On the non-equilibrium segregation of Boron in 9-12%Cr steels

The addition of only 20-150 ppm of boron to martensitic 9-12%Cr steels has a strong enhancing effect on their creep strength, and boron-added martensitic creep strength enhanced steels have been introduced in power plant constructions. Boron is found to redistribute in the martensitic steels during cooling after normalizing in a non-equilibrium segregation process. In this work we attempt to quantify boron segregation during solution treatment in a martensitic 9%Cr steel containing 108 ppm of boron by atom probe tomography and transmission electron microscopy. The investigated material was a solution treated at 1100C for 1 h and cooled in air. Plate-like boride of Cr2B was found on a prior austenite grain boundary (PAGB). The distribution of B, Cr, Fe, Co, Mo, C and other elements in boride and matrix was quantified. Boron is considered to segregate onto PAGBs by non-equilibrium mechanism