Effect of microalloying additions on the hot ductility of cast FeNi36

Abstract

Hot tensile tests were carried out on FeNi36 (Invar alloy) over the temperature range of 850–1150°C and at strain rates of 0.001–1s−1. The hot ductility improved by increasing temperature and strain rate. The improvement of hot ductility at high temperatures was attributed to the influence of dynamic recrystallization. On the other hand, increasing strain rate could reduce the time for the propagation of voids and therefore postpone final fracture. The influences of microalloying additions from Al, Zr, Ti and Mg on the hot ductility were investigated by similar hot tensile tests. The results showed that the addition of Al as a microalloying element could harm the hot ductility. It was associated with the increase of the stacking fault energy of the base metal, thereby decreasing the tendency for dynamic recrystallization. In addition, the aluminum oxide particles could act as the preferential sites for cracking. The addition of Zr degraded the hot ductility. It was attributed to increasing the stacking fault energy of the base metal and thereby decreasing the tendency for dynamic recrystallization. The hot ductility was improved by the addition of Ti. The positive effect of Ti was attributed to its effect on tie-up of S in TiS compounds. Magnesium was found as the most effective microalloying addition which could remarkably improve the hot ductility of the base metal. The influence of Mg on reacting with S thereby avoiding the grain boundary weakness was found responsible for the remarkable improvement of hot ductility. By adding 0.003wt.% Mg the amount of S in the material decreased from 0.0022 to 0.0011wt.%.