Microstructure Characterization of Oxide Dispersion Strengthened Steels Containing Metallic Chromium Inclusions after High-Temperature Thermal Aging

Abstract

Microstructure characterizations of 9Cr-oxide dispersion strengthened (ODS) steels were carried out after high-temperature thermal aging to reproduce the anomalous microstructure change that occurred in the BOR-60 irradiation test — formation of abnormally coarse and irregular precipitates a few tens of micrometers in size. The 750°C thermal aging for 8,000 h produced the precipitate growth in normalized-and-tempered (NTed) 9Cr-ODS steels, however, the largest precipitates were only a few micrometers. The furnace-cooling (FC) heat treatment produced the matrix structure change from tempered martensite to α-ferrite, but the precipitate size after the thermal aging was in the same level as that in the NTed 9Cr-ODS steel. In the case of NTed defective 9Cr-ODS steel containing metallic Cr inclusions, coarse and irregular precipitates were formed nearby metallic Cr inclusions after the 750°C thermal aging for 8,000 h. In the case of FCed defective 9Cr-ODS steel containing metallic Cr inclusions, the 750°C thermal aging for 8,000 h produced the pronounced growth of coarse and irregular precipitates, which was equivalent to the anomalous microstructure change in the BOR-60 irradiation test. Based on the analyses using energy dispersive X-ray spectrometry (EDX) and electron backscattered pattern (EBSP), coarse and irregular precipitates were identified as M23C6.