RU2294822C2 - Method of automatic argon-arc welding of austenitic steel pipes - Google Patents

Abstract

FIELD: mechanical engineering; argon-arc welding.

SUBSTANCE: invention relates to methods of automatic argon-arc welding of austenitic steel pipes in manufacture of important constructions, such as high-pressure pipes used at nuclear power stations. Proposed method includes mechanical preparation of welding surface zone, veeing edges to miter of 3.0-3.5 mm and subsequent multiple-pass welding with nonconsumable negative electrode using filler wire. Welding edges are veed to miter of 2.7-3.0 mm. Welding at first pass is carried out with heat input of 0.35-0.55 MJ/m by impulse current. Welding at second pass is done with heat input of 0.6-0.86 MJ/m by impulse current at cross oscillations of electrode. Welding at third and subsequent passes is carried out with heat input of 0.62-1.16 MJ/m by steady current with cross oscillations of electrode.

EFFECT: increased capacity art welding and quality of weld joint owing to reduction of intercrystalline cracking of weld joint.

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Description

The invention relates to the field of welding, in particular to methods of welding steels of the austenitic class, and can find application in the manufacture of critical structures, such as high pressure pipelines operating at nuclear power plants.

A characteristic operational defect of the welded joints of pipelines of type Д _у = 300 of the multiple forced circulation circuit of a nuclear power plant is intergranular corrosion cracks in the heat-affected zone. In the USA, the problems of intergranular corrosion of austenitic steels under tension attracted the close attention of specialists already in 1974 and has since turned into one of the most important problems associated with the operation of American nuclear power plants [Rumyantsev V.V. “Pipelines at nuclear power plants: increasing their reliability and durability”, Nuclear Technology Abroad, 1993, No. 3, p.3-8]. The appearance and development of intergranular stress corrosion in the heat-affected zone of welded joints is associated with the influence of the following factors: the tendency of austenitic steels to sensitization during the thermal welding cycle; the presence of a corrosive medium (oxygen-containing reactor water); the effect of the corresponding stresses; the presence of conditions for the initiation of cracks (concentrators, welding defects, etc.).

One way to reduce the influence of these factors is to reduce tensile stress in the weld zone, especially residual welding stresses, by choosing the appropriate modes and welding technology. Reducing the linear energy of welding also contributes to obtaining low values of stresses on the inner surface due to the compensating effect of shrinkage from subsequent passes [V.I. Makhnenko, V.M.Sheker, G.F. Rozynka, N.I. Pivtoran “Residual welding stresses in zone of ring welded joints of pipelines made of austenitic steels ", Automatic welding, 1998, No. 11 (548)].

The closest analogue of the claimed technical solution is a method for producing a welded joint, including mechanical preparation of the welding zone, cutting edges with a horizontal platform (whisker) with a bevel angle of 9 ÷ 15 °, width and thickness of a horizontal platform of 3.0 ÷ 3.5 mm, filling cutting during multi-pass automatic argon-arc welding with non-consumable electrode using filler wire [Shmeleva I.A. and others. Arc welding of steel pipe structures. - M.: Mechanical Engineering, 1986, S. 60-74].

A disadvantage of the closest analogue is the inability to obtain a high-quality welded joint on large-diameter pipelines from austenitic steel.

The problem solved by the invention is improving the quality of the welded joint.

The essence of the invention lies in the fact that in a method of automatic argon-arc welding of austenitic steel pipes, including mechanical preparation of the surface of the welding zone, cutting edges with a mustache 3.0 to 3.5 mm wide and subsequent multi-pass welding with a non-consumable direct current direct current electrode using filler wire, it is proposed to carry out the cutting of edges with a mustache 2.7–3.0 mm thick, weld the first pass with linear energy 0.35–0.55 MJ / m at pulse current, welding in perform the next pass with linear energy of 0.6 ÷ 0.86 MJ / m at pulse current with transverse vibrations of the electrode, and welding the third and subsequent passes with linear energy of 0.62 ÷ 1.16 MJ / m of stationary current with transverse vibrations of the electrode .

The choice of the thickness of the mustache 2.7 ÷ 3.0 mm in combination with the known width of the mustache 3.0 ÷ 3.5 mm allows, on the one hand, to apply the filler wire without increasing the linear energy of welding already when performing the first pass, on the other hand, ensure full penetration of the mustache. The use of filler wire and pulsed welding in the first pass at an average linear energy of the welding arc of 0.35 ÷ 0.55 MJ / m allows, firstly, to increase the content of the ferrite phase in the root bead and to improve its shape, and secondly, to reduce tensile welding stresses in the heat affected zone (HAZ) of welding. Improving the shape of the root bead is achieved by redistributing the ratio of factors determining it in favor of surface tension forces. An increase in the role of surface tension forces allows the welding of vertical fixed joints to reduce the gain from the inside in the lower position, the meniscus in the ceiling position and to exclude fusion of edges in vertical sections. When welding horizontal joints, it is possible to reduce the likelihood of undercuts in the root of the seam. Reducing tensile welding stresses in the HAZ is achieved by increasing the plastic properties of the weld metal. The indicated welding parameters allow reducing the bevel angle of the edges to values of 8 ÷ 10 °, thereby reducing the mass of the deposited metal and the total residence time of the HAZ in the region of least stability of austenite. Reducing the HAZ residence time in the region of least austenite stability is also achieved due to the optimal combination of the linear energy of the welding arc in each pass, the number of passes. The value of the linear energy of the welding arc was determined in accordance with GOST 13585 according to the formula:

Дж/м

J / m

where I is the welding current, A;

U _D - arc voltage, V;

V is the welding speed, m / s;

η - effective efficiency of the thermal action of the arc (for welding in argon η≅0.65).

The choice of the indicated values of the linear energy of the welding arc has the following meaning. The lower linear energy of the welding arc in the first pass is due to the need for complete penetration of the weld root. The value of the upper limit is determined by the need to keep the weld pool from escaping. In the second pass, an increase in the linear energy of the welding arc is associated with transverse vibrations of the electrode and an increase in the path made by the arc per unit time. The lower boundary of the linear energy of the welding arc is due to the condition of overlapping successively superimposed adjacent transverse rollers. The upper limit is determined by the need to maintain the required shape of the weld pool and eliminate burn through metal. The increase in linear energy in the third and subsequent passes is associated with an increase in the amplitude of transverse vibrations of the electrode.

The welding method is as follows. Cut the edges of the pipes to be welded with a mustache of 2.7–3.0 mm thick and 3.0–3.5 mm wide. The bevel angle of the edges is chosen equal to 8 ÷ 10 °. The welded joint is tacked by manual arc welding with a non-consumable electrode at a current of direct polarity using a filler wire with a given content of the ferrite phase. Set the welding machine to a welded joint. Argon shielding gas is supplied into the internal cavity with a flow rate of 6 ÷ 8 l / min. The first pass is performed on a pulsed current of direct polarity with a continuous or stepwise method of moving the electrode and feeding the filler wire to the front of the weld pool. The value of the average linear energy of the welding arc is selected in the range of 0.35 ÷ 0.55 MJ / m, while the nominal value of the linear energy of the welding arc with a continuous method of movement should be lower than with a step-by-step method of movement. This circumstance is associated with the compression of the temperature field across the axis of the weld during continuous movement of the welding arc, with the stepwise method of moving the electrode during the current pulse, the electrode is stationary and the temperature field has close to circular isotherms on the surface of the whisker. The second pass is performed on a pulsed current with transverse vibrations of the electrode in the groove and arc delay at the edges of the groove. When the arc is delayed at the edge, the speed of continuous movement of the electrode along the axis of the seam remains constant. The value of the average linear energy of welding in the second pass is chosen in the range of 0.6 ÷ 0.86 MJ / m. The arc current and pause time occur at the time of transverse movement of the electrode. The current and pulse time of the arc are at the time the electrode moves along the edge. The arc travel time along the edge is selected in the range of 0.35 ÷ 0.51 s, depending on the spatial position of the welded joint. The third and subsequent passes are performed with direct current of direct polarity with transverse vibrations of the electrode in the groove with an arc delay time at the edges of 0.2 ÷ 0.7 s. The value of the average linear energy in the third and subsequent passes is selected in the range of 0.62 ÷ 1.16 MJ / m. The specified welding method was tested on control welded joints and used to repair defective butt welded joints of Du 300 pipelines from austenitic steel 08X18H10T of power unit 4 of Leningrad NPP. Specific welding modes for fixed butt joints of pipelines of Du 300 are given in tables 1 and 2. Table 1 shows the automatic argon-arc welding of vertical fixed joints of pipelines. Table 2 shows the modes of automatic argon-arc welding of horizontal non-rotating butt joints of pipelines.

As a result of using the proposed welding method, the time for welding was reduced, and the sensitization of HAZ metal by the thermal welding cycle was reduced to an acceptable level, which ensured satisfactory resistance of welded metal to intergranular stress corrosion cracking under operating conditions of nuclear power plants.

Claims (1)

A method for automatic argon-arc welding of austenitic-grade steel pipes, including mechanical preparation of the surface of the welding zone, cutting edges with a mustache 3.0-3.5 mm wide and subsequent multi-pass welding with a non-consumable direct current electrode of direct polarity using a filler wire, characterized in that the cutting of edges is carried out with the implementation of a mustache with a thickness of 2.7-3.0 mm, the welding of the first pass is performed with a linear energy of 0.35-0.55 MJ / m at pulse current, the welding of the second pass is performed with linear 0,6-0,86 th energy MJ / m at a pulse stream at transverse oscillations of the electrode, and the third and subsequent welding pass is performed with heat input 0,62-1,16 mJ / m fixed current with transverse oscillations of the electrode.