Effects of Root Opening on Welding Speed, Hardness, Root Penetration and Tensile Strength using E7018 Welding Electrode by Weaving Technique on E250 Plates

Abstract

The weldability of E 250 mild steel plates which are extensively used in penstock pipes of hydropower depends upon different parameters like welding current, voltage, travel speed, electrode angle and other parameters. Mostly, E7018 electrode is used for welding by Shielded Metal Arc Welding (SMAW) process. Root opening is the gap between the two metals in a butt joint that are to be welded. Welding performed in workshops are done in controlled and qualitative environment but during installation in field due to geological conditions like high slopes, difficulty in handling pipes in vertical tunnels and inclined tunnels the welding may not be performed as done in workshop but the design requirement is same. Four sets of samples are prepared by fitting two samples each of 300 mm length x 300 mm width x 10 mm thickness with root opening of 2 mm, 4 mm, 6 mm and 8 mm respectively with bevel angle of 60 degrees, single groove and root face of 2 mm. Welding has been performed by certified welder, accordance to ASME Section IX, with the use of E7018 electrode with 2.5 mm diameter by SMAW process and weaving technique. The welding speed decreased while welding from 2 mm to 8 mm. The welding speed decreased from 81.31 mm/min to 76.23 mm/min, 66.58 mm/min and 55.54 mm/min at root opening of 2 mm, 4 mm, 6 mm and 8 mm respectively. Hardness of the material indicates the changes in property of the material after being welded. The hardness in weld metal increased from 125.6 HV to 127.3 HV, 136.8 HV and 139.2 HV and hardness in heat affected zone increased from 104.3 HV to 106.8 HV, 107.7 HV and 108.6 HV at root opening of 2 mm, 4 mm, 6 mm and 8 mm respectively. Complete root penetration of 10 mm was found while welding in root opening of 4 mm whereas minimum root penetration of 8.8 mm, 8.1 mm and 6.9 mm was found while welding with root opening of 2 mm, 6 mm and 8 mm respectively. The root penetration decreased with an increase of root opening. With the increase in root opening, heat input has to be increased. As, the experiment is carried in constant current and voltage, the heat input required for 6 mm and 8 mm root opening is less causing less root penetration in welds. There is no effect in minimum required tensile strength of welded zone when welding is performed by E7018 welding electrode in E250 grade materials by weaving technique varying the root opening from 2 mm to 8 mm. The rupture is found to be in parent metal in all the samples