The effect of grain boundary misorientation on sensitization and carbide precipitation in 304 stainless steel.

摘要

Stainless steels, although readily applied in industrial applications, are nonetheless susceptible to stress corrosion cracking (SCC) due to the depletion of chromium at or around the grain boundary to form {dollar}rm Msb{lcub}23{rcub}Csb6{dollar} precipitates in the temperature range of 500-850{dollar}spcirc{dollar}C (a phenomenon known as sensitization). This research will attempt to ascertain the general effect of grain boundary energy on the precipitation behavior with respect to differing grain boundary misorientations in 304 stainless steels.; The materials utilized in this study varied in carbon content (0.011, 0.025, 0.05, 0.07%C) and were heat treated at 670{dollar}spcirc{dollar}C for 10 and 50 hours and strained (0, 10, and 20% true strain). Sensitization measurements were obtained with an Electrochemical Potentiokinetic Reactivation (EPR) test and {dollar}rm Msb{lcub}23{rcub}Csb6{dollar} precipitation was observed with a transmission electron microscope (TEM).; The studies performed at the 50 hour heat treatment time suggest that lower carbon contents have few boundaries with carbides on them and occur only at large misorientations. In contrast, higher carbon contents have a higher grain boundary carbide density that occurs over a broader range of misorientations. This demonstrates that higher carbon contents do not need the extremely large energies generally associated with large misorientations to nucleate precipitates. Experiments performed at a lower aging time of 10 hours and those that were deformed to 20% true strain were found to have similar results.; Carbon content comparisons on non-coherent boundaries (that represent a constant interfacial energy site) revealed that precipitation occurs on the non-coherent step for higher carbon contents ({dollar}ge{dollar}0.025%C) but not for the lowest carbon content (0.011%C). This very fundamental observation affirms the conclusion that thermodynamics play a crucial role in carbide formation.; These conclusions all lead to the concept of a "critical nucleation energy", {dollar}rmgammasb{lcub}gb(crit){rcub},{dollar} that is necessary for the formation of a {dollar}rm Msb{lcub}23{rcub}Csb6{dollar} precipitate. The upper limit of this energy is the lowest energetic site where precipitation does occur (non-coherent twin boundary) and the lower limit is the energetic site where precipitation does not occur (coherent twin boundary); 16 mJ/m{dollar}rmsp2