| High pressure pipes is an important part in the large-scale industrial equipments, such as ammonia synthesis industry, petroleum hydrocracking industry and power generation industry, and20steel is one of the most widely used steel for pipes. Pipe failures, even explosion accidents, are caused by the strain aging embrittlement in recent years, resulting in huge economic losses and casualty. According to investigation, the manufacturing technology of in-service20steel pipes widely used in fertilizer enterprises are identical to the incident pipes, which means these pipes have the same risk factors. According to statistics, the total length of high pressure pipes of20steel is about100,000meters in ammonia synthesis industries and ammonia synthesis industries. All these20steel pipes have the strain aging embrittlement problem caused by the manufacturing defect, low requirements, as well as unsuitable process control. Therefore, it is significance to carry the research on the regularity and mechanism among the in-service conditions, microstructure and mechanical properties. Basing on the study results, non-destructive detection method of strain aging is proposed, which has great significance to optimize the production process and improve the using safety of high pressure pipes.The strain aging embrittlement of high pressure pipe is directly related to the steel heat treatment, cold deformation, aging time and aging temperature, carbon content and nitrogen content in the steel. Based on the analysis and research on the influencing factors of strain aging, they are determined that manufacturing conditions and other production states of strain aging. The properties of some in service pipes are also studied and analyzed. A set of argon protective equipment is made to prevent the thermal oxidation. Working principles of the equipment is that air is pumped out by a vacuum pump and the argon gas is filled in to keep the argon atmosphere. Then the microstructure at the same position of the sample can be observed before and after heating. The observation and analysis methods are in situ observation and multi-point observation in micro structure change after strain aging with OM, SEM and TEM. The material hardness, impact absorption energy and tensile properties are tested. Test specimens along the pipe axis direction are prepared in accordance with the relevant standards. Metallographic samples are made from different composition pipes that suffer normalizing, annealing and under annealing respectively. Lamellar-type pearlite is found. However, the lamellar distance and the grain size are different. The research results show that they are related to the heating temperature and the cooling rate. The microstructure doesn't change with pre-treatment and plastic deformation and strain aging for an hour. The microstructure change is observed in situ. Slip lines and slip bands in pearlite can be observed in plastic deformation state. The microstructure has no obvious change after strain aging. Iron nitride superlattice phase are found in the strain aging embrittlement material through TEM, it shows that interstitial atoms distribute long-range order, that is to say, the material atoms distribution is changed from short-range order into long-range order when the strain aging embrittlement occurs. The stress relief annealing pearlite change from lamellar-type to spherulitic after the strain aging or normalizing. The conversion depends on the heat treatment process and the degree of plastic deformation of materials. The microstructure shows no significant change in manufacture hot rolled state and then stress relief annealing.Vickers hardness test is carried on microstructures' samples. Hardness value under every heat treatment and plastic is obtained. Statistical analysis of the hardness shows its evolution law. The hardness varies from each other when normalizing, annealing and under annealing are taken respectively. The cooling rate also affects the hardness a lot, while the material compositions have little effects. The hardness value increases after plastic deformation on the basis of pre-treatment, while the value after strain aging becomes larger than the one after plastic deformation. The value also increases with the increase of the plastic deformation amount and the nitrogen content. The grain size also has effect on the hardness. Comparing to the pre-treatment state, the hardness value is almost the same after strain aging with no plastic deformation and it shows the strain aging affects the hardness only with strain exiting. The hardness of hot rolled material grows larger, and it shows cold deformation exits. This phenomenon also means that the finishing temperature is lower than the normalizing temperature. Annealing can dramatically reduce the increased hardness value caused by the aging. The depressed degree is related to the annealing temperature. The hardness after stress relief treatment is proportional to the nitrogen content and it is also related to the grain size. The pre-treatment process almost has no effect on material hardness. Charpy V-notch impact tests are carried on almost all the material mentioned above under room temperature. Parts of these impact fracture surfaces are observed with SEM. Tensile test is also carried out under room temperature. The results show the material of normalizing and annealing meet the standard require value. The hot rolled and the under annealing materials don't meet the standard value. The impact absorption energy is related to the material compositions after plastic deformation and the value is a little lower than the pre-treatment ones. But it still meets the standard requirement. Almost all the material of strain aging doesn't meet the required impact toughness. The material of10%strain aging of every heat pro-treatment is extremely brittle. The yield stress increases with the plastic deformation after strain aging. The yield ratio is proportional to the plastic deformation. The plastic reserve decreases with the increase of plastic deformation. Strain aging doesn't affect the strength increase along, the material strength after aging increases only with the existence of cold deformation. The material impact absorption energy can recover to the standard value after stress relief annealing. The yield ratio decreases a lot after annealing. The pre-treatment has great effect on the yield strength, but has little effect on the tensile strength. Fracture morphology has larger difference, which reflects the difference of the mechanical properties.The analysis by the author shows the corresponding relationship between the microstructure and mechanical properties of the hardness, impact properties and tensile properties. Therefore, other mechanical properties except for hardness can be infered with hardness and microstructure, and thus it can be judged whether the strain aging embrittlement overtake. According to the evolution rule of strain aging, non-destructive detection method of in-service20steel pipe strain aging embrittlement is proposed on the basis of analysis and inference on microstructure characteristics, hardness and the other corresponding mechanical properties of straight pipe without plasticity, strain embrittlement pipe, strain aging embrittlement pipe and elbow with plasticity and stress relief annealing. The non-destructive detection method is based on hardness and microstructure changes, and is proved accurate and reliable with the results of applying to two steel pipes.Control of test results in the text, as well as a variety of data about20steel pipe mastered by the author and China's national standards, amendment suggestions are proposed to modify the standard rules about nitrogen content, heat treatment after rolling, heat treatment after bending and impact test.The corresponding relations among the manufacturing conditions, microstructure and mechanical properties can provide significant guiding value to improve the metallurgical quality of high pressure pipes and the rolling and forming process conditions. The non-destructive detection method proposed in this paper can be directly used for the detection of the in service pipes, which can prevent the accidents caused by the train aging embrittlement and its incidental hydrogen embrittlement and stress corrosion, also can eliminate the hidden danger in dangerous and ensure the life property safety. The proposed modification to the current standard can improve the national standards of high pressure pipes, which would eliminate and reduce the hidden danger caused by the train aging embrittlement at source. The work involved in this paper has great theoretical reference significances and values on material basic research, such as material preparation of metal component, mechanical behavior, environment interaction, injury and failure mechanism. The proposed non-destructive detection method can be used to provide qualitative analysis and semi-quantitative analysis on in service pipes, while it still needs more support data and practical proves to provide quantitative analysis.
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