1992 Volume 78 Issue 6 Pages 934-940
Creep fracture modes of the SUS321H steel have metallographically been examined over the periods up to about 15×104 h at temperatures between 525 and 900°C in order to draw a creep fracture mechanism map of a SUS321H type steel.
The creep fracture modes have been divided to one transgranular creep fracture at shorter rupture time and three types of intergranular creep fracture ; the cracking at the surface of the specimen, the creep cavitation and the cracking at sigma/matrix interface.
The fracture due to proceeding of surface crack was observed in the wide ranges of temperatures and stresses in SUS321H steel. As this fracture mode due to the surface cracking was not observed in other steels, this mode seems to be characteristic this steel. In this steel grain boundary carbides of M23C6 disappears gradually by change of the carbides from M23C6 to TiC within grain during creep. This disappearance of the grain boundary carbides suggests that the acceleration of the grain boundary sliding rate leads to a marked formation of the surface cracking.
The fracture due to the creep cavitation was observed in the temperature range lower than 550°C. In these lower temperatures, the many carbides are still remains at the grain boundaries and the creep cavities occures at the interface of the grain boundary carbides and austenite matrix.
The fractures due to cracking at the interface of sigma/matrix were observed at longer rupture times, similar to SUS304H and SUS316H steels.