Fracture Resistance Improvement of Micro Alloy Steel Weld in Four Wire Submerged Arc Welding by applying One and Two Thermal Cycles

The high strength micro alloyed steels befor welding are prepare with hot or cold rolling. After welding, the weld metal region heat-treated to provide good mechanical properties. The heat treatment is annealing and reheating to different peak temperatures then cooling down very slowly in the furnace to achieve the best fracture strength. In this research, the Charpy impact were used to evaluate the fracture resistance of the weld of metals. The optical microscopy and scanning electron microscopy were used for study of the microstructure before and after apply of thermal cycles. According to results the optimum resistance to fracture was shown in the cycle of annealing to peak temperature of 950 °C and reheating in the second cycle to temperature peak of 650 °C. The thermal cycle of the final microstructure consists of pearlite/ bainite and polygonal ferrite along with a large number of metal oxide impurities. Evaluation of fracture surfaces indicate that the microstructure of pearlite/bainite and polygonal ferrite as compared with only ferritic/pearlitic has shown more ductile fracture. The samples with the highest fracture resistance has shown low hardness in comparison with other samples that were heat treated.