Determination Optimum Boron Quantity Effect on Low Temperature Impact Energy of Mild Steel Weld Metal that Fabricated by Submerge Arc Welding (SAW)

This study was carried out to determine the optimum of boron quantity affect on low temperature impact energy of the mild steel weld metals fabricated by submerge arc welding (SAW). For this reason some plates of ST 37 in 10× 150 × 200 mm dimensions prepared and welded by SAW. Boron as the oxide powder، added and mixed with flux powder. After welding the OM، SEM، impact and micro hardness tests were carried out for different samples of the weld metals. The results of the chemical analysis of different samples confirmed presence of 20، 30، 60، 70 and 110 ppm boron in weld metals. The OM result shows that in sample containing 20ppm boron، acicular ferrite was increased and polygonal ferrite، grainboundary and widmenstatten ferrite was decreased. Relative to original samples by increasing of the boron from 20 to 110ppm often polygonal، grainboundary and widmenstatten ferrite were formed. The microhardness test result showed that sample contained 20 ppm boron has the highest hardness compared to other samples. The impact test results showed that the high impact energy for sample contains 20 ppm boron at the three ambient، zero and -20 °C. The fractography result showed that، brittle fracture share for sample contains 20 ppm boron with the highest impact energy was 10 percent and for sample contains 110 ppm boron with the lower impact energy about 50 percent.