Abstract. Keywords: pipe welding; GMAW; root pass; WiseRoot+; penetration; welding speed. Introduction

Transcription

1 Advanced pipe welding with gas metal arc welding P. Jernström, J. Uusitalo Kemppi Oy, Kempinkatu 1, Lahti, Finland Abstract Gas metal arc welding (GMAW) is the most widely used arc welding process for joining of metals in demanding pipe welding applications in the petrochemical industry. For such applications, open root welding is used in situations that preclude welding from both sides of the material. External welding without copper backing has become possible through the introduction of advanced power source technology. The paper describes a novel process called WiseRoot+ designed for further improving the quality and productivity of pipe girth welding. Welding test results clearly show the beneficial effect of the new process on arc stability, spatter formation, weld penetration formation, and welding speed. Smooth, consistent root passes with complete penetration and sidewall fusion were obtained. The welding speeds employed were three to four times higher than those used in gas tungsten arc welding (GTAW). Keywords: pipe welding; GMAW; root pass; WiseRoot+; penetration; welding speed Introduction Over the last five decades, developments in pipe welding have enabled welds to be made with consistent high quality and efficiency (see Ref. 1). External one-sided welding without copper backing has become possible on account of the introduction of advanced power source technology. Many manufacturers of welding equipment have recently introduced products designed specifically for this purpose. For example, the Lincoln surface tension transfer process (STT) can be used for single-sided open root welding in any position (see Ref. 2). Today, Fronius, Miller, and EWM have corresponding products on the market. Kemppi s WiseRoot+ process involves very precise measurement of voltage, which serves as the input for the current control. Once the power source has recognised a short circuit, a controlled increase in current triggers the transfer of a droplet of filler metal from the wire. When the current measurement is exactly right, the current is dropped, before the filler-metal droplet falls and a short circuit ends. A short circuit ending at a point of low current produces smooth transfer of filler metal, with no spatter. After the short circuit is broken, a pulse is created in the current that heats the welding pool, but it does not cause transfer of filler metal (see Figure 1).

2 Figure 1: The behaviour of the current and voltage in the WiseRoot+ process over one short-circuit cycle. The WiseRoot+ process can be used for open root welding of pipes and plates from one side without backing. In welding of a stationary horizontal pipe, a downhill technique is used in the 12-to-6 o clock position. With a vertical pipe, welding can be carried out also in the PC position. In welding of plates, the PA, PC, PE, and PG welding positions can be used. WiseRoot+ has been designed with ease of use in mind. A suitable welding programme is selected on the basis of the filler-metal wire and shielding gas used. After this, the user can select the desired wire feed speed, and the welding programme selected handles adjustment of all the other parameters. Also, the welder can use a fine-tuning option to change the effect of the arc s heat input on the weld metal. This fine tuning affects the root penetration. Particularly in welding of a stationary pipe in the 3-to-6 o clock position (see Figure 2) with a relatively large root gap, root concavity may occur (as shown in Figure 3). This is caused by excessively high heat input, with the weld pool flowing inward from the root. In such a situation, the fine-tuning control can be adjusted in the negative direction, to decrease the heat input. This produces an acceptable weld also on the root side (see Figure 3). In general, the setting is needed for wider root gaps, to keep the larger weld pool from heating up too much. The + setting of the fine tuning is used for small root gaps, to ensure that the groove flanks melt in a narrow groove. Figure 2: Welding positions for a fixed pipe. Figure 3: Root concavity and an acceptable weld.

3 For the reasons mentioned above, it might be necessary to adjust the welding settings as the position changes during welding of a stationary pipe. With Kemppi FastMig X equipment, this can be done without interruptions in the welding: the MatchChannel feature allows the welder to change the memory channel on the fly, without interrupting the welding. The optimal parameters for the various positions can be saved in the MatchChannel memory. By making it possible to weld in every position with optimal parameters, MatchChannel delivers additional quality and productivity both: Stopping and restarting the welding is detrimental to productivity, because the end of the weld must be ground down before welding continues. Most welding defects occur at points where welding was restarted, so MatchChannel improves quality in this respect too. It also improves the start and finish of welding if higher values have been saved for the heat parameters in a different memory channel (via fine tuning with the + setting). This helps to reduce the risk of a welding defect. If welding is started or stopped on top of a finished weld or a tack weld, the edge of the previous weld must be rounded, to avoid welding defects. WiseRoot+ was developed for welding the root passes of butt joints. The shape and type of the groove can be selected to suit the thickness of the work piece. With a thickness of 5 mm or below, square joints and a root gap of 0-3 mm can be used, depending on the thickness. With thicker base metals, other types of butt joint can be used. For example, when the thickness is 10 mm, a single-v preparation (see Figure 4) with a gap and root face is recommended. One of the advantages offered by the WiseRoot+ process is that sufficient penetration can be achieved consistently, no matter how narrow the gap. The gap can be made as small as 2 mm while the root face can be increased to 2 mm. This improves productivity in two ways: the joint volume decreases, and welding of the root pass can be carried out with higher values for the parameters. Figure 4: Single-V preparation for a base metal thickness 10 mm. For thicker work pieces, it is more cost-effective to use a compound bevel single-v preparation or a single-u preparation (as shown in Figure 5), both of which help to decrease the joint volume. For a U joint, the gap is usually 0 mm and the root face 1-2 mm. For a U joint, the welding parameters have to be set much higher and automated welding is recommended because of the high welding speed. Figure 5: Examples of joint preparations for thick base metals (>12 mm)

4 In some cases, the gap may grow larger than in the examples presented in the diagrams here. This does not cause problems for WiseRoot+, which is tolerant of wide gaps. Of course, when the gap is greater than recommended, productivity suffers, because the welder has to use lower values for the welding parameters. Welding tests have been carried out successfully with gaps of up to 10 mm. Although such wide gaps are not recommended, they may still arise in challenging applications wherein a precise fit cannot be guaranteed. The process is well suited to the welding of root passes for steel. Welding programme packages are available for the welding machine that can be selected for the use of various filler materials in the welding of structural steel, stainless steel and high alloyed steels. Welding programmes for solid-steel wire in Imperial-system sizes (1.045 inches, or 1.14 mm) and with both mixed argon and CO 2 gases are available also. This type of wire is commonly used in, for example, Russia when one is welding root passes for gas pipelines. The WiseRoot+ process was developed specifically for welding the root passes of butt joints. In the product development, the primary focus was on the root welding of pipe butt joints in all positions. Experiments The welding tests were performed with the WiseRoot+ process, which is a standard feature of Kemppi s FastMig X multifunctional GMAW equipment (shown in Figure 7). The objective of the test was to evaluate the feasibility of the new process for the welding of horizontal fixed X60 steel pipe with an outside diameter of 600 mm. The wall thickness of the pipe was 12 mm with a single-v preparation. The gap width was between 2 mm and 3 mm, with the root face varying from 1.5 to 2 mm. Table 1 presents the welding parameters used in the experiments. Figure 7: The welding set-up for the experiments.

5 Table 1: Welding parameters used in the experiments Results and Discussion The welding tests demonstrated the beneficial effect of the new process on arc stability, weld pool control, penetration formation, and welding speed. Smooth, consistent root passes with complete penetration and sidewall fusion were obtained (see Figure 8). The maximum welding speed allowing acceptable weld quality (level B according to ISO 5817) was three to four times higher with the WiseRoot+ process than in welding using the GTAW process. In addition, the results with WiseRoot+ clearly showed that it is easier to compensate for variations in the gap width than with a conventional short circuiting process. Increased gap tolerance enables joints to be prepared and fitted together less accurately, saving time and reducing the costs of preparations for welding. Figure 8: Macrograph of a root pass welded with the WiseRoot+ process.

6 Conclusion The results of successful experiments with the novel welding process described and evaluated in this paper can be summarised in the following three statements: 1. The WiseRoot+ process can be used for open root welding of pipes and plates from one side without backing. In welding of a stationary horizontal pipe, a downhill technique is used in the 12-to-6 o clock position. 2. The welding tests demonstrated the new process s beneficial effect on arc stability, spatter formation, weld pool control, penetration formation, and welding speed. Smooth, consistent root passes with complete penetration and sidewall fusion were created in the experiments. 3. Additionally, the findings clearly showed that it is easier to compensate for variations in gap width than with conventional short circuiting processes. References 1. Yapp, D. (2011). Pipelines International, March High productivity pipe girth welding: Developments in mechanised arc welding of pipelines, p DeRuntz, B.D. (2001). Surface Tension Transfer welding in manufacturing. Selected Paper presented at the National Association of Industrial Technology Conference, Detroit Michigan, pp