Tritium and decay helium effects on cracking thresholds and velocities in stainless steel

Crack initiation and propagation were studied in three tritium-exposed stai nless steels. The purpose was to measure cracking thresholds and velocities as a function of helium concentration in Type 21-6-9 stainless steel and c ompare the results to earlier measurements on Types 316L and 304L steels. F racture toughness specimens were cut from forgings, fatigue-cracked and exp osed to tritium at 423 K and 31 MPa; The samples were aged for selected tim es at 273 K to " build-in " He-3 from tritium decay. Tritium concentrations ranged from 0-2600 atomic parts-per-million (appm) and He-3 concentrations ranged from 0-600 appm. The samples were step-loaded at room temperature i n air using a screw-driven mechanical testing machine and held at fixed dis placement until crack initiation was detected. Crack propagation was monito red by continuously recording the drop in load until crack arrest. Threshol d stress intensity was calculated from the load and the crack length at the end of the test. Crack velocities were determined from the load-time recor ds and compliance relationships and verified on some samples using a DC pot ential-drop technique. The crack path was along grain and twin boundaries. For 21-6-9, the threshold for cracking decreased with increasing helium con centrations from about 90 MPa-m(1/2) (50 appm helium) to 25 MPa-m(1/2) (600 appm helium). Steady-state-crack velocities averaged 10(-7) m/s and was no t strongly dependent on helium concentration. The data show that embrittlem ent of tritium-exposed stainless steels is a form of hydrogen embrittlement made worse by the hardening of the microstructure from nanometer-sized hel ium bubbles that build-in with tritium decay.