Effect of vacuum arc remelting and processing parameters on structure and properties of high purity niobium

Niobium and niobium alloys are normally produced by vacuum arc remelting (VAR) process. In the present work, the effect of various process parameters such as arc voltage, arc current (melt rate) and fill ratio on arc stability during VAR process and solidification structure of remelted niobium ingot is investigated. The effect of thermomechanical processing of VAR ingot on microstructure and mechanical properties is also investigated. It has been established that arc voltage is a function of arc length. For melting of niobium in 110 and 150 mm diameter crucibles, the optimum arc voltage is in the range of 34-37 V. It has been observed that the arc voltage needed for melting niobium is much higher than that used for melting stainless steels, reactive metals and superalloys. It has also been observed that for a given arc voltage, fill ratio has no significant influence on the melt rate of consumable electrode. However, arc current has significant influence on the melt rate and consequently on the solidification structure of the ingot. Thermomechanical processing of the VAR ingot is shown to result in significant increase in room temperature strength and ductility. However, the processing temperatures do not have any significant influence on the tensile properties.