Microstructural Degradation and Creep Fracture Behavior of Conventionally and Thermomechanically Treated 9% Chromium Heat Resistant Steel
- 254 Downloads
The microstructural degradation and the creep fracture behavior of conventionally and thermomechanically treated Grade 91 steel were investigated after performing small punch creep tests. A remarkable reduction in creep ductility was observed for the samples thermomechanically treated in comparison to those conventionally treated under the tested conditions of load (200 N) and temperature (700 °C). A change in the fracture mechanism from a ductile transgranular fracture to a brittle intergranular fracture was observed when changing from the conventionally treated to the thermomechanically treated processing condition, leading to this drop in creep ductility. The change in the fracture mechanism was associated to the localized concentration of creep deformation, close to coarse M23C6 carbides, at the vicinity of prior austenite grain boundaries (PAGB) in the thermomechanically treated samples. The preferential recovery experienced at the vicinity of PAGB produced the loss of the lath structure and the coarsening of the M23C6 precipitates. The electron microscopy images provided suggest that the creep cavities nucleate in these weak recovered areas, associated to the presence of coarse M23C6. After the coalescence of the cavities the propagation of the cracks was facilitated by the large prior austenite grain size produced during the austenitization which favors the propagation of the cracks along grain boundaries triggering the intergranular brittle fracture. This fracture mechanism limits the potential use of the proposed thermomechanical processing routes.
KeywordsCreep resistant steels Thermomechanical treatment Creep fracture behavior Microstructural degradation Small punch creep tests Ausforming
Authors acknowledge financial support to Ministerio de Economia y Competitividad (MINECO) through in the form of a Coordinate Project (MAT2016-80875-C3-1-R). Authors also would like to acknowledge financial support to Comunidad de Madrid through DIMMAT-CM_S2013/MIT-2775 project. The authors are grateful for the dilatometer tests by Phase Transformation laboratory. J.Vivas acknowledges financial support in the form of a FPI Grant BES-2014-069863. This work contributes to the Joint Programme on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA).
- 2.R.L. Klueh, K. Ehrlich, F. Abe, J. Nucl. Mater. 191–194(Part A), 116–124 (1992)Google Scholar