RU2344909C2 - Arc welding method for magnetised bodies during recovery operations - Google Patents

Abstract

FIELD: electricity; processes.

SUBSTANCE: method includes removal of defected area, installation of weld joints, local concentration of magnetic field counteracting to residual magnetic field and joints welding. Concurrently with welding, the welding area is affected by magnetic field along the normal to the weld and arc axis. At the same time variable magnetic field is created by superimposed coil coated with asbestos tape and energised at 50 Hz frequency. Besides variable magnetic field amplitude near welding surface is 80-100% from the residual intensity of magnetisation within the gap.

EFFECT: simple welding process and stable quality of weld joints for magnetised bodies without demagnetisation.

2 cl, 3 dwg

Description

The invention relates to the field of electric butt welding and can be used for welded magnetized joints of pipelines.

In-line magnetic flaw detectors are widely used to assess the technical condition of the linear part of the main pipelines and to identify defective sections, which include rare-earth permanent magnets with a high specific energy of the magnetic field, which leads to the formation of a strong residual magnetization in the pipe body.

When carrying out repair and restoration welding work, on accidentally hazardous areas identified during the control process, the residual magnetic field of the pipeline, acting on the welding arc, violates the stability of its combustion and the formation of the weld. The indicated complex of phenomena was called the “magnetic blast” effect.

Induction values were measured during repair work on pipelines with a diameter of 1220, 1020, 720 and 530 mm, most of which were diagnosed by magnetic flaw detectors. After removal of the defective sections, the induction at the free ends of the pipelines that underwent magnetic defectoscopy was 15–25 mT. After joining for welding defect-free inserts (coils), the induction in the groove increased by 3-5 times and reached the level of 50-120 mT. Induction at the free ends of pipelines that did not pass flaw detection was 2-8 mT, it was distributed extremely unevenly around the pipe circumference, and after docking with a coil it increased to a level of 8-20 mT with the same uneven distribution.

Known designed demagnetizing device (RU) [S.A. Volokhov, P.N.Dobrodeev, G.I. Mamin Experience of demagnetization of pipes on main pipelines using the latest technology. // ICDS / http://www.uatechnology.org]. Its power circuit is powered from the same welding unit that is used for pipe welding; the control circuit is powered from an AC network with a frequency of 50 Hz, 220 V. The pulses of magnetic fields necessary for demagnetization are created by a winding wound around the pipe. For ease of installation, it is composed of sections of a multicore cable connected by connectors. The process of demagnetization is controlled from a remote control panel. The level of magnetic induction in the gap between the welded edges is controlled by a magnetometer with a measurement range of 1-200 mT.

During 1999-2000 Nine types of switchgear were put into operation: at the Pridneprovsk trunk oil pipelines GAO, the Druzhba MNO (Ukraine) and the Druzhba State Public Technical Park (Belarus).

As a result of control welding of pipe joints, it was found that the level of induction in the groove should be no more than 8-10 mT. The welder does not notice the presence of a magnetic field with an induction level of up to 8 mT; in the induction range of 8–10 mT, the welder observes an increase in the excitement of the metal in the weld pool, with induction above 10 mT, metal spraying from the weld zone begins, and with induction above 30 mT, welding impossible.

According to the results of the timing of 10 different cases of demagnetization, the process duration averaged about 20 minutes: up to 10 minutes - installation of the winding, up to 2 minutes - demagnetization and control of the magnetic field level, up to 8 minutes - dismantling of the winding. The installation and dismantling of the winding are carried out by two welders, the connection of power cables to the welding unit and a mobile power station - their staff, demagnetization and control - one of the welders.

In MSTU them. N.E.Bauman for several years, work was carried out on the use of magnetic fields to control the welding process. To hold the weld pool, use a transverse magnetic field perpendicular to the axis of the seam and arc (Rybalchuk A.M. Formation of the weld seam with a magnetic field. // “Welder Professional”, 2005. No. 5, p. 9-10). The superposition scheme of two transverse fields preserves the stability of the arc, but is structurally quite complicated.

When arc welding with alternating current of increased frequency under the action of industrial magnetic fields, the current polarity is changed (RU 2245231, V23K 9/09) depending on the magnitude of the perturbing effect of the external magnetic field at the moment of reaching the critical deviation of the arc from the position coaxial with the electrode, which is determined by comparing the voltage on the arc with the reference voltage in the interval of the current flow period of the corresponding polarity. The disadvantage of this welding method is the complexity of the process.

As a prototype, a method of welding magnetized pipelines during repair and restoration works (RU 2237562, V23K 31/02) was adopted, including the removal of a defective zone, installation in its place equal to the size of the defect-free section, demagnetization of welded joints by magnetization reversal of the butt joint zone and welding joints, magnetization reversal of the material of the end zone of the joint is carried out before installing the defect-free section and welding until the effect of the “magnetic blast” is eliminated in the material by concentration of the magnetic field in the local zones mechanical shear pipeline to a value equal to the actual magnitude of the residual magnetic field of the pipeline, and its value is created in the end material of the core residual magnetic field which counteracts the magnetic field to said pipeline, then injected into the joint defect-free portion. The disadvantage of this welding method is the complexity of the process and the increased energy consumption for demagnetization of welded joints.

The objective of the invention is to develop a welding method that provides a simplification of the process and a stable quality of welded joints for magnetized objects without demagnetization.

The problem is solved in that in a method of welding magnetized objects without demagnetization, including the removal of the defective zone, the installation of welded joints, the concentration of the magnetic field in the local zone that counteracts the residual magnetic field, the welding of the joints, the objects are welded to the welding area with a magnetic field normal to the weld and axis of the arc simultaneously with welding. In this case, an alternating magnetic field is created due to the superimposed coil, which is coated with asbestos tape and fed with a frequency of 50 Hz. In addition, the amplitude of the alternating magnetic field at the welding surface of the object is 80-100% of the residual magnetization in the gap.

The essence of the proposed method is illustrated by drawings: figure 1 - diagram of the welding of pipelines; figure 2 is a diagram of the welding of plates and figure 3 is a sample of the weld when inducing a magnetic field in the gap between the plates of 50 MT (view from two sides).

A method of welding a magnetized pipeline is that at the ends of the ferromagnetic pipes 1 (Fig. 1), the defective zones are removed, they are installed coaxially and with a gap of 2 ... 3 mm. A coil 2 is laid on and welded onto the welding area, which is covered with asbestos tape and is powered from the transformer 3. The amplitude of the alternating magnetic field is set to 80 ... 100% of the residual magnetization of the gap, measured using a teslameter. Electrode 4 is introduced into the joint and welding is performed from apparatus 5

To implement the welding process of magnetized products, the magnetic system of Fig. 2 was assembled, in which permanent magnets 2 with pole tips 3 were mounted on yoke 1, on which plates 4 made of steel St20 with dimensions 170 × 140 × 10 mm with welded ends were laid . The gap between the plates was set equal to the diameter of the welding electrode 5. The magnetic field in the gap between the plates was 45-50 mT.

For welding, a coil of copper flexible wire 6, consisting of 210 turns, was applied to the joint of the plates 4, to which voltage from a network of 220 V, 50 Hz was supplied through a transformer 7. To protect the wire from temperature effects, the coil is wrapped with asbestos tape. Welding was carried out from a DC apparatus 8 of type UDTU - 251 with a welding electrode 5 of type LB52U with a diameter of 4 ... 5 mm and a welding current of 160 A. Welding was carried out at a voltage of 60 V on coil 6, while the amplitude of the magnetic field along the axis of coil 6 at the surface weldable plates 4 was 35 MT.

The alternating magnetic field generated by coil 6 produces a magnetization reversal in the direction of the axis of coil 6 (if the magnetic field and current are parallel, then they do not interact), and on the other hand, under the action of the Lorentz force (the action of a magnetic field on a charge moving at an angle to the field) pulls the plasma arcs into the cord. Both factors combined provide the possibility of welding in the presence of a magnetic field in the gap between the parts to be welded.

The cross section of the coil 6 wire, the number of turns and the current must be such that the amplitude of the magnetic field induction at the surface of the parts being welded is comparable with the magnetic field in the gap of the parts being welded (80 ... 100%).

Coil 6 is superimposed on the welding zone and, if necessary, is held by hand or secured in any way (for example, using wire braces, permanent magnets), then welding is performed within the window of coil 6. Depending on the size of the coil 6, it is possible to weld along the length of up to 0.5 m without rearranging it.

The re-installation time of coil 6 is not more than 20 seconds.

It is known that the deflection of the arc is caused by the superposition of an external transverse magnetic field on its own circular field of the arc in the circuit. In that part of the circuit where the lines of force coincide, an excess pressure is created and the arc deviates towards a weakening field, where the lines of force are directed towards each other. Since AC is used to power the arc, the resulting electromagnetic force acting on the arc is alternating and the arc oscillates in both directions from the equilibrium position with the frequency of the alternating current.

The tests (see figure 3) showed that the developed technology provides quick, accurate welding, economical, simple and can be recommended for industrial applications when repairing pipelines with residual magnetization. The results of assessing the state of the weld metal and the joint according to the criteria of fractography (nature of the break) and microanalysis of the weld sections along the length and cross-section (root, sintering boundary, weak zone, pore and other indicators of welding properties according to the requirements of table 12 RD 03-614-03) indicate about the good condition of the compounds according to the proposed method.

Claims (2)

1. The method of arc welding of magnetized objects during repair work, including the removal of the defective zone, installation with a gap of the weld joint, the creation of an alternating magnetic field in the local zone of the weld joint that counteracts the residual magnetic field of the magnetized objects, and welding of the joint, characterized in that the arc welding is carried out without demagnetization, for which they create a magnetic field normal to the seam and arc axis simultaneously with welding, while setting the amplitude of the alternating magnetic field at the weld rate of the object is 80-100% of the residual magnetization in the joint gap. 2. The method of arc welding according to claim 1, characterized in that in order to create an alternating magnetic field, a coil coated with asbestos tape is applied to the welding area and a voltage of 50 Hz is applied to it.

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