Microstructural evolution and characterization of a ferroniobium alloy

Niobium in the form of a ferroniobium alloy is added during the steel-making process to improve the mechanical properties of steel, but it may contain phase(s) with high melting temperatures that may be slow to melt or dissolve. It has been suggested that these phases lead to the presence of coarse Nb-rich particles in the resultant steel, which may adversely affect the mechanical properties. In the present study, electron microscopy and differential scanning calorimetry (DSC) were used to identify phases and microstructural evolution of a commercial grade ferroniobium alloy. The ferroniobium alloy was composed of two main phases, that is, Nb-rich solid solution and μ phase (Fe₇Nb₆). The μ phase (Fe₇Nb₆) was formed as a result of three reactions and exhibited three different morphologies based on their formation temperatures (proeutectic intermetallic, eutectic intermetallic and eutectoid intermetallic). The intermetallic that formed via the eutectic reaction was slightly Nb-rich and was heavily faulted. The Nb-rich eutectic portion of the Fe-Nb binary-phase diagram was also modified based on the DSC results to incorporate the effect of impurities in commercial alloys. On the basis of solidification study, the estimated cooling rate for the as received ferroniobium alloy was 10 K/min.