Evolution of damage in tertiary creep of type 316(N) SS weld metal

Abstract This paper reports analysis of the tertiary creep data for SS 316(N) weld metal considering strain (rather than exposure time) as the parameter “driving” creep damage, e ˙ / e ˙ 0 ; e ˙ is the instantaneous true creep rate, and e ˙ 0 its value for the undamaged material. The early part of tertiary creep, which is the dominant fraction of the creep life, is well described by the relation, ln e ˙ = ln e ˙ 0 + k ( e - e 0 ) ν , e ⩾ e 0 ; k and ν are constants, and e0 a threshold strain corresponding presumably to nucleation/growth of voids to stable sizes. The quantitative formulation did reflect the significant modulation of creep damage kinetics by the in situ transformation of δ-ferrite in the weld metal to embrittling phases. As long as the extents of δ-ferrite transformation during creep covers a relatively narrow range for a given temperature, the parameters k, ν and e0 were independent of stress while e ˙ 0 varied as a power of stress. The rupture strength for tr = 105 h at 823 K could be predicted satisfactorily using this formulation.