Introduction

The technology of argon arc welding (AAW) is widely used in the manufacture of pipelines for various purposes, including coil systems of the radiant section of hydrocarbon cracking units [1,2,3,4]. At present, HP-series austenitic alloys with carbide hardening are used as coil materials, on which basis pipes and fittings are produced by centrifugal casting and static casting, respectively. The coil systems of pyrolysis plants are operated under extreme conditions, including elevated temperatures of 800 – 900°C with local overheats up to 1150 – 1200°C, loads of 5 – 10 MPa, and aggressive corrosive environments [5,6,7]. The equipment of pyrolysis furnaces is designed for a nominal service life of 100,000 h; however, their actual service life comprises 30,000 – 60,000 h depending on the quality of materials and the severity of operating conditions. Welded joints are assumed to be the weakest areas of the pyrolysis furnace [1, 2, 4, 8, 9], largely due to the transformation of the alloy structure during welding, which accelerates its degradation during subsequent high-temperature operation [10,11,12,13,14,15].

The studies [1,2,3, 8, 9, 16, 17] revealed that the microstructure of a welded joint made of HP40NbTi alloy exhibits significant heterogeneity in terms of phase morphology and chemical composition. The highest inhomogeneity is observed in the heat affected zone (HAZ), which contains a varied-grained austenitic matrix with pronounced silicon segregation, as well as dispersed inclusions of G-phase (Nb6Ni16Si7) in the vicinity of large eutectic carbides based on chromium and niobium. Formation of a G-phase in the structure of HP40NbTi alloy during welding is an unfavorable factor causing the nucleation and development of grain boundary cracks under prolonged high-temperature loading of the material, which leads to a significant decrease in its performance characteristics [2, 3, 12, 18,19,20,21,22].

Therefore, the service life of radiant coil systems can be extended by inhibiting G-phase formation in the structure of welded joints made of HP alloys by optimizing the technological parameters of welding. This task represents a complex multifactor problem demanding the application of simulation and optimization methods.

In this part of the article, we set out to simulate and study experimentally the process of multi-pass argon arc welding of centrifugally cast pipes made of HP40NbTi heat-resistant alloy.

Methods of Study

Industrial centrifugally-cast pipes with a length of 2 m, a diameter of 115 mm, and a wall thickness of 14 mm made of HP40NbTi alloy were studied. The features of MIG/MAG welding of pipe sections in continuous and pulsed modes of the current source were investigated. A single vee butt joint with a bevel angle of β = 37° and a gap of b = 2 mm was used to cut pipe edges for welding (Fig. 1). The weld was formed in several passes. No preheating of pipe edges was carried out prior to welding.

Fig. 1
figure 1

Scheme of pipe edge cutting for welding.

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Numerical simulation of the AAW process was performed in the environment of the LS-DYNA computing package. The input data for simulating the product material included the chemical composition of the alloy and filler wire, as well as their mechanical and thermal-physical properties. The function of undefined material based on activation temperature was also used [23, 24]. A double ellipsoidal source with a normal (Gaussian) distribution of specific heat power was used as a heat source to simulate the molten metal pool [25]. The following model parameters of a double ellipsoidal heat source recommended for analyzing arc welding processes were used [25, 26]: af = 4 mm, ar = 12 mm, b = 8 mm, c = 3.5 mm, ff = 0.45, fr = 1.55, where ff and fr are coefficients determining the ratio of heat introduced into the frontal and tail parts of the ellipsoid, respectively; af , ar , b, c are the normal distribution radii corresponding to these parts of the ellipsoid. The chemical composition of pipes and filler wire is shown in Table 1. HP40NbTi alloy is characterized by the following physical properties: a liquidus temperature of 1425°C; a solidus temperature of 1370°C; a hardening temperature range of 40 – 55°C; a thermal expansion coefficient of 10.1 × 10 –6 – 17.1 × 10 –6 K–1; a thermal conductivity coefficient of 55.4 – 75.5 W/(m · K); a specific thermal capacity of 456 J/(kg · K); and a density of 8000 kg/m3. The mechanical properties of HP40NbTi alloy are provided in Table 2.

Table 1 Actual Chemical Composition of Pipe Metal and Filler Material
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Table 2 Mechanical Properties of HP40NbTi Alloy under Static Tension
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For structural studies, specimens with dimensions of 10 × 10 × 15 mm were cut out from different sections of the pipe in longitudinal and transverse directions using a high-speed blade. Metallographic studies and x-ray mapping were carried out using a Carl Zeiss Axiovert40light microscope, a TESCAN VEGA 2 LM scanning electron microscope equipped with a field emission gun, and an Inca X-Max-50energy dispersive x-ray spectrometer. To carry out light microscopy studies, the specimens were subjected to electrolytic etching in 10% aqueous oxalic acid solution for 30 sec.

Simulation and Experimental Results

Simulation

The coupled thermomechanical approach to the modeling of welding processes consists in the coordination of thermal and mechanical calculation modules, when the intermediate calculation results are exchanged after each computational cycle. The task of thermomechanical simulation consists in determination of the temperature field of the welded structure, as well as its displacement and stress fields. Thermal analysis is based on a thermal balance equation obtained according to the principle of energy conservation and using the finite element method for its discretization. As a result of solving continuum mechanics equations, the distribution of stresses within the computational domain and the displacement vector are found. The as-obtained thermomechanical model provides a set of possible solutions, with the coordinates being the parameters of the welding mode.

Let us consider the results of simulating the process of argon-arc welding of centrifugally-cast tubes from HP40NbTi alloy under two different modes.

Technological mode 1 (continuous). This AAW mode includes eight passes, each performed in continuous current mode. The parameters of technological mode 1 are presented in Table 3.

Table 3 Welding Mode 1 and Experimental Values of Maximum and Minimum Metal Temperature by Passes
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Postprocessing in LS-DYNA generated animation files of the technological process for each pass. Figures 2 and 3 show the results of calculating the temperature and displacement fields, as well as equivalent von Mises stresses, in the weld pool area and HAZ during the third and seventh passes to illustrate the process of simulating welding in accordance with technological mode 1. In addition, Fig. 2b and d present temperature variation curves in three HAZ points, where thermocouples were subsequently installed to validate the model experimentally.

Fig. 2
figure 2

Temperature field (a, c) and temperature change by passes (b, d ) in welding mode 1: a, b ) third pass, \({T}_{\mathrm{max}}^{\mathrm{calc}}\) = 813°C; c, d ) seventh pass, \({T}_{\mathrm{max}}^{\mathrm{calc}}\) = 717°C.

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Fig. 3
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Calculation of displacement fields in μm (a, c) and equivalent von Mises stresses in MPa (b, d ) in the third pass (a, b ) and seventh pass (c, d ) in welding mode 1.

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Technological mode 2 (pulse continuous). According to this mode, the welding operation includes nine passes (Fig. 4), with passes 3 – 6 performed in the pulse current mode, and the other passes performed in the continuous mode. Simulation is considered to be completed when the condition of melt volume constancy in the weld pool, upon its movement along the seam, is fulfilled. The parameters of technological mode 2 are presented in Table 4.

Fig. 4
figure 4

Distribution of weld layers in the model. Welding mode 2.

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Table 4 Welding Mode 2 and Experimental Values of Maximum and Minimum Metal Temperature by Passes
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Figures 5 and 6 illustrate the calculation results of temperature and displacement fields in the weld pool area and HAZ during the second and fourth passes to illustrate the process of technological welding mode 2. In addition, Fig. 5b and d present temperature variation curves in three HAZ points, where thermocouples were subsequently installed to validate the model experimentally.

Fig. 5
figure 5

Temperature field (a, c) and temperature change by passes (b, d ) in welding mode 2: a, b ) second pass, \({T}_{\mathrm{max}}^{\mathrm{calc}}\) = 702°C; c, d ) fourth pass, \({T}_{\mathrm{max}}^{\mathrm{calc}}\) = 625°C.

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Fig. 6
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Calculations of displacement fields in μm during the second (a) and fourth (b ) passes in welding mode 2.

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Experiment

Multi-pass argon arc welding of centrifugally-cast tubes from heat-resistant steel HP40NbTi under two technological modes, continuous (Table 3) and pulse continuous (Table 4), was carried out to validate the results obtained by numerical simulation. Argon of 99.9% purity with a flow rate of 10 L/min was used as a shielding and supporting gas for the welding arc.

The temperature field of the product was evaluated by thermocouples of TCA type (chromel–alumel), which were immersed up to the full depth of drilled holes. Recording of thermal cycles was carried out using Paragraph PL20 electronic digital recorders, allowing temperatures to be measured at a rate of 10 measurements per second. The recorder readings were processed using the ARC data viewer software package; the construction and analysis of thermal cycles were performed in the Office XL 2010 program.

Technological welding mode 1 (continuous). Welding was carried out in accordance with the parameters specified in Table 3. Filling of the root weld layer was performed in two stages, with passes from each edge of the segment to the top of the pipe. When welding a pipe with V-shaped cut edges (Fig. 1), a ∅3.2 mm electrode and welding wire (Table 1) of continuous cross-section (2.4 mm for the root weld and ∅3.2 mm for the ñover weld) were used. A two-layer weld was performed in several passes using the standard welding parameters: a voltage of 12 V, a current of 70 A for the root pass (soft mode), and a current of 110 A for the formation of the second layer.

A scheme of thermocouple holes for determining the temperature field of the product is shown in Fig. 7. The values of maximum and minimum temperature obtained from thermocouple readings for eight passes in the welding process are given in Table 3. As an example, Fig. 8 shows experimental dependences of temperatures on welding duration for the third and seventh passes, obtained from thermocouple readings.

Fig. 7
figure 7

Schemes of holes 1 – 12 for thermocouples in the pipe (a) and their cross-sections (b – d ) for welding mode 1: b) hole depth hhole = 7.3 mm, distance from the edge hedge = 8.0 mm, distance from the corner plane of the edge hcp = 2.0 mm; c) hhole = 2.5 mm, hedge = 4.5 mm, hcp = 2.0 mm; d ) hhole = 4.6 mm, hedge = 6.0 mm, hcp = 2.0 mm.

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Fig. 8
figure 8

Temperature variation curves during the third (\({T}_{\mathrm{max}}^{\mathrm{calc}}\) = 811°C) (a) and seventh (\({T}_{\mathrm{max}}^{\mathrm{calc}}\) = 731°C) (b) passes for welding mode 1, constructed based on thermocouple readings in points 1 – 15.

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The simulation (Fig. 2) and experimental (Fig. 8) results on determining the temperature field for mode 1 are presented in Table 5. The difference between the calculated and experimental (measured by thermocouples)temperature values does not exceed 2%.

Table 5 Calculated and Experimental Temperatures Tmax at Three HAZ Points During the Third and Seventh Passes in Welding Mode 1
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The results of determining the temperature field of the welded joint during welding by continuous mode 1 show that:

  • in passes 1 – 4, a significant difference of ∆T = 360 – 270°C between the maximum and minimum temperature values across the cross-section and diameter of the pipe is observed. This increases stresses in the weld seam (WS) and HAZ, thereby having a negative effect on the formed structure of the welded joint;

  • maximum temperature values were registered in passes 3 and 4, comprising 811 and 826°C, respectively.

Therefore, in the process of continuous welding, the maximum metal temperature of the HAZ is observed in passes 3 – 7, ranging from 740 to 830°C. These temperatures intensify the transformation of niobium carbide into an undesirable G-phase in HP40NbTi alloy [27, 28]. A quantitative analysis of the microstructure and phase composition of the welded joint showed the presence of a pronounced heterogeneity of silicon distribution in the γ-phase both in the WS and, specifically, in the HAZ. Here, silicon forms significant segregations along the boundaries of primary γ-grains near the eutectic carbides of niobium and chromium. In some HAZ areas, mainly near the boundary with the WS, light gray inclusions comparable in size to carbides are formed in the alloy structure at the carbide/matrix and NbC/CrmCn interphase boundaries (Fig. 9). Elemental analysis showed that these inclusions are enriched in Si, Ni, and Nb with the following variable chemical composition, wt.%: 6.7 – 9.4 Si, 29.2 – 32.4 Ni; 20.9 – 33.6 Nb, 21.4 – 29.2 Cr, 7.4 – 10.7 Fe, 0.02 – 0.07 Ti. In terms of stoichiometry, such a composition is similar to the G-phase.

Fig. 9
figure 9

Microstructure (EBSD) of HP40NbTi alloy in the HAZ near the WS of a welded joint produced by mode 1. Chemical composition of intermetallide, wt.%: 31.15 Ni, 29.11 Cr, 21.36 Nb, 10.61 Fe, 7.74 Si, 0.03 Ti.

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The presence of these inclusions in the weld structure accelerates the formation of a G-phase in HP40NbTi alloy at high-temperature operation, thus reducing the serviceability of the welded joint [14, 18, 29,30,31].

Technological mode 2 (pulse continuous). Welding was performed in accordance with the parameters given in Table 4. Filling of the root weld was implemented in two stages; passes were made from each edge of the segment to the top of the pipe. Passes 1 and 2 – 8 were performed using ∅2 mm and ∅3.2 mm electrodes, respectively. The root weld and the ninth cover weld were performed using ∅2.4 mm and ∅3.2 mm continuous cross-section welding wire, respectively (Table 1).

The scheme of thermocouple holes for determining the temperature field of the product is shown in Fig. 10. The values of maximum and minimum temperature measured by thermocouples in the process of nine welding passes in mode 2 are given in Table 4. As an example, Fig. 11 shows graphical dependences of temperature on welding duration, obtained based on thermocouple readings during the second and fourth passes.

Fig. 10
figure 10

Scheme of thermocouple holes (a) and installed thermocouples (TC) (b ) for welding mode 2: 1 ) TC23, 2 ) TC2; 3 ) TC4; 4 ) TC27; 5 ) TC10; 6 ) TC0; I ) one-side section; II ) two-side section.

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Fig. 11
figure 11

Temperature variations during the second (Tmax = 699°C) (a) and fourth (Tmax = 619°C) (b ) passes in welding mode 2, constructed based on thermocouple readings.

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The simulation (Fig. 5) and experimental (Fig. 11) results on determining the temperature field for welding mode 2 are presented in Table 6.The difference between the calculated and experimental (measured by thermocouples) temperature values does not exceed 2%.

Table 6 Calculated and Experimental Temperatures Tmax at Three HAZ Points During the Second and Fourth Passes inWelding Mode 2
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An analysis of the obtained experimental data shows that, compared to welding mode 1, welding mode 2 provides a smaller temperature gradient, which not only has a beneficial effect on the weld metal structure, but also reduces stresses in the product material. During pulse passes 3 – 6 in mode 2, the welding temperature does not exceed 650°C, with the maximum temperatures being observed in the process of continuous passes 2, 7, 8, and 9. However, even in these passes, the temperature does not exceed about 700°C (Table 4), which is significantly lower than in welding mode 1 (Table 3).

The results of determining the temperature field of the welded joint during welding mode 2 show that:

  • the difference between maximum and minimum temperature values in passes 5 – 9 does not exceed ∆T = 100 – 150°C, which reduces stresses in the WS and HAZ significantly and has a positive effect on the formed structure of the welded joint;

  • no significant temperature variations across the cross-section of the welded joint are observed during the welding process;

  • maximum temperatures were recorded in passes 2, 7, 8, and 9, i.e., at the stages of continuous welding. These temperatures did not exceed 700°C, being more than 100°C lower than during welding mode 1.

Hence, the pulse current mode of welding reduces the maximum metal temperature in the HAZ to 600 – 700°C, which should inhibitor sharply reduce the transformation of niobium carbide into a G-phase in the structure of HP40NbTi alloy [27, 28]. An analysis of the microstructure and phase composition of the welded joint produced by mode 2 showed both the presence of a weakly-expressed inhomogeneity of silicon distribution in the γ-phase in some WS and HAZ areas in the vicinity of eutectic niobium and chromium carbides and the absence of intermetallic phase inclusions (Fig. 12). These findings confirm a significantly weaker development of niobium carbide transformation when the pulse welding mode is used. This should have a favorable effect on the serviceability of the welded joint.

Fig. 12
figure 12

HP40NbTi alloy microstructure in the HAZ near the boundary with the first weld bead of the welded joint produced by mode 2 in backscattered electrons (a) and distribution maps of Cr (b ), Fe (c), Ni (d ), Nb (e), Ti ( f ), and Si (g ).

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Discussion

The results of an experimental study into the effect of AAW modes on the structure of welded joints made of HP40NbTi alloy were found to be consistent with those obtained by simulating the technological process of welding. The temperature field, which is a function of technological welding parameters, was established to have a determining influence on the welded seam structure and HAZ. The use of pulsed AAWreduces heat input and inhibits the formation of an intermetallic G-phase, which may trigger a premature failure of the welded joint during high-temperature operation of pipelines made of HP40NbTi alloy.

In order to obtain a welded joint with a given structure and properties, the heat input value in each specific case (material and geometry of the product) can be regulated by changing the technological mode parameters. Numerical simulation of the AAW process in the LS-DYNA software environment is an effective tool for solving the problem of optimization of technological parameters. This package can be considered as a virtual experimental stand for simulating different welding modes.

The proposed thermomechanical welding model facilitates determination of the thermomechanical state of the studied system at given process parameters and, as a consequence, prediction of structural-phase transformations in the product material. The numerical simulation of the AAWprocess also enables graphic visualization of temperature, stress, and displacement fields during the entire virtual study, as well as evaluation of the influence of residual stresses and thermal deformations on the final shape of the product.

Numerical simulation of the AAW process allows an optimal technological welding mode to be determined: the space of possible solutions is filled with test points, the coordinates of which are the mode parameters. As a result, the optimal point in the space of possible welding modes is identified.

Conclusions

  1. 1.

    Numerical simulation in the LS-DYNA software environment, intended for modeling welding processes, represents an effective tool for obtaining a welded joint with a given structure and properties. Finite element modeling of thermal welding processes based on a double ellipsoidal heat source allows the kinetics of thermal effects on welded products to be predicted, including for pulse welding modes.

  2. 2.

    The conducted experimental study into the effect of argon arc welding on the structure and phase composition of a pipe welded joint made of HP40NbTi alloy confirmed the validity of numerical simulation of welding processes based on the coupled thermomechanical approach.

  3. 3.

    The reduction of heat input by means of pulse-continuous welding mode inhibits the transformation of niobium carbide into an intermetallic G-phase in the weld metal and heat-affected zone of the welded joint.

  4. 4.

    The obtained simulation and experimental results confirm the feasibility of using pulse modes when welding pipe joints made of HP40NbTi alloy.