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EP-4276442-B1 - METHOD FOR TESTING SULFIDE STRESS CORROSION CRACKING OF STEEL MATERIAL

EP4276442B1EP 4276442 B1EP4276442 B1EP 4276442B1EP-4276442-B1

Inventors

  • SAKIMOTO TAKAHIRO
  • HANDA TSUNEHISA

Dates

Publication Date
20260513
Application Date
20220118

Claims (4)

  1. A method for testing sulfide stress corrosion cracking of a steel material, comprising: a crack introduction step of introducing, into a test piece (10) that is collected from a steel material to be evaluated, a fine initial crack (11) that extends in a sheet thickness direction from a surface (10a) of the test piece (10) and is 2 µm to 1000 µm in width and 1 µm to 1000 µm in initial length; a 4-point bending sulfide stress cracking (SSC) test step of performing a 4-point bending SSC test in which the test piece (10) into which the initial crack (11) has been introduced in the crack introduction step is immersed in a test solution in a 4-point bent state; and a stress intensity factor calculation step of calculating a stress intensity factor K ISSC value based on a state where the initial crack (11) has propagated in the sheet thickness direction by the 4-point bending SSC test step, wherein, in the crack introduction step, a plurality (first to n th , n = 2 or more) of the initial cracks (11) that extends in the sheet thickness direction from the surface (10a) of the test piece (10) is introduced along a row direction, in the 4-point bending SSC test step, the test piece (10) into which the plurality, that is first to n th , of the initial cracks (11) has been introduced is immersed in the test solution in a 4-point bent state, in the stress intensity factor calculation step, the stress intensity factors K ISSC values of the respective remaining initial cracks (11) are calculated from Expression (1) below based on the initial lengths and propagated crack lengths of a plurality, that is first to k-1 th and k+1 th to n th , of remaining cracks in a sample in which any one initial crack (11), that is k th among first to n th , among the plurality, that is first to n th , of the initial cracks (11) has been ruptured due to propagation in the sheet thickness direction by the 4-point bending SSC test step, [Math. 1] K ISSC 1 = F ⋅ σ π a 1 + b 1 , K ISSC 2 = F ⋅ σ π a 2 + b 2 , ⋯ , K ISSCk − 1 = F ⋅ σ π a k − 1 + b k − 1 , K ISSCk + 1 = F ⋅ σ π a k + 1 + b k + 1 , ⋯ , K ISSCn = F ⋅ σ π a n + b n here, K ISSC1 to K ISSCk-1 and K ISSCk+1 to K ISSCn are stress intensity factor K ISSC values of the respective remaining initial cracks (11), F is a shape factor, σ is a bending stress that is applied to the test piece (10) by the 4-point bending SSC test step, a 1 to a k-1 and a k+1 to a n are measured initial lengths of the respective remaining initial cracks (11), and b 1 to b k-1 and b k+1 to b n are measured propagated crack lengths of the respective remaining initial cracks (11).
  2. The method for testing sulfide stress corrosion cracking of a steel material according to claim 1, wherein, the initial lengths of the plurality, that is first to n th , of the respective initial cracks (11) introduced by the crack introduction step are measured, and, when a relationship between the initial lengths of the plurality, that is first to n th , of the respective measured initial cracks (11) and intervals between the initial cracks (11) adjacent to each other satisfies Expression (2), the 4-point bending SSC test step is performed, d 1 / a 1 ≥ 5 , d 1 / a 2 ≥ 5 , d 2 / a 2 ≥ 5 , … , d n − 2 / a n − 1 ≥ 5 , d n − 1 / a n − 1 ≥ 5 , and d n − 1 / a n ≥ 5 here, a 1 to a n are the initial lengths of the plurality, that is first to n th , of the respective initial cracks (11), and d 1 to d n-1 are the intervals between the initial cracks (11) adjacent to each other.
  3. The method for testing sulfide stress corrosion cracking of a steel material according to claim 2, wherein, in the stress intensity factor calculation step, the propagated crack lengths in the sheet thickness direction of the plurality, that is first to n th , of the respective initial cracks (11) in the 4-point bending SSC test step are measured, among the plurality, that is first to n th , of initial cracks (11), an initial crack (11) in which the crack has propagated longest in the sheet thickness direction is designated as the any one initial crack (11), that is k th among first to n th , in the target to be ruptured, and, when the initial length and propagated crack length of the any one initial crack (11), that is k th among first to n th , in the target to be ruptured satisfy Expression (3), the any one initial crack (11), that is k th among first to n th , in the target to be ruptured is determined to rupture, and the stress intensity factor K ISSC values of the respective remaining initial cracks (11) are calculated from Expression (1), 0.8 t ≤ a k + b k ≤ 1.0 t here, a k is the measured initial length of the any one, that is k th among first to n th , of initial crack (11) in the target to be ruptured, b k is the measured propagated crack length of the any one, that is k th among first to n th , of initial crack (11) in the target to be ruptured, and t is a sheet thickness of a test piece (10).
  4. The method for testing sulfide stress corrosion cracking of a steel material according to claim 3, wherein, in the stress intensity factor calculation step, when the stress intensity factor K ISSC values of the respective remaining initial cracks (11) have been calculated, the calculated stress intensity factors K ISSC values of the respective remaining initial cracks (11) are adopted in a case where the relationship between the initial lengths and propagated crack lengths of the respective remaining initial cracks (11) and the intervals between the adjacent cracks satisfies Expression (4) below, d 1 /(a 1 + b 1 ) ≥ 5, d 1 /(a 2 + b 2 ) ≥ 5, d 2 /(a 2 + b 2 ) ≥ 5, ..., d k-2 /(a k-1 + b k-1 ) ≥ 5, d k-1 (a k-1 + b k-1 ) ≥ 5, d k (a k+1 + b k+1 ) ≥ 5, d k+1 /(a k+1 + b k+1 ) ≥ 5, ..., d n-1 /(a n-1 + b n-1 ) ≥ 5, d n-1 /(a n + b n ) ≥ 5 here, a 1 to a k-1 and a k+1 to a n are the measured initial lengths of the respective remaining initial cracks (11), that is first to k-1 th and k+1 th to n th , b 1 to b k-1 and b k+1 to b n are the measured propagated crack lengths of the respective remaining initial cracks (11), that is first to k-1 th and k+1 th to n th , and d 1 to d n-1 are the intervals between the cracks adjacent to each other.

Description

Technical Field The present invention relates to a method for testing sulfide stress corrosion cracking of a steel material such as a steel pipes for line pipes. Background Art Fluids produced from deep-water oil fields or gas fields, which have been being developed in recent years, often contain corrosive hydrogen sulfide. In such an environment, high-strength steel comes to break due to the occurrence of hydrogen embrittlement called sulfide stress cracking (hereinafter, referred to as SSC). As the development of deep-water oil fields or gas fields progresses, an SSC resistance of steel pipes for line pipes in use has become important. In the past, there was a problem of gas leakage attributed to SSC immediately after pipeline operation or the like, and there has been a demand for a test technology for accurately evaluating the SSC resistance of materials in use from the viewpoint of safety. Ordinarily, the SSC resistance of materials is evaluated by a double cantilever beam (DCB) test according to a method D specified in National Association of Corrosion Engineers (NACE) Standard TM0177-2005 of NPL 1. In addition, a stress intensity factor KISSC value under a sulfide corrosion environment is calculated from the measured value of this DCB test. The stress intensity factor KISSC value that is obtained by the DCB test is an index indicating the lowest K value at which cracks are capable of propagating under a given corrosion environment (the intensity of a stress field at a crack tip portion), and it means that, as the larger this value, the lower the cracking susceptibility in the given corrosion environment. Citation List Non Patent Literature NPL 1: NACE Standard TM0177-2005 Relevant prior art can be found in document JP2008051632 which describes a stress-corrosion-cracking progress test-device method wherein a test piece having an initial crack is immersed in a test solution in a 4-point bending test. Summary of Invention Technical Problem By the way, in the DCB test according to the method D specified in NPL 1, it is necessary to collect a test piece from a steel pipe or a steel sheet in a direction in which a sheet thickness of the steel pipe or the steel sheet matches the thickness of the test piece due to dimensional restrictions, and a crack introduction direction of the test piece at that time is a surface direction perpendicular to a sheet thickness direction. However, since SSC in a steel pipe for line pipes occurs from the surface of the material, an actual crack propagation direction is the sheet thickness direction. Ordinarily, steel pipes manufactured by rolling have different microstructure distribution and strength and toughness characteristics in a sheet thickness direction and in a sheet thickness perpendicular direction due to microstructure control by rolling. Therefore, the KISSC value obtained by the DCB test cannot be said to represents steel sheet characteristics in a direction in which actual SSC occurs, and, in actual environments, there is a risk of the occurrence of unexpected SSC, which may lead to serious accidents. Due to such a fact, there has been a strong demand for a method for obtaining the KISSC value of a material in a direction in which actual SSC occurs. Therefore, the present invention has been made to solve this conventional problem, and an object of the present invention is to provide a method for testing sulfide stress corrosion cracking of a steel material enabling the obtainment of the stress intensity factor KISSC value of a steel material to be evaluated in the sheet thickness direction. Solution to Problem In order to solve the above-described problem, a method for testing sulfide stress corrosion cracking of a steel material according to the present invention is as specified in claim 1. Advantageous Effects of Invention According to the method for testing sulfide stress corrosion cracking of a steel material according to the present invention, it is possible to provide a method for testing sulfide stress corrosion cracking of a steel material enabling the obtainment of a stress intensity factor KISSC value in the sheet thickness direction of a steel material to be evaluated. In addition, the same aspect can also be applied to a method for testing ammonia stress corrosion cracking in tanks and vessels for ammonia, not part of the claimed invention. Brief Description of Drawings FIG. 1 is a flowchart for describing the flow of a treatment in a method for testing sulfide stress corrosion cracking of a steel material according to an embodiment of the present invention;FIG. 2 is a flowchart for describing the details of a step S5 (stress intensity factor calculation step) in the flowchart of FIG. 1;FIG. 3 is a perspective view of a state where a plurality of initial cracks has been introduced into a test piece to be tested by the method for testing sulfide stress corrosion cracking;FIG. 4 is a cross-sectional view for describing dimensions of an initial crack i