Investigation of creep damage in alloy 800H using ultrasonic velocity measurements

Results of ultrasonic velocity measurements on creep tested specimens of austenitic Alloy 800H are presented. It is shown that different influences have to be taken into account in order to correctly describe the observed behavior of the velocity during creep. At high stresses resulting in short lifetimes, the measured velocity changes can be attributed mainly to the plastic deformation of the material whereas at lower stresses the influence of physical discontinuities (micropores, microcracks) becomes dominant. Additionally, a purely thermal ageing effect due to carbide precipitations is noticed. Damage patterns are demonstrated using microradiography as well as metallography. From the results obtained it is suggested that the ultrasonic velocity becomes sensitive to microstructural damage at the transition range from secondary to tertiary creep. Concerning practical applications, this could allow the nondestructive detection of creep damage at a relatively early stage. (IZFP)