Experimental Research on Submerged Multi-Arc Welding of API-5L-70X Steel

Transcription

1 Experimental Research on Submerged Multi-Arc Welding of API-5L-70X Steel 1 LUIGI RENATO MISTODIE, 2 EMIL CONSTANTIN, 1 COSTICA VOICU, 1 VIRGIL TEODOR 1 Department of Manufacturing Engineering Dunarea de Jos University of Galati 47 Domneasca St., Galati 2 Romanian Welding Society 30 Mihai Viteazu Blv., Timisoara ROMANIA luigi.mistodie@ugal.ro Abstract: - The research focuses on the behaviour of API-5L-X70 steel, a high strength steel used in pipelines manufacturing, which was subjected to submerged mono and multi-arc welding. Due to the advantages of the welding process in comparison with other manufacturing processes, welding process is more and more applied in pipelines manufacturing field. Combining the number of wires and electrical arcs many variants of the submerged welding process were developed. It is obvious that multi-arc welding leads to a high productivity and efficiency. On the other hand, welded joints free flaws are required, in order to increase the safety of gas supplying and to reduce the possible incidents which could affect the environment and working conditions. That is why, an investigation of the welding process effects on the thermal, metallurgical and mechanical changes in the base material is compulsory. Two welding process variants were applied during the experimental programme. One variant consists of a single electrical arc and the other welding variant includes two arcs and three wires. A comparative study on mechanical properties of the welded joints performed by the both welding variants is deeply described in this paper. Tensile strength, yield strength, toughness and hardness testing were made in order to assess the behaviour of this steel grade during the welding process. Finally, the experimental results are graphically processed and comparatively discussed. Based on the results of the investigation, important conclusions are drawn in the end of the paper. Key-Words: - API-5L-X70 steel, submerged mono-arc and multi-arc welding, mechanical properties, testing 1 Introduction Due to the advantages of the welding process in comparison with other manufacturing processes, welding process is more and more applied in pipelines fabrication. Presently, the submerged welding process is preferred by the welding technologies designers and technologists for the achievement of the pipelines longitudinal welds. Many variants of the submerged welding processes, combining the number of wires and arcs, are described in the literature [1] - [2]. It is obvious that multiarc welding has a positive impact on the process productivity. Nevertheless, an investigation of the welding effects on the thermal, metallurgical and mechanical changes in the base material is compulsory. When multiarc welding is applied, the total heat is a sum of heats developed by each arc which has a specific role and influence on the welded joint properties and appearance (shape, dimensions). For instance, when three electric arcs welding is applied, the role of each arc is described below [3], [4]: the first arc is responsible for the weld root achievement. A good correlation between primary welding parameters is required in order to avoid the defects occurrence; the second arc is needed to fill the gap between sheets. Usually, the amperage is lower than that used at first electric arc, in order to obtain a suitable width of the weld and to keep the penetration previously achieved; the third arc finalizes the weld shape and its dimensions. The wire comes into the melted metal pool performed by the first and second electric arcs. Therefore, lower amperage and a higher voltage are set so that an appropriate weld width is obtained. Two multiarc welding variants of the three electric arcs welding are applied in the metallic structures industry: DC-AC-AC in Europe and AC- ISBN:

2 AC-AC in USA and Japan. Taking into account the amperage used for the weld root, a correlation between the amperages is recommended in Japanese literature [5]. Also, there are correlations between the pipe thickness on one side and the melting coefficient, heat input and welding speed on the other hand [6] - [8]. The equipment welding producers focussed on the productivity increasing. For example, ESAB is one of the leaders on the equipment welding market and developed many multi-wires welding technologies such as twin welding, tandem welding, one arc welding with warm wire and twin-tandem welding with warm wire. Applying multi-wires welding with six simultaneous arcs, the deposition ratio could reach 100 kg/h [10]. Combining the advantages of different welding technologies, then productivity, deposition rate, penetration and base metal dilution are monitored and controlled. The optimal welding technology will be established, as far as the diameters of wires, welding parameters and optimal distance between electric arcs. A high performance of the mechanical properties of welded joints can be achieved, using adequate welding materials and several welding passes, simultaneously performed in separate welding pools. The result is a thermal selftreatment, with an important effect on the quality of longitudinal welded joints of pipeline sections. A special welding flux will be used to increase the number of crystallizing centres in the welding pool and to refine the final structure of the weld. Finally, applying the multiarc welding procedure to the pipelines manufacturing, an improvement of plasticity and a better control of the brittle compounds, which could occur in the weld and in HAZ, are expected. different diameters and chemical composition of the filler material can be combined within the welding process; AC power supplies are cheaper, robust and easy maintenance; the successive heating and cooling produce extensions and the slag can be easily detached; reduced stresses and strains of the welded joint. Still, high productivity of the process is achieved only if the metallic structures are large or very large. Besides, the heat introduced in the base metal produces welding pool with large dimensions that tends to flow. Because of this phenomenon, ceramic, copper or flux backing is needed in order to stop the melted pool flow. One or two wires can be used when monoarc submerged welding procedure is applied. When double arc submerged welding is applied, distinct welding pools are formed and higher deposition rate and penetration are achieved. The distance between arcs is set at 100 mm. The schematic representation of the welding system, presented below, describes the principle of mono and multiarc submerged welding. Depending on the welding variant, one or two welding torches, one to three wires feed systems, one to three welding wire spools, one or two welding hoppers and one or two power supplies are combined in order to carry out the experiments. 2 Experimental program The experimental program, including mono arc and multiarc submerged welding, was conducted using the welding installation presented in the figure 1. The advantages of then welding system are related to the following performances: high productivity; high deposition rate; welding of medium and large components thickness; high heat generated by the electric arcs which involves higher penetration and lower cooling speed; Fig.1. Schematic representation of the welding installation [9] Welding in AC current, energy savings and a better stability of the electrical arc are obtained. As it can be noticed, two AC power supplies are included in the welding system. The first power ISBN:

3 supply (1) is a three-phase AC one, which is connected to the base metal and two wires, ensuring high heat input and therefore a proper penetration. The second power supply (2) is a mono-phase one connected to the base metal and the third wire, ensuring a suitable deposition and a good root filling. At multiarc submerged welding, the filler metal is supplied from distinct wire feed systems. Due to the wire feed system design different wire diameters can be used. Each electric arc is protected by the flux provided through distinct welding flux hopper. A welding flux vacuum cleaner was positioned behind the welding arcs in order to recover the welding flux and to ensure its continuous recirculation. The experimental program focused on an investigation of each welding process influence on the mechanical changes of the welded joint. Two thickness values (17.5mm and 19.1mm) of API-5L- X70 steel sheets have been investigated. OE-S2Mo welding wire and O19 welding flux were used in the research. A set of three samples was performed for each combination of welding variant and plate thickness. According to the API-5L standard, X- groove configuration has been prepared before welding the sheets. The root was continuously welded by gas metal arc welding (GMAW) procedure. Then, longitudinal butt welds were made on the both sides of the joint. The welding conditions, including each welding variant, plate thickness (s) and regimes parameters (amperage I, voltage U, welding speed s w ), are presented in the tables 1 and 2. The values of I 1 and I 2, U 1 and U 2 are characteristic to the first welding arc, respectively the second welding arc. The symbols of d 1 and d 2 represent the selected wires diameter for the first welding arc, respectively the second welding arc. Table 1.The welding parameters corresponding to the mono welding process Welding variant Monoarc welding Plate thickness s [mm] Wire diameter d [mm] 17.5 d = d = 4 Sample Side I [A] U [V] S w [cm/min] Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Table 2.The welding parameters corresponding to the multiarc welding process Welding variant Double arc welding Plate thickness [mm] Wire diameter [mm] d 1 = 3.2 d 2 = 4.0 d 1 = 3.2 d 2 = 4.0 Sample Side I 1 [A] I 2 [A] U 1 [V] U 2 [V] S w [cm/min] Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom Top Bottom ISBN:

4 After the welding phase, the samples were examined by non-destructive test (NDT) and those with defects were excluded from the investigation. The defect-free specimens were mechanically machined transversally to the welding direction and subject to tensile strength, toughness, yield strength and hardness tests. 3 Results and discussion In order to make an analysis of the mono and multiarc welding process influence on the mechanical proprieties changes of the material supposed to be weld, initial tensile strength, yield strength, toughness and hardness of the base material (BM) have to be known. The mechanical characteristics of API-5L-X70 base metal are presented in table 3. The comparative study between monoarc and multiarc (double) submerged welding variants reveals that the behaviour of this steel is different when one or double electric arcs act on the parent metal. The first electric arc seems to have a preheating role on the base metal, leading to a less impact of the welding process on it. To have a general view on the mechanical properties changes, the average results of the tensile, yield strength, toughness and hardness tests are comparatively presented in the tables 3 and 4. The tests were done according to ASTM standards. All samples were fractured in the base material, not in weld or HAZ, proving that the welding technologies were rightly designed and the couple wire-flux correctly selected. The tests for toughness testing were performed at 0 o C on specimens with notches located in different regions of the joint (weld and HAZ). As a conclusion, better results of the mechanical characteristics, meaning a less impact of the welding process, have been achieved when double arc submerged welding was applied. Table 3. API-5L-X70 steel proprieties according to the analysis report R m [N/mm 2 ] R p02 [N/mm 2 ] KV [J/cm 2 ] HV Table 4. Experimental results when monoarc welding is applied Plates thickness [mm] R m [N/mm 2 ] R p02 [N/mm 2 ] KV [J/cm 2 ] Tensile testing Weld 152 / HAZ 166 Fracture BM Weld 148 / HAZ 154 Fracture BM Table 5. Experimental results when double arc welding is applied Plates thickness [mm] R m [N/mm 2 ] R p02 [N/mm 2 ] KV [J/cm 2 ] Tensile testing Weld 178 / HAZ 180 Fracture BM Weld 165 / HAZ 171 Fracture BM 3.1. Tensile Strength Testing This destructive test is used to measure the strength of a welded joint. The usual requirements for the tensile strength of welds are that the specimen shall pull not less than 90 percentage of the base metal tensile strength. The samples of dimensions 350x50x17.5mm and 350x50x19.1mm were subjected to the tensile strength testing. Few of the samples are presented in the figure 2. To obtain a suggestive image of the welding process influence on the material behaviour from the mechanical point of view, a calculus to obtain the average values. All fractures were located in the parent metal as presented in the tables above. Fig.2. Tensile strength vs. thickness ISBN:

5 That is why the average obtained values are approximately equal with tensile strength of the parent metal, no matter thickness or welding variant. The average values of tensile strength and yield strength vs. thickness and welding variant are illustrated in the charts shown in the figure 2 and figure 3. A slight decrease of yield strength was noticed in the welded joints performed by the both welding variants. Still, the influence of the welding process is less when double arc welding is applied. In this case a decrease from 485N/mm 2 to 472N/mm 2 (when s=17,5mm) or to 480N/mm 2 (when s=19,1mm) is observed. When monoarc submerged welding process is used the decrease is lightly higher from 485N/mm 2 to 465N/mm 2 (when s=17,5mm) or to 470N/mm 2 (when s=19,1mm). A high decrease of toughness was noticed in the welded joints performed by the both welding variant (Fig.5). Still, the influence of the welding process is less when double arc welding is applied. In this case a decrease from 245J/cm 2 to 178J/cm 2 in weld and to 180J/cm 2 in HAZ (when s=17,5mm), or to 165J/cm 2 in weld and to 171J/cm 2 in HAZ (when s=19,1mm) is observed. When monoarc submerged welding process is used, the decrease is higher from 245J/cm 2 to 152J/cm 2 in weld and to 166J/cm 2 in HAZ (when s=17,5mm), or to 148J/cm 2 in weld and to 154J/cm 2 in HAZ (when s=19,1mm) is observed. Fig.5. Toughness vs. thickness Fig.3. Yield strength vs. thickness 3.2. Toughness Testing The test is used to determine the energy absorption on the surface unity in fracturing the specimen and consists of impact loading a three-point bend bar which contains a relatively blunt notch. This specimen type characterises very well the propagation effect of the cracks in the welded joint. The tests for toughness testing were performed at 0ºC on specimens of 100x15x17.5 mm and 100x15x19.1 mm dimensions, with notches located in different regions of the joint (weld and HAZ) as figure 4 shows. The lowest impact notch value from the joint is achieved in the weld and that is due to the casting structure obtained in the weld. Besides, overheating effect is responsible the decrease of plasticity propriety in HAZ of the joint Hardness Vickers HV 10 Testing Vickers hardness HV10 measurements were carried out using a NEOPHOT 32 microscope to assess the metallurgical modifications in the fusion zone, HAZ, and BM areas, according to the indentation pattern illustrated in figure 6. The Vickers HV10 hardness has the highest values in the fusion weld area, no matter thickness or welding variant (Fig.7). An increasing of hardness is registered in HAZ too, but still lower than in the weld. The metallurgical process caused by welding involves structural modifications and overheating important influences on the mechanical performances of the welded joint. Fig.4. Samples subjected to toughness testing Fig.6. Vickers hardness HV10 indentation pattern ISBN:

6 Acknowledgement This work was supported by the Romanian National Authority for Scientific Research, CNDI UEFISCDI, through grant 27/2012, project number PN-II-PT-PCCA Fig.7. Vickers hardness HV10 vs. thickness 4 Conclusions Based on the conducted work, the following main conclusions can be drawn: When multiarc welding is applied, the total heat is a sum of heats developed by each arc which has a specific role and influence on the welded joint properties and its appearance: the first arc is responsible for the weld root achievement. An accurate and good correlation between primary welding parameters is required in order to avoid the defects occurrence; the second arc is needed to fill the gap between sheets. Usually, the amperage is lower than that used at first electric arc, in order to obtain a suitable width of the weld and to keep the penetration previously achieved; the third arc finalizes the weld shape and its dimensions. The wire comes into the melted metal pool performed by the first and second electric arcs. Therefore, lower amperage and a higher voltage are set so that an appropriate weld width is obtained. The comparative study between monoarc and multiarc (double) submerged welding variants reveals that the behaviour of API-5L-X70 pipelines steel is different when one or double electric arcs act on the parent metal. The first electric arc seems to have a preheating role on the base metal, leading to a less impact of the welding process on its; Better results of the mechanical characteristics, meaning a less impact of the welding process, have been achieved when double arc submerged welding was applied. Finally, it is obvious that multiarc welding has la positive impact on the productivity and the investigation reveals the less impact of the multiarc submerged welding on the mechanical properties of the base material. References: [1] Babu S. P. K., Natarajan S., Influence of heat input on high temperature weldment corrosion in submerged arc welded power plant carbon steel, Materials & Design, 29(5), 2008, pp [2] Datta, S., Bandyopadhyay, A. and Pal, P. K., Application of Taguchi philosophy for parametric optimization of bead geometry and HAZ width in submerged arc welding using a mixture of fresh flux and fused flux, International Journal of Advanced Manufacturing Technology, 36, 2008, pp [3] Gunaraj, V. and Murugan, N., Prediction and Comparison of the Area of the Heat Affected Zone for the Bead-no-Plate and Bead-on-Joint in Submerged Arc Welding of Pipes, Journal of Materials Processing Technology, 95(1-3), 1999, pp [4] Joarder, A., Saha, S. C. and Ghosh, A. K., Study of Submerged Arc Weld Metal and Heat Affected Zone Microstructure of Plain Carbon Steel, Welding Journal, 1991, pp [5] NN, Pipes and tubes of nippon steels, Nippon Steel News, br. 326, 2005, [6] Datta S., Sundar M., Bandyopadhyay A., Pal P. K., Nandi G., Roy, S. C., Statistical modeling for predicting bead volume of submerged arc butt welds, Australasian Welding Journal, 51(2), 2006, pp [7] Kanjilal P., Pal T.K., Majumdar S.K., Combined effect of flux and welding parameters on chemical composition and mechanical properties of submerged arc weld metal, Journal of Materials Processing Technology, 171, 2006, pp [8] Karaoglu S., Secgin A., Sensitivity analysis of submerged arc welding process parameters, Journal of Material Processing Technology, 202(3), 2008, pp [9] Mistodie L.R., Rusu C.C., Constantin E., Scutelnicu E., Voicu C., Comparative experimental study on mono and multiarc submerged arc welding of API 5L-X70 steel, Proceedings of the European Federation for Welding - EWF EUROJOIN 8 Conference, May 2012, Pola-Croatia, pag , ISBN [10] NN, Pipeline catalogue, ESAB. ISBN: