Refinement of cast microstructure of A517 steel by addition of TiB2

The effect of TiB₂ addition on microstructure refinement of the as-cast and reheated A517 steel has been investigated. 0.1 wt.% TiB₂ addition resulted in a reduction in equiaxed γ grain size from 990 ± 183 to 116 ± 35 μm and an increase in the volume fraction of equiaxed γ grain region from 5% to 67% in the as-cast A517 steel ingots. Microstructure analysis identified TiN particles rather than TiB₂. This is attributed to the low thermodynamic stability of TiB₂, leading to its decomposition into free Ti and B elements at an elevated temperature. Then, chemical reaction between the free Ti and residual nitrogen in the liquid resulted in the formation of TiN. Hence, it is considered that TiN acted as heterogeneous nucleation sites for the δ-ferrite. This initiated the refinement and columnar to equiaxed transition of δ-dendrites. As a result, the subsequently formed γ grains were correspondingly refined. Such microstructure refinement led to improvement of the yield strength and ultimate tensile strength of the as-cast A517 steel. However, the reheating of the as-cast A517 steel resulted in a marginal microstructure refinement in the samples with low TiB₂ addition. This is attributed to the limited pinning effect of the coarse TiN particles formed during casting process. Consequently, the tensile properties of the reheated A517 steel remained unaffected by the TiB₂ addition.