RU2363877C2 - Pipe made of martensite stainless steel for oil wells - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy field, particularly to manufacturing of pipe from martensite stainless steel for usage in humid condition, containing gaseous carbon dioxide. Oil-and-gas pipe is manufactured from steel, containing in wt %: carbon from 0.005 up to 0.1, silicon from 0.05 up to 1, manganese from 1.5 up to 5, phosphorus maximum 0.05, sulphur maximum 0.01, chrome from 9 up to 13, nickel maximum 0.5, molybdenum maximum 2, copper maximum 2, aluminium from 0.001 up to 0.1, nitrogen from 0.001 up to 0.1, iron and unavoidable admixtures are the rest. Pipe allows variety of deplated by chrome section sunder surface without implementation of tempering operation.

EFFECT: it is provided deceleration of corrosion cracking process under tension without formation on surface of passivating film.

3 cl, 2 tbl, 1 ex

Description

FIELD OF TECHNOLOGY

The present invention relates to a martensitic stainless steel pipe for oil wells, and more particularly, to a martensitic stainless steel pipe intended for use in a humid environment containing carbon dioxide gas.

BACKGROUND

Oil and natural gas extracted from oil and gas wells contain corrosive gas such as gaseous carbon dioxide and gaseous hydrogen sulfide. In such a moist environment containing gaseous carbon dioxide, martensitic stainless steel pipes having high corrosion resistance are used as oil and gas field and pipeline pipes. More specifically, 13Cr stainless steel pipes are commonly used, typically API 13Cr steel pipes. The 13Cr stainless steel pipe is resistant to corrosion by exposure to gaseous carbon dioxide, since it contains about 13% Cr, and has a martensitic structure, since it contains about 0.2% C.

In recent years, deeper oil and gas wells have been researched and developed. An oil and gas field pipe (hereinafter simply referred to as “OCTG”), intended for use in a deep well in a moist environment containing gaseous carbon dioxide, should have a high strength of 655 MPa or more and a high impact strength. In the presence of a moist environment containing gaseous carbon dioxide at high temperatures in the range from 80 ° C to 150 ° C, there is a risk of corrosion of the active type, which causes cracking resulting from stress corrosion (hereinafter referred to simply as "SCC"), therefore high resistance to SCC.

When using 13Cr stainless steel pipes in deep wells with a high-temperature wet environment containing carbon dioxide gas, the following disadvantages are observed.

1. Due to the high content of C, the required impact strength cannot be obtained when the strength is increased to 655 MPa or more.

2. In the manufacturing process, a 13Cr stainless steel pipe is quenched and tempered and, as shown in FIG. 1, chromium carbides 50 are formed in the structure after tempering. The Cr depleted region 60, i.e., the low Cr region, is formed at the periphery of the chromium carbide 50 or at the grain boundary. The depleted Cr region 60 increases the possibility of SSC formation. Therefore, a 13Cr stainless steel pipe having a Cr depleted portion of 60 is not resistant to the SCC required when used in a deep well with a high temperature, moist, carbon dioxide gas atmosphere.

For this reason, an improved 13Cr martensitic stainless steel pipe has been developed that can be used in a deep well with a high temperature, moist, carbon dioxide gas atmosphere. The improved 13Cr martensitic stainless steel pipe has a higher SCC resistance than the 13Cr stainless steel pipe due to the passive film on the surface formed by the addition of an alloying element such as Mo and Cu and a C content of 0.1% or less. This is because, as shown in FIG. 2, almost no chromium carbide is released in the structure after tempering due to the low C content, provided that the tempering conditions are correct.

Since instead of C a larger amount of Ni is contained as an austenitic forming element, which is also an austenitic forming element, the martensitic structure is preserved even at a low content of C. Therefore, a pipe made of improved 13Сr stainless steel has the high strength and toughness required when it is used in a high-temperature wet containing gaseous carbon dioxide medium.

A traditional 13Cr stainless steel pipe is quenched and tempered to obtain the required strength, however, in order to reduce production costs, a 13Cr martensitic stainless steel pipe has been developed that is obtained without tempering after rolling (hereinafter referred to as the “martensitic stainless steel pipe without tempering”). A stainless steel martensitic steel pipe without tempering is described in JP 2003-183781 A, JP 2003-193203 A and JP 2003-129190 A. According to the publications cited, the desired strength and toughness can be obtained even in the absence of tempering.

However, as a result of research, the authors of the present invention found that the resistance to SCC of a martensitic stainless steel pipe without tempering is lower than the resistance of a traditional pipe of improved 13Cr martensitic stainless steel. As shown in FIG. 3, the Cr depleted portion is not formed on the inside of the pipe at a distance of more than about 100 μm from the surface of the martensitic stainless steel pipe without tempering, however, the Cr depleted portion 60 is formed to a depth of about 100 μm from the surface.

The chromium-depleted region 60 below the surface is formed after hot working. More specifically, chromium depleted portion 60 is formed when secondary scale is formed after rolling and chrome under the surface is absorbed by such dross, or chromium carbide 50 is formed below the surface due to graphite used as a rolling lubricant, so chromium depleted portion 60 is formed around carbide 50 chromium. The traditional improved 13Cr martensitic stainless steel pipe is tempered after rolling, therefore, such a chromium depleted section 60 below the surface is eliminated during the tempering process, however, martensitic stainless steel pipe is produced without tempering without tempering, so many chrome depleted sections 60 remain under surface in its original form.

Non-tempered martensitic stainless steel pipe described in JP 2003-193204 A has high SCC resistance. However, a smooth specimen was used in the described tests to evaluate SCC resistance, i.e. sample with polished surface. More specifically, SCC resistance of a sample including a Cr depleted site below the surface has not been evaluated. The inventors of the present invention conducted SCC tests using research samples including a Cr depleted site below the surface in accordance with the described conditions and found that the SCC resistance of samples including a Cr depleted site below the surface was lower than that of a smooth research sample.

Therefore, when a martensitic stainless steel pipe is used in a deep well without tempering, including many chromium-depleted portions 60 below the surface, SCC may occur in a high-temperature wet environment containing carbon dioxide gas.

As a method for removing such Cr depleted sites below the surface, shot blasting and / or pickling can be used. However, such treatments increase production costs. Even after such treatment, there is still the possibility that the chromium-depleted areas below the surface may remain depending on the processing conditions.

SUMMARY OF THE INVENTION

An object of the present invention is to provide martensitic stainless steel oil and gas pipes with high SCC resistance, irrespective of the presence of a chromium depleted region below the surface.

The authors of this invention found that in the case when a passive film is not formed, the Ni content is not more than 5% wt. and the content of Mn from 1.5% wt. up to 5% wt. provide high resistance to SCC, regardless of the presence of a chromium depleted area below the surface. Below is a description of the necessary conditions.

1. The lack of formation of a passive film.

The inventors of the present invention believe that in a humid carbon dioxide-containing gas, SCC can be limited to uniform corrosion with a low corrosion rate without the formation of a passive film, rather than a passive film formed on the surface of the steel. When forming a passive film, part of it can be destroyed by extraneous forces, such as exposure to wire and grains of sand, chloride ions or the like, even when Mo or Cu is added to strengthen the passive film. As shown in FIG. 4, when part of the passive film 2 is destroyed on martensitic stainless steel 1, the surface 3 without the passive film 2 serves as the anode, and the passive film 2 serves as the cathode. As a result, the corrosion current is concentrated at the surface 3, contributing to the occurrence of local corrosion. That is, the likelihood of SCC increases. In the event that the formation of the passive film 2 does not occur, the concentration of the corrosion current can be prevented and therefore local corrosion can be limited. In a moist environment containing gaseous carbon dioxide, in the event that the upper limit of the Cr content is 13% by mass, and the content of each of Mo and Cu is not more than 2% by mass, the passive film 2 is not formed.

2. The Ni content is not more than 0.5% wt.

Even without a passive film, when forming on the steel surface a portion with high solubility and a portion with low solubility from a microscopic point of view, the surface may be unevenly corroded. With increased uneven corrosion, SCC may occur at the boundary between the high solubility region and the low solubility region.

For this reason, the inventors of the present invention immersed a plurality of martensitic stainless steel samples having chromium-depleted regions in an aqueous solution of chloride (NaCl) with a saturated concentration and examined the relationship between metal ions eluting from steel and solubility on the surface of the steel. Many grades of martensitic stainless steel were used, the Cr content of which is from 9% to 13%, and the Cu content is no more than 2%, without a passive film. The Ni content in various grades of steel varies.

As a result of research, the authors of the present invention for the first time found that in the absence of the formation of a passive film and with a Ni content of not more than 0.5% wt. SCC can be prevented if there is a chrome depleted area below the surface.

As shown in FIG. 5, the surface of martensitic stainless steel without a passive film is uniformly corroded. At the same time, Fe ions and Cr ions elute from the surface of the steel, lowering the pH of the solution. Therefore, the pH of the solution in the surface regions 10 and 11, where Fe ions and Cr ions elute, decreases.

Meanwhile, Ni ions, eluted from the surface, prevent the solution from dropping. Therefore, the pH of the solution in the surface regions 12 and 13, where Ni ions elute, is higher than the pH of the solution in the surface regions 10 and 11. Therefore, as shown in FIG. 6, the solubility of the surface regions 12 and 13 is low, and the solubility of the surface regions 10 and 11 is high. As a result, corrosion spreads on the surface areas 10 and 11, while the surface is unevenly corroded. In the event that corrosion does not spread uniformly from a microscopic point of view, SCC most likely occurs at the boundary between the high solubility region and the low solubility region, as in region 15.

In the martensitic stainless steel described above without a passive film, uneven corrosion propagates due to the presence of Ni and SCC occurs. The probability of SCC is more dependent on the Ni content than on the presence of a chromium depleted site. Therefore, by decreasing the Ni content, local corrosion can be prevented despite the presence of chromium depleted areas below the surface, so that the occurrence of SCC can be prevented.

3. The content of Mn is from 1.5% wt. up to 5.0% wt.

Since Ni is capable of causing SCC, its content is preferably reduced. However, if the content of Ni as an austenite-forming element is reduced, martensite is formed, as well as δ-ferrite. δ-Ferrite not only reduces the strength and toughness of steel, but can also cause the formation of SCC arising from the intermediate phase between martensite and ferrite. Therefore, by decreasing the Ni content, the Mn content, as an austenite-forming element, can be increased to limit the formation of δ ferrite, as a result of which the formation of SCC arising from the intermediate phase can be prevented.

In view of the foregoing, the authors have developed the following invention.

The martensitic stainless steel oil and gas pipe according to this invention contains, by weight, from 0.005% to 0.1% C, from 0.05% to 1% Si, from 1.5% to 5% Mn, maximum 0.05% P , maximum 0.01% S, from 9% to 13% Cr, maximum 0.5% Ni, maximum 2% Mo, maximum 2% Cu, 0.001% to 0.1% Al and 0.001% to 0.1 % N, with the remainder being Fe and unavoidable impurities, the pipe having a chromium depleted region below the surface.

In this case, the chromium-depleted region below the surface is a region with a Cr concentration of 8.5% by weight in steel. or less, while such areas are scattered, for example, in the region from the surface to a depth of less than 100 microns in the direction deep into the steel. For example, a chromium depleted region is formed at the periphery of chromium carbide or at the grain boundary. The chromium depleted region is determined, for example, by the following method. A sample in the form of a thin film is obtained from an arbitrary part in a section from the surface to a depth of less than 100 μm toward the inner part of the martensitic stainless steel pipe. Such a sample, for example, is obtained using a focused ion beam device (FIB). A thin film sample is analyzed using a transmission electron microscope (TEM) and analyzing the concentration of Cr in the test area using an energy dispersive X-ray analyzer (EDS) attached to the TEM so that the presence of the Cr site can be determined.

The martensitic stainless steel oil and gas pipe of the present invention does not have a passive film formed on the surface in a high temperature wet environment containing carbon dioxide gas. The Ni content causing cathode formation is limited. Therefore, as shown in Fig. 7, in the OCTG of martensitic stainless steel according to this invention, the occurrence of local corrosion in a high-temperature wet environment containing carbon dioxide gas can be prevented despite the presence of a Cr depleted area below the surface - the entire surface is uniformly corroded at a low speed. The content of Mn, an austenitic element like Ni, is increased so that the structure becomes martensitic and the formation of δ ferrite is limited. Due to this, the occurrence of SCC from the intermediate phase can be prevented. Therefore, the martensitic stainless steel OCTG of the present invention has high SCC resistance.

The martensitic stainless steel oil and gas pipe according to this invention preferably further comprises at least one of the following elements: from 0.005% to 0.5% Ti, from 0.005% to 0.5% V, from 0.005% to 0.5% Nb, from 0.005% to 0.5% Zr.

In this case, each of the mentioned elements is connected to C in steel, forming a fine carbide. Therefore, the toughness of steel increases. It should be noted that the addition of these elements does not affect the resistance to SCC.

The martensitic stainless steel oil and gas pipe according to this invention preferably further comprises at least one of the following elements: from 0.0002% to 0.005% B, from 0.0003% to 0.005% Ca, from 0.0003% to 0.005% Mg and from 0.0003% to 0.005% of the rare earth element.

In this case, each of the added elements mentioned improves the hot working ability. It should be noted that these elements do not affect the resistance to SCC.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram illustrating the structure of 13Cr stainless steel.

2 is a schematic diagram illustrating the structure of an improved 13Cr martensitic stainless steel.

Figure 3 is a schematic diagram illustrating the structure of martensitic stainless steel without tempering.

4 is a schematic diagram illustrating the occurrence of SCC in martensitic stainless steel having a passive film formed thereon.

5 is an image illustrating the initial

corrosion stage of steel containing Ni and Cr.

6 is a view illustrating corrosion of steel containing Ni and Cr.

7 is a view illustrating the corrosion of OCTG from martensitic stainless steel according to this invention.

PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION

Further, the implementation of the present invention is described in detail.

1. The chemical composition

A martensitic stainless steel pipe according to an embodiment of the present invention has the following composition. Hereinafter, "%" regarding the elements means "% wt".

C: from 0.005% to 0.1%

Carbon improves steel strength. On the other hand, if the C content is excessive, it causes an excess of chromium carbide, and SCC occurs in such carbide. Therefore, the content of C is from 0.005% to 0.1%, preferably from 0.01% to 0.07%, more preferably from 0.01% to 0.05%.

Si: 0.05% to 1%

Silicon is effectively used to deoxidize steel. On the other hand, Si is a ferrite-forming element, and therefore an excessive Si content causes the formation of δ-ferrite, which lowers the toughness of steel. Therefore, the Si content is from 0.05% to 1%.

Mn: 1.5% to 5%

Manganese is an austenite-forming element and contributes to the formation of a martensitic structure. According to the present invention, the content of Ni, which is also an austenitic forming element, is reduced, therefore, the content of Mn is preferably increased in order to convert the steel structure to martensitic and to obtain high strength and toughness.

Moreover, Mn contributes to improved SCC durability. Manganese is able to limit the formation of δ-ferrite and prevent the occurrence of SCC from the intermediate phase between δ-ferrite and martensite.

Excessive Mn, on the other hand, reduces toughness. Therefore, the Mn content is from 1.5% to 5%, preferably from 1.7% to 5%, more preferably from 2.0% to 5%.

P: 0.05% or less

Phosphorus is a contaminant. Phosphorus, which is a ferrite-forming element, forms δ-ferrite and lowers the toughness of steel. Therefore, the content of P is as low as possible. The content of P is 0.05% or less, preferably 0.02% or less.

S: 0.01% or less

Sulfur is a contaminant. Sulfur, which is a ferrite-forming element, forms δ-ferrite in steel and lowers hot workability of steel. Therefore, the S content is as low as possible. The content of S is 0.01% or less, preferably 0.005% or less.

Cr: 9% to 13%

Chromium contributes to the improvement of corrosion resistance in a moist environment containing gaseous carbon dioxide. Chromium is also able to slow down the corrosion rate when corroding the entire surface of the steel. On the other hand, Cr is a ferrite-forming element and its excessive content causes the formation of δ-ferrite, which reduces hot workability and toughness. Therefore, the Cr content is from 9% to 13%.

Ni: 0.5% or less

According to this invention, nickel is an undesirable impurity. As indicated above, Ni ions limit the decrease in pH of the solution and, therefore, resistance to SCC. Therefore, the Ni content in the martensitic stainless steel pipe according to this embodiment is preferably as low as possible. Therefore, the Ni content is 0.5% or less, preferably from 0.25% or less, more preferably 0.1% or less.

Mo: 2% or less

Cu: 2% or less

On the oil and gas pipe of martensitic stainless steel according to this invention, a passive film is not formed, and the entire surface is corroded at a low speed. Molybdenum and copper stabilize and enhance the passive film, so their content is preferably as low as possible. Therefore, the total content of Mo and Cu is 2% or less. The Mo content is preferably 1% or less, and the Cu content is also 1% or less.

Al: 0.001% to 0.1%

Aluminum is effectively used as a deoxidizing agent. On the other hand, an excess Al content increases the content of non-metallic inclusions in steel, thereby lowering its toughness and corrosion resistance. Therefore, the Al content is from 0.001% to 0.1%.

N: 0.001% to 0.1%

Nitrogen is an austenite-forming element and limits the formation of δ-ferrite, thus turning the steel structure into martensitic. On the other hand, too high an N content unnecessarily increases strength and reduces toughness. Therefore, the N content is from 0.001% to 0.1%, preferably from 0.01% to 0.08%.

It should be noted that the balance is Fe and contaminants.

The martensitic stainless steel pipe according to this embodiment further comprises one of the following elements: Ti, V, Nb and, if necessary, Zr. The following is a description of the mentioned elements.

Ti: 0.005% to 0.5%

V: from 0.005% to 0.5%

Nb: 0.005% to 0.5%

Zr: from 0.005% to 0.5%

Each of these elements combines with C, forming a fine carbide and improving the toughness of steel. These elements also limit the formation of Cr carbide, thereby preventing a decrease in the amount of Cr solid solution. Provided that the content of each of the mentioned elements is from 0.005% to 0.5%, the effective achievement of such advantages is ensured. It should be noted that the excessive content of such elements increases the amount of carbides formed, which leads to a decrease in the toughness of steel.

The oil and gas pipe made of martensitic stainless steel according to this option additionally includes one of the following elements: B, Ca, Mg and, if necessary, rare-earth metals (rare-earth metal). The following is a description of the mentioned elements.

B: 0.0002% to 0.005%

Ca: 0.0003% to 0.005%

Mg: 0.0003% to 0.005%

REM: from 0.0003% to 0/005%

It should be noted that these elements contribute to the improvement of the workability of steel in a hot state. Provided that the content of each of the above elements does not go beyond the ranges presented above, such advantages are effectively achieved. It should be noted that an excessive content of such elements lowers the toughness of steel and reduces the corrosion resistance in a correlating medium. Therefore, the content of all these elements is preferably from 0.0005% to 0.003%, more preferably from 0.0005% to 0.002%.

2. Method of production

A melt for steel having the chemical composition described above is obtained by melting in a blast furnace or in an electric furnace. The resulting molten steel is subjected to a degassing process. The degassing process can be carried out using AOD (argon-oxygen refining) or VOD (decarburization with oxygen in vacuum). Alternatively, AOD and VOD may be combined.

Degassed molten steel by continuous casting is formed into a continuous casting material. A continuous casting material is, for example, a slab, bloom or billet. Alternatively, the molten steel may be in the form of ingots obtained by the ingot casting method.

A slab, bloom or ingot is converted into billets as a result of hot processing. In addition, preforms can be obtained by hot rolling or hot forging.

Billets obtained by continuous casting or hot working are subjected to further hot processing and form a martensitic stainless steel pipe for oil wells. As a hot processing method, the Mannesmann process is used. For example, a Mannesmann process using a mill for rolling tubes on a mandrel, a Mannesmann process using a pilger mill for periodic rolling, a process using an Assel-Mannesmann mill, or the like can be carried out, for example. Alternatively, a Ugine-Sejournet hot extrusion process, as well as a method of forging pipes, such as the Ergard method, can be used as hot processing. The heating temperature during hot processing is preferably from 1100 ° C. to 1300 ° C. This is because when the heating temperature is too low, hot processing is difficult. Too high a temperature, δ-ferrite is formed, which impairs mechanical properties or corrosion resistance. The final temperature of the material during hot processing is preferably from 800 ° C to 1150 ° C.

After hot working, the steel pipe is cooled to room temperature. The pipe may be cooled by air or water.

After cooling, the steel pipe is not tempered. It should be noted that after cooling to room temperature after hot rolling, the steel pipe can be subjected to heat treatment for solid solution. More specifically, after cooling to room temperature, the steel pipe is heated to a temperature of from 800 ° C. to 1100 ° C., heating is performed for a set period, and then it is cooled. The heating period preferably lasts from 3 to 30 minutes, although it is not particularly limited. It should be noted that after heat treatment on a solid solution, vacation is not carried out.

The chromium-depleted region is formed below the surface of the oil and gas pipe from martensitic stainless steel obtained by the above steps, and secondary scale is formed on the surface. Secondary scale can be removed by treatment with metal shot or the like.

Example 1

Materials were obtained samples having the chemical compositions shown in table 1, and tested for strength, toughness and resistance to SCC.

Steel was smelted having the chemical compositions shown in table 1. As follows from table 1, the chemical compositions of the materials of samples 1-11 are within the range of chemical compositions according to this invention. The materials of samples 1 and 2 have the same chemical composition. However, in the materials of samples 12-15, the content of any of the elements is beyond the scope of this invention.

The molten steel from the materials of samples 1 and 3-15 is cast into ingots. The obtained ingots are heated for two hours at a temperature of 1250 ° C, and then by forging they are turned into round billets. Round billets are heated at a temperature of 1250 ° C for an hour and heated round billets are stitched and lengthened at the Mannesmann mill for rolling pipes on a mandrel, thus obtaining a large number of seamless steel pipes (oil and gas and pipe pipes). Seamless steel pipes after elongation are cooled with air and materials are taken from them for samples. Secondary scale appears on the inner surface of the air-cooled sample materials.

The material of sample 2 was prepared as follows. Steel having the chemical composition shown in Table 1 is melted, and then seamless steel pipes are made from it in the same manner as from other sample materials. Then, seamless steel pipes are heat treated for solid solution. More specifically, seamless steel pipes are heated at a temperature of 1050 ° C. for 10 minutes, and then rapidly cooled.

Secondary scale is removed from the inner surface of some of the many seamless steel pipes obtained from each sample material by treating them with metal shot (hereinafter, seamless steel pipes will be referred to as "steel with scale removed"). In other seamless steel pipes, the secondary scale present on their inner surfaces remains intact (hereinafter, such steel will be referred to as “steel with secondary scale”). Briefly, two types of seamless steel pipes were obtained from each sample material.

Steel with secondary scale and steel with removed secondary scale are examined for the presence / absence of chromium depleted areas under the inner surfaces. More preferably, a thin film sample is obtained from a portion within 100 μm of the inner surface of the secondary scale steel using a focused ion beam machine (FIB). A thin film sample is examined using a transmission electron microscope (TEM) and analyzing the concentration of Cr in the test area using a 1.5 nm beam emitted by an energy dispersive X-ray analyzer (EDS) attached to the TEM. As a result of observations using TEM, it was found that all seamless steel pipes have Cr depleted sections under their internal surfaces.

Using the obtained sample materials, determine their strength and resistance to SCC.

1. Strength Test

In order to determine the strength of the materials of the samples, tensile test specimen No. 4 is obtained from each material of the sample according to JIS Z2201. According to JIS Z2241, tensile tests were carried out on specimens in the form of round rods and their yield strengths (MPa) were established.

2. SCC resistance test

Each of the samples with a bend of the beam in four places is obtained from steel with secondary scale and steel with removed scale from each material of the samples and the samples are subjected to cracking tests resulting from stress corrosion in a high-temperature moist environment containing gaseous carbon dioxide.

Each of the samples has a length of 75 mm, a width of 10 mm and a thickness of 2 mm in the length direction of the seamless steel pipe, with one surface of each sample (75 mm × 10 mm × 2 mm) serving as the inner surface of the steel pipe. A sample having a surface with a scale (a surface with a secondary scale) is obtained from steel with a secondary scale, and a sample having a surface from which a scale has been removed by treatment with metal shot (a surface with a descaled scale) is obtained from steel with a descaled scale.

Samples are subjected to bend tests in four places. More specifically, according to ASTM G39, 100% real voltage is applied to each sample. At the same time, tensile stress is applied to the surface with the secondary scale and to the surface with the removed scale. Then, the samples are immersed in a 25% aqueous NaCl solution saturated with gaseous CO ₂ under a pressure of 30 bar and kept at a temperature of 100 ° C. The test duration is 720 hours.

After testing, the cross section of each of the samples is examined for the presence / absence of cracking visually and under an optical microscope with a 100-fold increase. The chemical compositions of the surfaces are determined using an X-ray spectral analyzer based on the energy dispersion method (EDX) in order to determine the presence or absence of a passive film on the surfaces of the samples after testing, and the compounds formed on the surfaces are subjected to x-ray analysis.

3. Test results

The test results are shown in table 2. The yield strength in table 2 is indicated in MPa. The SCC resistance designation “O” means no cracking, and “X” means cracking.

As follows from the above data, each of the materials of samples 1–11 in the form of oil and gas and pipeline pipes has a yield strength above 758 MPa and sufficient strength despite the absence of a tempering process. It should be noted that the material of sample 2, subjected to heat treatment for solid solution, also has high strength.

The materials of samples 1-11 were tested for impact strength, and it was found that the materials of samples 6-8 containing at least one of such elements as Ti, V, Nb and Zr have a higher viscosity than the materials of samples 1 -5. More specifically, the vTrs of sample materials 6–8 are higher than the vTrs of other sample materials by 10 ° C or more, with vTrs being the break point for the Charpy-V test.

After the manufacture of the pipes, the materials of samples 1-11 were

examined visually for the presence / absence of defects, as a result of which it was found that the materials of samples 9-11, containing at least one of such elements as B, Ca, Mg and REM, have higher machinability than the materials of samples 1-8 .

Moreover, steel with scale and steel without scale of the materials of samples 1-11 did not crack in the SCC resistance tests and showed high resistance to SCC. According to the results of EDX and x-ray analysis after testing for SCC in the materials of samples 1-11, a passive film is not formed. More specifically, amorphous Cr and Fe based amorphous substances, probably formed as a result of corrosion, were found on the surfaces of the materials of samples 1–11 after SCC tests.

Meanwhile, the materials of samples 12-15 had SCC both in steel with scale and in steel with removed scale. More specifically, the strength of the material of sample 12 was too high due to the high C content in it, while it had SCC, probably caused by the formation of δ ferrite, due to its low Mn content. Sample material 13 had SCC, probably caused by an unstable passive film formed due to its high Mo content. The material of sample 14 had SCC due to its high Ni content. The material of sample 15 had SCC due to its high content of Ni, N, and Cu.

Although the present invention has been described and illustrated in detail, it is understood that this was done only for the purpose of illustration and example, and should not be construed as limiting. This invention can be implemented in various modified forms without violating its essence and scope.

Claims (3)

1. Oil and gas pipe made of martensitic stainless steel, containing, wt.%: Carbon from 0.005 to 0.1, silicon from 0.05 to 1, manganese from 1.5 to 5, phosphorus maximum 0.05, sulfur maximum 0.01 , chromium from 9 to 13, nickel up to a maximum of 0.5, molybdenum up to a maximum of 2, copper up to a maximum of 2, aluminum from 0.001 to 0.1 and nitrogen from 0.001 to 0.1, while the balance is iron and inevitable impurities, while the mentioned oil and gas the pipe is not tempered and has many chrome-depleted sections below the surface. 2. The pipe according to claim 1, characterized in that it is made of steel, optionally containing at least one of the following elements: titanium from 0.005 to 0.5%, vanadium from 0.005 to 0.5%, niobium from 0.005 to 0 , 5% and zirconium from 0.005 to 0.5%. 3. The pipe according to claim 1 or 2, characterized in that it is made of steel, optionally containing at least one of the following elements: boron from 0.0002 to 0.005%, calcium from 0.0003 to 0.005%, magnesium from 0 , 0003 to 0.005% and rare-earth metals from 0.0003 to 0.005%.

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