Improving the efficiency of welded joints of the "insert pipe-plate" type

. The article is devoted to the analysis of the causes of damage formation in the transition zone from the seam metal to the base metal of butt welded joints. A method for increasing fatigue strength and improving the properties of compounds of this type is proposed. The analysis of cases of fatigue failures of welded joints operating under repeated static loads indicates that cracks most often originate in the transition zone from the seam to the base metal. For this reason, the geometric parameters of the zone of maximum stress concentration should be taken into account when predicting performance. To analyze the operability of welded joints, which did biaxial loading on a breaking machine test. The operability of welded joints of the "insert pipe-plate" type in the initial state is approximately 4 times lower than the operability of the base metal by remelting the transition zone without electrode oscillations (samples 5 and 6) make up half of the operability of the base metal. By remelting the transition zone with an oscillating electrode with regulation of the linear energy along the width of the seam (samples 7 and 8) are practically not inferior to the operability of the base metal. 1 The design of the installation, which allows for automatic remelting of the transition zone from the seam to the base metal with transverse arc vibrations, providing a coupling radius R = 4 – 6mm. 2 Reduction of stress concentration and regulation of the zone of thermal influence at the junction of the seam with the base metal made it possible to increase the resistance to the origin and development of destruction of welded joints "weld-sheet" to almost similar characteristics of the base metal.


Description of the research object
The article is devoted to the analysis of the causes of damage formation in the transition zone from the seam metal to the base metal of butt welded joints.A method for increasing fatigue strength and improving the properties of compounds of this type is proposed.

Justification of the purpose of the study
Properly designed welded joints of structures operating under static loads, as a rule, provide equal strength with the base metal.However, when working in the re-static loading mode, the endurance limits of untreated welded butt joints can be only half, and joints with angular seams are 1/4 -1/6 of the endurance limit of the base metal [1][2][3][4].
The analysis of cases of fatigue failures of welded joints operating under repeated static loads indicates that cracks most often originate in the transition zone from the seam to the base metal (an example of such destruction is shown in Figure 1) despite the strict rationing of requirements for the size of seams and permissible welding defects.This is due to a number of adverse factors that reduce the performance of welded joints, the main of which are: stress and strain concentration, residual stresses, structural changes associated with the thermal welding cycle, as well as possible welding defects.For this reason, the geometric parameters of the zone of maximum stress concentration should be taken into account when predicting performance.The magnitude of the maximum stresses depends on global stresses, which are determined by the normative parameters of the geometric dimensions of welded joints and seams (specified by GOST standards, normals) and on local stresses, mainly depending on the radius of transition of the seam to the base metal R, which is not regulated by regulatory documents and, as practice shows, can vary from 0.1 to 2 mm.[5][6][7][8][9].
The increase in the durability of welded joints of the " insert pipe-plate " type is mainly achieved by reducing the stress concentration or by creating favorable residual compression stresses at the transition points from the seam to the base metal.In factory conditions, the method of melting the seam boundary by an electric arc burning in an argon medium without filler wire, developed at the E.A. Paton NPP, is widely used.The advantage of the method is the possibility of melting the seam boundary in any spatial position.
The disadvantages include the relatively small radius of the galtel R = 0.5 ... 3 mm and the deep shape of the zone of thermal influence formed during melting.

Research method
To eliminate these shortcomings, a device has been developed at the "MiASP" DSTU, which serves to melt the seam boundaries with an arc burning in an argon medium and performing transverse oscillations of the electrode with a synchronous change in the linear energy of the arc.The change in the value of the linear energy during melting of the seam boundary is achieved by increasing the speed of transverse movement of the welding electrode during its transition from the seam to the base metal and slowing down the movement when the non-melting electrode is located above the seam metal.For this purpose, a special-shaped cam kinematically connected to a welding torch is used in the transverse oscillation mechanism.The melting scheme of the transition zone from the seam to the base metal is shown in Figure 2 Fig. 2. The diagram of the gallon rollers for the transition from the seam metal to the base metal, where 1 -DC electric motor; 2 -reducer; 3 -special-shaped cam that oscillates the electrode and synchronously changes the linear arc energy; 4 -lever mechanism; 5 -burner for argon arc welding with a non-melting tungsten electrode.
The developed method allows to obtain a smooth outline of the transition profile from the seam to the base metal with a radius R = 4 -6mm, shown in Figure 3a.At the same time, a minimum zone of thermal influence is observed on the base metal at the galtel site, which is shown in Figure 3b.Welded joints tested under biaxial bending conditions under repeated static loading in 3% NaCl solution are presented in Table 1.The methodology and design of the installation for testing the base metal and welded joints are described in detail in [10].

Research results
Comparative tests of the base metal and welded joints of the "insert pipe-plate" type: remelted by manual argon arc welding without electrode vibrations and remelted by automatic argon arc welding with transverse electrode vibrations are presented in Table 1.The operability of welded joints of the "insert pipe-plate" type in the initial state (samples 3 and 4) is approximately 4 times lower than the operability of the base metal.Welded joints with remelting of the transition zone without electrode oscillations (samples 5 and 6) make up half of the working capacity of the base metal.
Welded joints of the "insert pipe-plate" type with remelting of the transition zone by an oscillating electrode with regulation of the linear energy along the width of the seam (samples 7 and 8) are practically not inferior to the operability of the base metal.

Conclusions
1.The developed technology is implemented without significant costs at machine-building enterprises and especially at those where arc welding in protective gases is used.2. The design of the installation has been developed, which allows for automatic remelting of the transition zone from the seam to the base metal with transverse arc vibrations, providing a coupling radius R = 4 -6mm.3. Reduction of stress concentration and regulation of the zone of thermal influence at the junction of the seam with the base metal allowed to increase the resistance to the origin and development of destruction of welded joints "insert pipe-plate" to almost similar characteristics of the base metal.

Fig. 1 .
Fig. 1.Destruction of the transition zone from the seam metal to the base metal of the welded joint

Fig. 3 .
Fig. 3. Welded joint" a) the appearance of the seam interface with the base metal; b) transition zone from the seam metal to the base metal after melting The mode of execution of galtel rollers on welded joints is as follows: current I = 120 A; arc voltage U = 8-12V; welding speed V = 8-10 m / hour; frequency of transverse oscillations of the electrode 40-60 min-1; oscillation amplitude 10mm; diameter of the tungsten electrode 3 mm.Welded joints tested under biaxial bending conditions under repeated static loading in 3% NaCl solution are presented in Table1.The methodology and design of the installation for testing the base metal and welded joints are described in detail in[10].

Table 1 .
Test results of the base metal and welded joints of the "weld-sheet" type, under conditions of biaxial re-static bending in a corrosive environment (3% NaCl solution).