King Mongkut s University of Technology Thonburi, Bangkok, Thailand

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1 Key Engineering Materials Online: ISSN: , Vol. 545, pp doi: / Trans Tech Publications, Switzerland Welding procedure development for welding of high strength carbon steel cladded with austenitic stainless steel 316L by using overmatching filler metal. Nusara Tiyasri 1,a, Bovornchok Poopat 2,b 1, 2 Department of Production Engineering, Welding Engineering Program, Faculty of Engineering, King Mongkut s University of Technology Thonburi, Bangkok, Thailand a nusara.tiyasri@gmail.com, b bpoopat@yahoo.com Keywords: Clad line pipe, 316L, clad plate, peel back, hot crack, Nickel base filler Abstract. This work aims to develop welding procedure for small diameter longitudinal welded clad pipe made from clad plate. High strength carbon steel base metal bonded with 316L stainless steel clad layer was used in this study. The dissimilar materials at the weld joint and accessibility limitation of small diameter present difficulty in welding process selection to achieve weld soundness. The joint and welding sequence are designed to avoid solidification cracking. Nickel base over matching filler is used on the clad side. Typical joint configuration is double V groove weld without clad peel back to minimize the number of passes inside the pipe. Firstly, welding is done on the carbon steel side by using Shielded Metal Arc Welding (SMAW) and Submerged Arc Welding (SAW) with carbon steel electrodes. Then, welding on the clad side is done by using ERNiCrMo-3 filler metal. Two different procedures for the clad side are studied. The first procedure is to weld the clad side by using Gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding pulse current (GMAW-pulse) and another procedure is to weld the clad side by using the SAW procedure. Hot cracking was observed in the case of SAW procedure at the clad weld centerline due to high heat input and high level of dilution. Mechanical properties and microstructure are evaluated. Clad weld by use of GTAW and GMAW-pulse could give sound weld metal. The tensile and yield strength of all weld metal were found to be greater than that of base metal and 100% shear failures were observed. Charpy impact energy of weld and HAZ at - 10ºC was found to be over 100 joules. Hardness of weld and HAZ area are surveyed over the weld cross section to determine local hardening. Additionally intergranular corrosion testing was carried out on the clad weld side and then bend testing was done. No crack was observed. Therefore, GTAW and GMAW-pulse clad weld procedure could give required properties according to clad line pipe standard, reduce cost of production and increase productivity compared to the peel back method. Introduction Longitudinally welded clad pipe is made from clad plate. A clad metal plate is made by bonding two metal plates with different characteristics to each other. This clad metal plate provides corrosion resistance on the high alloy clad side together with high strength from the carbon steel backing. Clad pipe composite materials are widely used for the transport of corrosive fluids in flow lines and associated risers in oil and gas production systems. Clad pipe normally comprises a steel pipe with a 2-3mm thick internally clad layer of Corrosion resistant alloy (CRA) material [1, 2]. The common manufacturing route of longitudinal welded clad pipe is seam welding firstly on the carbon steel side and then on the clad side to complete the weld [2]. The most common welding method for clad metal where both side are accessible is done by peeling back 8 to 9.5 mm of the CRA layer from the adjacent surfaces at edge of joint to avoid melting the CRA while welding the carbon steel side which can lead to cracking. Carbon steel is welded by using carbon steel electrodes. Then the CRA is restored in the peeled-back area. This method will have high risk of discontinuities and slow productivity in small ID clad pipe. [3]. Some manufacturers successfully use the Electro Slag Welding (ESW) process with wide strip filler metal to complete clad restoration on the peel back area with low dilution and to meet chemistry requirement for clad metal All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-13/05/16,10:09:41)

2 Key Engineering Materials Vol layer but ESW is a more suitable option for large diameter pipe since the welding touch arrangement is not able to fit within ID pipe [4]. When access to the inside of the pipe is limited another method is to weld from outside using a CRA filler throughout the weld [3]. This method seems to be expensive and gives low productivity. In clad metal or dissimilar-metal welding, the filler metal must be able to accept dilution by base metals without producing a crack-sensitive microstructure. [5] Nickel base weld filler metal has a FCC structure throughout the temperature range of the solid phase and can provides resistance to various corrosive environments. Therefore, nickel-base electrodes are widely used for the cladding and joining dissimilar metals [6,7]. This study is intended to improve productivity and cost reduction for longitudinal welding of small diameter clad line pipe. Two different welding procedures on the clad side are accomplished in this study. The first welding procedure is to weld one pass on the clad side by SAW and another procedure is to weld the clad side by GTAW and GMAW pulse. Experimental setup and procedure Material and groove design. The material used in this study is roll bond clad plate of 12.7 mm thick high strength carbon steel base metal bonded with 3.0 mm thick austenitic stainless steel grade 316L. The chemical composition of each layer is shown in Table 1. This clad plate is formed into pipe with internal diameter 297 mm by pressing. The edge bevel is prepared as double V groove without clad peel back as shown in Fig. 1 a). The groove is designed to allow double side welding and the depth of groove on the clad side is a little deeper than 316L clad layer to prevent the clad layer melting into the carbon steel which may cause hot cracking of weldments. This groove design also aims to reduce the number of welding passes on the clad side, reduce risk of weld defects, improve productivity and reduce welding cost compared to the peel back method and the CRA full joint welding method Table 1: Chemical composition of the studied clad plate and filler metal (in wt.%) Description C Mn Si Ni Mo Cr Ti Nb Fe _ Carbon steel base metal Balance 316L clad layer Balance ERNiCrMo-3 (SAW) ERNiCrMo-3(GTAW&GMAW-p) Table 2: Welding parameters Welding sequence Current Volt Travel speed (cm/min) Heat Input (kj/cm) _ Carbon steel side welding SMAW 130A 26V SAW 350A 34V _ Clad side welding procedure -1 SAW 450A 33V _ Clad side welding procedure -2 GTAW 280A 18V GMAW-pulse (*) 220A 30V (*) Base current 120A, peak current 410 A, pulse width 2.3ms, frequency 125Hz. _ Welding procedure. Two pipe samples were welded first on the carbon steel side (outside of pipe) with the same welding procedure using carbon steel filler metal. SMAW E8018-G, size 3.2 mm, was used as a first pass for pipe fit up and root pass. Then cap passes were completed by using SAW with high strength carbon steel filler metal A5.23 EG (OERLIKON SD3 1Ni ¼Mo) with a diameter of 2.4 mm and OERLIKON OP121TTW flux. SAW was selected to weld on the carbon steel side because this process delivers high deposition rate and the welding machine was easy to set up for line pipe welding. Carbon steel electrode matching with base material strength was used in this study. For the clad side, Nickel base filler metal ERNiCrMo-3 with chemical composition shown in Table 1 was used. In the first procedure, the clad side was welded by the SAW process using ERNiCrMo-3 and Oerikon OP76 welding flux. Welding was completed within one pass only.

3 184 Materials Science and Technology VII In the second procedure, the GTAW process was used as the first pass on the clad side to control weld penetration and to ensure complete fusion. GTAW with wire size 1.2 mm, wire feed rate 0.4 m/min., 2%Th Tungsten electrode 3.2 mm and Argon shielding gas with flow rate18l/min was set in this study. Then GMAW-pulse was used for cover passes on the clad side. Welding parameters for GMAW were set as; wire size 1.2 mm, wire feed speed 8.6 m/min and 25%Helium+75%Argon. For GTAW and GMAW-pulse, the welding torch was fixed to a welding boom inside the pipe. The pipe was longitudinally moved past the welding torch. Welding direction was controlled by a welding operator via video camera using remote control. The weld soundness, microstructure and mechanical properties were investigated. Furthermore welding cost and time for each procedure was also compared with that of the conventional method. Other welding parameters for each procedure are shown in table 2. Results and discussion Centerline cracking was visually observed on clad pipe that was welded by SAW as shown in Fig.1 b). Since SAW delivered relatively high heat input, it gave deeper penetration and high dilution from base metal (carbon steel). This procedure would tend to cause cracking. Fig. 1 c) shows a macro section of clad weld made by using GTAW and GMAW pulse. GTAW and GMAW pulse gave a satisfactory sound weld with a smooth bead and no sign of cracking was observed. Microstructures of each pipe sample were studied. Figure 2 a) shows SAW weld on clad side revealing as columnar dendrites. The Crack propagated along the weld centerline. The crack occurred at a dendrite grain boundary and the crack follows the colomnar interdendritic boundaries which indicated that the crack was occurred at the last stage of weld solidification or it can identify as solidification crack [8]. On the other hand, The GTAW and GMAW pulse clad weld showed no sign of cracks and microstructure of weld revealed as equiaxed grains as shown in Fig. 2 b). (b) (c) (d) Figure 1 a) Joint preparation b) Macro cross section of clad side welded by SAW c) Macro cross section of clad side welded by GTAW and GMAW-pulse d) Bend test samples SEM examination at higher magnification showed precipitate particles aligned along the interdendritic regions and crack area of SAW clad weld as shown in Fig. 2 c), which appeared as light phase along dendrite boundary. EDS analysis indicated Nb precipitation as shown in Fig. 2 d) and depletion of Nb in the dendritic matrix as shown in Fig. 2 e). It was found that weld cracking in Nickel base weld metal can be associated with second phase particles in the weldment. For example, 625 alloy (UNS N06625) weld solidifications exhibits L (γ + NbC) and L (γ + Laves) eutectic-type reactions leading to cracking in Ni base weld metal.[9,10,11]. The presence of second phase and their size and distribution could be affected by heat input during welding [12]. The processes and welding parameters used on SAW gave high heat input and this caused

4 Key Engineering Materials Vol solidification cracking due to slow cooling rate. It allowed enough time for interdendritic segregation of Nb resulting in cracking at the last stage of solidification. GTAW and GMAW pulse used lower heat input with fast cooling rate resulting in less time for Nb segregation. Heat input is one of the major factors that can cause solidification cracks in nickel base weld metal thus a low heat input welding procedure must be considered necessary for clad joining. The ductility of weld metal was tested by side bend tests at 180 degree with 50 mm plunger. It showed satisfactory results without any opened defect or crack as shown in Fig.1 d). Tension test results of transverse and longitudinal welds exceeded yield and UTS of the base metal which met DNV requirement as shown in Table 3. Ductility was also found to be acceptable to Det Norske Veritas (DNV) standard. (b) (c) (d) (e) Figure 2 a) Microstructure of SAW clad weld crack area at 500X. b) Microstructure of GTAW and GMAW pulse clad weld at interface area between weld metal, carbon steel and 316L clad at 100X. c) SEM micrograph at 2000X indicated EDS analysis location at the particle (Spectrum 1) and dendrite matrix (Spectrum 2), d) EDS analysis at precipitate particle (Spectrum 1), e) EDS analysis at dendrite matrix (Spectrum 2)

5 186 Materials Science and Technology VII Table 3. Tension test result Description Yield strength(mpa) Tensile strength(mpa) Elongation(%) Longitudinal weld 520, , Transverse weld - 620, Charpy impact tests were carried out to determine toughness at difference locations; weld metal, fusion line (FL), 2 mm from FL (FL+2), 5 mm from FL(FL+5) and base metal. Charpy impact testing was performed at -10 o C. The results showed that the absorbed energies for all test locations were found to be greater than 100 J and broken specimens appeared to fail by shear fracture. Impact resistance at the weld metal was found to be the lowest compared to other locations because the weld metal had higher strength than that of heat affected zone (HAZ) and base metal. The Vickers hardness was surveyed over weld cross sections on both the clad side weld by SAW and the GTAW&GMAW pulse according to ASTM E92 with HV10 used to determine hardness over weld and HAZ region, Fig. 3 a) shows hardness levels at 1.5 mm depth below the clad surface and Fig. 3 b) shows hardness values at 1.5 mm below the clad weld-carbon steel interface. Since weld metal has higher strength, the hardness of clad weld produced by GTAW&GMAW was higher than that of HAZ, 316L clad metal and carbon steel base metal. (b) Figure 3 a) Hardness at 1.5 mm below clad surface. b) Hardness at 1.5 mm below clad interface

6 Key Engineering Materials Vol The SAW clad weld gave lower hardness than that of HAZ and base metal due to high dilution and high heat input. Corrosion resistance testing of transverse weld metal produced by GTAW&GMAW pulse was performed on the clad side according to A262 Practice E. Intergranular corrosion testing was conducted to determine the susceptibility of weldments and HAZ of austenitic stainless steel to intergranular attack associated with the precipitation of chromium-rich carbides due to heat applied by welding. No intergranular corrosion or fissures were observed on the test sample after bending to 180 degree. Summary This work proposes a welding procedure for small diameter longitudinal welded clad pipe made from clad plate. The dissimilar materials at the weld joint and accessibility limitation of small diameter presented difficulty of welding process selection to achieve weld soundness. GTAW and GMAW-pulse welding provided sound clad weld metal and met service requirements of the DNV clad line pipe standard by using nickel base over matching filler metal on the clad side. The tensile and yield strength of all weld metal were found to be greater than that of base metal and 100% shear fractures were observed. Charpy impact energy of weld and HAZ at -10ºC was found to be over 100 joules. Hardness on weld and HAZ areas was surveyed over weld cross sections to determine local hardening. It is also possible to reduce cost of production and increase productivity due to the reduction in number of passes and machining requirement of the peel back method. Corrosion resistance of weld and HAZ were found to be acceptable for service. Use of the SAW process on clad weld resulted in solidification cracking. The cracking is believed to be related to Nb segregation, and possible precipitation of a Nb rich phase, in the interdendritic regions. This segregation is likely to be more severe when a high heat input process such as SAW is used. References [1] Sayee Raghunathan, Dave Harvey., 2007, Project Outline: Improved Welding. Inspection and Integrity of Clad Pipeline Girth Welds, TWI Ltd. Cambridge, UK, +44 (0) [2] Liane M.Smith, Mario Celant, CASTI Handbook of Cladding technology, 2nd edition, CASTI Publishing Inc. pp [3] Lynn C. Leblanc. Fabrication and Welding of Solid CRA and CRA-Clad Materials. American Welding Society, [4] Mark Fryer, Development of UOE Clad Linepipe, Corus Tubes Energy Business, UK [5] American Society of metal, 1993, ASM hand book Volume 6, Welding brazing and soldering. [6] American Welding Society, 2001, Welding Handbook Volume 1, Welding Science and Technology, 9th edition, American Welding Society, Miami, Florida. [7] M. Sireesha, Shaju K. Albert, V. Shankar, S. Sundaresan. A comparative evaluation of welding consumables for dissimilar welds between 316LN austenitic stainless steel and Alloy 800, Journal of Nuclear Materials 279 (2000),pp [8] Sindo Kou, Welding Metallurgy, 2nd Edition John Wiley & Sons, Inc. 9. J.N. Dupont, Solidification of an Alloy 625 Weld Overlay. Metallurgical and Materials transactions, Volume 27A, November [9] J. N. Dupont, C. V. Robino and A. R. Marder, Solidification and Weldability of Nb-Bearing Superalloys. [10] Vani Shankar, K.Bhanu Sankara Rao, S.L. Mannan, Microstructure and mechanical properties of Inconel 625 supperalloy, Journal of Nuclear material 288 (2001) pp [11] Dong Jin Lee, Youn Soo Kim, Yong Taek Shin, Contribution of Precipitate on Migrated Grain Boundaries to Ductility-Dip Cracking in Alloy 625 Weld Joints, KIM and Springer, Vol. 16, No. 5 (2010). pp [12] I.L.W. Wilson, R.G. Gourley, R.M. Walksak, and G.J. Brick. The effect of Heat Input on microstructure and cracking in alloy 625 weld overlays, Westinghouse electric Corporation Electro mechanical division, Cheswick avenuem Pennsylvania.

7 Materials Science and Technology VII / Welding Procedure Development for Welding of High Strength Carbon Steel Cladded with Austenitic Stainless Steel 316L by Using Overmatching Filler Metal /