Low Temperature Impact Toughness of Structural Steel Welds with Different Welding Processes

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1 KSCE Journal of Civil Engineering (2015) 19(5): Copyright c2015 Korean Society of Civil Engineers DOI /s TECHNICAL NOTE Structural Engineering pissn , eissn Low Temperature Impact Toughness of Structural Steel Welds with Different Welding Processes Hyun-Seop Shin*, Ki-Tae Park**, Chin-Hyung Lee***, Kyong-Ho Chang****, and Vuong Nguyen Van Do***** Received January 25, 2014/Revised June 7, 2014/Accepted June 24, 2014/Published Online January 5, 2015 Abstract Influence of welding process and welding consumable on the impact toughness at low temperatures of the Heat Affected Zone (HAZ) and the weld metal in a structural steel weldment was investigated. A comparison of the low temperature impact toughness was made between the welded joints fabricated by Shielded Metal Arc Welding (SMAW) and Flux Cored Arc Welding (FCAW) processes, respectively. The Charpy impact tests along with the microstructural observations and the hardness measurements were carried out to derive the effective welding method to guarantee the higher impact toughness of the HAZ and the weld metal at low temperatures. Standard V-notch Charpy impact specimens were prepared and tested under dynamic loading condition. Variation of the Charpy impact energy with respect to the test temperature and that of the hardness across the welds were presented and correlated with the microstructure and the welding process. Analysis of the results unveiled that the weld metal of the FCAW joint has a little higher low temperature impact toughness owing to the higher nickel content, whilst the HAZ of the SMAW joint has much superior impact toughness at low temperatures attributed to the lower heat input; thus the efficient welding method to ensure higher low temperature impact toughness of the HAZ and the weld metal is to employ a low heat input welding process using a welding consumable with high nickel content. Keywords: low temperature impact toughness, heat affected zone, weld metal, welding consumable, welding process 1. Introduction In the past decade, the increasing demand for the natural resources such as oil and gas has prompted the construction of steel structures in cold regions. The application of steel structures in cold environments requires clarifying whether the steels satisfy the required impact toughness at low temperatures, since steel becomes more vulnerable to brittle fracture by impact loading as the ambient temperature goes down. Welding is essential for the fabrication of steel structural members. Thus, the low temperature impact toughness of the weld metal, the HAZ (Heat Affected Zone) and the base metal of the welded steel structures constructed at cold regions should be evaluated so as to secure the structural integrity of the welded parts. Moitra et al. (2002) investigated the microstructural effects on the fracture toughness at low temperatures of the HAZ of 9Cr 1Mo steel welds through the simulated HAZ specimens. The low temperature impact toughness was estimated in terms of the upper shelf energy and the ductile-to-brittle transition temperature. Bayraktar et al. (2004) examined the impact toughness of the weld metal and the HAZ for pipeline laser welds by using a new type of impact tensile testing to have correct evaluation of the laser weld toughness properties at low temperatures. Ibrahim et al. (2010) performed a comparative study on the fracture behavior of austenitic and duplex stainless steel weldments at low temperatures, which were fabricated by both shielded metal arc welding and tungsten inert gas welding, through instrumented impact testing. Recently, Lee et al. (2012) carried out an experiment to assess the low temperature impact toughness of multi-pass butt-welded high strength TMCP (Thermo-Mechanical Controlled Process) steel welds. The experiment included the microstructural observation and the Charpy impact test of the HAZ and the weld metal. The experimental investigation was also conducted on conventional structural steel welds for comparison. They insisted that in order *Member, Senior Researcher, Structural Engineering Research Division, Korea Institute of Construction Technology, Goyang , Korea ( hsshin@kict.re.kr) **Research Fellow, Structural Engineering Research Division, Korea Institute of Construction Technology, Goyang , Korea ( ktpark@kict.re.kr) ***Member, Assistant Professor, The Graduate School of Construction Engineering, Chung-Ang University, Seoul , Korea (Corresponding Author. ifinder@cau.ac.kr) ****Member, Professor, Dept. of Civil and Environmental & Plant Engineering, Chung-Ang University, Seoul , Korea ( changkor@cau.ac.kr) *****Assistant Professor, Dept. of Civil Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam ( dovanvuong28584@gmail.com) 1431

2 Hyun-Seop Shin, Ki-Tae Park, Chin-Hyung Lee, Kyong-Ho Chang and Vuong Nguyen Van Do to guarantee the low temperature impact toughness of the welded joints, adoption of an appropriate welding process and suitable welding electrode should be made. It has been known that welding process and welding consumable have considerable effects on the performance of steel welded joints. Reddy et al. (1998) studied the resistance against projectile penetration of the HAZs and the weld metals in highstrength low-alloy steel weldments fabricated by three different welding processes. The ballistic performance of the weldments was explained on the basis of the microstructures, the hardness gradients across the weldments and the thermal efficiencies of the three welding processes. Magudeeswaran et al. (2008) investigated the influence of welding process and welding consumable on the transverse tensile and impact properties of armour grade quenched and tempered steel joints and reported that welding process and welding electrode significantly affect the transverse tensile strength and the impact toughness of the welded joints. However, they focused on the weld metal properties, and thus limited information on the HAZ impact toughness was provided. Moreover, in their work, the test temperature was confined to room temperature; hence the effects that welding process and welding consumable have on the low temperature impact toughness of structural steel welds are still unknown. Ren et al. (2009) explored the effects of alloying elements in welding wires and welding process on the microstructures and low-temperature impact toughness of weld metals. They indicated that optimal contents of alloying elements in welding electrode together with an appropriate welding heat input can improve the low temperature impact toughness of weld metals. However, their work was limited to the weld metal toughness of the submerged arc welded pipeline steel. In this study, the low temperature impact toughness of structural steel welded joints fabricated with different welding processes was evaluated through the Charpy impact test along with metallurgical observation and hardness measurement in order to find out the effective welding method to guarantee the higher impact toughness of the HAZ and the weld metal at low temperatures. The welding processes employed are Shielded Metal Arc Welding (SMAW) and Flux Cored Arc Welding (FCAW), which are representative welding processes used in construction, nuclear power plants and ship buildings and require different welding heat input and different welding electrode during implementation. The weld fillers used are welding consumables for steels for low temperature use. Effects of the welding process and the welding consumable on the impact toughness of the welded joints at low temperatures were examined based on the absorbed energies, the microstructures and the hardness; thus the efficient welding method was derived. Table 1. Chemical Composition and Mechanical Properties of the Base Material used Chemical composition (mass, %) Base metal C Si Mn P S EH Mechanical properties Base metal Ultimate Charpy Yield Stress Elongation strength impact energy (%) (J) EH ( 40 o C) for use in cold environments. Table 1 shows the chemical composition and the mechanical properties of the base metal based on the mill test certificate. Typical ferrite + pearlite features are revealed in the base metal microstructure in which the portion of ferrite is much larger (see Fig. 1). Bevel butt joint configuration with the root gap of 6 mm, as shown in Fig. 2, has been prepared for joining the plates in order to secure the notch position at the weld metal and the HAZ in the impact test specimen. Two weld specimens were constructed, i.e. one was fabricated by SMAW process with 15 welding passes using an AC arc welding machine and the other was joined by FCAW process with 9 welding passes employing a CO 2 semi-automatic welding machine. The welding consumables used to fabricate Fig. 1. Microstructure of the Base Metal 2. Materials and Methods The base material used in this study is EH36 TMCP steel plate with 20 mm thickness, which is equivalent to ASTM A131 steel. The TMPC steel is widely used in ship buildings and is guaranteed Fig. 2. Joint Configuration and Weld Specimen Size 1432 KSCE Journal of Civil Engineering

3 Low Temperature Impact Toughness of Structural Steel Welds with Different Welding Processes Table 2. Chemical Compositions and Mechanical Properties of the Welding Consumables used Chemical composition (mass, %) Welding electrode C Si Mn P S Ni Ti B S-76LTH (4ø) Supercored 81-K2 (1.4ø) Mechanical properties Welding electrode Yield Stress Ultimate strength Elongation (%) S-76LTH Supercored 81-K the weld specimens were S-76LTH for SMAW process and Supercored 81-K2 for FCAW process, which were low-hydrogen welding electrodes and were produced in accordance with the AWS A5.5 and AWS A5.29 specifications, respectively. The weld fillers deposited are suited for steels targeted to low temperature use. The chemical compositions and mechanical properties of the welding consumables are presented in Table 2. Prior to welding, welding electrode should be baked to reduce the diffusible hydrogen content of the weld metal. In this study, the electrodes were dried in 350 ~ 400 o C oven for two hours, and kept warm in 100 ~ 150 o C incubator after the heating. The welding conditions and process parameters used in the fabrication of the respective weld specimen are typical of industrial practice and are given in Table 3. After the welding was over, microstructural analyses were carried out using the OLYMPUS PME3 optical microscope. Conventional metallographic procedures were followed to prepare the specimens for microstructural examinations. The samples were extracted from the weld metal, the HAZ and the base material of the respective weldment. Then, they were polished and etched by a 2% Nital solution for about 20 ~ 30s. Moreover, Vicker s microhardness testing machine (with diamond pyramid indenter of 10 kg transverse load) was used for measuring the hardness across the welds. The hardness was then correlated with the microstructure and the welding process. The Charpy impact tests were carried out using the Tinius Olsen Charpy impact machine with standard Charpy V notch specimens ( mm) machined as per KS B 0809 (2001). In order to obtain the effective welding process to ensure the superior impact toughness at low temperatures of the steel welded joints, specimens were extracted from the weld metal (a) and the HAZ (b) as shown in Fig. 3. The impact test specimen was cut with the centerline of its height corresponding to that of the plate thickness, and the notch face of the specimen was chosen perpendicular to the surface of the weld piece, with the location of the notch measured relative to the centerline of the root gap or relative to the fusion line (Lee et al., 2014). This allowed for impact tests where the notch was in the weld metal and in the HAZ, respectively. The impact tests were conducted in accordance with KS B 0810 (2003) and KS B 0821 (2007) Table 3. Welding Conditions and Process Parameters PASS (a) Weld Specimen Made by SMAW Process Current (A) Voltage (V) Speed (cm/min) Heat Input (KJ/cm) ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ PASS (b) Weld Specimen Fabricated by FCAW Process Current (A) Voltage (V) Speed (cm/min) Heat Input (KJ/cm) Remarks Welding polarity: AC Preheat Not applied Inter-pass Up to 150 o C Remarks Welding polarity: DC(+) Preheat Not applied Inter-pass Up to 150 o C Fig. 3. Notch Locations of the Impact Test Specimens standard specifications. The tests were performed in the temperature range from -60 o C to -30 o C at intervals of 10 o C including Vol. 19, No. 5 / July

4 Hyun-Seop Shin, Ki-Tae Park, Chin-Hyung Lee, Kyong-Ho Chang and Vuong Nguyen Van Do room temperature (20oC) with the impact velocities between 5 and 5.5 m/s. Three specimens were tested at each temperature as recommended by the specifications, and each specimen was held for approximately 10 min at the low testing temperature before testing began to make sure the temperatures evenly distributed throughout the specimen (Lee et al., 2014). 3. Results The optical micrographs taken at the weld metals and the HAZs of the welded joints with different welding processes are displayed in Figs. 4 and 5, respectively. Note that the location of the microstructure in the HAZ or in the weld metal coincides with that of the notch in the corresponding impact test specimen. The micrograph taken at the weld metal zone of the SMAW joint exhibits acicular ferrite and tangled ferrite along the grain boundaries, whereas the weld metal of the FCAW joint consists of dendrite having acicular ferrite in patches. The HAZ microstructures of the SMAW and FCAW joints show acicular ferrite morphology. Nevertheless, fine acicular ferrite features in the HAZ of the SMAW joint while coarse acicular ferrite features in Fig. 5. Microstructures at the HAZs: (a) SMAW Joint and (b) FCAW Joint the FCAW counterpart are revealed. Moreover, the content of acicular ferrite is much higher in the HAZ of the SMAW joint. The hardness across the weld cross section which covers the weld region, the HAZ and the base metal are presented in Table 4. Referring to the measurements, the weld metal and the HAZ of the SMAW and FCAW joints have superior hardness than the base material, indicating that there is no softening zone in the joints. Moreover, it can be seen that the hardness values in the weld metal and the HAZ of the SMAW joint are higher than the corresponding FCAW counterparts. The Charpy impact test allows the material properties for service temperatures to be determined experimentally in a simple manner. Fig. 6 shows the Charpy V-notch impact toughness requirements Table 4. Hardness Data for the Base Metal, the HAZ and the Weld Metal (HV10) Joint Fig. 4. Microstructures at the Weld Metals: (a) SMAW Joint and (b) FCAW Joint SMAW FCAW 1434 Location Base metal HAZ Weld Metal KSCE Journal of Civil Engineering

5 Low Temperature Impact Toughness of Structural Steel Welds with Different Welding Processes Fig. 6. Impact Testing Requirements (ASME, 2004) in joules for the standard impact specimens made of carbon and low alloy steels with respect to the maximum nominal thickness depending on the minimum specified yield strength (ASME, 2004), which are not given in KS specifications. The graph is crucial in terms of the fact that it provides the minimum criteria for the use of steel in low atmospheric temperatures. This means that if the impact toughness of the steel exceeds the required value specified in the graph at specific temperature, it can be safely used in the temperature. Variations of the absorbed energies with respect to the five test temperatures for the weld metals and the HAZs of the SMAW joint and the FCAW joint are tabulated in Tables 5 and 6, respectively. The impact toughness requirements Joint SMAW FCAW Table 6. Charpy Impact Test Results for the HAZs Test temperature 30 o C 40 o C 50 o C 60 o C 30 o C 40 o C 50 o C 60 o C Charpy fracture energy (J) 400 Maximum: 366 Minimum: 328 Average: 342 Maximum: 387 Minimum: 304 Average: 337 Maximum: 326 Minimum: 300 Average: 316 Maximum: 320 Minimum: 1 Average: Maximum: 320 Minimum: 255 Average: 9 Maximum: 250 Minimum: 205 Average: 225 Maximum: 215 Minimum: 185 Average: 203 Maximum: 175 Minimum: 115 Average: 142 Impact toughness requirement (J) Joint SMAW FCAW Table 5. Charpy Impact Test Results for the Weld Metals. Test temperature -3 o C -40 o C -50 o C -60 o C -30 o C -40 o C -50 o C -60 o C Charpy fracture energy (J) 147 Maximum: 124 Minimum: 105 Average: 112 Maximum: 124 Minimum: 102 Average: 110 Maximum: 62 Minimum: 52 Average: 55 Maximum: 40 Minimum: 31 Average: Maximum: 175 Minimum: 103 Average: 134 Maximum: 153 Minimum: 112 Average: 137 Maximum: 66 Minimum: 60 Average: 64 Maximum: 48 Minimum: 34 Average: 40 Impact toughness requirement (J) are also given in the tables. 4. Discussion From the results given in Tables 5 and 6, it can be found that the weld metals and the HAZs of the joints fabricated using the SMAW and FCAW processes satisfy the requirements up to the test temperature of -60 o C. It indicates that both the welded joints can be safely adopted up to the temperature. As shown in the micrographs, the weld metals of the SMAW and FCAW joints exhibit acicular ferrite with different second phase ferrite, respectively. It is worth noting that the weld metal microstructure in the SMAW joint show more acicular ferrite. Acicular ferrite is the phase most commonly observed as austenite transforms during the cooling of low alloy steel weld deposits. It is the most preferable microstructure as it enhances the strength and toughness of the weld metal. The interlocking nature of acicular ferrite, together with its fine grain size, provides the maximum resistance to crack propagation by cleavage (Yang et al., 1993). Hence, the weld metal with higher acicular ferrite microstructure has higher hardness and toughness. Generally, manganese and molybdenum are added to the alloy to promote the formation of the acicular microstructure upon transformation from austenite (Junhua et al., 2004). The higher content of manganese in the weld metal chemistry of the SMAW joint contributes the larger formation of acicular ferrite and thus results in the higher hardness in the weld Vol. 19, No. 5 / July

6 Hyun-Seop Shin, Ki-Tae Park, Chin-Hyung Lee, Kyong-Ho Chang and Vuong Nguyen Van Do metal. Nevertheless, it is observed that the weld metal of the FCAW joint has a little higher impact toughness at low temperatures than that of the SMAW joint, which is attributed to the higher nickel content of the Supercored 81-K2 electrode (see Table 2). It has been reported that the weld metal toughness can be increased by an increase of nickel content (Magudeeswaran et al., 2008; Parker and Stratford, 1999) which is indispensable composition to the weld filler targeted to steels for low temperature use. It is a common practice to correlate the HAZ properties with the heat input during welding. The difference in the ferrite morphology in the HAZs of the SMAW and FCAW joints is due to the difference between the heat input during the fabrication of the welded joints. The formation of acicular ferrite is controlled by welding heat input, i.e. the cooling rate is a governing parameter that determines the formation of acicular ferrite-based microstructure. If the heat input is higher, i.e. the cooling rate is lower, the content of acicular ferrite will be less, and vice versa. Moreover, the higher heat input leads to the coarse microstructure. In the present investigation, the average heat input of 13.5 kj/cm was recorded during the fabrication of the SMAW joint, whereas the average heat input during the FCAW process was 17.3 kj/cm. Thus, the higher heat input during the fabrication of the FCAW joint promotes the coarser acicular ferrite features and lesser acicular ferrite content in the HAZ compared to the SMAW counterpart. Hence, the HAZ of the FCAW joint has much inferior low temperature impact toughness than that of the SMAW joint in terms of higher upper-shelf energy and lower ductile-to-brittle transition temperature. In addition, the lower heat input of the SMAW joint favored the higher hardness in the HAZ and weld metal compared to that of the corresponding FCAW counterparts. The above results suggest that the more effective welding process to ensure the higher fracture toughness of the HAZ and the weld metal at low temperatures is the SMAW process. 5. Conclusions In this study, an experimental program which included metallographic observation, hardness measurement and the Charpy impact test for steel welds fabricated by different welding processes and welding consumables was carried out to find out an effective welding method to secure higher impact toughness of structural steel welded joints at low temperatures. The impact specimens were extracted from the HAZs and the weld metals. Standard V- notch Charpy specimens were prepared and tested under dynamic loading condition. Variation of the Charpy impact energy with respect to the test temperature and that of the hardness across the welds were presented and correlated with the microstructure and the welding process; thus effects of the welding process and the welding consumable on the low temperature impact toughness of the HAZ and the weld metal were analyzed in detail. Based on the experiments and discussion of the results, the following conclusions can be made. 1. Weld metal of the FCAW joint has a little higher low temperature impact toughness than that of the SMAW joint owing to the higher nickel content of the welding electrode used in the FCAW process, despite the larger formation of acicular ferrite in the weld metal of the SMAW joint. 2. HAZ of the SMAW joint has much superior impact toughness at low temperatures than that of the FCAW joint due to the higher acicular ferrite content attributed to the lower heat input during the welding process. 3. An efficient welding method to ensure higher low temperature impact toughness of the HAZ and the weld metal is to employ a low heat input welding process using a welding consumable with high nickel content. Acknowledgements This research was supported by a grant from a Strategic Research Project (Development of High Performance Material & Rapid Construction Technology for Extreme Environment) funded by the Korea Institute of Construction Technology. References ASME (2004). Impact testing requirments, Boiler & Pressure Vessel Code Sec. VIII Div. I. Bayraktar, E., Hugele, D., Jansen, J. P., and Kaplan, D. (2004). Evaluation of pipeline laser girth weld properties by Charpy (V) toughness and impact tensile tests. Journal of Materials Processing Technology, Vol. 147, No. 2, pp , DOI: /j.jmatprotec Ibrahim, O. H., Ibrahim, O. S., and Khalifa, T. A. F. (2010). Impact behavior of different stainless steel weldments at low temperatures. 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