Damage assessment of creep affected weldments of a Grade 91 header component after long-term high temperature service

Martensitic Grade 91 heat-resistant steel has been used successfully within power plant construction over the last decades. In this paper, results of a service failure investigation conducted on two parts cut from a creep-damaged header component are presented. After an operational time of about 78,000 h, first cracks, located near the weld toe at the flanks of several stubs, were found in 2006. After repair work, consisting of eliminating the cracks by grinding followed by magnetic particle inspection, the header component went into service for another 9,000 h with unfilled excavations until its replacement. The examinations show typical problems of 9–12 % Cr steels correlating with the Type IV mechanism like extensive pore and cavity formation in the area of the fine-grained heat-affected zone. The state of damage of the welds as well as the adjacent materials—and what this implies for a temporary extended operation of a damaged component—was investigated using hardness measurements, light optical (LOM) and scanning electron microscopy (SEM). It is shown that a repair by open defect-removal excavations eliminates the cracks found within the creep damaged material but implies varied stress conditions in adjacent areas which might favour accelerated creep failure during further operation. Further, the location, size and density of creep cavities were analysed. Microscopic investigations indicate that the main damage has accumulated in an equiaxed grain structure showing significantly smaller grain sizes, which is also shown by electron back scatter diffraction (EBSD) analysis.