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Fujio TSUKAMOTO and Takashi SUZUKI Synopsis: The "hot-twist ductility" of highly alloyed austenitic steels such as AISI type 309, 310 and 330 was studied. Effects of alloying elements, melting practices, and grain size of these steels on the "hot-twist value" at testing temperatures ranging from 1,000 to 1,300•Ž were mainly investigated by torsion testing at a rate of 200 rpm. Fully austenitic specimens showed twist value curves of a convex type at testing temperatures, while two-phase specimens had much lower twist values than the former, nearly constant through all testing temperatures. With the former specimens, a transcrystalline fracture was observed at temperatures lower than that showing maximum twist value, while an intercrystalline fracture was shown at temperatures higher than that. In the case of type 310 steel, the higher the carbon content of specimen, the lower was its twist value. On the other hand, the maximum twist value was obtained with 0.2% C specimen in the case of type 309 steel. This difference was attributed to the fact that the ductility of these steels depended upon the stability of austenitic structure. Besides, effects of Si, Ni, Cr, N and Nb contents on twist value were explained. Comparing vacuum-melted with air-melted specimens, the effect of melting atmosphere was not so clear than that of alloying elements. Addition of small quantity of Al improved hot-twist ductility, while addition of large amount of Al as well as B up to 0.01% in the form of FeB much impaired ductility. In the case of high-C steels, the increase of annealing temperature which resulted in grain-coarsening decreased the hot-twist value owing to the increase of solubility of C atoms, while no change of the hot-twist value were observed with very low-C steels. Therefore, it was concluded that grain size itself did not affect the hot ductility.

Hot-Twist Ductility of High Cr-Ni Austenitic Steels