Behaviour of helium bubbles in helium-doped stainless steel weldments for nuclear power plant

Abstract

Summary This paper describes an investigation into the behaviour of helium bubbles in helium-doped austenitic stainless steel weldments by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and numerical calculations. The stainless steel was helium ion implanted and welded with a YAG laser apparatus. Helium ion implantation in the sample was performed with an 8 MeV implantation apparatus. The samples were doped with 1.0 × 1019 atoms/m2 at 5 MeV and then doped with 2.45 × 1019 atoms/m2 at 6 MeV. The results obtained may be summarised as follows. 1. At a low laser power, large helium bubbles of 3–4 μm size are formed at the weld bead/base metal interface, i.e. on the weld metal side of the weld bond region. The amount of weld metal increases, and the helium concentration decreases, with an increasing laser power. The helium bubbles therefore tend to have a diameter as small as under 1 μm. The results of the numerical calculations suggest that the helium concentration in the weld metal is more than 10 appm. Helium-free samples contain no helium bubbles. 2. Helium bubbles with a diameter of less than 0.2 μm are also formed at the grain boundaries in the heat-affected zone (HAZ). The helium bubble region increases with an increasing laser power. 3. The results of the TEM observations suggest that fine helium bubbles of under 0.2 μm size are formed at the dendrite cell boundaries in the weld metal. Their size tends to increase with an increasing laser power. 4. When helium ion implanted samples are laser-welded, helium bubbles are formed in the weld metal and HAZ. Any increases in the laser power and welding speed decrease the helium concentration. Even at helium concentrations of 2.9 and 3.8 appm, helium bubble-induced microcracking occurs at the dendrite cell boundaries in the weld metal.