Quantitative evaluation method for aging of S30432 austenitic stainless steel pipe

Abstract

The invention provides a quantitative evaluation method for aging of an S30432 austenitic stainless steel pipe. The quantitative evaluation method comprises the following steps: acquiring a metallographic structure morphology graph and percentage elongation after fracture of the S30432 austenitic stainless steel pipe to be evaluated; determining the aging level of the S30432 austenitic stainless steel pipe according to the obtained metallographic structure morphology graph and the percentage elongation after fracture; the metallographic structure morphology graph comprises a light corrosion morphology graph and a heavy corrosion morphology graph. The method provided by the invention can reliably and accurately evaluate the aging degree of the 30432 austenitic stainless steel pipe, and can be used for guiding a metal inspector to accurately evaluate the aging grade of the S30432 steel pipe after high-temperature service so as to assist a power plant to do next metal supervision work, so that a pipe explosion non-stop event caused by too serious material aging is avoided, and the service life of the 30432 austenitic stainless steel pipe is prolonged. The safety and reliability of the generator set are improved.

Description

Technical Field

[0001] The invention belongs to the technical field of austenitic stainless steel aging assessment, and in particular relates to a quantitative assessment method for aging of a S30432 austenitic stainless steel pipe. Background Art

[0002] The working environment of the heating surface tubes of the boiler of thermal power units is harsh. Under the long-term action of high-temperature flue gas and high-pressure steam, material degradation is inevitable, resulting in a continuous decline in safety of use. In this regard, the power generation field often judges the degree of material degradation by analyzing the aging state of the steel pipe organization, providing an important basis for formulating reasonable supervision measures.

[0003] S30432 austenitic stainless steel pipe is the main material used for high-temperature heating surfaces such as superheaters and reheaters of domestic ultra-supercritical units. Despite its excellent overall performance, as the service life of the units increases, more and more S30432 steel pipes begin to show significant aging. Although there are general aging rating standards DL / T884-2019 "Technical Guidelines for Metallographic Inspection and Assessment of Thermal Power Plants" and special aging rating standards for austenitic steel DL / T1422-2015 "18Cr-8Ni Series Austenitic Stainless Steel Boiler Tube Microstructure Aging Rating Standard", they are only applicable to aging grade assessment of Chinese grades such as 07Cr19Ni10, 07Cr19Ni11Ti, 07Cr18Ni11Nb, 08Cr18Ni11NbFG and foreign grades that are the same or similar to these steels. For new austenitic stainless steel grades such as S30432 (domestic corresponding grade 10Cr18Ni9NbCu3BN), the comparison charts and aging feature descriptions provided by this standard cannot correspond to the actual microstructure of the S30432 steel pipe, and it is difficult to use it as an accurate basis for aging rating.

[0004] Due to the lack of effective targeted aging rating methods in the industry, different inspectors can only rely on personal experience or understanding when rating the aging of S30432 austenitic stainless steel pipes, resulting in deviations in the assessed aging grades. In severe cases, the material degradation degree of the S30432 heating surface pipes may be misjudged, and incorrect supervision and management measures may be taken, resulting in aging pipe burst accidents, threatening the long-term reliable operation of the unit. Therefore, it is urgent to develop an aging rating method for S30432 austenitic stainless steel pipes to accurately assess their aging degree and provide a reliable basis for their technical supervision and management. Summary of the invention

[0005] The object of the present invention is to provide a quantitative assessment method for the aging of S30432 austenitic stainless steel pipes. The quantitative assessment method is a simple, accurate and effective aging rating method for S30432 steel pipes based on material plasticity indicators and organizational characteristics. The method is used to guide metal inspectors to accurately assess the aging level of S30432 steel pipes after high-temperature service, so as to assist power plants in the next step of metal supervision, avoid pipe bursts and non-stop events due to excessive material aging, and improve the safety and reliability of generator sets.

[0006] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0007] The present invention provides a quantitative evaluation method for aging of S30432 austenitic stainless steel pipe, the quantitative evaluation method comprising:

[0008] Obtain the metallographic structure morphology and elongation after fracture of the S30432 austenitic stainless steel pipe to be evaluated;

[0009] Determine the aging level of the S30432 austenitic stainless steel pipe according to the obtained metallographic structure morphology and elongation after fracture;

[0010] The metallographic structure morphology diagram includes a light corrosion morphology diagram and a heavy corrosion morphology diagram.

[0011] It is worth noting that the present invention selects a variety of ultra-supercritical boilers with current mainstream structural forms, cuts a large number of S30432 steel pipes at different locations to conduct laboratory microstructure characteristics and room temperature tensile properties research, and analyzes the corresponding relationship between the two, and finds that with the increase of aging degree, the room temperature tensile strength and yield strength of the S30432 steel pipe do not show a significant decrease, and always meet the standard requirements, while the plasticity index elongation after fracture maintains a good linear relationship with the aging degree. Therefore, the quantitative evaluation method of the present invention combines the two results of metallographic structure morphology and elongation after fracture to judge the aging degree of austenitic stainless steel pipes, avoiding the interference caused by different corrosive agent ratios or corrosion degrees, and can simply, accurately and effectively know the aging degree of austenitic stainless steel, avoiding the occurrence of pipe burst and non-stop events due to excessive material aging, and improving the safety and reliability of the generator set.

[0012] In addition, the method provided by the present invention clarifies the dividing lines of different aging degrees of S30432 steel pipes, and the descriptions of key organizational characteristics corresponding to aging levels 1 to 5 are obviously different. At the same time, a comparative graph is given for intuitive display, and the plasticity index elongation after fracture is used as a supporting basis, which is convenient for metal detection personnel to conduct aging assessment of S30432 steel pipes with unified standards, filling the gap in the industry.

[0013] As a preferred technical solution of the present invention, the elongation after fracture is measured using GBT 228.1-2021 "Tensile test of metallic materials Part 1: Room temperature test method".

[0014] As a preferred technical solution of the present invention, the method for obtaining the metallographic structure morphology image comprises the following steps:

[0015] The S30432 austenitic stainless steel tube to be evaluated was polished, cleaned and corroded in turn, and then observed using a metallographic microscope to obtain a metallographic structure morphology diagram.

[0016] Preferably, the etching treatment includes a light etching treatment and a heavy etching treatment.

[0017] It is worth noting that, in the present invention, a light corrosion morphology image is obtained through a light corrosion treatment, and a heavy corrosion morphology image is obtained through a heavy corrosion treatment.

[0018] Preferably, the polishing treatment comprises: polishing the S30432 austenitic stainless steel pipe to be evaluated with 120#, 240#, 600#, 1000#, 1500# and 2000# sandpaper in sequence, and then mechanically polishing with 2.5 μm diamond grinding paste.

[0019] Preferably, the cleaning process comprises cleaning the surface with an alcohol cotton ball.

[0020] Preferably, the corrosion treatment time is 5 to 30 seconds, for example, it can be 5 seconds, 8 seconds, 11 seconds, 14 seconds, 17 seconds, 20 seconds, 23 seconds, 27 seconds or 30 seconds, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 10 to 20 seconds.

[0021] Preferably, the metallographic etching agent used in the corrosion treatment includes ferric chloride, analytical grade hydrochloric acid and a solvent.

[0022] As a preferred technical solution of the present invention, the solvent used in the light etching treatment includes an alcohol aqueous solution.

[0023] Preferably, the concentration of the alcohol aqueous solution is 40-60wt%, for example, it can be 40wt%, 44wt%, 48wt%, 52wt%, 56wt% or 60wt%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably it is 50wt%.

[0024] Preferably, the solid-to-liquid ratio of ferric chloride, analytical pure hydrochloric acid and alcohol aqueous solution used in the light corrosion treatment is 1g:10mL:70-100mL, for example, it can be 1g:10mL:70mL, 1g:10mL:75mL, 1g:10mL:80mL, 1g:10mL:85mL, 1g:10mL:90mL, 1g:10mL:95mL or 1g:10mL:100mL, but is not limited to the listed values, and other values ​​not listed within the numerical range are equally applicable.

[0025] Preferably, the solvent used in the heavy etching treatment comprises analytical grade alcohol.

[0026] Preferably, the solid-to-liquid ratio of ferric chloride, analytically pure hydrochloric acid and analytically pure alcohol used in the heavy corrosion treatment is 5g:50mL:50-60mL, for example, it can be 1g:10mL:50mL, 1g:10mL:52mL, 1g:10mL:54mL, 1g:10mL:56mL, 1g:10mL:58mL or 1g:10mL:60mL, but is not limited to the listed values, and other values ​​not listed within the numerical range are equally applicable.

[0027] As a preferred technical solution of the present invention, the aging level of the S30432 austenitic stainless steel pipe includes five levels, the first level is the unaged level, the second level is the slightly aged level, the third level is the moderately aged level, the fourth level is the severely aged level, and the fifth level is the fully aged level.

[0028] As a preferred technical solution of the present invention, when the elongation after fracture A of the S30432 austenitic stainless steel is ≥45.0%, it is evaluated as an unaged grade; the elongation after fracture A ≥45.0%, for example, can be 45.0%, 46.0%, 47.0%, 48.0%, 49.0% or 50.0%, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0029] Preferably, when the light corrosion morphology cannot be displayed; and the twin boundaries in the heavy corrosion morphology are complete and clear, and a small amount of point-like NbC precipitation phases are dispersed in the S30432 austenite grains and grain boundaries, it is evaluated as a non-aging level.

[0030] Preferably, the average size of the point-like NbC precipitates is less than 0.2 μm, for example, 0.19 μm, 0.18 μm, 0.17 μm, 0.16 μm or 0.15 μm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0031] It is worth noting that when the unaged S30432 austenitic stainless steel pipe is subjected to light corrosion treatment, the metallographic structure cannot be displayed.

[0032] As a preferred technical solution of the present invention, when the elongation A of the S30432 austenitic stainless steel is 40.0-45.0% and not 45.0%, it is assessed as a mild aging level; the elongation A is 40.0-45.0%, for example, it can be 40.0%, 41.0%, 42.0%, 43.0%, 44.0%, 44.5% or 44.9%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0033] Preferably, when the austenite grain boundaries in the light corrosion morphology are clear and continuous, and there is no obvious precipitation phase in the grain; and the twin grain boundaries in the heavy corrosion morphology are clear but the number is reduced, the number of point-like NbC precipitation phases in the grain increases, and point-like M precipitation phases are intermittently precipitated at the grain boundaries. 23 When C6 precipitates, it is rated as a light aging level.

[0034] Preferably, the average size of the point-like NbC precipitates is less than 0.2 μm, for example, 0.19 μm, 0.18 μm, 0.17 μm, 0.16 μm, 0.15 μm or 0.14 μm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0035] Preferably, the dot M 23 The average size of the C6 precipitated phase is less than 0.4 μm, for example, it can be 0.38 μm, 0.35 μm, 0.3 μm, 0.25 μm, 0.2 μm or 0.15 μm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0036] As a preferred technical solution of the present invention, when the elongation A of the S30432 austenitic stainless steel is 35.0-40.0% and not 40.0%, it is evaluated as a moderate aging level; the elongation A is 35.0-40.0%, for example, it can be 35.0%, 36.0%, 37.0%, 38.0%, 39.0%, 39.5% or 39.9%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0037] Preferably, when the austenite grain boundary in the light corrosion morphology begins to melt, a complete and clear morphology is still retained; and the twin boundary in the heavy corrosion morphology is still clear, the austenite grain boundary is obviously coarsened, and the granular M 23 When the C6 precipitation phase is distributed in a chain at the grain boundary, the point-like carbide precipitation phase in the grain grows, and a small part develops into a granular carbide precipitation phase, it is assessed as a moderate aging level.

[0038] Preferably, the granular M 23The average size of the C6 precipitated phase is ≥0.4 μm, for example, it can be 0.4 μm, 0.42 μm, 0.44 μm, 0.46 μm, 0.48 μm, 0.50 μm or 0.52 μm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0039] Preferably, the average size of the granular carbide precipitation phase is 0.2-0.5 μm, for example, it can be 0.2 μm, 0.23 μm, 0.26 μm, 0.29 μm, 0.32 μm, 0.35 μm, 0.38 μm, 0.41 μm, 0.44 μm, 0.47 μm or 0.5 μm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0040] As a preferred technical solution of the present invention, when the elongation A of the S30432 austenitic stainless steel is 29.0-35.0% and not 35.0%, it is assessed as a severe aging level; the elongation A is 29.0-35.0%, for example, it can be 29.0%, 30.0%, 31.0%, 32.0%, 33.0%, 34.0%, 34.5% or 34.9%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0041] Preferably, when the austenite grain boundaries in the light corrosion morphology are further ablated, the local morphology has disappeared; and the twin boundaries in the heavy corrosion morphology are mostly dispersed, and some of them keep their original shape, the austenite grain boundaries are further coarsened, and the granular M 23 The C6 precipitate phase is distributed in a chain at the grain boundary, and a small amount of blocky σ phase precipitates at the grain boundary. When the granular carbide precipitate phase increases, it is assessed as a severe aging level.

[0042] Preferably, the granular M 23 The average size of the C6 precipitated phase is ≥1.0 μm, for example, it can be 1.0 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm or 2.0 μm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0043] Preferably, the average size of the blocky σ phase is less than 4 μm, for example, it may be 3.8 μm, 3.6 μm, 3.4 μm, 3.2 μm, 3.0 μm, 2.8 μm or 2.6 μm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0044] Preferably, the average size of the particulate carbide precipitation phase is ≥0.5μm, for example, it can be 0.5μm, 0.52μm, 0.54μm, 0.56μm, 0.58μm, 0.6μm or 0.62μm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0045] As a preferred technical solution of the present invention, when the elongation after fracture A of the S30432 austenitic stainless steel is less than 29.0%, it is evaluated as a fully aged grade, and the elongation after fracture A less than 29.0%, for example, can be 28.5%, 28%, 27.5%, 27%, 26.5%, 26%, 25.5% or 25%, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0046] Preferably, when only a small amount of traces of austenite grain boundaries remain in the light corrosion morphology, and the twin boundaries disappear in the heavy corrosion morphology, there are a large number of granular M 23 When C6 precipitation phase and a large amount of blocky σ phase exist, it is rated as complete aging level.

[0047] Preferably, the granular M 23 The average size of the C6 precipitated phase is ≥1.0 μm, for example, it can be 1.0 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2.0 μm or 2.2 μm, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0048] Preferably, the average size of the blocky σ phase is ≥4 μm, for example, it may be 4 μm, 4.2 μm, 4.4 μm, 4.8 μm, 5.0 μm, 5.2 μm, 5.4 μm or 5.6 μm, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0049] As a preferred technical solution of the present invention, when the S30432 austenitic stainless steel is used as the heating surface tube, the aging level at a depth of 0.5 mm on the smoke-facing side is taken as the final evaluation result.

[0050] Preferably, when the metallographic structure morphology is partially uneven, the aging level of the same aging level structure occupying more than 80% of the total observation area is taken as the final evaluation result.

[0051] It is worth noting that when the metallographic structure of the S30432 austenitic stainless steel pipe is between two levels, it is allowed to use half-level representation. For example, when the metallographic structure of the S30432 austenitic stainless steel pipe is between level 1 and level 2, it can be rated as level 1.5.

[0052] In addition, when the aging grade of the S30432 austenitic stainless steel pipe is rated as 1 to 3, the elongation after fracture A of the S30432 austenitic stainless steel pipe is ≥35% (in line with the standard requirements), and it can continue to be used safely; when the aging grade of the S30432 austenitic stainless steel pipe is rated as 4, it is necessary to conduct fixed-point tracking inspections on the S30432 austenitic stainless steel pipe in a cycle of 3 years to continuously understand its aging status; the remaining life of the cuttable pipe between levels 4 and 5 (4.5 levels) is evaluated; when the aging grade of the S30432 austenitic stainless steel pipe is rated as 5, the elongation after fracture of the S30432 austenitic stainless steel pipe is significantly lower than the standard requirements, the organizational characteristics are close to the burst state, and the safety is difficult to guarantee. It is recommended to replace it as soon as possible.

[0053] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] (1) The quantitative assessment method provided by the present invention clarifies the dividing line of different aging degrees of S30432 steel pipes. The key characteristics of the organization corresponding to aging levels 1 to 5 are obviously different. At the same time, a comparative map is given for intuitive display, and the plasticity index elongation after fracture is used as a supporting basis, which is convenient for metal detection personnel to uniformly standardize the aging assessment of S30432 steel pipes, filling the gap in the industry;

[0056] (2) After the test personnel have graded the aging of the S30432 steel pipe according to the quantitative assessment method provided by the present invention, they can obtain the degree of performance degradation without cutting the pipe to measure its tensile properties. Through the provided plasticity index and suggestions, they can give reasonable supervision measures to the unit to avoid pipe failure due to bursting or premature replacement, which wastes the potential of steel pipe use;

[0057] (3) In the quantitative assessment method provided by the present invention, the light corrosion morphology diagram focuses on displaying the grain boundary characteristics, while the heavy corrosion morphology diagram can better display the intracrystalline characteristics. The inspector can choose according to the situation, or combine the two to conduct aging grade assessment to avoid interference caused by different corrosive agent ratios or corrosion degrees. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] FIG1( a ) is a heavily corroded morphology of a 1# steel pipe (unaged grade) provided in Example 1, at a magnification of 500 times;

[0059] FIG1( b ) is a heavily corroded morphology of the 1# steel pipe (unaged grade) provided in Example 1, at a magnification of 1000 times;

[0060] FIG2( a ) is a light corrosion morphology image of the 2# steel pipe (mild aging level) provided in Example 1, with a magnification of 500 times;

[0061] FIG2( b ) is a light corrosion morphology image of the 2# steel pipe (mild aging level) provided in Example 1, with a magnification of 1000 times;

[0062] FIG2( c ) is a heavily corroded morphology of the 2# steel pipe (mild aging level) provided in Example 1, at a magnification of 500 times;

[0063] FIG2( d ) is a severely corroded morphology of the 2# steel pipe (mild aging level) provided in Example 1, at a magnification of 1000 times;

[0064] FIG3( a ) is a light corrosion morphology image of the 3# steel pipe (moderate aging level) provided in Example 1, with a magnification of 500 times;

[0065] FIG3( b ) is a light corrosion morphology image of the 3# steel pipe (moderate aging level) provided in Example 1, with a magnification of 1000 times;

[0066] FIG3( c ) is a severely corroded morphology of the 3# steel pipe (moderate aging level) provided in Example 1, at a magnification of 500 times;

[0067] FIG3( d ) is a severely corroded morphology of the 3# steel pipe (moderate aging level) provided in Example 1, at a magnification of 1000 times;

[0068] FIG4( a ) is a light corrosion morphology image of the 4# steel pipe (severe aging level) provided in Example 1, with a magnification of 500 times;

[0069] FIG4( b ) is a light corrosion morphology image of the 4# steel pipe (severe aging level) provided in Example 1, with a magnification of 1000 times;

[0070] FIG4( c ) is a severely corroded morphology of the 4# steel pipe (severely aged grade) provided in Example 1, at a magnification of 500 times;

[0071] FIG4( d ) is a severely corroded morphology of the 4# steel pipe (severely aged grade) provided in Example 1, with a magnification of 1000 times;

[0072] FIG5( a ) is a light corrosion morphology image of the 5# steel pipe (completely aged level) provided in Example 1, with a magnification of 500 times;

[0073] FIG5( b ) is a light corrosion morphology image of the 5# steel pipe (completely aged level) provided in Example 1, with a magnification of 1000 times;

[0074] FIG5( c ) is a heavily corroded morphology of the 5# steel pipe (completely aged level) provided in Example 1, at a magnification of 500 times;

[0075] FIG5( d ) is a heavily corroded morphology of the 5# steel pipe (completely aged level) provided in Example 1, at a magnification of 1000 times;

[0076] Figure 6 This is a heavily corroded morphology image of a new spare tube in stock for the final superheater provided in Example 2, with a magnification of 1000 times;

[0077] Figure 7 This is a light corrosion morphology image of the last stage superheater 21-17# tube provided in Example 2, with a magnification of 1000 times;

[0078] Figure 8 This is a light corrosion morphology image of the 7-12# tube of the final superheater provided in Example 2, with a magnification of 1000 times;

[0079] Fig. 9 This is a light corrosion morphology image of the 7-9# tubes of the final superheater provided in Example 2, with a magnification of 1000 times;

[0080] Fig.10 This is a light corrosion morphology image of the final superheater 21-9# tube provided in Example 2, with a magnification of 1000 times. DETAILED DESCRIPTION

[0081] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0082] Example 1

[0083] This embodiment provides a quantitative assessment method for aging of S30432 austenitic stainless steel pipe, the quantitative assessment method comprising:

[0084] (1) Provide spare S30432 austenitic stainless steel pipes, marked as 1#; and provide steel pipes at different positions in the final superheater of a power plant made of S30432 austenitic stainless steel, marked as 2# to 5#;

[0085] (2) GBT 228.1-2021 "Tensile test of metallic materials Part 1: Room temperature test method" was used to test the elongation after fracture of 1# to 5# steel pipes, and the elongation after fracture A of 1# steel pipe was 51.5%, the elongation after fracture A of 2# steel pipe was 42.5%, the elongation after fracture A of 3# steel pipe was 37.5%, the elongation after fracture A of 4# steel pipe was 32.5%, and the elongation after fracture A of 5# steel pipe was 27.5%;

[0086] (3) The 1# to 5# steel pipes were lightly corroded by using a metallographic etching agent of ferric chloride with a solid-liquid ratio of 1g:10mL:85mL, analytical pure hydrochloric acid and an alcohol aqueous solution with a concentration of 50wt%, and then observed by a metallographic microscope, and the light corrosion morphology images as shown in Figures 2(a), 2(b), 3(a), 3(b), 4(a), 4(b), 5(a) and 5(b) were obtained;

[0087] (4) The 1# to 5# steel pipes were subjected to heavy corrosion treatment respectively using a metallographic etching agent of ferric chloride, analytical pure hydrochloric acid and analytical pure alcohol with a solid-liquid ratio of 5g:50mL:50mL, and then observed using a metallographic microscope, and the heavy corrosion morphology images as shown in Figures 1(a), 1(b), 2(c), 2(d), 3(c), 3(d), 4(c), 4(d), 5(c) and 5(d) were obtained;

[0088] (5) Analyze the elongation after fracture obtained in step (2), the light corrosion morphology obtained in step (3), and the heavy corrosion morphology obtained in step (4) to evaluate the aging of 1# to 5# steel pipes:

[0089] 1# steel pipe: The elongation after fracture A of 1# steel pipe is 51.5%. According to Figures 1(a) and 1(b), the twin boundaries are complete and clear under heavy corrosion, and a small amount of point-like NbC precipitation phases with an average size of less than 0.2μm are dispersed in the austenite grains and grain boundaries. Therefore, 1# steel pipe is evaluated as unaged grade.

[0090] 2# steel pipe: The elongation after fracture of 2# steel pipe is 42.5%. According to Figure 2(a)-(d), under light corrosion, the austenite grain boundaries are clear and continuous, and there is no obvious precipitation phase in the grain; under heavy corrosion, the twin grain boundaries are clear but the number decreases, the number of point-like precipitation phases in the grain increases, and the average size remains at <0.2μm; the grain boundary intermittent precipitation of point-like M with an average size of <0.4μm 23 C6 precipitation phase, thus the 2# steel pipe is assessed as slightly aged;

[0091] 3# steel pipe: The elongation after fracture A of 3# steel pipe is 37.5%. According to Figure 3(a)-(d), under light corrosion, the austenite grain boundary begins to melt, but still retains a complete and clear morphology; under heavy corrosion, the twin boundary is still clear, and the austenite grain boundary is obviously coarsened; the granular M with an average size of ≥0.4μm 23 The C6 precipitate phase is distributed in a chain at the grain boundary, the point-like precipitate phase grows in the grain, and a small part develops into a spherical or granular shape, with an average size of 0.2μm≤<0.5μm, so the 3# steel pipe is evaluated as a moderate aging level;

[0092] 4# steel pipe: The elongation after fracture A of 4# steel pipe is 32.5%. According to Figure 4(a)-(d), under light corrosion, the austenite grain boundaries are further melted and the local morphology has disappeared; under heavy corrosion, the twin boundaries are mostly dispersed, and some of them keep their original shape, and the austenite grain boundaries are further coarsened; the granular M with an average size of ≥1.0μm 23 The C6 precipitate phase is distributed in a chain at the grain boundary, and a small amount of blocky σ phase with an average size of less than 4μm is precipitated at the grain boundary. The spherical or granular precipitate phase increases in the crystal, and the average size is ≥0.5μm. Therefore, the 4# steel pipe is evaluated as a severe aging level;

[0093] 5# steel pipe: The elongation after fracture A of 5# steel pipe is 27.5%. According to Figure 5(a)-(d), only a few traces of austenite grain boundaries remain under light corrosion; twin boundaries disappear under heavy corrosion; a large number of granular M with an average size of ≥1.0μm exist in the grains and grain boundaries. 23 C6 precipitation phase, and there are a lot of blocky σ phases with an average size of ≥4μm, so the 5# steel pipe is assessed as fully aged.

[0094] Example 2

[0095] This embodiment provides a quantitative assessment method for aging of S30432 austenitic stainless steel pipe, the quantitative assessment method comprising:

[0096] (1) Provide the spare S30432 austenitic stainless steel pipes in stock for the spare final superheater of a power plant; provide the 9th pipe from the left of the 21st row of the S30432 final superheater of a power plant that has been overheated and aged for a long time and burst, which is recorded as 21-9#; provide the partial pipes of the 21st row and the 7th row from the left of the power plant, which are recorded as 21-17#, 7-12#, and 7-9# respectively;

[0097] (2) The steel pipe provided in step (1) is tested for elongation after fracture by using GBT 228.1-2021 "Tensile test of metallic materials - Part 1: Test method at room temperature", and the elongation after fracture A of the spare S30432 austenitic stainless steel pipe is 47.5%, the elongation after fracture A of the 21-9# steel pipe is 21.5%, the elongation after fracture A of the 21-17# steel pipe is 38.5%, the elongation after fracture A of the 7-12# steel pipe is 33.0%, and the elongation after fracture A of the 7-9# steel pipe is 27.0%;

[0098] (3) A metallographic etching agent of ferric chloride, analytical pure hydrochloric acid and 50 wt% alcohol aqueous solution with a solid-liquid ratio of 1 g:10 mL:85 mL was used to perform light corrosion treatment on the 21-17#, 7-12#, 7-9# and 21-9# steel pipes provided in step (1), and then a metallographic microscope was used to observe the results. Figure 7-Figure 10 Light corrosion morphology of

[0099] (4) The steel pipe provided in step (1) is subjected to heavy corrosion treatment using a metallographic etching agent of ferric chloride, analytically pure hydrochloric acid, and analytically pure alcohol in a solid-liquid ratio of 5 g:50 mL:50 mL, and then observed using a metallographic microscope to obtain the following: Figure 6 The heavy corrosion morphology of the spare S30432 austenitic stainless steel pipe shown;

[0100] (5) Analyze the elongation after fracture obtained in step (2), the light corrosion morphology obtained in step (3), and the heavy corrosion morphology obtained in step (4) to perform aging assessment on the steel pipe provided in step (1):

[0101] Spare S30432 austenitic stainless steel pipe: The elongation A of the steel pipe is 47.5%, and according to Figure 6 It can be seen that the twin boundaries are complete and clear under heavy corrosion, and a small amount of point-like NbC precipitation phases with an average size of less than 0.2μm are dispersed in the austenite grains and grain boundaries. Therefore, the steel pipe is evaluated as unaged grade;

[0102] 21-17# steel pipe: The elongation after fracture of the steel pipe is 38.5%, and according to Figure 7 It can be seen that under light corrosion, the austenite grain boundaries begin to melt, but still retain a complete and clear morphology, and its morphology is similar to Figure 3 (a) and Figure 3 (b). Therefore, the 21-17# steel pipe is evaluated as a moderate aging level;

[0103] 7-12# steel pipe: The elongation after fracture A of the steel pipe is 33.0%, and according to Figure 8 It can be seen that under light corrosion, the austenite grain boundaries are further melted, the local morphology has disappeared, and its morphology is similar to Figure 4 (a) and Figure 4 (b). Therefore, the 7-12# steel pipe is evaluated as a severe aging level;

[0104] 7-9# steel pipe: The elongation after fracture A of the steel pipe is 27.0%, and according to Fig. 9 It can be seen that: under light corrosion, only a small amount of traces remain on the austenite grain boundary; under heavy corrosion, the twin boundary disappears, and its morphology is similar to Figure 5(a) and Figure 5(b), so the 7-9# steel pipe is evaluated as a completely aged grade;

[0105] 21-9# steel pipe: The elongation after fracture A of the steel pipe is 21.5%; and according to Fig.10 It can be seen that: under light corrosion, only a small amount of traces are retained on the austenite grain boundary, and 200mm below the burst hole of the pipe, creep holes have already appeared in the σ phase in the material; therefore, the 21-9# steel pipe is assessed to be at the fully aged level.

[0106] Allowable stress test:

[0107] The 1# to 5# steel pipes provided in Example 1 were subjected to an allowable stress test, with a safety factor of 1.5. The results are shown in Table 1:

[0108] Among them, the allowable stress test is calculated based on the 100,000-hour endurance strength at 650°C and a safety factor of 1.5; the 100,000-hour endurance strength is obtained by extrapolating the 5,000-8,000-hour endurance strength data using the isotherm method.

[0109] Table 1

[0110]

[0111] According to the data in Table 1:

[0112] The evaluation results obtained by the method provided by the present invention are consistent with the performance of ASMEⅡ Material D (i.e., the performance of ASME Boiler and Pressure Vessel Code International Specification Ⅱ Material D) and the requirement of DL / T438-2023 standard 9.3.19 that "the material evaluation and life assessment should be carried out for the 18Cr-8Ni series austenitic heat-resistant steel pipe when the organizational aging reaches level 4". This further proves the reliability, accuracy and stability of the quantitative evaluation method for aging of S30432 austenitic stainless steel pipe provided by the present invention.

[0113] In summary, the method provided by the present invention can reliably and accurately assess the aging degree of 30432 austenitic stainless steel pipes, and can be used to guide metal inspectors to accurately assess the aging level of S30432 steel pipes after high-temperature service, so as to assist power plants in the next step of metal supervision, avoid pipe bursts and non-stop incidents due to severe material aging, and improve the safety and reliability of generator sets.

[0114] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A quantitative assessment method for aging of S30432 austenitic stainless steel pipe, characterized in that: The quantitative assessment method includes: Obtain the metallographic structure morphology and elongation after fracture of the S30432 austenitic stainless steel pipe to be evaluated; Determine the aging level of the S30432 austenitic stainless steel pipe according to the obtained metallographic structure morphology and elongation after fracture; The metallographic structure morphology diagram includes a light corrosion morphology diagram and a heavy corrosion morphology diagram.

2. The quantitative evaluation method according to claim 1, characterized in that: The method for obtaining the metallographic structure morphology image comprises the following steps: The S30432 austenitic stainless steel pipe to be evaluated was polished, cleaned and corroded in turn, and then observed using a metallographic microscope to obtain a metallographic structure morphology diagram; Preferably, the corrosion treatment includes light corrosion treatment and heavy corrosion treatment; Preferably, the corrosion treatment time is 5 to 30 seconds, preferably 10 to 20 seconds; Preferably, the metallographic etching agent used in the corrosion treatment includes ferric chloride, analytical grade hydrochloric acid and a solvent.

3. The quantitative evaluation method according to claim 2, characterized in that: The solvent used in the light etching treatment includes an alcohol aqueous solution; Preferably, the concentration of the alcohol aqueous solution is 40-60wt%, preferably 50wt%; Preferably, the solid-to-liquid ratio of ferric chloride, analytically pure hydrochloric acid and alcohol aqueous solution used in the light corrosion treatment is 1 g:10 mL:70-100 mL; Preferably, the solvent used in the heavy corrosion treatment includes analytical grade alcohol; Preferably, the solid-to-liquid ratio of ferric chloride, analytically pure hydrochloric acid and analytically pure alcohol used in the heavy corrosion treatment is 5g:50mL:50-60mL.

4. The quantitative evaluation method according to any one of claims 1 to 3, characterized in that: The aging level of the S30432 austenitic stainless steel pipe includes five levels, the first level is the non-aging level, the second level is the light aging level, the third level is the moderate aging level, the fourth level is the heavy aging level, and the fifth level is the complete aging level.

5. The quantitative evaluation method according to claim 4, characterized in that: When the elongation after fracture A of the S30432 austenitic stainless steel is ≥45.0%, it is evaluated as the non-aging grade; Preferably, when the light corrosion morphology cannot be displayed; and the twin boundaries in the heavy corrosion morphology are complete and clear, and a small amount of point-like NbC precipitation phases are dispersed in the S30432 austenite grains and grain boundaries, it is evaluated as the non-aging level; Preferably, the average size of the dot-like NbC precipitated phase is less than 0.2 μm.

6. The quantitative evaluation method according to claim 4, characterized in that: When the elongation after fracture A of the S30432 austenitic stainless steel is 40.0-45.0% and not 45.0%, it is evaluated as a light aging grade; Preferably, when the austenite grain boundaries in the light corrosion morphology are clear and continuous, and there is no obvious precipitation phase in the grain; and the twin grain boundaries in the heavy corrosion morphology are clear but the number is reduced, the number of point-like NbC precipitation phases in the grain increases, and point-like M precipitation phases are intermittently precipitated at the grain boundaries. 23 When C6 precipitates, it is rated as a light aging level; Preferably, the average size of the dot-like NbC precipitates is less than 0.2 μm; Preferably, the dot M 23 The average size of the C6 precipitated phase is less than 0.4 μm.

7. The quantitative evaluation method according to claim 4, characterized in that: When the elongation after fracture A of the S30432 austenitic stainless steel is 35.0-40.0% and not 40.0%, it is evaluated as a moderate aging level; Preferably, when the austenite grain boundary in the light corrosion morphology begins to melt, a complete and clear morphology is still retained; and the twin boundary in the heavy corrosion morphology is still clear, the austenite grain boundary is obviously coarsened, and the granular M 23 When the C6 precipitate phase is distributed in a chain at the grain boundary, the point-like carbide precipitate phase in the grain grows, and a small part of it develops into a granular carbide precipitate phase, it is evaluated as a moderate aging level; Preferably, the granular M 23 The average size of C6 precipitate phase is ≥ 0.4 μm; Preferably, the average size of the granular carbide precipitation phase is 0.2-0.5 μm.

8. The quantitative evaluation method according to claim 4, characterized in that: When the elongation after fracture A of the S30432 austenitic stainless steel is 29.0-35.0% and is not 35.0%, it is evaluated as a severe aging level; Preferably, when the austenite grain boundaries in the light corrosion morphology are further ablated, the local morphology has disappeared; and the twin boundaries in the heavy corrosion morphology are mostly dispersed, and some of them keep their original shape, the austenite grain boundaries are further coarsened, and the granular M 23 The C6 precipitate phase is distributed in a chain at the grain boundary, and a small amount of blocky σ phase is precipitated at the grain boundary. When the granular carbide precipitate phase increases, it is rated as a severe aging level; Preferably, the granular M 23 The average size of C6 precipitate phase is ≥1.0μm; Preferably, the average size of the blocky σ phase is less than 4 μm; Preferably, the average size of the granular carbide precipitation phase is ≥0.5 μm.

9. The quantitative evaluation method according to claim 4, characterized in that: When the elongation after fracture A of the S30432 austenitic stainless steel is less than 29.0%, it is evaluated as a fully aged grade; Preferably, when only a small amount of traces of austenite grain boundaries remain in the light corrosion morphology, and the twin boundaries disappear in the heavy corrosion morphology, there are a large number of granular M 23 When C6 precipitates and a large amount of blocky σ phase exists, it is assessed as a complete aging level; Preferably, the granular M 23 The average size of C6 precipitate phase is ≥1.0μm; Preferably, the average size of the bulk σ phase is ≥ 4 μm.

10. The quantitative evaluation method according to any one of claims 1 to 9, characterized in that: When the S30432 austenitic stainless steel is used as the heating surface tube, the aging level at a depth of 0.5 mm on the smoke-facing side is taken as the final evaluation result; Preferably, when the metallographic structure morphology is partially uneven, the aging level of the same aging level structure occupying more than 80% of the total observation area is taken as the final evaluation result.

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