Interpretation of the role of forest dislocations and precipitates in high-temperature creep in a Nb-stabilised austenitic stainless steel

The creep response of type 347 austenitic stainless steel was investigated by means of short-term constant-load tests at 750°C. The material was tested in annealed condition or after annealing and 10% prestraining. The results clearly show that cold working significantly increases creep strength at 750°C only at applied stresses larger than 150 MPa. Data were compared with results available in the literature for type 304 steel. The differences between the two materials are discussed in the light of the current theories of dislocation creep. In particular, the marked increase in creep strength observed in Nb-containing steels after testing the two materials in annealed condition was attributed to a fast precipitation of NbC nucleated on dislocations. The different creep response of prestrained steels are also attributed to the different kinetics of the precipitation of second-phase particles (M23C6 in type 304 and NbC in type 347).