Study on the head of stainless steel pressure vessel
The austenitic stainless steel head of pressure vessel will produce various defects in the process of manufacturing and use, which will affect the quality of pressure vessel and endanger the safe operation of pressure vessel. Therefore, various types of defects and causes of stainless steel pressure vessel head in the process of manufacturing and use are summarized and analyzed, and the technical measures to eliminate and repair the defects are put forward.
Pressure vessel head is one of the main pressurized components of pressure vessel, and austenitic stainless steel is a high-quality material for manufacturing pressure vessel head. However, from the manufacturing and usage in recent years, austenitic stainless steel head products have produced defects such as cracks, bulging, over-burning, creasing, thinning oversize, scratching, pulling, cratering, etc. In order to reduce the generation of defects, it is necessary to analyze the causes of their generation, and propose the main measures to control the generation of defects, and develop a reasonable reworking process for defects that can be repaired.
1. Austenitic stainless steel head defects and cause analysis
The processing and forming process of austenitic stainless steel head includes material acceptance, surface treatment, leveling, scribing, cutting, welding, weld height repair and grinding, pressing and forming, nondestructive testing, heat treatment, trimming and other processes. However, during the processing, the changes of material, thickness, processing process and the proficiency of workers’ operation may produce various defects. The analysis of head defects and causes from the inspection of each process is as follows.
1.1 Analysis of crack defects and causes
Cracking of austenitic stainless steel head is mainly caused by the following aspects: First, the process of turning the straight edge of the head causes process hardening, generates the third residual stress, the inner surface of the straight edge section is subjected to large tensile stress to produce macroscopic internal stress, the spinning speed of the cold spinning process in the process of turning the straight edge is too fast, the pressure exerted by spinning is too high, and the stamping speed of the cold stamping process is too fast, the upper and lower die gap is not properly controlled may Cracks may occur; secondly, port cracks are mainly formed due to uneven cutting surface in the billet cutting process and stress concentration in the pressing process; thirdly, coarse grains on the fusion line in the heat affected zone of the weld are subjected to external force to produce cracks; in addition, stress concentration on the bitten edge of the weld in the head drum pressing and stamping process may also form cracks along the fusion line.
1.2 Analysis of bulging defects and causes
Head bulging refers to the deformation of the local area of head surface bulging outward, which is usually produced in the hot forming process. Analysis of causes: Firstly, the rounded transition zone is subjected to tangential compressive stress in stretching, which destabilizes the plate and generates bulging; secondly, the uneven heating of the head blank causes uneven deformation of metal to increase, thus generating bulging.
1.3 Analysis of over-burning defects and causes
Over-burning refers to the phenomenon of plate burnout when the local heating temperature exceeds the initial pressure temperature during head hot forming. In the hot forming process, uneven temperature field of heating furnace can easily lead to local overburning of head. When using coal furnace or coke furnace for heating, the temperature inside the furnace is not easy to control and over-burning phenomenon often occurs, so coke and coal heating furnace should not be used for stainless steel heads. For carbon steel head, although the standard is not specified, it is not suitable to use coal and coke furnace for heating.
1.4 Analysis of defects and causes of excessive thinning
For stamping head, the wall thickness is thinned the least when the bottom of head is subjected to die pressure and friction force; the compressive stress in the upper part of straight edge section by crimping ring is greater than the tensile stress in the extension of rounded transition zone, and the thickness increases; the wall thickness is thinned the most when the rounded transition zone is under the joint action of tensile stress and die pressure. For spinning head, during the process of pressing drum, the billet is constantly pounded by the pressing drum head, and the thinning amount is greater than that of stamping head, and the wall thickness uniformity is poorer. As long as the process is properly controlled, the process thinning is controllable. The main reasons for the occurrence of excessive thinning are: firstly, the pressure of crimping circle is too large and the billet stretching freedom is small; secondly, the poor finish of billet and die and the poor effect of lubricant cause the billet stretching resistance and poor stretching effect; thirdly, the uneven wall thickness thinning due to the poor control of pressing drum process.
1.5 Analysis of scratching defects and causes
Scratching of head is a surface damage defect in the process of head processing, and scratching phenomenon may appear on both inside and outside surfaces. The main reasons for scratching are: firstly, the surface of the mold is not smooth enough and there are sharp defects; secondly, when hot pressing, a thick oxidized layer is formed on the surface of the billet and it falls off during the pressing process, causing scratching; thirdly, improper operation during the transfer process can also cause external surface scratching.
1.6 Cracking defects and the causes of analysis
Pulling crack is the tensile force exceeds the strength limit of the material resulting in cracking. The reasons for pulling crack are: one is low elongation of material and excessive sulfur and phosphorus content; two is excessive pressure of crimping circle, which causes pulling crack of blank when stamping; three is poor lubrication effect and uneven friction between die and blank causing pulling crack of head; four is too fast stroke of die when stamping and too fast spinning speed when spinning flanging, which has caused transverse pulling crack in round and smooth transition area.
1.7 Analysis of crater defects and causes
Crater is the local or whole circumference depression formed during the processing of head. The main reasons for generating pits are: Firstly, the mold position is not correct. In stamping process, if the die is eccentric and the gap size is different on both sides, on the side with small gap, the blank is not easy to be pressed down and the die is stuck when it runs to that part and pressed down forcibly, it will lead to serious local thinning on that side to form crater; secondly, when spinning, improper pressing drum process will also lead to crater generation, especially for thin wall head, large head deformation and uneven pounding of drum head may lead to local crater.
2. Preventive measures of stainless steel pressure vessel head defects
During the pressing process of stainless steel pressure vessel head, the force is complicated and various kinds of defects are easily generated. According to the reasons of different defects formation, making corresponding preventive measures is an important link to ensure the quality of the head.
2.1 Preventive measures for cracking defects
Different measures are taken for different situations of stainless steel pressure vessel head cracking, straight edge cracking is mainly controlled by spinning speed and pressure, stamping speed; strict control of cutting process, smooth grinding of blank edges to prevent port cracking; cracking of welding seam and heat affected zone, strict control of welding process parameters, slow down the cooling speed, appropriate increase in weld shape factor, as far as possible using small current multi-layer Multi-pass welding, to avoid cracks in the center of the weld, to prevent edge biting, control the welding height, and reasonable heat treatment of the weld area.
2.2 Preventive measures for bulging defects
Measures to prevent the generation of bulging include: the use of secondary stretching process to reduce residual stress; the use of tapered crimp ring; improve the heating quality of the blank, so that the blank is uniformly heated. Measures taken to prevent the generation of overburning include: using electric, gas and oil furnaces with uniform temperature fields, placing the head in the center of the furnace as much as possible, arranging thermocouples of electric furnaces and nozzles of oil and gas furnaces evenly, and conducting regular inspection of the temperature field in the furnace. The following measures should be taken to prevent creasing: add pressure ring, especially for pressure vessel factory with self-made head, which lacks professional equipment; control upper and lower die clearance; control processing technology, and materials not easy to be formed can be formed by multiple stretching.
2.3 Preventive measures for defects of thinning overrun
The control of thinning is mainly in the manufacturing process, the overall thinning to consider whether the ingredient thickness is sufficient, the size of the blank can ensure that there is sufficient crimping width; control of local thinning to ensure the correctness of the pressure drum process and the skill of the workers operation.
2.4 Preventive Measures for Scratch Defects
In order to reduce scratching defects, the surface of blank should be cleaned before pressing the head to avoid adhering hard objects from scratching the surface in the stamping process; the upper and lower dies should be cleaned regularly to reduce the adhesion of oxidized skin; and the dies should be checked regularly for breakage.
2.5 Preventive measures for cracking and cratering defects
The measures for cracking control include: strictly control the quality of materials, the mechanical properties, chemical composition, metallographic organization and process performance indexes of materials conform to the technical standards; workers master the processing technology, control the stamping speed and pressure, and frequently apply lubricating oil on the contact surface of upper and lower pressure rings to prevent the pulling and cracking of the head. The main purpose of preventing craters is to ensure the upper die is in the center of the crimp ring to prevent eccentricity; there should be measuring tools on site and never estimate by eyes.
3. Analysis of repair measures of stainless steel pressure vessel head defects
In the processing of stainless steel pressure vessel head, defects are inevitable, and some defects, although exceeding the standard, do not have much impact on its safety performance after repair. Based on the principle of energy saving and consumption reduction, repair should be carried out as much as possible under the premise of ensuring the quality of the head. Some defects cannot be repaired or the mechanical properties of the head cannot be guaranteed after repair, so they should be scrapped. Crack defects can be eliminated by grinding. Adopt ultrasonic nondestructive testing to determine the location and length of defects, then use grinding wheel to polish, starting from both ends of cracks, and polish the defective position into U-shaped groove, then carry out surface inspection to check whether the defects are completely removed until it is confirmed that they are completely removed, and when the polishing depth exceeds 5% or 2mm of steel thickness, it should be welded. Bulging defects can be repaired by hydraulic back-topping method, but serious bulging deformation will cause uneven stress level in all areas of the head and uneven mechanical properties in all parts of the head, which should not be repaired. Over-burning defects may lead to changes in metallurgical organization, decrease in mechanical properties of materials, overheating, over-burning and burnout definition difficulties, and it is recommended that the head be scrapped. Slight creases can be polished and eliminated; scratches and local pits can be polished and patched depending on the situation; the whole circumferential pits cannot be repaired and should be scrapped.
Cracks, bulging, over-burning, creasing, over-thinning, scratching, pulling, cratering and other defects will be produced in the manufacturing and use of stainless steel pressure vessel head. The main reasons for the defects can be summarized as follows: poor material control; improper process execution, improper stamping speed and pressure control, eccentric die, incorrect welding parameters are all such; equipment does not meet the requirements of processing heads, including uneven heating furnace temperature field, damaged die, etc. In order to minimize the generation of these defects, the following points should be done in head manufacturing: strict control of materials; formulation of reasonable process; strengthening training of operators; strict implementation of process discipline; regular inspection of processing equipment; reduction of influence of external factors.
Source: China Head Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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