Helium embrittlement in type 304 stainless steel

Helium was uniformly injected into small tensile samples of type 304 (austenitic) stainless steel to concentrations of 1 × 10⁻⁷ and 3 × 10⁻⁵ atom fraction helium. Subsequent tensile testing above 540 °C revealed progressive ductility loss—as measured by total elongation at rupture—with increasing temperature. This effect was more severe in samples with the high helium content where elongations at 760 °C were a third or less of those of control samples. Yield and tensile strengths remained unaltered by the presence of helium.Above 650 °C, grain boundary sliding, which results in intergranular cracking, becomes important. The cracks begin as voids on carbide particles which act as obstacles to grain boundary sliding. In the presence of helium, bubbles attach themselves to these carbide particles and serve as void nuclei, thereby accelerating the process. Bubbles were also more prevalent on grain boundaries, dislocations and inclusion particles than isolated in the matrix.