Analysis of research status on forming processes and quality control of heavy pressure vessel thick-wall sealing head

Large pressure vessels have been a hot spot for research because of their wide use occasions, many kinds of forming processes and high manufacturing difficulties. In addition, the key technologies of quality control in the production process of each forming technology are briefly outlined, and the causes of forming defects such as wrinkles and cracks in several typical forming processes are analyzed and some main control measures are summarized. In addition, the key technologies of quality control in the production process of each forming technology are briefly outlined, the causes of forming defects such as wrinkles, cracks, etc. in several typical forming processes are analyzed and some main control measures are summarized.

As a main pressurized element widely used in boilers, chemical vessels, oil tanks, nuclear reactors, missiles and artificial satellites, etc., heads are gaining more and more attention. The head processing technology is generally divided into integral forming and flap forming. According to the processing method, there are mainly stamping and spinning forming and rarely used explosion forming and gas (liquid) pressure expansion forming [2-4]; the processing technology of split-flap forming generally refers to split-flap pressing and welding forming. At present, the domestic commonly used or integral forming, and some research has been conducted in split-flap forming. The research on these two processes is mainly based on numerical simulation analysis of pressure vessel head forming process to propose a more reasonable and optimized forming scheme or further optimize the forming parameters on the basis of the original process, so as to replace the old production process and realize simple manufacturing of high quality heads. Therefore, this paper mainly reviews the commonly used domestic integral forming process and flap forming process and equipment parameters, and introduces the quality control of forming process.

1. Integral forming process

Integral forming process of thick-walled head of large pressure vessel is the most common preparation process, which includes stamping technology, spinning technology and partial explosion forming technology. Generally speaking, for standard series heads (e.g. diameter Φ1000mm), stamping is generally used; for processing large and medium-sized non-standard series heads (e.g. diameter Φ1025mm) or very large diameter heads (e.g. diameter Φ8000mm), spinning technology is preferred [2].

1.1 Hot stamping technology

Hot stamping of head belongs to the compound forming process of drawing-drawing-expanding-bending of sheet metal. During the whole forming process, the sheet material starts to deform with the contact tangent point of the top point of the convex die under the action of the pressure of the convex die, and when the convex die continues to move downward, the pressing surface gradually moves from the center to the edge of the sheet material, so that the upper surface of the convex die and the sheet material are completely fitted to form the head. As shown in Figure 1, the whole forming area of the sheet (AD area) can be divided into two types of deformation, deep drawing and swelling, due to different deformation properties, where: BC area is the deep drawing deformation area, where the material is subject to tangential compressive stress; CD area is the swelling area, where the material is subject to tangential tensile stress; C is a critical point where the compressive stress is 0. The location of this critical point changes with the change of a series of crimping forces and other conditions [4].
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Fig. 1 Hot stamping of thick-walled head
The whole blank undergoes violent metal plastic rheology under complex deformation, and the stresses, strains and metal flow states generated in different parts of the workpiece are extremely complex, which are very likely to cause the following defects:

  • (1) Wrinkling (internal wrinkling) in the overhanging sidewall part due to insufficient tensile stress in the blank;
  • (2) Wrinkling (external wrinkling) in the flange part due to too much tangential compressive stress [5]; 
  • (3) Wrinkling (external wrinkling) due to the radial and tangential tensile stresses in the sheet (3) dome cracking due to excessive radial and tangential tensile stresses on the sheet, resulting in excessive thinning of the material near the apex of the die [6]. For thick-walled head, because the tensile stress in dome forming is difficult to control, it is more likely to produce defects such as excessive thinning of wall thickness.

Therefore, mastering the metal deformation law and accurately grasping the ratio of drawing depth and expansion bending are the effective control of defects such as wrinkling and rupture in hot press forming of head. The manufacturing process of overall hot stamping and forming of thick-walled head of large pressure vessel is generally as follows: the billet which has been inspected, scribed, cut and notched is heated, then the heated workpiece is centered on the lower die, the edge of the workpiece is pressed with the crimping ring and the press is started, when the punch and the workpiece are in contact, the master cylinder is started to press the punch downward, and the head is finally formed when the workpiece has completely passed the lower die. Finally, the head wrapped on the punch is removed by lifting the punch and the crimping ring at the same time and by the stripping device (blocking iron), and the stamping is completed. In the process of manufacturing thick-walled heads, presses with larger tonnage are often used due to the large size of stamped heads. At present, hydraulic presses and water presses are often used as stamping equipment in the production of pressure vessel head stamping. It mainly depends on the capacity of presses (punching pressure, column pitch, opening height and closing height, etc.), shape and size of head blank and the result of considering its manufacturing economy from head batch.

1.2 Spinning technology

Thick-walled head spinning technology is a new field in the research of head forming technology at home and abroad in recent years. Due to the advanced, economical and practical nature of spinning process and the advantages of less investment in equipment, simple tooling, less material, low production cost and no limitation of head size by tooling compared with stamping [7], spinning has been developed considerably in recent years and has become a new field in metal pressure processing, especially for processing large and medium-sized non-standard series of heads with incomparable advantages of other processing methods. The advantages are unparalleled by other processing methods. However, there are also disadvantages such as large steel thinning, large deviation of head shape, easy to produce cracks during spinning, process hardening and possible aging cracks in the process of use [7]. In the production process, spinning can be divided into cold spinning, warm spinning and hot spinning depending on the temperature. The cold spinning process for manufacturing pressure vessel heads refers to the spinning and forming of rough slabs at room temperature, which has the advantages of economy, speed and flexibility, but due to the limitation of material plasticity, not many head materials can be formed by cold spinning. It is difficult to use cold spinning when the head thickness is thick or the material strength is high; it is easy to produce instability, wrinkling and cracking for forming heads with wall thickness much smaller than the diameter; cold spinning of heads at the edge of equipment capacity will also cause problems such as material delamination and surface micro-cracking. Generally, the metal is heated to a temperature below the recrystallization temperature or forging temperature, which is called warm spinning. Warm spin forming technology is mainly used for forming medium-thick plate heads and heads of some materials that are difficult to be cold deformed. Hot spinning is done by heating the metal to a temperature above the recrystallization temperature or forging temperature. According to different heating methods, hot spinning can be divided into 2 types: one is to heat the slab to a certain temperature first and then spin it on the hot spinning machine; the other is to clamp the slab on the hot spinning machine first, then heat the deformation area of the slab locally and spin it to shape. The head spinning usually adopts the spinning wheel to press the surface of the sheet and form it continuously along the surface of the sheet, Figure 2 shows the head spinning process mechanism [8]. The process of spinning is mainly divided into undercutting – bevel processing – welding – 9 No.2 Hua Kaixuan et al: Analysis of the current situation of forming process and quality control of thick-walled head of large pressure vessels – grinding of weld residual height – pressing drum – hot spinning -Beveling of head -Processing -Shape and dimensional inspection -UT, RT (ultrasonic inspection, radiographic inspection) -Test plate performance test -Warehousing [2].
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Figure. 2 Thick-walled head spinning mechanism
The spinning equipment for forming process is mainly spinning machine, and the capability and accuracy of spinning mechanism for head preparation is one of the signs of its high level. The radial thrust of the spinning wheel of the newly developed large CNC spinning machine in China can reach 1000kN, the diameter of the processed parts can reach Φ2600mm, the spindle speed is 20-100r-min-1, and the diameter of the rotary table can reach Φ2800mm. the test shows that the wall thickness difference of the spinning formed parts on this equipment is only (3±0.06)mm [9].

1.3 Explosive forming of head

Explosive forming is a kind of high-energy rate forming method that uses the explosive blast effect to process the head blank. As its forming process is nearly adiabatic process, the heat generated by plastic deformation does not have time to dissipate, retained inside the forming head, so that the plastic deformation generated by the process of hardening can be partially eliminated, which is one of the advantages of other mechanical forming process can not be replaced. In the forming process, as the explosion of explosives can produce impact force of up to more than 100,000 atmospheres and temperature of thousands of degrees, these high temperatures and pressures are transmitted to the head blank steel plate through the elliptical earth layer, forming a kind of spherical shock wave spreading around, thus producing elliptical plastic deformation and making the head blank reach the required geometric size under the constraint of the explosive tire tool. Explosive forming of heads can be generally divided into two types of forming and non-molding according to the forming tool [10]. Head molding, that is, forming die is machined to have the specific precise shape required for forming and fixed in the middle of the medium, then the head plate material is placed on the mold and fixed, the explosive is placed at a high H distance from the plate material, the surrounding are filled with medium uniformly and densely, through the shock wave generated by the explosive, the blast wave and the gas products generated by the explosion to form the plate material to be formed in close contact with the mold to form the head, as shown in Figure 3a shown [10]. Thick-walled head explosion non-mold forming is also known as free forming, this forming method generally uses an open mold, as shown in Figure 3b [10]. Its spinning equipment with the process of head explosion forming is mainly spinning machine, and the ability and accuracy of spinning machine to prepare heads is one of the signs of its level. The latest domestic large CNC spinning machine spin wheel radial thrust up to 1000kN, processing parts diameter up to Φ2600mm, spindle speed 20 ~ 100r-min-1, rotary table diameter up to Φ2800mm. test shows that the difference in wall thickness of the spinning formed parts on this equipment is only (3 ± 0.06) mm [9]. 1.3 Explosive forming of heads Explosive forming is a high-energy rate forming method that uses explosive blast to process the head blank. Since the forming process is nearly adiabatic, the heat generated by plastic deformation is not dissipated in time and retained inside the formed head, thus allowing the partial elimination of the process hardening that accompanies plastic deformation, which is one of the irreplaceable advantages of other mechanical forming processes. In the forming process, as the explosion of explosives can produce impact force of up to more than 100,000 atmospheres and temperature of thousands of degrees, these high temperatures and pressures are transmitted to the head blank steel plate through the elliptical earth layer, forming a kind of spherical shock wave spreading around, thus producing elliptical plastic deformation and making the head blank reach the required geometric size under the constraint of the explosive tire tool. Explosive forming of heads can be generally divided into two types of forming and non-molding according to the forming tool [10]. Head molding, that is, forming die is machined to have the specific precise shape required for forming and fixed in the middle of the medium, then the head plate material is placed on the mold and fixed, the explosive is placed at a high H distance from the plate material, the surrounding are filled with medium uniformly and densely, through the shock wave generated by the explosive, the blast wave and the gas products generated by the explosion to form the plate material to be formed in close contact with the mold to form the head, as shown in Figure 3a shown [10]. Explosive non-mold forming of thick-walled heads is also called free forming, and this forming method generally uses open molds, as shown in Fig. 3b [10]. The main difference between this method and the head explosion molding is that the explosive, the mold and the shape of the sheet are adjusted according to the different impact forces.
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Fig. 3 Moulding model (a) and non-moulding model (b) for thick-walled head explosion forming

1.4 Die rotary forging method

In terms of integral forging of heads, only a few developed countries such as Japan and France have such integral head forming technology in the international arena [11]. The more advanced method of integral forging of large thick-walled heads proposed in China is the partial concave die forging method proposed by Professors Ren Yunlai and Nie Shaomin of Yanshan University [12], see Figure 4 [11]. In this method, the pre-drawn formed billet is placed on a convex die that can be rotated at intervals, and the convex die is rotated at staggered intervals every time the upper partial concave die is pressed down; the upper partial concave die is pressed down continuously causing the thickness dimension of the head to decrease and its diameter to increase until it is finally forged and formed. The above forging method can forge heads with the largest possible size on the existing hydraulic press, but cannot forge heads with flanges. Guo Changli et al [11] of Taiyuan University of Science and Technology used numerical simulation technology to simulate the rotary forging process of flanged heads, which first upsets the flat blanks and then rotates and crushes them at staggered intervals through a special half-moon anvil upper die, and every time the half-moon anvil upper die is pressed down, it rotates at staggered intervals until the blanks on the lower die are crushed into shape, and finally the crushed heads are flanged and pressed to determine the final shape of the head is determined.
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Fig. 4 Schematic diagram of partial concave die rotary forging method
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Fig. 5 Schematic diagram of equal rotary down forging method
In both partial concave die rotary forging method and half-moon anvil equal rotary down forging method, the metal in the bottom area is subjected to additional stretching of the metal on both sides, which can lead to the thinning of the head, so the forming quality can be better controlled by changing the rotation angle of the convex die and the amount of die down pressure. Several forming processes of thick-walled heads above have their own characteristics: stamping to form whole one-time deep-drawn large-diameter heads often requires large tonnage and large table press equipment as well as bulky and expensive molds, so the cost is higher, but the efficiency is high and it is suitable for mass production; spinning method can use general-purpose spinning machine without special molds (tireless spinning), so the cost is relatively low and the efficiency is lower than stamping. process; explosion forming method has the advantage of simple die structure, no need for upper die and large stamping equipment, low cost, especially for some difficult to deform the metal, the disadvantage is low efficiency, only applicable to single-piece production, after forming often also need to calibrate the shape [13]; rotary die pressing method, is a newer process used at home and abroad, because the method omits the ingot rolling into steel plate process, production It has the highest efficiency [14], and the mechanical properties of the formed head are basically isotropic and have excellent internal quality.

2. Flap forming process

In the manufacturing of large thick-walled pressure vessel heads, the head blanks are also often formed by flap pressing and welding. This kind of head consists of a large spherical top and multiple curved flaps, and the spherical top and curved flaps should be cold pressed and formed first; then prefabricated bevels are made, and finally group welding is carried out on the welded body, the schematic diagram of which is shown in Fig. 6.
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Figure. 6 head split flap welding and forming
The process flow [15] is as follows: confirming the slab material of the welded head – roughing down according to the shape of the head – hot stamping forming – heat treatment – ultrasonic inspection – scribing cutting and pressing – gas cutting Balance – Processing of weld bevel – Assembly of head – Welding of head – Appearance inspection, correction and NDT magnetic particle inspection. However, there are disadvantages such as complicated design, calculation and material removal of head slice, long process route, many kinds of dies, high cost, large welding workload, and not easy to guarantee accuracy after welding.

3. Key technologies of forming quality control

3.1 Control of head mold forming defects

The traditional production process of head is hot stamping forming, in which the process of violent deformation of billet will lead to serious instability of head stamping deformation after forming, which is mainly manifested in two aspects: wrinkling and rupture, which is mainly caused by the uncoordinated ratio of drawing depth, expansion and bending of forming conditions in stamping process, that is, the stress and strain states of each part of forming process are different. The proportion of deepening will cause wrinkling, expansion and bending will easily form thinning and even rupture of the head. For hot stamping forming of thick-walled heads, since the wall thickness is very large and not easy to produce pulling crack, more attention is paid to how to favorably transform the excess metal in numerous triangular areas on the flat blank into spherical heads to reduce the thinning and punching pressure in the larger area near the top of the sphere so as not to make the thinning super poor and pulling crack due to excessive radial tensile stress [16]. Therefore, in the actual production of thick-walled heads, since most materials are in overhang during the forming process, the solution of hot stamping defects of thick-walled heads mainly focuses on how to avoid wrinkling in the overhanging area and flange area, and thinning and pulling crack in the top area of expansion.

  • (1) The factors affecting wrinkling are: control of crimping force, friction conditions, stamping speed, blank size [17], in addition, the critical C point position in Fig. 1 can be shifted to the flange direction by setting deepening tendons and changing the structure of spherical convex die;
  • (2) The measures to solve the dome thinning and cracking are: appropriately grasp the drawing direction in the drawing process, increase the contact surface between the convex die and the blank, as well as reasonably arrange the crimping, uniform In the design of the die, the drawing tendons are reasonably set, the rounded corners are appropriately increased, and the focus is on the strength and deformation of the material itself, the gap between the upper and lower dies, the geometry, the process parameters (heating temperature and crimping force size), etc. [18]. In the literature [19], the finite element simulation of the hot drawing process of large head is combined with the energy method in macroplasticity theory and the theoretical model for the calculation of drawing force is established, which verifies the correctness of the established theoretical model and thus provides a method and theoretical basis for the study of effective process control in the hot drawing process of large head. The literature [20] studied hot stamping and forming of spherical heads and discussed the design of forming dies, determination of process parameters, selection of forming equipment and changes of head dimensions after forming. For the integral forging process, the uneven wall thickness of the formed head is mainly caused by the thinning of the head due to the additional stretching of the metal in the bottom area by the metal on both sides, so the elimination of the bottom thinning amount mainly relies on changing the number of downward presses of the upper die and the contact area with the billet metal. In the literature [21], by optimizing the upper anvil rotary forging integral head process through simulation and analysis, the shape of upper anvil for forming large thick-walled heads is determined to be saddle-shaped, and the single circumferential pressing amount of the motion trajectory of the upper anvil and the rotation angle of the upper anvil after each pressing are determined, which effectively optimizes the wall thickness variation in the forming process of thick-walled heads and improves the forming quality of forged heads.

3.2 Control of wrinkling in spinning process

The thick-walled head spinning process is very easy to produce wrinkles, and the main influencing factors are:

  • (1) Blank size and material properties;
  • (2) Key equipment parameters such as shape of spinning wheel (diameter and radius of corner), gap between core die and spinning wheel of spinning wheel installation angle;
  • (3) Process parameters such as spinning temperature, number of spinning passes, trajectory of the spinning wheel, degree of deformation and the point reached by each pass of the spinning wheel, feed volume and feed speed and heat treatment [7, 22], which can also lead to difficult control and consistency of forming dimensions if these factors are not selected properly.

Especially for some heads with small thickness to diameter ratio, as their wall thickness is much smaller than the diameter, it is very easy to produce instability, wrinkling and cracking during spin forming. Since most of the spinning process parameters of thick-walled heads are still selected empirically [22], it causes difficulties in selecting forming parameters on the one hand, and has a significant impact on product quality on the other. In the literature [23], the influence of spin wheel parameters on the forming process was studied in depth and the relationship between spin wheel parameters and wall thickness of head spinning was obtained by finite element model analysis, which is conducive to improving the non-flattening degree and roundness of the head by increasing the spin wheel feed ratio, choosing suitable spin wheel fillet radius or spin wheel mounting angle; reducing the spin wheel feed ratio, decreasing the spin wheel fillet radius and increasing the spin wheel mounting angle can improve the flange deformation Reducing the feed ratio of rotary wheel, decreasing the radius of rotary wheel angle and increasing the mounting angle of rotary wheel can improve the flange deformation and even eliminate the defects such as expansion and material bulging, thus making the wall thickness distribution of head more uniform.
In the literature [24], the influence of process parameters such as different trajectory of pass curves and spin wheel mounting angles and feed ratios on the quality of multi-pass deep drawing and spinning was investigated and analyzed through experimental comparison. The results showed that the most uniform wall thickness of the spun parts was obtained by using the rotary wheel trajectory with convex and concave round-trip feed without die advancement, and the larger the installation angle and feed ratio of the rotary wheel trajectory, the more likely the workpiece was wrinkled.

3.3 Explosive forming process parameters

Select some important parameters of the explosive forming process improper selection of the head forming success or failure has a great impact, explosive forming process with the advancement and diffusion of shock waves, pressure energy is a gradual change in the process. The height of explosive position can also be changed according to the thickness and diameter of the plate, so only by coordinating and calculating the corresponding value can a head with qualified quality be formed. In order to better control the quality of explosively formed heads, attention should be paid to control the height of explosive position, depth of medium, amount of explosive and size of plate material related to the forming degree of each part of heads; in addition to the influence of the above parameters, reasonable selection of parameters such as tightness of explosive package, shape and finish of pull ring and pressing force of crimp ring should also be made. Explosive forming explosives are mainly plastic explosives, flexible explosives, general industrial explosives, etc. The detonation performance, burst speed and mechanical properties of different formulations of each type of explosives are also different, and should be selected according to the shape of the head piece and the use of performance [25]. Especially for the choice of crimping conditions is important. Explosive forming crimping device is often used crimping ring, through the adjustment of the crimping ring and its accessories clamps on the plate material to apply crimping force size also directly affects the quality of the parts, such as forming depth, the amount of thinning of the top, internal and external wrinkles. Increase the crimping force, it will increase the expansion ratio of explosive deep-drawing parts, thus increasing the amount of thinning, forming depth decreases, but help prevent internal and external wrinkles. Decrease the crimping force, the opposite situation. In practical engineering applications, the crimping force is not easy to grasp, often based on experience [26].

3.4 Crack prevention in welding process

Cracking is the most important welding defect in welded joints in the process of head splicing, which is extremely harmful and one of the causes of sudden damage to welded structures and containers causing catastrophic accidents many times. Therefore, by choosing suitable welding materials, formulating reasonable welding process and selection parameters and conducting welding process evaluation [27], defects such as weld cracks can be prevented to the greatest extent so as to control the quality of assembled welded heads. Preheating before welding, layer temperature control, post-heat hydrogen elimination treatment and heat treatment are important measures to prevent welding cracks in the welding process.

  • (1) Preheating and layer temperature control before welding. Before welding and welding process of heating range is small, easy to cause the preheating temperature is not enough and uneven; preheating temperature control is not good or welding process of preheating temperature is not effectively monitored, are very likely to cause cracks. The cooling rate of the welded parts, the temperature difference between the welded area and the welded parts, the welding strain rate and the constraint of the welded structure are related to the preheating before welding [28].
  • (2) Post-heat hydrogen elimination treatment. Post-weld timely post-heat hydrogen elimination treatment is conducive to preventing cold cracking, accelerating the escape of diffused hydrogen from the weld and heat-affected zone in the molten metal, and effectively reducing residual stresses to a certain extent.
  • (3) Post-weld heat treatment. After the plate is stamped and formed, the quality of the formed head and the performance of the weld seam are better guaranteed, and the proper heat treatment process parameters are used to process the welded joints, so that the mechanical indexes of the welded joints are greatly improved, and a better match of strength, hardness and impact toughness is achieved while improving the strength and eliminating the weld stress, so as to obtain a better overall performance and thus avoid the appearance of cracks as far as possible [14].

4. Conclusion

There are many kinds of forming processes for large thick-walled pressure vessel heads, but each process has its own characteristics. When choosing the process, we should pay attention to the feasibility of the process and also consider the selection and adjustment of each parameter in the process as much as possible. Through the analysis of the above process and quality control, we can give some help to the head forming process.

Author: Hua Kaixuan, Yu Xiaolu, Wang Ke Zhi

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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