In this study, defect formation mechanisms in the ERW (electric resistance welding) process for API pipe production were investigated. The results showed that defects observed in the weld joint of ERW pipe are a main factor in the deterioration of the mechanical properties of welded joints. From systematical research, it was clear that the crucial main defects of ERW pipe are caused by large inclusions with complex compositions after the steel making process and penetrators formed during ERW welding with excessive heat input condition. In order to guarantee the toughness of the weld joints, after theoretical and experimental considerations ERW pipe weld defect reducing methods can be recommended. According to previous test results, it was clear that the hook cracks caused by large inclusions were reduced by selecting the slag composition ratio of CaO and Al2O3 at the eutectoid point during steel making process. On the other hand, in case of the penetrator formation type defects, it was defined that by controlling of the heat input in the optimization range was the best solution for decreasing the penetrator formation.

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