Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel

Transcription

1 I.O. Oladele et al.: Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel 11 Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel Isiaka O. Oladele a,ψ, Okikiola G. Agbabiaka b, and Nimota A. Yakubu c Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure. Ondo State. Nigeria; a wolesuccess2000@yahoo.com; b okisagbabs@yahoo.com c yakubu.nimota@gmail.com Ψ Corresponding Author (Received 29 December 2015; Revised 12 March 2016; Accepted 27June 2016) Abstract: The effect of welding processes using Gas Tungsten Arc welding (GTAW) and Shielded Metal Arc Welding (SMAW) on the mechanical properties of SA516 steel of varying thicknesses was investigated in this research. GTAW and SMAW were carried out on the selected steel of 3, 6, and 10 mm thicknesses. Mechanical tests such as tensile, flexural and hardness were carried out on the materials base metals, heat affected zones and the weld joints. Microstructural examination was also observed for the materials of each welding process. The research outcomes depict that GTAW aid best properties in terms of tensile and hardness while SMAW is suitable for good flexural property. Also found out was that 3 mm thickness had the best performance in GTAW while 10 mm thickness had the best performance in SMAW. This may be due to the mode of heat input from the welding processes. Keywords: Welding processes GTAW, SMAW, SA516 steel, Mechanical Properties ISSN The Journal of the Association of Professional Engineers of Trinidad and Tobago Vol.44, No.2, October/November 2016, pp Introduction In the engineering industry, welding is widely used in the fabrication of pressure vessels, ships, maintenance, and repair of parts and structures (Butt et al, 2012). Among the various methods for joining metals, welding is one of the most convenient and rapid methods available ranging from simple steel brackets to nuclear reactors (Blondeau, 2008). Welding is dependable, efficient and economical for permanently joining similar metals. It is widely used in all sectors or manufacturing, from earth moving equipment to the aerospace industry (ANSI, 2005; Arthur, 1985). In recent years, there are quite a number of welding processes that differ greatly in the manner in which heat and pressure are applied, and the type of equipment employed. The most common welding processes are shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). All of these methods utilise an electric power supply to generate the arc which melts the base metal(s) to form a molten pool (Guy and John, 1974; Hari and Pandey, 2013). The filler wire is either added automatically (GMAW) or manually (SMAW & GTAW) and the molten pool is allowed to cool. All of these methods use some type of fluxes or gases to create an inert environment in which the molten pool can solidify without oxidizing (Jodh and Surlnder, 2014). SA516 carbon steel is a standard material specification for pressure vessel application usually at medium and lowered temperature services (Brownmac, 2016). In spite of its widely usage, SA516 carbon steel has some crucial chemical elements (see Table 1) with properties ideally suitable for use in the oil, gas and petrochemical industry (Masteel, 2016). Whilst in use in service, there are problems associated with the SA516 welds such as cracks and high hardness of HAZs due to excessive heating of the material during welding (Harold, 1986). However, to achieve a quality product, certain welding parameters must be met such as welding current, thicknesses, absence of inclusion, adequate heating and cooling rate (Harold, 1986). Thus, in order to understand the failure mode of SA516 steels, there is a need to investigate the response of different thicknesses of this material when joined with the commonly used welding processes. In this research, GTAW and SMAW were used to investigate the effect of thickness on the mechanical properties of selected welded samples. The outcome of the research gave detailed insight into the influence of optimal heat input on the mechanical properties of the selected steel and the welding procedures specifications (GTAW and SMAW) with the best performance for further applications. 2. Materials and Methods SA516 carbon steel was selected for the welding operation was in accordance to the ASME specification for carbon steel pressure vessel plates for moderate and lowtemperature services (ANSI, 2005; ASTM, 2014). The specification covers carbon steel plates intended primarily

2 I.O. Oladele et al.: Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel 12 for services in welded pressure vessels where improved notch toughness is important. Plates under this specification are available in four grades (Grade U.S SI) having different strength level. However, this research utilized grade 70 with strength range MPa with chemical composition as shown in the Table Electrode composition The chemical composition of the mild steel electrode for SMAW process used for the weld and filler metal for GTAW is shown in Table 2. Table 1. Metal (SA516) chemical composition Analysis % C Si Mn P S Cr Mo Ni Al Cu Nb N Sn Ti V Cr MoNi Cu Min Specification Max Cast/Heat Table 2. SMAW electrode, chemical composition and GTAW filler metal composition (Horbat, 2015). AWS Suffix C Mn Si Mo P S Cu others E7018 A ER70S Ti, Zr, Al 2.2 Welding Experimental Procedures The selected steel samples were cut into 15 x 20 mm using a razor cutting machine with a carbon cutting disc followed by prepping (< 3 mm groove) of one side of each plates using an oxy acetylene gas flame for the purpose of the weld pull deposit. The prepped surfaces were smoothened with grinding machine using a carbon grinding disc before welding the samples. The pictorial view of the welded sample is as shown in Figure 1. these operations, three (3) samples each of 3, 6, and 10 mm of SMAW was obtained. Table 3. Order of the welding current followed for SMAW with the use of E70181 electrode, following ESAB (E70181 Manufacturer Specification) Plate Thickness Electrode Amperage Pass (mm) Diameter (mm) Range (I) Rooting Filling Capping Rooting Filling Capping Rooting Filling Capping Plate 1. Welded Samples Operation of the SMAW specimens The prepped side of the test piece, 3 mm thick plate of two 15 x 20 mm was set beside each other for welding. The SMAW welding machine was connected to the power source, an arc was struck to start the deposition of the filler rod on the groove prepared for weld pool. The sample was then allowed to cool and solidify in air. A similar procedure was followed for the 6 mm and 10 mm thick plates as shown in Table 3, at the completion of However, it is noteworthy that the use of 2.4 mm (gauge 12) for the rooting of the specimens facilitates good penetration and avoids the specimen from moving apart while 3.2 and 4.0 mm eliminate the hydrogen embedded during rooting to avoid hydrogen cracking defect Operation of the GTAW specimens The prepped side of the test piece of 3 mm thick plate of two 15 x 20 mm was set beside each other for welding. The GTAW welding machine was prepared to the power source (connections of the negative terminal hearth to the metal work bench, the positive terminal to the welding nozzle and the gas hose connected to the positive terminal). An arc was strike to start the deposition of the filler rod on the groove prepared for weld pool). The

3 I.O. Oladele et al.: Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel 13 shielding gas covers the filler metal deposited from the atmospheric defect. The sample was allowed to solidify in the air and the same procedure was repeated for 6 mm and 10 mm thick plate as shown in Table 4. At the end of these operations, three (3) samples each for 3, 6, and 10 mm of GTAW were produced (see Figure 1). Table 4. Order of the welding current followed for GTAW with the use of ER70S2 electrode, following ESAB ER70S2 Manufacturer Specification Plate Electrode Amperage Pass Thickness (mm) Diameter (mm) Range (I) Rooting Filling Capping Rooting Filling Capping Rooting Filling Capping 2.3 Mechanical Testing (a) Determination of Tensile Properties of the samples Tensile test samples were prepared in accordance with ASTM A370 (ASTM, 2014) using a universal testing machine Su Zhou Long Sheng, China. Three (3) samples were tested as shown in Figures 2 (a b) with a load of 300 KN and crosshead speed of 5 mm/min correlating with an initial strain rate of 0.98 s 1, respectively. The specimen was mounted by its ends into the holding grips of the test apparatus. The tensile testing machine was designed to elongate the specimen at a constant rate, and to continuously and simultaneously measure the instantaneous applied load with a load cell and the resulting elongations using an extensometer. The applied load permanently deformed and fractured each sample into two parts as shown in Figures 2(cd). Figure 2. (a) Machined GTAW tensile specimens (b) Machined SMAW tensile specimens (c) Fractured GTAW tensile specimens (d) Fractured SMAW tensile specimens (b) Determination of Rockwell hardness properties of the samples This method consists of indenting the test material with a hardened steel ball indenter using 6392 Indentec hardness testing machine. Rockwell hardness test produces a much smaller indentation more suited for hardness traverses. Test samples were cut into 9 x 9 mm for weld joint (WJ), heat affected zone (HAZ), and parent metal (PM) as shown in Table 4. The indenter was forced into the test material under a preliminary minor load of 60 kgf which gradually increased to 100 kgf and 150 kgf. The test was repeated thrice on WJ, HAZ and PM and the readings were recorded as displayed by the hardness testing machine as shown in Table 5. Table 5. Rockwell Hardness Test specimen identification Sample Weld joint 9 x 9 mm Heat Affected Zone 9 x 9 mm Parent Metal 9 x 9 mm 3 mm GTAW mm GTAW mm GTAW mm SMAW mm SMAW mm SMAW 1 1 Base Metal 1 (c) Determination of Flexural Properties of the samples The flexural test was carried out by using Testometric universal testing machine in accordance with ASTM E19092 (Gene, 2014) standard test method for flexural properties of welded samples. The flexural test was performed at the speed of 100 mm/min. Three samples were tested for each representative samples from where the average values for the test samples were used as the illustrative values. (d) Metallographic Examination Test samples were cut into 9 x 9 mm each for all the welding zones tested and were subsequently ground, polished and etched for about 30 seconds with 2% natal solution. Optical micrographs were observed on a Nikon Optiphot metallurgical microscope using Nikon D70 digital camera. Optical microscopy was used to view the microstructures of the 3 welding zones; weld joint (WJ), heat affected zone (HAZ) and the parent metal (PM), of the weldments for all the samples tested as shown in Table 6. Table 6. Metallographic specimen identification Sample Weld joint Heat Affected Zone Parent Metal 3 mm GTAW mm GTAW mm GTAW mm SMAW mm SMAW mm SMAW 1 1 BASE METAL 1

4 I.O. Oladele et al.: Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel Results and Discussion 3.1 Tensile Test In Figure 3, it was observed that both GTAW and SMAW followed similar trend on the welded samples. It was noticed that as the material thickness increases, the tensile strength at yield of the welded materials decreases and the lower the materials thickness the higher the yield strength values. This effect is due to the number of passes of the welded samples, as 3 mm thick plate required minimal numbers of filling before capping which enhanced the ductility of the welded joint such that the lower the thickness of the metal the longer the plastic behavior the material exhibits before it fractures. It can also be depicted from the graph that 3 mm GTAW and 3 mm SMAW gave the best tensile strength at yield with a value of MPa and MPa. performance compared to SMAW samples. The tensile modulus of 3mm GTAW sample is MPa followed by 3 mm SMAW welded sample with a value of MPa. Next to these was welded sample from 6 mm GTAW which has a value of MPa. In all, the tensile modulus of all the samples indicated good capping of the samples. Figure 4. Tensile Strength at Peak of GTAW, SMAW and BASE METAL Figure 3. Tensile Strength at Yield of GTAW, SMAW and BASE METAL Figure 4 depicts that for GTAW samples, the lower the thickness of the samples the higher the tensile strength at peak while SMAW samples deviate from that order as 6 mm SMAW sample had a value of MPa followed by 3 mm SMAW sample with a value of MPa. This may be attributed to insufficient filling of 3 mm SMAW before capping. In general, 3 mm GTAW sample gave the best tensile strength at peak which is about MPa followed by 6 mm SMAW sample with a value of MPa and 3 mm SMAW MPa. The base metal had a value of 485 MPa lower than 3mm GTAW, 6 and 3 mm SMAW welded samples tensile strength at peak which gave the best result due to the filler metal, welding electrode, shielding gas and the applied heat during the welding processes. Figure 5 shows the tensile modulus of the test samples. From the results, it was observed that as the thickness of the material increases the tensile modulus of the welded materials decreases. The results showed that the thickness of the material had an inverse effect on its tensile modulus. It was observed that 3 mm sample possesses the highest values from the two welding procedure used. However, GTAW samples gave the best Figure 5. Tensile Modulus of GTAW, SMAW and BASE METAL 3.2 Flexural Test Figure 6 shows the flexural strength of the samples. From the results, it was observed that as the thickness of the material increases the flexural strength increases. It was also noticed that out of the two welding procedures adopted, GTAW gave the best performance to SMAW. Considering the result, 10 mm SMAW sample had a value of MPa, followed by 10 mm GTAW sample and base metal with a value of MPa and MPa, gave the best flexural strength. However, GTAW welding process followed the order of the higher the thickness, the higher the flexural strength while SMAW gave similar interpretation but no difference in the value of 3 and 6 mm SMAW.

5 I.O. Oladele et al.: Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel 15 dependent on the materials thickness during GTAW welding processes. Figure 6. Flexural Strength at peak of GTAW, SMAW and Base metal samples 3.3 Hardness Test The Rockwell hardness test results of the various samples are shown in Tables 7 and 8. Table 7. Rockwell Hardness result of GTAW and SMAW Sample HRC (150 kgf) HRC trend 3 mm WJ GTAW mm HAZ GTAW mm WJ GTAW mm HAZ GTAW mm WJ GTAW mm HAZ GTAW mm WJ SMAW mm HAZ SMAW mm WJ SMAW mm HAZ SMAW mm WJ SMAW mm HAZ SMAW Base Metal Table 8. Welded Joint, Heat Affected Zone and Base Metal Sample Hardness trend Welded Joint Sample Trend hardness HAZs Samples Trend hardness 3 mm WJ GTAW 3 3 mm HAZ GTAW 2 6 mm WJ GTAW 2 6 mm HAZ GTAW 1 10 mm WJ GTAW 1 10 mm HAZ GTAW 3 3 mm WJ SMAW 5 3 mm HAZ SMAW 4 6 mm WJ SMAW 4 6 mm HAZ SMAW 6 10 mm WJ SMAW 6 10 mm HAZ SMAW 5 Base Metal 7 Base Metal 7 Figure 7. Rockwell Hardness result of GTAW, SMAW and Base metal 3.4 Microstructural Analysis As shown in Figures 8 and 9, welding zones for both SMAW and GTAW welding processes are denoted as: (i) welds joint (ii) heat affected zone and, (iii) base metal. From the micrographs, ferrite and pearlite phases were revealed as light and dark regions, respectively. The microstructural analysis of the test samples shows that the welded joints have even distribution of pearlite within the ferrite as seen in plates (ai) and (bi) for both welding processes. This is the reason for the observed good hardness property in these zones. However, at the heat affected zones there are clusters of pearlite in some regions within the ferrite as seen in plates (aii) and (bii) which is responsible for the low hardness than other zones. At the base metal zone (plate iii), uniform distribution of pearlite within the ferrite was noticed with a good mechanical property as seen in the results shown in Figures 3, 4 and 5. Figure 7 shows that all the samples welded joints have a higher hardness value than their respective heat affected zones followed by the base metal due to heat input gradient during welding at three different points of welded samples (WJ, HAZ and base metal). GTAW welding process gave the best hardness results in the following order: 10 mm GTAW (44.4 kgf), 6 mm GTAW (44.2 kgf) and 3 mm GTAW (39.4 kgf). This is attributed to the shielding argon gas used during the welding process that aid higher heat input than SMAW which also signifies that the welded joint hardness is directly Figure 8. The micrographs of various zone; weld joint, heat affected and base metal for 3 mm samples: (aiii) GTAW x200 and (biii) SMAW x200

6 I.O. Oladele et al.: Influence of Welding Process and Thickness on the Mechanical Properties of SA516 Steel 16 Figure 9. The micrograph of various zone; weld joint, heat affected and base metal for 3 mm samples: (iii) Base metal 4. Conclusion The welding procedures adopted (GTAW and SMAW) with varying SA516 steel samples of 3, 6, and 10 mm thicknesses have been investigated with respect to their tensile, flexural and hardness properties. From the research outcomes, the following conclusions can be drawn. The two welding processes revealed that GTAW gave the best performance with respect to tensile and hardness properties while SMAW gave the best response with respect to flexural or bending strength property. These responses were due to the mode of heat input from the selected processes and the thickness of the metals. It was noticed that 3 mm thickness had the best performance in GTAW while 10 mm thickness was with the best performance in SMAW. However, it was observed that hardness property from both GTAW and SMAW welded joints were higher than that of their respective heat affected zones and base metals due to the presence of well dispersed hard phase of pearlite in the soft ferrite phase at the weld joints as evident in the micrographs. References: ANSI (2005), Z49.1: Safety in Welding, Cutting and Allied Processes, The American National Standards Institute/American Welding Society, p. 53. Arthur, J.C. (1985), Introduction to Robotics, Macmillan Publishing, NY, p ASTM (2014), Standards E8 and E8M, Standard Test Methods for Tension Testing of Metallic Materials, ASTM International, Pennsylvania, USA Blondeau, R. (2008), Metallurgy and Mechanics of Welding Processes and Industrial Applications, 1st Edition, John Wiley and Sons, USA Brownmac (2016), ASME SA516 Grade 70 and ASTM A516 Grade 70, Accessed 27th April 2016 from Butt, M.T.Z., Ahmad, M.S. and Azhar, M. (2012), Characterisation for GTAW AISI 316 to AISI 316 and SA516 Grade 70 steels with welded and prewelded annealing conditions, Journal of Quality and Technology Management, Vol.8, No.2, pp Gene Mathers (April, 2014), Guided Bend Test for Ductility of Weld. ASTM E19092, ASTM International, Pennsylvania, USA Guy, G.A. and John, J.H. (1974), Element of Physical Metallurgy, 3rd edition, AddisonWesley Publishing, p.393. Hari, O.M.. and Pandey, S. (2013), Effect of heat input on dilution and heat affected zone in submerged arc welding process, Indian Academy of Sciences Sa dhana, Vol.38, Part 6, p Horbat (2015), Horbat Filler Metal, American Welding Society, July, Accessed from Jodh, S. and Surlnder S, B. (2014), Effect of welding speed on depth of penetration during arc welding of mild steel plates, International Journal of Research in Mechanical Engineering and Technology, Vol.4, No.2, p Masteel (2016), ASME SA516 Grade 70, Accessed 27th April 2016 from Schmeilski, Harold L. (1986), Factors Affecting Inservice Cracking of Weld zone in Corrosive Service, The National Board of Boiler and Pressure Vessel Inspectors. Authors Biographical Notes: Isiaka Oluwole Oladele holds a Master s Degree and Ph.D. in the area of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Ondo State, Nigeria. Dr. Oladele has supervised many undergraduates and postgraduatesresearch projects and has published in local and international journals and conference proceedings. Agbabiaka Okikiola Ganiu graduated as the overall best student with a First Class Degree Honors in Metallurgical and Materials Engineering and is currently undergoing his Master s Degree programme in the Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Ondo State, Nigeria. He is also serving as a Graduate/Research Assistant in the aforementioned Department and has collaborated with other senior researchers to carry out research in the field of composites development. Yakubu Adenike Nimota was a graduate of the Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Ondo State, Nigeria. Her research interest is in the area of welding and corrosion of metals and alloys.