Effect of an Intermediate Process on the Microstructure and Mechanical Properties of HT9 Fuel Cladding

This study investigated the effect of an intermediate process on the microstructure and mechanical properties of a ferritic-martensitic tube for a fuel cladding tube in a sodium-cooled fast reactor (SFR). Cold drawing and heat treatment of the HT9 tube (12Cr-1MoWV) below the tempering temperature without a normalizing process was conducted (Route 1), whereas conventional normalizing and tempering conditions were also used for a comparison (Route 2). The results showed that the microstructure gradually transformed into the work-hardened structure during the Route 1 process, maintaining higher hardness throughout the reduction step. On the other hand, the microstructure of the Route 2 process gradually turned into a tempered martensite structure, whose hardness gradually increased throughout the step. M23C6 type of precipitates were generated regardless of the manufacturing route, where the size of precipitates in the Route 1 process was controlled to keep the high temperature mechanical property.