Fracture Toughness of CF8 Castings at Four Kelvin
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INTRODUCTION
C A S T stainless steels are being considered for structural applications in high-energy superconducting devices operating at 4 K. 1 Castings offer economic advantages over wrought products where large, complex shapes are required. Owing to microstructure, however, the mechanical properties of castings are often inferior compared with those of wrought products. There are few published studies of the mechanical properties of cast stainless steels at cryogenic temperatures. In particular, there are no fracture toughness data for stainless steel castings at 4 K. Since fracture toughness is a critical design parameter for any large cryogenic structure, a brief study was conducted to provide such data for CF8 castings. CF8 stainless steel is the cast counterpart of wrought AISI 304. The base composition for both AISI 304 and CF8 is: Fe, 18 to 20 pct Cr, and 8 to 12 pct Ni. But AISI 304 is virtually 100 pct austenite (y) at room temperature, whereas CF8 typically contains a body-centered-cubic second phase, delta (6)-ferrite, within an austenitic matrix. The amount of &ferrite is controlled by manufacturing and composition variables; from 0 to 40 pct 8-ferrite may be present in CF8, although typically there is not more than 20 pct. 1 Delta-ferrite in the austenite matrix generally increases the strength and lowers the fracture toughness of welds and castings, 2 especially at cryogenic temperatures. 3 At room temperature the 8-ferrite is considered advantageous, serving as a strengthener and grain refiner while improving weldability. At cryogenic temperatures the 8-ferrite is relatively hard and brittle, and, as demonstrated quantitatively in this paper, even small amounts of 8-ferrite (3.2 to 14.5 pct) can have a detrimental effect on fracture toughness. This work was performed at the National Bureau of Standards (NBS), subsequent to work initiated at Brookhaven National Laboratory (BNL). A previous study at BNL provided tensile strength and ductility properties for a series of E.L. BROWN is Research Professor with the Department of Metallurgical Engineering, Colorado School of Mines, Golden, CO 80401. T. A. WHIPPLE is Senior Research Engineer with Rockwell International, Rocky Flats Plant, P. O. Box 464, Golden, CO 80401. R. L. TOBLER is Metallurgist, Fracture and Deformation Division 562, National Bureau of Standards, Boulder, CO 80303. Manuscript submitted July 27, 1981.
METALLURGICALTRANSACTIONS A
CF8 castings at temperatures from 300 to 4 K, as well as Charpy impact data at 300 and 77 K. Experimental difficulties in specimen holding and temperature control owing to dynamic loading prevented the measurement of Charpy impact energy at 4 K, which is the critical temperature for the intended engineering applications. Consequently, J-integral fracture toughness measurements were performed at NBS, using state of the art procedures which have been successfully applied in recent years to wrought stainless steels and their weldments. BNL cooperated by providing fracture test specimens from the same CF8 ca
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