Precipitation and mechanical properties of Nb-modified ferritic stainless steel during isothermal aging

The influence of isothermal aging on precipitation behavior and mechanical properties of Nb-modified ferritic stainless steel was investigated using Thermo-calc software, scanning electron microscopy and transmission electron microscopy. It was observed that TiN, NbC and Fe2Nb formed in the investigated steel and the experimental results agreed well with the results calculated by Thermo-calc software. During isothermal aging at 800 °C, the coarsening rate of Fe2Nb is greater than that of NbC, and the calculated average sizes of NbC and Fe2Nb of the aged specimen agreed with the experimental results. In addition, the tensile strength and micro-hardness of the ferritic stainless steel increased with increased aging time from 24 h to 48 h. But aging at 800 °C for 96 h caused the coarsening of the precipitation, which led to a decrease of tensile strength and micro-hardness.