Forging Strain Rate and Deformation Temperature Effects on the Fracture Toughness Properties of Type 304L Stainless Steel Precharged with Tritium

Abstract

Forged austenitic stainless steels are used as the materials of construction for pressure vessels designed to contain tritium at high pressure. These steels are highly resistant to tritium-assisted fracture but their resistance can depend on the details of the forging microstructure. In this study, the effects of forging strain rate and deformation temperature on the fracture toughness properties of Type 304L stainless steel were studied. Forgings were produced from a single heat of steel using four types of production forging equipment -hydraulic press, mechanical press, screw press, and high-energy-rate forging (HERF). Each machine imparted a different nominal strain rate during the deformation. The objective of the study was to characterize the J-Integral fracture toughness properties as a function of the industrial strain rate and temperature. The second objective was to measure the effects of tritium and decay helium on toughness. Tritium and decay helium effects were measured by thermally precharging the as-forged specimens with tritium gas at 340 MPa and 350°C and aging for up to five years at -80°C to build-in decay helium prior to testing. The results of this study show that the fracture toughness properties of the as-forged steels vary with forging strain rate and forging temperature. The effect is largely due to yield strength as the higher-strength forgings had the lower toughness values. Tritium exposures reduced the fracture toughness values remarkably to fracture toughness values averaging 10-20% of as-forged values. Forging strain rate and temperature had little or no effect on fracture toughness properties after tritium exposure.