A TEM investigation of M23C6 carbide precipitation behaviour on varying grain boundary misorientations in 304 stainless steels

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Transmission electron microscopy (TEM) along with electrochemical potentiokinetic reactivation (EPR) testing was performed on different grades of 304 stainless steel (0.01, 0.025, 0.05, and 0.07%C) in order to assess the sensitization and precipitation behaviour on different grain boundary misorientations. The materials were heat treated at 670°C for 50 h to subject the materials to the sensitization regime. The EPR data and TEM observations revealed that when the amount of carbon was increased the degree of sensitization increased along with the density of precipitates. Large angle misorientations (Θ > 15°) were prevalent in all the carbon content materials and the {1 1 0} grain surface orientation was found to be the major texturing orientation. The steels with lower carbon contents nucleated a few small precipitates on high angle grain boundaries, while larger amounts of carbides were observed on lower angle grain boundaries for the higher carbon contents. It was deemed that higher carbon contents required lower energies to nucleate and grow precipitates. A carbon content threshold was found (above 0.05% C) in which precipitates fully saturate the grain boundary. Precipitation followed the energies of different types of boundaries. The highest energy boundary (general random grain boundary) nucleated precipitates first, then precipitation followed on non-coherent twin boundaries, and was not observed on coherent twin boundaries. A “critical nucleation energy”, γgb(crit.), was therefore found to exist at which precipitation will occur on a boundary. This value was found to be in the range of 16 mJ m−2 < γgb(crit.) < 265 mJ m−2 which corresponds to the energies of special boundaries (coherent and non-coherent portions of twins respectively) at the ageing temperature of 670 °C.

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