RU2635035C1 - Method for production of pipes - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes forming edge portions of a sheet, forming a hollow blank from the sheet, and welding the edges with a longitudinal seam. Elimination of metal pipes damage in areas close to the seams is provided by the fact that when forming the edge portions of the sheet from which a pipe is formed, these sections are provided with larger value of curvature than the nominal curvature of the pipe. In these sections, the curvature equals to 1.04-1.10 of the pipe curvature. The portions of increased curvature are moulded over the length equal to (4.0÷5.0) h, where h is thickness of sheet, and the same length of transition zones is provided from the portions of increased curvature to the sections of nominal curvature of the pipe.

EFFECT: production of welded pipes with longitudinal weld seam.

2 cl, 5 dwg

Description

The invention relates to metallurgy and can be used for the manufacture of pipes of various sections from both steel and other metals and alloys.

A known method of manufacturing pipes according to the patent of Germany No. 3840938, MKI 5 V21D 39/08 from 1990, which provides for the formation of pipe blanks from sheets with subsequent welding of the edges and the implementation after this operation of the distribution of the pipe - increase its diameter.

It should be noted that not always the implementation of this method provides a satisfactory pipe strength.

Also known is a method of manufacturing pipes according to the patent of the Russian Federation No. 2162758, MKI B21C 37/12 from 1999. The method involves applying depressions along the edges of the sheet to 0.5 of its thickness and subsequent molding of the pipe from this sheet.

After that, the workpiece is welded with two longitudinal seams offset along the perimeter of the pipe relative to each other.

The method also includes forming protrusions on one of the edges of the sheet and depressions on its other edge at the junction of the edges of the sheet, followed by welding with two longitudinal welds.

This method provides an increase in pipe strength, however, its implementation requires rolling of the edge sections of the sheet along its entire length, which requires the use of powerful rolling mills and leads to a significant increase in the cost of pipes.

As the closest analogue of the prototype can be adopted by the method of copyright certificate of the USSR No. 1657249, MKI 5 V21C 37/12 from 1991

This method includes forming from a sheet by plastic deformation of the pipe billet, as well as crimping the edges of the sheets collected when forming the pipe with a gap.

However, this method requires the use of powerful presses for crimping the edges of the sheets, which requires very high costs for the manufacture and operation of equipment. Therefore, the cost of the pipes is very high, and issues related to increasing strength are of great importance. Currently, welded pipes are widely used for the main pipeline transport of oil, gas, and other substances. They transport 100% of the produced gas and 80% of oil. Pipes with diameters of 1020 mm, 1220 mm, 1420 mm and more are produced by plastic bending and subsequent welding from steels of strength classes K38 - K65 and X42 - X80 up to 18 m long with wall thicknesses up to 52 mm and more, for example, in the process of step-by-step molding.

However, the number of accidents due to pipe destruction is very significant: annually due to pipe destruction caused by fatigue cracks and stress corrosion, 10-15 million tons of oil flows from oil pipelines (out of 305 million tons produced in the Russian Federation).

From oil losses, losses exceed $ 270 million per year, see V.N. Shinkin, “Resistance of materials for metallurgists”, Moscow, Publishing House Dom MISiS, 2013 - (p. 637-638).

In the same monograph, it is noted that: “Most of the destruction of gas pipelines with a diameter of 1420 mm from X70 steel occurs in areas located up to 200 mm from a longitudinal weld”. It is in these zones near the welds that are areas of reduced strength, in which cracks occur, leading to the destruction of pipes.

Permissible stresses in weld zones are usually recommended to be reduced with respect to the base metal. Often, a coefficient is recommended that determines the decrease in stresses equal to 0.9 (for butt welded joints), see, for example, I.A. Birger, B.F. Shorr, G.B. Iosilevich “Strength calculation of machine parts”. Directory. Moscow, Mechanical Engineering, 1979 (in the indicated monograph it is recommended “For critical structures after welding, it is necessary to anneal (in argon or in vacuum) to relieve residual stresses”; page 116 of this monograph. It is easy to assess how much the cost of pipes will change if they are carried out annealing in argon atmosphere or in vacuum).

Effective methods of processing zones of welds with pressure (forging, ultrasonic treatment, shot peening, etc.) are also effective. But all these methods, with their high efficiency, significantly increase the cost of manufacturing pipes of main pipelines.

It should be agreed with the conclusion that “Welded joints are the weakest point of any pipeline” (see Valiulin I.R., Soloviev E.A. et al. “Welding Production”, No. 5. 2016. P. 3-8).

The basis of this invention is to provide increased reliability and strength of pipes, without significantly increasing the cost of the equipment used.

This problem is solved in that when implementing the method, which includes stamping the edge sections of the sheet, forming a hollow workpiece from it by bending and welding its edges with longitudinal welds, two curved zones of increased curvature are formed at each of the edges of the workpiece when stamping these sections of the sheet, and the one closest to the edge of the sheet is deformed to a value of curvature equal to 1.04-1.10 of the curvature of the pipe being manufactured, and the adjacent zone with curvature equal to the curvature of the pipe, with a smooth transition from one zone to another.

In addition, it is provided that sections of edge zones with a curvature increased by 1.04-1.10 times are formed on a length equal to (4.0 ÷ 5.0) h, where h is the sheet thickness and the same length is performed transition zones to the nominal pipe curvature.

An example implementation of the method is illustrated by drawings, see FIG. 1 - FIG. 5.

In FIG. 1 shows a diagram of the implementation of the first operation — stamping of the marginal zones of the sheet, and FIG. 2 is an example of a rolling process - stepwise bending of a sheet with the formation of a pipe blank from it.

FIG. 3 illustrates the final step of forming a workpiece for welding, and in FIG. 4 shows an example of a pipe after welding with one weld. In FIG. 5 shows a pipe with two longitudinal welds.

The following notation is used in these figures: 1 - a sheet from which a welded pipe is made and a diagram of the stamping of its edge sections is shown. In these sections 2 and 3, the necessary curvature of the edge sections of the sheet is provided.

During step forming, the movement of the sheet is provided by the rollers 4, and the workpiece is supported on the rollers 5 and 6, with guide rollers 7 and 8. The bend implements the punch 9 through the lever 10 from the movable support node 11.

The compression of the edge sections of the workpiece 1 is achieved using punches 12 and 13 in the curved zones defined by the angles α, FIG. 3. The drive of these punches provide the movement of the beam 14.

The cross section of the pipe 15 illustrates the position of the weld 16, and the pipe 17 is made of two workpieces that are connected by welds: 18 and 19.

Here is the operational description of the method.

The first operation is the stamping of the marginal zones of the sheet, from which the pipe is then formed by bending. The edges of the sheet 1 are deformed with dies, forming in the dies 2 and 3 the edge zones of the sheet for the future pipe. Here, in the zones defined by angles α, the curvature is increased, and zones with angles β are transitional: in them, the curvature decreases to the nominal value of the pipe curvature, which is constant in the zones defined by the angles γ.

, то к ним примыкают зоны, определенные углами β. Это участки переменной кривизны, которая уменьшается здесь от

до

. Длина этой переходной зоны равна длине зоны постоянной кривизны (равной

. Далее в третьей зоне, характеризуемой углом γ, кривизна постоянная и равна номинальной кривизне трубы

. Ее длина обычно равна (0,2÷0,3)R и здесь формируют кривизну, которую невозможно создать последующим изгибом.The stamps are made in such a way that if the curvature of the zones corresponding to the angles α is equal to 1.04-1.10 pipe curvature equal to

, then zones defined by angles β adjoin them. These are sections of variable curvature, which decreases here from

before

. The length of this transition zone is equal to the length of the zone of constant curvature (equal to

. Further, in the third zone, characterized by the angle γ, the curvature is constant and equal to the nominal curvature of the pipe

. Its length is usually equal to (0.2 ÷ 0.3) R and here form a curvature that cannot be created by a subsequent bend.

The deformation of the edge zones can be carried out simultaneously by two stamps on both edges of the sheet, or sequentially: first, deforming in the stamps one edge zone of the sheet, and then the second (at the opposite edge of the sheet).

The second operation is “step-by-step molding”, i.e. sequential movement of the workpiece and its bending during stops.

The movement of the workpiece along a predetermined path (circular arc) is provided by the rollers 4, and the rollers 5 and 6 are the supporting ones (Fig. 2).

The deformed workpiece is sequentially (in the intervals between bending deformation cycles) moved, and its trajectory is determined by rollers 7 and 8.

When the workpiece stops, the punch 9 through the lever 10 and the beam 11 moves downward, bending the workpiece (stationary). After that, the workpiece is moved to the next "step", determined by the distance between the rollers 5 and 6. Like the first operation, the second is usually carried out at "room" temperature (18-25 ° C). It is impossible to realize the bending of the ends of the workpieces shorter than the distance between the rollers 5 and 6 and this explains the need to give them the required curvature by a separate stamping operation (in dies 2 and 3).

The third operation is the compression of the edge sections of the workpiece with punches 12 and 13 having a curvature equal to 1.04 ÷ 1.10 of the curvature of the formed pipe to increase the accuracy of the position of the edges of the workpiece before welding, the compression is carried out by the movement of the beam 14.

The next, fourth operation is welding the edges of the pipe 15 with a longitudinal weld 16 (along the generatrix of the billet 1, curved in the shape of a cylinder (Fig. 4). For large thicknesses of steel sheets more than 20 mm, it is better to weld with two seams: “from the inside” and “ outside "the workpiece. Their connection will provide the formation of a seam 16. If the pipe is made of two blanks, then the pipe 17 is connected by two longitudinal seams 18 and 19, Fig. 5.

Further operations: fifth - expansion (expansion of the pipe by the pressure of punches on the inner surface of the pipe), testing and control are carried out by conventional methods. If possible, carry out tempering when heated to 500-600 ° C to reduce residual stresses in the metal arising after welding.

It is known that defects in the welding process can occur in the weld zone: lack of fusion, incomplete fusion, cracks, gas bubbles or pores, which can significantly reduce the strength of the pipe during operation and lead to its destruction. Inevitable fluctuations in the magnitude of the pipe curvature have a significant effect on strength.

For pipes loaded with internal pressure, the strength is determined not by the function itself, which characterizes its profile, but by the curvature of this function, which depends on the first and second derivative functions. Therefore, for small deviations from the nominal value of the pipe profile, the oscillations along the perimeter of its curvature can significantly exceed the oscillations of the function itself.

Let the pipe, due to the inaccuracy of its configuration, have some ellipticity and its shape does not correspond to a circle of radius R, but to an ellipse with half shafts: “ a ” and “b” (b>R> a ).

It is known that the equation of an ellipse with semiaxes a , b has the form

,

,

and the radius of curvature of this line

.

.

Therefore, the curvature is

.

.

For y = 0, x = ± a

,

,

and for y = ± b, x = 0

,

,

and therefore the ratio of the moduli of these quantities of curvature is

.

.

, то отношение величин модулей кривизны соответствует кубу этого числа и будет равно 1,33 и в столько же раз максимальное напряжение превысит ее минимальную величину. Действительно, для трубы радиусом R и толщиной h имеет место формула П. ЛапласаIf the ratio of the axes of the ellipse is equal to

, then the ratio of the magnitudes of the curvature moduli corresponds to a cube of this number and will be equal to 1.33 and the maximum voltage will exceed its minimum value by the same amount. Indeed, for a pipe of radius R and thickness h, the P. Laplace formula holds

,

,

where p is the pressure, σ ₁ , σ _{2 are the} main stresses (tension) R ₁ , R ₂ are the main radii of curvature.

As R ₂ → ∞, R = R ₁

,

,

where p is the pressure of the gas or liquid in the pipe.

Formula (5) can be written as

and for ellipse

.

.

The ratio of maximum voltage to minimum will be at variable curvature

.It is clear that for a perfectly round section of the cylinder and a = b, A = 1 and therefore

.

, приведет к локальному повышению напряжений в 1,03³÷1,05³=1,093÷1,16 раз, что может существенно снизить прочность трубы.Even a slight difference in diameters with a small ellipticity of the pipe, for example

, will lead to a local increase in stresses of 1.03 ³ ÷ 1.05 ³ = 1.093 ÷ 1.16 times, which can significantly reduce the strength of the pipe.

возможно увеличение напряжений в 1,15³=1,52 раза, что существенно уменьшит прочность и надежность трубы. Неравнопрочность и наличие участков увеличенных напряжений и уменьшенной прочности приводит к необходимости увеличивать запасы прочности и, следовательно, массу трубопроводов.If, in the weld zone, an “angular section” with small flat zones is formed, then

a voltage increase of 1.15 ³ = 1.52 times is possible, which will significantly reduce the strength and reliability of the pipe. The unevenness and the presence of sections of increased stress and reduced strength leads to the need to increase the margin of safety and, consequently, the mass of pipelines.

In the proposed method, separation of sections of reduced strength (near welds) and increased stresses (in areas of lower curvature) is ensured, since it is in the areas of possible reduction in strength that the curvature is increased and therefore it is here that the possibility of increasing stresses is excluded.

If the curvature is less than 1.04, equalization of stresses in the pipe under the influence of internal pressure will not be ensured.

Exceeding the upper limit of curvature equal to 1.10 will result in zones near the weld being less than their average values, but stresses will increase significantly in other areas around the pipe perimeter, for example, near the intersection of the pipe profile with the vertical coordinate axes (axes y , Fig. 4, 5).

If the length of the section of increased curvature at the edges of the sheet is less than 4.0 h, where h is the thickness of the pipe, then sections of small curvature may appear near the weld, where increased stresses will occur. An increase in the length of these zones over 5.0 h will lead to a useless increase in the size of the stamping zones at the edges of the sheet, where there is no increased stress.

The aforementioned confirms the optimality of the proposed intervals: an increase in the curvature of the weld seams; and the lengths of the sections of increased curvature in the zones at the welds.

Here is an example of the implementation of the proposed method.

In the manufacture of a pipe with a diameter of 1220 mm from steel of strength class K 60 with a wall thickness of 20 mm, failure occurs for a pipe without defects at an internal pressure of 4.56 MPa (see Shinkin V.N. Resistance of materials for metallurgists. Moscow. House MISiS. 1914. S. 648).

, что обеспечит повышение надежности трубы в эксплуатации.But due to the influence of defects, the pipe can collapse in areas near the weld at a pressure lower by 3.6%. The implementation of this section of the pipe with a curvature increased by 1.06 times will lead to a decrease in stresses in the zone near the weld by 1.06 ³ -1.19 times. The dimensions of this zone will be assumed equal to 4.5h = 4.5⋅20 = 90 mm and this will increase the strength of the metal zones, in which it is possible to arrange welding defects that reduce strength, with a margin

That will provide increased reliability of the pipe in operation.

In the manufacturing process of welded pipes by stamping, bending and welding, some deviations in the configuration of the workpieces and the resulting pipes are inevitable. But in pipes, the stress state under the influence of internal pressure is determined not directly by the pipe dimensions, but by its curvature, i.e. stresses depend on the first and second derivatives of the function that determines the shape of the pipe. This leads to significant fluctuations in the strength characteristics.

On the other hand, zones of reduced strength near welds may appear. With an unfavorable arrangement of these zones, when they coincide with the zones of reduced curvature of the pipe, destruction in these zones of the metal of the pipe becomes very likely and this reduces the reliability and durability of the pipes.

This method involves increasing the curvature of the pipes precisely in those areas where the most likely occurrence of defects and the destruction of the metal. Where defects can occur, the curvature of the pipe is increased, which is where it provides stress reduction and significantly increases the reliability of the pipes during their operation.

At first glance, a pipe of circular cross section of constant thickness, loaded with internal pressure, is an example of an equally strong part.

But the presence of a weld (or several seams) violates the equal strength.

The proposed method with the formation of pipes of variable curvature, increased in the zones of the welds, essentially restores the condition of equal strength of the metal throughout the volume of the pipe.

Claims (2)

1. A method of manufacturing pipes, including stamping the edge sections of the sheet, forming a hollow billet from it and welding its edges with a longitudinal weld, characterized in that when stamping each edge section of the sheet, two curved zones are formed, and the zone adjacent to the edge of the sheet is deformed to the curvature equal to 1.04 ÷ 1.10 of the nominal curvature of the pipe being manufactured, and the other zone to the curvature equal to the nominal curvature of the pipe, with a smooth transition zone between them. 2. The method according to p. 1, characterized in that a zone with a curvature equal to 1.04-1.10 of the nominal curvature of the pipe is formed at the edge of the sheet with a length equal to (4.0-5.0) h, where h is the thickness of the sheet, and the same length perform the transition zone.

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