H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iSTEEL JOINTS ... 477–484 COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iSTEEL JOINTS WELDED BY SMAW AND GTAW PRIMERJALNE [TUDIJE MIKROSTRUKTURE IN @ILA VOSTI ZV ARNIH SPOJEV MED PLO[^AMA IZ MALO OGLJI^NEGA JEKLAZ9%N i ,IZDELANIMI Z SMAW IN GTAW Haiyang Zhu 1,2 , Xuebing Yang 3 , Kun Liu 1 , Xiaoyong Wang 4 , Yuhang Du 1 , Jiasheng Zou 1* 1 School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, China 2 School of Metallurgical Engineering, Suzhou Institute of Technology, Jiangsu University of Science and Technology, Suzhou, China 3 Zhenjiang Zhongchuan Hitachi Zosen Machinery Co., Ltd., Zhenjiang, China 4 School of Metallurgical Engineering, Jiangsu University of Science and Technology, Zhangjiagang, China Prejem rokopisa – received: 2024-01-17; sprejem za objavo – accepted for publication: 2024-06-17 doi:10.17222/mit.2024.1096 The microstructure and impact toughness of9%Nisteel joints welded by shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) were investigated. The two kinds of weld metal were mainly composed of cellular dendrites and a granular precipitated phase. The grains and cellular dendrites of the GTAW weld metal were smaller than those of the SMAW weld metal. The precipitates in the SMAW weld metal were stripe-shaped while those in the GTAW weld metal were rod-shaped; the number of precipitates in the GTAW weld metal was lower than in the SMAW weld metal. The low-temperature impact tough- ness of the GTAW weld metal was better than that of the SMAW weld melt. Keywords:9%N isteel, SMAW, GTAW, microstructure, impact toughness Avtorji v ~lanku opisujejo raziskavo mikrostrukture in udarne `ilavosti zvarnih spojev plo{~ debeline 12 mm iz malo oglji~nega jekla z 9% Ni, ki so bili izdelani z oblo~nim varjenjem z opla{~eno kovinsko elektrodo (SMAW; angl.: shielded metal arc weld- ing) in postopkom oblo~nega varjenja z volframovo elektrodo (GTAW; angl.: gas tungsten arc welding) v reaktivnem oziroma za{~itnem plinu. Med varjenjem sta nastali dve vrsti mikrostruktur v zvarnih spojih, ki sta v glavnem sestavljeni iz celi~ne dendritne in granularne precipitatne faze. Kristalna zrna in celi~ni dendriti GTAW zvarov so bili manj{i kot tisti, ki so nastali med varjenjem s postopkom SMAW. Izlo~ki (precipitati) v SMAW zvarih so imeli obliko trakov medtem, ko so imeli v GTAW zvarih obliko pali~ic. [tevilo (koncentracija) izlo~kov v GTAW zvarih je bilo manj{e od {tevila izlo~kov v SMAW zvarih. Nizko temperaturna udarna `ilavost GTAW zvarov je bila bolj{a (vi{ja) od `ilavosti SMAW zvarov. Klju~ne besede: jeklo z 9 % Ni, oblo~no varjenje z opla{~eno kovinsko elektrodo (SMAW), oblo~no varjenje z volframovo elektrodo (GTAW), za{~itna/reaktivna atmosfera, mikrostruktura, udarna `ilavost 1 INTRODUCTION Liquefied natural gas (LNG) as a clean and green en- ergy fuel is gradually replacing coal and oil, leading to a rapid development of cryogenic storage-tank equip - ment. 1–3 Currently,9%N isteel is the main material for manufacturing LNG cryogenic storage tanks. Compared with austenitic stainless steel and aluminum alloy, 9 % Ni steel has high strength, low thermal conductivity and thermal expansion coefficient, and good low-tempera- tures toughness. 4,5 In addition, good economic perfor- mance makes it an ideal material for cryogenic equip- ment. 6 In the market,9%N isteel is commonly supplied in three states: double normalizing and tempering (NNT), quenching and tempering (QT), and two-phase region quenching and tempering (QLT). 7,8 Welding is a necessary process for manufacturing LNG storage tanks. Welding consumables for9%N i steel have been developed, transitioning from ferritic types to high nickel alloys. This change is attributed to the good fracture toughness of austenitic nickel-based al- loy at low temperature. 9–11 Currently, the welding tech- niques for9%N isteel include shielded metal arc weld- ing (SMAW), submerged arc welding (SAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), flux cored arc welding (FCAW), laser beam welding (LBW), electron beam welding (EBW), etc. 12–18 Many scholars conducted comparative studies on9%N i steel welded with different welding techniques. For ex- ample, Peng et al. 19 investigated the micro-segregation, microstructure, and bending properties of9%N isteel weldments by SAW and SMAW. El-Batahgy et al. 20 stud- ied the weldability of9%N isteel using LBW and GTAW. Gook et al. 21 investigated the effect of welding parameters on the weld formation, microstructure and tensile strength of9%N isteel welded in a single pass using three welding techniques: autogenous laser weld- ing, laser cold wire welding and hybrid laser-arc weld- Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 477 UDK 669.14:621.791 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(4)477(2024) *Corresponding author's e-mail: zjzoujs@just.edu.cn (Jiasheng Zou) ing. The consumables for welding9%N isteel com- monly include Ni-Cr-Fe and Ni-Cr-Mo nickel-based alloys. The typical welding consumables for SMAW are ENiCrMo-3 and ENiCrMo-6 electrodes, while ERNiCrMo-3 and ERNiCrMo-4 wires are used for GTAW. 22 Huang 23 studied the microstructure and me- chanical properties of joints welded by SMAW and GTAW using the ENiCrFe-9 and ERNiCrMo-4 filler metals, respectively. However, few comparative studies o n9%N isteel joints welded by SMAW and GTAW us- ing the ENiCrMo-6 and ERNiCrMo-4 filler metals are found. For this study, two kinds of9%N isteel joints were welded by SMAW and GTAW using welding elec- trode ENiCrMo-6 and welding wire ERNiCrMo-4, re- spectively. The differences in the microstructure and im- pact toughness of the two welded joints were investigated. 2 EXPERIMENTAL PART The base material wasa9%N isteel plate with the QT state and a thickness of 12 mm. Figure 1 shows the optical microstructure of9%N isteel with tempered martensite. Weld plates were produced by SMAW and GTAW. The welding consumables were an ENiCrMo-6 welding electrode for SMAW with a diameter of 3.2 mm and an ERNiCrMo-4 welding wire for GTAW with a diameter of 2.4 mm. The chemical compositions of the base metal and welding consumables are presented in Table 1. The size of the welding plate was (300 × 240 × 12) mm, and the welding groove angle was 60°, as shown in Figure 2. A blunt edge of 2 mm was reserved for the SMAW plate, and a total of 5 layers and 8 passes were produced, with 3 layers on the front and 2 layers on the back. Single-sided welding with a double-sided forming process was used for the GTAW plate with a to- tal of 6 layers and 10 passes. The welding parameters are shown in Table 2. The welding heat input was calculated using Q = U × I/S, where Q, U, I and S represented the heat input, arc voltage, welding current, and welding speed, respectively. The heat input of SMAW was H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iSTEEL JOINTS ... 478 Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 Table 1: Compositions of the base metal and welding consumables (w/%) Alloy C Si Mn P S Cr Ni Mo Nb Cu Fe W 9%Ni 0.05 0.27 0.66 0.005 0.002 0.03 9.12 0.002 – 0.01 – – ENiCrMo-6 0.03 0.31 2.73 0.005 0.01 12.36 66.37 5.82 1.12 0.05 5.70 1.39 ERNiCrMo-4 0.01 0.03 0.39 0.006 0.005 14.98 58.44 15.77 – 0.02 5.85 3.32 Table 2: Parameters of the two welding techniques Technique Current (A) Voltage (V) Speed (cm min –1 ) Gas flow rate (l min –1 ) Interlayer temper- ature (°C) Heat input (kJ cm –1 ) SMAW 130–140 24–28 12–16 – < 100 14.7–15.6 GTAW 150–160 12–15 10–13 12 < 100 10.8–11.1 Figure 2: Schematic diagram of welding: a) SMAW, b) GTAW Figure 1: Microstructure of the base metal 14.7-15.6 kJ cm –1 , while the heat input of GTAW was 10.8–11.1 kJ cm –1 . The heat input of SMAW was about 4k Jc m –1 higher than that of GTAW. After the completion of welding, the microstructure and impact toughness were analyzed. The metallo- graphic and impact samples were cut using a wire elec- trical discharge machine. After grinding and polishing, the weld zone of the metallographic sample was etched electrolytically ina5%chromic acid solution at a volt- age of 4.5 V for 10 s, then the base metal was etched by a 4 % nitric alcohol solution. The metallographic microstructure was observed by a Zeiss Axio Imager A2m optical microscope (OM). The grain size of the weld metal was studied with Oxford C-Nano electron backscatter diffraction (EBSD). The EBSD specimens were firstly mechanically polished, then prepared with vibration polishing for 2–3 h using an Al 2 O 3 agent. The precipitated phase was analyzed with a JSM-6510LA scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). A V-notch impact test was carried out by a JB-300B pendulum impact testing machine at –196 °C. The position and dimensions of the impact sample are shown in Figure 3. The notch position was located at the cross-section of the weld zone. The fracture mechanism of the impact specimens was ana- lyzed with SEM. 3 RESULTS AND DISCUSSION 3.1 Macrostructure and microstructure The macrostructures of the welded joints obtained with SMAW and GTAW are shown in Figures 4a and 4b. Macroscopic metallography showed that no defects such as crack, slag, incomplete penetration and incom- plete fusion were present in the two welded joints. Fig- ures 4c and 4d show a cellular dendritic solidification mode that occurred in the SMAW and GTAW weld metal. It can be seen that the dendrite branches are of a bright color, while black particles exist among these den- drites, attributed to the residual liquid metal distributed between these branches during the crystallization process and the presence of a serious dendrite segregation. Comparing the dendrite sizes and spacing types in the two weld metals, it was found that the GTAW den- H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iS TEEL JOINTS ... Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 479 Figure 3: Position and dimensions of the impact specimen Figure 4: Macrostructure and microstructure of the weld metal: a), c) SMAW, b), d) GTAW drites were finer and the distance between adjacent den- drites was smaller than in the SMAW sample. To deter- mine the grain size and grain orientation, EBSD was applied to each weld metal and the results are shown in Figure 5. Orientation maps were produced for the sec- tions transverse to the welding direction. Inverse pole figures (IPF) showed that the width of grains in the SMAW weld metal was larger than in the GTAW weld metal. The microstructures of the partially melted zone (PMZ) and heat affected zone (HAZ) of the two welded joints are shown in Figures 6a and 6c. An obvious pla- nar crystallization occurred in the PMZ, and the width of PMZ was a few tens of microns. Due to a higher heat in- put, more base-metal fusion occurred, and the width of the PMZ in the SMAW joint was larger than that in the GTAW joint. The crystal morphology was affected by temperature gradient G and crystallization rate R.A tt h e bonding line, temperature gradient G was large and crys- tallization rate R was small; it was hard to induce consti- tutional supercooling, so a planar crystal was formed. With the solidification taking place from the bonding line to the center of the weld pool, temperature gradient G gradually decreased, while crystallization rate R grad- ually increased, and the constitutional supercooling zone gradually increased, so the crystal morphology changed from the planar crystal to cellular dendrite. 24 H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iSTEEL JOINTS ... 480 Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 Figure 6: Microstructures of welded joints: a), b) SMAW, c), d) GTAW Figure 5: EBSD maps of the weld metal: a) SMAW, b) GTAW H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iS TEEL JOINTS ... Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 481 Figure 8: SEM images and EDS spectrums of GTAW weld metal: a), b) SEM images, c), d) EDS spectrums Figure 7: SEM images and EDS spectrums of SMAW weld metal: a), b) SEM images, c), d) EDS spectrums Figures 6b and 6d show the coarse grain HAZ (CGHAZ) of the two welded joints. Grains grew coarse due to rapid overheating and repeated thermal cycling. The higher heat input in the SMAW joint induced a lon- ger time at high temperature, resulting in coarser lath martensite. 3.2 Precipitated phase Figure 7a shows a SEM image of the SMAW weld metal. A large amount of precipitated phase is distributed in the matrix. The highly magnified image in Figure 7b shows that the intragranular precipitates formed discon- tinuous stripes with sharp ends. EDS analysis spectrums of these precipitates are presented in Figures 7c and 7d, and the contents of elements are presented in Table 3.I t can be seen that these precipitates are rich in Nb and Mo. Figures 8a and 8b show SEM images of the GTAW weld metal. It is obvious that these precipitates are dis- tributed in interdendritic regions. Unlike the SMAW weld metal, the number of precipitates in this case is smaller, and the shape of precipitates mainly resembles rods with elliptical ends. This might be related to the chemical composition and cooling condition. The EDS results in Figures 8c, 8d and Table 3 show that the pre- cipitates are rich in Mo and W. The nickel-based welding electrode ENiCrMo-6 for SMAW contained a certain amount of Nb and Mo, while the nickel-based welding wire ERNiCrMo-4 for GTAW contained a certain amount of Mo and W (Table 1). These elements exhibit large atomic radius differences with Ni. For example, Mo and Nb, with their large atomic radii, tended to segregate in interdendritic regions, which resulted in a pronounced concentration gradient between dendritic and interdendritic regions, thus promoting the formation of (Nb, Mo)-rich and (Mo, W)-rich precipitates. 19,25,26 3.3 Impact toughness The results for the impact absorbing energy and lat- eral expansion at –196 °C for the two weld metals are shown in Figure 9. The average impact absorbing energy and lateral expansion values were used to characterize the impact toughness, where the lateral expansion re- ferred to the sum of the maximum expansion on both sides of the specimen after impact fracture. When a spec- imen was subjected to a plane stress during the impact process, cracks were generated, and their propagation was squeezed outward, finally forming a lateral expan- sion. It can be seen that the average impact absorbing en- ergy and lateral expansion of the SMAW weld metal are 82 J and 1.25 mm, respectively, while their values for the GTAW weld metal are 110 J and 1.58 mm, respectively. The impact absorbing energy of the GTAW weld metal is about 30 J higher than that for SMAW, and the lateral ex- pansion of the GTAW weld metal is 26 % higher than that for SMAW. The results indicate that the low temper- ature toughness of the GTAW weld metal at –196 °C is better than for SMAW. Figures 10a and 10d show SEM images of the im- pact fractures of the SMAW and GTAW weld metal, re- spectively. The fracture surfaces were uneven and a large plastic deformation occurred. Highly magnified images of the fracture show two kinds of appearances: One in- cludes dimples parallel to the length direction of cellular crystals, as shown in Figures 10b and 10f, indicating that the width of cellular dendrite in the SMAW weld metal is significantly larger than that for GTAW. The other form includes equiaxed dimples and cleavage fac- ets, as shown in Figures 10c and 10e. The SMAW frac- ture exhibits a few dimples and some cleavage facets, while more dimples and fewer cleavage facets are ob- served in the GTAW weld metal. It can be inferred that the fracture was perpendicular to the length direction of cellular crystals. A higher impact toughness of the GTAW weld metal is attributed to the finer cellular den- dritic crystals which increased the resistance to crack propagation. Moreover, the morphology and quantity of precipitates also influenced the toughness. As the SMAW weld metal had more precipitates with sharp ends than the GTAW weld metal, a high stress concentra- H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iSTEEL JOINTS ... 482 Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 Table 3: EDS results for the precipitated phases from Figures 7b and 8b (w/%) Weld metal Sites C Si Cr Mn Fe Ni Nb Mo W SMAW 1 14.41 0.75 8.45 2.93 12.19 37.77 8.16 6.39 – 2 16.85 0.53 7.97 1.91 11.17 35.38 10.14 6.76 – GTAW 3 18.24 – 10.43 – 8.04 24.90 – 32.19 6.21 4 18.81 – 10.50 – 7.86 25.97 – 31.82 5.04 Figure 9: Impact absorbing energy and lateral expansion of SMAW and GTAW weld metal tion and local plastic deformation occurred around these precipitates, which decreased the crack propagation re- sistance in the interdendritic region. 27 4 CONCLUSIONS The microstructure and impact toughness of9%N i steel welded by SMAW and GTAW were studied. The main conclusions were as follows: (1) The cellular dendrites of weld metal and the lath martensite of the CGHAZ in the GTAW joint were finer than those for SMAW, and the grain size of the GTAW weld metal was smaller. H. ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iS TEEL JOINTS ... Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484 483 Figure 10: Fracture surfaces of impact specimens: a), b), c) SMAW; d), e), f) GTAW (2) Precipitated phases were discontinuously distrib- uted in the interdendritic region of weld metals. The pre- cipitates in the SMAW weld metal mainly resembled stripes while rods were observed in the GTAW weld metal. The quantity of precipitates in the SMAW weld metal was higher than in the GTAW weld metal. (3) The impact toughness of the GTAW weld metal at -196 °C was higher than for SMAW. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 52105351), China Postdoctoral Science Foundation (Grant No. 2022M722928), Natural Science Foundation of Jiangsu Province (Grant No. BK20200997& BK20210890), and Jiangsu Provincial Double-Innovation Doctoral Program (Grant No. JSSCBS20210991). 5 REFERENCES 1 J. Pospí{il, P. Charvát, O. Arsenyeva, L. Klime{, M. [pilá~ek, J. J. Kleme{, Energy demand of liquefaction and regasification of natural gas and the potential of LNG for operative thermal energy storage, Renew. Sustain. Energy Rev., 99 (2019), 1–15, doi:10.1016/j.rser. 2018.09.027 2 D. J. Oh, J. M. Lee, B. J. Noh, W.S. Kim, Ryuichi-Ando, Toshiyuki- Matsumoto, M. H. Kim, Investigation of fatigue performance of low temperature alloys for liquefied natural gas storage tanks, Proc. Inst. Mech. Eng., Part C: J. Mech. Eng. Sci., 229 (2015) 7, 1300–1314, doi:10.1177/0954406215569255 3 Y . Zhu, W. Mu, Y . Cai, D. Xin, M. 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ZHU et al.: COMPARATIVE STUDIES ON MICROSTRUCTURE AND TOUGHNESS OF9%N iSTEEL JOINTS ... 484 Materiali in tehnologije / Materials and technology 58 (2024) 4, 477–484