RU2530486C1 - Method of inspecting stress corrosion cracking resistance of pipe steel - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of inspecting stress corrosion cracking resistance of pipe steel comprises preparing cylindrically shaped samples, applying voltage and exposing said samples to the testing medium; wherein the samples are subjected to preliminary tensile deformation with stretch ratio of 1-10%; applying a load, the value of which is 50-80% of the yield point, and placing the samples into the testing medium with pH 2.5-5 for 180-360 hours; breaking down the samples on air by stretching on a breaking machine, and judging stress corrosion cracking resistance from the difference in mechanical properties of steel in the initial state and after testing; wherein stress corrosion cracking resistance is judged from the degree of change in plasticity, which is calculated using the formula: $$\xi =\frac{{\delta }_5^0-{\delta }_5^H}{{\delta }_5^0}\cdot 100\%,$$

where $${\delta }_5^0$$

is relative elongation in the initial state; $${\delta }_5^H$$

is relative elongation after testing, wherein steel for which the value ξ ranges from 0 to +10% is assigned class 1 resistance, steel for which the value ξ is greater than +10% or ranges from minus 10% to 0% is assigned class 2 resistance and steel for which the value ξ is minus 10% is assigned class 3 resistance.

EFFECT: high information value and reliability while shortening the duration of testing for stress corrosion cracking resistance, taking into account the susceptibility of steel to nonuniform plastic deformation, and enabling the classification of steel according to stress corrosion cracking resistance.

2 cl, 2 tbl

Description

The invention relates to the field of quality control of steel products intended for use in aggressive environments that have a corrosive effect on metals.

One of the most damaged objects as a result of corrosion is equipment exposed to aggressive environments, of which hydrogen (free, unbound) and other aggressive components are a component. Such equipment includes pipelines for transporting oil and gas, including trunk pipelines, as well as infield, tanks, downhole and other types of equipment (including for the chemical and oil refining industries).

One of the main types of corrosion failure of such equipment is stress corrosion cracking (SCC, stress corrosion).

In relation to steel gas pipelines, the important stages of this type of destruction are:

- the occurrence of foci of local corrosion on the surface of the pipe upon its contact with the soil electrolyte according to the classical electrochemical mechanism with the subsequent generation of stress-corrosion cracks,

- development of SCC cracks by the mechanism of anodic dissolution of the metal at the mouth of the crack or hydrogen embrittlement (due to the ingress of hydrogen into the steel from the corrosive medium), which at a certain stage can receive anomalous acceleration and lead to through long-term destruction of the pipeline.

These ideas about the SCC process do not fully take into account the role of the stress-strain state, as well as the processes of plastic microdeformation of the surface and then deeper layers of the metal, which lead to the exhaustion of the ductility reserve of individual sections, to the appearance and development of cracks.

So, the main condition for the occurrence of the first stage of CSF - the initiation of cracks, is the presence of anomalies on the surface of the pipes that cause an inhomogeneous stress distribution in the pipe section: the spread of the mechanical properties of the metal, the thickness of the sheet blank, dents, the displacement of the edges in the weld zone, etc. Other abnormal areas that form on the surface as a result of the contact of the metal structural elements present on it, exhibiting corrosive activity in aqueous media (non-metallic inclusions, structural and segregation heterogeneity), with a soil electrolyte, are foci of corrosion.

During a long stay under load (during pipeline operation) of metal with abnormal areas on the surface, an inevitable leveling of the stress field occurs, which is realized by slowly proceeding plastic deformation of the metal in places of anomalies. The result of the exhaustion of the reserve of ductility of metal in individual microvolumes is the nucleation of microcracks.

The conditions for the development of the second stage of SCC, in addition to the presence of germ cracks on the metal surface, are access of the corrosive medium to the metal surface, as well as hydrogen evolution from the corrosive medium as a result of chemical and electrochemical processes. This stage involves the sequential alternation of two processes: local anodic dissolution (LAR) and hydrogen embrittlement (VO), while both processes are prepared and accompanied by an active plastic deformation. The process of microplastic deformation of the layer is intensified by the action of hydrogen. Contrary to the widespread belief that embrittlement takes place in hydrogen-containing media, in the initial period at low concentrations, hydrogen can contribute to microplastic deformations. But these microplastic deformations, as well as embrittlement processes, lead to the exhaustion of the plasticity reserve and to the accelerated development of the SCC process.

The presented model ideas about the SCC process indicate a significant role in its development of steel’s propensity for uneven microdeformation in contact with the medium and the feasibility of its determination to assess the resistance of steel to SCC.

A known method for assessing the tendency of pipe grades of steel to stress corrosion, including exposure to a test sample of a hydrogen-containing corrosive medium, while previously prints are applied to the test sample with diamond, a load is applied within 0.85-0.95 of the yield strength of the steel, determine the coefficient of non-uniformity of surface microdeformation (K _n ) according to the formula:

, $$K_H=\frac{{\sum }_{R\mathrm{but}sb}}{{\displaystyle \sum _{i=\mathrm{one}}^n\Delta l_i}}$$

,

where ^∑ scatter - the total amount of the spread of deformation of the sections between the prints,

^Δ l _i - the elongation between the prints,

- общая сумма удлиненных участков между отпечатками, и по коэффициенту К_н оценивают склонность стали к стресс-коррозии: при значении К_н в пределах 0,05÷0,12, марка трубной стали не склонна к КРН, при значении К_н в пределах 0,12÷0,17 на трубной стали появляются стресс-коррозионные повреждения, не представляющие опасности в условиях длительной эксплуатации, а при значении К_н более 0,17 марка трубной стали склонна к КРН, а в качестве водородсодержащей коррозионной среды используют 3% раствор хлорида натрия, подкисленный соляной кислотой до pH 2-2,3. Способ позволяет повысить коррозионную стойкость магистральных трубопроводов в условиях, вызывающих стресс-коррозию. (патент RU2299420, МПК G01N 17/00, опубликован 20.05.2007). $${\displaystyle \sum _{i=\mathrm{one}}^n\Delta l_i}$$

- the total amount of elongated sections between the prints, and the coefficient of K _n assess the tendency of steel to stress corrosion: when the value of K _n in the range of 0.05 ÷ 0.12, the grade of pipe steel is not prone to SCC, when the value of K _n within 0 12 ÷ 0,17 appear on the tubular steel stress-corrosion damage, not dangerous in conditions of prolonged operation, and a value of more than 0.17 K _n stamp pipe steel is prone to SCC as well as hydrogen-containing corrosive medium using 3% chloride sodium acidified with hydrochloric acid to a pH of 2-2.3. The method improves the corrosion resistance of pipelines in conditions that cause stress corrosion. (patent RU2299420, IPC G01N 17/00, published May 20, 2007).

The disadvantage of this method is the need for a large number of measurements carried out with a hand tool, during which errors may occur. In addition, the method allows to assess the unevenness of surface microdeformation only in local areas, while real steels are characterized by significant heterogeneity of the microstructure.

The closest analogue of the claimed invention is a method for assessing the resistance of steel against stress corrosion cracking, which consists in the fact that samples are taken from the products, cylindrical samples are made to which stress is applied and exposed to an aggressive environment. The sample is kept in an aggressive environment under constant load for 720 hours. The level of applied stress is in the range from 0.6 to 0.95 of the yield strength of steel, depending on the requirements of regulatory documentation. The criterion for the resistance of steel may be the maximum value of the applied stress at which the sample did not fail within 720 hours, or the fact of the absence of failure at a certain fixed load (most often 0.8 of the yield strength of steel), also after exposure to aggressive environment for 720 hours (NACE Standard TM 0198-98 method. Standard Test Method Slow Strain Rate Test Method for Screening Corrosion-Resistant Alloys (CRAs) for Stress Corrosion Cracking in Sour Oilfield Service, p.1 - 16 - prototype).

The disadvantage of this method is its low sensitivity, long test duration and the inability to rank steels close in mechanical characteristics, containing hydrogen traps of different efficiencies, which largely determine the steel's resistance to stress corrosion. In addition, under the specified test conditions, for some steels, the processes of microplastic deformation may not develop, which also reduces the reliability of the assessment of resistance against SCC.

The problem to which the invention is directed, is to create a method for controlling the resistance to stress corrosion cracking under stress of steels intended for pipes of gas mains and other types of equipment operating under conditions leading to the ingress of hydrogen into the metal.

The technical result of the present invention is to increase the information content and reliability while reducing the duration of the test for resistance to corrosion cracking, taking into account the tendency of steel to inhomogeneity of plastic deformation, as well as the possibility of ranking steels according to resistance classes against stress corrosion cracking.

The specified technical result is achieved by the fact that in the method for controlling the resistance of pipe steels against stress corrosion cracking, which consists in producing cylindrical samples to which stress is applied and subjected to a test medium, according to the invention, the samples are subjected to preliminary tensile deformation with degrees 1- 10%, then a load is applied, the value of which is 50-80% of the yield strength, and the samples are placed in a test medium with a pH value of within 2.5-5 for 180-360 hours, after which the samples are destroyed in air by tensile testing on a tensile testing machine, and resistance to stress corrosion cracking is judged by the difference in the mechanical properties of the steels in the initial state and after testing. About resistance to stress corrosion cracking is judged by the degree of change in ductility, which is calculated by the formula:

, $$\xi =\frac{{\delta }_5^0-{\delta }_5^H}{{\delta }_5^0}\cdot \mathrm{one\; hundred}\%$$

,

- относительное удлинение в исходном состоянии;where - $${\delta }_5^0$$

- elongation in the initial state;

- относительное удлинение после испытаний, $${\delta }_5^H$$

- elongation after testing,

steel, for which the value of ξ is from 0 to + 10%; belong to the 1st class of resistance,

steels for which the ξ value is more than + 10% or from minus 10% to 0% are assigned to the 2nd resistance class,

steels for which the ξ value is less than minus 10%; belong to the 3rd class of resistance.

The essence of the claimed invention lies in the fact that there is a simulation of two processes inherent in the operation of gas pipelines: local anode dissolution (LAR) and hydrogen embrittlement (VO), while both processes are prepared and accompanied by an active plastic deformation. The localization sites of microplastic deformations can be imperfections of the crystal lattice, metallurgical heterogeneity of steel.

Preliminary tensile deformation with a total degree of 1-10% is necessary to intensify the plastic flow in the most stressed sections of the metal even before hydrogen enters the metal, which is typical for the beginning of the first stage of SCC. When the degree of deformation is less than 1%, plastic deformation will be realized unevenly in volume only in certain sections of the metal. An increase in the degree of deformation of more than 10% will exceed the actually possible degrees of deformation that can occur in the pipeline before hydrogen enters the steel.

The application of a load, the value of which is 50-80% of the yield strength, is necessary to create a stress-strain state characteristic of the nucleation and development stages of SCC cracks. It is precisely on the basis of the requirements for providing loads in the pipeline of not more than 80% of the yield strength that steel is selected for the pipeline, its diameter and wall thickness of the pipe are calculated for specific operating conditions. A load of less than 50% of the yield strength does not provide an average level of stress characteristic of the operating conditions of the pipeline.

Exposure of samples in a test medium with a pH value in the range of 2.5-5 for 180-360 hours ensures that hydrogen enters the steel during testing, sufficient to create a stress-strain state in metal areas with structural elements that are traps for hydrogen. At a pH value of more than 5, for a given test duration, and also for a test duration of less than 180 hours, the development of steel degradation processes associated with hydrogen entering the steel and the development of microplastic deformations will not be sufficient to assess the steel's resistance to SCC. At a pH value of less than 2.5, the mechanisms of steel destruction change and become characteristic of environments with a high content of hydrogen sulfide. In this case, other structural elements begin to play a decisive role in the destruction, which begins to occur by the mechanism of hydrogen cracking, than in the SCC processes, which reduces the reliability of the results.

The claimed distinguishing features (parameter "change in elongation") and criteria for ranking the results are determined on the basis of numerous experiments empirically.

Examples of the invention

From pipe steels of grades K60-K65, the chemical composition of which is given in Table 1, cylindrical samples 90 × 5 mm in size were made, 7 pieces per each steel variant. One sample from the variant was tested immediately to determine elongation in the initial state. Each of the remaining samples was subjected to preliminary deformation by stretching from 1 to 5%, respectively. Next, the samples were placed in a closed cell with a solution of 5% NaCl, 0.4% acetic acid, pH 2.9. A load of 80% of the yield strength was applied to the samples. Tests were conducted 240 hours. Then, the samples were destroyed in air by a tensile method and the relative elongation was determined for each sample of one variant and compared with the value of this parameter in the initial state. The steel resistance to SCC was judged by the largest change in elongation. We also tested the samples according to the prototype (NACE Standard ТМ 0198-98 method. Standard Test Method Slow Strain Rate Test Method for Screening Corrosion-Resistant Alloys (CRAs) for Stress Corrosion Cracking in Sour Oilfield Service, p.1-16). For this, one cylindrical sample was taken from each variant and tested under a load of 80% of the yield strength for 720 hours in an aggressive NACE environment. The criterion of steel resistance to SCC is the fact of the destruction of the sample during aging. The test results of the samples according to the developed methodology and prototype are shown in table 2. The tests were carried out on pipe steels of grades K60-K65, the chemical composition of which is shown in table 1.

Table 1 The chemical composition of the studied steels No. Steel The content of elements, wt.% FROM Si Mn P S Cr Ni Al Ti V Nb one K60 0,090 0.26 1,52 0.009 0.002 0,036 0,03 0,037 0.019 0.06 0.04 2 K65 0,063 0.22 1,69 0.008 0.002 0.24 0.23 0,034 0.017 0.04 0,066 3 K60 0,065 0.26 1,58 0.008 0.002 0.019 0.17 0,038 0.015 0,03 0,045 four K65 0.059 0.25 1,53 0.008 0.002 0.22 0.20 0,041 0,025 0.02 0,055

table 2 The test results of the samples according to the methodology for assessing the tendency of pipe grades of steel to corrosion and mechanical damage with heterogeneity of surface microdeformation and the prototype. Arr No. The fact of the destruction of the sample (test method prototype) The value of the preliminary deformation,% The relative elongation in the initial state,% Residual elongation after testing,% The degree of change in elongation% Elongation Resistance Class one Yes 2 19 22 -18.9 3 2 No 2 twenty 21 -6.4 2 3 No 2 26 21 +18 2 four No 3 17 17 0 one

It was established that the steels of compositions 3 and 4 have a fundamentally different character of changes in properties in the hydrogenation process from compositions 1 and 2. In the first case, an increase is observed, and in the second, a decrease in elongation, which may be due to the accumulation of unfavorable forms of hydrogen in the metal. However, as shown above, both a decrease and an increase in the relative elongation after testing are unfavorable factors for the development of SCC.

The degree of change in elongation showed the highest resistance of steel K65 of composition 4 - class 1, the lowest - steel K60 of composition 1 - class 3, steels of compositions 2 and 3 showed an intermediate resistance class - class 2.

The results of the tests carried out by the prototype method, in part, correlate with the results of the proposed method. The steel sample No. 1 was destroyed during the tests on the prototype and showed the 3rd resistance class according to the developed technique. Samples of steels No. 2-4 did not collapse when tested according to the prototype, however, according to the developed method, they showed different resistance classes. The developed technique makes it possible to more accurately rank steel in terms of resistance to SCC.

Thus, the invention provides an increase in information content and reliability while reducing the duration of testing for resistance to corrosion cracking, taking into account the tendency of steel to inhomogeneity of plastic deformation, as well as the possibility of ranking steels according to resistance classes against stress corrosion cracking.

Claims (2)

1. A method of controlling the resistance of pipe steels against stress corrosion cracking, which consists in the manufacture of cylindrical samples to which stress is applied and subjected to a test environment, characterized in that the samples are subjected to preliminary tensile deformation with degrees of 1-10%, then apply a load, the value of which is 50-80% of the yield strength, and place the samples in a test medium with a pH value in the range of 2.5-5 for 180-360 hours, after which the samples are destroyed air stretching method, and a resistance to stress corrosion cracking is judged by the difference in mechanical properties of the steels in the initial state and after the test. 2. The method according to claim 1, characterized in that the resistance to stress corrosion cracking is judged by the degree of change in ductility, which is calculated by the formula:
$$\xi =\frac{{\delta }_5^0-{\delta }_5^H}{{\delta }_5^0}\cdot \mathrm{one\; hundred}\%$$

,
where - $${\delta }_5^0$$

- elongation in the initial state;
$${\delta }_5^H$$

- elongation after testing,
steel, for which the value of ξ is from 0 to + 10%, is assigned to the 1st resistance class,
steels for which the ξ value is more than + 10% or from -10% to 0% are assigned to the 2nd resistance class,
steel, for which the value of ξ is less than minus 10%, belong to the 3rd class of resistance.