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In this paper, effects of preheating and type of electrode (E8010 and E8018) on microstructure and mechanical properties of 5L X70 steel welded joints were investigated. The microstructure of joint zone and fracture surface was analyzed by light optical microscope and scanning electron microscope equipped with energy dispersive spectroscopy. Hardness mapping and tensile test were also performed to find the relationship between microstructure and mechanical properties. The results showed a consistency between the hardness variation data and the microstructure of joint zone. Moreover, hardness mapping revealed coarse and fine grain subregions in the heat affected zone which were not detected in the micrographs. The tensile test indicated that the non-preheated sample, welded by cellulosic E8010, had the minimum value in the strength and the ductility of welded steel. The fractography also determined that size and distribution of strengthening phases affected the fracture mode of welded specimens.