Abnormal Phases in High W Content Nickel Base Superalloys and Phase Control

Abstract

In this paper, the forming conditions and harmful effects of the abnormal phases such as M 6 C carbides, (W, Mo) and in a series of high W content K9, K20 and 601 alloys, which are suitable for forging die, have been studied. The abnormal phases precipitated from liquid belong to primary phases. The large blocky M 6 C formed in as-cast alloys, as the total amount of (W+Mo) and carbon content exceeded 18 wt% and 0.15 wt% respectively, while the carbon content less than 0.06 wt% promoted the formation of primary and phases. The addition of elements such as Co or Ru and the rapid solidification exhibited a decreased tendency to precipitate M 6 C carbides. The primary M 6 C carbide is the most stable phase among the three abnormal phases and is not removed until the full melting of alloy. Contrarily, the and are unstable phases and can transform to M 6 C carbides. The primary M 6 C carbides severely deteriorate the mechanical properties of the alloys, and the stress rupture life of the alloys at 1100 /118MPa decreased over 50%, as the volume fraction of primary M 6 C in alloys was higher than 1.5%. It was very difficult to avoid the formation of primary M 6 C for a heavy section forging die cast by this series alloys, therefore the compositional modification for these alloys was performed.