RU2230641C1 - Method for lowering residual welding stresses of pipeline - Google Patents

Abstract

FIELD: power machine engineering, namely processing pipelines of nuclear power plants after welding them, other industry branches.

SUBSTANCE: method comprises steps of radially reducing pipelines in zone spaced from welded butt; performing pipeline reduction by winding onto pipeline preliminarily tensioned high strength filament. It allows to decrease residual stresses on inner surface of pipeline.

EFFECT: prevention of corrosion cracking of pipelines of nuclear power plants.

1 dwg, 1 ex

Description

The invention relates mainly to the field of power engineering and can be used in subsequent processing of pipelines of nuclear power plants after welding and in other branches of technology.

A known method of reducing residual welding stresses of a pipeline by redistributing them due to induction heating, i.e. heating the pipeline from the outside using an inductor to a controlled temperature, while inside the pipeline is washed with cold water. (Interim report on the sub-step: “Analysis of recommendations for preventing cracking of welded joints of pipelines made of austenitic steel (based on NUREG 0313)”, International Center for Nuclear Safety, Moscow, Russia, 1997, p. 72).

The disadvantage of this method is the use of complex expensive and bulky equipment, which is difficult in the conditions of existing nuclear power plants.

There is also a method of reducing residual welding stresses in a metal of a weld of a pipeline welded end-to-end with one another by a circumferential seam, which consists in mechanical compression of the pipeline in the radial direction using an annular device, axially split and driven by an external mechanism. In the process of compression, stress redistribution occurs along the length and cross section of the pipeline, which leads to a decrease in the level of axial tensile stresses on the inner surface of the pipeline in the weld area (US Patent No. 4, 612, 071, IPC: C 21 D 8/00, from 09.16. 86 g.).

The disadvantages of this method are the significant time for preliminary preparation of the compression process necessary for measuring the cross section of the pipe and the selection of special expensive ring gaskets, which increases material costs and the time spent by personnel in the radioactive zone. In addition, the relatively large dimensions of the device do not allow its use in hard-to-reach places of the existing nuclear power plant.

The claimed solution allows to reduce residual welding stresses on the inner surface of the pipeline and thereby prevent corrosion cracking of the pipelines of nuclear power plants made of austenitic steel. This eliminates the need for measuring the cross section of the pipe, the selection and use of special ring gaskets, individually selected for each joint. This significantly reduces the time for preliminary preparation of the compression process, which in the conditions of nuclear power plants reduces radiation loads on personnel. In addition, the probability of damage to the pipeline in the event of an error in the selection of special ring gaskets even on the pipeline with significant ovality is eliminated, which significantly increases reliability. To implement the proposed method, the consumption of material (thread) is about 2 kg per one joint at a cost of 2000 rubles / kg, which compared with the known method leads to increased efficiency.

A method is proposed for reducing the residual welding stresses of a pipeline, including compressing the pipeline in a radial direction at a distance from the welded joint, wherein the compression is performed by wrapping the pipeline with a pre-tensioned high-strength thread.

The invention is illustrated in the drawing, which shows a diagram of the implementation of the proposed method.

The scheme includes a pipeline 1 having a circumferential weld 2, a winding of high-strength filament 3.

The method is as follows. The pipe 1 at a distance of “X” from the weld 2 is wrapped with a high-strength thread. The thread in the winding process is maintained in tension, as a result of which tensile stresses are created in it. The stresses from the individual turns are summed up, as a result of which the thread forms a compression pipe 1 ring 3. The threads and tows of fiber on a different basis have high mechanical properties and allow to obtain high contact pressures on the winding surface and thereby achieve redistribution of stresses along the cross section and length of the pipe 1 , which, in turn, leads to a decrease in the level of axial tensile stresses on the inner surface of this pipeline in the region of the weld 2.

The height of the winding “N” depends on the tension of the thread during winding, the mechanical characteristics of the pipe material and the amount of necessary reduction of residual stresses on the inner surface of the pipe 1 in the zone of the weld 2. The distance from the center of the seam to the center of the winding “X” is chosen so that the weld zone was in the region of maximum reduction of tensile stresses on the inner surface of the pipeline during winding. This distance for a particular pipeline material, its diameter and wall thickness is determined by calculation or experimentally using strain gauges, by measuring stresses on the inner surface of the test pipeline during winding.

An example of a specific implementation of the proposed method. For testing, a pipe was used with an external diameter of 325 and a wall thickness of 16 mm from 08Kh18N10T steel. For winding, an armos fiber bundle “Armos-600”, TU6-12-172, manufactured by OJSC “KAMENSKVOLOKNO”, was used. The width of the winding was 57 mm, height 23 mm. The tension of the tow during winding was ~ 80 kg. Moreover, the decrease in the outer diameter of the pipe was 2.8 mm, which is ~ 1%. Strain gauges glued on the inner surface of the pipeline recorded a decrease in the level of axial stresses by more than 220 MPa, which is quite enough to prevent corrosion cracking of the pipeline in the welded joint zone. The distance from the center of the winding to the center of the region of reduction of tensile stresses on the inner surface of the pipeline was 60 mm.

Claims (1)

A method of reducing residual welding stresses of a pipeline, including compressing the pipeline in a radial direction at a distance from the welded joint, characterized in that the compression is carried out by winding the pipeline from a previously stretched high-strength thread.