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Abstract: Austenitic stainless steel pressure vessel heads can develop various defects during manufacturing and use, which affects the quality of the pressure vessels and endangers their safe operation. Therefore, a summary analysis is conducted on the various types of defects that occur in stainless steel pressure vessel heads during manufacturing and use, as well as their causes, and technical measures for eliminating and repairing these defects are proposed. Keywords: stainless steel ； Pressure vessel ； headstock ； Manufacturing ； Defects Introduction The pressure vessel head is one of the main pressure-bearing components of a pressure vessel, and austenitic stainless steel is an excellent material for manufacturing such heads. However, based on manufacturing and usage in recent years, austenitic stainless steel head products have developed defects such as cracks, bulging, over-sintering, wrinkles, excessive thinning, scratches, tearing, and pits. To reduce the occurrence of defects, it is necessary to analyze their causes and propose key measures to control their formation, as well as establish appropriate rework procedures for those defects that can be repaired. 1 Defects in austenitic stainless steel heads and analysis of their causes. The processing and forming process of austenitic stainless steel heads includes procedures such as material inspection, surface treatment, leveling, marking, cutting, welding, trimming of weld height, pressing into shape, non-destructive testing, heat treatment, and edge trimming. However, during the processing stage, variations in materials, thickness, processing techniques, as well as the skill level of the workers can all lead to various defects. The defects in the heads and their causes are analyzed based on the inspection results of each process. 1.1 Analysis of crack defects and their causes The main reasons for cracks in austenitic stainless steel heads include the following: Firstly, work hardening occurs during the process of straightening the edges of the head, resulting in third-type residual stresses; the inner surface of the straightened section is subjected to high tensile stresses, leading to macroscopic internal stresses. Excessive spinning speed during the cold spinning process, too high pressure applied during spinning, as well as excessive stamping speed during the cold stamping process and improper control of the gap between the upper and lower dies can all cause cracks ； Secondly, port cracks mainly arise due to uneven cutting surfaces during the blank cutting process; stress concentration occurs during the pressing process, which leads to the formation of these cracks ； Thirdly, the grains at the fusion line in the heat-affected zone of the weld are coarse, and cracks arise as a result of external forces. Additionally, stress concentration occurs at the areas where the weld has undercutting during the process of head bulging and stamping, which can also lead to the formation of cracks along the fusion line. 1.2 Analysis of bulging defects and their causes: Bulging of the head refers to the outward deformation of a local area on the surface of the head, and it generally occurs during the hot forming process. Cause analysis: First, the smooth transition zone during stretching is subjected to tangential compressive stress, causing the sheet to become unstable and develop bulges ； Secondly, the uneven heating of the head blank leads to increased uneven deformation of the metal, thereby causing bulging. 1.3 Analysis of overburning defects and their causes: Overburning refers to the phenomenon of sheet material damage that occurs when the local heating temperature during the hot forming of end caps exceeds the starting pressure temperature. In the hot forming process, uneven temperature fields in the heating furnace can easily lead to local over-sintering of the head. When using coal furnaces or coke furnaces for heating, it is difficult to control the temperature inside the furnace, and overheating often occurs; therefore, stainless steel end caps should not be heated using coke or coal furnaces. For carbon steel heads, although the standards do not specify it, heating with coal furnaces or coke ovens is also not advisable. 1.4 Analysis of defects related to excessive wall thickness reduction and their causes: For stamped heads, the bottom part of the head is subjected to mold pressure and friction, resulting in the smallest reduction in wall thickness ； The compressive stress exerted by the blanking ring on the upper part of the straight-edge section is greater than the tensile stress resulting from the transition zone with a smooth curvature, leading to an increase in thickness ； Under the combined action of tensile stress and die pressure, the smooth transition zone experiences the greatest wall thickness reduction. For spun heads, during the drumming process, the blank is continuously struck by the drumming head, resulting in a greater thickness reduction compared to stamped heads, and poorer wall thickness uniformity. As long as the process is properly controlled, process thinning is controllable. The main reasons for excessive thinning include: first, excessive pressure from the flanging ring, resulting in limited freedom of deformation for the blank ； Secondly, the poor finish of the blanks and molds, along with inadequate lubrication, result in high stretching resistance for the blanks and poor stretching performance ； Third, poor control of the drum pressing process leads to uneven thinning of the wall thickness. 1.5 Analysis of scratch defects and their causes: Scratches on the head are surface damage defects that occur during the processing of heads, and scratches can appear on both the inner and outer surfaces. The main causes of scratches are: first, the surface of the mold is not smooth enough, with sharp defects ； Secondly, during hot pressing, a thick oxide layer forms on the surface of the blank; this layer peels off during the pressing process, causing scratches ； Third, improper handling during transportation can also cause scratches on the outer surface. 1.6 Analysis of tearing defects and their causes Tearing occurs when the tensile force exceeds the material’s strength limit, resulting in cracking. The reasons for cracking include: first, the low elongation of the material, and second, excessive levels of sulfur and phosphorus ； Second, the pressure of the blanking ring is too high, causing the blank to crack during stamping ； Third, the lubrication effect is poor; uneven friction between the mold and the blank causes cracking of the head ； Fourth, the mold stroke was too fast during stamping, and the spinning speed was too high during spinning and flanging, resulting in transverse cracking in the smooth transition zone. 1.7 Pitting defects and analysis of their causes: Pitting refers to local or circumferential depressions that form during the processing of end caps. The main reason for the formation of pits is: first, the mold is not in the correct position. In the stamping process, if the die is eccentric and the gaps on both sides are not equal, it is difficult to press the material in the side with the smaller gap. When the die moves to that area, it gets stuck; forcing pressure down will result in severe thinning of that area and the formation of dents ； Secondly, during spinning, improper drum pressing techniques can also lead to the formation of pits, especially in thin-walled end caps; as these end caps undergo significant deformation, uneven pounding by the drum head may result in localized pits. 2 Preventive measures for defects in stainless steel pressure vessel heads. During the pressing process of stainless steel pressure vessel heads, complex stress conditions arise, which can lead to various types of defects. Taking appropriate preventive measures based on the causes of different defects is an important step in ensuring the quality of end caps. 2.1 Preventive measures for crack defects: Different measures are taken depending on the various causes of cracks in the heads of stainless steel pressure vessels; cracks in the straight-edge sections are primarily controlled through the spinning speed and pressure, as well as the stamping speed ； Strictly control the cutting process, and polish the edges of the blanks smoothly to prevent cracks at the ends ； To prevent cracks in the weld seam and heat-affected zone, it is necessary to strictly control the welding process parameters, slow down the cooling rate, appropriately increase the weld shape factor, use low current and multiple passes of welding whenever possible to avoid cracks at the center of the weld seam, prevent undercutting, control the weld thickness, and apply appropriate heat treatment to the weld area. 2.2 Preventive measures for bulge defects Measures to prevent the occurrence of bulges include: using a secondary stretching process to reduce residual stresses ； Use a conical blank holder ； Improve the heating quality of the blank to ensure uniform heating of the material. Measures taken to prevent overheating include: using electric furnaces, gas furnaces, and oil furnaces with a uniform temperature field; placing the furnace lids as close to the center as possible; arranging the thermocouples in electric furnaces and the nozzles in gas and oil furnaces evenly; and conducting regular inspections of the temperature field inside the furnace. To prevent the formation of wrinkles, the following measures should be taken: install pressure rings, especially in pressure vessel factories that use self-made end caps and lack professional equipment ； Control the gap between the upper and lower dies properly ； By controlling the processing techniques, materials that are difficult to shape can be drawn multiple times. 2.3 Preventive measures for defects related to excessive thinning. The control of thinning mainly lies in the manufacturing process; when considering overall thinning, it is necessary to ensure that the thickness of the material used is sufficient, and that the dimensions of the blank allow for enough bending width ； Controlling local thinning requires ensuring the accuracy of the drum pressing process and the skill of the operators. 2.4 Preventive measures against scratch defects: To reduce scratch defects, the surface of the blank should be cleaned thoroughly before pressing the head, in order to prevent hard objects from sticking to the surface and causing scratches during the stamping process ； Regularly clean the upper and lower molds to reduce the adhesion of scale ； Regularly check the mold for any damage. 2.5 Preventive measures for cracking and pit defects: The measures taken to control cracking include strictly controlling the quality of the materials, ensuring that their mechanical properties, chemical composition, microstructure, and process-related characteristics meet the requirements specified in the technical standards ； The workers are skilled in the processing techniques, can control the stamping speed and pressure effectively, and regularly apply lubricant to the contact surface between the upper and lower pressing rings to prevent cracking of the end cap. To prevent the formation of pits, it is essential to ensure that the upper die is positioned at the center of the flanging ring in order to avoid eccentricity; measuring tools should be available on site, as relying on visual estimation is not advisable. 3 Analysis of repair measures for defects in stainless steel pressure vessel heads. During the processing of stainless steel pressure vessel heads, the occurrence of defects is inevitable; although some of these defects exceed acceptable limits, repairing them has little impact on their safety performance. In line with the principle of energy conservation and reduced consumption, repairs should be carried out as much as possible while ensuring the quality of the head. Some defects cannot be repaired, or repairing them does not guarantee the mechanical properties of the head, in which case it must be scrapped. Crack defects can be removed by grinding. Ultrasonic non-destructive testing is used to determine the location and length of the defects, after which grinding is carried out using a grinder. Grinding starts from both ends of the crack, with the defect area being shaped into a U-shaped groove. Subsequently, surface inspection is performed to verify that the defect has been completely removed; this process continues until it is confirmed that the defect is fully eliminated. If the grinding depth exceeds 5% of the steel’s thickness or 2 mm, repair welding must be performed. Bulging defects can be repaired using hydraulic pushing back, but severe bulging deformation can cause uneven stress levels across different areas of the head, as well as uneven mechanical properties in various parts of it, making repair impractical. Over-sintering defects can cause changes in the microstructure and a decline in the mechanical properties of the material. It is difficult to distinguish between overheating, over-sintering, and burnout; therefore, it is recommended to discard such end caps. Minor wrinkles can be removed by polishing ； Scratches and local dents can be polished and rewelded as appropriate ； Pits that remain throughout the week cannot be repaired and should be scrapped. 4 Conclusion During the manufacturing and use of stainless steel pressure vessel heads, defects such as cracks, bulging, over-burning, wrinkles, excessive thinning, scratches, tearing, and pits can occur. The main reasons for the defects that occur can be summarized as: inadequate control of materials ； Poor process execution, improper control of stamping speed and pressure, mold eccentricity, and incorrect welding parameters all fall into this category ； The equipment does not meet the requirements for processing heads, including an uneven temperature field in the heating furnace and damaged molds. To minimize the occurrence of these defects, the following measures should be taken in the manufacturing of end caps: strictly control the quality of materials entering the production process ； Develop a reasonable process ； Strengthen operator training ； Strictly enforce process discipline ； Conduct regular inspections of processing equipment ； Reduce the impact of external factors.