Fracture Toughness of CF8 Castings at Four Kelvin

The first fracture toughness measurements for CF8 stainless steel castings in liquid helium at 4 K are reported. Single-phase (austenite) and duplex (austenite + δ-ferrite) castings were tested. On the basis of estimates from J-integral data, the plane-strain fracture toughness (K_lc) of castings containing 3.2 to 14.5 pct δ-ferrite ranged from 84 to 179 MPa · m^l/2 at 4 K. In contrast, a fully austenitic casting (0 pct δ-ferrite) exhibited a K_lc, value of 331 MPa · m^l/2, which is nearly equivalent to the toughness of a wrought AISI 304 stainless steel of a similar strength. Light and scanning electron microscopy studies indicate that the inferior toughness of castings containing δ-ferrite may be attributed to the brittleness of this body-centered-cubic phase at cryogenic temperatures and its distribution in the microstructure. The relative stability of the austenitic phase with respect to martensitic phase transformation may also play a significant role.