RU2533403C2 - Method for electrophysical processing of welded joints in metal structures - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: method involves passing of sign alternating pulses of electric current through the joint and heat-affected zone. Current pulses are passed in the joint cooling zone where temperature of metal is lower than the Curie point temperature by 50-100 degrees Celsius. The current pulses are passed perpendicularly to the welded joint. Duration of the current pulses amounts to 1.5-2.5 seconds and intervals between the pulses - to 1-3 seconds.

EFFECT: invention allows for the reduction of energy consumption and labour time at removing residual welding stresses, mainly, of longitudinally electric welded pipes of big diameter.

2 dwg

Description

The invention relates to mechanical engineering and can be used in various industries to increase the reliability of welded joints of steel structures and increase their service life.

There is a method of increasing the bearing capacity of machine parts, in which the heating of defective zones is carried out by a pulsed current for 0.01-0.5 seconds (USSR author's certificate No. 1015561, CL. B23P 6/04). Due to the concentration of current in the region of defects, a temperature gradient is created that causes thermoelastic compression stresses. In this case, localization of defects in the crystal structure, which are stress concentrators, occurs.

However, the existing method of reducing stress is designed to process areas of metal whose dimensions are incommensurably smaller than the cross section through which current flows. It is practically impossible to solve the problem of reducing residual stresses in welded joints of large diameter pipes or other structures in a known manner.

There is a method of reducing residual stresses in welded joints of large metal structures, including electrophysical processing (ETF) of sections with increased residual stresses by passing a packet of alternating current pulses through these sections, the current pulses having the shape of an isosceles trapezoid and a pause between current pulses of 4-6 seconds ( declaration patent for the invention of Ukraine No. 53559A, CL B23P 6/04, S21D 1/8).

The disadvantage of this method is the significant energy and time required to reduce residual stresses.

The aim of the present invention is to reduce energy costs and reduce the working time necessary to reduce residual stresses in welded joints, especially large-diameter longitudinally welded pipes.

This goal is achieved by the fact that in the proposed method of reducing residual stresses in welded joints, including EFO sections with increased residual stresses, in the process of welding through alternating electric current pulses of alternating duration (2, 0 ± 0.5) seconds and a pause between pulses 1-3 seconds.

Another difference is that electric current pulses are transmitted perpendicular to the weld.

The third difference is that current pulses are passed in the weld cooling zone, where the metal temperature is 50-100 degrees Celsius lower than the temperature of the Curie point.

Each of the listed signs differs from the signs of the known solutions used to reduce residual stresses, therefore, the proposed technical solution meets the criterion of "significant differences".

The above set of essential features of the proposed method of reducing residual stresses in welded joints allows you to create a positive effect and therefore determines the compliance of the proposed technical solution with the criterion of "novelty".

Indeed, as a result of plastic deformation during cooling of the weld, internal stresses and residual deformations occur. According to the principle of thermodynamic equilibrium, the system always tends to a state with a minimum of total free energy, therefore internal stresses and residual strains tend to an equilibrium static state. Any violation leads to a redistribution of stresses, which again leads the system to an equilibrium state.

Starting with crystallization, various types of defects (dislocations) arise in metals. All influences during metal processing (thermal, mechanical, chemical, etc.) are associated with the transformation of its dislocation structure. Plastic deformation of metals has a thermofluctuation nature and is the result of elementary shifts in crystals when a dislocation moves in a slip plane, which can become blocked at fixing centers (potential or energy barriers). An increase in internal stresses is accompanied by an increase in the dislocation density due to their accumulation on the stoppers.

The ability of a metal to deform is limited by the exhaustion of plasticity in individual microvolumes, where the centers of destruction occur. Further deformation of the metal without destruction is possible only after heat treatment (annealing). At temperatures of recrystallization annealing, thermal fluctuations always exceed the height of the energy barriers in the lattice and dislocations overcome the stoppers. Microplastic deformation is carried out due to the potential elastic energy accumulated in the process of preliminary plastic deformation of the metal, and there is a structural rearrangement of a relaxation nature (a decrease in the dislocation density and residual stresses).

In contrast to heat treatment during ETF, a non-thermal transformation of the dislocation structure occurs due to a decrease in the height of energy barriers as a result of the interaction of the electromagnetic field of the electric current with the electromagnetic field of a real metal crystal lattice. This interaction occurs, first of all, at the head dislocations of nonequilibrium dislocation groups (in clusters), which are located on the eve of the detachment from the stopper, and the current pulse initiates their discharge. Dislocation disruption from stoppers occurs and the dislocation density decreases (relaxation of residual stresses).

If ETF is performed at elevated metal temperature, then the transformation of the dislocation structure will occur: a) thermal - due to an increase in thermal fluctuations with increasing metal temperature; b) non-thermal - by reducing the height of the energy barriers in the lattice when exposed to electric current. Obviously, with an increase in thermal fluctuations, dislocation disruption from the stoppers is provided with a lower decrease in the height of energy barriers, that is, with a reduced current density of the ETF. The crystallization of ferromagnetic metals (with the appearance of magnetic properties) is characterized by the temperature of the Curie point, above which the magnetic properties disappear. Therefore, when the temperature of the cooling metal is close to the temperature of the Curie point, the efficiency of the electric current will be higher than that of the metal at low temperature.

The present invention, including the ETF of a metal that crystallizes when the weld is cooled, reduces the electric current density necessary to reduce the dislocation density and create an equilibrium metal structure in order to reduce residual stresses.

Experimental verification

Testing the method of reducing residual stresses was carried out by experimental studies.

The test was carried out by a comparative assessment of the emerging residual welding stresses (AIS): a) without ETF; b) with ETF in the process of electric arc welding. The level of residual stresses formed was monitored using the PKON SMF portable non-destructive testing device (TU U 33.2-30976520-002: 2008). The set parameters of the ETF current were provided using the DS10D installation (TU U 31.6-30976520-001-2001).

The studies were carried out on a cylindrical pipe with an external diameter of 270 mm, a wall thickness of 7 mm, a length of 1.2 m, the material was steel 20. A longitudinal section was made mechanically along the length of the pipe (along the axis). The level of AOS in the HAZ was controlled at 13 points uniformly distributed along the seam length. The measurements were carried out using the PKON SMF instrument in relative units. The OCH level measured in a welded steel 20 sample after heat treatment (calibration of the device) was taken as a unit.

The length of the weld was conditionally divided into three sections:

seam section 1 - control points 0-4;

seam section 2 - control points 4-8;

seam section 3 - control points 8-13.

Electric arc welding of the longitudinal seam of the pipe was carried out in the direction from point 0 to point 13, the welding current was 150-180 A (Figure 1).

Welding of weld section 1 was performed without ETF. Welding of weld sections 2 and 3 was carried out under the influence of the ETF current. The ETF current was supplied using special equipment and passed across the weld so that the weld zone and HAZ were processed. ETF current parameters: the amplitude value of the current pulse is 980 A, the duration of the current pulse (2.0 ± 0.5) s, the duration of the pause between pulses 1-3 s, the polarity of the current pulses is alternating. An increase in the efficiency of ETF was achieved due to a local increase in current density in the weld zone and HAZ. To do this, when welding the weld section 3, to increase the efficiency of the ETF, the current connection points approached the weld axis.

After the seam cools down at the control points (1-13), the OSH level in the HAZ was measured. The results are presented in Figure 2.

Experimentally established:

1. A reduction in the level of residual stresses by 50-60% was obtained with ETF pulses of electric current lasting (2.0 ± 0.5) seconds and a pause between pulses of 1-3 seconds.

2. An increase in the efficiency of ETF and the lowest value of the level of residual stresses were obtained at minimum distances to the point of current supply, which confirms the need to pass EPO currents perpendicular to the weld.

3. The smallest value of the residual stresses was obtained with the ETF of the zone of cooling and crystallization of the weld metal, where the temperature is approximately 50-100 ° C lower than the temperature of the Curie point.

Using the proposed method for reducing residual stresses in welded joints of metal structures, including the ETF of the weld and HAZ during cooling, provides significant savings in energy costs and working time compared to existing methods.

The proposed method of reducing residual stresses in welded joints is an effective method to increase the reliability of welded structures and increase their service life. It allows to reduce the level of residual stresses and improve the mechanical properties of the metal of welded joints without after welding.

The proposed method is most effective for local processing of welds of large structures, which is impossible in other ways. The weld zone is processed directly in the process of electric arc welding of metal structures.

Claims (1)

The method of electrophysical processing of welded joints of metal structures, including passing alternating electric current pulses through the seam and the heat affected zone, characterized in that the current pulses are passed in the weld cooling zone, in which the metal temperature is 50-100 degrees Celsius lower than the Curie point temperature, while pulses of electric current lasting 1.5-2.5 seconds with pauses between pulses 1-3 seconds pass perpendicular to the weld.

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