Cause Analysis on Surface Cracks of Thread of 27CrMoTi Steel Casing Pipes

Crack defects were found on surface of thread of 27CrMoTi steel casing pipes produced by one plant after turning process. Causes for cracks were analyzed by methods of chemical composition analysis, macroscopic observation, analysis of scanning electron microscope and energy spectrum, metallographic inspection and so on. The results show that:there were a great amount of inclusions in the interior of surface cracks of thread of 27CrMoTi steel casing pipes, and the inclusions were caused by slag entrapment of mould powder for continuous casting crystallizer; the cracks initiated and propagated to be surface cracks as a result of stress concentration during hot-rolling.