Forming of single layer welded barrel joint
What is the barrel joint of single-layer roll welded pressure vessel?
Single-layer volume welding pressure vessel cylinder section, by the steel plate by scribing, under the material (such as steel plate width is not enough, but also from the first small plate welded into a large plate), in the coil machine rolled into shape.
This structure due to the manufacturing process and tooling simple, high production efficiency, wide range of applications, reliable quality, not only the most widely used, but also become a variety of other structures (such as multi-layer wrap, lap, steel belt around the wrong etc.) pressure vessel cylinder section or cylinder body forming basis.
Rolled into shape, is the process of continuous three-point bending of the plate with the coil machine.
Coiling machine has three rollers, four rollers and horizontal, vertical points.
Rolling temperature has a cold volume, hot volume, warm volume.
Briefly described below.
General three-roller rolling machine working principle as shown in Figure 15.6.1-1.
The three rollers of the coil machine according to the triangle arrangement, the upper rollers can be lifted, the lower rollers used to transport steel.
This kind of plate rolling machine is called symmetrical three-roller plate rolling machine, it is simple in structure, cheap, in the medium and small size container manufacturing in common application.
Its main shortcomings, is a straight edge at each end of the plate can not be bent.
The straight edge section is handled by the following methods.
Figure 15.6.1-1 symmetrical three-roller plate rolling machine working sketch
(1) reserved straight edge section to be rolled round after the straight edge section will be removed, see Figure 15.6.1-1.
(2) Pre-bend the straight edge section with a suitable curvature die under the press, see Figure 15.6.1-2.
1 – lower die; 2 – steel plate; 3 – upper die
Figure 15.6.1-2 pre-bending using a press
(3) the use of curvature appropriate and sufficiently rigid template pressure pad on the coiling machine for pre-bending, as shown in Figure 15.6.1-3.
1 – the lower roll; 2 – pad; 3 – steel plate; 4 – the upper roll
Figure 15.6.1-3 using a three-roller coil pre-bending
(4) using the point-by-point press bending method for pre-bending, as shown in Figure 15.6.1-4.
1 – pressure head; 2 – steel plate; 3 – support
Figure 15.6.1-4 pre-bending using the point-by-point press bending method
All of the above methods require additional equipment and tooling, increasing the complexity of the process or causing material waste.
Figure 15.6.1-5 is the working principle of the symmetrical four-roller rolling machine, the upper roll for the active roll, the lower roll can be adjusted vertically up and down to move, both sides of the auxiliary roll can be independently adjusted position.
In this machine can complete the pre-bending and roll round all the work.
It has the advantage of high precision, easy to mass production, high efficiency, the disadvantage is the higher cost of equipment.
China’s large pressure vessel manufacturing plants have introduced 8m four-roller (horizontal) plate rolling machine; cold volume maximum thickness of about 40mm, hot volume maximum thickness of 100mm ~ 120mm, can roll the minimum inside diameter of the cylinder section for 920mm (its main roller diameter of 900mm), can roll the maximum length of the cylinder section for 8m.
8m plate rolling machine is developed for the steel plate perpendicular to the rolling direction rolled using electroslag welding of the long cylinder section, in fact, very few domestic use of this long cylinder section.
Roll roll is too long, in order to ensure its stiffness to increase the roll diameter, due to roll are forged with alloy steel, which not only makes the roll plate machine costly, and can not use its roll small diameter barrel section.
Figure 15.6.1-5 four-roller plate rolling machine working principle diagram
Later developed a successful pre-bending plate material three-roller rolling machine, called asymmetric three-roller rolling machine, its two lower rollers can be separate for vertical direction movement, with the method of lifting the lower rollers on the steel plate pre-bending, its working principle, see Figure 15.6.1-6. This kind of rolling machine structure is simple, due to less a roll, the cost is much lower than the four-roller rolling machine.
Figure 15.6.1-6 asymmetric three-roller plate rolling machine working principle diagram
In recent years, through technical improvements and development, China’s own design and manufacture of a variety of types and pre-bending large, medium and small CNC three-roller rolling machine steadily pushed into the market, to meet the rapid development of China’s pressure vessels and other manufacturing industries (the number of manufacturing units to increase the number of vessels and other products large scale) needs.
The common feature of these three-roller rolling machine is to roll the plate when the three rolls to the positive triangle arrangement, pre-bending to the asymmetric arrangement.
Among them, the small and medium-sized up-roller universal three-roller scrolling machine because of low cost, compact structure, in the manufacture of medium and small containers and other widely used; horizontal down-regulation three-roller scrolling machine because of the strong rolling capacity and mainly used in the manufacture of large containers and so on.
Four-roller scrolling machine technology is also faster, horizontal downward adjustment type four-roller scrolling machine, the upper roll universal type four-roller scrolling machine because of its own advantages in wind power, automobile manufacturing, construction machinery, offshore oil platforms and other bulk manufacturing industry applications more, the container industry because of a single piece of small batch manufacturing and less application.
At present, China’s pressure vessel manufacturing industry has a large number of cold rolled steel plate thickness of 150mm or more large CNC coil machine, ranking steadily in the world.
Horizontal downward adjustment type CNC coil machine rolled thick plate, extra-thick plate capacity has been repeatedly improved, the current cold roll capacity has reached 360mm × 3500mm (plate thickness × plate width), some technical indicators to reach the advanced level of similar international products.
With the continuous integration with foreign technology, the fully hydraulic horizontal down-regulated CNC three-roller coiler will become the mainstream model in the future market.
With the rapid development of tanker, storage tank, steel pipeline and other industries, China’s small coil machine will develop in the direction of thin edge, extra-long type, small diameter, multi-curvature and CNC.
With the continuous improvement of technology and the application of fully automatic intelligent terminals, a variety of large, medium and small coiling machine is moving towards the direction of intelligent equipment.
All of the above introduced are horizontal coiling machine.
The common defect of all kinds of horizontal coiling machine is: when the hot roll, with the bending deformation of the steel plate when heating the oxide skin has fallen off, due to the high hardness of the oxide skin, in the rolling process is pressed into the steel surface, it is difficult to deal with, its indentation destroyed the surface quality of the cylinder section.
Vertical plate rolling machine is no such problem, in addition, vertical plate rolling machine rolled wall thickness of thin large diameter barrel section, the steel plate will not collapse due to its own rigidity and lack of collapse.
The shortcomings of the vertical plate rolling machine is that the steel plate in the rolled friction with the ground, the large diameter thin-walled barrel section may cause the upper and lower arc inconsistency.
According to the capacity of the plate rolling machine, the strength of the steel, thickness and the size of the diameter of the cylinder section, the rolling temperature is divided into room temperature, high temperature, medium temperature, that is, cold volume, hot volume, warm volume three.
Cold roll, that is, the rolled barrel section at ambient temperature.
Cold roll has accurate shape and size, easy to operate, low cost, steel rolled when its surface quality is not damaged (no oxide indentation) and other advantages, in addition, because the cold roll will not cause the thickness of steel thinning, so as long as the steel thickness δs (i.e., feeding thickness), can also ensure that the actual thickness after forming is not less than the design drawings marked minimum forming thickness or nominal thickness.
Therefore, as long as the conditions permit should be preferred to cold roll.
Hot roll, that is, in the recrystallization temperature above the rolled barrel section.
More than the coil machine cold roll, warm roll capacity of thick steel plate need to be hot roll, rolled usually need to be heated to forging temperature (generally in about 1000 ℃, depending on the steel and tooling process level and slightly different), so that the steel plate has a better plasticity, easy to roll.
The main shortcomings of hot rolls are.
- (1) heating increases the manufacturing cost.
- (2) high-temperature rolling is not easy to operate.
- (3) oxidation is serious, oxidation skin indentation will make the surface quality damage.
- (4) hot roll will lead to thinning of the thickness of the steel plate, the length becomes longer.
GB150-1998 only puts forward the following principle requirements: determine the processing margin according to the manufacturing process to ensure that the thickness of the convex head and hot rolled cylinder section after forming is not less than the nominal thickness of the part minus the negative deviation of the steel plate.
In order to meet this requirement, the manufacturing unit often has to purchase steel plates with thickness specifications greater than the nominal thickness, and the difference between the two is commonly known as the second rounding.
In the design process, the design thickness plus the negative deviation of the steel thickness upward rounding to the standard specification thickness of steel (i.e., nominal thickness) rounding amount, commonly known as the first rounding.
The first rounding amount in the design of the opening reinforcement can still be used as reinforcing metal, while the second rounding amount in many cases can only lead to a waste of material.
In order to avoid this phenomenon, GB/T150.1 requirements: the nominal thickness of the container components and the minimum forming thickness should generally be marked on the design drawings; GB/T150.4 also requires: manufacturing units should be based on manufacturing processes to determine the processing margin to ensure that the actual thickness of the pressurized components after forming is not less than the minimum forming thickness marked on the design drawings.
It should be said that the requirements from the standard (GB/T150.1) has solved the problem of the second rounding, but some design units of the design drawings are often not marked cylinder minimum forming thickness, at present in China’s pressure vessel design, manufacturing mostly separate, that is, in the design, manufacturing were completed by two units, the second rounding phenomenon is difficult to completely avoid.
If the design drawings marked the minimum forming thickness, not only can avoid the second rounding, the first rounding amount may also be used as part of the thickness of steel.
If the designer based on design experience and manufacturing experience, in the design of the need to consider the amount of processing thinning, and in the drawings are marked and explained, the manufacturing unit in the purchase of steel thickness (i.e., the thickness of the material) can also avoid the phenomenon of the second rounding.
Temperature volume, that is, between the cold volume and hot volume temperature rolled barrel section.
It seeks to avoid cold, hot volume of the short, take the long of the two, the steel plate heated to the recrystallization temperature below (heating temperature according to the steel to determine, for carbon steel, low alloy steel, heating temperature generally does not exceed 500 ℃) of a temperature after the roll, not only can make the steel plate to obtain a slightly larger plasticity than the cold state, but also to avoid serious oxidation of the steel surface.
For cold rolled barrel section, attention should be paid to the need to restore the performance of heat treatment.
Cold-formed carbon steel and low-alloy steel, will be cold hardening and make the strength rise, plasticity, toughness, certain high-alloy steel (such as non-stable austenitic stainless steel, austenitic-ferritic stainless steel) will also be cold hardening and organizational changes, corrosion resistance is reduced, so the deformation of the barrel section should be heat treatment after forming, in order to restore the performance lost due to cold hardening.
On the non-stable austenitic stainless steel, austenitic-ferritic stainless steel cold deformation of the organizational changes, see the literature “pressure vessel stainless steel”.
Limited by China’s steel smelting and rolling technology level and other factors, GB150-1998 using the relative deformation as a judgment of cold forming or warm forming of the cylinder whether the need to restore performance heat treatment, and limited to carbon steel and low-alloy steel, and the amount of deformation between the two and even low-alloy steel deformation limits are not the same.
The degree of cold hardening (that is, the degree of loss of material properties) and the type of material and the size of the deformation, but the need to restore performance heat treatment after cold forming or warm forming, but also with many other factors, such as the nature of the medium, the thickness of the material and the amount of forming thinning, performance, etc..
According to China’s steel manufacturing level and years of engineering practice, with reference to the United States ASME and other foreign standards and norms, GB/T150.4 using the size of the deformation rate as a judgment of cold forming or warm forming of the cylinder whether the need to restore performance heat treatment based on the requirements of cold forming pressure components deformation rate of more than 5% of carbon steel, low-alloy steel, deformation rate of more than 15% of austenitic stainless steel (when the design temperature below -100 ℃, or higher than 675 ℃, the deformation rate control value of 10%), meet any one of the following conditions, should be formed after the heat treatment to restore the performance of the material.
- (1) containers containing extremely toxic or highly hazardous media.
- (2) the drawings indicate that there is stress corrosion of the container.
- (3) for carbon steel, low-alloy steel, thickness greater than 16mm before forming.
- (4) of carbon steel, low-alloy steel, after forming the thinning amount greater than 10%.
- (5) for carbon steel, low-alloy steel, materials required to do impact testing.
Tube section forming for one-way stretching, see Figure 15.6.1-7. deformation rate (%) = 50δ [1-(Rf/Ro)]/Rf (δ for the plate thickness, that is, the thickness of steel, also known as the thickness of the material, mm; Rf for the forming of the radius of the middle surface, mm; Ro for the forming of the radius of the middle surface before (for the flat plate ∞), mm.
When step-by-step cold roll forming, if no intermediate heat treatment is carried out, the deformation rate of the formed part is the sum of the deformation rate of each step forming; if intermediate heat treatment is carried out, the sum of the deformation rate of the formed part before and after intermediate heat treatment is calculated respectively.
Figure 15.6.1-7 One-way stretch forming
Cold forming to restore performance heat treatment, for carbon steel and low alloy steel, restore material performance heat treatment for recrystallization annealing treatment; for high alloy steel (such as austenitic stainless steel, austenitic – ferritic stainless steel, ferritic stainless steel), restore material performance heat treatment to restore the material supply state heat treatment.
GB/T150.4 stipulates that: if hot forming changes the material supply heat treatment state, should be re-heat treatment to restore the material supply heat treatment state;.
GB/T150.4 also provides: When the use of materials required to be consistent with the heat treatment state of the supply, the entire manufacturing process shall not destroy the heat treatment state of the supply, otherwise the heat treatment should be repeated.
For example, with the steel manufacturing unit to provide tempered steel (quenched + tempered) manufacturing containers, the choice of manufacturing units should pay attention to examine the ability to roll, such as the Department of hot roll, hot forming not only more than the recrystallization temperature, but also will certainly exceed the original tempering temperature, the destruction of the steel tempering state, the manufacturing unit should be re-quenched treatment, which not only causes waste, and most manufacturing units of heat treatment equipment and Technical level and steel manufacturing units compared to a certain gap.
Of course, if the hot forming temperature does not exceed the tempered steel (quenched + tempered), normalized + tempered steel original tempering temperature or normalized steel after welding heat treatment temperature, that is, not change the material supply heat treatment state, may not restore the material supply state heat treatment, but if the material performance degradation or reduced, should be restored to the material performance heat treatment or material supply state heat treatment.
With the current roll plate machine roll capacity, such cases are not many, manufacturing units according to the ability to roll plate machine, either cold roll or lower temperature under the temperature roll, recrystallization annealing treatment, or (destruction of the material supply state) hot roll, to restore the material supply state heat treatment.
For warm volume of the barrel section, forming temperature is lower, cold hardening effect still exists, it is appropriate to refer to GB/T150.4 cold forming pressure components of the heat treatment conditions and requirements to determine whether the need for restoration of performance heat treatment after forming.
Forming temperature is higher (but does not exceed the recrystallization temperature), if the material performance deterioration or reduced, should be restored to the material performance heat treatment or material supply state heat treatment to restore material performance.
Cylinder section cold roll, warm roll after forming the need to restore material properties through heat treatment, or warm roll, hot roll after forming the need to restore the material supply heat treatment state through heat treatment, should be prepared according to TSG21, GB/T150.4 conditions of the parent material heat treatment specimens and test samples and sample inspection and evaluation.
Rolled cylinder and processing of longitudinal weld bevel can be applied after the longitudinal weld (is a class A welded joints), so that the production of a cylinder section is basically complete.
Processing of both ends of the cylinder section of the circumferential weld bevel, with the circumferential weld (a class B welded joints) will be several cylinder section and the upper and lower heads, etc. assembled together, that is, constitute a pressure vessel shell.
In the round, longitudinal, circumferential welding process, certain shape and size deviations will inevitably occur, such as A, B class welded joints of misalignment, welded joints and axial angles, shell straightness and roundness, etc.. The following are the reasons for their generation, preventive measures and the main requirements of the standard (GB/T150.4).
The misalignment of the welded joints of the cylinder section A
Barrel section A class welding joint misalignment, manifested as the joint on both sides of the steel plate failed to align, it is a local abrupt change in shape, will cause additional bending moment.
Rounding quality is not high is the main reason for misalignment.
In the barrel section A welding joint before welding, you can use some correction tools to make the wrong edge in a certain degree to be corrected, as shown in Figure 15.6.1-8, in the barrel section on the welded Г-shaped iron or door-shaped iron, into the bevel wedge to force the bevel alignment after spot welding fixed.
It should be noted that the quality of pre-bending directly affects the quality of rounding.
Thick-walled barrel section pre-bending caused by the wrong side of the counterpart is more difficult to correct, need to try to avoid.
Because of the improvement of the coil machine capacity and roll technology, according to the strength of the steel plate, thickness, tube section diameter size and manufacturing experience, etc. may not be pre-bending.
The stress of the inner wall of the container is greater than the outer wall, theoretically speaking, the wrong side of the amount should be measured in the inner wall, but taking into account the thin-walled container inside and outside the wall stress difference is not large, in order to facilitate the operation, the wrong side of the barrel section is generally measured outside the wall, as shown in Figure 15.6.1-9 in the above figure.
The qualified index of the wrong side should be controlled by two aspects together: one is the percentage of steel thickness, one is the absolute value, to avoid the wall thickness of the container wrong side is too large.
Figure 15.6.1-8 correction of the wrong side Figure 15.6.1-9 A class of welded joints against the wrong side
Composite steel containers are different, it is not only required in the inner wall (i.e., cladding side) detection (see Figure 15.6.1-9 in the following chart), and the amount of misalignment of the mouth of the qualified indicators are also much stricter than other containers, the main purpose is to prevent the impact of the cladding material anti-corrosion effect due to excessive misalignment.
Welded joints formed in the circumferential angles
The angularity of the welded joint is a local shape change caused by uneven shrinkage of the weld metal during cooling, such as becoming peach-shaped (convex) or waist-shaped (concave), which leads to the generation of additional bending moment and the emergence of local stress in the cylinder.
The angles formed in the ring direction of the welded joint can be corrected as far as possible using some of the following methods.
One is to take certain measures in the welding, such as the choice of X-shaped or double U-shaped bevel, the inner wall welded one after the outer wall and then welded one, and so on until welded full, so that the contraction deformation of the inner and outer welding channel as far as possible to offset each other, reduce the angles.
This measure will increase the difficulty of bevel processing and welding.
Second, the A class welding joints after the completion of the welding rounding.
Rounding in the coil machine, so that the welded joint parts in the coil machine repeatedly bending deformation, in order to minimize the effect of angular, and roll round the same, rounding is also divided into cold school, temperature school, hot school. The use of electroslag welding joints should be hot school, hot school is equivalent to the same time the electroslag welding seam normalized.
As with the wrong side, the corner of the qualified indicators are also controlled by the percentage of steel thickness and the absolute value of the two aspects together.
According to the outer convexity or concavity of the corner, respectively, using the inner or outer sample plate to measure the size of the corner.
The diameter of the inner and outer sample plate circular part should be equal to the inner diameter and outer diameter of the cylinder respectively.
Obviously, the size of the sample plate (i.e., the chord length of the sample plate) on the angular measurement results are influential, therefore, the standard (GB/T150.4) on the sample plate chord length is specified as 1/6 inner diameter Di and not less than 300mm, see Figure 15.6.1-10, to ensure comparability of measurement results.
Figure 15.6.1-10 circumferential angles at the welded joint
The misalignment of the B class welded joints
The reasons for the misalignment of the butt of Class B welded joints are often related to the following factors.
- (1) The straight edge section of the head (no straight edge section refers to the port, same below) and the adjacent cylinder section or the inner circumference between the adjacent cylinder sections are not consistent.
- (2) The straight edge section of the head or the cylinder section is not round.
- (3) There is local concave and convex deformation at both ends of the straight edge section or cylinder section of the head.
However, since it is difficult to measure the inner circumference of the straight side section of the head and the cylinder section, in fact, the outer circumference of the straight side section of the head and the cylinder section is measured for the convenience of operation to check the consistency of the inner circumference and avoid the resulting alignment of Class B welded joints. The misalignment of edge is exceedingly poor.
It is difficult to correct the misalignment formed by inconsistent inner circumference between straight edge section of head and adjacent cylinder section or adjacent cylinder section. Therefore, various processing allowance reserved, including factors affecting inner circumference such as elongation of steel plate during hot coiling and welding shrinkage, should be precisely considered when scribing, and carefully controlled in the process of material removal, bevel processing, welding and forming.
Before the promulgation of special standard for head for pressure vessel (JB/T4746-2002), in order to control the misalignment between head and adjacent cylinder section, the vessel manufacturing unit often has to wait for the head to arrive at the factory and then undercut the adjacent cylinder section according to the measured value of the outer circumference of the straight side section of the head. .
JB/T4746-2002 improves the requirements on forming dimensions of head on the basis of the special standard for head instead. Since its promulgation and implementation, the cylinder section of container manufacturing unit does not need to match with the size of head, and the amount of misalignment between head and adjacent cylinder section can be guaranteed to avoid the above unreasonable phenomenon.
The current pressure vessel head standard (GB/T25198-2010 “Pressure Vessel Head”) follows the requirements of JB/T4746-2002 on forming size of head, etc. and reduces the tolerance of outer circumference and inner diameter of part of head based on nominal diameter and material thickness of head, and also requires the use of full-size sample with clearance. The shape deviation of ellipsoidal, disc and spherical heads is checked by full-size sample with clearance, and the forming dimension of heads is better guaranteed.
In a word, keeping the consistency of the inner circumference of the straight side section of the head with the inner circumference of the adjacent cylinder knuckle and the consistency of the inner circumference between each adjacent cylinder knuckle as much as possible is the basis for reducing the amount of misalignment of the butt joint of class B welded joints.
The incompatibility between the straight edge of the head and the cylinder section is also the reason for the misalignment of the B class welded joints, and the support can be added inside the head to force the support round when needed, and the support can be removed after the B class joints are properly welded.
Some manufacturing units in the group of B class joints, the long axis of the adjacent cylinder section and the long axis (short axis and short axis) relative to each other, this practice can reduce the B class welded joints of the wrong side, but the group welding after the completion of the roundness of the container has a negative impact.
Foreign advanced assembly process is the long axis of the cylinder section and the short axis of another cylinder section relative to each other, the specific practice is: the outer wall of a point of the two adjacent cylinder sections will be aligned and spot welded, and then the use of hydraulic clamps, clamps, rib plates and other methods to align the outer wall of another point and spot welded, and so on to finally complete the assembly of B class joints.
The advantage of this practice is not only to reduce the wrong edge of the counterpart, but also because the two adjacent cylinder section long axis to short axis assembly, assembly of the long axis becomes shorter, short axis becomes longer, to ensure the roundness of the container.
The use of this practice should pay attention to two points: First, the process of spot welding should be assessed qualified process, and the use of measures to make the spot welding metal into a qualified part of the product weld; Second, the removal of clamps, pulling ribs should be repaired after the surface of the weld marks, if necessary, should also be magnetic particle or penetration testing to determine the repaired surface without impermissible defects.
As with Class A welded joints, the qualification index of the butt misalignment of Class B welded joints is also controlled by both the steel thickness percentage and the absolute value.
Class B welded joints of the wrong side of the mouth, generally should be measured on the outer wall of the container, while composite steel plate containers should be measured on the inner wall of the container (i.e., the cladding side), and qualified indicators are stricter than the general container, see Figure 15.6.1-11.
Figure 15.6.1-11B type of welded joints against the wrong side of the amount
There are two ways to deal with the wrong edges of class A or B welded joints of cylinder body and head caused by unequal thickness: one is to trim the edges of thick plates before grouping the cylinder section or head, and the other is to use overlay welding to bevel the edges of thin plates after grouping, generally using the trimming method.
Three points should be noted when adopting overlay welding method.
First, the process of overlay welding should be assessed as a qualified process.
Second, when welding the bevel of the circumferential weld, while surfacing the edge of the thin plate, or welding the circumferential weld (A or B class welded joints) and then surfacing the edge of the thin plate to facilitate nondestructive testing and post-weld heat treatment.
Third, the overlay parts need to be all (100%) nondestructive testing, nondestructive testing methods and radiation or ultrasonic testing technical grade, qualified level with the A or B class welded joints, overlay parts also need to use ultrasonic testing methods to check the fit according to the requirements of the overlay welding, etc., to determine the overlay metal and its intersection with the base material does not allow the presence of defects.
Welded joint axial formation of the angles
Welded joints axial formation of the angles, is a class B welded joints due to uneven post-weld shrinkage and deformation, as shown in Figure 15.6.1-12.
As with the angles formed in the circumferential direction of the welded joint, the contraction deformation of the inner and outer weld channel should be offset as much as possible to reduce the angles formed in the axial direction of the welded joint when welding B class joints.
Figure 15.6.1-12 axial formation of angles in welded joints
GB/T150.4 stipulates that the length of not less than 300mm straightedge to check the axial formation of the angles of welded joints, and its eligibility index is also controlled by the percentage of steel thickness and absolute value of the two aspects together.
The roundness of the shell
The design calculation of cylindrical container (stress-strain analysis) is based on the cross-section of its shell is circular.
Due to the inevitable existence of certain errors in manufacturing, the cross-section of the shell can not be a geometric circle, the actual shape of the cross-section and the difference between the geometric circle called the shell of the roundness, it is clear that the greater the shell of the roundness, after bearing pressure on the shell wall of the actual stress level and the state and theoretical calculation of the value of the difference is also greater.
The standard roundness of the concept to measure the shell of roundness.
The concept of roundness tolerance in GB/T1182-2018/ISO1101:2017 “Product Geometry Technical Specification (GPS) geometric tolerances shape, direction, position and runout tolerance marking” standard means that when a single measured element is between two concentric circles with radius difference less than or equal to the given tolerance value, its roundness is qualified.
The concentric position and radius values of these two circles depend on their radius difference being the smallest possible value.
Here for the convenience of detection with diameter difference instead of radius difference, that is, the so-called shell roundness refers to the shell of the same section of the maximum inside diameter and the minimum inside diameter difference, as shown in Figure 15.6.1-13, some of the previous standards will be roundness called ellipticity, which is not appropriate, the characteristics of the ellipse is the long axis (maximum inside diameter) and the short axis (minimum inside diameter) perpendicular to each other, and the shape of the shell cross-section is not necessarily so.
Figure 15.6.1-13 roundness of the shell
Some previous standards require checking the roundness of the barrel section.
The roundness of the cylinder section and the roundness of the shell are two different concepts, the role of the two is different, the timing of the inspection is also different.
The purpose of checking the roundness of the barrel section is to reduce the amount of misalignment, as described in the section of B class welded joints with wrong edges, it is measured after welding the barrel section A class joints and rounding, is the intermediate control link of the manufacturing process; check the roundness of the shell, the purpose is to make the actual stress level of the shell and the state and theoretical calculation results as far as possible, it should be measured after the completion of the assembly of the container, is the quality control of the final product.
China’s standards are gradually approaching to foreign advanced standards, focusing on the final quality of the product, the intermediate process links should not be more restrictions, that is, under the premise of ensuring product quality, to give the manufacturing side more room for innovation.
GB/T150.4 requirements is the roundness of the shell.
Shell roundness, due to the container pressure (internal pressure, external pressure and vacuum) of the different detection methods and qualified indicators are different.
Inner pressure container is mainly a matter of strength.
With the gradual increase in internal pressure, the roundness of the shell will be adjusted from “not round” to “round” direction, roundness deviation on the container stress distribution is mainly in the initial stage of pressure, when the pressure is close to the working pressure, the stress state in the container wall and the theoretical Calculated value is basically the same, which has been confirmed by a large number of stress test results, that is to say, the deviation of the roundness of the internal pressure vessel is not very harmful.
Internal pressure vessel shell roundness of the qualified indicators by the following two aspects of common control: one is the percentage of the inner diameter of the container; the second is the absolute value.
External pressure and vacuum container damage in the form of destabilization.
With the increase of external pressure, the shell will not only adjust from “not round” to “round” direction, and only more and more “not round”, until the sudden loss of the original shape under the action of external pressure stability.
Therefore, the deviation of shell roundness to the external pressure and vacuum containers will be more harmful than the internal pressure containers, that is to say, the external pressure and vacuum containers shell roundness should be more stringent requirements.
Before the introduction of GB150, China’s pressure vessel design standards and manufacturing standards are separate, when the main manufacturing standards (JB741) to the pressure and vacuum vessel roundness deviation requirements (≤ 0.5% Di) than the internal pressure vessel (≤ 1% Di) increased by 1 times.
GB150 pressure vessel design, manufacturing, inspection and acceptance requirements are unified in a standard, in the pressure and vacuum vessels for stability checks will be long cylinders and short cylinders are considered separately, it is clear that the manufacture of long cylinders or short cylinders are used in the same indicator (0.5% Di) to require the roundness of the shell is not reasonable enough.
GB150 in order to unify the design and manufacturing requirements, the provisions of the sample and check the chart of the way to check the external pressure and vacuum vessel shell roundness.
GB/T150.4 in GB150-1998 on the basis of a clear external pressure and vacuum containers according to the basic shape of the internal pressure container check (shell roundness), and then use the sample plate to check the local bump and depression, control the external pressure and vacuum container roundness.
The above briefly introduces the main defects that are easily produced when forming and assembling single-layer roll-welded cylinders, corrective measures and relevant requirements of the standard.
In order to improve efficiency and ensure quality, domestic and foreign have developed some mechanized assembly equipment, such as domestic had developed a hydraulic external support cylinder B type joint assembly device, can meet the diameter of 1m ~ 3.2m, wall thickness of 60mm below the container assembly, improve the labor conditions, to ensure the quality of assembly.
Single-layer pressure welding cylinder section of cold and hot processing forming, grouping, welding and shape and size deviation control and requirements, in addition to process equipment, and single-layer volume welding cylinder section of the same, will not repeat.
Non-ferrous metal materials and their composite plate single roll welded tube section of the cold, hot processing forming and container (including composite steel plate) manufacturing process of shape and size deviation generated by the causes, preventive measures and single roll welded steel tube section similar.
Non-ferrous metal material barrel section is generally used in cold roll, can also be hot roll, whether the need (to restore the state of supply of materials) heat treatment and container manufacturing process shape and size deviation control requirements, according to the relevant product standards.
Cladding for non-ferrous metal material composite plate tube section, the preferred choice of cold roll, the use of hot roll or warm roll should ensure that the use of grass-roots materials in accordance with the provisions of product standards, while taking into account the performance of the cladding; container manufacturing process shape and size deviation control requirements, according to the relevant product standards.
Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.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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