Metallographic inspection of chemical pressure pipelines

Guidelines for metallographic inspection 1. Purpose and scope of application 1.1 Purpose Starting from the construction of thermal power plants, we must control the quality of metal materials and ensure the safe operation of various pipes and components and their welded joints within the scope of metal technical supervision of thermal power plants. 1.2 Scope of application: Applicable to the metallographic inspection of high-temperature pipes and components, pressure-bearing pipes and components, and their welded joints in thermal power plant equipment in the power system. 2. Comply with relevant standards and regulations. 2.1 The scope and quantity of inspection of high-temperature pipes and components, pressure-bearing pipes and components, etc. should comply with the relevant provisions of the "Metal Supervision Regulations for Thermal Power Plants DL438-91". 2.2 The inspection scope and number of welded joints should comply with the relevant provisions of the "Code for Electric Power Construction and Acceptance (Welding of Thermal Power Plants) DL5007-92". 2.3 The inspection work must comply with the relevant provisions of the "Safety Work Procedures for Electric Power Construction (Heat Engine Installation)" SDJ162-82. 2.4 The inspection work must comply with the relevant regulations of (84 Electrical Engineering No. 146). 2.5 The determination of non-metallic inclusions can be carried out according to 2.6 The microstructure grain size can be assessed according to 2.7 The banded structure, Widmanstatten structure and free cementite can be determined with reference to 2.8 The decarburization layer can be determined with reference. 2.9 The spheroidization level standard can be evaluated by referring to the "12Cr1MoV steel spheroidization level standard (draft)" "15CrMo steel pearlite spheroidization reference level" and "20 carbon steel pearlite spheroidization reference level". 2.10 The graphitization standard of carbon steel can be evaluated with reference to 2.11 The qualification standards for macro and micro metallographic inspection of welded joints, which can be evaluated according to the relevant regulations of . 3. Requirements for metallographic inspection personnel 3.1 Metallographic inspection personnel include metallographic inspection technicians and metallographic inspection engineers. 3.2 Requirements for metallographic inspection technicians 3.2.1 According to the project situation, fully grasp the metallographic inspection work items and workload, formulate work plans and formulate technical measures. 3.2.2 Participate in and guide the metallographic inspection work in the project, and be responsible for the quality of the inspection. 3.2.3 Provide necessary theoretical guidance, training and assessment of practical operating techniques, and summarize and exchange new test methods for metallographic inspection workers to improve metallographic inspection analysis technology and skills. 3.2.4 Carry out necessary analysis and testing on the steel types encountered for the first time. 3.2.5 Compile and review test reports in a timely manner and do a good job in sorting and archiving. Requirements for metallographic inspectors 3.3.1 Only after passing the training and examination on professional theoretical knowledge and practical skills can inspectors undertake corresponding metallographic inspection work and prepare test reports. 3.3.2 Inspectors should understand the material, welding process and heat treatment of the inspected parts. 3.3.3 Inspect the inspected parts determined in the inspection work items. Ensure that the inspection is timely, the operation is correct, and the conclusion is accurate. 4. Responsibilities 4.1 Accept the metallographic inspection order, conduct the inspection, and issue an inspection report. 4.2 The construction office is responsible for entrusting the metallographic inspection and provides cooperation during the inspection process. 5. Main instruments and equipment: desktop microscope or large workpiece microscope; metallographic polishing machine; film enlarger. 6. Work procedures 6.1 Preparation before inspection 6.1.1 After receiving the order, learn about the inspection location and inspection conditions (power supply, shelf, etc.) from the entrusting party. 6.1.2 Understand the steel number, specifications, heat treatment status, welding parts and welding process of the inspected material, and clarify the purpose of the test. 6.1.3 For steel types encountered for the first time in the project, heat treatment process performance tests and metallographic structure evaluation and analysis can be carried out if necessary. 6.1.4 It is best to intercept inlaid samples from the straight sections of the main steam pipeline and high-temperature reheat steam pipeline. The sample number must correspond to the pipeline layout number during installation. 6.1.5 Welded joints that require post-weld heat treatment should be inspected after heat treatment. 6.1.6 Before the inspection, the test plan should be determined based on the purpose of the inspection, the materials to be inspected and the test conditions. Corresponding measures should be taken for some external factors that affect the test results to ensure the accuracy of the analysis. 6.2 Macroscopic inspection 6.2.1 Purpose: To inspect macroscopic defects of pipelines and components and their welded joints. 6.2.2 Methods 6.2.2.1 Observe the object being inspected with the naked eye or a magnifying glass less than 10 times (usually acid leaching test is used) 6.2.2.2 The acid leaching test can use hot acid etching or cold acid etching. 6.2.2.3 According to the purpose of inspection, determine the location, inspection surface and quantity of the sample to be intercepted. The sample of the welded joint should include the weld, heat affected zone and base metal. 6.2.2.4 The sample can be cut out by hand saw, grinding wheel cutting, mechanical processing or gas cutting. 6.2.2.5 The smoothness of the surface to be inspected should be based on the purpose and requirements of the inspection, and the corresponding mechanical processing method should be selected. If necessary, sandpaper of different particle sizes can be used to grind it step by step. Generally, it can be ground to No. 500 water sandpaper or No. 01 metallographic sandpaper. 6.2.2.6 Before acid leaching, the surface of the sample to be inspected must be free of scratches and local defects, and there should be no oil on the surface. If necessary, it can be cleaned with gasoline, alcohol and benzene. 6.2.2.7 According to the chemical composition of the inspected piece and the purpose of the inspection, determine the composition of the etching solution, the etching temperature and the etching time, so that defects can be accurately displayed. 6.2.2.8 The acid leaching solution should be stored in acid-resistant containers, and the acid leaching test should be conducted in a fume hood. 6.2.2.9 When the sample is put into the acid leaching solution, the surface to be inspected should not be in contact with the utensils or other samples, and should be completely immersed in the acid solution to ensure uniform etching. The etching solution can be used continuously, but its composition must ensure reliable test results. 6.2.2.10 After the acid leaching is completed, use acid-resistant iron pliers or acid-resistant rubber gloves (during cold immersion) to remove the sample from the etching solution, and immediately rinse it in flowing water (hot or cold water). At the same time, use a brush to brush off the corrosion products on the surface of the sample. But be careful not to stain, scratch, or touch the inspected surface with your hands. After brushing, you can also use 2% NAOH or 3-5% Na2CO3 solution to clean, and finally blow dry. 6.2.2.11 After acid leaching of the sample, if it is found that there are water stains or other contaminants on the surface due to unclean brushing, the acid leaching solution can be put in again for slight etching, and then brushed and rinsed again. ； If it is found that the etching is too shallow, the etching can be continued until it meets the requirements. If it is found that the etching is too deep, at least 1mm of the surface to be inspected must be removed from the sample, and then grinded and etched again according to the above operation method. 6.2.2.12 After acid leaching, the sample should be inspected and photographed in time. If it needs to be stored temporarily, it should be placed in a desiccator, but the storage time should not be too long: if it needs to be stored for a long time, oil or transparent paint can be applied to the surface to prevent rust, and then stored in a desiccator. 6.2.2.13 The quality assessment of acid leaching specimens should be carried out in accordance with relevant standards and technical conditions. If defects such as folding, cracks, pores and incomplete welding are found during inspection, a simplified diagram or photographic record can be drawn. When taking pictures, the magnification should be noted and the size and location of the defects should be measured to facilitate the assessment of the quality of the inspected parts. 6.2.2.14 To ensure safety, operators must wear overalls or rubber aprons, glasses, masks, rubber gloves, etc. when working. 6.3 Microscopic inspection 6.3.1 Purpose: Analyze the microstructure morphology, distribution and grain size of metal materials, and judge and determine the quality of metal materials. 6.3.2 Method: On-site, a portable large workpiece microscope can be used for inspection or peritoneal technology; in the laboratory, an optical microscope or an electronic eye can be used to inspect intercepted samples. 6.3.3 Interception of specimens 6.3.3.1 The direction, location and number of sample interceptions should be determined according to the purpose of the inspection. They must be representative and characterize the characteristics of the material being inspected. 6.3.3.2 The sample should be intercepted to avoid changes in the sample structure due to heating or deformation of the metal material. For samples intercepted by gas cutting, all heat-affected zones must be removed. 6.3.3.3 The interception of welded joint specimens shall be carried out in accordance with the relevant regulations. 6.3.3.4 For forged and supercooled deformed materials, in order to identify the type and grade of non-metallic impurities and the grade of the band-like structure, longitudinal samples should generally be intercepted from the center of the surface; in order to identify the structural changes from the outer wall to the inner wall of the specimen, the distribution of inclusions on the cross section, the depth of the decarburized layer and surface quenching layer, etc., transverse samples should generally be intercepted. 6.3.3.5 For damaged parts, the interception of the sample should include both the main damaged parts and the undamaged parts for comparison. 6.3.3.6 To cut out the sample of the main steam and reheat steam straight pipe, small triangular pieces can be cut out with a hand saw before beveling the pipe end to be welded. The size can generally be about 1-1.5cm along the circumferential width of the pipe and the thickness of the pipe wall. 6.3.4 Inlay of the sample 6.3.4.1 The mounting method can be used when the sample size is too small or the edge needs to be checked. 6.3.4.2 The mounting of the sample can be done by mechanical mounting method, hot pressing mounting method, condensation mounting method, etc. 6.3.5 Grinding of the sample 6.3.5.1 Method: The sample can be ground manually or with an automatic sample grinder. 6.3.5.2 step 6.3.5.2 .1 Grinding: The cut sample is first ground smooth with a grinding wheel to prepare for the next sandpaper grinding. The sample must be cooled with water during grinding to prevent the metal structure from changing due to heat. 6.3.5.2 .2 Polishing 6.3.5.2 ..2.1 Manual polishing: After grinding with a grinding wheel, clean and blow-dry the sample, grind it by hand on various sandpapers from coarse to fine. The sandpaper must be laid flat on flat glass, metal or board. From coarse sandpaper to fine sandpaper, every time you change the sandpaper, the sample must be turned at a 90-degree angle to the old grinding marks. Vertically, grind in one direction until the old grinding marks completely disappear and the new grinding marks are uniform. At the same time, wash the sample with water or ultrasonic waves. Your hands should also be washed at the same time to avoid bringing the roving grains to the fine gauze paper. When grinding the sample, be careful not to use too much force or too long each time. 6.3.5.2 .2.2 Mechanical grinding and polishing: Place sandpaper of different numbers from coarse to fine on a mechanical grinding machine and grind them in sequence. 6.3.5.2 .3 Polishing: Throw away the wear marks on the sample to achieve a mirror surface without grinding defects. The polishing method can be mechanical polishing, chemical polishing, electrolytic polishing, etc. 6.3.3.2 .3.1 Mechanical polishing can be divided into two steps: rough polishing and fine polishing. For rough polishing, the sample polished with sandpaper can be moved to a polishing machine equipped with nylon, woolen, etc. The polishing material can be fine particles of alumina, emery, etc. The polishing time is 2-5 minutes. After polishing, wash it with water and blow dry. For fine polishing, the sample that has been polished by sandpaper can be moved to a polishing machine equipped with nylon silk, velvet, etc. When performing fine polishing on the disk, the force should be light. Polishing must be done from the center to the edge of the disk, and a small amount of grinding powder suspension should be added from time to time. At the completion stage of polishing, the sample can be polished in the opposite direction of the rotation direction of the polishing disk. Generally, the sample is polished until the wear marks of the sample are completely removed and the surface looks like a mirror. After polishing, wash with water and blow dry so that there will be no water marks or dirt residue on the surface. 6.3.3.2 .3.2 Electrolytic polishing Electrolytic polishing is based on the principle of anodic dissolution. The sample is the anode, and stainless steel or other materials are the cathode. The conditions of electrolytic polishing are determined by voltage, current, temperature and polishing time. 6.3.3.2 .3.3 Chemical polishing relies on the uneven dissolution of chemical reagents on the surface of the sample to gradually obtain a bright surface. However, it can only make the surface of the sample smooth and cannot meet the requirements for a flat surface. It has a good polishing effect on pure metals. 6.3.5 Sample etching The ground sample can be immersed in the etching agent in the glassware, or can be gently wiped with a cotton ball dipped in the etching agent, or the surface of the sample can be polished with a dropper. The etching time depends on the nature of the metal material, the concentration and temperature of the etching solution, the purpose of the inspection and the magnification of the microscopic inspection, so that it can be clearly displayed under the microscope. The details of the metal structure are appropriate. It is not advisable to use excessive etching to increase the degree of tissue. When observed at high magnification, the degree of etching is slightly lighter than when observed at low magnification. After etching is completed, take it out immediately and quickly clean it with water. Then clean it with absolute alcohol and blow dry with hot air. The prepared sample should have no wear marks, pits, pits, water marks, etc., and the structure should be clear and representative. 6.3.6 Microscopic examination of specimens. Microscopic inspection includes inspection before etching and inspection after etching. Before etching, the sample is mainly inspected for cracks, non-metallic inclusions and defects caused during the sample preparation process. ； After etching, the microstructure of the sample is mainly examined. 6.3.6.1 Microscopic inspection of pipes and components: 6.3.6.1 .1 Identify the types of non-metallic inclusions and microcracks in materials, observe their shape and distribution, and measure their number and size. 6.3.6.1 .2 Identify the composition of the microstructure of the inspected piece, the morphology, distribution and quantity of various structures. Evaluate the grain size, banded structure, non-metallic inclusions, Widmanstatten structure, spheroidized structure, decarburization layer, etc. 6.3.6.1 .3 Identify the structural characteristics and determine the status of the heat treatment process. 6.3.7 Microscopic inspection of welded joints 6.3.7.1 Analysis of weld structure 6.3.7.1 .1 The weld structure has the characteristics of continuous crystallization and growth. Common weld structures include columnar crystals, dendrites, and isopericrystals. Welds with the same composition have very different properties due to different crystal structure shapes. 6.3.7.1 .2 During multi-layer welding, acoustic phase change recrystallization occurs because the subsequent welding layer reheats the previous welding layer, resulting in differences in the organizational morphology of various parts of the weld area. 6.3.7.1 .3 The welding process is a continuous rapid cooling process. After a certain cooling rate is exceeded, non-equilibrium structures----------bainite and martensite will be obtained. 6.3.7.2 Analysis of heat-affected zone organization: 6.3.7.2 .1 The heat-affected zones of welded joints are mainly: local melting zone. Overheating zone (coarse grain zone). Normalizing or recrystallization zone (fine grain zone). Incomplete normalizing or incomplete recrystallization zone. The organizational morphology and characteristics of each zone depend on the peak temperature of the welding thermal cycle, heating rate, high temperature residence time and subsequent cooling rate. Partial melting zone and overheating zone are the areas with the greatest changes in structure and performance, and are the focus of analysis for metallographic inspection of welded joints. 6.3.7.2 .2 Partial melting zone: This zone is narrow and adjacent to the fusion line. Due to severe overheating, large grains, and severe uneven chemical composition, the performance deteriorates and is the weakest zone in the welded joint. 6.3.7.2 .3 Overheated zone: The obvious feature of this zone is that the grains are the coarsest, and its organizational morphology is basically the same as that of the partially melted zone. It is a sensitive zone for reheating cracks. 6.3.7.2 .4 Recrystallization area: The grains in this area are fine and uniform, and it is the area with the best microstructure and normal temperature mechanical properties in the welded joint. 6.3.7.2 .5 Incomplete crystallization zone; this zone is located in the temperature range A1~A3 and has the characteristics of part of the ferrite dissolving into austenite. The austenite decomposes during cooling, but the original untransformed ferrite remains. For some low-alloy steels, due to the high content of carbon and alloying elements in this part of the austenite, it may transform into high-carbon martensite after rapid cooling, resulting in a mixed structure of martensite and ferrite. This structure has poor performance. 6.3.7.3 Analysis of matrix structure. For dissimilar steel welded joints, the base metal and heat-affected zones on both sides of the weld must be inspected. 6.4 Metallographic photography 6.4.1 Metallographic photography can be divided into macro photography and micro photography. The magnification of macro photography is generally 1-10 times, and the magnification of micro photography is generally 100-1000 times. 6.4.2 Macro photography refers to photographing the appearance or local macro details of the inspected object, which requires clear outlines, a rich three-dimensional sense, and no unnecessary shadows. The quality of the film is mainly related to the selection of the shooting field of view and background, the configuration of the lighting, exposure conditions, darkroom processing and other related factors. When shooting, narrow the aperture to increase the depth of field, and the effect will be better. 6..4.3 Microphotography: It is used to photograph the characteristics of the microstructure and defects in the inspected parts. The subject is required to be representative or typical, realistic and clear. The selection of the lens depends on the required magnification, which is generally to make full use of the resolution of the microscope objective. The total magnification should not be greater than 1000 times the numerical aperture of the objective. Try to use a photography lens with a smaller magnification and a larger length. A camera obscura can be used to make the image have better flatness; the light source used for photography needs to be adjusted appropriately, and the light emitted needs to be stable and of sufficient intensity; the exposure time of the film depends on factors such as the nature of the film and the intensity of light in the sample. If necessary, the segmented exposure method can be used for testing: when taking pictures on site, increase the lighting brightness as much as possible and shorten the exposure time to avoid blurring the film image due to vibration. 6.4.4 Darkroom processing 6.4.4.1 Film development should generally be carried out in total darkness, but a dark green safety light can be used for short-term observation. The temperature of the developer and the development time should be carried out according to the film instructions. Generally, the development temperature is about 20 degrees. During operation, the film can be moistened with clean water, and then put into the developer. After the development is completed, That is to say, move it into clean water or fixing solution to stop the film, and then put it into the fixing solution. The fixing time is usually 20-30 minutes. During the development and fixing processes, the film must be facing up and completely immersed in the solution. Shake it in time. Do not touch the photographic part of the film with your fingers or clips.: 6.4.4.2 When printing, select appropriate photographic paper, exposure time and light intensity according to the condition of the film. The exposure time should be such that the subtle image lines in the darker parts of the film can be clearly displayed. The developed photographic paper can be stopped in water or fixing solution, and then placed in the fixing solution. The fixing time is generally 15-30 minutes. The fixed photo should be rinsed in flowing water, and after rinsing, the photo should be dried and polished. 6.5 Assessment: Evaluate according to 2.5----2.11. 7. Fill in the report 7.1 Fill in the test report according to the test records, fill in the report strictly, and no forgery is allowed. 7.2 Every item in the test report must be filled in, and no omissions are allowed. 7.3 The report must be filled in neatly, without alteration, and kept clean. 7.4 The test report must be accompanied by inspection record drawings when necessary. 7.5 The pasting of photos must be neat and standardized. 7.6 The report must be made in duplicate and handed over to a specialist for processing.

Periodic Inspection of Industrial Pipelines (i.e. Pipe Regulations) Article 31 The following pipelines should generally select representative parts for metallographic and hardness inspection and random inspection. (1) Carbon steel and ferritic stainless steel pipes with operating temperatures greater than 370°C ； (2) Molybdenum steel and chromium-molybdenum steel pipes with operating temperatures greater than 450°C ； (3) Low alloy steel and austenitic stainless steel pipes with operating temperatures greater than 430°C ； (4) Carbon steel and low alloy steel pipelines transporting hydrogen-facing media with operating temperatures greater than 220°C. Article 33 For pipelines whose service life is close to or has exceeded the design life, metallographic inspection or hardness inspection shall be carried out during inspection. If necessary, samples shall be taken for mechanical performance test or chemical composition analysis. Article 41 The material of pipeline components shall comply with the design and use requirements. If it is inconsistent with the original design, the material is unknown or the material is deteriorated, the safety status level is divided as follows: 3. Material Deterioration and Damage Spheroidization, graphitization, creep damage, hydrogen corrosion, intergranular corrosion, stress corrosion and fatigue damage of materials can generally be graded according to the following regulations: (1) If the material has mild spheroidization, it can be rated as level 2. When moderate spheroidization or more serious spheroidization occurs, it can be rated as level 3 or 4. The evaluation of the spheroidization degree of materials is carried out in accordance with the requirements of the current industry standards "No. 20 Steel Pearlite Spheroidization Rating Standard for Thermal Power Plants" DL/T 674 and "12Cr1MoV Steel Spheroidization Rating Standard for Thermal Power Plants" DL/T 773. (2) If the material undergoes mild graphitization, it can be rated as level 3. When moderate graphitization or more serious graphitization occurs, it will be rated as level 4. The assessment of the graphitization degree of materials is carried out in accordance with the requirements of the current industry standard "Carbon Steel Graphitization Inspection and Rating Standard" DL/T 786. (3) For creep damage, if the metallographic examination only finds creep cavities in the material, it can be rated as level 3, and when creep cracks exist, it can be rated as level 4. (4) For hydrogen corrosion, if only hydrogen corrosion decarburization occurs but no cracks are found, it can be rated as level 3. When hydrogen corrosion cracks appear, it can be rated as level 4. (5) If intergranular corrosion occurs in the material, but no cracks are found, the material can be rated according to the maximum intergranular corrosion depth using the local thinning assessment method in Article 43. If cracks are found, it will be rated as level 4. ； (6) When the pipeline has stress corrosion, fatigue damage and cracks generated during use are found after inspection, it will be rated as Level 4.

Generally, the following situations need to be done: 1. Standardized. For example, what inspections should be carried out on materials used where pressure vessels are required to be used? ； 2. The drawings have regulations. For example, there are drawings that stipulate that the piece should undergo metallographic inspection. ； 3. When materials according to grade requirements enter the factory for re-inspection. For example, it is generally stipulated that level three forgings should undergo metallographic inspection. ； 4. When there are doubtful materials with grade requirements that need to be re-inspected. For example, if there is a problem with this batch of materials during production, the problem may involve the structure and metallography of the material, and re-inspection is required. ； 5. When the materials produced need to be graded. For example, the process stipulates that the workpiece material must meet the metallographic grade requirements in order to be qualified.