Effects of Heat Treatment and Aging on Microstructural and Mechanical Properties of RAFM Steel

This study investigated the effects of normalizing and tempering on microstructures and mechanical properties of reduced activation ferritic/martensitic steel with Y and Zr. The thermal stability and mechanical properties of the steel, subjected to two normalizing and one tempering (intermediate) treatment, were examined during aging at 823 K for 5000 h. OM, SEM and TEM were employed to observe microstructures and precipitates. Hardness, tensile and impact properties were measured using a microhardness tester, electronic universal material testing machine and pendulum impact testing machine. The results show that the optimal treatment was identified at 1323 K × 30 min + 1193 K × 30 min, yielding strength of 642 MPa at room temperature (RT) and a ductile-brittle transition temperature (DBTT) of 193 K. When the tempering temperature was raised from 963 K to 1063 K, the yield strength of the steel decreased from 724 MPa to 504 MPa, elongation increased from 17.9% to 25.6%, and DBTT decreased from 228 K to 221 K. The intermediate treatment (1323 K × 30 min + 1193 K × 30 min + 1028 K × 90 min) resulted in fine carbides, enhancing mechanical properties. The steel was stabilized to martensite after aging for 5000 h at 823 K. The Laves phase content increased during the aging process and became stable after aging for 2500 h. The yield strength of the steel after aging for 5000 h was 638 MPa (RT) and 308 MPa (923 K), with an impact energy of 313 J at RT and a DBTT of 204 K. Initial fine grain and M₂₃C₆, pinning of Y-Zr-O and precipitation of Laves phases were responsible for good mechanical properties and microstructural stability of the alloy.