Round bar tensile test specimen for sulfide stress corrosion cracking test of a steel, test method for sulfide stress corrosion cracking of steel, and seamless steel pipe having excellent resistance to sulfide stress corrosion cracking

Abstract

A round bar tensile test specimen for a sulfide stress corrosion cracking test of a steel, the specimen including a parallel section, a shoulder section, and a grip section. A sectional shape of the shoulder section being formed by a curve having two or more radii of curvature. A radius of curvature R1 (mm) of a portion of the curve adjacent to the parallel section being 15 mm or more, and the radius of curvature R1 (mm) satisfying (0.22119)R1100 in terms of load stress (MPa) of the sulfide stress corrosion cracking test. A length X1 (mm) of the portion of the curve having the radius of curvature R1 in a longitudinal direction of the test specimen satisfying X1{(r/8)(R1r.sup.2/4)}. Additionally, other radii of curvature of the curve being smaller than the radius of curvature R1.

Claims

1. A round bar tensile test specimen for a sulfide stress corrosion cracking test of a steel, the specimen comprising: a parallel section; a grip section; and a shoulder section disposed between the parallel section and the grip section, a sectional shape of the shoulder section being formed by a single continuous curve having two or more radii of curvature, wherein a radius of curvature R1 (mm) of a portion of the curve adjacent to the parallel section is 15 mm or more, the radius of curvature R1 (mm) satisfies formula (1) below in terms of load stress (MPa) of the sulfide stress corrosion cracking test:
(0.22119)R1100(1) a length X1 (mm) of the portion of the curve having the radius of curvature R1 in a longitudinal direction of the test specimen satisfies formula (2) below:
X1{(r/8)(R1r.sup.2/4)}(2) r: radius (mm) of the tensile test specimen in the parallel section, and other radii of curvature of the curve are smaller than the radius of curvature R1.

2. A test method for sulfide stress corrosion cracking of a steel, the test method comprising the steps of: applying a constant load stress (MPa) to a round bar tensile test specimen according to claim 1 immersed in a test solution, the specimen being sampled from a seamless steel pipe; evaluating resistance of the specimen to sulfide stress corrosion cracking; and determining whether resistance of the specimen to sulfide stress corrosion cracking exceeds a predetermined threshold.

3. The test method for sulfide stress corrosion cracking of a steel according to claim 2, wherein the seamless steel pipe has a yield strength of 110 ksi grade that is 758 MPa grade or higher.

4. A seamless steel pipe comprising the round bar tensile test specimen evaluated in the method according to claim 2 that has a resistance to sulfide stress corrosion cracking that exceeds the predetermined threshold.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically illustrates a shape of a round bar tensile test specimen of the present disclosure.

(2) FIG. 2 illustrates the dimensions of a known round bar tensile test specimen.

DESCRIPTION OF EMBODIMENTS

(3) A sulfide stress corrosion cracking test of a steel targeted in the present disclosure is a constant-load test conforming to NACE TM 0177 Method A. A round bar tensile test specimen including a parallel section, a shoulder section, and a grip section as illustrated in FIG. 1 is used.

(4) In the round bar tensile test specimen of the present disclosure, as illustrated FIG. 1, the shoulder section is formed by a curve having two or more radii of curvature R1, R2, R3, and the like. The radius of curvature R1 (mm) of a portion of a curve adjacent to the parallel section is 15 mm or more and satisfies formula (1).

(5) R1: 15 mm or more, and
(0.22119)R1100(1)

(6) (: load stress (MPa) in the test)

(7) The radius of curvature R1 is 15 mm or more so that a stress-concentrated portion is not formed near the boundary between the parallel section and the shoulder section. There is a correlation between the failure in the shoulder section and the load stress in the test. As the load stress increased in the test, the radius of curvature in the shoulder section needs to be increased to decrease the stress gradient in the shoulder section, thereby preventing the failure in the shoulder section. Therefore, the radius of curvature R1 of a portion of the shoulder section adjacent to the parallel section is limited to (0.22119) mm or more in terms of the load stress a. When the radius of curvature R1 is (0.22119) mm or more, the failure in the shoulder section can be prevented. If the radius of curvature R1 is excessively large, an increase in the sectional area of the test specimen in the shoulder section is small, that is, a decrease in the load stress in the shoulder section is small. Consequently, a region which is close to the parallel section and to which the load stress is applied is widened, which induces the failure in the shoulder section. Therefore, the radius of curvature R1 is limited to 100 mm or less and is preferably 80 mm or less. Accordingly, the radius of curvature R1 (mm) of a portion of a shoulder section curve adjacent to the parallel section is limited to a value which is 15 mm or more and satisfies the formula (1).

(8) The length X1 (mm) of a curved portion having the radius of curvature R1 in the longitudinal direction of the test specimen is adjusted so as to satisfy formula (2) below in terms of the radius r (mm) of the parallel section and the radius of curvature R1 (mm).
X1{(r/8)(R1(r.sup.2/4))}(2)

(9) (r: radius (mm) of the parallel section)

(10) If X1 is lower than the value on the right-hand side in the formula (2), a stress concentrated in the shoulder section increases and a desired effect of preventing the failure in the shoulder section cannot be produced. In a region of the shoulder section in which the failure in the shoulder section may occur, the above-described radius of curvature R1 needs to be maintained to prevent the failure in the shoulder section.

(11) Radius of Curvature of Shoulder Section Curve: Two or More

(12) The shoulder section is formed by a curve having two or more radii of curvature. In a region of the shoulder section close to the grip section, the radius of curvature of the shoulder section is smaller than the radius of curvature R1 of a portion of a shoulder section curve adjacent to the parallel section, and the curve that forms the shoulder section is a curve having at least two radii of curvature. If the number of radius of curvature of the curve that forms the shoulder section is increased, the working of the test specimen becomes complicated. Furthermore, even if the number of radii of curvature is increased to more than 3, a marked effect is not expected. Therefore, the number of radii of curvature is preferably up to 3.

(13) A large radius of curvature of the shoulder section decreases the stress gradient near the grip section, which easily causes the failure near the grip section. As the radius of curvature of the shoulder section increases, the length of the shoulder section increases and the full length of the test specimen increases. Such a change in the full length of the test specimen requires changes in the test cell and jig, resulting in inefficiency. Thus, the radius of curvature of a portion of the shoulder section near the grip section is smaller than the radius of curvature R1 of a portion of the shoulder section adjacent to the parallel section. Consequently, the failure on the grip section side can be suppressed while at the same time the full length of the test specimen can be reduced. Note that the radius of curvature other than the radius of curvature R1 is preferably 15 mm or more and 40 mm or less.

(14) A high-strength seamless steel pipe for oil wells having a yield strength of 110 ksi grade (758 MPa grade) or higher and used in a wet hydrogen sulfide environment (sour environment), which is one of steels to be tested in the present disclosure, preferably has, for example, the following composition.

(15) (a) A high-strength seamless steel pipe for oil wells has a composition containing, on a mass % basis, C: 0.20 to 0.50%, Si: 0.05 to 0.40%, Mn: 0.3 to 1.5%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.10%, N: 0.006% or less, Cr: 0.1% to 1.5%, Mo: 0.5 to 3.0%, V: 0.01 to 0.3%, Nb: 0.002 to 0.05%, B: 0.0003 to 0.0030%, O (oxygen): 0.0040% or less, and Ti: 0.001 to 0.025%, with the balance being Fe and unavailable impurities.

(16) (b) The high-strength seamless steel pipe for oil wells has the composition of (a) further containing, on a mass % basis, Ti: 0.003 to 0.025%, and the contents of Ti and N are adjusted so that Ti/N: 2.0 to 5.0 is satisfied.

(17) (c) The high-strength seamless steel pipe for oil wells has the composition of (a) or (b) further containing, on a mass % basis, one or more selected from Cu: 1.0% or less, Ni: 0.10% or less, and W: 3.0% or less.

(18) (d) The high-strength seamless steel pipe for oil wells has the composition of any of (a) to (c) further containing, on a mass % basis, one or two or more selected from Ca: 0.0005 to 0.0050%, Zr: 0.0005 to 0.03%, and Mg: 0.0005 to 0.0025%.

(19) The high-strength seamless steel pipes for oil wells of (a) to (d) are produced by, for example, forming a material (billet) having the above-described composition into a seamless steel pipe by hot working, then cooling the seamless steel pipe at a cooling rate higher than or equal to that of air cooling so that the surface temperature reaches a temperature of 200 C. or less, and then performing a tempering treatment in which the resulting seamless steel pipe is heated to a temperature range of 600 to 740 C.

(20) In some cases, after the cooling and before the tempering treatment, the seamless steel pipe is reheated to a temperature range of Ac3 transformation point or more and 1000 C. or less, a quenching treatment in which the surface temperature is rapidly decreased to a temperature of 200 C. or less is performed at least once, and then the tempering treatment is performed.

EXAMPLES

(21) A test specimen was sampled from the high-tensile seamless steel pipe for oil wells having the composition shown in Table 1, and machining was performed to obtain a round bar tensile test specimen for a sulfide stress corrosion test which had the dimensions shown in Table 2.

(22) The test specimen is sampled by the following method conforming to API SPECIFICATION 5CT, but the method is not limited thereto as long as the manufacturer and the purchaser come to an agreement. Specifically, as described in API SPECIFICATION 5CT Annex D, the sampling frequency of the test specimen is each heat treatment heat. A position having the highest average hardness among an inner surface, an outer surface, and a central portion of the seamless steel pipe is a sampling position of the test specimen, and the test specimen is sampled in the longitudinal direction of the steel pipe.

(23) Although not shown in Table 2, all the test specimens (Test Nos. 1 to 12) had a full length of 115.0 mm and a length in the parallel section of 25.4 mm. The radius of the grip section was 4.0 mm in Test Nos, 1 to 3, 5 to 7, 9, 11, and 12 and 5.55 mm in Test Nos. 4, 8, and 10.

(24) The steel pipes No. A, No. B, and No. C were steel pipes having a yield strength of 758 MPa (110 ksi) or higher. The steel pipe No. D was a steel pipe having a yield strength of lower than 758 MPa. A sulfide stress corrosion test was conducted using the obtained round bar tensile test specimen. The test was conducted in conformity with NACE TM 0177 Method A using an NACE solution (hydrogen sulfide-saturated 5% NaCl+0.5% CH.sub.3COOH solution) at 25 C. by applying a constant load for a maximum of 720 hours. For some of the test specimens, the test was continued to 840 hours to perform more severe evaluation. Note that three test specimens were prepared in consideration of variation. Table 3 shows the test results. When the failure did not occur until 720 hours, an evaluation of (Good) was given. When the failure occurred before 720 hours, the position of the failure was checked. When the failure occurred in the parallel section, an evaluation of (Good) was given because proper evaluation was conducted. When the failure occurred in the shoulder section or the grip section, an evaluation of (Poor) was given because proper evaluation was not conducted. When an evaluation of (Good) was given to all the three test specimens, a judgment of (Pass) was given. When an evaluation of (Poor) was given to at least one of the three test specimens, a judgment of (Reject) was given.

(25) TABLE-US-00001 TABLE 1 Steel pipe Chemical composition (mass %) No. C Si Mn P S Ni Cr Mo Cu Nb A 0.26 0.22 0.82 0.008 0.001 0.01 1.45 0.63 0.02 0.02 B 0.28 0.15 0.64 0.012 0.001 0.15 0.92 1.38 0.01 0.01 C 0.30 0.23 0.44 0.008 0.001 0.01 0.98 0.95 0.45 0.02 D 0.24 0.30 0.55 0.009 0.001 0.01 0.87 0.75 0.01 0.03 Tensile properties Yield Tensile Steel Chemical composition (mass %) strength strength pipe sol. YS TS No. V Ti B Al Ca O N (MPa) (MPa) A 0.08 0.001 0.0014 0.069 0.0020 0.0009 0.0042 786 854 B 0.06 0.015 0.0020 0.035 0.0002 0.0012 0.0056 885 926 C 0.05 0.008 0.0025 0.035 0.0001 0.0010 0.0033 883 970 D 0.03 0.013 0.0016 0.033 0.0012 0.0016 0.0045 747 830

(26) TABLE-US-00002 TABLE 2 Shape of test specimen Yield Radius of curvature in shoulder section Length in shoulder section stress Load Radius in Value on Satisfac- Value on Satisfac- Steel of steel stress parallel left side tion of right side tion of Test pipe YS in test section r R1 in formula formula R2 R3 X1** in formula formula No. No. (MPa) (MPa) (mm) (mm) (1)* (mm) (1)* (mm) (mm) (mm) (2)*** (mm) (2)*** Remarks 1 A 786 645 3.175 30 22.9 (Yes) 20 4.5 3.3 (Yes) Inventive Example 2 A 786 645 3.175 20 x(No) 9.3 2.6 (Yes) Comparative Example 3 B 855 690 3.175 35 32.8 (Yes) 20 4.5 3.6 (Yes) Inventive Example 4 B 855 690 3.175 75 (Yes) 30 20 7.3 5.4 (Yes) Inventive Example 5 B 855 690 3.175 25 x(No) 5.6 3.0 (Yes) Comparative Example 6 B 855 690 3.175 35 (Yes) 20 3.5 3.6 x(No) Comparative Example 7 B 855 690 3.175 60 (Yes) 16.5 4.8 (Yes) Comparative Example 8 B 855 721 3.175 105 39.5 x(No) 30 20 9.0 6.4 (Yes) Comparative Example 9 C 883 664 3.175 40 27.0 (Yes) 20 5.5 3.9 (Yes) Inventive Example 10 C 883 724 3.175 60 40.3 (Yes) 25 15 7.0 4.8 (Yes) Inventive Example 11 D 724 648 3.175 40 37.2 (Yes) 15 7.1 3.9 (Yes) Inventive Example 12 D 724 648 3.175 15 x(No) 7.1 2.2 (Yes) Comparative Example *(0.22 119) R1 100 . . . (1) **Length of a portion of a shoulder section having radius of curvature R1 in the longitudinal direction of a test specimen ***X1 {(r/8) (R1 r.sup.2/4)} . . . (2)

(27) TABLE-US-00003 TABLE 3 Test result Steel Failure Test pipe time No. No. (hour) Failure position Evaluation Judgement Remarks 1 A 720* (Good) (Pass) Inventive Example 720* (Good) * Failure did not occur 720* (Good) even after 840 hours 2 A 686 Shoulder section x(Poor) x(Reject) Comparative Example 720** (Good) ** Failure occurred in 621 Shoulder section x(Poor) shoulder section after 760 hours 3 B 288 Parallel section (Good) (Pass) Inventive Example 720 (Good) 664 Parallel section (Good) 4 B 720 (Good) (Pass) Inventive Example 312 Parallel section (Good) 720 (Good) 5 B 303 Shoulder section x(Poor) x(Reject) Comparative Example 475 Shoulder section x(Poor) 625 Shoulder section x(Poor) 6 B 720*** (Good) x(Reject) Comparative Example 691 Shoulder section x(Poor) *** Failure occurred in 588 Shoulder section x(Poor) shoulder section after 748 hours 7 B 541 Grip section x(Poor) x(Reject) Comparative Example 315 Shoulder section x(Poor) 498 Shoulder section x(Poor) 8 B 563 Shoulder section x(Poor) x(Reject) Comparative Example 702 Shoulder section x(Poor) 553 Shoulder section x(Poor) 9 C 720 (Good) (Pass) Inventive Example 720 (Good) 720 (Good) 10 C 720 (Good) (Pass) Inventive Example 720 (Good) 720 (Good) 11 D 720 (Good) (Pass) Inventive Example 720 (Good) 720 (Good) 12 D 720 (Good) (Pass) Comparative Example 720 (Good) 720 (Good)

(28) In all Inventive Examples in which the round bar tensile test specimen within the scope of the present disclosure was used, when failure occurred, the failure occurred in the parallel section, an evaluation of (Good) was given, proper evaluation for resistance to sulfide stress corrosion cracking could be performed, and the judgement was (Pass). In Comparative Examples in which the round bar tensile test specimen outside the scope of the present disclosure was used, the failure occurred in the shoulder section or the grip section, proper evaluation was not performed, an evaluation of (Poor) was given, and the judgement was (Reject).

(29) Test No. 1, No. 3, No. 4, No. 9, and No. 10, the shape of the test specimen satisfies the scope of the present disclosure (the formula (1) and the formula (2) are satisfied and R2 or R3 is specified), and failure does not occur in the shoulder section. In Test No. 2 and No. 5, the radius of curvature R1 of a portion of the shoulder section adjacent to the parallel section is below the scope of the present disclosure, and the failure occurs in the shoulder section. In Test No. 6, the length X1 of a portion of the shoulder section adjacent to the parallel section, the portion having the radius of curvature R1, is below the scope of the present disclosure, and the failure occurs in the shoulder section. In Test No. 7, the number of radii of curvature in the shoulder section is one, which is outside the scope of the present disclosure, and the failure occurs in the shoulder section or the grip section. In Test No. 8, the radius of curvature R1 of a portion of the shoulder section adjacent to the parallel section is above the scope of the present disclosure, and the failure occurs in the shoulder section.

(30) In Test No. 11 and No. 12, proper evaluation is performed not only in the case where the shape of the test specimen satisfies the scope of the present disclosure (Test No. 11) but also in the case where the shape of the test specimen is outside the scope of the present disclosure (Test No. 12) because the steel pipe has a low yield strength of less than 758 MPa (110 ksi). Thus, according to the present disclosure, it is found that, in particular, when the steel pipe has a high yield strength of 758 MPa (110 ksi) or higher, proper evaluation for resistance to sulfide stress corrosion cracking can be performed.

(31) In No. 1 of Inventive Example, the test was completed without causing failure even after 840 hours. However, in Nos. 2 and 6 of Comparative Example, although the test specimen did not undergo the failure after 720 hours, the failure occurred in the shoulder section before 840 hours.

(32) Among inventive Examples (Test No. 1, No. 3, No. 4, No. 9, No. 10, and No. 11) in which the round bar tensile test specimens within the scope of the present disclosure were used, the high-tensile seamless steel pipes for oil wells (steel pipes No. A, No. C, and No. D) from which the round bar tensile test specimens (Test No. 1, No. 9, No. 10, and No. 11) in which all of the three test specimens did not undergo failure before a particular time passed (herein, 720 hours) were sampled are provided with a test result failure did not occur before a particular time passed (e.g., 720 hours) in the sulfide stress corrosion cracking test of the present disclosure. The test result may be provided by mentioning the test result on a mill sheet of high-tensile seamless steel pipes for oil wells or attaching a label that mentions the test result to high-tensile seamless steel pipes for oil wells.