1 Introduction

S355PNH steel is currently the most recognized structural steel applied in manufacturing of medium-power gas pipelines. The steel is used for production of a pressure equipment, including devices operating at elevated temperatures. It is adapted in pressure tanks, boiler tanks, storage tanks and heat exchangers, in oil and gas projects. The high short-term tensile strength (approx. 650 MPa [1]) is the result of its chemical composition and microstructure, characterized by a higher carbon content and several times higher percentage of titanium in relation to unalloyed structural steels, as well as niobium and boron content at the level of 0.5% [2]. These elements provoke the formation of carbides, titanium carbides-nitrides: TiN, Ti (C, N) and niobium NbC in the dominant ferritic structure, deteriorating weldability of the discussed steel [3,4,5]. Metal inclusions promote formation of welding defects and imperfections, which mainly include sticking and the lack of proper fusion. In the case of this type of construction material, the high elongation value exceeding 20% must be noted, what is especially favored in case of transmission installations, which, due to factors related to changes in the soil leading to shifting of the pipe fastening system, prevents the occurrence of a violent breakage and unsealing [6, 7]. Therefore, it is extremely important to obtain at least the same mechanical properties and microstructural features as observed in the parent material, when welding joints are made of the discussed above steel grade. Otherwise, a joint with lower mechanical properties than the native material will have a lower mechanical resistance to the applied operating conditions, a higher tendency to cracks, including those of a brittle nature, and a reduced possibility of transferring any additional loads resulting from geometrical changes in the fastening system [8, 9].

To obtain a proper, high-quality joint, all welding parameters must be set very precisely: current, arc voltage, welding speed, beveling method, type of electrode wires, and especially gas mixtures [10, 11]. The authors of publications indicate that all thermodynamic conditions of the process affect the properties of the welded joint. These conditions include the linear energy of welding, the conditions of heating the joint before welding and the conditions of its solidification and cooling after welding [12, 13].

To reduce welding stresses when joining P355NH steel, it is recommended to limit the linear energy during welding to the level of 4.4 kJ/cm and to control the interpass temperature [14,15,16,17]. There are no precise guidelines, however, defining the exact interpass temperature for P355NH steel depending on the thickness of the welded sheet. Therefore, it was decided to determine the influence of interpass temperature and the influence of argon shielding mixtures containing helium on the properties of joints of a thin-walled tubular structure with a thickness of 3.6 mm. The obtained joints made of P355NH steel were examined against any welding defects and non-conformities. In addition, the best process parameters were selected to ensure the creation of joints that lack welding defects and incompatibilities and are characterized by the desired mechanical properties.

2 Welded material and joining technology

The fine-grained P355NH steel becomes increasingly used in the construction and reconstruction of gas pipelines, due to its high tensile strength at the level of 650 MPa and acceptable relative elongation at the level of 25%. The ratio of the tensile strength to the yield point plays a significant role for safety reasons in pre-critical state of the structure, its high value indicates a relatively large reserve of the stress value, when the stress due to operating conditions exceeds the yield point. In the case of the tested steel grade, the value of this ratio was at the level of 1.8.

Table 1 Chemical composition of the P355NH steel
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The microstructure of the P355NH steel, in terms of parent material, included mostly ferrite and a small amount of perlite, presented in Fig. 1. Taking the tensile strength of the dominant microstructure component of 300 MPa [14] into account, it can be stated with a great certainty that the tensile strength value of the completed material is the effect of a high content of carbon and titanium, which result in turn in the formation of carbides and carbonitrides.

Fig. 1
figure 1

Microstructure of the P355NH steel in delivery condition

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The TIG welding process (141 according to PN-EN ISO 4063 norm) was selected for welding of pipes with an outer diameter of 88.9mm and a thickness of 3.6 mm, made of P355NH steel. The most important additional material includes TIG 12.64 (W4Si) welding rod with a length of 1 m and a diameter of 2.4mm. The additional material meets the EN ISO 636-A W4Si1 classification.

Chemical composition of the welding wire (Table 2) differs from the chemical composition of the welded steel. Elements that increase the strength (especially Cr, Ti, V) and elements improving the plasticity of the joint (especially Ni, Mo) should be noted (Table 3).

Table 2 Welding wire W4Si—chemical composition (%)
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Before joints from sheets with a thickness of \(t= 3.6\) mm have been started to weld, a V-chamfer was performed, as shown in Fig. 2.

Fig. 2
figure 2

Preparation of metal sheets before welding (sheet thickness \(t = 3.6\) mm, outer pipe diameter \(D = 88.9\) mm)

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Fig. 3
figure 3

The order of applying stitches in the joint

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For the shielding gas, the following mixtures were selected based on the EN ISO 14175 norm: Ar- 20% He, Ar- 10% He and pure argon. The gas flow rate was set at the level of 9 l/min in each case. The tungsten non-consumable electrode had a diameter of 2 mm. The joints were made at a room temperature of \(+\) 20 \(^{\circ }\)C. The joint had a four-stitched character. The sequence of laying the subsequent layers of the joint is presented in Fig. 3. All four layers were made with the same parameters. The welding parameters are summarized in Table 3.

Table 3 TIG welding parameters
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The temperatures of the interstitial layers within the range of 230–260 \(^{\circ }\)C were verified.

3 Research methods

The technical quality verification of the welded joint was performed in two stages. The first stage included non-destructive, while the second stage destructive testing.

The non-destructive testing (NDT) included the following types of tests:

  1. (a)

    VT - visual examination with an eye armed with a magnifying glass at 3\(\times \) magnification—the tests were carried out in accordance with the requirements of PN-EN ISO 17638 norm, evaluation criteria according to EN ISO 5817 standard,

  2. (b)

    MT- magnetic particle testing—the tests were carried out in accordance with PN-EN ISO 17638 norm, and their assessment in accordance with EN ISO 5817 standard, using a magnetic flaw detector test device type REM—230.

  3. (c)

    PT - penetration tests, according to PN-EN ISO-3452-1 standard, penetration time 20 min, development time 20 min,

  4. (d)

    UT- ultrasonic testing, according to PN-EN ISO-15614-1 norm, with the use of Olimpus EPOCH 600 detector and frequency of 4 Hz. Tested according to the PN-EN ISO 2400 formula.

Devices operating under pressure require quality checking with the ultrasonic method (UT) to determine any non-conformities or welding defects. The analysis of the results obtained through non-destructive testing allowed to select joints for destructive testing, that included temporary tensile strength and impact toughness. Furthermore, the samples were examined structurally with the use of a light microscope (LM). The tests were carried out in accordance with the PN-EN ISO 9016 2021 standard.

Destructive testing (DT) consisted of the following trials:

  1. (a)

    Tensile test as defined by the PN-EN ISO 6892-1 standard,

  2. (b)

    Impact toughness test according to the requirements of the PN-EN ISO 7438 standard,

  3. (c)

    Hardness test according to the guidelines of the PN-EN ISO 6507-1 standard,

  4. (d)

    Bending test in accordance with the requirements of PN-EN ISO 7438 standard.

  5. (e)

    Examination of the microstructure (light microscopy) as defined by the standards PN-EN ISO 17639: 2013-12, PN-EN ISO 15614-1.

  6. (f)

    Scanning electron microscope (SEM) images were taken with a Supra 35 (Zeiss). The accelerating voltage was 20 kV. To obtain images of the surface topography, the secondary electron detector (by the in-lens detector) was used. The microscope is additionally equipped with the X-ray energy-dispersive spectroscopy detector from EDAX.

4 Research results and analysis

The obtained TIG joints with different three shielding gas mixtures with various helium (He) content (0%, 10%, 20%) were tested. The results of the non-destructive testing of produced joins with the use of various welding parameters are presented in Table 4.

Table 4 Non-destructive test results
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It was found that for correct welding of 3.6-mm-thick sheets made of P355NH steel, most important is controlling the temperature of the interpass layers, which should be at the maximum level of 230 \(^{\circ }\)C. The preheating temperature at 260 \(^{\circ }\)C was found as too high, since welding defects and non-conformities such as incorrect fusion and gluing appeared in the joints. Additionally, it was observed that the use of Ar-10% He gas mixture enables obtaining correct joints in more variants of the process (change of linear energy, interpass temperature).

The next stage of the research included verification of the mechanical properties of joints made with different linear energies and with the use of different shielding gases. For further destructive tests (evaluation of mechanical properties), only correctly prepared joints (Q1, Q2, Q3, Q9, Q10, Q11, Q12, Q17, Q19) were taken into account. Table 5 shows the immediate tensile strength of the tested joints (average of 3 measurements).

Table 5 The strength tests results of the examined joints
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The table data show the possibility to obtain high tensile strength of the joint (at the level of 500 MPa) simultaneously with an acceptable relative elongation (over 20%). Such a result was obtained only when a shielding gas a mixture of Ar-10% He was applied. The received result is relatively close to the mechanical parameters of the tested steel as a parent material.

Table 6 Bending test results of a join produced with various shielding gas mixtures
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In most cases, no cracks were found in the joint and in the HAZ, both on the face and root side of the weld (Table 6). It was observed that the most favorable results were obtained with the application of the Ar-10% He sheathing mixture, as the greatest number of joints obtained under the cover of this mixture were free of cracks once the bending test has been performed (Q9, Q10, q11, Q12). It was noticed, that in one of the tested joints made with the use of Ar (Q17) and Ar-20% He (Q2) mixture, also no cracks appeared after the bending test. After that, it was decided to check the impact strength of the joints that passed the bending test (Q2, Q9, Q10, Q11, Q12, Q17). The impact tests were performed at temperatures of 0 \(^{\circ }\)C and − 10 \(^{\circ }\)C.

The hardness measurements showed that the tested joint had a 35% higher hardness than the parent material, as shown in Fig. 4. The increase in hardness was visible both in the heat-affected zone and in the weld, reaching min. 207 MPa compared to the native material with the result not exceeding 160 MPa, as shown in Fig. 5. A similar quality result was obtained when testing tensile strength, which, in combination with hardness test values, indicated the high quality of the produced joint—exceeding the manufacturing quality of the parent material.

Fig. 4
figure 4

Vickers hardness of parent material, HAZ and weld

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Fig. 5
figure 5

Ultimate tensile strength (UTS) at approaching by means of the following relationship UTS \(=\) 3.5 HB and data shown in Fig. 4 recalculated on values of Brinell hardness (HB)

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Table 7 data shows that all joints met the minimum requirements for steels used in pressure equipment, i.e., impact strength above 30 J at temperature of − 10 \(^{\circ }\)C and impact strength not lower than 40 J at 0 \(^{\circ }\)C.

Table 7 Three-point bending tests results of the joint made with various shielding gas mixtures
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It can be seen that the best plastic properties had a joint (Q9) made in the shield of Ar-10% He gas mixture, since it was the only one that met the first impact strength class, i.e., a minimum of 47 J at a temperature of − 10 \(^{\circ }\)C, as shown in Fig. 6.

Fig. 6
figure 6

The influence of He content in the TIG method shielding gas on the joint impact strength

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It should be noted the mentioned joint presented a relatively low susceptibility to low temperature, manifested by the small reduction in the values of breaking energy. It means this kind of the joint (Q9) is not significantly sensitive to negative temperature. This kind of data is located directly above the results for the higher resistance of a joint (Q3) on changes in KV values due to temperature lowering, confirming the joint quality, as shown in Fig. 7.

Fig. 7
figure 7

Differences between energy values absorbed in the impact test of the produced joint, carried out at two temperature values: 0 \(^{\circ }\)C and − 10\(^{\circ }\)C

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Therefore, further joint quality evaluation was carried out with the use of sample Q9. The results of the microstructural tests are presented in Figs. 8 and 9.

Fig. 8
figure 8

Partial normalization area in HAZ. They recrystallized only at a lower transformation temperature (perlite, areas enriched with Mn) creating fine-grained ferritic-pearlitic micro-areas. (nital etching)

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Fig. 9
figure 9

Grain stitch in the joint (Q9). At the boundaries of the former austenite grain, GBF (grain boundary ferrite) and SPF (side place ferrite) ferrite are visible. Inside of the grains of the former austenite a fine needle AF (acicular ferrite) ferrite appears. The non-metallic inclusions are visible (nital etching)

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The microstructure analysis shows that ferrite with various morphologies became the dominant phase in the joint, with a large share of GBF (grain boundary ferrite) and SPF (side place ferrite) ferrite. A small compactness of the fine-grained acicular ferrite AF was observed. The ferritic phase most often nucleates as GBF at the grain boundary, as SPF in contact with GBF, and as AF inside the former austenite grain according with the model (Fig. 10).

Fig. 10
figure 10

Model of growing the morphological ferrite phase: AF( acicular ferrite), GBF (grain boundary ferrite) and SPF (side place ferrite) [own study]

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The most common starting places for the formation of the AF phase are non-metallic inclusions. The condition for the formation of the AF phase on non-metallic inclusions is both the appropriate chemical and phase composition of the inclusion, its size, coherence of lattice parameters of the precipitation with the phases, as well as the values of the thermal conductivity coefficients of the structure elements. These mechanisms determine the structure obtained when welding austenitic steels (Fig. 11).

The analysis of the obtained results of structural tests allows to confirm the formation of fine, evenly distributed non-metallic inclusions (small black zones in the weld presented in Fig. 9), which favor the formation of a fine-needle ferritic structure. X-ray microanalysis determined that the observed inclusions are TiN (titanium nitrides) and have a size of 1\(-\)1.5 µm (Fig. 12). According to the literature data [18,19,20], it can be concluded that both the size of the inclusions, their distribution, and, above all, the lattice parameters of TiN are conducive to the nucleation of the AF phase.

Fig. 11
figure 11

Main mechanisms and location of nucleation of coniferous ferrite on inclusions: a grain boundaries are the main starting points of ferritic phase nucleation b–c inclusions are strong starting points for ferritic phase nucleation. d The inclusion surface acts as an inert surface for nucleation, e the lack of crystal lattice fit, f the coherence of the ferrite crystal lattice and inclusions reduces the energy at the phase interface between ferrite and inclusions compared to the boundary between the inclusion and the austenite. g The strain energy near the inclusion increases due to differences in the thermal expansion of austenite and inclusion, which leads to a decrease in the activation energy of the ferritic phase nucleation. h Local depletion of hardening elements such as carbon or manganese at the austenite grain boundary, leading to an increase in the driving force of nucleation of ferrite from austenite [own study]

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Titanium nitrides TiN that enable the nucleation of the AF ferrite due to the lattice fit are also visible.

Fig. 12
figure 12

The presence of non-metallic titanium inclusions in sample Q9 (20 kV)

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Based on the analysis of Fig. 12, it can be seen that the size of the titanium non-metallic inclusions in the Q9 junction generally does not exceed 2 µm. The size and arrangement of the inclusions are very important for obtaining good mechanical properties of the joint. The test results indicate a strong relationship between the structure and the mechanical properties of the joint. The observed inclusions in the weld metal can be treated as dispersed phases in the composite material. The strengthening effect is obtained as a result of the coherent precipitations, which is consistent with literature data [21].

5 Summary and conclusion

The article aimed to verify the effect of helium added to the argon shielding mixture in the MIG process when welding P355NH steel on medium-power gas pipelines. Welding of steel intended for pressure pipelines is a responsible task and a major material and technological challenge. Depending on the thickness of the joint and the weight of the welded structure, welding recommendations may change. Various process parameters must be carefully studied to obtain joints of very good quality. The mechanical properties of the joint are affected by the thermodynamic conditions of the process. It is important to establish the correct preheating temperature and to establish the temperature of the inter-pass layers. At the same time, a lot of thermodynamic welding parameters were analyzed. The most important are welding speed, welding current, preheating temperature, interpass temperature and finally the role of shielding gases (three different shielding gases). The choice of shielding gas can play an important role. In the case of TIG welding, the addition of helium may be important as a gas that cools the joint more than argon. For this purpose, the joints were made both in pure argon shield and in the shield of Ar-10% He and Ar-20% He mixtures. The influence of the interpass temperature at the temperature of 230 \(^{\circ }\)C and 260 \(^{\circ }\)C was simultaneously examined. The results of non-destructive tests clearly indicated that the interpass temperature should be at the level of 230 \(^{\circ }\)C. The most appropriate shielding gas is the Ar-10% He mixture. Numerous non-destructive and destructive tests have been performed. The most important role in the assessment of weldability was played by: tensile strength test, bending test, impact test, hardness tests and evaluation of the full metallographic structure using light microscope and scanning microscope.

Due to the low carbon content in the structure of the steel and the weld, the ferritic structure is dominant. The high presence of titanium in this steel contributes to obtaining good mechanical properties, as inclusions are formed that can strengthen the weld metal. The inclusion size, distribution and form depend on the thermodynamic conditions of welding. The most beneficial size and distribution of titanium inclusions were obtained during welding in the Ar-10% He shield. The favorable structure translated into good mechanical properties (tensile strength, impact strength).

The following conclusions were made:

  1. 1.

    It is possible to obtain the correct joint made of P355NH steel by controlling the welding linear energy and the temperature of the interpass layers.

  2. 2.

    During MIG welding of P355NH steel, it is recommended to control the interpass temperature, which cannot exceed 230 \(^{\circ }\)C.

  3. 3.

    It is possible to obtain the first class of impact strength if an argon shielding mixture containing 10% He is applied

  4. 4.

    It is possible to obtain the tensile strength of a joint made of P355NH steel at a minimum level of 500 MPa while maintaining ductility.

  5. 5.

    In the tested welds, it was observed that the dominant phase was a coarse-grained ferrite. GBF ferrite in combination with other microstructure components such as SPF ferrite, AF ferrite, and especially titanium nitrides contributes to increase of the immediate tensile strength.