Observations of Gas Bubbles in an Austenitic Stainless Steel Charged with3H

Abstract

Nitronic 40 is a high-strength austenitic stainless steel. Investigations are in progress to explore the possibility of using this material as a high-strength substitute for 300 series stainless steels, especially in hydrogen environments. There is also interest in determining the influence of helium on the mechanical properties of this material from the viewpoint of fusion reactor systems. The alloy is nominally (in weight percent) 21 Cr, 6 Ni, 9 Mn, and 63 Fe and is strengthened by 0.35 wt % N. Smooth bar tensile specimens approximately 3.5 mm in diameter with 25 mm gage lengths were machined from high energy rate forged (HERF) stock. These specimens were then gas phase charged with 2H at 64 MPa pressure at 470 K or 3H at 47.5 MPa at 345 K for 1450 h. Due to the nature of the charging the 3H distribution was nonuniform, ranging from approximately 1300 ppm at the specimen surface to less than 5 ppm at a depth of 0.5 mm from the surface. Following charging, the specimens were stored at ˜250 K for 5.5 years. Approximately one-third of the tritium decayed to 3He during this period, yielding a maximum of 900 ppm of 3He at the surface of the specimens. Tensile tests at 298 K revealed a significantly greater loss in ductility in the specimens charged with 3H. Following tensile testing TEM specimens of suitable size were sectioned near the fracture surface, thinned using conventional techniques for radioactive specimens, and examined. Electron microscopy was performed in a Philips EM 400T equipped with a field emission gun.