Effect of Nb precipitate coarsening on the high temperature strength in Nb containing ferritic stainless steels

The effect of Nb precipitate coarsening on the high temperature strength in Nb containing ferritic stainless steels was investigated. Hot tensile tests were carried out after aging at 700 °C for different times using a Gleeble 1500. High temperature strength decreased with increasing aging time. During the aging, Nb(C,N), Fe2Nb and Fe3Nb3C were precipitated out. The coarsening rate of Fe2Nb was much faster than that of NbC due to the incoherent interface between Fe2Nb and the Fe matrix, which was in contrast to the semi-coherent interface between NbC and the Fe matrix. The observed coarsening behavior well agreed with the calculated coarsening behavior of precipitates by Ostwald ripening equation. It was found that the coarse rod shaped Fe2Nb formed during aging was very detrimental to the high temperature strength in Nb containing steels.