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As an effective and significant method for evaluating crack arrestability,drop weight tear test(DWTT)results become even more important for those high toughness steels with large thickness since it is more difficult to assure enough ductile fracture in a severe environment or with a larger thickness.Separation is a commonly seen phenomenon during drop weight tear testing in high toughness linepipe steels.A small numbers of separations are believed not to affect the running ductile fracture resistance,while severe separation may be harmful for fracture or burst of pipes.However,the occurrence of separation may be related to multiple factors.It is helpful to find methods to eliminate separations with a clear understanding about separation for development of high grade high toughness linepipe steels.In this paper,a quantitative relationship was built up betweent microstructures and separations and according to the quantitative analysis data and stress analysis,the essential reasons causing the occurrence of separation in high toughness plate linepipe steels are discussed and summarized.