Effect of B and Ce on the grain boundary segregation of Mo-rich phases in super-austenitic stainless steels

Mo-rich precipitation in super-austenitic stainless steels has significant effects on their hot workability and corrosion resistance. The occupation tendencies of Mo and other alloying elements at Σ3(112), Σ5(210), and Σ9(114) grain boundaries of fcc-Fe were calculated through the first-principles method, and the influence of B and Ce on the grain boundary segregation of alloying elements was analyzed. Then, microscopic mechanism for the precipitation in super-austenitic stainless steels containing B and B + Ce was also discussed. The results show that the segregation tendencies of Cr and Ni to grain boundaries are extremely weak, while those of Mo and Ce are materially stronger, and they are apt to segregate at Σ5(210) and Σ9(114) grain boundaries. Mo, in particular, exhibits a wider segregation area and tends to segregate to the whole grain boundary regions. However, after being located at grain boundaries, B and B + Ce make the segregation of Mo at these grain boundaries more difficult, which is beneficial to adjusting the grain boundary segregation of Mo. Moreover, the calculated results agree well with the experimental results. Through microstructure characterization, the micro-alloying of B and B + Ce can indeed inhibit the Mo-rich precipitation in super-austenitic stainless steels, especially for B + Ce, reducing a great number of large precipitates along grain boundaries to discontinuous and finer precipitates.