Microstructural evolution and change in hardness during creep of NF709 austenitic stainless steel

Microstructural evolution and the change in hardness during creep deformation of NF709 austenitic stainless steel were investigated. Creep tests were carried out at 650 degrees C for 2932 h under a load of 210 MPa for comparison with aging specimen at 650 degrees C for 3000 h. The hardness results indicated that applied stress during creep process induced hardness increase. Analysis of longitudinal section microstructure showed that the creep damage caused by pores and the grain boundary hardening caused by elongated grains could be the factors leading to hardness differences. The G phase dispersedly precipitated in intragranular and interacted with dislocations during creep process, indicating strain hardening.