Effect of weld thermal cycle and restraint stress on helium bubble formation in stainless steels

Helium bubble structure was examined in helium-implanted stainless steels after applying thermal cycles and tensile stresses using a weld thermal and stress cycle simulator. SUS304 specimens implanted with helium ions to 5 appm were heated at 1473 K for 2 s in an argon gas atmosphere. The heat-up rate and cooling rate were controlled to be 90 and 130 K/s, respectively. Tensile stresses ranging from 0 to 4 MPa were applied immediately after reaching a temperature of 1473 K. TEM observation revealed that bubble formation occurred even after short annealing times and that the size of the helium bubbles was strongly dependent on the tensile stress during heating.