Metallographic techniques for condition assessment and life prediction in SP249 guidelines

The first phase of component assessment according to the SP249 Generic Guidelines comprises a code-based calculation of life consumption and a review of operating and maintenance history. Should the component prove unacceptable on any of the indicated criteria, then the Guidelines direct the assessor to move to a hands-on inspection, involving conventional NDE methods and various metallographic techniques. This paper outlines the latter approach. Microstructural degradation and corresponding thermal softening of the base metal can result in a variety of microstructural changes in the steel, such as changes in composition, structure, structure, size and spacing of carbides; in ferrite composition; in solid-solution strength; and in lattice parameter. Carbide characteristics, measured by direct observation or indirectly by hardness measurement, have proved the most sensitive indicator of thermal degradation. Creep damage (cavitation) can be measured directly. The two major applications of replication techniques are (1) the study of microstructure (creep cavitation, gain size, etc.) using surface replication and optical microscopy, and (2) the examination and identification of small second-phase particles by extraction replica techniques, as, for example, for the purpose of interparticle spacing determination.