Creep deformation and fracture behavior of types 316 and 316L(N) stainlesssteels and their weld metals

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.