RU2638476C1 - Method for manufacturing welded straight-seam pipes of titanium alloys - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes forming of a strip into a pipe blank in the form of oval cylinder, welding and calibration. Forming is performed after conventional division of strips by width into two equal parts and additional separation of each half of width into extreme sections of approximately ≤2.5 mm size and three successively arranged and approximately equal inner portions: peripheral, middle and central which are successively processed by the same forming radius R_f equal to (0.8-0.95) from the radius of the tube (R_t), in three successive stages. At the first step, the peripheral portions are bent to the fold radius R_f with simultaneous bending of edges with radius equal to (0.65-0.75) R_t, and the central and middle sections are bent with radius of back bending with a value which does not take the strip out of elastic deformations area. At the second stage gradual folding of the central sections is carried out up to the bend radius R_f, the middle sections are treated with radius which does not take the strip out of elastic deformations area. At the third stage, the middle sections are treated up to radius R_f under condition of gradual forced stretching of central and peripheral sections to radius providing an ellipse form with a larger horizontal axis in the range of 1.02÷1.1 with respect to the vertical axis at the forming mill outlet.

EFFECT: increased accuracy of geometric sizes of pipes.

3 dwg

Description

The invention relates to the production of longitudinally welded pipes, in particular to the production of thin-walled tubes of small diameter from titanium alloys welded by argon-arc welding.

Welded straight-seam pipes are produced by forming a metal sheet (strip). In pipe production, the requirements of increasing corrosion resistance, reducing weight, and increasing the durability of pipe products have always been relevant. High strength steel materials and titanium alloys are used to meet these requirements. The high corrosion resistance and specific strength of titanium alloys, despite their high cost, stimulate their wider use in the nuclear industry, energy, shipbuilding, and the chemical industry.

To ensure the correct shape of the profiles of titanium alloy pipes, it should be noted that the Young's modulus of titanium or titanium-based alloy is approximately half the modulus of carbon steel. Therefore, to achieve an effect comparable in effect to the molding of a steel profile at room temperature, it is necessary to provide a twice as high level of elastoplastic deformation.

There is a method of bending parts over a large radius in the stamp, in which the radius of the punch is known to be smaller than the radius in the part after bending (Reference stamp designer: Sheet stamping. / Under the general editorship of LI Rudman. - M.: Engineering, 1988 p. 210-211). This is due to the fact that when bending along a large radius, in addition to a large spring angle, an elastic change in the radius of curvature occurs. The radius of curvature of the punch is calculated by the formula according to which it depends on the thickness of the material, the bending radius and the coefficients of the mechanical characteristics of the material. The calculation according to the formula is approximate and cannot be used in the design of the geometry of the rolls used to form the tube blanks.

A known method of forming a pipe billet, in which the curvature (radius of molding) is distributed according to a complex empirical formula (B. Zhukovsky and others. The production of pipes by electric resistance welding. M .: Metallurgizdat, 1953).

The disadvantage of the described analogue is that the condition of the peripheral sections of the pipe billet is not controlled, which leads to plastic distortion of the billet profile, the appearance of corrugations at its edges and, as a result, the production of a poor-quality welded pipe.

A known method of manufacturing an electric-welded straight-seam pipe, comprising forming from a tape a tube stock in the form of an oval cylinder with a slit, the minor axis of which is equal to the diameter of the subsequent round welding gauge, heating the edges and welding them by crimping in a welding gauge, the workpiece is formed to obtain symmetrical parts with radii equal to the radius of the welding gauge, the centers of which are located on the major axis of the oval cylinder, and the ratio of the major axis to the minor axis is 1.03-1.30 (RF patent No. 2232655, IPC VC 37/08, publ. 20.07.2004) - prototype. The method provides high strength and quality of the weld, and also increases the yield.

The disadvantages of this method are:

- low accuracy during bending, due to the fact that the size of the spring depends on many factors, the main of which are the mechanical properties of the bent material (the harder the material, the greater the spring angle), the bending radius (the larger the radius, the greater the spring angle), material thickness (the thinner the material, the greater the spring angle).

The prototype method is not optimized for the manufacture of thin-walled pipes (the ratio of diameter to wall thickness D / s≥40.0) from titanium alloys, since it does not take into account their specific properties. In particular, the spring angle during molding of titanium alloy billets is 2 or more times greater than the spring angle of similar steel alloy billets. In the production of titanium pipes, corrugation occurs at the edges of the workpiece.

The aim of this invention is the manufacture of titanium alloys of high-quality thin-walled welded pipes, with a ratio of diameter to wall thickness D / s≥40,0.

The technical result is the manufacture of pipes with accurate geometric dimensions and a high-quality weld due to the reducing effect of springing of the workpiece material and rational selection of the geometric dimensions of the tool in the technological process of pipe formation before welding.

A method of manufacturing welded longitudinal seam pipes from titanium alloys, including forming a strip into a tube blank in the form of an oval cylinder, welding and calibration, and forming is performed after conditionally dividing the strip into two equal parts in width and additionally dividing each half of the width into extreme sections of size ≤2, 5 mm and three consecutive approximately equal internal sections: peripheral, middle and central, which are sequentially treated with the same molding radius R _f equal to ( 0.8-0.95) from the radius of the pipe (R _t ), in three successive stages: at the first stage, the peripheral sections are bent to the bend radius R _f with the simultaneous bending of the edges with a radius equal to (0.65-0.75) R _t , and the central and middle sections are bent with a radius of inverse inflection with a value that does not remove the strip from the region of elastic deformations, at the second stage, the central sections are gradually bent to the bending radius R _f , while the middle sections are treated with a radius that does not remove the strip from the elastic region deformations, at the third stage vayut middle portions to a radius R _p, provided that the progressive forced demoulding the central and peripheral portions with a radius of providing at the outlet from the forming mill elliptical shape with a larger horizontal axis in the range 1.02 ÷ 1.1 with respect to the vertical axis.

The invention is illustrated by drawings, where in FIG. 1 shows a molding scheme in step 1; FIG. 2 is a diagram of molding in step 2; FIG. 3 is a diagram of the molding in stage 3.

The essence of the invention consists in taking into account the specifics of forming thin-walled pipes from titanium alloys and using techniques that reduce the influence of harmful factors in the manufacture of these products.

It is known that low accuracy during bending (product molding) is due to the fact that the amount of springing depends on the following main factors:

- mechanical properties of the bending material (the spring angle of titanium alloys is more than twice the angle of steel spring),

- bending radius (the larger the radius, the greater the spring angle),

- the thickness of the material (the thinner the material, the greater the angle of springing) (Handbook of sheet metal stamping. VP Ostrovsky. - M.: MASHGIZ, 1957, p. 75).

During bending, plastic residual deformation does not penetrate the entire thickness of the strip: the surface layers are deformed plastically, the internal ones are elastic.

When removing external loads, the zones of plastic deformation tend to fix the sheet in a bent state, and the zones of elastic deformation will tend to return it to its original state before bending, as a result of which a springing effect will occur. Springing changes the curvature of the sheet and the bending angle.

The method uses an ovalizing tool, since the pairs of upper and lower rolls form cross sections of the strip in the form of an oval along the axes of the stands. A feature of ovalization is the use of molding radii R _f smaller than the radius of the finished pipe R _t . The radius of the molding is calculated by a certain coefficient from the range of 0.8 ÷ 0.95 of the radius of the finished pipe, depending on the diameter and wall thickness.

To reduce the size of the areas of springing, the strip is conditionally divided in half, both halves are divided symmetrically relative to each other into the extreme sections of size ≤2.5 mm (A4) and three consecutive approximately equal internal sections: peripheral (A3), middle (A2) and Central (A1), which are sequentially processed R _f in three stages. This allows you to reduce stresses arising from springing, to values that do not lead to the formation of corrugations, the displacement of the edges of the profile and, as a consequence, the manufacture of low-quality welded pipes.

The development of the strip begins in the first molding stand with peripheral sections, which are bent by the molding radius R _f equal to (0.8-0.95) from the radius of the pipe (R _t ) with simultaneous bending of the edges with a radius equal to (0.65-0.75 ) R _t , and the central and middle sections are bent with a radius of inverse inflection with a value that does not remove the strip from the region of elastic deformations.

The obtained grooved profile with rounding on the sides with radii, the sizes of which are close to the final equal dimensions of the molded workpiece before welding, eliminates the need for additional deformation of these sections in subsequent passes. Due to this, the non-uniformity of longitudinal deformations over the cross section is reduced. Bending the middle part of the strip in the direction opposite to the curvature of the pipe allows you to:

- increase the stability of the profile during further molding,

- due to the forced initial dilution of the molded lateral sides, it makes it possible to uniformly and sequentially bend the middle part of the pipe billet in subsequent passes into the closed loop of the pipe of the final shape with a given curvature.

At the second stage, the central sections (A1) are gradually worked out to a radius R _f , provided that the middle sections (A2) are machined with a radius that does not remove the strip from the region of elastic deformations, and the peripheral sections (A3 and A4) have already been worked out.

The bending of the central sections, with bending radii R _f , at the second stage gives the mirror symmetry of the two parts of the workpiece relative to the vertical axis. The gradual development of these sections provides a monotonous bending and reduces the risks of corrugation.

In the third stage of processing, the middle sections (A2) are bent to a radius R _f , provided that the central (A1) and peripheral (A3) sections are gradually forced to form a radius that provides an ellipse shape with a larger horizontal axis in the range 1 at the exit from the forming mill , 02 ÷ 1.1 with respect to the vertical axis. This ratio of the ellipse axes is selected empirically and ensures optimal conditions for joining the edges of the workpiece during welding, which in turn guarantees a high-quality weld.

The decoding of the central (A1) and peripheral (A3) sections of the workpiece leads to sequential stretching-compression of the parts of the workpiece, which are located on the inner side of the neutral section, and compression-stretching of the parts of the workpiece, which are located on the outside of the neutral section of the workpiece. Thus, the plastic deformation of the metal leads to a decrease in the elastic limit during repeated deformation of the opposite sign (Bausinger effect) (Arkulis G.E., Drogobid V.G. Plasticity Theory. - M .: Metallurgy, 1987, p. 159). As a result, the springing of the material of the obtained part decreases sharply, because the subsequent stretching of the preform after compression or its compression after pre-stretching leads to a decrease in the yield stresses of the material and, consequently, to a decrease in the elastic component of the deformation (i.e., a decrease in springing and, consequently, an increase in the accuracy of the geometric dimensions of the preform).

An example of a specific implementation.

Purpose of work: Production of a welded pipe ∅25.4 mm with a ratio of diameter to wall thickness D / s = 63.5 of titanium alloy Gr2 meeting the requirements of ASTM B338 standard, strip width 80.4 mm.

The molding radius was adopted calculated equal to 0.87-0.88 of the radius of the pipe R _t = 12.7 mm and amounted to R _f = 11.1-11.3 mm.

Before welding, the pipe billet had a predetermined cross-sectional profile; the edge divergence before welding corresponded to the calculated one and amounted to 2–4 mm.

The calibration rolls installed on the mill were made in one radius and adjusted to the output size of ∅025.36-∅25.38 mm. The maximum allowable size of the welded pipe is ,525.527 mm, and the minimum allowable size is в25.273 mm in accordance with ASTM B338.

After welding and calibration were performed:

- metallographic control;

- distribution test, distribution size 22%, no cracks or tears, in accordance with ASTM B338;

- flattening test, there are no cracks or tears on the external and internal surfaces, in accordance with ASTM B338;

- tensile test, yield strength 310 MPa, temporary resistance 422 MPa, elongation of 44.2%, corresponds to ASTM B338.

The measurements of the outer diameter of the pipes leaving the line had a maximum of 25.49 mm and a minimum of 25.32 mm, which meets the requirements of ASTM B338.

This method allows the manufacture of pipes in accordance with international standards.

Claims (1)

A method of manufacturing welded longitudinal seam pipes from titanium alloys, including forming a strip into a tube billet in the form of an oval cylinder, welding and calibration, characterized in that before forming, they conditionally divide the strip into two equal parts in width with the additional separation of each of them into extreme sections of size ≤2.5 mm and three consecutively located approximately equal internal peripheral, middle and central sections, which are formed with a radius R _f equal to (0.8-0.95) from the radius of the pipe R _t in three consecutive at the first stage, the peripheral sections are bent to a radius R _f with the simultaneous bending of the extreme sections to a radius equal to (0.65-0.75) R _t , and the central and middle sections are bent with a radius of inverse bending, which makes it possible to maintain strip in the field of elastic deformations, at the second stage, the central sections are gradually bent to a radius R _f and the middle sections to a radius that allows the strip to be saved in the region of elastic deformations, and at the third stage, average joints of radius R _f with the gradual forced molding of the central and peripheral sections to a radius that ensures the formation of a tube stock with a cross section in the form of an ellipse with a horizontal axis equal to 1.02 ÷ 1.1 of the vertical axis.

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