Microstructural evolution of ASTM grade 91 after long-term creep testing

Abstract

Modified 9Cr–1Mo steel (ASTM grade P/T91) is conventionally applied in the construction of high efficiency supercritical and ultra-supercritical boilers, reaching metal temperatures up to 600–610 °C. As a part of the characterisation process of commercial pipes, Tenaris has performed over the years several long-term creep tests, with duration up to over 100,000 h, for temperatures between 550 and 650 °C. Microstructural characterisation has been performed on broken specimens, including analysis of the state of precipitation after TEM imaging of extraction replicas. The evolution of M23C6 coarse carbides, and MX carbo-nitrides, as well as the nucleation of new phases, such as Laves and Z are described in this paper. Observations reveal Laves phase nucleation and growth peak at 600 °C, whereas almost no particles were observed after ageing at higher temperature; moreover, transformation of MX carbo-nitrides into Z-phase is also faster at 600 °C, and slows at higher temperature.