Optimization procedures for GMAW of bimetal pipes

Journal of Materials Processing Technology 211 (2011) 1112–1116

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Optimization procedures for GMAW of bimetal pipes A.M. Torbati a , R.M. Miranda b,∗ , L. Quintino c,d , S. Williams a , D. Yapp a a

Cranfield University - Cranfield, MK43 0AL, United Kingdom UNIDEMI, Departamento de Engenharia Mecânica e Industrial, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c IST-UTL Instituto Superior Técnico, Lisboa, Portugal d IDMEC, Institute of Mechanical Engineering, TULISBON, Lisboa, Portugal b

a r t i c l e

i n f o

Article history: Received 3 November 2010 Received in revised form 11 January 2011 Accepted 13 January 2011 Available online 21 January 2011 Keywords: Butting bimetal pipes Duplex stainless steels Weld penetration Rapid arc GTAW

a b s t r a c t Autogenous gas tungsten arc welding (GTAW) and pulse rapid arc gas metal arc welding (GMAW) of butting bimetal (Bubi) pipelines were studied. GMAW was carried out from the outside of the pipe while GTAW was done from the inside to prevent lack of penetration and to promote a smooth internal weld bead surface. Current, welding speed, electrode diameter, shielding gas and orbital positions were defined as variables. The requirement for the GTA weld was to achieve 2 mm penetration depth without undercutting. The required penetration was difficult to achieve due to the outwards flow pattern in the molten pool driven by the Marangoni effect as a result of low sulphur content. It was shown that, under optimised conditions, it was possible to obtain sound welds with proper geometry and defect free. The conditions needed were a combination of current of 170 A, welding speed of 200 mm/min and an electrode angle of 30◦ , with shielding gas protection of He–25%Ar for narrow groove welding of a J-beveled pipe. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Bi metallic materials are required for pipelines working in conditions with corrosive effect. High pressure, temperature and cyclic stress result in the formation of corrosion cracking, namely in zones with stress concentration. Butting bimetal pipelines are an innovation dated from the beginning of the nineties which improves the life of the welded construction without higher costs. A relevant solid stainless steel pipe would cost 25–40% more than a bubi-pipe according to Bingen and Martensen (2004). The bubi-pipe consists of a corrosion resisting pipe which is telescopically aligned inside a pipe in carbon-manganese material. The tight bonding between the two pipes is achieved by hydraulic expansion. Compared with the metallurgical clad pipe, the bubi pipes offer a wide range of material combinations for both the inner and outer pipe and remarkable price advantages. The thick layer of carbon steel provides adequate strength for the pipelines, while the thin layer, which is normally 1.5–3 mm thick, provides internal corrosion resistance. The liner thin layer of corrosion resistant alloy (CRA) can be in austenitic stainless steel, duplex or super duplex stainless steel or nickel based alloy according to Banse (1998). The use of duplex stainless steels in bimetal pipelines brings advantages related with the particular characteristics of high

∗ Corresponding author. E-mail address: [email protected] (R.M. Miranda). 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.01.013

strength and good corrosion resistance of these steel grades. The excellent combination of strength and corrosion resistance in duplex stainless steels (DSS) is due to their strict composition control and microstructural balance though the ferrite–austenite ratio, but it is often upset in DSS weld metals owing to the rapid cooling rates associated with welding. To achieve the desired ferrite–austenite balance, and hence properties, either the weld metal composition and/or the heat input need to be controlled (Nowakiy Lukojc, 2005; Urena et al., 2007). Welding defects can also occur, during and after welding, which require a good control of the welding procedure. Gas tungsten arc welding and gas metal arc welding are processes used for this type of steel in the oil and gas pipelines. Defects as lack of penetration and cracking, namely in orbital welding, shall be avoided. When butt welding, root pass is the most critical, since it must fully penetrate and prevent contamination of the corrosion resistant liner, so, moisture and dirt contaminations must be avoided. Sabapathy et al. (2003) and Wahab et al. (2005) emphasized the importance of root pass in pipeline failure. First pass into the liner pipe results in a fine back surface inside the pipe and then other passes are conducted to fill the groove. Fig. 1 shows the joint preparation. Developments in arc welding processes are strongly related with the need to increase productivity and guarantee the desirable welding quality and integrity. GMAW welding is one of the most used processes in industry both in manual and robotized welding. The search for higher productivity and better quality has led to the development of many variants of this process.

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Fig. 1. Joint prepartaion for orbital welding.

In order to increase the productivity of the GMAW welding processes several methods have been applied, such as: • Optimization of shielding gas composition; • Development of new waveforms and more efficient power sources; • Unconventional welding process setting parameters control. These methods have lead to several variants of conventional GMAW as the TIME, tandem, rapid arc variants summarized in a recent study from Pepe et al. (2010). A high melting rate is associated with significantly higher productivity levels and, consequently, results in lower costs of production. This is noteworthy of interest for any industry, specially for oil and gas pipelines. A higher melting rate involves not only a larger quantity of filler material melted, but also a higher arc heat input which may have unfavourable influence on the weld mechanical properties. In welding it is, therefore, necessary to be cautious and take into account the chemical composition and the mechanical properties of the material. This all the more true in duplex stainless steel, where high heat inputs can deteriorate the properties of the base material, in the heat affected zone. RapidArc is a process where a good control of heat input is achieved. This variant of GMAW consists in a refined pulse process, specially designed to achieve faster welding speeds comparable with the traditional pulse GMAW. The RapidArc waveform facilitates low voltage welding at high welding speeds, from 1 to 1.5 m/min as illustrated in Fig. 2 adapted from Lincoln Electric Company. In traditional pulse the arc length is longer to avoid spatter, which limits the welding speed. The RapidArc waveform keeps arc length short and the precise control of the short circuit cycle eliminates spatter. Other advantages of this process compared with conventional GMAW are claimed to be a reduction of incidence of undercutting by 75%, and a reduction of 10–30% of the cycle time due to the higher welding speeds leading to cost savings as referred by Pepe (2010). The lower spatter levels with this process can also reduce labor costs for post-weld part and fixture clean-up as from Torbati et al. (in press). The objective of this work was to develop a root pass procedure by combining a rapid arc GMAW external weld applied from the outside of the pipe with a high quality GTAW weld applied from the inside of the pipe. The GTA was added to assure full penetration with a smooth transition between the weld and the base material on the inner surface to improve corrosion and fatigue resistance. Additionally, attention was given to the interface between the two passes in order to prevent gaps which had a detrimental effect on pipe performance under operating conditions (Fig. 3).

Fig. 2. Schematic diagram of waveform and metal transfer mechanism RapidArc.

2. Experimental procedure 2.1. Materials and equipment The material used in this study was an ASTM A240 S32506 duplex stainless steel pipe with 254 mm internal diameter and 14 mm wall thickness. Tables 1 and 2 present the compositions of the base material and the wire filler material applied in GMAW, respectively. The power source used for GTA welding was a MIGATRONIC A/S GMA COMMANDER which is a three phase machine based on inverter technology. The power source used for GMA welding was a Lincoln 455/STT which has the capability of supplying power for GMA, pulsed GMA, STT and pulsed RapidArc. It also has a feeding system to provide filler wire. This power source was used in the synergic mode which means that the voltage and the current are altered automatically to synchronise with the defined wire feed speed to sustain the stability of the arc. Process data was recorded by an oscilloscope Yokogawa DL 750 Scopcorder. It recorded the electrical data from the power supply and monitored the current, voltage and wire feed speed during the welding trials. With these values it was possible to calculate the heat input for the different welding conditions studied. The manipulator was a rotator that held the pipe with three jaws at any angle from 0◦ to 90◦ to the horizontal axis while revolving around the pipe’s axis of rotation clockwise or anti clockwise. It has 90◦ freedom of rotation around the X axis to hold the pipe in different spatial positions and inclinations. The jaws have 360◦ freedom of rotation around pipe’s central axis of rotation. A gas flow mixer PRC-2000/3000 programmable ratio control system automatically controlled the flow of multiple gases. It can Table 1 Chemical analysis of S32506 base material (wt.%). C

Si

Mn

P

S

Cr

Mo

Ni

W

0.015

0.32

0.44

0.025

<0.005

25.0

3.19

6.34

2.00

Table 2 Chemical analysis of 2205 wire material for GMAW (wt.%). C

Si

Mn

N

Cr

Ni

Mo

0.02

0.5

1.6

0.17

23.0

8.5

3.1

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Fig. 3. Schematic representation of the project objective.

mix two or three gas types in order to achieve different proportions for shielding. After welding samples were cut and prepared for metallographic analysis after polishing and etching with Kalling reagent. Macrostructure and weld defects were observed using an optical microscope NIKON PRYSM connected to a computer through a KYF55B JVC video camera for data acquisition. 2.2. Welding procedure investigation Fig. 4 shows the experimental setup used to perform the orbital welds with a continuously rotating torch. The torch was fixed for any position defined and the pipe rotated to simulate the upwards and downwards travel. The procedure was to first study the GTA process in the downhand position for selected variables such as current, welding speed, shielding gas and electrode angle in order to achieve suitable penetration depth and weld bead profile. These parameters were then investigated in a complete set of orbital welding positions. The GMA process was then developed for both full and partial penetration of the J preparation that was used. Finally, the optimised parameters were applied to the pipe to demonstrate the successful combined GMA and GTA process. For welding in a narrow groove joint from both sides, one GMA root pass was done from outside with 2205 duplex steel wire filler followed by a remelting GTA weld run, from the inside. The ends of the pipes were prepared and beveled and butts were tack welded to hold the joint for welding. Fig. 5 shows the joint preparation design. The optimum combination of parameters was obtained from preliminary trials and applied on an actual beveled pipe.

Fig. 4. Experimental setup for GTA welding of the pipe.

Fig. 5. J-bevel joint preparation.

3. Results and discussion 3.1. GTA process studies The main parameters studied were: current, travel speed, torch tip angle and gas mix. Their effects on penetration depth, weld bead width, top bead profile and defect level were examined. Fig. 6 shows the effect of welding current at two different travel speeds on the penetration depth. Weld beads with smooth top surfaces were obtained with currents up to 200 A, although the welds were very wide with low aspect ratios. The width was of 6.5 mm and the aspect ratio (weld bead depth to width ratio) was below 0.2 at 150 A. In the region of 100–200 A, the penetration depth increases only slowly with current. This is because of the unfavourable outward fluid flow caused by surface tension as described by Mills et al. (1998) and more recently by Lu et al. (2005) for TIG and Lu et al. (2003) for ATIG welding. At higher currents penetration depths begin to increase more rapidly due to the depressing force caused by the high arc or vapour pressure. However, to obtain penetration depths greater than the required 2 mm currents of more than 250 A, but then the weld profiles show unacceptable undercut on the top surface.

Fig. 6. Penetration depth as a function of current for two different travel speeds for the GTA process.

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Fig. 8. Variation of penetration depth in mm with position of welding. Fig. 7. Effect of electrode angle on penetration depth for a current of 150 A and two different travel speeds.

In order to improve the penetration depth while avoiding the undercut, the effect of electrode angle was investigated at low currents. Fig. 7 shows the effect of electrode angle on penetration depth for two travel speeds and a current of 150 A. It can be seen that penetration depth increases with decreasing electrode angle but it is still insufficient to reach the value required. Various other gas mixes were investigated in order to increase the penetration depth at low to medium current intensities. The first gas mixtures investigated contained 30% and 75% helium with the balance argon. At all currents and travel speeds there was some increase in penetration but the aspect ratio of the welds was not improved. The increase in penetration is due to the higher ionization potential of helium leading to higher heat input into the weld. At the low to medium currents needed to avoid undercut, the low aspect ratio weld profile is caused by unfavourable outward fluid flow. Leinonen and Karjalainen (1990) and Heiple and Roper (1982) have observed this problem in these types of materials which is due to the low level of surface active elements such as sulphur and oxygen. To reverse the direction of the Marangoni flow, oxygen can be introduced into the shielding gas as studied by Lu et al. (2004a,b). This is often done by adding carbon dioxide which disassociates into oxygen and carbon. Several mixtures were investigated including 0.7% and 2% CO2 in pure argon with limited success. A much larger increase in penetration depth was obtained for a mixture of 1.5% CO2 , 54% helium and balance argon as observed by Shanping et al. (2005). For example, at a current of 150 A and 200 mm/min travel speed the penetration depth increased to 2.2 mm with this mixture compared to 1.0 mm for pure argon. The weld width did not change significantly either, so the aspect ratio increased to 0.29 from 0.15. However, undercut appeared on the top surface. There was also a problem with these CO2 mixtures in that the electrode rapidly became contaminated leading to arc wander and instability. This occurs so rapidly that not even a complete weld could be made round a pipe. The next investigation was concerned with the effect of position of welding when going round the pipe. Tests were carried out with a welding current of 150 A, a gas mix of 75% He and 25% Ar and welding speeds of 200 and 400 mm/min. The welds produced at 400 mm/min often showed unacceptable undercutting. Fig. 8 shows the variation in penetration depth with position of welding for a travel speed of 200 mm/min. The penetration depth varied between 1 and 1.4 mm, but there was no obvious correlation with

Fig. 9. Macrographs of full penetration GMA root welds in narrow groove J-prep. Travel speed: 500 mm/min; wire feed speed: 8.4 m/min.

position and this probably represented the natural variation of the process. From this investigation the optimum parameters for the GTA internal root pass were determined to be those shown in Table 3 and these were used for the pipe welding trials. 3.2. Combined GMA and GTA welding of root pass in pipe with J-prep A first set of trials were conducted on a prepared pipe in order to establish satisfactory conditions to achieve full or near full penetration for the GMA process. A range of travel and wire feed speeds were investigated. An example of a full penetration weld made with a travel speed (TS) of 500 mm/min and wire feed speed (WFS) of 8.4 m/min, contact tip to work distance (CTWD) and a pure argon gas flow of 16 l/min is shown in Fig. 9. For the combined process made with the external GMA weld followed by the internal GTA weld, several values of parameters

Table 3 Welding parameters for root pass with GTAW. Current (A) 170

Travel speed (mm/min) 200

Shielding gas He–25%Ar

Flow rate (l/min) 14

Electrode angle ◦

30

Electrode gap (mm) 2

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Fig. 10. Macrographs of the combined GMA and GTA welding process showing good root welds free of geometrical and metallurgical defects. Left picture: weld 1; right picture: weld 2. Table 4 Welding parameters for root pass with GMAW. Weld no.

Set WFS (m/min)

TS (mm/min)

CTWD (mm)

Gas (l/min)

WFS (m/min)

Current (A)

Voltage (V)

Power (kW)

Heat input (J/mm)

1 2

9.8 9.8

800 1000

11 11

16 16

9.23 9.25

187 188

23.4 23.1

4.95 4.95

370 300

were investigated. The GMA process parameters were adjusted so that a partial penetration weld was made. The GTA pass from the inside then ensured that a full penetration weld was made with no defects and with excellent weld geometry on the internal root face. Examples are shown in Fig. 10 for the welding conditions for the GMAW process parameters shown in Table 4 and for the GTAW process in Table 3. 4. Conclusions In the present work, autogenous GTA pipe welding and pulse rapid arc gas metal arc (GMA) welding of the pipes were studied with the view of optimising orbital welding in butting bimetal pipes in duplex stainless steel. The goal was to achieve adequate penetration on the inner surface after a first GMAW pass from the outside. The following conclusions were drawn: (1) The welds made in this material have low penetration depths and low aspect ratios due to the unfavourable outward flow patterns caused by surface tension driven flow (Marangoni effect). (2) At low to medium currents, increasing penetration depth only increases slowly with current, due to the outward flow pattern. (3) When current exceeds a value such that the depression force overcomes the surface tension force, a more significant increase in penetration is obtained. However, this results in unsatisfactory weld surfaces and undercutting. (4) Sharper electrode angles result in higher penetration which is also a result of Marangoni effect. (5) Helium introduces more heat to the fusion zone and slightly increases penetration. However, it cannot increase the weld aspect ratio and welds are fairly wide. Nevertheless, welding under shielding of helium–argon mixtures produces high quality welds. (6) Adding CO2 to an argon/helium mix increased penetration, but caused rapid electrode deterioration and eventually arc wandering. (7) A combination of a current of 170 A, travel speed of 200 mm/min, an electrode angle of 30◦ and He–25%Ar as the shielding gas were suitable for narrow groove welding of a J-beveled pipe in flat position.

(8) By using the combined GMAW external pass and the GTAW internal high quality root welds were produced with no welding defects in the inner layer in duplex stainless steel. References Banse, J., 1998. New material alternative: stainless and nickel bonded pipes. Stainless Steel World 10 (3), 48–49. Bingen, M., Martensen, E., 2004. BuBi-pipe [butting-bimetal-pipe]—the intelligent cost-saving solution. Stainless Steel World 16, 14–17. Heiple, C.R., Roper, J.R., 1982. Mechanism for minor element effect on gta fusion zone geometry. Welding Journal 61 (4), 97. Leinonen, J.I., Karjalainen, L.P., 1990. Unexpected weld pool profiles in GTA [TIG] welding with oxidising shielding gas. Recent trends in welding science and technology. In: David, SA., Vitek, J.M. (Eds.), Proc. 2nd Int. Conf. Gatlinburg, TN, 14–18 May 1989. ASM International, Materials Park, OH, USA, pp. 387–390. Lu, S.P., Fujii, H., Sugiyama, H., 2003. Marangoni Convection and Welding Penetration in ATIG Welding. Transactions of JWRI 32 (1), 79–82. Lu, S.P., Fujii, H., Nogi, K., 2004a. Weld shape comparison with iron oxide flux and Ar–O2 shielding gas in gas tungsten arc welding. Science and Technology of Welding and Joining 9 (3), 272–276. Lu, S.P., Fuji, H., Nogi, K., 2004b. Marangoni convection and weld shape variations in Ar–O2 and Ar–CO2 shielded GTA [TIG] welding. Materials Science and Engineering A 380 (1/2), 290–297. Lu, S.P., Fujii, H., Nogi, K., 2005. Influence of welding parameters and shielding gas composition on GTA [TIG] weld shape. ISIJ International 45 (1), 66–70. Mills, K.C., Keene, B.J., Brooks, R.F., 1998. Marangoni effects in welding. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 356 (1739), 911–925. Nowakiy Lukojc, A., 2005. Structure and properties of the heat affected zone of cuplex steel welded joints. Journal of Materials Processing Technology 164, 1074–1081. Pepe, N., 2010. Advances in gas metal arc welding and application to corrosion resistant alloy pipes. PhD thesis. Cranfield University. Pepe, N., Quintino, L., Pires, I., Miranda, R., Yapp, D., 2010. Applications of Innovative Variants in MIG/MAG Welding, IIW – XII – 1989 – 10. Sabapathy, P.N., Wahab, M.A., Painter, M.J., 2003. Numerical models of in-service welding of gas pipelines. Journal of Materials Processing Technology 14 (2), Special issue. Shanping, L., Hidetoshi, F., Kiyoshi, N., 2005. Effects of CO2 shielding gas additions and welding speed on GTA weld shape. Journal of Materials Science 40 (9), 2481–2485. Torbati, A.M., Miranda, R.M., Quintino, L., Williams, S., in press. Welding bimetal pipes in duplex stainless steel. International Journal of Advanced Manufacturing Technology, doi:10.1007/s00170-010-2889-7. Urena, A., Otero, E., Utril, M.V., Munez, C.Y., 2007. Weldability of 2205 duplex stainless steel using plasma arc welding. Journal of Materials Processing Technology 183, 624–631. Wahab, M.A., Sabathy, P.N., Painter, M.Y., 2005. The on set of Pipewall Failyre during “in service” welding of gas pipelines. Journal of Materials Processing Technology 108, 422–441.