restricted.”

“This raw material must be specified by each supplier and may need varying mechanical characteristics in the hot-roiled steel to make coiled tubes of varying mechanical property. The properties of the raw material increase in value, and so does the production cost. The strip-to-strip welding used in the assembly of the “long strip” is well-known. The joining area of the coiled tubing is affected by the way it is ERW formed/welded. The performance of the strip welds will be affected by the increasing properties of the coiled tubing. This is due to the fact that welding destroys the refinement created during hot rolling and there is no simple heat treatment post weld capable of regenerating both toughness and tensile properties. This zone generally restores tensile, but it also reduces toughness and fatigue life. The current industrial route is capable of producing high-strength coiled tubing but at a higher cost and with poor relative performance to strip welds joints with respect to the pipe body.

A full body heat treatment is another option for making coiled tubing. This heat treatment is used on a material that has been made into a pipe in the “green” state. Because the heat treatment conditions are still determining its properties, this treatment is applied to a material that has been formed into a pipe in the so-called?green? state. The steel chemistry and heat treatment conditions are key factors in determining the final product’s properties. The coiled tubing can be made uniform in length by combining steel composition, welding material, and heat treatment. This eliminates the need for a strip-to-strip joint that is so critical for high-strength conventional coiled tubes. Although this general idea has been used before, it was not successfully applied to high-strength coiled tubing (yield strength between 80 and 140 ksi). This is because heat treatment at high line speeds (which is required to achieve high productivity) will often require complex and extended facilities. If the right chemistry is used and the heat treatment conditions are chosen, this process can be simplified.

“Choosing the right chemistry for an industrial heat treatment facility with reasonable dimensions is dependent on an understanding of the many variables that impact coiled tubing performance. These include: a) Axial mechanical properties, b) Uniformity in Microstructure and Properties. c) Toughness. d) Fatigue Resistance. e. Sour Resistance.

“Below is a description of chemistry that can be used to heat treat coiled tubing. This is largely outside the limits set forth by API 5ST. (Max.C0.16%, Max.Mn:1.2% (CT70-90) Max.Mn:1.65 (CT100-110), Max.P:0.02%, (CT70-90) Max.P:0.025 (CT100-CT110). Max.S:0.005, Si.Max:0.5).”

“Embodiments in this disclosure refer to a coiled-steel tube and methods for producing it. In some embodiments, the tube can have a yield strength greater than 80 Ksi. The tube’s composition can be as low as 0.16-0.35 Wt. % carbon. 0.30-2.00 wt. % manganese, 0.10-0.35 wt. % silicon, upto 0.005 wt. % sulfur, upto 0.018 wt. % phosphorus. The remainder is iron and other impurities. A final microstructure can be added to the tube. This could include a mixture of bainite and tempered martensite. The final microstructure of the tube may also contain a mixture of bainite and tempered martensite.

“Disclosed is a coil steel tube made from a plurality welded strips. The tube can contain base metal regions, weld joints and their heat affected areas and can have a yield strength greater that about 80 ksi. It can also include a composition consisting of iron and 0.17-0.35 Wt. % carbon, 0.30-2.00 wt. % manganese, 0.10-0.30 wt. % silicon, 0.010-0.040 wt. % aluminum up to 0.10 wt. % sulfur, up to 0.010 wt. % sulfur, and up to 0.015 wt.

“In some embodiments, there is additional composition up to 1.0 wt. % chrome, up to 0.5% wt. % molybdenum up to 0.0030 Wt. % Boron, up to 0.0.030 wt. % titanium, upto 0.50 wt. Copper, up to 0.50 Wt. % nickel, upto 0.1 wt. % nickel, up to 0.1% vanadium, and up to 0.0050wt. % oxygen and up to 0.05 wt. % calcium.”

“In certain embodiments, the composition may contain 0.17 to 0.30 wt. % carbon, 0.30 – 1.60 wt. % manganese from 0.10 to 0.20 weight % silicon, upto 0.7 wt. % chromium up to 0.5 wt. % molybdenum, between 0.0005 and 0.0025 wt. % Boron, 0.010 to 0.0025 wt. Titanium, 0.25 to 0.35 Wt. Copper, 0.20 to 0.35 Wt. % nickel, 0.04 to 0.35 wt. % Niobium, up 0.04 wt. % vanadium up to 0.0015wt. % oxygen, upto 0.03 wt. % calcium up to 0.003wt. % sulfur, up to 0.003 wt. % phosphorus.”

“Some embodiments allow the tube to have a minimum yield strength as low as 125 ksi. The tube may have a minimum yield strength as low as 140 ksi in some embodiments. The minimum yield strength for a tube may be between 125 and 140 ksi in some embodiments.

“In certain embodiments, the final microstructure may contain at least 95 volume% tempered martensite within the base metal regions, weld joints and heat affected zones. Some embodiments. The final grain size can be below 20 mm in the base metal areas, weld joints and heat affected zones. Some embodiments. The tube may have a final grain size below 15 in the base metal areas, the weld joints and the heat affected zone.

“Some embodiments can include bias welds in the weld joints. The fatigue life of bias welds in some embodiments can exceed 80%. Some embodiments allow for a percentage hardness of a joint that includes its heat affected zone to be as high or lower than the hardness of the base material.

“In some cases, welding strips can include bias welding. Some embodiments allow for the formation of a tube by forming a line joint. The method may also include coiling the tempered oil on a spool. Some embodiments allow for austenitizing to form a grain size of less than 20 mm in the base metal areas, weld joints, or heat affected zones.

“In certain embodiments, the composition may also contain tip up to 1.0 wt. % chrome up to 0.5 wt. % molybdenum, up to 0.0030 Wt. % Boron, up to 0.0.030 wt. % titanium, upto 0.50 wt. Copper, up to 0.50 Wt. 1% nickel, up to 0.01 wt. % Niobium, upto 0.15 wt. % vanadium up to 0.0050wt. % oxygen and up to 0.05 wt. % calcium.”

“In certain embodiments, the composition may contain 0.17 to 0.30 wt. % carbon, 0.30 – 1.60 wt. % manganese from 0.10 to 0.20 weight % silicon, upto 0.7 wt. % chromium up to 0.5 wt. % molybdenum, between 0.0005 and 0.0025 wt. % Boron, 0.010 to 0.25 wt. % titanium, 0.25 – 0.35 wt. Copper, 0.20 to 0.35 Wt. % nickel, 0.04 to 0.35 wt. % Niobium, up 0.04 wt. % vanadium up to 0.00015 Wt. % oxygen, upto 0.03 wt. % calcium up to 0.003wt. % sulfur and as high as 0.010 wt. % phosphorus.”

“The yield strength of tempered tubes can be greater than or equal 125 ksi in some embodiments. The minimum yield strength for tempered tubes can be 140 ksi in some embodiments. The minimum yield strength of tempered tubes can be between 125 to 140 ksi in some embodiments.

Coiled Tubing is a raw material that is made in a steel shop from hot-rolled strips. Microstructural refinement and controlled rolling are used to ensure high strength and toughness. For pipe production, the strips are cut longitudinally to the desired width and then joined end to end using a joining process (e.g. To form a longer strip, you can use Friction Stir Welding and Plasma Arc Welding. The ERW process is used to form the tube. Performance of the final product is measured by: a. axial mechanical properties; b. uniformity of microstructures and properties; c. toughness; d. fatigue resistance; e. sour resistance, etc. The traditional processing method results in coiled tubing’s mechanical properties. This is due to the combination of hot-rolled strip properties with modifications made during welding operations and tube formation. These properties are not as good as the coiled tube performance, which is what we have just listed. Because the welding process used for joining the strips modifies the as-rolled microstructure so that final properties can still be affected even after heat treatments are applied. Heterogeneities in microstructure, as well as the presence of brittle constituents along the welds, are associated with decreased fatigue life and poor sour performances. A new route should at minimum include a complete body heat treatment. The route has been described but not specified. This disclosure describes the raw material characteristics and chemistries that are combined with appropriated welding procedures and heat treatment conditions will produce a quenched, tempered product that is highly efficient in both strip and pipe joining welds. This material is suitable for coiled tube because it has been chosen not only for its relative cost but also to maximize fatigue life in the specific conditions of operation of coiled tubes (low cycle fatigue under simultaneous axial load and internalpressures).

This disclosure is about a high-strength coiled tubing (minimum yield strength of 80 ksi to140 ksi) that has a longer low-cycle fatigue time than standard products as defined by API5ST. This disclosure also improves resistance to Sulfide Stress Cracking. The right selection of steel chemistry, and the appropriate processing conditions are key to this remarkable combination. As disclosed in U.S.App. No. US201210186686 A1. US201210186686 A1. This disclosure focuses on the steel chemistries used to create a quenched, tempered coiled tube with the mentioned properties. While certain mechanical properties can be created by heat treatment of a base material of a particular composition are well-known, this particular application of coiled tubing uses a raw material with specific chemical to minimize the negative effect of specific variables, such as segregation patterns on the specific properties.

The coiled tube’s increased resistance to fatigue is one of its most important features. Because coiled tubes are used in standard field operations, they are frequently spooled and unspooled often. This causes cyclic plastic deformations which can eventually lead to failures. Low cycle fatigue causes deformation to be more concentrated at the microscopical level in soft material regions. Cracks can easily grow and nucleate if brittle constituents are present in these strain concentration areas. Heterogeneous microstructures, which have softer regions than localized deformation, can lead to a decrease in fatigue life. They are also associated with brittle constituents that nucleate or propagate cracks. These micro-structural features are found in the Heat Affected zone of welds (HAZ). Some types of pipe bodies microstructures also have the above-mentioned characteristics. They are made up of a mix of hard and non-hard constituents like ferrite, pearlite, and bainite. This case, strain is located in the softer ferrite near the border with bainite. Cracks can then be nucleated and propagated. This microstructure is present in high strength coiled tubes.

“To avoid strain localization in low cycle fatigue, the microstructure must be homogeneous across the pipe body and joints as well as at the microscopic scale. A microstructure made of tempered martensite is best for low carbon steels. This is essentially a ferrite matrix containing a uniform and fine distribution of carbides. The chemistry selection and processing conditions described hereafter aim to create a homogeneous microstructure in tube body, bias welding and ERW line that is at least 90% tempered and preferably more than 95% tempered.

“Moreover, tempered martensite can produce extremely high strength grades better than standard coiled-tube microstructures (composed ferrite pearlite, and bainite). For which, very expensive alloying additions must be made to achieve yield strengths greater than about 125 Ksi, highly costly alloying additives are required.”

Tempered martensite has a higher SSC resistance than structures that contain bainite.

The best steel chemistry is the one that can produce heat-treated coiled tubing with a FBHT. It can be described as: Carbon (wt% C), Manganese, (wt% Mn), Silicon(wt% Si), and Molybdenum. You can also set upper limits on certain impurities like Sulfur (w.% S), Phosphorus, and Oxygen.

“This disclosure uses tempered martensite as the final structure. The steel chemistry is different from other coiled tube art due to the higher carbon content. (See API 5ST for more information. Maximum Carbon allowed for Coiled Tubing is 0.16%). This allows for the creation of desired microstructures through a FBHT that includes at least one cycle each of austenitization and quenching.

“The terms ‘approximately? and?about? are used herein. The amounts used in this document are close to the stated amount, but still achieve a desired function. For example, the terms “approximately”, “about”, and “substantially” may be used. It could refer to an amount that is less than 10%, less than 5 percent, less than 11%, less than 0.1%, or less than 0.01% from the stated amount.

Carbon is an element that can be inexpensively added to steel. It improves hardenability and promotes carbide precipitation during heat treatment. It is possible to form large amounts of bainite during heat treatment if carbon hardenability is lower than 0.17%. It is difficult to achieve a yield strength of 80 ksi or more with the required fatigue life and SSC resistance due to the formation of bainite. The API5ST maximum carbon limit for coiled tubing is 0.16%. This makes it unsuitable for heat treatment. Conventional coiled tube microstructures contain large amounts of bainite, which can reduce toughness, fatigue life, and SSC resistance in higher strength grades. Coiled tubings that have a minimum yield strength of 110 Ksi or less.

Steels with more carbon than 0.35% will be less weldable. They are more likely to have brittle constituents, cracks, and other problems during welding, and post-weld heat treatments. Higher carbon content may lead to significant amounts of retained ausstenite that are converted into brittle constituents by tempering. These brittle constituents can reduce fatigue life and SSC resistance. The C content of steel compositions can vary from 0.1% to 0.3%, or preferably between 0.0.17% and 0.0.30%.

“Manganese addition improves hardenability, strength. Mn contributes to the steelmaking process’s deoxidation, as well as sulfur control. It may be difficult for you to achieve the desired strength level if your Mn content is below 0.30%. Large segregation patterns can be formed as Mn content rises. Segregated Mn areas may form brittle constituents in heat treatment, which can reduce toughness and fatigue. These segregated areas can also increase material susceptibility for sulfide stress cracking. The Mn content in the steel composition should be between 0.30% and 2.0%. Preferably, it should range between 0.30% and 1.60%. More preferably, it should range between 0.30% and 0.80% for applications that have an enhanced SSC resistance.

Silicon is an element that can be added to steel. It has a deoxidizing and strength-enhancing effect. The toughness of some embodiments may drop if the Si content is greater than 0.30%. Large segregation patterns can also be formed. The Si content in the steel composition can vary from 0.10% up to 0.30%, or preferably between 0.10% and 0.20%.

“Chromium addition increases the steel’s hardenability, tempering resistance and toughness. You can use Cr to partially replace Mn to increase strength and reduce fatigue life. Because of its effect on hot forming loads, Cr can be a costly component that makes coiled tubing more difficult. In some embodiments, Cr is reduced to 1.0% and preferably to 0.7%.

“Molybdenum” is an element that can be added to steel to increase its strength and prevent softening during tempering. This resistance to tempering makes it possible to produce high-strength steels with a lower Mn content, increasing fatigue life and SSC resistance. Mo additions can also be used to reduce the segregation and grain boundaries of phosphorous, which may improve resistance to inter-granular cracking. This ferroalloy can be expensive so it is desirable to lower the maximum Mo content in the steel composition. In certain embodiments, the maximum Mo content is approximately 0.5%.

“Boron” is an element that can be added to steel in a strong way to increase its hardenability. B, for example, may increase hardenability by inhibiting formation of ferrite in quenching. Some embodiments use B to improve hardenability (i.e. to quench a structure of at least 90% martensite in steels with reduced Mn content to increase fatigue life and resistance to SSC. The B content should not be less than 0.0005 wt. These embodiments may make it difficult to achieve the desired hardenability. If the B content is too high, then coarse boron carbides can form at grain boundaries, which could adversely affect toughness. In an embodiment, the B content in the composition should not be less than 0.0030%. In another embodiment, the B content ranges from 0.0005% to 0.025%.

Titanium, an element that can be added to steel to increase the steel’s effectiveness by fixing nitrogen impurities (TiN), and inhibiting the formation Boron nitrides. In some cases, it might be difficult to achieve the desired effect of Boron on steel hardenability if the Ti content is too low. However, TiC and TiN may form if the Ti content exceeds 0.03 wt.%. This can adversely affect ductility and toughness. In certain embodiments, the Ti concentration may be as low as 0.030%. Other embodiments may have a concentration of Ti ranging from 0.010% to 0.025%.

“Considering that the manufacture of coiled tubing with low mechanical properties benefits from low temperature resistance. The addition of B and Ti increases hardenability, but does not increase tempering resistance. It allows the production of 80ksi grades without the need for prolonged soaking during tempering. This results in an increase in productivity. The limit to producing coiled tubing in heat treatment lines is the length required to soak the material properly during tempering. This is why the use of Ti and B is especially important for low yield strength coiled tube production.

“Copper is an element which is not required in some embodiments of the steel alloy. Cu is sometimes required for some applications in coiled tubing to increase atmospheric corrosion resistance. In certain embodiments, the Cu content in the steel composition might be less than 0.50%. Other embodiments may have Cu concentrations ranging from 0.2% to 0.3%.

Nickel is an element that increases the strength and durability of steel. Cu can be added to the steel composition. Hot shortness, also known as hot rolling defects, can be avoided by using Ni. But Ni is expensive and in some embodiments the Ni content in the steel composition may be less than or equal 0.5%. Other embodiments may have Ni concentrations ranging from 0.2% to 0.3%.

“Niobium, an element that can be added to steel compositions may refine the austenitic grains size of the steel when it is reheated into the austenitic area. This will result in an increase in strength and toughness. Nb can also precipitate during tempering, which increases steel strength through particle dispersion toughening. An embodiment of the steel composition’s Nb content may range from 0% to 0.10%, or preferably 0% to 0.04%.

Vanadium can be added to steel in order to increase its strength by carbide precipitations during tempering. If the V content of the steel composition exceeds 0.15%, vanadium carbide particles can form in large quantities, which may result in a decrease of steel’s toughness. In certain embodiments, the steel’s V content is restricted to 0.15% or less, and preferably, to 0.10%.

Aluminum is an element that can be added to steel compositions. This has the ability to reduce the steel’s oxidizing effects and refine the steel’s grain size. An embodiment may have a lower Al content than 0.010% to make steel more susceptible to oxidation and displaying high levels of inclusions. Other embodiments may result in the formation of coarse precipitates that reduce the steel’s toughness. The Al content of the steel composition can vary from 0.010% to 0.040%.

“Sulfur is an element which causes steel’s toughness and workability to decrease. In some instances, the steel composition’s S content is restricted to 0.010% or less.

“Phosphorus, an element that reduces the steel’s toughness, is a component. The steel composition should have a minimum P content of 0.015% and preferably 0.010%.

“Oxygen could be an impurity in the steel composition, which is primarily made up of oxides. The steel’s impact properties are affected by increasing O content in an embodiment of the steel composition. In certain steel compositions, an O content that is less than or equals 0.0050 wt%, or preferably less than or equivalent to 0.0015 Wt%, is desirable.

Calcium, an element that can be added to steel compositions may increase toughness by altering the shape of sulfide additions. An embodiment may have a minimum Ca-S content ratio of Ca/S>1.5. Other embodiments of the steel mixture may not contain excessive Ca. The steel composition may have a maximum Ca content of approximately 0.05% or preferably 0.03%.

“The content of unavoidable impureties including. However, N. Pb and Sn, Ass, Sb, Bi, and the like should be kept to a minimum. The properties of steels made from the steel compositions described in the present disclosure, such as strength and toughness, may not be significantly impaired if these impurities remain below certain levels. One embodiment may have a N content less than 0.010%. Preferably, it will be less than or equal 0.008%. Another embodiment may have a Pb content less than or equaling 0.005%. A further embodiment. The Sri content of the steel composition can be less or equal to 0.02%. An additional embodiment may have the As content of steel composition less than or equaling 0.012%. Another embodiment may have a Sb content less than or equaling 0.008%. A further embodiment may have a Bi content less than or equaling 0.003%.

“The final product specification and industrial facility constraints will determine the steel chemistry that is selected for this disclosure. It is not possible to obtain large soaking times during tempering in induction heat treatments lines. Because Mn addition can reduce fatigue life and SSC resistance by forming large segregation patterns, it will be minimized whenever possible. To replace Mn, Cr and Mo will be used in a lesser extent. The body heat treatment will remain as simple as possible. These elements can increase carbide stability and softening resistivity, which could lead to long soaking times in tempering. These elements should be used for higher strength grades (for instance Grade 110 and above), where tempering resistance is desirable. They should not be used in lower grades (Grade 80), for which it would be impractical to use long industrial heat treatment lines.

“Grade 80 will have the lowest grades and it will be preferred to use B and Ti microalloyed additives, in combination with appropriate C contents. These elements can be used to achieve good hardenability, without high Mn additions. Tempering resistance is not affected by B and Ti. To achieve the desired strength, a simple and quick tempering treatment is possible.

“The industrial processing route that corresponds to this disclosure will be described in the following paragraphs. The focus is on Full Body Heat Treatment (FBHT).

Raw material for coiling tubing is made in a steel shop from hot rolled strips. The wall thickness of the strips can vary from 0.08 to 0.30 inches. The steel supplier may use controlled rolling to refine the as-rolled microstructure. An important microstructural refinement is not necessary for the as roiled strip because the disclosure microstructure and mechanical characteristics are almost entirely defined by the final FBHT. This flexibility in hot rolling allows for lower raw-material costs and allows you to use steel chemistries that are not possible with more complex hot rolling processes (in general controlled roll can only be applied to low carbon micro-alloyeds steels).

The steel strips are cut longitudinally to the required width for pipe production. The strips are then joined end-to-end by welding (e.g. To make a longer strip, you can use Friction Stir Welding (or Plasma Arc Welding). These welded strips can be used to form a pipe, such as an ERW process. The typical outer diameter of a coiled tube is between 1 and 5 inches. Pipe lengths range from 15,000 feet to 40000 feet.

The Full Body Heat Treatment (FBHT), is then applied after the pipe has been formed. This heat treatment aims to create a homogeneous microstructure that contains at least 90% tempered Martensite and the remainder being bainite. This microstructure has a uniform carbide distribution, grain size below 20 mm?preferably below 15?m? and provides good combination of strength, toughness, strength and low cycle fatigue. This microstructure can also be used to increase resistance to Sulfide Stress cracking (SSC), as it is more resistant than conventional structures made of ferrite, pearlite, and large volumes of upper bainite.

“EXAMPLES”

“Example A”: Chemistry Selection to Increase Hardenability

“As previously mentioned, this disclosure’s microstructure is composed at least 90% of tempered martensite and a homogenous distribution fine carbides. The rest is bainite. This microstructure permits the production of a coiled tub with the desired combination: high strength, increased low cycle fatigue life, and better SSC resistance.

“The tempered martensite can be obtained after the pipe has been formed using ERW. If further refinement is required to improve SSC resistance, the heat treatment can be repeated up to two times. Because subsequent austenization and quenching cycles reduce the size of prior austenitic grains, as well as martensite blocks and packets.

0.15 0.28 TinSTD2 0.14 0.80 0.33 0.55 0.10 0.27 0.27 Nb?TinSTD3 0.14 0.80 0.34 0.57 0.32 0.22 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.22 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.28 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.27 0.35 0.33 0.55 0.33 0.32 0.32 0.32 0.32 0.32 0.22 0.28 0.22 0.32 0.22 0.28 TnCMn1 1.17 2.20 0.17 2.00 0.17 2.20 0.20 0.20 0.17 2.00 0.20 0.20 0.20 0.20 0.35 0.20 0.20 0.20 0.20 0.35 0.20 0.20 0.20 0.20 0.20 0.35 0.20 0.20 0.35 0.28 0.15 0.28 Ti\nSTD2 0.14 0.80 0.33 0.55 0.10 0.17 0.27 Nb?Ti\nSTD3 0.14 0.80 0.34 0.57 0.32 0.22 0.28 Nb?Ti\nCMn1 0.17 2.00 0.20 ? ? ? ? ?\nCMn2 0.25 1.60 0.20 ? ? ? ? ?\nBTi1 0.17 1.60 0.20 ? ? ? ? B?Ti\nBTI2 0.25 1.30 0.20 ? ? ? ? B?Ti\nCrMo1 0.17 1.00 0.25 1.00 0.50 ? ? ?\nCrMo2 0.25 0.60 0.20 1.00 0.50 ? ? ?\nCrMoBTi1 0.17 0.60 0.20 1.00 0.50 ? ? B?Ti\nCrMoBTi2 0.24 0.40 0.15 1.00 0.25 ? ? B?Ti\nCrMoBTi3 0.24 0.40 0.15 1.00 0.50 ? ? B?Ti\nCrMoBTi4 0.26 0.60 0.15 0.50 0.25 B?Ti”

The following guidelines were derived from Table A2’s analysis of experimental data:

“C?Mn Steels: Hardenability is mainly dependent on Carbon and Manganese Additions. When C is below the lower limit (CMn1 Steel), about 2% Mn can be used for the desired hardenability. Mn, however, is an element that can produce strong segregation patterns which may reduce fatigue life. The Mn addition in higher carbon formulations is then decreased. When carbon concentration is around 0.25%, 1.6%Mn is sufficient to reach hardenability (CMn2 Steel).

B?Ti steels: These alloys are plain carbon and microalloyed with Boron or Titanium. Mn can be further decreased due to the Boron effect’s increase in hardenability. To achieve hardenability for Carbon below the lower limit, approximately 1.6% Mn is possible. 1.3% Mn is sufficient to attain the hardenability (BTi2 Steel) when carbon concentration is less than 0.25%.

“Cr.Mo steels: These steels contain Cr and Mo additions, which are useful for increasing tempering resistance. They can be used to produce ultra-high strength grades. Cr and Mo are elements that increase hardenability, so Mn may be reduced. Cr and Mo, however, are expensive additions that decrease the steel’s hot workability. Their maximum contents are 1% and 0.5% respectively. One example of Carbon with the lower limit is that 1% Mn can achieve the CR90 (CrMo1) if the steel has been microalloyed with Bi?Ti. A further reduction of Mn to 0.6% is possible (CrMoBTi1).

“Example B”: Chemistry Selection for Different Coiled Tube Grades

The results show that increased carbon and Cr?Mo can be used to achieve higher grades. Particularly. Low Carbon content means Grade 140 cannot be achieved using standard chemistries as described in API5ST. A lean chemistry that has low carbon is the best option to achieve Grade 80. To ensure good hardenability, B?Ti microalloying additives can be used (e.g. BTi1 chemistry).

“It is important that we mention that to produce martensitic structures using the standard steels (STD1, STD2 or STD3), it was necessary for laboratory to use higher quenching rates than those at the mill in order to achieve this. If we reduce the cooling rate to the industrially acceptable level, no coiled tube grades are possible with conventional steels through the FBHT process route.

“Example C”: Chemistry Selection to Reduce Negative Impacts of Segregation during Solidification

“During steel solidification, alloying elements tend not to be diluted in liquids due to their higher solubility than the solid (?). ferrite or austenite). Two types of non-uniform chemical composition patterns are formed by solute rich areas upon solidification: macrosegregation or microsegregation.

“Microsegregation is caused by freezing the solute-enriched fluid in the interdendritic space. It is not a problem as the microsegregation effects can be removed by subsequent hot work. Macrosegregation, on the other hand is a non-uniformity in chemical composition of the cast section at a larger scale. You cannot eliminate it by hot working or soaking at high temperatures. It produces the centerline separation band in the case of the disclosure.

“A prominent central segregation band must be avoided because:

“Brittle constituents such as non-tempered martensite can appear in this area as a result welding operations (biasweld and ERW), see FIGS. 5A-B). These unwanted constituents can be removed by the full body heat treatment. The tube can be bent between heat treatment and welding, resulting in a failure during industrial production.

“After FBHT, the remnant of central segregation bands is a region enriched substitutional solutes (as Mo, Si, and Mn) with a lower density of coarse carbides that the rest of material. This area is vulnerable to nucleate cracks in low cycle fatigue as shown in FIGS. 6-7. 6-7.

“Although macrosegregation cannot be eliminated, it can be minimized by selecting the right steel chemistry.

“Based on EDX measurements of samples corresponding to a wide variety of steel chemistries and alloying elements, the enrichment factors at central segregation bands were calculated for different alloying element. Table C1 shows the results. Table C1 shows the enrichment factors (EF). These are the ratios of each element’s concentration in the central band to that which corresponds to the average matrix. These factors are dependent on the thermodynamic partition coefficient of liquid and solid, and diffusivities during solidification.

“TABLE C1nEnrichment Factors (EF) at the central band correspondnto different substitutional alloy elements.

Table C1 clearly shows that some elements have a strong tendency for solidification to segregate, such as Si and Cu. However, Cr and Ni have low enrichment values. Ni is an expensive addition. Cr can be used when a higher hardenability or tempering resistance is required without creating strong segregation patterns.

The enrichment factors provide information on the expected increase in concentration for each element within the central segregation bands. Not all elements have the same effect on the tendency of material to form brittle constituents in heat treatment or welding. The tendency to form brittle constituents during heat treatment is higher for elements with higher hardenability. Important to note that elements with high diffusion coefficients such as Carbon or Boron can segregate during solidification but are homogenized by hot rolling. They do not form brittle constituents at the segregation bands.

“Example A shows that Manganese is the most hardenable metal. This can be derived from the analysis of CCT diagrams. This is in contrast to Carbon and Boron which have very small segregation patterns following hot rolling. Si and Cu, on the other hand, have a strong tendency towards segregation but do not play a significant role in hardenability. Due to its high enrichment factor, and significant effect on hardenability Mn addition must be decreased as much as possible to reduce the negative effects of macrosegregation.

Because of its impact on hardenability, high Mn content is usually added to steel compositions. This disclosure reveals that hardenability is mainly achieved by higher Carbon additions, so Mn concentrations can generally be reduced. Boron and/or Chromium can also be used to reduce Manganese. You can see examples in Table C2, which displays the critical cooling rate (CR90), for various steel compositions based on CCT diagrams (data taken directly from Example A). To achieve hardenability in steels with about 0.25 percent Carbon, Mn can reduce from 1.6% to 1.3% by adding Boron. If Cr?Mo is added, it can further be reduced to 0.4%.

“Example D”: Homogenization in Microstructure

“As mentioned previously, the fatigue life for coiled tubing depends on microscopical characteristics as well as microstructural heterogeneities. Combining soft and hard micro-constituents can lead to plastic strain localization which is the driving force behind crack nucleation. This section compares the microstructures of coiled tubing obtained using the API 5ST standard production method with those that correspond to the chemistry and processing conditions described in this disclosure.

“A standard coiled tube grade 110 (yield strength between 110 Ksi and 120 Ksi) was used as a reference material. It is listed in Table A1 with the chemistry STD2 which is within API 5ST specifications. This standard material was compared with a coiled tubing of the same quality made using chemistry BTi2and using the FBHT.

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