Abstract
The effect of microstructure on tensile properties and creep fracture has been investigated using austenitic 21-4N steel.
In tensile tests of 21-4N steel, the ductility decreased owing to a fracture of the grain boundary reaction (GBR) nodules in the temperature range from 20°C to 500°C, while it increased by inhibition of grain boundary sliding due to the nodules at temperatures above about 600°C. 0.2% proof stress decreased with an increasing amount of GBR, but tensile strength did not decrease very much except at large amount of it.
It was found in 21-4N steel that recovery at elevated temperatures was more difficult to occur in the GBR nodules with rod-like precipitates than in the grains with spherical precipitates. In steels with less than 36% of GBR, a remarkable decrease in flow stress occurred at lower temperatures when the amount of plastic deformation increased. An increase in strain rate of tensile test at 600°C resulted in a large decrease in ductility and a noticeable increase in flow stress at smaller amount of plastic deformation when it caused a fracture of the nodules.
In notched creep rupture tests of 21-4N steel at 700°C, a ductile fracture at the interface of the nodules occurred in the notch-strengthened steel with GBR nodules, while grain boundary facet was observed in the notch-weakened one without the nodules. Furthermore, it was considered in 21-4N steel without the nodules as well as in SUS 304 steel that the creep fracture was caused by the grain boundary sliding and the deformation of grains, even under the creep condition where round type cracks were found at grain boundary.
