Effect of minor elements on hot workability of nickel-base superalloys

Abstract

Studies have been performed on the effect of five minor elements (S, Ca, Mg, Y, and Zr) on the hot workability of three solid-solution strengthened, nickel-base superalloys: Inconel 600, Hastelloy-X, and a Ni–Cr–W type alloy (NSC-1) for heat-exchanger pipes in high-temperature gas reactors. The hot workability was evaluated from the reduction of area in hot tensile tests using a Gleeble testing machine. Round bar specimens were prepared from near surface portions of alloy ingots melted and cast by vacuum induction melting, vacuum arc remelting, or electroslag remelting. When the sulphur content was increased, a ductility dip appeared for each alloy between 950° and 1150°C; the greater the sulphur content, the deeper the dip. At the same time, the fracture appearance became increasingly intergranular. By adding a small amount of Ca, Mg, Y, or Zr, the hot workability of these alloys changed drastically, and it was found that controlled addition of these minor elements gave a superior hot ductility. Defining ∆S as ∆S=%S–0·8 × %Ca–0·3 × %Mg–0·5 × %Y–0·1 × %Zr (wt-%) the hot ductility of these alloys changed systematically as follows, irrespective of the three refining processes: the ductility dip diminished and excellent ductility was achieved with 0·003>∆S>–0·004. The hot ductility decreased gradually with ∆S<–0·004, and became extremely poor when ∆S was larger than 0·003. Investigation of fractured surfaces with Auger electron spectroscopy and electron probe microanalysis showed that when ∆S was positive, atomically segregated sulphur was present on the intergranular fractured surfaces, while when ∆S was negative, atomic sulphur was not detected, but metallic sulphide compounds were observed both in grain interiors and on grain boundaries. The hot workability of nickel-base superalloys, therefore, can be related to the ∆S value, and in order to improve the hot workability of these alloys, minor elements should be added so that ∆S becomes nearly zero. By the controlled addition of Ca, Mg, Y, and Zr, a newly developed Ni–Cr–W type alloy (NSC-1) has been successfully processed.