Abstract
The effect of multiple-mechanical-thermal-treatment (MMTT) on low- and high-temperature mechanical properties of niobium-stabilised austenitic steels has been studied. MMTT consisted of 2% plastic prestrain at room temperature of the solution treated material followed by ageing at 800 C for 1/2h to promote the precipitation of Nb(C,N). The 5-MMTT cycle material appears to give best combination of strength and ductility. Electron microscopy (HVEM) studies have shown that MMTT introduces finely dispersed particles of Nb(C,N) which stabilise and support a comparatively high dislocation density. MMTT strengthening is, therefore, due to the combined contribution of dispersion and dislocation substructure hardening. The low-temperature strength of the MMTT material was found to be inferior to the cold-worked material, because of extensive recovery occurring during the thermal stage of each MMTT cycle. The high temperature strength (hot tensile and creep) was, on the other hand, found to be appreciably superior for the MMTT material as compared with other comparable treatments. The substantially improved creep resistance is, however, accompanied by poor creep ductility, which has been correlated with void formation at precipitate particles and inclusions during the prestraining stage of each MMTT cycle. Variable stress creep tests performed during this work revealed inconsistencies in recovery creep measurements and theoretical creep models based on the concept of internal stress. The results indicate that anelastic relaxation processes play a dominant role in such tests.