Evaluating the Effectiveness of Magnetic-Pulse Treatment for Healing Continuity Defects in the Metal of Oil and Gas Pipelines

Abstract

This research paper addresses the issues in evaluating the effectiveness of magnetic-pulse treatment for healing continuity defects in the metal of oil and gas pipelines. A theoretical analysis of the magnetic-pulse action mechanism on continuity defects in the metal was carried out. The results of experimental studies of the effect of magnetic-pulse action on continuity defects of thick-walled samples, cut from used gas pipelines containing microcracks with different geometries, are also presented. The samples were processed under two different technological operating modes of the magnetic-pulse unit: the applied energy was 10 kJ for the first mode and 20 kJ for the second mode. The state of the cracks’ microstructure before and after the magnetic pulse treatment was studied using an optical microscope. As a result of the studies, it was found that magnetic-pulse treatment led to local heating of the crack tips, which was confirmed by the formation of a heat-affected zone in the vicinity of the crack tips. The temperature at the crack tips reached the metal’s melting point at the applied energy of 20 kJ, whereas at the energy of 10 kJ, signs of metal melting were not noted. In the course of the conducted experiments, it was found that the cracks were not completely eliminated after magnetic-pulse treatment; however, the edges of the crack tips melted, with subsequent filling by molten material. Magnetic-pulse treatment resulted in blunting of the crack tips, as their shape became smoother. It was established that the geometry and shape of the crack tip have significant influences on the effectiveness of this technology, as a narrow and sharp crack tip required less energy to reach the metal’s melting point compared to smoother one. The effect of magnetic pulse treatment on the microstructure of pipeline metal and its strength characteristics was also studied. It was found that this treatment leads to structural changes in the area of the crack tip in the form of grain refinement and subsequent strengthening of the pipeline metal.

1. Introduction

In the technological processes of manufacturing and during the operation of pipelines, continuity defects (including point and linear defects, such as delamination, cracks of various nature, etc.) occur in their structure. The nucleation, growth, and accumulation of these defects in the metal structure lead to a decrease in the ability of the metal to resist deformation due to the loss of plasticity [1,2,3,4].

One peculiarity of continuity defects is that these defects during loading cause a concentration of stresses near their tips, leading to a rapid increase in their number and geometric dimensions, followed by the merging of the latter and the formation of large discontinuities. As a result, the operational properties and service lifespan of structures located under the load decrease [5,6,7,8,9]. Additionally, the shape of the defect tips has proved to be a significant factor influencing the crack growth rate and the metal’s fracture toughness. It has been observed that cracks with sharp tips tend to exhibit higher stress concentrations compared to cracks with smoother, rounded shapes. This higher stress concentration can result in accelerated crack propagation and increased susceptibility to failure. Therefore, cracks with sharp geometries are generally considered more dangerous, and they require careful monitoring and treatment strategies to prevent structural failures [10,11,12].

Oil and gas pipelines are known to be constantly in a complex stress state since they are always exposed to multiple variables of static loads (for example, pressure fluctuations, hydraulic shocks, vibration processes, temperature effects, etc.), providing favorable conditions for damage accumulations and hence leading to destruction. Analyses of the causes of accidents in pipelines indicate that the occurrence, accumulation, and propagation of microdefects in the metal are the main reasons for the decrease in their operational properties and their reliability, thus causing their failure [13,14,15,16,17,18].

Consequently, early detection of metal continuity defects and their elimination before they reach a critical size are urgent tasks, the solution of which will significantly improve the operational properties of the working elements of pipelines and extend their lifespan.

Repair of gas and oil pipelines in most cases is performed by replacing defective sections. However, this technology is expensive and requires much repair work and time [19,20,21,22,23]. At the same time, to reduce the time and cost of repair work, a number of repair methods have been developed that allow for restoring defective metal sections without the need to replace them. The methods used to eliminate discontinuities in metal structures include electron beam processing, heat treatment, processing with laser shock pulses, and processing with high-energy pulsed current [24,25,26,27]. However, experience has shown that these methods are only effective for eliminating surface defects due to the small penetration depth. Moreover, these methods can only be applied in laboratory conditions [28,29,30].

One of the modern methods for eliminating continuity defects in metal structures under load is magnetic-pulse treatment (MPT). The principle of this method is that the processed metal is exposed to high-energy magnetic-pulse action. The mechanism for eliminating discontinuities when a metal is exposed to a high-energy electromagnetic pulse has not been sufficiently studied and requires further research. Meanwhile, it was found that, when metal is exposed to a high-energy electromagnetic pulse near the tips of defects, along with mechanical action, the concentration of the electromagnetic field occurs, leading to heating of the material, and, as a result, partial melting of the metal in the zone of maximum concentration—at the tip of the defect [31,32].

In these research studies [33,34,35], mathematical models were proposed for studying the physical mechanisms of restoring the continuity of metal structures by a high-energy electromagnetic pulse. From the proposed models, it follows that, when a metal is exposed to a high-energy electromagnetic pulse near cracks, rapid, non-uniform local heating occurs, accompanied by thermal expansion, and as a result, large compressive stresses arise, leading to closing of crack edges and slowing of their further development. Computer modeling has been based on samples containing cracks with a regular shape since building a model that accurately simulates real cracks is an unattainable task. This limitation makes it difficult to predict the effectiveness of this technology for treating samples containing real cracks with an irregular shape using numerical modeling. Therefore, to obtain a more accurate assessment of the effectiveness of MPT, it is necessary to conduct experimental studies of standard samples containing cracks that have formed during a long-term operation.

Experimental confirmation of the elimination of defects in metal structures by high-energy electromagnetic action was obtained by [36]. The study showed that microdefects of titanium plates were “healed” after electromagnetic impact. Meanwhile, the closure of defects occurred under certain characteristics (value, duration, etc.) of the impulse action.

In the research [37,38], experimental studies of volume changes in microdefects and restoration of the continuity of samples made of various steels, after exposure to high-energy electromagnetic pulses, were carried out. It was found that, due to the electromagnetic effect, the continuity of the material structure was restored. In this case, not only were primary microdefects “healed” but also defects that arose during the process of plastic deformation.

Based on the analysis of the scientific papers devoted to the issue of the effect of magnetic-pulse treatment on metal continuity defects, it should be noted that experimental studies were carried out only on thin metal plates (up to 1 mm thick) made of alloys with low-alloyed pipe steel, titanium alloy, and aluminum alloy [39,40,41,42]. These experiments did not allow for studying the effectiveness of magnetic-pulse treatment depending on the depths of defects and their localization relative to the surface of the gas pipeline, due to the significantly greater thickness of actual industrial pipelines.

This study involved the magnetic-pulse treatment of thick-walled samples cut from decommissioned main gas pipelines. The use of samples with standard thickness showed a high ability of induced currents to penetrate deep into the metal, as evidenced by the melting of the metal observed at the crack tip, located at a depth of 14 mm from the sample surface. This finding expands the application field of the applied technology not only for surface and near-surface defects but also for deeper defects. On the other hand, the presence of real branching cracks with various shapes and sizes provided insights into the behavior of induced currents since it was noted that current lines tend to concentrate on tips with more regular, narrow shape.

Previous related experimental studies of the possibility of eliminating continuity defects were based on defect closure as a result of plastic deformation occurring in the vicinity of the crack under the influence of impulse pressure, formed in the working zone of the inductor and propagating within the treated metal. However, until today, there have been no experimental confirmations of the findings from previous analytical research that healing continuity defects can occur through the melting of their tips. In the light of this research gap, the presented experimental results, confirming the possibility of localized welding of crack tips using magnetic-pulse treatment, can be considered the primary innovation of this work.

2. Background

MPT is carried out on serial high-energy magnetic-pulse units (MPU), which create high-energy electromagnetic pulses with various parameters (intensity, duration, frequency of discharge current, etc.) [43].

The principle of operation of a magnetic-pulse unit is based on the direct conversion of electrical energy stored by energy storage devices into an electromagnetic field acting in the working zone of the magnetic-pulse unit inductor.

Thus, the sample is subjected to intense, complex thermo-mechanical action. On the one hand, the magnetic field pressure acts on the metal surface in the form of compression waves propagating in the metal and causing plastic deformation of discontinuities and a decrease in their size [44,45]. On the other hand, because the electric current tends to flow in the direction with the least resistance, combined with a sudden change in direction at the crack tips, a concentration of induction current lines occurs at the crack tip, leading to an increase in their density.

The concentration of eddy currents leads to non-uniform local heating of defect tips accompanied by thermal expansion, and as a result, thermal compressive stresses ${\sigma }_T$ appear in the defect contour, leading to crack closure [46,47].

The thermal stresses can be determined by the formula:

$${\sigma }_T=E_0{a}_L{T}_a$$

(1)

where $E_0$ is Young’s modulus in Pa; $a_L$ is the coefficient of thermal expansion in °C⁻¹; and $T_a$ is the temperature in °C.

The temperature near the crack tips may, depending on the parameters of the MPT and the properties of the processed metal, increase up to the melting point of the metal. In this case, a crater is formed, with a diameter that can reach several millimeters. Molten metal is ejected from the tip into the cracks under the action of compressive stresses. As a result, the cracks are welded with molten material and their further propagation is prevented.

Figure 1 shows the mechanism of concentration of induced currents at the crack tip.

The density of induction currents in the processed metal $j_m$ depends on many parameters [48,49,50]:

$$j_m=f(W_H,U_H,C,{\eta }_M,{\eta }_a,{\Delta }_s,{\Delta }_I,l_p,S_I,h_g,\tau ,{\rho }_{})$$

(2)

where $W_H$ is the stored energy in storage capacitors, measured in kJ; $C$ is the electrical capacity of storage devices, measured in F; $U_H$ is the operating voltage of storage devices, measured in kV; ${\eta }_M$ is a dimensionless coefficient, which characterizes the transfer of electrical energy flowing in the discharge circuit to electromagnetic energy; ${\eta }_a$ is a dimensionless coefficient, which expresses the energy lost due to the active resistance of the discharge circuit; ${\Delta }_S$ is the penetration depth of the induction currents in the processed metals in mm; ${\Delta }_I$ is penetration depth of the magnetic pulsed field through the inductor thickness in mm; $l_p$ is length of the working part of the inductor in mm; $S_I$ is inductor working area in mm²; $h_g$ is the gap between the inductor and the processed metal in mm; $\rho$ is specific electrical resistance of the processed material in Ohm.m; and $\tau$ is discharge current duration in μs.

The density of induction currents at the crack tip $j$ can be determined by the formula [51]:

$$j=\frac{{\mu }_o{\eta }_M\sqrt{{\Delta }_s.\omega .C_V.\gamma }}{(1+k).{\tau }^{0.5}}$$

(3)

where $\omega$ is operating frequency of the discharge current, flowing in the inductor coil in kHz; ${\mu }_0$ is magnetic permeability of vacuum, ${\mu }_0$ = 1257 μH/m; $C_V$ is the specific heat capacity of the material in J.k⁻¹; $\gamma$ is the specific electrical conductivity of the material in S/m; $k=r/a$ is a dimensionless coefficient characterizing the ratio of the defect opening to its length, where r is defect opining in mm; and a is defect length in mm.

Formula (3) shows that the density of induced currents near the crack tip depends not only on the thermo-physical properties of the metal but also on the geometry of the crack and its shape. Since the shape and size of the defect significantly influence the distribution of the induced current density, smoother and broader shapes can create more favorable conditions for current flow, leading to a more uniform distribution of current lines, consequently decreasing in their concentration. Meanwhile, at sharp tips, an abrupt change in the direction of the currents occurs, followed by their concentration in this zone and, accordingly, an increase in their density.

It should be noted that the pressure and temperature at the crack tips can exceed the limits of metal fracture at a high stored energy of the MPU. Accordingly, an important point in MPT is the determination of the optimal modes of MPU and the limits of their application in accordance with the properties of the treated metal [52,53,54]. Thus, the determination of the optimal MPU operating modes and the limits of their application is an urgent task that requires further research.

3. Materials and Methods

3.1. Sample Preparation

As a material for research, a defected section was cut out from a gas pipeline made of 17G1S low-alloy steel in the area of a transverse butt-welded joint, and a sector fragment was prepared (Figure 2).

The cut fragment contained a macro-crack in the fusion zone that extended to the fragment surface, and the crack initiation point was attributed to a welding defect (incomplete root penetration of the weld). The crack grew rapidly during pipeline operation due to internal pressure of gas. The crack length was 240 mm (Figure 2). This main gas pipeline has been in operation for 25 years, and it has dimensions of Ø1220 × 18.0 mm.

Figure 2 shows a fragment of the gas pipeline.

From the macro-crack zone (zone (a), Figure 2), a sample 55 × 15 mm in size was prepared for a microsection using a band saw.

The study sample contained a central crack that propagated from the outer surface of the gas pipeline into the depth of the metal within the heat-affected zone formed during welding. The crack had a depth of 13 mm, and the average opening was 2.3 mm (Figure 3). The crack branched out in two directions, the tips of which were assigned numbers 1 and 2 (Figure 4).

The lateral surface of the sample was ground and polished with a LaboPol-60- Struers Corporation-Copenhagen, Denmark grinding machine. As polishing agents, a polycrystalline suspension based on 3-μm silicon oxide was used. Figure 4 shows the sample after the grinding and polishing processes, and the zones for detailed studies are marked on it.

3.2. Research Methodology and Characteristics of the Instruments Used

The initial state (before MPT) of the crack microstructure at tips №1 and №2 was examined using an Olympus BX51-Olympus Corporation-Hamburg, Germany optical microscope. After determining the initial state of the crack at the tips №1 and №2, the sample was subjected to magnetic pulse action using an MPU-40 kJ magnetic pulse unit. Sample processing was carried out in two different technological modes: in the first mode, the stored energy in the capacitors of the MPU was 10 kJ; and in the second mode, the energy was 20 kJ. After each stage of processing, the state of the microstructure of the crack at tips №1 and №2 was re-examined using the Olympus BX51-Olympus Corporation-Hamburg, Germany optical microscope. Also, after magnetic-pulse processing with an energy of 20 kJ, the metal hardness near crack tip №2 and in the base metal zone (zone №3) was measured using a Matsuzawa hardness tester mmt—x at a 100-g force load.

The parameters of the MPU-40 kJ magnetic pulse unit are presented in

Table 1. MPU-40 kJ parameters.

Stored Energy (kJ)	Charge Voltage (kV)	Discharge Frequency (kHz)	Mass (kg)	Dimensions (M)
40	1–20	34	1500	1.950 × 1.700 × 1.860

.

The sample was processed using a multi-turn cylindrical inductor, the coil of which is made of copper, the inner diameter of which is 70 mm, and the number of turns of which is five. The oscillogram of the discharge current flowing through the inductor circuit was recorded using a Rogowski coil current sensor-SU-Samara-Russia with sensitivity of 5.63 kA/V. The current sensor was connected to a LeCroy WaveSurfer 44Xs-A-LeCroy corporation-Hamburg, Germany digital oscilloscope with a bandwidth of 400 MHz to display the obtained oscillogram.

Figure 5 shows the cylindrical inductor with a processed sample placed inside it, as well as the current sensor connected to the terminals of the inductor.

4. Results and Discussions

Figure 6 shows an image of the initial state of crack end №1, taken by an Olympus BX51-Olympus Corporation-Hamburg, Germany optical microscope at 50× magnification on an unetched sample surface. A microcrack extending inward from the end of №1 into the metal can be observed, with a length of 711.2 μm and an average opening of 65 μm.

Figure 7 shows the initial state of the microcrack tip extending from the end of №1; the image was taken with an Olympus BX51-Olympus Corporation-Hamburg, Germany optical microscope at 500× magnification.

During magnetic-pulse treatment at 10 kJ, an oscillogram of the discharge current flowing in the inductor circuit was recorded (Figure 8).

Figure 8 shows that the discharge current had a sinusoidal damped character with a maximum amplitude of 76 kA and a duration of 700 μs.

After carrying out magnetic-pulse treatment at 10 kJ, the state of crack tips №1 and №2 was examined using an Olympus BX51-Olympus Corporation-Hamburg, Germany optical microscope.

Figure 9 shows the image of crack end №1 after MPT with energy of 10 kJ, obtained with an optical microscope at 50× magnification.

The state of the microcrack tip extending from end №1 after MPT at 10 kJ is shown in Figure 10.

Comparing Figure 7 and Figure 10, it can be noted that the MPT-10 kJ mode caused local heating, leading to point melting of the edges of the microcrack and the formation of oxides. Furthermore, there was observed separation of micro-areas from the edges of the crack tip (indicated by number 1 in Figure 10), which could be attributed to the impact of high-pulsed pressure generated in the working zone of the inductor on the metal surface as a result of magnetic field repulsion of the inductor and induction currents.

Figure 11 shows the image of crack end №2 after magnetic-pulse treatment with an energy of 10 kJ, obtained by an optical microscope at 100× magnification.

In Figure 11, the consequences of a significant increase in temperature can also be observed but without any signs of melting on the edges. The results obtained can be explained by the smoother shape of tip №2, as it has a smaller depth than its opening, leading to scattering of induction currents and, as a result, the metal temperature during processing under the 10-kJ mode not reaching the value necessary for melting the metal.

During magnetic-pulse treatment at 20 kJ, an oscillogram of the discharge current flowing in the inductor circuit was recorded (Figure 12); the maximum amplitude of the discharge current was 88 kA, and the pulse duration was 800 μs.

The microstructure of tips №1 and №2 after the second treatment mode (with energy of 20 kJ) was re-examined using an optical microscope. Figure 13 shows the image of tip №1 after MPT with energy of 20 kJ, obtained at 100× magnification.

Figure 13 and Figure 14 present the consequences of local heating in the microcrack region, demonstrating the emergence of a heat-affected zone. Tempering colors of steel appeared along the microcrack edges. The temperature at the microcrack tip reached the melting point of steel, as confirmed by the melting of “island” micro-regions that had separated from the microcrack edges after the first MPT mode (with energy of 10 kJ), as shown in Figure 10, as well as crystallized micro-splatters of molten metal on the micro-grinding surface (Figure 14) As a result of the melting of these microsections in the area of microcrack №1, metal redistribution occurred—the microcrack edge was filled with metal with the formation of oxide films, but a spherical pore was formed near the tip of №1, which is a less dangerous defect compared to the sharp end of the crack.

Figure 13 demonstrates that the metal melting occurred in the vicinity of the microcrack tip, while in the microcrack initiation zone (zone a), there were no signs of metal melting. This outcome can be attributed to the concentration of induction currents at the crack tip. This observation confirms the results of numerical simulations in related research, indicating that the highest activity of the induced electromagnetic fields occurs precisely at the crack tip.

Figure 15 shows the state of tip №2 after MPT with energy of 20 kJ, taken with an optical microscope at 100× magnification.

In Figure 15, there is also observed a melting of the microsection in the area of tip №2, resulting in the formation of a crater. This finding can be explained by the density of induction currents reaching the necessary values to raise the temperature to the melting point of the metal. As a result, an increase in the length of the microcrack was noted. On the other hand, the crack tip became blunted as the rough relief of the crack edge became smoother. Blunting of the crack tips is well known to reduce stress concentrations in their vicinity, playing a crucial role in determining the material’s behavior during fracture. The reduction in stress concentration decreases the risk of further crack propagation and positively affects the material’s deformation behavior.

Based on the obtained Figure 10, Figure 11, Figure 13, Figure 14 and Figure 15, it can be noted that the effectiveness of the applied technology largely depends on the geometry and size of the crack. Since at the sharp tip of №1 with a size of several micrometers, the melting of the metal at the crack edges occurred with their subsequent filling with molten metal, at the smoother tip №2, with a size of tens of micrometers, the scattering of induction currents occurred with a subsequent decrease in temperature, which required a greater amount of energy to achieve the metal melting point. This observation corresponds to the mathematical model proposed in [51], which stated that the density of induction currents near the crack tips is inversely proportional to the degree of defect opening.

In addition, the complex configuration of crack tip №2 created unfavorable conditions for the flow of induced currents, resulting in the scattering of their lines with a subsequent decrease in their density.

Based on the obtained results, it can be concluded that this study corresponds to the findings of previous works [36,37,38,39,40,41,42] and enables the expansion of this technology’s application beyond thin plates to include thick samples, provided that the operating parameters of the MPU are appropriately selected in accordance with the geometry of the processed defects. In addition, it is worth noting that the experimental results validate the findings of the numerical simulations conducted in related studies [30,31,32]. Numerical simulation was carried out based on a numerical, coupled model related to the interaction of a high-energy pulsed electromagnetic field with a pre-damaged thermal elastoplastic material containing defects, such as flat intergranular microcracks with linear dimensions on the order of 10 µm. The model used considered the melting and evaporation of the metal, as well as the dependence of its physical and mechanical properties on the temperature. The system of equations was solved numerically using the finite element method on adaptive lattices using the alternative method of Euler–Lagrange.

The temperature change graphs obtained through modeling revealed that the point of greatest impact during MPT was the crack tip, where an increase in temperature up to the metal melting point was observed, corresponding to the observation shown in Figure 13, Figure 14 and Figure 15 that the magnetic-pulse treatment resulted in high, non-uniform local heating of defect tips up to the metal melting point.

Moreover, injection of molten metal into the crack is noted, which can be explained by the action of compressive thermal stresses that arose in the vicinity of the crack tips as a result of heat treatment. These findings confirm the results of related studies carried out in works [46,47] that non-uniform local heating in the vicinity of the crack tips causes compressive thermal stresses, leading to the ejection of molten metal from the edges of the cracks toward the interior.

In one study [55], based on the finite element method, a mathematical model was proposed for calculating the stress intensity factor (K) for cracks located in a stress concentration zone in elements of metal structures. The dependence of the intense stress coefficient (K) on the crack shape was found.

For semi-elliptical cracks, a dimensionless coefficient Q was assigned:

$$Q=1+1.464{(\frac{a}{b})}^{1.65}$$

(4)

where Q is a dimensionless coefficient that characterizes the shape of the crack; a is the crack length in mm; and b is the crack width in mm.

Figure 16 shows the idealized semi-elliptical surface crack geometry.

Figure 17 presents the dependence of the stress intensity factor on the crack shape.

Figure 17 shows that the crack shape significantly affects the stress intensity factor at its tip since more elongated cracks cause more stress concentrations at their tips than cracks with a more rounded shape. The observed findings can be explained by significant changes in metal geometry near a sharp crack tip resulting in a significant stress gradient, which consequently leads to increased stress, as stress is a function of its gradient. On the other hand, a rounded shape leads to a more uniform distribution of stress and, consequently, a decrease in the stress intensity factor.

Figure 13, Figure 14 and Figure 15 demonstrate that MPT resulted in blunting of the crack tips, leading to their smoothing. Based on this observation and the correlation obtained in work [55], it can be concluded that the blunting of crack tips leads to a decrease in the stress intensity factor, consequently reducing the risk of further crack propagation.

The microhardness of the metal was measured in the region of tip№2 (in the heat-affected zone) and in zone №3 (base metal). Figure 18 shows the location of the microhardness measurement points near tip №2; the figure was obtained at 50× magnification. The results of microhardness measurements are presented in

Table 2. Microhardness of the sample metal in zones №2 and №3 after MPT with energy of 20 kJ.

№ Measuring Point	Base Metal Microhardness (Zone №3), HV	Microhardness in Zone of the Tip №2, HV
1	243	322
2	277	400
3	239	351
4	276	426
5	287	364
Average value, HV	264.4	372.6

.

After conducting microhardness measurements, it was found that the metal microhardness near the crack tip increased by 1.5 times compared to the microhardness of the base metal after MPT with 20 kJ. The results obtained can be explained by rapid local heating followed by rapid cooling causing a local thermal treatment in the heat-affected zone, leading to an increase in the hardness of the metal.

In the study [56], the dependence of fracture toughness on steel hardness was studied for 6-5-2 type high speed steel (0.85% C, 6% W, 5% Mo, 2% V) quenched from various austenitizing temperatures. All the samples had fatigue pre-cracks with lengths of 2 mm.

As a result of the study, it was found that fracture toughness decreases with increasing material hardness, which in turn reduces the material’s ability to resist crack propagation because an increase in hardness is usually accompanied by a more brittle metal structure and a decrease in the metal’s ability to absorb deformation energy during crack growth, making the material more susceptible to failure.

The analysis of the results of studies conducted in works [55,56] complicates the resolving of the task presented in our research since magnetic-pulse treatment leads to the blunting of crack tips and a decrease in stress in their vicinity. On the other hand, the metal becomes more brittle and less resistant to destruction.

To substantiate the obtained results, metallographic studies of the metal microstructure were carried out in two areas: in the heat-affected zone near tip №2 and the base metal outside the heat-affected zone (zone №3).

Figure 19 shows the microstructure of the base metal outside the heat-affected zone (zone №3) after MPT at 20 kJ.

Figure 19 illustrates the initial microstructure of 17G1S low-carbon steel in the base metal zone (outside the heat-affected zone), displaying a coarse-grained ferrite–pearlite structure.

The initial microstructure near crack tip №2 before magnetic-pulse treatment was studied using an optical microscope (Figure 20).

Comparing Figure 19 and Figure 20, it can be observed that the structure of the metal remained ferrite–pearlite, while grain recrystallization has occurred, followed by grain refinement. This phenomenon can be explained by crack tip №2 being located within the heat-affected zone of welding, where the structure was subjected to high temperatures.

Figure 21 shows the microstructure of the heat affected zone near tip №2 after MPT at 20 kJ.

Comparing Figure 20 and Figure 21, further grain recrystallization is observed after processing, resulting in a decrease in grain size, while the structure remains ferrite–pearlitic. This observation can be attributed to the temperature in the crack tip zone exceeding the austenitic transformation temperature (Ac3), followed by cooling and transformation into a finer-grained ferrite–pearlite structure.

The microstructure has become finer compared to the microstructure of the base metal (the grain size of the steel has significantly decreased), which, as known, leads to metal strengthening. This outcome is further confirmed by a 1.5-fold increase in metal microhardness near the crack tip compared to the base metal.

5. Conclusions

An analysis of the causes of accidents on oil and gas pipelines indicates that the occurrence, accumulation, and propagation of continuity defects in the pipeline metal are the main reason for the decrease in their reliability and premature failure. These defects cause stresses concentration, especially at crack tips, leading to their rapid growth until complete destruction.

Based on the analysis of the methods used to eliminate continuity defects of metal structures, it can be concluded that, today, there are no repair methods that could be used in practice to repair pipelines without the need to replace defective sections, which could significantly reduce the cost and time needed for repair works.

As a result of a theoretical study of the mechanism of the influence of magnetic-pulse impact on metal continuity defects, it was found that the process of “healing” of continuity defects in the case of their early detection occurs due to the concentration of induction currents at the tips of the crack, leading to significant local heating, accompanied by thermal expansion, causing thermal compressive stresses working to close cracks.

Experimentally, a significant increase in temperature at the crack tips during magnetic-pulse treatment was noted, which was confirmed by the formation of a heat-affected zone in the in the vicinity of crack tips №1 and №2. The obtained results validated the findings of the numerical simulations conducted in studies [30,31,32] that, when the metal is exposed to a high-energy electromagnetic pulse, rapid non-uniform local heating occurs near the crack tip.

During the magnetic-pulse treatment with energy of 10 kJ, the separation of microsections from the crack edges in the zone of tip №1 was also noted. This phenomenon can be explained by the emergence of pulsed pressure in the working zone of the inductor as a result of the repulsion of the magnetic fields of the discharge current and induction currents. This observation confirms the results of numerical simulations carried out in studies [46,47].

It was also found that, during MPT with energy of 10 kJ, a melting point of the microcrack edges was observed only at the narrow sharp tip (tip №1), where a high concentration of induction currents occurred. At the same time, at the tip with smoother edges (tip №2), there were no signs of metal melting. However, during MPT with energy of 20 kJ, melting of the crack tips edges was noted. In the case of narrow sharp tip №1, the edges melted with subsequent filling by molten metal. In the case of broad crack tip №2, the crack grew and became blunted. Thus, it can be concluded that the geometry and shape of the crack are decisive factors during the application of magnetic-pulse treatment. The sharper and narrower that the crack tip is, the higher that the density of the induction currents is, and consequently, the higher that the heating of the crack tip is. It is also worth paying attention to tip№1 having a more regular shape compared to tip №2, which could have influenced the degree of concentration of induction currents and, therefore, the treatment parameters necessary to achieve the desired effect. Thus, these results indicate the need for further studies in the field of crack geometry to better understanding the influence of the shape and size of cracks on the effectiveness of the applied technology.

It was noted that metal melting occurred at the crack tips with an opening of several micrometers, while in far distant areas from the tips with opening of tens of micrometers (closer to the microcrack initiation zone), signs of melting were absent (Figure 13).

Based on this observation, it can be concluded that the applied technology can be effective in healing defects at early stages of their growth before they reach a critical size, after which the cracks become unstable, and the only possible decision is to replace the defective section. In this regard and before applying this technology, regular monitoring of cracks using non-destructive methods, such as ultrasonic inspection or visual examination, is required. This method can help to track the growth of cracks and determine when they become unstable.

Measurements of the steel microhardness showed that the hardness of the metal near the tip increased by 1.5 times compared to the base metal, which can be explained by rapid local heating and subsequent rapid cooling causing a local thermal treatment in the heat-affected zone, which led to structural changes. Based on obtained images №20 and №21, structural changes in the form of a grain size decrease can be observed. However, to fully understand the nature of the microstructure and its changes in the heat-affected zone and far from it, further research is required.

Studies of the crack tips; state have shown melting of the crack tips edges with subsequent filling with molten metals. As a result, the shape of the crack tip became smoother, reducing the risk of stress concentration and the likelihood of further crack growth. On the other hand, structural changes were noted in the form of a decrease in grain size, which led to hardening of the metal in the vicinity of the crack tips, as evidenced by the 1.5 times increase in microhardness after magnetic-pulse treatment. It is well known that a decrease in grain size leads to a decrease in ductility and a decrease in the plastic strain capacity of the material. As a result, the materials’ fracture toughness and damage tolerance decrease.

Thus, the aforementioned contradiction complicates the task of assessing the effectiveness of the applied technology in preventing further crack growth, as the obtained results do not provide a clear understanding of the mechanism of crack behavior after treatment. Therefore, and to obtain a more accurate assessment of the effectiveness of the applied technology in preventing further crack growth, additional mechanical testing, including tensile and fracture toughness tests, should be carried out. Such tests can provide a more complete understanding of the effectiveness of magnetic-pulse treatment in healing cracks and preventing their further propagation.