Evolution of microstructure and mechanical properties of 9Cr ferrite/martensite steels with different Si content after long-term aging at 550 °C

Abstract

In view of developing novel alloys for applications in advanced Generation IV lead-cooled fast reactor (LFR) fuel cladding materials, four 9Cr ferrite/martensite (F/M) heat resistant steels with different Si content were developed. In the present work, the long-term thermal aging of 9Cr F/M steels was carried out in air at 550 °C for 500, 1000, 2000 and 5000 h. The microstructural evolution was investigated based on OM, SEM, EBSD and TEM technologies. The changes of mechanical properties after thermal aging were evaluated by room temperature tensile test. During long-term aging, the microstructures of all four steels exhibited highly thermal stability, and M23C6 carbides and MX carbonitrides also remained unchanged. Meanwhile, after aging for 5000 h, a new Fe2W-type Laves phase formed in all four steels with an average size of 94.8 ± 17 nm in 9Cr-1Si steel. The tensile properties of all samples aged for 2000 h exhibited almost no change compared with the as-received ones. However, after thermal exposure for 5000 h, the strength of all four 9Cr F/M steels increased due to the precipitation strengthening effect of the small Laves phase particles. Silicon exerted a certain degree of solid solution strengthening effect for 9Cr F/M steels. The volume fraction of Laves phase increased with the increase of Si content. As the Si content was 1%, residual high-temperature ferrite was found in the microstructure, but it has no detrimental effect on the mechanical properties of 9Cr F/M steels studied in the present work. Comprehensively considering the microstructure and mechanical properties, the suitable addition of Si content did not exceed 0.7%.