Control of precipitation behavior in reduced activation steels by intermediate heat treatment

► MX precipitates instead of M 23C 6 during intermediate treatment at 850 °C. ► The volume fraction of M 23C 6 decreases due to the decrease of carbon content in the matrix during tempering. ► The decrease of volume fraction of M 23C 6 causes the decrease of the mean size of M 23C 6 from 150 nm to 70 nm. ► The fine carbides have vital contribution to mechanical properties during the service. A special intermediate heat treatment was studied in order to obtain dispersed distribution of fine M 23C 6 carbides in reduced activation steels. The results indicated that MX carbonitrides rather than M 23C 6 precipitated first in the steel with intermediate heat treatment at 850 °C, which led to a decrease of carbon concentration in the supersaturated martensitic matrix and correspondingly a reduced volume fraction from 2.6% to 0.6% and mean size of M 23C 6 from 150 nm to 70 nm. The dispersed fine carbides were beneficial to the enhancement of the hardness in the steel. The intermediate heat treatment was proposed for an improvement of creep properties in reduced activation steel.