Fracture Toughness of CF8 Castings at Four Kelvin |
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Authors: | E L Brown T A Whipple R L Tobler |
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Affiliation: | 1. Department of Metallurgical Engineering, Colorado School of Mines, Golden, CO, 80401 2. Rockwell International, Rocky Flats Plant, P. O. Box 464, 80401, Golden, CO 3. Fracture and Deformation Division, 562, National Bureau of Standards, 80303, Boulder, CO
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Abstract: | 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 (Klc) of castings containing 3.2 to 14.5 pct δ-ferrite ranged from 84 to 179 MPa · ml/2 at 4 K. In contrast, a fully austenitic casting (0 pct δ-ferrite) exhibited a Klc, value of 331 MPa · ml/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. |
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