Abstract:The resent work in the Department of Superalloys and Itermetallic Composites, Institute of Metal Research, about the effect of nitrogen (N) on the microstructure and properties of cast superalloys was summarized. The control of N cast superalloys melting process was discussed. The results show that N content increases with the increase of Cr content in the virgin alloys, and the increment of recycle times or revert proportion in reverted alloys. Increasing N content considerably reduces the mechanical properties of both cast Ni-based superalloys and Co-based superalloys. In cast Ni-based superalloys, N reacts mainly with Ti to form TiN particles in the melt, which acts as profuse nucleus for subsequent carbides nucleation and growth at a higher temperature than in the normal solidification. These blocky carbides with large size can block the dendrite channels, and reduce the fluidity of the final liquid fraction and the permeability of the solidification structure. Consequently, increasing N content results in a greater degree of microporosity, which leads to a significant decrease in the mechanical properties of cast Ni-based superalloys. In Co-based superalloy, high N content will extend the solidification window during the solidification process, and then leads to the coarse carbide eutectic structure by increasing the quantity of the interdendritic Cr₇C₃ carbide eutectic, which restrains the M₂₃C₆ particles precipitates in the interdendritic regions. This microstructure is conducive to promote the initiation and propagation of the cracks and decrease the mechanical properties of cast Ni-based superalloys.

Key words: cast superalloy; Ni-based superalloy; Co-based superalloy; nitrogen; effect mechanism