Effect of nitrogen content,titanium content and prior deformation on creep behaviour of nitrided austenitic steels

The creep behaviour of nitrided austenitic steels containing 0.5–2.4 wt.-%Ti has been studied at 750°C. A nitrogen content in excess of that required to form Ti(Cr, Nb)N decreased the creep rate at high stresses owing to precipitation during the tests. The increased strength with increasing titanium content could be related to an increase in the back stress which was of similar magnitude to the Orowan stress. Prior cold work decreased the primary creep. In constantload tests the minimum creep rate was reduced by prior cold work but in constant-stress tests there was little reduction in minimum creep rate until prior elongation was greater than 5%. After a prior elongation of 10%, both the decrease in primary creep elongation and the decrease in secondary creep rate were about an order of magnitude. Both effects are attributed to the retention of high dislocation densities and the slow recovery rates associated with the close spacing and dendritic nature of the particles. A simple conversion of constant-load to constant-stress tests was used and was consistent with the creep of these steels obeying a Garofalo type equation.