Effects of alloying elements on single crystal growth and microstructure of austenitic stainless steels

It has been attempted to improve the resistance to stress corrosion cracking by manufacturing single crystalline austenitic stainless steels. However, single crystal growth was observed to depend on alloying elements of steels. In this paper, relations among single crystal growth, alloying elements and microstructure were studied. Casting experiments were carried out for 26 kinds of steels which contained 10.5-20.5 mass% chromium, 8.4-20.9 mass% nickel and 0-4.7 mass% molybdenum ail with carbon contents below 0.03 mass%. Single crystals were manufactured by using the mold withdrawal unidirectional solidification method which provided large size single crystals. Growth of single crystals and their microstructures were closely depended on the (Cr-eq/Ni-eq) ratio. Single crystal casting were obtained for steels with (Cr-eq/Ni-eq) less than 1.83. These single crystals were fully austenitic for (Cr-eq/Ni-eq) less than 1.5 while contained mixtures of the continuous austenitic single crystal matrix and ferrite phases for (Cr-eq/Ni-eq) between 1.5 and 1.83. Furthermore, for Mo added steels, phases enriched with the element were observed at gamma/delta grain boundaries by TEM. On the other hand, single crystal growth was not observed for steels with (Cr-eq/Ni-eq) higher than 1.87 because their microstructures were not dendritic, but rather of the lathy structure type.