MICROSTRUCTURAL STUDY OF CREEP-RUPTURE IN A 12-PERCENT CHROMIUM FERRITIC STEEL

Creep cavitation and rupture of a 12% Cr-Mo-V steel has been studied using advanced analytical techniques. By electronically superposing etched and unetched micrographs of cavitated regions, it has been possible to correlate cavity size and form to location within the complex microstructure of this material. The results indicate that cavitation occurs not only on former austenite grain boundaries (FAGBs) but also within the former austenite grains. The analysis also indicates, however, that the cavities lying on some of those FAGBs which are nearly perpendicular to the applied stress are larger and closer spaced than those within the grains and on other FAGB facets. Calculations of cavity size using a constrained cavity growth model are shown to agree well with the cavity size measurements for three different strains. The success of the constrained cavity growth description implies that cavitating facets in 12% Cr-Mo-V steel quickly shed load to other regions of the microstructure and thus act like microcracks long before cavity coalescence becomes evident. The possible role of sulphur in the nucleation of cavities has also been investigated. From Auger mapping results, it is concluded that larger concentration of sulphur are typically present where cavity nucleation takes place. This suggests that sulphur facilities cavity nucleation in this material.