OPTIMIZATION OF WELDING PARAMETERS FOR STRUCTURAL INTEGRITY OF AISI 1018 LOW CARBON STEEL WELDMENTS

Abstract

This research was undertaken to study the effect of bevel angle and welding heat input on the mechanical properties and microstructures of American Iron and Steel Institute (AISI) 1018 low carbon steel weldments for structural integrity enhancement through electric arc welding technique. 10 mm and 20 mm flat plates; and 10 mm pipe were prepared with a butt joint geometry at varying bevel angles (30°, 45°, 60° and square butt joint), welded at constant voltage and varying current. Optimization of welding parameters was performed using One-Factor-At-A-Time (OFAAT) for the design of experiment and results analysis. The current was varied between 70 – 120 A for 10 mm plate, and 170 - 185 A for 20 mm plate and 10 mm pipe. The hardness of the weldment, Heat Affected Zone (HAZ) and base metal were done using 490 MN major load with dwell time of 10 s while tensile cap was 50 kN. A V-shaped notch, 2 mm deep, with 45° angle and 0.25 mm radius along the base and maximum value 300 J were used for the impact test while 50 N load was applied with sliding distance 3 mm respectively and sliding speed of 3 mm/s for the wear test. The microstructures of the weldments, HAZ and the base metal were characterized using Optical Microstructures (OM) - (100X) and Scanning Electron Microscopy (SEM) analysis. Fractography of the fracture samples were also carried out. The findings from the research revealed that; at constant voltage, varying current and bevel angle the highest heat input was found to be 13.61 kJ/mm at 60° bevel angle for 10 mm plate; 62.50 kJ/mm at 60° bevel angle for 20 mm plate and 106.00 kJ/mm at 45° bevel angle for 10 mm pipe. Results on mechanical properties gave maximum hardness of 199.5 Hv at the HAZ of 45° bevel angle for untreated sample of AISI low carbon steel for 10 mm plate and maximum tensile strength of 410.92 MPa at yield for as – welded sample while sample with 60° bevel angle has the maximum tensile strength of 308.90 MPa at yield for the heat treated (annealed) sample.\The maximum ultimate tensile strength (UTS) was attained for 45° bevel angle untreated sample while 60° bevel angled sample has maximum ultimate tensile strength for the heat-treated samples. It was observed that sample with 60° bevel angle has the maximum tensile strength of 1210.97 MPa at yield and 771.01 MPa at break for the as – welded sample of 20 mm plate. The 45° bevel angled sample has the maximum tensile strength of 1134.50 MPa at yield and 731.32 MPa at break for the as – welded sample of 10 mm pipe. The characterization of the weldments showed from the OM and SEM evaluation showed that there were presence of ferrite and pearlite in the microstructures with good mechanical and microstructural properties for structural integrity enhancement. The characterization of the weldments showed from the OM and SEM evaluation that there were presence of ferrite and pearlite in the microstructures. There was an increase in the hardness value in the HAZ area. The formation of fine martensite phase is due to the fast cooling that caused increased hardness for good mechanical and microstructural properties with enhanced structural integrity. Wear results results concluded that higher bevel angle had lower coefficient of friction, better adhesion and good wear resistance in comparison to smaller bevel angle. There were occurrence of delamination and ploughing in the wear profile. The fractography analysis showed a mix-mode of ductile-brittle fracture with evidence of dimples, microvoids and cleavages in the weldment of AISI 1018 low carbon steel. The impact toughness increased with low hardness. Investigation revealed that the fractures were caused by blow holes, slag inclusion and inadequate welding pool in some areas thereby resulting into cracks and finally fractures. Finally, the optimization of the welding parameter-heat input depends on the welding current, arc voltage, travel speed, welding length and actual time of welding which in turn affects the cooling rate using OFAAT method of design of experiment. However, the cooling rate is also directly affected by the bevel angle, the thickness of the weldment and post-weld heat treatment in which 45° bevel angle was found to give the optimum mechanical properties and microstructures for structural integrity enhancement.