Effect of temperature and pressure on cavitation damage in sodium

Three materials with widely different mechanical properties, L-605, Stellite 6B, and AISI 316 stainless steel were subjected to accelerated cavitation damage in sodium at 800 F (427 C). L-605 was tested also at 400 F (204 C) and 1200 F (649 C). A magnetostrictive vibratory apparatus was used. Argon was introduced over the liquid sodium to maintain pressures ranging from 1 to 4 atm (1 to 4 × 105 N/m2). Cavitation damage was determined by volume loss measurements, and metallographic studies were made. It was found that increasing pressure significantly increased cavitation damage to all materials at all temperatures tested. When the volume-loss-rate data were normalized to consider only the heavily damaged area of the specimens, the average volume loss rate increased as a power function of pressure (at pressures above approximately 2 atm (2 × 105 N/m2)). This result of increasing damage with increasing pressure implies that, in fluid systems where cavitation occurs in high pressure regions, damage to components may be much greater than would be expected normally from cavitation tests conducted at atmospheric pressure. L-605, the one material tested at all three temperatures and all four pressures, exhibited greater damage at 800 F (427 C) than at 400 F (204 C), or 1200 F (649 C) at all pressures. At 800 F (427 C) the single temperature at which all materials were tested over the entire range of pressures, the relative ranking of the materials with respect to resistance to cavitation damage was the same regardless of pressure. In order of increasing damage, it was Stellite 6B, L-605, and AISI 316 stainless steel. This result, together with the fact that the damage rate increased with increasing pressure, suggests that laboratory cavitation damage testing can be greatly accelerated by conducting such tests at higher pressures. Metallographic examination of axially sectioned specimens of all the materials tested at 800 F (427 C) revealed severe undercutting of the surface and transgranular cracking. Surface deformation was indicated by the appearance of slip bands in all materials after testing at 800 F (427 C). Stellite 6B showed extensive cracking of carbides beneath the surface, and L-605 exhibited some bending of twins near the surace.