Creep characterization of type 316LN and HT-9 stainless steels by the K-R creep damage model

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model, A, B, k, m, lambda, r, and q were determined for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation, For type 316LN stainless steel, lambda=epsilon (R)/epsilon* and lambda (f)=epsilon/epsilon (R) were 3.1 and increased with creep strain. The creep curve with lambda =3.1 depicted well the experimental data to the full lifetime and its damage curve showed a good agreement when r=24. However for the HT-9 stainless steel, the values of lambda and lambda (f) were different as lambda =6.2 and lambda (f) = 8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.