Analysis method for steam pipeline welding seam cracking reason

Abstract

According to the analysis method for the steam pipeline welding seam cracking reason, through combination of staged gradient corrosion and an image analysis technology, material defects are quantified, layered stripping energy spectrum detection and fractal dimension calculation are adopted, the interaction between oxidation corrosion and instantaneous cracking is distinguished, a grain size matching index and five-dimensional scoring system is provided, and the analysis result of the steam pipeline welding seam cracking reason is obtained. And carrying out dynamic coupling analysis on multiple parameters such as material structure difference, insufficient welding heat input and abnormal component gradient. According to the method, quantitative judgment of the failure mode is achieved through multi-dimensional data fusion, the problems that a traditional method is high in one-sidedness and large in error rate are effectively solved, stress corrosion and other cracking reasons can be rapidly positioned, unplanned shutdown and economic losses caused by weld joint failure are remarkably reduced, and the method is suitable for systematic safety evaluation of the high-temperature and high-pressure steam pipeline.

Description

Technical Field

[0001] The present invention relates to the fields of power engineering and material failure analysis, and in particular to a method for analyzing causes of cracking of steam pipeline welds. Background Art

[0002] In the power industry, power plant main steam pipelines are subject to long-term high temperatures (500-600°C), high pressures (10-30 MPa), and vibration loads. Their safety and reliability directly impact unit operation. Welds are the weak link in pipelines, and cracks can easily lead to leaks.

[0003] The current analysis method has the following defects:

[0004] The lack of quantitative evaluation indicators for material structure differences (such as grain size matching index) makes it difficult to assess the risk of interfacial stress concentration;

[0005] The detection of grain boundary component segregation and tiny abnormal particles relies on high-cost electron microscopy analysis (such as Auger spectroscopy), which makes field application difficult;

[0006] Fracture corrosion analysis lacks dynamic delamination technology and cannot distinguish the interaction between continuous oxidation and instantaneous cracking;

[0007] The one-sidedness of a single detection method (such as hardness test) leads to an error rate of more than 40% in failure mechanism determination.

[0008] Therefore, a method is needed to comprehensively analyze the causes of steam pipeline weld cracking. Summary of the Invention

[0009] A first aspect of the present disclosure provides a method for analyzing the cause of cracking of a steam pipeline weld, comprising the following steps:

[0010] The elbows of steam pipes were subjected to phased gradient corrosion treatment to obtain their void density, grain boundary segregation and mechanical properties;

[0011] Calculating the grain size matching index of the straight pipe and the elbow of the steam pipeline, analyzing the crack propagation path of the steam pipeline in combination with impact metallographic inspection, and obtaining the gradient slope of the steam pipeline;

[0012] Performing layered peeling energy spectrum detection on the fracture of the steam pipe, and calculating the fractal dimension and oxygen-sulfur ratio of the crack path of the steam pipe;

[0013] The void density, grain boundary segregation, gradient slope, fractal dimension and oxygen-sulfur ratio are input into a pre-established analysis function to calculate the cause of the weld cracking of the steam pipeline.

[0014] In combination with the first aspect, the staged gradient corrosion process includes:

[0015] Pre-corroding the elbow with a 4% nitric acid alcohol solution for a first preset time to reveal a matrix tissue outline;

[0016] corroding the elbow using a ferric chloride solution for a second preset time;

[0017] The cavity density of the elbow is counted.

[0018] In combination with the first aspect, the calculation formula of the grain size matching index is:

[0019]

[0020] Where D is the grain size grade, HV is the Vickers hardness, and the subscripts pipe and elbow represent the corresponding parameters of straight pipes and elbows, respectively.

[0021] In combination with the first aspect, the fractal dimension calculation uses a box counting method to analyze the crack path in the electron microscope image, and when the fractal dimension D is greater than 1.65, it is determined to be stress corrosion cracking.

[0022] In combination with the first aspect, the analysis function is:

[0023] SCC index =0.3X1+0.25X2+0.2X3+0.15X4+0.1X5,

[0024] Among them, X1 is the hole density, X2 is the grain boundary segregation, X3 is the gradient slope, X4 is the fractal dimension, and X5 is the oxygen-sulfur ratio.

[0025] Beneficial Effects: This disclosure provides a method for analyzing the causes of cracking in steam pipeline welds. This method quantifies material defects by combining staged gradient corrosion with image analysis technology. It uses layered peeling energy spectrum detection and fractal dimension calculation to distinguish the interaction between oxidative corrosion and transient cracking. It proposes a grain size matching index and a five-dimensional scoring system, dynamically coupling multiple parameters such as material structure differences, insufficient welding heat input, and abnormal composition gradients for analysis. This method achieves quantitative determination of failure modes through multi-dimensional data fusion, effectively addressing the high one-sidedness and high error rate of traditional methods. It can quickly locate cracking causes such as stress corrosion, significantly reducing unplanned downtime and economic losses caused by weld failure. It is suitable for systematic safety assessments of high-temperature and high-pressure steam pipelines. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The present invention is a flowchart of a method for analyzing the causes of cracking of a steam pipe weld according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] Exemplary embodiments are described in detail herein, with examples illustrated in the accompanying drawings. When the following description refers to the drawings, identical numerals in different drawings represent identical or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments are not intended to represent all embodiments consistent with the present disclosure.

[0028] The terms used in the embodiments of the present disclosure are for the purpose of describing specific embodiments only and are not intended to limit the embodiments of the present disclosure. The singular forms "a," "the," and "the" used in the embodiments of the present disclosure and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise. It should also be understood that the term "and / or" used herein refers to and includes any or all possible combinations of one or more associated listed items.

[0029] It should be understood that although the terms first, second, third, etc. may be used to describe various information in the embodiments of the present disclosure, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from each other. For example, without departing from the scope of the embodiments of the present disclosure, the first information may also be referred to as the second information, and similarly, the second information may also be referred to as the first information. Depending on the context, the word "if" as used herein may be interpreted as "at the time of" or "when" or "in response to determining".

[0030] like Figure 1 FIG. 1 is a flow chart of a method for analyzing the cause of cracking of a steam pipe weld according to an embodiment of the present disclosure, comprising:

[0031] S101: Perform phased gradient corrosion treatment on the elbow of the steam pipe to obtain its void density and grain boundary segregation;

[0032] For example, the elbow of the steam pipe was first etched with 4% nitric alcohol for 10 seconds to reveal the matrix outline, and then etched again with ferric chloride solution (5g FeCl:50ml HCl:100ml ethanol) for 30 seconds to preferentially dissolve small abnormal particles that are not resistant to corrosion;

[0033] The unit area (0.5 mm 2 ) Number of holes, optional, when unit area (0.5mm 2 ) When the number of holes is greater than 200 per field of view or the maximum hole diameter is greater than 5 μm, the material corrosion resistance is judged to be unqualified;

[0034] In the backscattering mode of the scanning electron microscope, grain boundary segregation is obtained. Optionally, when the Mo content fluctuation at the grain boundary is greater than 1.5 wt% or the Cr / Mo atomic ratio is greater than 8.5, it is determined that the grain boundary is susceptible to corrosion.

[0035] S102: Calculating the grain size matching index of the straight pipe and the elbow of the steam pipe, analyzing the crack propagation path of the steam pipe in combination with impact metallographic joint inspection, and obtaining the gradient slope of the steam pipe.

[0036] Grain size matching index (GMI) calculation:

[0037]

[0038] Where D is the grain size grade number, HV is the Vickers hardness, and for example, when GMI>0.35, it is determined that the difference in structure between the straight pipe and the elbow leads to interfacial stress concentration;

[0039] Optionally, the non-metallic inclusion rating (rated according to GB / T10561, Class A, B, C, and D inclusions ≤ Level 2) and grain size (Level 5-6, in line with GB5310-2023) can be linked to the GMI index. If Class A inclusions are ≥ Level 2 and GMI > 0.35, the weld cracking risk level is judged to be high.

[0040] Furthermore, impact metallographic inspection is performed. After the impact specimen is broken, it is immediately immersed in liquid nitrogen for cooling. A cross section 2 mm below the fracture is directly cut at -196°C for metallographic sample preparation. The relationship between the crack propagation path and the angle between the martensite lath bundles can be observed in situ. When the proportion of cracks with an angle of less than 30° is greater than 60%, it is determined that the welding heat input is insufficient.

[0041] The gradient slope of the steam pipe is obtained by energy spectrum line scanning, including the concentration gradient curve of Cr and Mo elements on both sides of the fusion line. When the gradient slope is greater than 15% / μm or a concentration "depression area" appears (the lowest value is less than 80% of the base material content), it is determined that the welding dilution rate is abnormal.

[0042] S103: performing a layered peeling energy spectrum test on the fracture of the steam pipe, and calculating the fractal dimension and oxygen-sulfur ratio of the crack path of the steam pipe.

[0043] For example, the layered peeling energy spectrum analysis method is used. The fracture surface is first ground in 0.5 mm increments, and each layer is scanned by EDS. When the rate of decrease of the O element content from the surface to the inside is less than 5% / layer or the S element suddenly increases by more than 0.8% in the third layer, it is determined that continuous oxidation corrosion exists.

[0044] The box counting method was used to calculate the fractal dimension D of the crack path in the electron microscope photograph. When the fractal dimension D>1.65, it was determined to be stress corrosion cracking (traditional fatigue cracking D<1.55).

[0045] S104: Inputting the void density, grain boundary segregation, gradient slope, fractal dimension, and oxygen-sulfur ratio into a pre-established analysis function to calculate the cause of the weld cracking of the steam pipeline.

[0046] The analysis function is:

[0047] SCC index =0.3X1+0.25X2+0.2X3+0.15X4+0.1X5,

[0048] Among them, X1 is the hole density, X2 is the grain boundary segregation, X3 is the gradient slope, X4 is the fractal dimension, and X5 is the oxygen-sulfur ratio.

[0049] Exemplary, where:

[0050] X1 (hole density): >200 / mm 2 5 points;

[0051] X2 (grain boundary segregation): Cr / Mo>8.5, 4 points;

[0052] X3 (gradient slope): >15% / μm, 3 points;

[0053] X4 (fractal dimension): D>1.65, 2 points;

[0054] X5 (oxygen-sulfur ratio): O / S>50, 1 point.

[0055] If SCC index ≥3.5, it is determined to be stress corrosion dominated failure.

[0056] Furthermore, the staged gradient corrosion treatment includes:

[0057] Pre-corroding the elbow with a 4% nitric acid alcohol solution for a first preset time to reveal a matrix tissue outline;

[0058] corroding the elbow using a ferric chloride solution for a second preset time;

[0059] The cavity density of the elbow is counted.

[0060] The phased gradient corrosion treatment reveals the microscopic defects of the material through a step-by-step chemical corrosion method. First, the elbow is pre-etched for a short time (10 seconds) using a 4% nitric acid alcohol solution. Its low-concentration acidic environment can gently remove the surface oxide layer and clearly show the macroscopic outline of the matrix tempered martensite structure; then a secondary corrosion is carried out using a ferric chloride mixed solution (5g FeCl3:50ml HCl:100ml ethanol). The Cl in the solution is - Ions preferentially erode carbides or untempered martensite at the grain boundaries, causing tiny abnormal particles in the material (such as undissolved carbides, non-metallic inclusions, etc.) to form visible holes. The number of holes per unit area is counted using image analysis software. When the hole density exceeds 200 / mm 2When the number of micro defects increases, it indicates that there are significant micro-defects inside the material. Such defects can easily become the source of crack initiation under high temperature and high pressure environment, directly affecting the corrosion resistance and service life of the material.

[0061] Furthermore, the calculation formula of the grain size matching index is:

[0062]

[0063] Where D is the grain size grade, HV is the Vickers hardness, and the subscripts pipe and elbow represent the corresponding parameters of straight pipes and elbows, respectively.

[0064] The formula for calculating the Grain Size Matching Index (GMI) assesses interfacial stress risk by quantifying the microstructure differences between straight pipes and elbows. In the formula, D_pipe and D_elbow represent the grain size grades of the straight pipe section and elbow, respectively (rated according to the ASTM E112 standard), while HV_pipe and HV_elbow represent the Vickers hardness measurements of the corresponding locations. This formula reflects the effect of microstructure coarsening on mechanical properties through the grain size difference (D_pipe - D_elbow), while the hardness ratio (HV_elbow / HV_pipe) is used to correct for deviations caused by work hardening or softening. When GMI > 0.35, it indicates a significant mismatch in grain size and hardness between the straight pipe and elbow. This mismatch, under high-temperature cyclic stress, can lead to an increased stress concentration factor at the weld interface, accelerating crack initiation and propagation.

[0065] Furthermore, the fractal dimension calculation uses a box counting method to analyze the crack path in the electron microscope image, and when the fractal dimension D is greater than 1.65, it is determined to be stress corrosion cracking.

[0066] The fractal dimension calculation uses the box counting method to quantitatively characterize the crack path in the fracture electron microscope image. This method divides the crack morphology image into grids (boxes) of different sizes. The slope of the logarithmic relationship between the number of grids required to cover the crack and the grid size is the fractal dimension D. Due to the synergistic effect of the corrosive medium and mechanical stress, the expansion path of stress corrosion cracks exhibits a highly irregular branching structure (such as intergranular or a mixed morphology of transgranular and intergranular), resulting in a fractal dimension D value significantly higher than that of conventional fatigue cracks (D < 1.55). When D > 1.65, it indicates that the crack path complexity has reached the typical threshold for stress corrosion cracking, which can serve as a key criterion for distinguishing stress corrosion from other failure modes.

[0067] Furthermore, the analysis function is:

[0068] SCC index =0.3X1+0.25X2+0.2X3+0.15X4+0.1X5,

[0069] Among them, X1 is the hole density, X2 is the grain boundary segregation, X3 is the gradient slope, X4 is the fractal dimension, and X5 is the oxygen-sulfur ratio.

[0070] The five-dimensional scoring system (SCC_index) integrates multi-dimensional inspection data through a weighted algorithm to achieve a quantitative assessment of failure modes. In the formula, X1 (void density) reflects the density of material matrix defects, X2 (grain boundary Cr / Mo atomic ratio) characterizes the degree of grain boundary segregation, X3 (fusion line composition gradient slope) indicates abnormal weld dilution rate, X4 (fractal dimension) quantifies crack morphology, and X5 (oxygen-sulfur ratio) assesses environmental corrosiveness. The weights assigned to each parameter (0.3, 0.25, 0.2, 0.15, 0.1) are determined based on their contribution to stress corrosion susceptibility. For example, void density has the highest contribution because it is directly related to the corrosion resistance of the material itself. When SCC_index ≥ 3.5, it indicates that at least three core parameters (such as void density, grain boundary segregation, and fractal dimension) are simultaneously exceeded, which is consistent with the multi-factor coupling mechanism of stress corrosion cracking, thus avoiding misjudgment of a single indicator.

[0071] The above embodiments are only used to illustrate the technical solutions of the present disclosure, rather than to limit them. Although the present disclosure has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present disclosure, and should all be included in the scope of protection of the present disclosure.

Claims

1. A method for analyzing the causes of cracking of steam pipe welds, characterized in that: The following steps are involved: The elbows of steam pipes were subjected to phased gradient corrosion treatment to obtain their void density and grain boundary segregation; Calculating the grain size matching index of the straight pipe and the elbow of the steam pipeline, analyzing the crack propagation path of the steam pipeline in combination with impact metallographic inspection, and obtaining the gradient slope of the steam pipeline; Performing layered peeling energy spectrum detection on the fracture of the steam pipe, and calculating the fractal dimension and oxygen-sulfur ratio of the crack path of the steam pipe; The void density, grain boundary segregation, gradient slope, fractal dimension and oxygen-sulfur ratio are input into a pre-established analysis function to calculate the cause of the weld cracking of the steam pipeline.

2. The method according to claim 1, characterized in that The staged gradient corrosion process includes: Pre-corroding the elbow with a 4% nitric acid alcohol solution for a first preset time to reveal a matrix tissue outline; corroding the elbow using a ferric chloride solution for a second preset time; The cavity density of the elbow is counted.

3. The method according to claim 1, characterized in that The calculation formula of the grain size matching index is: Where D is the grain size grade, HV is the Vickers hardness, and the subscripts pipe and elbow represent the corresponding parameters of straight pipes and elbows, respectively.

4. The method according to claim 1, wherein The fractal dimension calculation adopts the box counting method to analyze the crack path in the electron microscope image, and when the fractal dimension D is greater than 1.65, it is determined to be stress corrosion cracking.

5. The method according to claim 1, wherein The analysis function is: SCC index =0.3X1+0.25X2+0.2X3+0.15X4+0.1X5, Among them, X1 is the hole density, X2 is the grain boundary segregation, X3 is the gradient slope, X4 is the fractal dimension, and X5 is the oxygen-sulfur ratio.

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