INTRODUCTION

Modern welding equipment is being improved each year through the use of the latest digital boards in power sources for welding and software, which in their turn improve the stability of the arc, which plays an important role in the transition of alloying elements to the weld pool. The chemical composition of the welded joint metal after welding affects the stability of welded structures in a corrosive environment and temperature fluctuations during operation. The question of high-quality alloying of the joint from the coated electrode rod still remains relevant.

Among the materials that meet the requirements for resistance to aggressive media and operating conditions, the most common is 12Kh18N10T high-alloy steel [1]. The use of such steel necessitates a special approach to the procedure of its welding. Due to the uneven chemical composition in the welded joint metal or the local concentration of alloying elements in its separate layers, intergranular corrosion may occur, which will lead to irreversible consequences [2, 3]. The articles [4, 5] consider the diffusion of alloying elements and the mixing of metals of different classes during welding. However, in these studies, an analysis of the change in the concentration of alloying elements in the base metal and the near-weld zone was performed, which cannot give a complete description of the multi-pass welded joint. The study of the weld metal needs visual inspection, microstructural analysis, mechanical tests, as well as the determination of the chemical composition in different parts of the welded joint. However, most often, only the base metal of a different class subjected to external loads is studied [6, 7].

The movement trajectory of chemical elements during welding of a sample of small thickness can inform only about the alloying of a narrow zone of the weld metal [8]. However, the literature presents the results of studies carried out specifically on samples of small thickness. In this regard, it is relevant to study the behaviour of alloying elements in the welded joint metal of various thicknesses and under different multi-pass welding modes. Each element during welding moves randomly in liquid metal; it is extremely difficult to control and direct such movement. One can only state the fact of the presence of an element in a certain place or its distribution under certain external influences [9, 10].

The results of assessing the influence of the welding amperage and the power source type on the transition of alloying elements into the deposited metal under different welding modes showed [11, 12] that it is necessary to study the redistribution of alloying elements in the weld not only depending on the amperage, but also on the spatial arrangement of the welded joint. If welding is carried out at different parameters and using different electrodes, it is possible to determine under which modes one or another element diffuses most completely in the passes. In [13, 14], the effect of the electrode type and the composition of its coating on the distribution of alloying elements in the weld in various directions was studied.

The objective of this research is to study the nature of alloying elements distribution in different areas of the welded joint metal and the heat-affected zone during manual arc welding of tubes made of 12Kh18N10T steel with coated electrodes.

METHODS OF STUDY

A hot-rolled tube with a diameter of 159 mm and a wall thickness of 6 mm from 12Kh18N10T steel was used for welding. Edge preparation was performed according to GOST 16037–80, joint S17. Manual arc welding of tube blanks was carried out with OZL-8, TsL-11 and TsT-15 electrodes using a LORCH HandyTIG 180 AC/DC ControlPro welding inverter.

The perimeter of the tube joint was conventionally divided into 6 segments every 45°. Welding was carried out manually at direct current of reverse polarity in accordance with the manufacturer’s recommendations (Table 1). The tube was divided into six segments, from which tubular samples produced in soft and rigid modes were made (three samples for each mode) (Fig. 1). In these samples, in accordance with the modes, the welded joints are made as follows: starting from the segments 2 and 5, the root pass is filled, then from the segments 3 and 4 to the end of the segments 6 and 1, the hot pass is filled, in the segments 6 and 1, the facing roller is filled. Each segment was marked according to Table 1.

TABLE 1. Marking of Welded Joint Samples from 12Kh18N10T Steel
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Fig. 1.
figure 1

Deposition scheme and marking of samples of a welded joint made of 12Kh18N10T steel: a) top view; b ) three-dimensional image of the joint; 1 – 6 are the segment numbers; I, II are the welding directions in soft and rigid modes, respectively; P is the pass; R is the root.

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Sample cutting and preparation were carried out on Delta Abrasi Met and Automet 250 machines (Buehler). Of the six segments of the welded joint, 18 samples for microstructure studies (three samples for each mode) and 6 samples for mechanical testing were made. The chemical composition of the weld and the elements distribution in different parts of the welded samples were also determined. The chemical composition of the welding electrode rod and the base metal was assessed on a Q4 TASMAN 170 Bruker optical emission spectrum analyser, and the composition of the welding slag and electrode coating was assessedon a Rigaku Nex CG x-ray fluorescence analyser.

The content of alloying elements (Cr, Ni, Ti, Nb, etc.) in polished and etched samples (sections) cut in the cross section of welds was determined on a Carl Zeiss EVO LS 10 scanning electron microscope with an Oxford Instruments X-Max 80 Detector (SDD).

The results of the chemical analysis of the coated electrode rods are presented in Table 2.

TABLE 2. Chemical Composition of Coated Electrode Rods
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RESULTS AND DISCUSSION

The typical sections of the welded joint were investigated: transitions from the weld metal to the base metal (for each bead) and transitions between the beads, taking into account the effect of applying subsequent beads. Measurement of the Cr, Ni, Ti, Nb content was carried out in limited segments of the welded joint (circles in Fig. 2). Based on the measurement results for each segment, the arithmetic mean value of the element concentration was calculated.

Fig. 2.
figure 2

Sections (in circles) for measuring the content of alloying elements in single-pass (a), double-pass (b ) welds and the near-weld zone of a welded joint made of 12Kh18N10T steel: BM is the base metal; WM is the weld metal; FL is the fusion line; BT is the bead transition.

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In the case of a single-pass weld, the measurements were carried out in areas located linearly in the direction from the base metal to the weld metal with the intersection of the fusion line (Fig. 2a ). For a two-pass weld, the lines for constructing measurement areas cover both the beads overlap and the transition from each bead to the base metal (Fig. 2b ). By analogy, measurements were also made in three-pass welds. Based on the results of x-ray spectral microanalysis, graphs of the change in the concentration of alloying elements (in %) over the cross section of the weld and base metal were plotted. Thus, the article discusses a qualitative change in the content of alloying elements.

Figure 3 shows the distribution curves of chromium and nickel over the cross section of welded joints made using different electrodes and under different welding modes.

Fig. 3.
figure 3

Change in the chromium (a, c, e) and nickel (b, d, f ) content in samples (see Table 1) of one- (a, b ), two- (c, d ), and three-pass (e, f ) weld metal when producing a welded joint from 12Kh18N10T steel using TsL-11, TsT-15, and OZL-8 electrodes (No is the serial number of the measurement area).

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Chromium distribution. In single-pass (Fig. 3a ) welding in rigid modes, the use of TsL-11 electrodes contributes to a uniform chromium distribution over the weld cross section. Soft welding modes cause depletion of the weld metal with chromium along the fusion line with the base metal. In two-pass (Fig. 3c ) welding in soft modes, the chromium concentration in the weld is 6% lower, and in rigid modes it is 2% higher between the beads than in the base metal. A multi-pass (Fig. 3e ) bead obtained under soft welding conditions is characterized by an increase in the chromium concentration in the weld during the transition from the first to the third bead, and rigid modes provide only a slight decrease.

After welding using TsT-15 electrodes, the chromium concentration in a single-pass weld is lower than in the base metal by 2% (under rigid modes) and by 5% (under soft modes). In a two-pass weld, obtained under soft modes, chromium is distributed evenly in all layers, and in rigid modes, the chromium concentration in the layers is 6% lower than in the first bead. After multi-pass welding, chromium is evenly distributed over the welded joint.

The use of OZL-8 electrodes in single-pass welding causes an increase in the chromium concentration in the weld metal by 6% relative to the base metal, regardless of the mode. During two-pass welding in rigid modes, the chromium concentration increases by 4% to the second bead, and soft modes stabilize its content. In three-pass welding, there is a tendency to reduce the chromium content, especially along the lines of inter-bead fusion.

Nickel distribution. With an increase in the nickel content in the weld, its resistance to corrosion cracking decreases. Figure 3b, d, and f shows the curves of changes in the nickel content in the beads in the process of their surfacing in different modes with various electrodes.

Single-pass (Fig. 3b ) welding with TsL-11 electrodes according to soft modes causes a significant decrease in the nickel content in the weld metal (by 5.5%). Rigid welding modes do not significantly change its content. Similar results were also obtained for two-pass (Fig. 3d ) welding. In the case of three-pass (Fig. 3f ) welding in rigid modes, the nickel content stabilizes, and soft modes contribute to an increase in its concentration in the weld.

The use of TsT-15 electrodes reduces the nickel content in a single-pass weld relative to the base metal by 1.0 – 1.5%, regardless of the welding mode. Soft modes stabilize the nickel content in a two-pass weld, while rigid modes reduce its concentration in the direction from the first bead to the second by 2%. In multi-pass welding in rigid modes, the same nickel content in the weld is detected and in soft modes it decreases, especially along the line of inter-bead fusion.

In single-pass welding with OZL-8 electrodes, the nickel concentration does not change significantly over the cross section of the weld metal. Soft modes in two-pass welding are characterized by uniform nickel distribution, and rigid modes cause an increase in its concentration by 1.5% from the first bead to the second. With a multi-pass welding, there is a tendency to reduce the nickel concentration from the first to the third bead, regardless of the welding modes.

Figure 4 shows the change in the titanium and niobium content in the metal of deposited beads at different stages of welding using different electrodes.

Fig. 4.
figure 4

Change in the titanium (a, c, e) and niobium (b, d, f ) content in samples (see Table 1) of one- (a, b ), two- (c, d ), and three-pass (e, f ) weld metal when producing a welded joint from 12Kh18N10T steel using TsL-11, TsT-15, and OZL-8 electrodes (No is the serial number of the measurement area).

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Titanium distribution. Regardless of the electrode brand and welding modes in the welded joint (Fig. 4a ), a critical decrease in the titanium content from the base metal to the weld through the fusion line is detected. At that, in the metal of welds produced using OZL-8 electrodes, there is a zero level of titanium content. Titanium in a small amount (0.1%) was detected in two-pass and multi-pass welds made with TsT-15 electrodes, and only in the root bead. The welding mode did not affect the titanium concentration in two-pass welds (Fig. 4c ). In multi-pass welding (Fig. 4e ), soft modes contributed to the uniform titanium distribution over the root bead, and rigid modes increased the titanium concentration in it to 0.3% with a subsequent decrease to 0.1% to the transition line and then stabilization at that level.

Niobium distribution. In soft single-pass (Fig. 4b ) welding modes with TsL-11 electrodes, niobium is uniformly distributed from the fusion line into the weld metal. Rigid modes increase the niobium concentration by 0.2%. In a two-pass (Fig. 4d ) weld, there is a tendency for the niobium level to decrease from the first to the second bead, regardless of the mode. In a three-pass weld (Fig. 4f ), in rigid modes, the niobium content does not change, and in soft modes, it increases significantly.

Carrying out single-pass soft welding with TsT-15 electrodes provides a uniform niobium distribution from the fusion line into the weld metal. Rigid modes stabilize the niobium content in the weld. In a two-pass weld, soft modes contribute to an increase in the niobium content in subsequent beads, and rigid modes contribute to its decrease. Multi-pass beads produced under rigid modes are characterized by a stable niobium distribution. Under soft modes, the niobium concentration in the second bead increases being stable over the bead and it decreases in the third bead with subsequent stabilization.

CONCLUSIONS

Manual arc welding of tubes made of 12Kh18N10T steel with TsL-11 electrodes in soft modes in one or two passes causes a decrease in all alloying elements in the weld metal, and the use of rigid modes is characterized by a stable concentration of weld elements both in one- and multi-pass modes. TsL-11 electrodes can be recommended as filling electrodes for two-pass welds under soft conditions and for multi-pass welds under rigid conditions.

The use of TsT-15 electrodes for welding reduces the content of the main alloying elements (chromium and nickel) in single-pass welds. In two-pass and three-pass beads, welding with these electrodes in soft modes contributes to the predominant stabilization of all alloying elements. The best performance is observed when producing a multi-pass weld in rigid conditions. These electrodes should not be used for single-pass welding of thin elements. Such electrodes can be recommended for producing multi-pass welds by soft welding.

OZL-8 electrodes can be used to produce single-pass welds in welding modes with a wide range of parameters. When two-pass welds are produced using OZL-8 electrodes, the concentration of elements is stabilized, and in multi-pass beads, regardless of the modes, the content of elements in the welds decreases. OZL-8 electrodes can be recommended for welding of products of small thickness or for producing root and facing beads when welding samples of large thickness.