Abstract
Structural changes during creep have been studied in a 20Cr-25Ni-Nb stabilised stainless steel using transmission electron microscopy. The application of multiple-mechanical-thermal-treatment (MMTT) produced in the steel a very fine cellular substructure supported by Nb(CN) particles. The cells formed during MMTT have been converted into subgrains during early stages of creep and elongated on further creep deformation at all the three stresses (70, 84, 95 N/mm[2]) examined. The elongation was more pronounced at the high stresses (84, 95 N/mm[2]) and due to the banded distribution of Nb(CN) the elongation occurred in the areas of subgrains where the density of particles was relatively lower and the width of the subgrains were comparable to the interspacing of the particle bands (-1mum). Dislocation analysis of the substructure indicated that most of the dislocations were close to screw orientation. The analysis of creep data at constant load and variable temperatures showed that two creep mechanisms were operating, one below and the other above a critical temperature, or at constant temperature a critical stress. It was found that the activation energy for the creep process operating below the transition point was about 300 kJ/mol and for that operating above it was about 630 kJ/mol. Over the stress range studied both activation energies remained stress independent. The transition temperature however decreased linearly with increasing creep stress. Experimental data indicated that the transition from one creep mechanism to the other took place over a narrow range of temperature or stress.