Abstract: Austenitic stainless steel pressure vessel heads will have various defects during the manufacturing and use process, which will affect the quality of the pressure vessel and endanger its safe operation of the pressure vessel. Therefore, various types of defects and causes of stainless steel pressure vessel heads in the manufacturing and use process are summarized and analyzed, and technical measures to eliminate and repair defects are proposed.
Keywords: stainless steel; pressure vessel; head; manufacture; defect
The head of the pressure vessel is one of the main pressure components of the pressure vessel, and austenitic stainless steel is a high-quality material for the manufacture of the head of the pressure vessel. However, from the perspective of manufacturing and use in recent years, austenitic stainless steel head products have defects such as cracks, bulges, over-burning, wrinkles, thinning exceeding the standard, scratches, cracks, and pits. In order to reduce the occurrence of defects, it is necessary to analyze the reasons for their occurrence and propose the main measures to control the occurrence of defects and formulate a reasonable repair process for the defects that can be repaired.
1 Defects and causes of austenitic stainless steel heads
The processing and forming process of austenitic stainless steel head includes material acceptance, surface treatment, leveling, scribing, cutting, welding, welding and grinding, press forming, non-destructive testing, heat treatment, trimming and other processes. However, in the process of processing, changes in material, thickness, processing technology and the proficiency of workers may produce various defects. From the inspection of each process, the head defects and causes are analyzed as follows.
1.1 Analysis of Crack Defects and Causes The causes of cracks in austenitic stainless steel heads mainly include the following aspects: First, work hardening is caused during the straightening process of the head, resulting in the third residual stress, and the inner surface of the straight edge section is subjected to Large tensile stress produces macroscopic internal stress. During the flanging process, the spinning speed of the cold spinning process is too fast, the pressure applied by spinning is too high, the stamping speed of the cold stamping process is too fast, and the gap between the upper and lower dies is not properly controlled. Cracks may occur; second, the cracks at the port are mainly caused by the uneven cutting surface during the cutting process of the billet, and the stress concentration during the pressing process; third, the fusion line in the heat-affected zone of the weld has coarse grains and is subject to an external force. In addition, the stress concentration of the undercut part of the weld during the drum pressing and stamping process of the head will also form cracks along the fusion line.
1.2 Bulge defects and cause analysis The head bulge refers to the outward bulging deformation of the local area of the head surface, which is generally produced in the thermoforming process. Analysis of the reasons: First, the smooth transition zone is subjected to tangential compressive stress during stretching, which makes the plate unstable and causes bulging; second, the uneven heating of the head blank causes the uneven deformation of the metal to increase, resulting in bulging.
1.3 Analysis of over-burning defects and their causes Over-burning refers to the phenomenon of sheet burning caused by the local heating temperature exceeding the initial pressure-temperature during the thermoforming of the head. In the hot forming process, due to the uneven temperature field of the heating furnace, it is easy to cause local overburning of the head. When using a coal furnace and coke furnace for heating, the temperature in the furnace is not easy to control, and the phenomenon of overburning often occurs. Therefore, coke and coal heating furnaces should not be used for heating stainless steel heads. For carbon steel heads, although the standard does not stipulate, it is not suitable to use coal stoves and coke ovens for heating.
1.4 Defects and causes of thinning exceeding the standard. For stamping heads, the bottom of the head is subjected to mold pressure and friction, and the wall thickness is reduced to the minimum; Thickness increases; the smooth transition zone reduces the maximum wall thickness under the combined action of tensile stress and die pressure. For the spinning head, during the drum pressing process, the blank is continuously beaten by the drum head, the thinning amount is larger than that of the stamping head, and the wall thickness uniformity is poor. Process thinning is controllable as long as the process is properly controlled. The main reasons for the thinning and exceeding the standard are: first, the pressure of the blank holder is too large, and the degree of freedom of blank stretching is small; second, the roughness of the blank and the mold is poor, and the lubricant effect is not good, resulting in high tensile resistance of the blank and poor stretching effect. ; Third, the poor control of the drum pressing process leads to uneven wall thickness reduction.
1.5 Scratching Defects and Cause Analysis Scratches on the head are surface damage defects during the processing of the head, and scratches may appear on the inner and outer surfaces. The main reasons for scratches are: first, the mold surface is not smooth enough, and there are sharp defects; second, during hot pressing, a thick oxide layer is formed on the surface of the billet, which falls off during the pressing process and causes scratches; third, improper operation during the transfer process also Can cause scratches on the outer surface.
1.6 Analysis of pull cracking defects and their causes Pull cracking is the cracking caused by the tensile force exceeding the strength limit of the material. The reasons for the cracking are as follows: first, the material elongation is low, and the content of sulfur and phosphorus exceeds the standard; second, the pressure of the blank holder is too large, which will cause the blank to be cracked during stamping; third, the lubrication effect is poor, and the friction between the mold and the blank is not enough. All-cause the cracking of the head; fourth, the die stroke is too fast during stamping, the spinning speed is too fast during spinning and flanging, and the smooth transition area has caused lateral cracking.
1.7 Analysis of pit defects and their causes pits are local or whole circumference depressions formed during the processing of the head. The main reasons for the pits are: First, the mold position is not correct. In the stamping process, if the die is eccentric, the gaps on both sides are different, and the blank is not easy to be pressed down on the side with the small gap. The die runs to this part and gets stuck and forcibly pressed down, which will cause the side to be severely thinned and form a pit. ; Second, during spinning, improper drum pressing process will also lead to pits, especially thin-walled heads. The deformation of the head is large, and the uneven drumming of the drum head may lead to local pits.
2 Preventive measures for defects of stainless steel pressure vessel heads
In the process of pressing the stainless steel pressure vessel head, the force is complex, and various defects are prone to occur. According to the reasons for the formation of different defects, taking corresponding preventive measures is an important link to ensuring the quality of the head.
2.1 Preventive measures for crack defects Different measures are taken according to different situations of stainless steel pressure vessel head cracks. The cracks in the straight side section are mainly controlled by the speed of the hemming, the pressure, and the punching speed; the cutting process is strictly controlled, and the edges of the blank are polished smoothly to prevent ports. Cracks occur; for cracks in the weld and heat-affected zone, the welding process parameters should be strictly controlled, the cooling rate should be slowed down, the shape factor of the weld should be appropriately increased, and low-current multi-layer multi-pass welding should be used as much as possible to avoid cracks in the center of the weld and prevent Undercut, control the welding height, and perform reasonable heat treatment on the weld area, etc.
2.2 Preventive measures for bulging defects Measures to prevent bulging include: adopting a secondary stretching process to reduce residual stress; adopting a tapered edge blanking ring; improving the heating quality of the blank so that the blank is heated evenly. The measures are taken to prevent overburning include: using electric furnaces, gas furnaces and oil furnaces with uniform temperature fields, placing the heads in the furnaces as much as possible in the center position, the uniform layout of the thermocouples of the electric furnaces and nozzles of the oil and gas furnaces, and checking the temperature field in the furnaces. Check regularly. To prevent the occurrence of wrinkles, the following measures should be taken: adding pressure rings, especially the pressure vessel factories with self-made heads, lack of professional equipment; controlling the gap between the upper and lower molds; controlling the processing technology, and the materials that are not easy to form can be stretched multiple times take shape.
2.3 Preventive measures for thinning and over-standard defects The control of thinning is mainly in the manufacturing process. For the overall thinning, it is necessary to consider whether the thickness of the ingredients is sufficient and whether the size of the billet can ensure sufficient blanking width; the control of local thinning must ensure that the pressing drum is The correct of the process and the proficiency of the workers.
2.4 Preventive measures for scratch defects In order to reduce scratch defects, before pressing the head, the surface of the blank should be cleaned to avoid sticking hard objects to scratch the surface during the stamping process; the upper and lower dies should be cleaned regularly to reduce oxide scale adhesion; Regularly check the mold for damage.
2.5 Preventive measures for cracking and pit defects The measures for cracking control are: strictly control the quality of the material, the mechanical properties, chemical composition, metallographic structure and process performance indicators of the material to meet the technical standards; workers are proficient in the processing technology and control the stamping. Speed and pressure, often apply lubricating oil on the contact surface of the upper and lower pressure rings to prevent the head from cracking. To prevent the generation of pits, the main purpose is to ensure that the upper die is in the center of the blank holder to prevent eccentricity. There must be measuring tools on-site, and do not rely on eye estimation.
3 Analysis of repair measures for stainless steel pressure vessel head defects
In the process of processing stainless steel pressure vessel heads, the occurrence of defects is inevitable. Although some defects exceed the standard, their safety performance will not be affected much after repair. In line with the principle of energy-saving and consumption reduction, on the premise of ensuring the quality of the head, it should be repaired as much as possible. Some defects cannot be repaired, or the mechanical properties of the head cannot be guaranteed after repair, and they must be scrapped. Crack defects can be eliminated by grinding. Use ultrasonic non-destructive testing to determine the position and length of the defect, and then use a grinding wheel to grind it. First, start grinding from both ends of the crack, grind the defect position into a U-shaped groove, and then carry out a surface inspection to check whether the defect is completely removed until it is confirmed that it is completely removed. When the depth exceeds 5% or 2mm of the steel thickness, repair welding should be carried out. The bulge defect can be repaired by hydraulic topping, but serious bulge deformation will cause uneven stress levels in each area of the head, and uneven mechanical properties of each part of the head, which is not suitable for repair. Over-burning defects may lead to changes in the metallographic structure, deterioration of the mechanical properties of materials, and difficulty in defining overheating, over-burning, and burning. It is recommended that the head be scrapped. Slight wrinkles can be removed by grinding; scratches and local pits can be ground and repaired according to the situation; the pits cannot be repaired and should be scrapped.
Defects such as cracks, bulges, over-burning, wrinkles, thinning exceeding the standard, scratches, cracks, pits, and other defects will occur during the manufacture and use of stainless steel pressure vessel heads. From the main causes of defects, it can be summarized as follows: material control is not strict; process execution is not in place, stamping speed and pressure are improperly controlled, mold eccentricity and incorrect welding parameters are all in this category; equipment does not meet the requirements for processing heads, including The temperature field of the heating furnace is uneven, and the mold is damaged. In order to reduce the occurrence of these defects as much as possible, the following points should be done in the manufacturing of the head: strictly control the materials and personnel; formulate a reasonable process; strengthen the training of operators; strictly implement the process discipline; carry out regular inspection of the processing equipment; Reduce the influence of external factors.
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