Grade 91 steel has been widely utilized in power plants over the last 20 years. Its specification for worldwide power plant construction has dramatically increased since the acceptance of Code Case 1943 for this material in 1983. Recent evaluation of a combination of ex-service Grade 91 steel components and virgin material has provided a unique opportunity to independently assess the performance of a combination of base metals and weldments. This approach has been grounded in the fundamental objective of identifying and quantifying metallurgical risk factors in 9%Cr creep strength enhanced ferritic (CSEF) steels and in Grade 91 steel in particular.

Identification of metallurgical risk factors is essential in the integration of a meaningful life management strategy for complex 9%Cr CSEF steels. In particular, there is a need to link the damage developed either in-service or through well-controlled testing to features in the material. These features may include, but not be limited to, carbonitride precipitates (and distribution of these phases along grain boundaries, e.g. M23C6 and MX-type), inclusions developed as a consequence of the steel making process (such as MnS, Al-rich or complex Ca-modified) and intermetallic phases which evolve in-service (Laves or Z-phase) or through product forming (AlN or BN). Identification and linking of these particles to damage is considered a critical need for widely used 9%Cr CSEF steels as Grade 91 steel can show widely variable behavior with respect to creep ductility whilst Grade 92 shows a marked and consistent trend to low creep ductility.

In this study, the evaluation of damage is detailed for an ex-service Grade 91 steel heat which exhibited a marked susceptibility to the evolution of in-service damage. This evaluation focuses on the evolution of HAZ damage under well-controlled, uniaxial creep test conditions using feature, cross-weld creep tests. For interrupted and failed samples, evaluation of damage was performed using state-of-the-art scanning electron microscopy (SEM) based techniques. Particular emphasis was made to link the post-test condition with the as-fabricated weldment and the parent material condition. It will be shown that there is a significant population of cavities associated with second phase particles, and particularly inclusions and intermetallic phases resulting from steel making and product manufacturing processes.

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