Metallographic analyses were carried out to clarify the micro-mechanism of Type IV failure at the outer edge of HAZ for W containing martensitic creep resistant 9 to 12% Cr steels. Microstructure observation and the creep rupture test determined the type IV failure location at fine grain HAZ through the welding thermal cycle simulation. Based on the comparison of three candidate mechanisms, grain size refinement, dislocation sub-structure evolution and precipitation morphology alteration, the microstructure evolution by thermal cycle of welding and PWHT was expected to be the main reason for the failure. TEM analysis of the microstructure elucidated the globular sub-grain microstructure formation by PWHT derived from the low dislocation density of the fine grain martensite with ambiguous lath structure. The precipitation morphology change is also one candidate for type IV failure. The quantification of the influence of V-Wing type MX disappearance at fine grain HAZ on creep rupture strength must be accurately estimated in further research.

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