RU2492037C1 - Method of decreasing residual strain in welded metal joints of pipelines - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: proposed method comprises applying the LF-vibrations by appropriate vibrators to in welding. Said LF vibrations are applied at frequency equal to intrinsic frequency of pipeline section with welded joint defined by define formula. Said frequency is calculated with allowance for pipeline section OD and ID, and spacing between the points of stiff attachment of pipeline section with welded joint.

EFFECT: higher quality of weld joint and metal near-seam layer.

2 dwg, 1 ex

Description

The invention relates to the field of welding, and in particular to methods for removing residual stresses arising in welded joints, including when welding pipelines.

A known method of relieving residual stresses in welded joints of metals (Patent JP 48-10709, dp 06.04.1973), in which the cyclic superposition of low-frequency vibrations on the crystallizing metal of the weld pool by means of creating these vibrations, which is located on the surface of the workpiece.

The disadvantage of this method is that the object of influence is only the weld seam of the connection, while the heat-affected zone of the base metal remains unused.

There is also known a method of arc welding (JP Patent No. 53012751A, dp 04.02.1978), in which during the welding process the crystallizing metal is affected by vibrational vibrations in the transverse weld direction, which allows to reduce residual stresses and obtain a fine-grained weld structure.

The disadvantage of this method is that the object of influence is only the weld seam of the connection, while the heat-affected zone of the base metal remains unused.

A known method of relieving residual stresses of welded joints of vessels and apparatuses, as well as their elements (Patent RU No. 2243272, dp 27.12.2004). By introducing elastic vibrations of the sound range using a heat generator, while heating the vessels from the inside by moving combustion products through them, the frequency of elastic vibrations is determined from the condition of vibration of the treated vessel at its own frequency, and the moment of resonance is determined by a jump-like increase in the amplitude of the vessel's own vibrations.

The disadvantage of this method is that the vibration processing is carried out by moving inside the welded joint of the combustion products from the heat generator, which is impossible to carry out during repair and welding work on pipelines.

A known method of reducing residual stresses in welded joints of metals (Patent RU No. 2424885, publ. 07.27.2011), selected as a prototype. The method includes applying a cyclic load to the crystallizing metal of the weld pool in the transverse direction. The cyclic load is applied by low-frequency vibrations in a vertical plane perpendicular to the axis of the weld. At the same time, at least two vibrators operating in antiphase and located symmetrically relative to the axis of the seam on each of the two welded edges are used to excite vibrations. Use vibrators operating at frequencies from 50 to 300 Hz with an amplitude of up to 0.8-1 mm. For welding structures having significant diametrical and / or longitudinal dimensions, additional pairs of vibrators are installed.

The disadvantage of this method is that it is proposed to carry out vibration processing in a wide range of frequencies without specifying a specific frequency, although it is known that the effectiveness of vibration processing depends on the used oscillation frequency.

The technical result of the invention is to improve the quality of the weld and the weld zone of the base metal of the welded joint of pipelines.

The technical result of the invention is achieved by the fact that in the method of removing residual stresses in the welded joints of metal pipelines, including the application of low-frequency vibrations by vibration devices to the weld and the metal heat-affected zone during welding, the frequency of forced vibrations supplied is equal to the natural frequency of the pipeline section with the welded joint ƒ ^∗ , which is calculated according to the well-known formula (Gorshkov L.K. Fundamentals of the theory of mechanical vibrations in exploratory drilling SPb .: SPGGI, 1998. - 109 s):

$${ƒ}^{*}={\nu }_n^2{\left(E\cdot J/\rho \right)}^{0.5}/\left(l^2\cdot 2\pi \right),$$

where ƒ ^∗ is the natural frequency of the pipeline section, Hz,

ν _n is the spectrum of characteristic numbers, for the calculation of which an approximate formula can be used

n=1, 2, 3… $${\nu }_n\approx \frac{2n+\mathrm{one}}{2}\pi ,$$

n = 1, 2, 3 ...

accordingly, ν ₁ = 4.71, ν ₂ = 7.85, ν ₃ = 11.00 ...,

E is the modulus of normal elasticity, Pa,

J is the axial moment of inertia of the pipe section of the selected size, m ⁴ ,

ρ is the calculated mass per unit length of the pipeline, kg / m,

l is the distance between the points of rigid fastening of the pipeline section, m

The calculation is carried out at the first value of the characteristic number ν ₁ = 4.71, since at the first frequency of natural vibrations of a rigidly fixed section of the pipeline with a welded joint, the welding place will resonate with a maximum amplitude, as a result of which the positive effect of external vibration on the quality of the welded joint will be maximum . The calculation formula takes the form:

ƒ ^∗ = 3.57 · (E · J / ρ) ^0.5 / l ²

The imposition of low-frequency vibrations by vibration devices on the weld and the heat-affected zone of the metal with a frequency of input vibrations equal to the natural frequency of the pipe section with the welded joint, provides vibrations of this section in the resonance mode, that is, with maximum amplitude. This ensures that significant welded stresses appear in the weld and the heat affected zone, the summation of which with existing residual stresses leads to local plastic deformations of the metal with the disappearance of residual stresses in the welded joint. Since the yield strength of metals decreases significantly with increasing temperature, the vibration of the welded joint during welding enhances this positive effect.

A schematic diagram of a method for pipelines is presented in figure 1. The section of the pipeline 1 where welding is to be performed, for example, to weld the fitting 2, is lifted on cables 3 above the bottom of the trench 4 using pipe layers 5. After cutting the required hole in section 1 and fixing the fitting there using “tacks” next to it on the pipe is installed with a clamp 6 with a vibration device 7 with a frequency controller 8. For welding, use the electrode holder 9 and the welding unit 10. As a vibration device 7, for example, an asynchronous electric motor can be used AC power with unbalance and frequency converter or standard electromagnetic device.

Based on the size of the pipeline and the distance l between the points of rigid fastening of the pipeline section 1 with a welded connection by ropes 3, the natural vibration frequency ƒ ^{∗ of the} section of the rigidly fixed section of the pipeline 1 with a welded joint is calculated by the formula. Using a vibration device 7 and a vibration frequency regulator 8, low-frequency vibrations are superimposed on a pipeline section with a welded joint with a frequency equal to the calculated frequency of its own vibrations, and welding is performed simultaneously with vibration.

As the calculations showed, the ƒ ^∗ values of the pipeline section 1, depending on the most common pipeline sizes and its length l, are in the frequency range from several tens to several hundred hertz (Figure 2).

Below is information about the effect of this method on the mechanical properties of welded joints from dissimilar steels, significantly differing in strength properties, to improve the quality of which after welding, currently using expensive and difficult to implement in the field heat treatment. In accordance with the results of metallographic and physical and mechanical tests of samples cut from welded joints from dissimilar pipeline steels (St3sp with 10G2FB and 20 with 16GS), differing in magnitude of σ _{B by} more than 80 MPa, the low-frequency vibrations used during welding favorably affect the structure of the weld, grinding grain, and mechanical properties (KCV at -20 ⁰ С, σ _B , σ _T ) of both the weld and the heat-affected zone of these steels. Moreover, the greatest effect — an increase in the KCV of the weld metal up to 105% —is achieved at the low-frequency oscillation frequency closest to the natural frequency of the welded section. As tests of such samples for low-cycle fatigue showed, the method increases the number of loading cycles up to 4 times that a welded joint from multi-strength steels can withstand to failure (from 0.25 · 10 ⁶ to ≥1 · 10 ⁶ cycles).

Example. We calculate the frequency of natural vibrations for the section of the steel pipe 1, which is being welded, with the nominal external and nominal internal diameters of the pipe indicated in FIG. 2. For the distance l, we take the distance between the points of hard fastening of the pipeline section with cables 3 when it is lifted from the bottom of the trench for the duration of repair and welding works. The value of the elastic modulus E in tension-compression of steel is considered equal to 200 GPa. The axial moments of inertia of the pipe sections of the analyzed sizes are calculated by the formula J = π (D ⁴ -d ⁴ ) / 64, where D and d are the outer and inner diameters of the pipeline section. When calculating the mass per unit length of the pipeline section ρ, we take the density of steel equal to 7850 kg / m ³ .

The calculation formula takes the form:

$${ƒ}^{*}=\frac{\mathrm{4,5}\cdot {10}^3}{l^2}\cdot \sqrt{\frac{D^{\mathrm{four}}-d^{\mathrm{four}}}{D^2-d^2}},\text{Hz}$$

The calculation results ƒ ^∗ for a rigidly fixed section of the pipeline with a length of 5 m and 10 m are presented in the table in figure 2.

If the section of the pipeline 1 with a welded joint lies at the bottom of the trench, the application of low-frequency oscillations with its own frequency in accordance with the method also gives a technical result. In this case, the length of the fixed section of the pipeline l should be taken as the length of the excavated section of the pipeline.

Claims (1)

A method of relieving residual stresses in welded joints of steel pipelines, comprising applying low-frequency vibrations to the weld seam and the metal heat-affected zone by vibration devices, characterized in that the application of low-frequency vibrations to the weld and metal heat-affected zone is carried out with a frequency equal to the natural frequency of the section pipeline with a welded joint, which is calculated by the formula:
$$f^{*}=\frac{\mathrm{4,5}\cdot {10}^3}{l^2}\cdot \sqrt{\frac{D^{\mathrm{four}}-d^{\mathrm{four}}}{D^2-d^2}},$$

where f ^* is the frequency of natural vibrations of the pipeline section with a welded joint, Hz;
D is the outer diameter of the pipeline section, m;
d is the inner diameter of the pipeline section, m;
l is the distance between the points of rigid fastening of the pipeline section with a welded joint, m

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