The effects of hydrogen on the creep rupture properties of fe-ni alloys

Abstract

Creep tests were run on Fe-Ni alloys with nominal compositions of 100 pct Ni, 75 pct Ni-25 pct Fe, 50 pct Ni-50 pct Fe, 25 pet Ni-75 pet Fe and 100 pct Fe. Test temperatures were 898, 1073 and 1198 K, and the stress levels ranged from 6.5 to 80.1 MPa—varying with temperature and composition. Tests were conducted in a hydrogen or helium atmosphere, and creep rates, specimen elongations and rupture lifetimes were recorded. Grain boundary sliding measurements were made on nickel specimens to determine the fraction of strain due to grain boundary sliding at rupture. An alternating atmosphere test was also conducted on nickel specimens to see if a change in test atmosphere while the test was in progress would change creep rates. Finally metallographic studies were made of the fracture surfaces of all the test specimens using a light microscope and SEM. Results of the tests showed that hydrogen reduced the creep rupture lifetime of the 100 pct Ni and 75 pct Ni-25 pct Fe by as much as 80 pct. The lower Ni alloys showed little or no effect. The alternating atmosphere test showed no change in the creep rates of the Ni specimens when the atmosphere was cycled. Grain boundary sliding measurements showed no significant difference in the fraction of total strain due to grain boundary sliding. Metallography revealed no clear differences between fracture surfaces of specimens tested in hydrogen or helium. Causes for the observed creep behavior modification are explored.