Creep Analysis of a Service-exposed 1.25Cr-0.5Mo Steel Using the Omega Method

In recent years, a more sophisticated methodology to predict the remnant creep life is demanded as the operating duration of existing high temperature components have exceeded their design lives in various industries. In that sense, the strain rate based life assessment could lead to a better solution than conventionally applied methods relying upon hardness measurements, observation of microstructures and so on because strain rate is the one of the most sensitive indicator of damage accumulation by creep. In the present work, authors have examined the applicability of the omega method to creep curve description and life assessment using a service exposed 1.25Cr-0.5Mo steel operated for twenty three years in a refinery. The materials examined were two kinds of parent materials (a normalized and tempered plate and a forged flange) and a weld metal fabricated by a Shield Metal Arc Welding technique. Despite relatively large cast to cast variations in magnitudes of the parameters termed ε0 and Ω in the Omega method, consistent correlation between these parameters and the creep properties such as rupture life, a magnitude of strain and ductility were found. As reported in the previous works, ε0 and Ω can be expressed by the equation which is similar to that for Norton's law. Using these relationships, the creep behavior in terms of time-strain correlation from a low strain regime to rupture and time to rupture was predicted with satisfactory accuracy in the testing conditions in the present work.