G. Sasikala et al., Creep deformation and fracture behavior of types 316 and 316L(N) stainlesssteels and their weld metals, MET MAT T A, 31(4), 2000, pp. 1175-1185
The properties of a nuclear-grade type 316(L) stainless steel (SS) alloyed
with nitrogen (316L(N) SS) and its weld metal were studied at 873 and 923 K
in the range of applied stresses from 100 to 335 MPa. The results were com
pared with those obtained on a nuclear-grade type 316 SS, which is lean in
nitrogen. The creep rupture lives of the weld metals were found to be lower
than those of the respective base metals by a factor of 5 to 10. Both the
base and weld metals of 316L(N) SS exhibited better resistance to creep def
ormation compared to their 316 SS counterparts at identical test conditions
. A power-law relationship between the minimum creep rate and applied stres
s was found to be obeyed for both the base and weld metals. Both the weld m
etals generally exhibited lower rupture elongation than the respective base
metals; however, at 873 K, the 316 SS base and weld metals had similar rup
ture elongation at identical applied stresses. Comparison of the rupture li
ves of the two steels to the ASME curves for the expected minimum stress to
rupture for 316 SS base and weld metals showed that, for 316L(N) SS, the s
pecifications for maximum allowable stresses based on data for 316 SS could
prove overconservative. The influence of nitrogen on the creep deformation
and fracture behavior, especially in terms of its modifying the precipitat
ion kinetics, is discussed in light of the microstructural observations. In
welds containing delta ferrite, the kinetics of its transformation and the
nature of the transformation products control the deformation and fracture
behavior. The influence of nitrogen on the delta ferrite transformation be
havior and coarsening kinetics is also discussed, on the basis of extensive
characterization by metallographic techniques.