A TEM INVESTIGATION OF M23C6 CARBIDE PRECIPITATION BEHAVIOR ON VARYING GRAIN-BOUNDARY MISORIENTATIONS IN 304 STAINLESS-STEELS

Transmission electron microscopy (TEM) along with electrochemical pote ntiokinetic reactivation (EPR) testing was performed on different grad es of 304 stainless steel (0.01, d0.025, 0.05, and 0.07% C) in order t o assess the sensitization and precipitation behaviour on different gr ain boundary misorientations. The materials were heat treated at 670 d egrees C for 50 h to subject the materials to the sensitization regime . The EPR data and TEM observations revealed that when the amount of c arbon was increased the degree of sensitization increased along with t he density of precipitates. Large angle misorientations (Theta > 15 de grees) were prevalent in all the carbon content materials and the {110 } grain surface orientation was found to be the major texturing orient ation. The steels with lower carbon contents nucleated a few small pre cipitates on high angle grain boundaries, while larger amounts of carb ides were observed on lower angle grain boundaries for the higher carb on 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 gr ain boundary. Precipitation followed the energies of different types o f boundaries. The highest energy boundary (general random grain bounda ry) nucleated precipitates first, then precipitation followed on non-c oherent twin boundaries, and was not observed on coherent twin boundar ies. A ''critical nucleation energy'', gamma(gb(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) < gamma(gb(crit.)) < 265 mJ m(-2) which corresponds to the energies of special boundaries (coherent and non-coherent portions of twins respectively) at the agei ng temperature of 670 degrees C. (C) 1998 Chapman & Hall.