Effect of the Grain Boundary Reaction on the Notched Creep Rupture Strength of the Austenitic Heat Resisting Steel

Abstract

An analysis of stress and strain during creep was made on the notched bar specimens of austenitic 21-4N steel by finite element method.
A decrease in the stress concentration at notch root due to creep deformation occurred more rapidly in the notch-strengthened ductile specimen with 36% of the grain boundary reaction (GBR) than in the notch-weakened brittle one of greater hardness (4%GBR). The equivalent strain at notch root was smaller than that of smooth specimen except in the early stage of creep in the ductile steel, but the former was always larger than the latter in the strongly notch-weakened one.
In the brittle specimen, an increase in nominal stress enhanced the creep deformation at notch root and consequently lowered rupture time ratio of the notched specimen to the smooth one. Almost of the rupture life was spent in the creep deformation before the initiation of grain boundary cracks in the notched specimen as well as in the smooth specimen, independent of amount of GBR. Amount of plastic deformation necessary to initiate cracks was larger in the ductile specimen than in the brittle one. This is considered to greatly affect the rupture life of the notched specimen.