Abstract
The objective of this study is to identify the causes of intergranular environmentally assisted cracking in non-chromium depleted stainless steels, a phenomenon which has been observed under irradiated and non- irradiated conditions. Special emphasis is placed on quantifying the possible effects of sulfur, phosphorus and nitrogen relative to chromium depletion and aqueous impurities in altering both the crack growth rate and cracking morphology in 288°C water.
Eleven custom and three commercial heats have been examined to date, including several containing exaggerated sulfur levels. Slow strain rate tests on smooth, cylindrical specimens and fracture mechanics, crack growth rate tests on 1-T CT specimens were performed in 288°C water. The resulting cracking morphology and crack growth rate data were compared with grain boundary characterization performed by Auger electron spectroscopy and analytical electron microscopy.
While competitive segregation to grain boundaries of sulfur, phosphorus and nitrogen was observed, the amount of intergranular cracking correlated well only with grain boundary sulfur segregation and not with phosphorus and nitrogen segregation. However, in no instance was a statistically significant enhancement in crack growth rate observed in the fracture mechanics specimens. This contrasts with the lower temperature Huey test where intergranular attack is strongly sensitive to phosphorus (not sulfur or nitrogen) segregation. Chromium depletion and/or aqueous impurities can have a much more pronounced effect on crack growth rates in stainless steels than do metallurgical segregants. However, since differences exist between long vs short cracks, especially in crack tip water chemistry, the role of metallurgical impurities on environmental cracking may be more pronounced during the short crack / “initiation” stage.
